REVIEW
The Utility of Sulfonate Salts in Drug Development
DAVID P. ELDER,1 ED DELANEY,2 ANDREW TEASDALE,3 STEVE EYLEY,3 VAN D. REIF,4 KARINE JACQ,5 KEVIN L. FACCHINE,6
ROLF SCHULTE OESTRICH,7 PATRICK SANDRA,5 FRANK DAVID5
1
GlaxoSmithKline Research and Development, Park Road, Ware, Hertfordshire SG12 0DP, United Kingdom
2
Reaction Science Consulting LLC, Princeton, New York 08540
3
Astra Zeneca, R&D Charnwood, Bakewell Road, Loughborough, Leicestershire LE11 5RK, United Kingdom
4
Formerly Schering-Plough, 556 Morris Avenue, Summit, New Jersey 07901-1330
5
Research Institute for Chromatography, Pres. Kennedypark 26, B-8500 Kortrijk, Belgium
6
GlaxoSmithKline Research and Development, Five Moore Drive, Research Triangle Park, North Carolina 27709-3398
7
F. Hoffmann-La Roche Ltd, Grenzacher Strasse, 4070 Basel, Switzerland
Received 3 September 2009; revised 13 November 2009; accepted 14 November 2009
Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.22058
ABSTRACT: The issue of controlling genotoxic impurities in novel active pharmaceutical
ingredients (APIs) is a significant challenge. Much of the current regulatory concern, has been
focused on the formation and control of genotoxic sulfonate esters. This is linked with the
withdrawal of Viracept (Nefinavir mesilate) from European markets in mid-2007, over concerns
about elevated levels of ethyl methanesulfonate (EMS). This issue has resulted in calls from
European regulators to assess risk mitigation strategies for all marketed products employing a
sulfonic acid counter-ion to ensure that the sulfonate esters that could be potentially formed are
controlled to threshold of toxicological concern (TTC)-based limits. This has even led to calls to
avoid sulfonic acids as salt counter-ions. However, sulfonic acid salts possess a range of
properties that are useful to both synthetic and formulation chemists. Whilst sulfonate salts
are not a universal panacea to some of the problems of salt formation they do offer significant
advantages as alternatives to other salt forming moieties under certain circumstances. This
review thus sets out to define some of the advantages provided through utilization of sulfonic
acids, explaining the importance of their retention as part of a thorough salt selection process.
ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:2948–2961, 2010
Keywords: alkylators; crystallinity; crystallization; formulation; hydrates/solvates; hygroscopicity; morphology; physicochemical properties; polymorphism
INTRODUCTION
In mid-2007 the European Medicines Agency suspended the marketing authorization of Viracept
(nelfinavir mesylate), an anti-viral medicinal product, owing to concerns over the presence of elevated
levels of ethyl methanesulfonate (EMS), in the drug
product.1 As a consequence of these public health
concerns, Swissmedic requested marketing authorization holders (MAHs) to perform risk assessments
on preparations containing sulfonic acid salts (e.g.,
Correspondence to: David P. Elder (Telephone: þ44-1920883658;
Fax: þ44-1920882679; E-mail: [email protected])
Journal of Pharmaceutical Sciences, Vol. 99, 2948–2961 (2010)
ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association
2948
mesylates (methanesulfonic acids), besylates (benzenesulfonic acids), and tosylates (toluenesulfonic
acids)) to check for the presence of sulfonate esters
and, if required, to take appropriate measures to
avoid them.2 The legal premise for this requirement
is a specific production statement relating to the
manufacture of mesylates within the European
Pharmacopoeia and the allowable levels are based
on the threshold of toxicological concern (TTC)
limits.3
The Coordination Group for Mutual RecognitionHuman committee (CMDh) also made a similar
request.4 MAHs were informed that any such changes
requiring amendment to the method of manufacture
or control of active pharmaceutical ingredient (API)
or drug product must be submitted to the competent
authorities using the established procedures, along
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
with a timetable for the various submissions of each of
the variations that would be required. CMDh
indicated that the risk assessments should be made
available upon request from any competent authority.
One potential unwarranted reaction by regulators
or indeed by industry might be to advocate the
restriction or replacement of sulfonate salt counterions with ‘‘less hazardous’’ alternatives. As there are,
in many instances, significant advantages offered by
sulfonate counter-ions, consequently their exclusion
would have a major impact on the development of
effective medicines. This current regulatory unease is
clearly linked to the problems associated with
Viracept. However, the root cause of this product
recall was a special case involving the unanticipated
contamination of the starting material (methanesulfonic acid) with ethanol5 resulting in the formation of
elevated levels of EMS over an extended time period.
This scenario is not at all representative of typical
processing conditions for salt-formation. This is
because the acid/base reaction involved in the
formation of the pharmaceutical salt is extremely
rapid and progresses to completion and is much more
favored than the potential side reaction between the
acid and the solvent (short chain alkyl alcohols). This
ensures that there is little opportunity of there being
any ‘‘free acid’’ left after the salt forming reaction,
which can subsequently react with the solvent to form
these potentially genotoxic sulfonate esters. Particularly, as the acid and base are typically present in
stoichiometrically equivalent levels.
During 2007 and 2008, a series of studies were
initiated by a Product Quality Research Institute
(PQRI) working group aimed at examining the
reaction between sulfonic acids and alcohols to
increase the understanding of this potential side
reaction. These studies, focused on factors that could
influence this reaction, including the effect of
temperature, the presence/absence of water and the
influence of a competing base, to mimic the conditions
used during salt formation processes. The aim of the
studies was to generate sufficient data to be able to
establish a kinetic model of the reaction, such that
under any given set of processing conditions the level
of sulfonate ester formed could be predicted and
ultimately used to develop effective control/purge
measures.
The initial results for MMS have now been
published.6 These studies have not only shown that
the level formed can be predicted but that under the
right processing conditions no detectable levels of
ester are formed. These conditions, where there are
equivalent levels of acid (counter-ion) and base (API)
present mirror those seen in typical pharmaceutical
salt formation processes. An obvious conclusion to be
drawn from this work is that control of sulfonate ester
formation can readily be achieved through simple
DOI 10.1002/jps
2949
process controls. Further details of the key conclusions from these studies are provided in the main
body of the text (see the Risk of Sulfonate Ester
Formation as a Function of Salt Formation Section)
These studies established a basis upon which
sulfonic acids can be used as counter-ions without
risk of sulfonate ester formation, but it is also
important to consider why sulfonic acids are an
integral part of any comprehensive salt selection
process. This article therefore aims to provide a
general overview of the utility of sulfonic acids as salt
forming counter-ions in drug development, particularly in the isolation and crystallization of intractable
drug substances.
UTILITY OF SULFONIC ACIDS AS
PHARMACEUTICAL SALTS
Frequency of Usage of Sulfonic Acid Salts
Salt formation with pharmaceutically acceptable
counter-ions is an extremely useful approach for
the optimization or modification of the physicochemical, processing (including formulation), biopharmaceutical (including safety and tolerability) or
therapeutic properties of ionizable drug substances.
Each of the individual salts of a particular drug
substance can be considered as a unique chemical
entity with its own distinctive physicochemical and
biopharmaceutical properties.7–9
It has been estimated that approximately half of all
of the active pharmaceutical substances (API) that
have been developed were ultimately progressed as
pharmaceutically acceptable salts and that salt
formation is an integral part of the development
process.10,11 Although some groups have tried to use
high throughput crystallization approaches to facilitate rapid resolution of solid-state issues,12 the
absence of any clear predictive relationships between
the physicochemical properties of the free base (or
free acid) and that of any of the resultant salts, means
that the selection of the best salt with the optimum
properties is at best a difficult, semi-empirical
task.13,14 This difficulty was illustrated by O’Connor
and Corrigan,15 who reported that the solubilities of a
series of structurally related amine salts could differ
by as much as 100-fold. This was attributed to several
factors, including crystal lattice energies and the
intrinsic pH of the resulting solutions.
Two extensive reviews by Berge et al.7,8 have
evaluated the relative frequency of usage of pharmaceutical salts up to 1993. This was taken from an
assessment of the drug monographs listed in Martindale,16 The Extra Pharmacopoeia, 30th Edition, 1993.
An abstracted form of the summary table for anionic
salt forming acids covering the usage of sulfonic acids
is provided in Table 1.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2950
ELDER ET AL.
Table 1. Frequency of Usage of Sulfonic Acid Salts
(Based on Berge et al.7,8)
Anion
Benzenesulfonate
Camphorsulfonate
Sterylsulfate
4-Chlorobenzenesulfonate
6,7-Dihydroxycourmarin-4methanesulfonate
1,2-Ethanedisulfonate
Laurylsulfate
Ethanesulfonate
Methanesulfonate
1,5-Naphthalenedisulfonate
Toluene 4-sulfonate
Total
Alternative
Name
Percenta
Besylate
Camsylate
—
Closylate
Cromesilate
0.26
0.59
0.07
0.07
0.07
Edisylate
Estolate
Esylate
Mesylate
Napsylate
Tosylate
—
0.20
0.13
0.13
3.20
0.20
0.39
5.31
a
Percent is based on total number of anionic salts in clinical use up to
1993.
The Need for Stronger Counter-Ions
Berge et al.7,8 showed that the relative frequency of
usage of sulfonic acids at the time their reviews were
performed, was fairly low, accounting for about one
twentieth of the total usage of anionic salts. The most
frequently used sulfonic acid salt (mesylate) was still
only the fifth (3.20%) most popular choice of anionic
salt forms, behind the hydrochloride (43.99%), sulfate
(5.82%), bromide (3.79%), and chloride (3.53%) salt
forms.
However, a more recent review by Serajuddin,17
which examined trends in salt form usage for those
medicinal products approved by the US Food and
Drug Administration (FDA) over a 12-year period
from 1995 to 2006 (120 in total) noted that mesylate
salt usage had increased significantly to second in the
order of ranking of anionic counter-ions. By 2006 this
comprised 10% of total usage. He also noted a general
increase in the usage of strong inorganic counter-ions,
for example, hydrochlorides, mesylates, hydrobromides/bromides, and sulfate/bisulfates, which had
increased to just over three quarters of the total usage
(79%). Following a similar trend, acidic drugs are
tending to use strong alkali counter-ions, for example,
NaOH and KOH in salt formation, and sodium and
potassium now account for just under three quarters
of the total cationic salt usage (74%).
Serajuddin17 ascribed the increase usage of strong
acids (and alkalis) to the general decrease in the
aqueous solubility of new drug candidates, previously
noted by Lipinski et al.18 This was in turn attributed
to combinatorial and high throughput chemistry
strategies. Lipinski19 reported that nearly one-third
of the medicinal compounds synthesized in academic
laboratories between 1987 and 1994 had solubilities
<20 mg/mL. Serajuddin17 stated that this solubility
value had decreased further in the intervening
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
decade and that one-third of medicinal compounds
had solubilities of <10 mg/mL and a further one-third
had solubilities between 10 and 100 mg/mL. As a
result stronger acids are required to form salts of such
drug substances.9
The impact of decreased solubility and how it
constrains the availability of acidic counter-ions,
restricting it to the strong inorganic acids, was
elegantly delineated by Serajuddin and Pudipeddi.20
The authors studied the pH solubility relationships of
two structurally related analogues; avitriptan and
BMS-181885 (see Fig. 1). Both were dibasic in nature
with identical pKa values of 8.0 and 3.6. However, the
intrinsic solubilities (So) of the respective free bases
differed by a factor of 10. Although, avitriptan
solubility was low (6 mg/mL); that of BMS-181885
was even lower, with a value of only 0.7 mg/mL. The So
values linked with their higher pKa values (8.0),
affected the respective pHmax values of the two
analytes. Avitriptan had a pHmax (pH of maximum
solubility) of ca. 5; whereas, in contrast BMS-181885
was one pH unit lower with a pHmax of ca. pH 4.
In the case of avitriptan this meant that a large
selection of potential counter-ions was available for
salt formation, including all of the carboxylic acid
counter-ions, for example, tartrate, citrate, acetate,
succinate, etc. However, in contrast, salt formation
was not theoretically feasible with any of the common
carboxylic acids for BMS-181885 and the use of strong
inorganic counter-ions, such as hydrochloric, sulfuric,
and sulfonic acids were required. Similarly, Chiang
et al.21 recently reported on a potent MK-2 inhibitor
with a pKa of about 4, and indicated that the optimum
salt form was the mesylate, based on evaluations of
crystallinity, melting point, and hygroscopic, as well
as physical stability.
Black et al.22 in an extensive study of ephedrine
evaluated 25 acidic anions of ephedrine, of which 17
were isolatable. The anions were selected to reflect
differences in charge distribution and stereochemistry and represented five different classes of anionic
counter-ions widely used in the pharmaceutical
industry; that is, carboxylic acids, dicarboxylic acids,
hydroxy acids, and strong acids (inorganic acids and
Figure 1. Structures of Two Analogues (Avitriptan and
BMS-181885) having identical pKa values (8.0 and 3.6), but
which have Intrinsic Solubilities (So) differing by a factor of 10.
DOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
sulfonic acids). The authors reported that the acid and
base dissociation constants (pKa) were the most
important variable and reiterated that for salt
formation to occur it was essential that the differences between the pKa’s of the conjugate acid and
base should be >2 pH units. They were also able to
highlight that the impact that the solvent has on
the pKa value is often not understood and that organic
solvents can change the pKa of the anion significantly.
Brittain23 showed that the use of log Ks (equilibrium constants) placed this empirical rule (that of
differences of >2 pH units between the pKa’s are
required for salt formation), on to a more scientific
basis. He supported these arguments by showing that
for drug substances with differing basicities, that as
the pKa of the base is reduced (e.g., pKa’s of 10.6, 8.55,
and 7.25, respectively) that the corresponding pKa of
the acid also needs to be reduced and that for the final
example (pKa 7.25) that salt formation will only occur
with strong acids (pKa < 3.0). He also showed that
similar arguments apply to acidic salts. In the case of
ibuprofen (pKa) (4.41) it would be expected that salt
formation will only occur with bases with pKa’s in
excess of 8.41. Thus, in addition to sodium and
potassium salts, ibuprofen could be expected to form
salts with erbumine (pKa 10.68), arginine (pKa 9.59),
lysine (pKa 9.48), ethanolamine (pKa 9.16), and
diethanolamine (pKa 8.71).
Even in cases where salts can be readily formed
with a range of counter-ions, those involving strong
counter-ions may still be preferable. Evidence of this
is provided by the following study relating to sertraline. Sertraline has a high pKa (9.16 0.12) and would
be expected to readily form salts with all potential,
acidic counter-ions, irrespective of acid strength.
However, Remenar et al.24 demonstrated that strong
acid counter-ions still showed a greater propensity to
form crystalline salts than the corresponding carboxylic acid salts.
They evaluated eight mono-protic acidic counterions of sertraline in a high throughput crystallization
screen, including three carboxylic acids and five
strong acids. They found that the strong acid counterions generally gave a higher percentage of hits (which
were defined as ‘‘percentage of samples containing a
given salt that yielded crystals’’) than the weaker
counter-ions. The percentage of hits for the eight salts
were; acetate (8%), benzoate (35%), lactate (22%),
hydrobromide (82%), benzenesulfonate (78%), toluenesulfonate (94%), methanesulfonate (69%), and
ethylsulfonate (8%). No reasons were provided as to
the low percentage value for ethylsulfonate, compared to the other sulfonate salts.
Other examples where sulfonic acids were preferentially selected include:
Gross et al.25 performed a salt formation evaluation
of the next generation H-1 antagonist, NBI-75043.
DOI 10.1002/jps
2951
They evaluated six counter-ions, three weak acids
(tartrate, fumarate, and maleate), and three strong
acids (hydrobromide, tosylate, and besylate) and
selected the besylate salt.
Bastin et al.26 reported that only a limited number
of stable and crystalline salts could be synthesized for
two very weak bases they investigated (pKa values of
<4.25), as opposed to a greater number for a stronger
base (pKa 5.3). Of the two weak bases studied only the
mesylate and hydrochloride salts could be isolated as
crystalline solids for RPR-111423 (pKa 4.25). For
RPR-127963 (pKa 4.10) it was possible to isolate two
carboxylic acid salts (citrate and tartrate) as well as
three strong acid salts (mesylate, sulfate, and
hydrochloride), although only the mesylate and
sulfate were deemed to be suitable for further
development. In contrast, RPR-200765 (pKa 5.3)
formed salts with a large number of counter-ions,
however even in this case the only stable salts were
those with strong acidic counter-ions; hydrochloride,
hydrobromide, methanesulfonate, and camphorsulfonate acids.
Stahl and Wermuth27 asserted that the prior
knowledge and experience had led to a reduction in
the numbers of safe counter-ions. They attempted to
categorize salts into three classes. Firstly, those that
have unrestricted use because of their physiological
prevalence, for example, hydrochlorides and sodium
salts. In the second category he placed those counterions that are not typically natural in occurrence, but
due to their prevalence in daily use have shown low
toxicity and good tolerability. All of the sulfonic acids
were placed in this category. Finally, the last category
includes those counter-ions where specific applications have driven their use or where they have
intrinsic pharmacological activity in their own right.
For example, the use of sweet-tasting cyclamate salts
to mask the bitter-tasting properties of many novel
APIs.
Salt Formation and Form (Polymorph) Considerations
From a salt formation and polymorphic form perspective, there can be significant advantages to
selecting a sulfonic acid salt over other strong acid
counter-ions, particularly the more ubiquitous hydrochloride salt.
Remenar et al.24 highlighted that seemingly minor
differences in the counter-ion can have a profound
effect on the number of polymorphs and solvates that
can be formed in the resultant salts. They reported
that these effects were not predictable and that a
greater understanding (high throughput crystallization allied with other characterization techniques)
can help to eliminate those salts with high polymorphic preference and focus appropriate resource on
the salt form with lesser polymorphic propensity.
They investigated the polymorphic propensity of the
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2952
ELDER ET AL.
selective serotonin re-uptake inhibitor, sertraline.
The authors indicated that there are reported to be 28
polymorphic forms of the commercial hydrochloride
salt and showed that the sulfonic acid salts had much
less propensity than the hydrochloride salt to form
multiple polymorphs. The besylate and tosylate salts
formed single polymorphic forms; whereas, the esylate
and mesylate salts formed two polymorphic forms.
Gross et al.25 looked at alternative salt forms of
NBI-75043, as the initially selected tartrate salt had
low melting point (948C), it was hygroscopic and had a
propensity to form solvates and hydrates. Subsequently, they selected the besylate salt as it crystallized in high purity and yield from multiple solvent
systems, gave the highest melting point (165–1688C)
and existed as a single polymorphic form.
Pharmaceutical salts frequently exist as hydrates13
and this can be problematical during secondary
processing, particularly wet granulation. It can be
difficult to monitor and control the anhydrate/hydrate
form during the wet massing, fluid bed drying, and
aqueous film coating subprocesses.28,29 Although, this
is a factor that could lead to de-selection of a
particular salt form (if there was a back-up salt with
perceived advantages); life is rarely that straightforward. As salt selection is a complex process with
many, often conflicting criteria, companies will often
progress salts that can form hydrates and manage the
consequences. Often the biggest challenge is knowing
whether to develop the anhydrate or hydrate form.
However, in contrast to other strong acid salts,
mesylates appear to have a much lower propensity to
form hydrates,9 which makes them an attractive salt
form for secondary processing, particularly wet
granulation. The reasons for this are not well
understood.
In relation to their studies of ephedrine Black
et al.22 reported that for the 17 salts of ephedrine that
were isolated and characterized, the strong acid
counter-ions, including sulfonic acid salts were
generally superior. They reported that 90% (9/10) of
these salts isolated from water and 100% (6/6 salts)
isolated from methanol were crystalline. There were
no hydrates or phase transitions reported when
strong acid counter-ions were utilized. In contrast,
only 53% (8/15) of the weak acid salts (carboxylic,
dicarboxylic, and hydroxy acids) isolated from water
and none of the salts (0/9) isolated from methanol
were crystalline. In addition, there were three
hydrates generated and four of the salts crystallized
from an initial amorphous phase after extended
storage periods, when weak acid counter-ions were
used.
Elder30 also confirmed the utility of mesylate salts
and their reluctance to form hydrates/solvates. He
evaluated the physicochemical and crystallographic
properties of several different salts of two different
heterocyclic model compounds. Both had different
basic cationic functionalities (a guanidine (GU) API
and imidazole (IM) API); and both were capable of
forming multiple protonated salts (e.g., mono-, di-, or
tribasic-salts) with various inorganic and organic
anionic counter-ions of differing stoichiometries.
For the guanidine model compound (GU), Elder30
determined the crystal structures of the free base
(1 hydrate and 2 isomorphous solvates), 11 different
mono-protonated and 4 different di-protonated salts
and showed that all were solvated/hydrated, with the
exception of the dimesylate salt. It can be seen from
Table 2 that the dimesylate salt is a higher melting
salt with high aqueous solubility (59.1 mg/mL),
forming a structure with a very high packing density
(Dc 1.50 g/cc). In fact, with the exception of the
ascorbate salt, where the packing is also very efficient
(Dc 1.54 g/cc), but is predicated on the presence of two
crystallographically independent cations in the unit
cell, the mesylate salt gave the highest packing
density of all of the salts studied. It can also be seen
Table 2. Summary of Physicochemical and Crystallographic Properties of a Novel Guanidine Heterocyclic API30
Salt
Base1H2O
Base1i-PrOH
Base1i-BuOH
Hemifumarate0.25EtOH
Monoacetate1H2O
Monomaleate1i-PrOH
Monomalonate1i-PrOH
HCl1H2O2i-PrOH
Monoascorbate1H2O
2HCl1H2O
Dimesylate
Sulfate1H2O
Phosphate1H2O
Solubility (mg/mL)
Density (g/cc3)
Mp (8C)
H-bond length (Å)
<0.001
<0.001
<0.001
0.1
3.8
—
3.1
1.9
20.9
2.4
59.1
<0.1
1.6
1.40
1.25
1.27
1.28
1.40
—
—
1.27
1.54
1.44
1.50
—
—
132.0–134.2
116.6–119.7
82.5–85.5
171.1–173.2
150.4–151.4
180.8–182.8
114.9–116.9
235.5–237.5
132.1–134.2
263.2–265.4
269.1–271.3
220.7–223.7
240.4–242.4
3.102 (6)a
3.068 (8)a
3.084 (8)a
2.977 (11)
2.901 (6)
—
—
3.273 (7)
2.766 (8)b, 3.024 (9)
3.135 (9), 3.027 (8)
2.827 (9), 2.747 (8)
—
—
i-PrOH, i-propanol; i-BuOH, i-butanol; EtOH, ethanol.
a
H-bonds incorporating oxygen of water/solvent molecules.
b
There are two crystallographically independent molecules in the unit cell, space group P1.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
DOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
that those salts with the highest packing densities
have commensurately shorter H-bond lengths.
Similarly, for the imidazole model compound (IM),
Elder30 evaluated the crystal structures of the free
base (one solvate), three different mono-protonated
and five different tri-protonated salts. In this case
the free base and all of the mono-protonated salts
were solvated and/or hydrated, with the exception of
the mesylate salt. For the tri-protonated salts, the
hydrochloride salt formed five hydrates of varying
stoichiometry. In contrast, the other tri-protonated
salts were not of diffraction grade quality, and it was
difficult to accurately assess the stoichiometry. In this
example (see Tab. 3) the mono-mesylate salt has the
highest packing density of the mono-protonated salts
studied (1.25 g/cc). However, in order for the trihydrochloride salts to achieve similar or higher
packing densities (1.24–1.31 g/cc), they needed to
utilize extensive hydration in their crystal lattices
and one to five water molecules were incorporated
into the crystal structures. It can also be seen that
those salts with the highest packing densities tend to
have at least one short H-bond (with the exception of
IM 3HCl2H2O) and that the IM mesylate salt has a
very short H-bond length (2.754(5) Å).
Even in those few reported cases where hydrate
formation is observed26,31 it would appear that the
mesylate salt still offers significant advantages over
the alternative salt forms. Engel et al.31 reported that
LY333531 hydrochloride existed in three hydrated
forms (anhydrate, monohydrate, and tetrahydrate)
compared with the single hydrated form of the
mesylate.
Processing Advantages
The melting point of pharmaceutical salts is often
intrinsically related to their physicochemical properties. In general APIs that exhibit low melting points
often exhibit plastic deformation during secondary
processing which can cause both caking and aggregation, and impacts on both flow and compressibility
performance.32 These in turn can have an adverse
impact on many critical quality attributes of the drug
product; for example, disintegration rates and the
2953
related in vitro dissolution rates, friability and
content uniformity. This is particularly true for low
dose medicinal products.33 The greater ionic content
of strong inorganic acid salts, for example, sulfonates,
sulfates, hydrochlorides, etc., usually ensures that
the resultant salt is less plastic in nature, which
facilitates better performance during both primary
and secondary processing.26
The more energy intensive the primary or secondary process then typically the more issues that can
arise as a result of plastic deformation of the API. Size
reduction, and in particular micronization, can result
in premature melting causing deposition of the drug
substance, often as an amorphous glass, on the
micronization chamber parts. Similar issues can arise
during tablet compression and filming of the punch
tips can occur, which can impact on processability and
overall manufacturability.8
Steele34 reported on the comparative crystallinity
of salts of a b-blocking drug candidate. He showed
that the sulfonate salts (besylate and tosylate)
demonstrated high crystallinity compared to the
hydrochloride salt, (see Tab. 4). Both of the sulfonate
salts micronized well and demonstrating good flow in
the mill chamber, with low levels of amorphous
formation and tight particle size distribution. In
contrast the hydrochloride salt did not micronize as
well and showed a wide particle size distribution
whilst the process was extremely dusty. However, the
crystallization process for the besylate salt was not
well controlled and it formed an oil upon scale-up.
Steele also evaluated the suitability of the salts for
use in a pressurized meter dose inhaler (pMDI)
formulation. The hydrochloride salt did not suspend
very well in propellant 227 (heptafluoropropane),
whereas, in comparison the tosylate salt formed a
much more stable suspension. On this basis the
tosylate salt was chosen for further development.
A good example of the processing advantages
inherent in mesylate salt forms was reported by
Bastin et al.26 RPR-200765 mesylate demonstrated
good handling properties, including good flow, during
secondary process development, allowing straightforward capsule and tablet development. These flow
Table 3. Summary of Physicochemical and Crystallographic Properties of a Novel Imidazole Heterocyclic API30
Salt
Base1toluene
Monohydrochloride1H2O
Monohydrochloride1acetone
Monomesylate
Trihydrochloride1H2O
Trihydrochloride2H2O
Trihydrochloride5H2O (I)
Trihydrochloride5H2O (II)
Trihydrochloride5H2O (III)
DOI 10.1002/jps
Solubility (mg/mL)
Density (g/cm3)
Mp (8C)
H-bond length (Å)
<0.001
>10
—
>10
>10
>10
>10
>10
>10
1.19
—
1.19
1.25
1.31
1.28
1.25
1.26
1.24
201–204.2
175.0–177.8
—
203.0–204.0
188.1–190.9
—
189.8–191.8
—
209.7–210.8
—
—
3.067 (3)
2.754 (5)
3.067 (3), 3.053 (3), 2.990 (3)
3.098 (8), 3.058 (7), 3.017 (9)
3.110 (20), 3.030 (20), 2.650 (30)
3.020 (10), 2.940 (10), 3.090 (10)
3.092 (4), 3.019 (4), 2.993 (7)
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2954
ELDER ET AL.
Table 4. Summary of Physicochemical Properties of Salts of RPR200765 (Based on Bastin et al.26)
Salt
Mp (8C)
Solubility (mg/mL)
pH (max)
Hygroscopicity
Hydrochloride
Hydrobromide
Mesylate
Camsylate
245–248
276–277
214
265–267
16.7
3.3
39.0
20.0
2.16
2.63
1.93
2.22
Hygroscopic, with multiple hydrated forms
Hygroscopic, with multiple hydrated forms
Nonhygroscopic with stable monohydrate form
Nonhygroscopic
properties were superior to the alternative salts
studied; that is, hydrochloride, hydrobromide, sulfate, and camsylate salts.
In another example, Bastin et al.26 also reported
that RPR-127965 mesylate had slightly better flow
properties at low relative humidity (RH), but sticking
was evident at high RHs. In contrast, the sulfate salt
showed slight sticking issues, but this problem
occurred at all relative humidities.
Engel et al.31 provided another interesting example
of how the ability to form a highly crystalline salt can
positively impact the quality of the API. This relating
to a candidate drug LY333531. From a primary
processing perspective the mesylate salt of LY333531
filtered extremely well and the crystallization process
afforded improvements in impurity profiles of the key
process related impurity, termed TRS. This was in
stark contrast to all of the hydrated forms of the
hydrochloride salt. The mesylate salt reduced the
levels of TRS to 2% compared with the hydrochloride
anhydrate (4.72% TRS) and hydrochloride monohydrate and tetrahydrate salts (9.8% TRS). From a
comparative purity perspective the free base starting
material had an initial impurity level of nearly 10%
TRS.
Hydrochloride and particularly di-hydrochloride
salts of weak bases can undergo disproportionation.8
The driving force for this reaction is the generation of
volatile hydrogen chloride gas that is either lost from
the system or reacts with other components of the
formulation/processing equipment leading to either
de-stabilization and/or processing issues.35,36 Narurkar et al.37reported on the rusting of tablet tooling
which was attributed to the liberation of hydrogen
chloride gas from the hydrochloride salt of the API. A
similar phenomenon also occurs during lyophilization
where the high vacuum/low temperature combination
tends to drive off labile hydrogen chloride gas from
the corresponding hydrochloride salts.8
In contrast, the disproportionation of the mesylate
salt and other sulfonic acid salts is much less
common, this is presumably because the by-products
of the disproportionation are relatively stable and
nonvolatile and therefore the main driving force for
the reaction is significantly reduced. There are only a
limited number of reports in the literature of mesylate
salt disproportionation, which speaks to the uncommon nature of the event. One of the few was recently
reported by Chiang et al.21 who investigated the
physical stability of a mesylate salt in various
nonclinical formulations.
Melting Point and Stability Advantages
The intrinsic stability of the API is generally related
to the melting point; which in turn is related to
increased crystallinity from the conjugate anion.45
Gould demonstrated that salts prepared from highly
aromatic sulfonic acids produced high melting salts.
In contrast, flexible aliphatic carboxylic acids such as
citrate yielded oils.
Sulfonate salts typically produce higher melting
point salt forms of the API compared to other salts of
the base, which in addition to processing advantages
can enhance stability. Elder30 showed that the
mesylate salt was the highest melting salt form of
the guanidine (GU) salts studied (see Tab. 2) and
showed elevated melting points (in excess of 2008C)
for the imidazole (IM) model compound (see Tab. 3).
Similarly, all of the strong acidic salts of RPR2007651 (see Tab. 5) and RPR-127965 (see Tab. 6)
showed melting points in excess of 2008C.26
Remenar et al.24 evaluated eight mono-protic acidic
counter-ions of sertraline. They found that the strong
Table 5. Summary of Physicochemical Properties of Salts of RPR127965 (Based on Bastin et al.26)
Salt
Free base
Hydrochloride
Citrate
Tartrate
Mesylate
Sulfate
Mp (8C)
Solubility (mg/mL)
pH (max)
Hygroscopicity
119.0–123.0
275.0
130.2–134.3
198.5–201.6
280.9–282.2
305.7–308.9
ND
3.9
0.8
0.9
108.0
ca. 50
ND
2.33
2.49
2.56
1.76
1.32
ND. Multiple hydrated forms
Nonhygroscopic, with 1 anhydrate and 2 monohydrates
Nonhygroscopic, with stable hemihydrate
Very hygroscopic, with unstable anhydrate
Nonhygroscopic with no hydrated or anhydrated forms
Nonhygroscopic with no hydrated or anhydrated forms
ND, not determined.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
DOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
2955
Table 6. Summary of Physicochemical Properties of Salts of Sibenadet (Based on Steele34)
Test
Solubility SLF (mg/mL)
Solubility pH 4 saline (mg/mL)
Hygroscopicity at 80%RH (%w/w)
Micronization
HCl
Besylate
Tosylate
0.17
0.8
0.9
Micronizes poorly;
dusty process; broad particle
size distribution
0.15
0.60
0.6
Micronizes well,
but some amorphicity;
tight particle size
distribution
0.12
0.90
0.2
Micronizes well,
but some amorphicity;
tight particle size
distribution
0.3
1.6
13.6
Poorly crystalline
0.5
1.3
2.6
Highly crystalline;
amorphous material
(postmicronization)
crystallises at 1008C or at >80%RH
N/A
Forms isopropanol solvate
N/A
20.19
8.07
175.13
11.20
Oils on scaling
35.41
þ4.07
123.48
15.03
0.4
1.4
3.0
Highly crystalline;
amorphous material
(postmicronization)
crystallises at 808C or at
>50%RH
No evidence of
polymorphism/solvate formation
Good
43.59
8.83
134.49
8.16
Particle size distribution (mm)
D (v, 0.1)
D (v, 0.5)
D (v, 0.9)
Crystallinity
Polymorphic form
Scalability
FPFa (%)
FPF (% change from initialb)
Mean Dose (mg)
Mean dose (% change from initialb)
a
b
Fine particle fraction (FPF).
After storage at 408C/75%RH for 1-month.
acid counter-ions generally gave higher melting
points than the weaker counter-ions. The melting
points of the different salts (and individual polymorphs) of sertraline were acetate (not reported),
benzoate (134 and 1558C), lactate (62 and 1508C),
hydrobromide (2668C), benzenesulfonate (1508C),
toluenesulfonate (2658C), methanesulfonate (196
and 2018C), and ethylsulfonate (96 and 1488C).
Propoxyphene provides an interesting example of
how the use of a sulfonic acid salt was able to address
an incompatibility issue within a combination product propoxyphene was successfully formulated as
the napyslate salt in a stable fixed dose combination
product with aspirin. This addressed the previously
seen incompatibilities of the propoxyphene hydrochloride with aspirin. The authors indicated that the
napyslate salt reduced the intrinsic wettability and
hygroscopicity of the propoxyphene thereby reducing
the potential for hydrolysis of the aspirin at the interface between the two APIs within the formulation.16
Solubility/Dissolution Advantages
Poor or inadequate solubility in aqueous and biorelevant media can often hinder and constrain the
development of oral and parenteral drug products.38
Typically, increasing the melting point has an
adverse effect on aqueous solubility because of
increasing crystal lattice energies.39 Interestingly,
sulfonic acid salts (particularly alkyl sulfonates such
as mesylates) tend to be an exception to this rule,
generally exhibiting both high melting points and
DOI 10.1002/jps
good solubility. Bighley et al.8 reported that the
increased prevalence of sulfonate salts was in part at
least because of their influence on dissolution rate
and indicated that this was because the mesylate
salts of APIs tend to be highly soluble.
Mesylate salts often offer significant solubility and
dissolution advantages even when compared with
other salts of strong acids. Streng et al.40 reported on
the relative solubilities of the lactate, phosphate,
hydrochloride, and mesylate salts of terfenadine.
They reported significant solubility differences ranging from 0.5 to 5.0 mg/mL. Both of the selected salts
of LY33353131 were Biopharmaceutics Classification
System (BCS) class II41 compounds (low solubility
and high permeability). The mesylate salt had a
slightly improved aqueous solubility compared to the
hydrochloride salt (0.5 mg/mL vs. 0.1 mg/mL), and
demonstrated a comparable increase in exposure in
the dog (2.6 times).
Bastin et al.26 reported on the salt screening of
RPR-200765, a drug being developed for the treatment of rheumatoid arthritis. The free base had
unacceptably low solubility (10 mg/mL) and commensurately low bioavailability in animal models. RPR200765 had a pKa of 5.3 making it amenable to salt
formation with a wide range of acidic counter-ions.
The authors established that RPR-200765 formed
stable salts with strong acids, forming the hydrochloride, hydrobromide, mesylate, and camsylate
salts. The summarized data are shown in Table 4.
The mesylate salt was selected for further developJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2956
ELDER ET AL.
ment on the basis of good solubility, rapid dissolution
rate, low hygroscopicity, and a stable and wellcontrolled polymorphic form.
Li et al.42 investigated the solubility and dissolution rates of the weak base, haloperidol and two of its
salts (hydrochloride and mesylate). Haloperidol has
a pKa of 8.0, an intrinsic solubility (So) of 2.5 mg/mL
and a pHmax value of around pH 5. Haloperidol
mesylate demonstrated faster dissolution rates over
the pH range 2–7 (0.87–2.04 mg/min/cm2) than the
corresponding hydrochloride salt (0.16–0.29 mg/min/
cm2) and the maximum solubility was also significantly greater over the same pH range (30.4 mg/mL)
compared to the hydrochloride salt (4.2 mg/mL). The
dissolution rate of the mesylate decreased significantly at pH values below 2, presumably caused by
the formation of the less soluble hydrochloride salt on
the surface of the mesylate salt, but it still retained a
slightly higher solubility than the corresponding
hydrochloride salt at this acidic pH.
Elder30 showed that the mesylate salt demonstrated the greatest solubility of all the guanidine
(GU) salts that were studied (see Tab. 2). In contrast,
Black et al.22 showed a less clear-cut picture for
salts of ephedrine. The sulfonate salts showed
solubilities in the range 0.26–2.25 M/L. The mesylate
salt did indeed show superior solubility compared to
the corresponding hydrochloride salt (1.60 M/L), but
the greatest solubility was shown by a carboxylate
salt (5.77 M/L); perhaps demonstrating the semiempirical nature of salt formation. The authors
concluded that it is solution chemistry, rather than
crystal lattice effects that were the determining
variables for the aqueous solubility of a series of
salts.
This was confirmed by Serajuddin17 who reported
on the comparative solubility of salts of avitriptan.
The mesylate salt had enhanced solubility (16.3 mg/
mL) compared to both the mono- and di-hydrochloride
salts (3.4 and 9.0 mg/mL, respectively), but that the
mesylate salt had similar solubility to the common
carboxylic acid salts, for example, tartrate, acetate,
lactate, and succinate (14.7–16.5 mg/mL).
Remenar et al.24 in their study of sertraline found
that the mesylate salt gave the highest solubility of all
of the salts evaluated. The solubilities of the various
salts of sertraline, in water at room temperature,
were acetate (3.3 mg/mL), benzoate (0.4 mg/mL),
lactate (1.9 mg/mL), hydrobromide (0.6 mg/mL), benzenesulfonate (0.3 mg/mL), toluenesulfonate (0.1 mg/
mL), methanesulfonate (4.2 mg/mL), and ethylsulfonate (1.7 mg/mL). Creasey and Green43 selected the
mesylate salt of pralidoxime to enhance the solubility
of the API.
The salts of some drug substances are surface
active and solubility can be enhanced via micellar
solubilization. Hussain et al.44 provided an interestJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
ing example of this. They reported that the hydrochloride salt of DuP 747 showed some surface activity,
but the critical micelle concentration (CMC) is not
achieved at the saturation solubility of this salt (3 mg/
mL). In contrast, the mesylate salt did form a micellar
solution resulting in significantly enhanced solubility
(60 mg/mL).
Benneker et al.45 claimed equivalent stability,
reduced hygroscopicity, and significantly enhanced
solubility of the mesylate and tosylate salts of
paroxetine (both >1000 mg/mL) versus the marketed
hydrochloride salt (4.9 mg/mL).
For parenteral delivery it is a prerequisite that the
drug substance is solubilized in the formulation
vehicle in order to address safety and local tolerability
considerations.38 Rowe et al.46 developed a Parenteral
Expert System and the authors indicated that
solubility was the most significant impediment to
the development of a viable parenteral medicinal
product. Even though the intrinsic solubility of the
API can be enhanced using solubilizers, co-solvents,
and buffers, etc., the absolute solubility is still very
important as the formulation can precipitate at the
injection site causing phlebitis. This phenomenon is
impacted by several variables; such as drug substance
solubility at physiological conditions, buffer capacity
of the formulation, relative pKa’s of API and buffer
and the initial concentration of the API.47
The preeminence of aqueous solubility was exemplified by Bastin et al.26 who reported on the salt
screening of RPR127963, a drug being developed for
the treatment of cardiovascular diseases. The free
base had low solubility. RPR127963 had a pKa of 4.1
making it amenable to stable salt formation with
strong acidic counter-ions, forming the hydrochloride,
mesylate, and sulfate salts. Interestingly, salts could
also be formed with citric and tartaric acids, but the
resulting solubilities were still too low for parenteral
and high dose oral formulations. The summarized
data are shown in Table 5.
Bastin et al.26 also indicated that the mesylate and
sulfate salts were the preferred candidates. However,
the sulfate was selected as the best candidate as the
molecule was being progressed as a parenteral asset
and the sulfate showed slightly better solubility in cosolvents, giving it a better chance of being developed
as a high dose, buffered parenteral formulation.
There are several reports of the mesylate salt being
selected in preference to the halide salt to facilitate
parenteral drug development, for example, neostigmine mesylate and ziprasidone mesylate.48 Kim
et al.49 also reported on a combination of salt
formation and formulation strategies to enhance
the solubility of a drug for parenteral dosage forms.
The mesylate salt of ziprasadone had an intrinsic
solubility of 0.89 mg/mL; however, this was inadequate to achieve the required solubilities for intraDOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
muscular administration (20–40 mg/mL). The concomitant use of complexation with b-cyclodextrin
sulfobutyl ether was necessary to achieve the
required solubility.
Even in those cases where the design intent is to
develop a less soluble salt form of an API, for use in
controlled release dosage forms, the sulfonates can
demonstrate great utility. The hydrophobicity of the
sulfonates can be readily modified by introduction of
bulkier and/or longer side chains, for example,
esylate, edisylate, besylate, tosylate, embonate,
napyslate, xinafoate, and estolate.7 Ware and Lu50
evaluated the solubility of different salt forms of
trazodone versus the marketed hydrochloride salt.
The tosylate salt had the lowest (and hence best)
solubility profile of the salts evaluated. Stephens51
utilized the napsylate salt of propoxyphene to
enhance its palatability, by reducing its aqueous
solubility, for use in oral liquid dosage forms. The
hydrochloride salt was too bitter for this application.
Biopharmaceutical Differences
Serajuddin and co-workers17,52,53 have all demonstrated the negative impact of the common chloride/
hydrochloride ion (HCl or NaCl) on the solubility and
dissolution rates of hydrochloride salts in physiological media. This also has adverse impacts on
parenteral drug development as 0.9% saline is a
commonly used isotonic diluent that has some
advantages over the alternative sugar diluents, for
example, glucose, sorbitol, or dextrose. This phenomenon typically translates into reduced exposure from
the hydrochloride salt versus alternative salts or the
free base.54–56
Rajogopolan et al.57 demonstrated significantly
enhanced solubility of the mesylate salt (440 mg/
mL) versus the hydrochloride salt (1 mg/mL) of the
anti-psychotic drug, 2,3,4,5-tetrahydro-8-(methylsulfonyl)-1-H3-benzazepin-7-ol. However, addition of
sodium chloride significantly reduced the solubility
down to 60 mg/mL, presumably due to the rapid
conversion to the corresponding lower solubility
hydrochloride, thus potentially precluding saline as
an isotonicity agent with this salt.
2957
Li et al.53 demonstrated that other alternative
strong acid salts, such as mesylate (or other
corresponding sulfonic acid salts), which have higher
aqueous solubilities than the hydrochloride salt may
also have certain in vivo advantages as well. This was
also attributed to the common ion effect that can
reduce the solubilities of hydrochloride salts in gastric
environments. Li et al.53 showed that the high
solubility and relatively high surface area of haloperidol mesylate salt resulted in enhanced dissolution
rates (<2-min in pH 2 simulated gastric media),
which were more rapid than the competing common
ion formation, resulting from the in situ conversion to
the lower solubility HCl salt.
Serajuddin17 reported that besylate and bisulfate
(hydrogen sulfate) salts of an investigational API also
showed no evidence of this competing equilibrium
(reduced solubility due to formation of the common
hydrochloride ion). He reported that for these
sulfonate salts amorphous gels with enhanced
solubility and dissolution rates were typically formed,
noting that there was also a necessity to ensure
that this did not negatively impact on disintegration
rates of the corresponding tablet or capsule dosage
forms.
Malek et al.58 reported that high molecular weight
sulfonic acid salts of several parenterally administered antibiotics produced sustained, but lower
plasma levels, whilst retaining higher lymph concentrations. Massey59 utilized a sulfonate salt of
cephalexin to enhance both the absorption and
stability of the API. The use of the embonate salt of
pyrantel facilitated the development of a long-acting
and stable oral suspension for topical treatment in the
gut.8
Steele34 reported on the in vivo comparisons of salts
of sibenadet, a combined D2/(2-adrenorecepotor agonist for the treatment of chronic obstructive pulmonary disease (COPD). The optimum salts of sibenadet
from a physicochemical perspective were prepared as
pMDI formulations and compared in an in vivo study
in rats over a 7-day period (see Tab. 7). The sulfonate
salts (tosylate and besylate) both gave enhanced
exposure versus the HCl salt, with the besylate salt
giving the greatest increases in both cmax (maximum
Table 7. Comparative In Vivo Pharmacokinetic Data for Selected Salts of Sibenadet (Based on Steele34)
Variable
Target dose (mg/kg/day)
Total inhaled dose (mg/kg/day)
MMAD (mm)
Lung burden dose (mg/kg/day)
Mean AUC (ng h/mL)a
Mean cmax (ng/mL)a
a
Vehicle
HCl
Tosylate
Besylate
0
0
N/A
0
N/A
N/A
ca. 2.4
2.70
5.1–5.6
0.16
13.1
11.9
ca. 2.4
3.21
4.2–4.4
0.24
16.2 (123.7%)a
13.4 (112.6%)a
ca. 2.4
3.17
4.2–4.8
0.22
22.0 (167.9%)a
18.1 (152.1%)a
Value in brackets is % increase versus HCl salt.
DOI 10.1002/jps
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2958
ELDER ET AL.
plasma concentration) and AUC (area under the
plasma curve).
Debs et al.60 compared three different salts
gluconate, lactate, and isoethionate (2-hydroxyesylate) of pentamidine for the pulmonary treatment of
pneumonia in AIDS patients. They found that all
three salts were retained in the lung for a fortnight
and gave high topical lung levels.
As reported previously in the solubility section,
Engel et al.31 showed that the mesylate salt of
LY333531 had a slightly improved aqueous solubility
versus the hydrochloride salt which equated to a
comparable enhancement in exposure in the dog.
Safety and Toxicology
Two recently reported studies have cast fresh doubt
on the issue of the toxicity of alky esters of sulfonic
acids. Müller et al.61 reported on extensive animal
studies performed on EMS that were deemed both
relevant and predictive of the human situation. These
studies clearly demonstrated that EMS exhibits
threshold-mediated responses for both chromosomal
damage and related mutagenicity at doses below
25 mg/kg/day.
Extrapolation of these data to humans shows a 370fold safety margin above the worst-case human
dosing scenario, associated with exposure linked to
contaminated Viracept, of 0.055 mg/kg/day and
demonstrates that any biological damage induced
by consumption of EMS can be corrected by the
bodies’ DNA repair mechanisms. It ultimately concluded that patients taking Viracept with elevated
levels of EMS have no increased risks of teratogenicity or carcinogenicity.
In addition, the CHMP genotoxic impurities guideline indicates that compound-specific assessments
can be made in the case of genotoxic substances for
which carcinogenicity data are available. Recent
mouse bioassay data on methyl methanesulfonate
(MMS) are sufficient to calculate a TD50 value
(31.8 mg/kg/day) and derive a virtually safe dose
(VSD) based on an increased risk of 1 in 100,000 in a
completely analogous way to the derivation of the
default TTC.62 Linear extrapolation of the MMS
TD50 gives a VSD of 0.64 mg/kg/day (equivalent to
38 mg/day in a 60 kg patient), compared to the much
more conservative value of 1.5 mg/day, based on the
TTC.
Interestingly, the issue of the formation and the
related toxicity of alky esters of strong acids is not
solely restricted to sulfonic acids, but is equally
germane to other acids of this class. Yang et al.63
recently reported on controlling the levels of the
genotoxins, methyl, and ethyl chloride, in amine
hydrochloride salts from alkyl alcohol solvents, for
example, methanol and ethanol.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
There are also some reported instances where
sulfonate salts appear to be safer than alternative
salts. Verbeek et al.33 reported on the relative safety
of two salts of the cardiovascular drug, amlodipine.
The maleate counter-ion can react with the API to
produce a toxic impurity; N-(2-(4(2-chlorophenyl)-3(ethoxycarbonyl)-5-(methoxycarbonyl)-6-(methyl1,4-dihydro-2-pyridyl)methoxy)ethyl)) aspartic acid.
In contrast, the favored salt form of the API (besylate
salt) could not form this impurity and was clearly
superior from a patient safety perspective.
The napsylate salt of propoxyphene was less toxic
than the corresponding hydrochloride salt in rodents,
presumably based on the reduced solubility of the
former salt.64
Risk of Sulfonate Ester Formation as a Function of
Salt Formation
As already outlined in the introduction, a series of
studies has been performed under the auspices of
PQRI focused on assessing the risk associated with
sulfonate ester generation during salt formation.6 A
key factor within these studies was the establishment
of the mechanism involved in formation of such
impurities. The most critical aspect of which is the
role played by acidity. For the reaction to occur it
requires a highly acidic environment as the first step
in the formation of a sulfonate ester involves
protonation of the alcohol (solvent) Effective removal
of the source of this acidity prevents the reaction from
occurring and thus eliminates the risk of formation.
The most effective way to ensure this is to employ a
stoichiometric amount of the pharmaceutical base
relative to the sulfonic acid employed as the counterion.
Even where such conditions as those described
above cannot be achieved it is still possible to
drastically reduce the levels of ester formed through
control over factors such as temperature/time and
water content.
Another important point to make regarding sulfonate ester formation is then even under ‘‘idealized’’
conditions, that is, those conducive to formation, that
is, high temperature/high acidity (no base present)/
anhydrous/extended reaction times the level of ester
formation is very low. For example, even after 24 h at
708C, the direct reaction between methane sulfonic
acid and ethanol yields <0.4 mol% EMS, in solution.
It is important to emphasize that final point as such
esters are freely soluble and thus the risk of presence
in the isolated final salt is further diminished.
Based on this work it has been possible to define a
series of simple and effective process controls that if
employed during salt formation processes involving
sulfonic acids and an alcoholic solvent ensure the
complete control over sulfonate ester formation.
These being:
DOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
(1) Avoid an excess of acid to minimize the potential for sulfonate ester formation.
(2) If an excess of sulfonic acid is needed, use the
minimum excess possible and conduct the salt
formation and isolation steps at the lowest
practical temperature.
(3) If possible, include water in the salt formation
and isolation procedures to shift the esterification equilibrium towards acid and alcohol.
(4) Avoid situations in which sulfonic acid and
alcohol are mixed and stored before use. If this
is unavoidable then any solutions should be
prepared at as low a temperature as possible
and hold times kept to a minimum.
(5) Control the purity of the input sulfonic acids.
This can, particularly in the case of methane
sulfonic acid, be a source of sulfonate esters.
CONCLUSION
The issue of controlling genotoxic and potential
genotoxic impurities in novel APIs is a significant
challenge. Much of the current regulatory concern,
particularly in Europe, has been focused on the
formation and control of potentially genotoxic sulfonate esters; particularly in light of the recent
withdrawal of Viracept from European markets, over
concerns about elevated levels of EMS. This has
resulted in calls from European regulators to assess
risk mitigation strategies for all marketed products
employing a sulfonic acid counter-ion to ensure that
the sulfonate esters that could be potentially formed
are controlled to TTC-based limits. Yet as highlighted
above recent studies conducted by PQRI have cast
considerable doubt over the extent of the risk posed by
the use of sulfonic acids and have clearly shown that
this can be effectively eliminated through application
of simple process controls.
Despite these safety concerns there are very real
pharmaceutical advantages inherent in the continued utilization of the sulfonic acids as strong acidic
counter-ions. In contrast to other salts mesylates
appear to show a reduced propensity to form
hydrates, which makes them an attractive salt form
for secondary processing, particularly wet granulation. Remenar et al.24 showed that sulfonic acid salts
had much less propensity than the commercial
hydrochloride to form multiple polymorphs. Whether
this is generally true or restricted to these specific
instances is not clear. There are too few examples to
make general claims, none the less there does appear
to be some indications that sulfonate salts may have
advantages over other strong acid counter-ions in
their ability to form a lesser number of polymorphic/
hydrated forms from the same pharmaceutical base.
DOI 10.1002/jps
2959
The greater ionic content of strong inorganic acid
salts, for example, sulfonates, sulfates, hydrochlorides, etc., usually ensures that the salt is less plastic
in nature facilitating better secondary processing.
Unusually, and beneficially, mesylate salts tend to
exhibit both the high melting point that are important
both from a processability and stability perspective
and the good aqueous solubilities that are important
from a bio-pharmaceutics perspective.
The hydrophobicity and the resultant physicochemical properties (particularly solubility) of the sulfonates can be readily modified by introduction of
bulkier and/or longer side chains, for example,
esylate, edisylate, besylate, tosylate, embonate,
napyslate, xinafoate, and estolate salts. This can be
used to good effect in the development of controlled
release, pulmonary or oral liquid products where
decreased solubility is an important factor.
In contrast, the greater solubilities typically found
with mesylates greatly facilitates parenteral drug
development especially when combined with formulation strategies. This is also allied with significant
advantages over the more common hydrochloride
salt(s) that can show reduced solubility and dissolution rates in the presence of the common chloride ion
(HCl or NaCl) in physiological media.
We have conducted a reasonably extensive, qualitative review of the literature to assess the advantages offered by sulfonic acid counter-ions in drug
development. We freely acknowledge that sulfonate
salts are not a universal panacea to address the entire
myriad of development issues of new chemical
entities (NCE). The optimum physical form for any
NCE needs to be individually assessed and optimized
and will be based on many different, often conflicting
criteria. However, sulfonate salts should not be
discounted during the initial salt assessment simply
due to perceived issues of safety (potential contamination with alkyl esters of sulfonic acids) and can, as
illustrated within this article, be a vital component of
a balanced and thorough salt and form optimization
process.
REFERENCES
1. European Medicines Agency. 2007. Press Release (20th September 2007) CHMP Assessment Report for Viracept, London.
EMEA/H/C/164/Z/109. European Medicines Agency Agrees on
Action Plan Following the Recall of Viracept and Recommends
Suspension of the Marketing Authorisation, London. EMEA/
275367/2007.
2. Mesylate Ester Type Impurities Contained in Medicinal Products, Swissmedic Department for Control of the Medicinal
Products Market, 23rd October 2007.
3. Guideline on the Limits of Genotoxic Impurities, Committee for
Medicinal Products (CHMP), European Medicines Agency,
London, 28 June 2006 (CPMP/SWP/5199/02, EMEA/CHMP/
QWP/251344/2006).
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
2960
ELDER ET AL.
4. Coordination Group for Mutual Recognition-Human committee
(CMDh), Request to Assess the Risk of Occurrence of Contamination with Mesilate Esters and Other Related Compounds
in Pharmaceutical, EMEA/CMDh/98694/2008, London, 27th
February 2008.
5. CHMP Assessment Report for Viracept, EMEA/CHMP/492059/
2007, 20th September 2007.
6. Teasdale A, Eyley S, Delaney E, Jacq K, Taylor-Worth K,
Lipczynski A, Reif VD, Elder DP, Facchine K, Golec S, Oestrich
RS, Sandra P, David F. 2009. Mechanism and Processing
Parameters Affecting the Formation of Methyl Methanesulfonate from Methanol and Methanesulfonic Acid, An Illustrative
Example for Sulfonate Ester Impurity Formation. Org Process
Res Dev 13:429–433.
7. Berge SM, Bighley LD, Monkhouse DC. 1977. Pharmaceutical
salts. J Pharm Sci 66:1–19.
8. Bighley LD, Berge SM, Monkhouse DC. 1995. Salt Forms of
Drugs and Absorption. In: Swarbrick J, Boylan JC, editors.
Encyclopaedia of pharmaceutical technology, Vol. 13. New
York: Marcell-Decker, Inc. pp 453–499.
9. Stahl PH, Wermuth G. 2002. Monographs on acids and bases.
In: Stahl PH, Wermuth G, editors. Handbook of Pharmaceutical Salts; Properties, Selection and Use (International Union
of Pure and Applied Chemistry), Chapter 12. New York: WileyVCH. pp 265–327.
10. Balbach S, Korn C. 2004. Pharmaceutical evaluation of early
development candidates ‘‘the 100 mg approach.’’ Int J Pharm
275:1–12.
11. Gardner CR, Almarsson O, Chen H, Morrisette S, Peterson M,
Zhang Z, Wang S, Lemmo A, Gonzales-Zugasti J, Monagle J,
Marchionna J, Ellis S, McNulty C, Johnson A, Levinson D,
Cima MJ. 2004. Application of a high throughput technologies
to drug substance and drug product development. Comp Chem
Eng 28:943–953.
12. Morissette SL, Almarsson O, Peterson ML, Remenar JF, Read
MJ, Lemmo AV, Ellis S, Cima MJ, Gardner CR. 2004. Highthroughput crystallisation: Polymorphs, salts, co-crystals and
solvates of pharmaceutical solids. Adv Drug Deliv Rev 56:275–
300.
13. Zhu H, Xu J, Varlashkin P, Long S, Kidd C. 2001. Dehydration,
hydration behaviour and structural analysis of fenoprofen
calcium. J Pharm Sci 90:845–859.
14. Anderson BD, Flora KP. 1996. Preparation of water soluble
compounds through salt formation. In: Wermuth CG, editor.
The practices of medicinal chemistry. London: Academic Press.
pp 739–754.
15. O’Connor KM, Corrigan OI. 2001. Comparison of the physicochemical properties of the N-(2-hydroxyethyl)pyrroloidine,
diethylamine and sodium salt forms of diclofenac. Int J Pharm
226:163–179.
16. Reynolds JEF, editor. 1993. Martindale: The extra pharmacopoeia, 30th edition. London: The Pharmaceutical Press. pp 1–
1428.
17. Serajuddin ATM. 2007. Salt formation to improve drug solubility. Adv Drug Deliv Rev 59:603–616.
18. Lipinski CA, Lombardo F, Dominy BW, Feeny PJ. 1997. Experimental and computational approaches to estimate solubility
and permeability in drug discovery and development settings.
Adv Drug Deliv Rev 44:3–25.
19. Lipinski CA. 2004. Solubility in water and DMSO: Issues and
potential solutions. In: Borchardt RT, Kerns EH, Lipinski CA,
Thakker DR, Wang B, editors. Pharmaceutical profiling in drug
discovery for lead selection. Arlington, Virginia: AAPS Press.
pp 93–125.
20. Serajuddin ATM, Pudipeddi M. 2002. Salt-selection strategies.
In: Stahl PH, Wermuth G, editors. Handbook of Pharmaceutical Salts; Properties, Selection and Use (International Union
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
of Pure and Applied Chemistry), Chapter 6. New York: WileyVCH. pp 135–160.
Chiang P-C, South SA, Daniels JS, Anderson DR, Wene SP,
Albin LA, Mourey RJ, Selbo JG. 2009. Aqueous versus nonaqueous salt delivery strategies to enhance oral bioavailability
of a mitogen-activated protein kinase-activated protein kinase
(MK-2) inhibitor in rats. J Pharm Sci. 98:248–256.
Black SN, Collier EA, Davey RJ, Roberts RJ. 2007. Structure,
solubility, screening and synthesis of molecular salts. J Pharm
Sci 96:1053–1068.
Brittain HG. 2007. Pharm Technol Strategy for the Prediction
and Selection of Drug Substance Salt Forms 31:78–88.
Remenar JF, MacPhee JM, Larsen BK, Tyagi VA, Ho JH,
McIlroy DA, Hickey MB, Shaw PB, Almarsson O. 2003. Salt
selection and simultaneous polymorphism assessment via high
throughput crystallization: The case of sertraline. Org Process
Res Dev 7:990–996.
Gross TD, Schaab K, Ouellette M, Zook S, Reddy JP, Schurtleff
A, Sacaan AI, Alebic-Kolbah T, Bozigan H. 2007. An approach
to early phase salt selection: Application to NBI-75043. Org
Process Res Dev 11:365–377.
Bastin RJ, Bowker MJ, Slater BJ. 2000. Salt selection and
optimisation for pharmaceutical new chemical entities. Org
Process Res Dev 4:427–435.
Stahl PH. 2002. Appendix by Wermuth G. In: Stahl PH, Wermuth G, editors. Handbook of Pharmaceutical Salts; Properties, Selection and Use (International Union of Pure and
Applied Chemistry). New York: Wiley-VCH. pp 329–350.
Morris KR, Griesser UJ, Eckhardt CJ, Stowell JG. 2001. Theoretical approaches to physical transformations of active pharmaceutical ingredients during manufacturing processes. Adv
Drug Deliv Rev 48:91–114.
Airaksinen S, Karjalainen M, Schevchenko A, Westermarck S,
Leppänen E, Rantanen J, Yliruusi J. 2005. Role of water in the
physical stability of solid dosage formulations. J Pharm Sci
94:2147–2165.
Elder DP. 1992. PhD thesis entitled, ‘‘Physicochemical and
Crystallographic Investigations into the Salt Formation of
Two Heterocyclic Drugs’’. University of Edinburgh, UK.
Engel GL, Farid NA, Faul MM, Richardson LA, Winneroski LL.
2000. Salt form selection and characterisation of LY333531
mesylate monohydrate. Int J Pharm 198:239–247.
Lee S, Hoff C. 2002. Large scale aspects of salt formation:
Processing of intermediates and final products. In: Stahl
PH, Wermuth G, editors. Handbook of Pharmaceutical Salts;
Properties, Selection and Use (international Union of Pure and
Applied Chemistry), Chapter 8. New York: Wiley-VCH. pp 191–
220.
Verbeek RK, Kanfer I, Walker RB. 2006. Generic substitution:
The use of medicinal products containing different salts and
implications for safety and efficacy. Eur J Pharm Sci 28:1–6.
Steele G. Pre-formulation—Candidate Drug Selection, Characterisation and Implications for Product Design, presented at
CPS Symposium 2006, Exploiting Emerging Formulation
Sciences Technologies for Pharmaceutical and Fine Chemicals
Manufacturing, 12–13th October 2006, Singapore, 2006, http://
www.ices.a-star.edu.sg/ices/CPSsymposium2006/index.do.
Lin S-L, Lachman L, Swartz CJ, Huebner CF. 1972. Preformulation investigation I: Relation of salt forms and biological
activity of an experimental hypertensive. J Pharm Sci 61:1418–
1422.
Chen TK, Thornton D, Ho MYK. 1990. Reduction of dimethyl
sulfoxide by dihydrohalide salts of pharmaceuticals. Int J
Pharm 59:211–216.
Narurkar AN, Purkatystha AR, Sheen P-C. 1985. Effect of
various factors on the corrosion and rusting of tooling material
used for tablet manufacture. Drug Dev Ind Pharm 11:1487–1495.
DOI 10.1002/jps
THE UTILITY OF SULFONATE SALTS IN DRUG DEVELOPMENT
38. Dressman J, Reppas C. 2007. Drug solubility: How to measure
it, how to improve it. Ad Drug Deliv Rev 59:531–532.
39. Gould PL. 1986. Salt selection for basic drugs. Int J Pharm 33:
201.
40. Streng WH, His SK, Helms PE, Tan HG. 1984. General treatment of the pH solubility profiles of weak acids and bases and
the effects of different salts on the solubility of a weak base.
J Pharm Sci 73:1679–1684.
41. Amidon GL, Lennernas H, Shah VP, Crison JRA. 1995. Theoretical basis for a biopharmaceutical drug classification: The
correlation of in vitro drug product dissolution and in vivo
bioavailability. Pharm Res 124:413–420.
42. Li S, Wong S, Sethia S, Almoazen H, Yoshi YM, Serajuddin AT.
Investigation and solubility and dissolution of a free base and
two different salt forms as a function of pH. Pharm Res 2005.
22:628–635.
43. Creasey NH, Green AL. 1959. 2-Hydroxyiminomethyl-Nmethylpyridinium methanesulphonate (P2S), an antidote to
organophosphorus poisoning. Its preparation, estimation and
stability. J Pharm Pharmacol 11:485–490.
44. Hussain MA, Wu L-S, Koval C, Hurwitz AR. 1992. Parenteral
formulation of the kappa agonist analgesic, DuP 747, via
Micellar solubilization. Pharm Res 9:750–752.
45. Benneker FBD, Van Dalen F, Lemmens JM, Peters T, Picha F.
4-Phenylpiperidine compounds, WOV 98/56787, 17th December, 1998.
46. Rowe RC, Wakerly MG, Roberts RJ, Grundy RU, Upjohn NG.
1995. Expert systems for parenteral development. PDA J
Pharm Sci Technol 49:257–261.
47. Simamora P, Pinsuwan S, Surakitbanharn Y, Yalkowsky SH.
1995. Studies in phlebitis VIII: Evaluations of pH solubilized
intravenous dexverapamil formulations. PDA J Pharm Sci
Technol 50:123–128.
48. Strickley RG. 2000. Parenteral formulations of small molecules
therapeutics marketed in the United States (Part III). PDA J
Pharm Sci Technol 54:152–169.
49. Kim Y, Oksanen DA, Massefski W, Jr., Blake JF, Duffy EM,
Chruynk B. 1998. Inclusion complexes of Ziprasidone mesylate
with b-cyclodextrin sulfobutyl ether. J Pharm Sci 87:1560–
1567.
50. Ware EC, Lu DR. 2004. An automated approach to salt selection for new unique trazodone salts. Pharm Res 21:177–184.
51. Stephens VC. US Patent 3,065,261, 1962.
52. Serajuddin ATM, Jarowski CI. 1985. Effect of diffusion layer
pH and solubility on the dissolution rate of pharmaceutical
bases and their hydrochloride salts: Phenazopyridine. J Pharm
Sci 74:142–147.
DOI 10.1002/jps
2961
53. Li S, Doyle P, Metz S, Royce AE, Serajuddin ATM. 2005. Effect
of chloride ion on dissolution of different salt forms of haloperidol. J Pharm Sci 94:2224–2231.
54. Perrin J, Vallner J. 1970. The effect of the anion on the
absorption of tetracycline from the rat stomach. J Pharm
Pharmacol 758–762.
55. Miyazaki S, Oshiba M, Nadai T. 1981. Precaution on use of
hydrochloride salts in pharmaceutical formulations. Chem
Pharm Bull 70:594–595.
56. Lewis C, Stern KF, Grady JE. 1965. Comparisons of laboratory
animals of the antibacterial activity and absorption of lincomycin hexadecylsulfamate and lincomycin hydrochloride. In:
Sylvester JC, editor. Antimicrobial agricultural chemotherapym 1964. Washington: American Society of Microbiology.
pp 13–17.
57. Rajogopolan N, Dicken CM, Ravin LJ, Randall CS, KrupinskiOlsen R. 1989. Solubility properties of the serotonergic agonist
2,3,4,5-tetrahydro-8-(methylsulfonyl)-1H-3-benazazepin-7-ol.
Drug Dev Ind Pharm 15:489–497.
58. Malek P, Kolc J, Herold M, Hoffman J. 1958. Lymphotrophic
antibiotics—‘‘Antibiolymphins.’’ In: Welsch H, Marti-Ibanez F,
editors. Antibiotics annual, 1957–1958. New York: Medical
Encyclopaedia, Inc. pp 546–551.
59. Massey EH. German Ofen. 2,138,049 (3rd February, 1972)
through Chem Abs 76: 117507f, 1972.
60. Debs R, Brunette E, Fuchs H, Lin E, Shah M, Hargis A,
Montogomery AB. 1990. Biodistribution, tissue reaction, and
lung retention of pentamidine aerosolisized as three different
salts. Am Rev Respir Dis 142:1164–1167.
61. Müller L, Gocke E, Larson P, Lavé T, Pfister T. Elevated Ethyl
Methane Sulfonate (EMS) in Nelfinavir Mesylate (Viracept,
Roche): Animal Studies Confirm Toxicity Threshold and
Absence of Risk to Patients, Poster LPBE1157, XVII International AIDS Conference, 03–08th August, 2008, Mexico City,
Mexico.
62. CPDB. 2008. Carcinogenic Potency Database. Methyl
methanesulfonate: http://potency.berkeley.edu/chempages/
METHYL%20METHANESULFONATE.html.
63. Yang Q, Haney BP, Vaux A, Riley Da, Heidrich L, He P, Mason
P, Tehim A, Fisher Le, Maag H, Anderson Ng. 2009. Controlling the genotoxins ethyl chloride and methyl chloride formed
during the preparation of amine hydrochloride salts from
solutions of ethanol and methanol. Org Process Res Dev 13:
786–791.
64. Emmerson JL, Gibson WR, Anderson RC. 1971. Acute
toxicity of propoxyphene salts. Toxicol Appl Pharmacol 19:
445–451.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 7, JULY 2010