Indi an Journal of Biochemistry & Biophysics
Vol. 37 , O ctober 2000, pp. 273-282
Minireview
Hydrophilic polymeric nanoparticles as drug carriers
P K Ghosh*t
Department of Biotechnology, Ministry of Science and Technology,
Block 2, C.G.O. Complex, Lod i Road, New Delhi 110 003
Received 6 April 2000; accepted 12 June 2000
Polymeric hydrophilic nanoparti cles have· attracted alle ntion because of th eir potential usefulness as drug-carriers in
target specifi c cel ls, ti ssues or nu cleus. Applications are ex pected throu gh injectable, oral, nasal or ocular routes.
Hydrophi li c nanoparticles have been prepared that are hydrophilic on th e surface as well as inside the core; particles have
also been made that are hydrophilic on the outer surface bu t are hydrophobic within the core. External stimu li responsive
hydrophilic nanoparticles are yet another new add iti o n to the spectra of new materials in drug delivery. Water soluble as
well as insolubl e drugs can be en trapped into these polymers by adjustin g the hydrophi li city/hydroph obicity of the core of
the nanoparticles. The review di scusses th e requirements that these hydrogel nanoparti cles must meet to be effecti ve in drug
delivery, and the importance o f the preparative met hods for controlling their sizes below I 00 nm , with narrow size
distribution.
Nanometre size particles popul arly known, as nanoparticles are understood to mean different ranges of
dimension of particles to different people. Some
investigators have considered the solid colloidal
particles of di ameter between 10 and 1000 nm 1' 2 as
the nanoparticles, while others have used terms like
nanophase and nanostructures to mean particles
havin g diameters ' below I 00 nm. More recently th e
term was used to mean particles with diameters of
50 nm or below 4 . In thi s arti cle, the term has been
used to mean parti cles of size between I 0 to I 00 nm .
Polymeric particles within this size range, loaded
with drugs, arc expected to be extensively
investigated during the coming years for targetin g
tissues, cell s and sub-cellular organs such as nucleus.
Applications are expected through inj ectable, oral ,
ocular or nasa l routes.
Polymeric nanoparticles have been extensively
investigated over the last two decades after the first
report was publi shed 5 in 1976. These particles as dru g
carriers or carri ers of tracer molecules have been
presented as so lid nanospheres where the act ive
substances have been incorporated either inside the
spheres or have bee n adsorbed on their surfaces or
*Author for corresponde nce.
Email: pk g hosh @dht.ni c.in ; g prasanta @hotmail.com
t vi e ws e xpressed in thi s paper are th ose of the auth or and they
have nothing to do with th e organi sation to which he belongs .
This work is dedicat ed to th e fond memory of late Prof Bimal K
Bachh awat , who in spired me for pursuin g scie nce with
dedi cation .
both. They have also been presented as nano capsules
where the active substances have either been
incorporated within the core or have been loaded on
the surface by physical adsorption or by chemical
bonding2 (Fig. I ) .
Site-specific dru g delivery is yet largely empirical
and the methods available are presently imperfect.
Non-toxic, nanosize polymers in controlled drug
delivery are nevertheless thought to be playing
important rol e, and it has been hypothes ised that
while the drug should be deli vered at rates that
maintain its optimum therapeuti c activity at the act ive
sites, concurrently the side effects should be brought
down to th e lowest leve l6 . Delivering drugs through
polymeric nanocarri ers is considered to assist in
reducing the adverse reactions and side effects. The
pol ymeric carriers have to be so chosen as to have
several characteristics that have been identified and
. db y some mvestt
.
.gators 6- tl as un d er:
summanse
(a) the polymers shou ld be compatible with th e body
in terms of ada ptability (non-tox ici ty and nonantigenicity) and should be biodegradable and
bio-compatible.
(b) the particles should preserve and protect th e
drugs and should not release them till they reach
the sites of acti on.
(c) the nanoparti cles should not interact or should not
have any harmful effects on the body cells or
ti ssues . .
INDI AN J BIOCHEM BIOPHYS , VOL. 37, OCTOBER 2000
274
e 8
- e
-..
•
/
Nanospheres ~
...
)
N'"""P'ul, i
~
/
• Drug substances entrnpped or adsorbed
-
Drug substances chemically bound
Fi g. !-Drugs or tracer mol ec ules loaded with polymeri c hydroph ilic nanoparticles in different fo rms
(adopted with modifi cations from ref. 2)
(d) th...., particl es shou ld be ab le to traverse the
intervenin g me mbran es.
(e) the particles should recogni se the s ites of action
and should get bound or associated with th e sites
of action.
(f) the dru gs should be re leased at rates so as to
achi eve the desired therapeutic effects o n a
continuous basis .
(g) afte r the re lease of the d ru gs, th e nan opart ic les
should be degraded or e limin ated from the body.
All the pol ymeric na no particle drug carri e rs th at
have been designed and in vest igated so fa r are to
produce the ta rget in g syste m to ac hi eve the above
characteristics .
Mechanisms of drug delivery
The po lyme ri c dru g carri e rs that have been
desi gned would deliver the drug at the ti ss ue s ite by
any one or more of the three gene ral physico1
c hemical mechanisms 12. 1. name ly (a) by th e swe llin g
of the po lyme r na no particles by hyd rati on fo ll owed
by re lease throu gh diffu sio n (b) by an enzy mat ic
reac tion leadi ng to rupture o r cleavage, leadin g to the
degradation of the polyme r at sites, thereby rel eas in g
the drug from the entrapped inne r core o r (c) by
c leavage of the drug fro m the polymer and its deadsorption/re lease fro m the swe ll ed nanopa rti c les .
Upon inj ection of a polymeric targeting nano syste m,
the fate of the part ic le within the body would be
gove rn ed by their parti c le s ize, sha pe, surface charge
and the surface hydrophob ic or hyd rophili c
c haracte ri sti cs. F urth er, the in vivo di stributi on and
accumu lation would be affected by the ph ys icoc he mi ca l properties of th e targetin g nanopartic les.
Characteristics of particles for targeting specific
sites in the body
The major c ha ll enge in the development of
po lymeric particulate dru g carri ers has been to c reate
co nditi ons so that the particles ca n evade the sys temic
scavenging machinery suc h as the reticuloendothelial
14 15
sys te m (RES ) ' while in c irculation in the body.
Phospholipid ves ic les (liposo mes). w hi c h are se lfasse mbl ed colloidal particles, have been wide ly
16 17
ex pl o red as dru g carri e rs ' . Th ey were not found
suitabl e for site spec ific drug targetin g as th ey were
c leared fas t by the RES of th e body. However, suc h
ves ic les coated with poloxamers (meth yl oxyrane
po lyme rs) of different mo lecul a r we ights were found
to be usefu l in evad in g the RES uptake and
co nseq ue ntly the c irc ulati o n time o f th e lipid ves ic les
co uld be prol onged in bl ood 2 ' 1R·l" . Coating the lipid
ves ic les w ith poloxa me rs and poloxamines re nde r th e
surface of the lipid vesic les hyd rophilic. This observation led investi gato rs to initiat efforts to modulate
the surface hyd rophili c ity of drug ca rrier partic les to
enabl e the evas io n of the system ic scavengers.
It is known th at ingested micro-particles of
diameter, 7 j..lm or above are filtered out by the
cap illary bed of the lun g. Such large part ic les cann ot
5 27
e nter the c irculato ry syste m of the bod / - . Smaller
GHOSH: HYDROPHILIC POLYMERIC NANOPARTICLES AS DRUG CARRIERS
particles get into circulation, but they are scavenged
by the macrophages in blood, spleen and bone
marrow. The kinetics and extent of scavenging are
fast and critical below certain sizes and depend on the
degree of their surface hydrophilicit/ 8. 32 .
Importance of preparative methods in controlling
the polymeric particle size
Presently , vast literature is available for the
preparation of polymeric nanoparticles of all ranges.
The polymeric materials used have been diverse as
polylactyllactate 33 , polyalkylcyanoacrylates 34'35 , polymethylmethacrylates36, different polymerised vinyl
derivatives bearing allyl, acryloyl, acrylamido and
methacrylamido groups 37 , acrylate or alkyl acrylate
terminated polyethylene oxide or polyethylene glycol,
or para vinylbenzyl terminated polyethylene glycol or
polyethylene oxide or mixtures there of38 , dextran 39,
gelatin 40 etc . Known methods are employed for the
polymerization of the monomers, di spersed in a
suitable manner. Usually, an oil in water (for an oil
soluble monomer) or water in oil (fo r water soluble
monomer) type dispersion is made, usi ng suitable
surfactants
like sodium
bis-2-ethylhexylsulfosuccinate (AOT), to enable the formation of desired
microemulsion droplets. For water soluble monomers
i.e. water in oil type microemulsion polymerization ,
water soluble initiators like ammonium, sodium or
potassium persulfates with act ivators like tetramethylethylene diamine (TEMED) are used. In order to
impart so me ri gidity to the nanoparti cles, cross-liking
agents likeN , N' meth ylenebisacrylamide (MBA) are
also used with the monomer. The loci of polymerization are th e droplets of the microemul sion
phase.
Polymer particles prepared by the conventional
emulsion polymerization methods produce highly
polydi sperse particles with diameters in the range of
hundreds to thousands of nanometres . It was shown
that th e probl ems of wide size range could he
overcome if polymerization was carried out in
microemulsion instead of in emulsion 41 as microemulsions are thermod ynamically stable and are in
monodi spersed phase. The polymerisati on of methacrylic acid was carried out in reverse microemul sion
to produce hi ghly monodispersed particle42 . The
quantity of water that could be solubilised into the
non-polar solvent in a reverse microemul sion is also
largely depend ant upon the type of the surfactant
molecul e used and it was shown 4-' that in water-AOT-
275
isooctane system, the reverse micellar droplets could
be made comparatively larger by increasing water to
AOT ratio. This is primarily due to a favourable
interface-packing factor emanating from the property
of free internal rotation of AOT, thereby causing
more water to be accommodated inside the droplet.
More water inside is tantamount to more free water
that can accommodate larger quantities of water
soluble extraneous materials inside the core water
droplet nano reactors, and the system would therefore
be suitable for the preparations of larger quantities of
precise nanoparticles.
The water droplet size in reverse micelles is
controlled mainly by three factors, namely, the choice
and the concentration of surfactants and the content
of water that can be associated with the concentration
of the surface active agents. Although several
investigations had been carried out on the preparation
of polymeric nanoparticles in reverse microemulsions44.52, the influence of water to surfactant ratio on
the size of nanoparticles was long overlooked.
Another important factor is the use of the ri ght
quantity of initiators to optimise the receipt of oneinitiator molecule at the most per microemul sion
droplet. Thi s was revealed from our recent work
where we could consistently prepare polymeric
hydrophilic nanoparticles 5-'· 54 of precise size range of
I 0 to I 00 nm. The polymerization was initiated
entirely into the individual aqueous core of reverse
micellar droplets of the microemul sion phase, but
there had also been inter droplet interaction in the
process. Our observation had been that about I 0-15
droplets take part in such inter-droplet interactions,
which lead to the formation of nanoparticl e
compounds/composites at the end of the polymerization process5-'· 54 .
Process of making polymeric hydrophilic nanoparticles of narrow-size distribution
The surfactant, sod ium bi s-2-ethylhexylsulph osuccinate, or Aerosol OT (i.e. AOT) was dissolved in
n-hexane (usually 0.03 M to 0.1 M of AOT
solution). Water-so luble monomer, N-vinylpyrrolidone, was used and the polymer was cross-linked
with MBA for nanoparticle preparation. Aqueou s
solutions of monomer, cross-linking agent, initi ator
and FITC-Dextran (FITC-Dx) were added to AOT
solution in hexane and the polymerization was carried
out following the standard procedure. Additional
amount of buffer may be required to be added in
276
IND IAN J BIOCHEM BIOPHYS , VOL. 37 , OCTOBER 2000
reverse micelles in order to get th e host micellar
droplets of desired size. In a typi cal experiment for
the preparation of nanoparticles of vin ylpyrro lidone
containing FJTC-Dx as an · encapsul ated marker
material (mol. mass 19.3 kDa), we had taken 40 ml of
0.03 M AOT soluti on in hexane in whi ch 316 Jll of
fres hl y distilled N-v in ylpyrrolidone, 100 Jll of MBA
(0.049 g/ml ) as cross-linking agent, 20 Jll of I%
ferrous ammo nium sulphate (FAS), 20 Jll of 11 .2 %
aqueous soluti on of TMED, 30 Jll of 20% am monium
persulph ate as initi ator and SO ~LI of marker
compound, FJTC-Dx ( 160 mg/ml) were added. The
soluti on was homogeneous and opticall y transparent.
Pol ymeri zati on was done in N2 atmos phere at 3S°C
for 8 hr in a thermostati c bat h with continuous
stirring. The above method produced PVP nanoparti cles cross-linked with MBA and containing
FJTC-Dx as encapsul ated material. The organi c
so lvent was eva porated off in a rotary eva porator and
the dry mass was res uspended in S ml of water by
soni cati on. Calculated amount of 30% CaCI 2 solution
was added drop by drop with continuous sti rrin g to
preci pitate the surfactan t as calc ium sa lt of bis (2ethylhexyl) sulphosuccinate, [Ca(DEHSS) 2] . The
centrifu ged ( I0,000 rpm for I0 min) aqueous soluti on
conta ·ns nanoparticl es, which was homogeneous and
transparent. The cake of Ca(DEHSS) 2 after centrifugatio n contains some nanoparticles adsorbed on it.
It was di sso lved in I 0 ml of n-hexane and the hexane
soluti on was was hed 2-3 times with I ml of water.
The phase separated aqueous layer was drain ed out
and added to the ori ginal centri fugate. The total
aqueous di spersion of nan oparti cles was lyophili sed
immedi ately to dry powder for subsequent use.
Lyoph ili sed nanoparticles are easily redispersibl e in
aqueous buffer, which was su bj ected to gel filtrati on
usin g Sephacryl S-200 ( I cmx 34 em) column preequil ibrated with SO mM phos phate buffer saline at a
flow rate of i 2 ml/hr at 2S°C, and th e separated
nanoparticles free from un-encapsu lated FJTC-Dx and
other unreacted compounds were directl y used for
bio-distribution an d oth er studies. The size of these
particles in buffer was fo und to be same before and
after lyophilisation.
The number of dro plets tak ing part in th e
interacti on depends upon the ini tia l density and size
of dropl ets, temperature of pol ymerization and the
quantity of initi ator used to tri gger po lymerization ;
th e smaller th e dro plet size, the lower the temperature
and lesser the concentration of the initiator (ideall y
one initiator particle per water droplet) , th e lesser is
the inter-droplet interacti on55 .
..
Fig. 2 shows the rel ationship between the initial
water droplet size (diameter) and the diameter of the
nanoparticles produced in our laboratory in hexaneAOT- water system usin g N-vinylpyrro lidone (VP) as
the monomer and N, N'-meth ylenebi s acrylam ide
(MBA) as the cross linking agent (3.2% w/w of VP)
and entrapp ing within the po lymer, FITC-dex tran
compl ex (mo l. mass 19.3 kDa) upto J.2 % w/w of the
polymeric materia1 56 . This study was carri ed out to
assess the entrap ment efficiency of the nano-polymeric particl es for the marker water so luble FJTCdex tran comp lex. This relati onship shows consistent
particle diameter and th erefore similar and reproducibl e degree of polymerization of the nanoparticl es ,
whi ch is ex plained from th e hypothesis that the
polymerisable monomers are packed together in
micellar d!foplets by th e action of the surfactant AOT
in hexane. Inter dropl et interaction is also limited
within a few water drop let nanoreactors. These
res ults are consistent with the ea rli er observations on
pol ymerisation of dimethylami noeth yl methacrylate
.tn benzene s7.ss as we II as tI1e mtcrcemu
.
Iston
. parttc
. Ies
produced from the polymeri sati on of ac rylamide in
toluene usi ng AOT as the surfactant 4 R.4'!. The
molecular mass of the res ultin g polymers was I06 to
I07 Da. As an exa mple, polyac ryl am ide nanoparti cles
300
l
250
1
E'
.s
...
200
Q)
G.i
E 150 .
nl
i5
Q)
0 100 .
'E
nl
a..
50
0 +-0
- - - . - -- ---.2
·
4
6
8
Droplet Diameter (nm)
Fig. 2-lntcr-relatio nship between th e fo rmation of the si ze of
FlTC-Dex tran ( 19.3 ki loda llo n) loaded nanopart ic les and th e size
of the start in g reverse mi cell ar water drop lets in which th ese
nanoparticles were prepared. [Loadi ng of F lTC-Dcxtran=3.2%
w/w of po lyme ri c material. MBA=l .2% w/ w or th e polymeric
mate ri al]
GHOSH: HYDROPHILIC POLYM ERI C NANOPARTICL ES AS DR UG CARRIER S
prepared in AOT reverse mi c roe mul s ion4 1 have been
reported to have size of 19 nm and mo lecul ar mass of
6
3.2x I 0 Da w hi c h can vary dependin g upon the
change in the compos iti on and the s ize o f re ve rse
mice ll ar dropl e ts. Probabl y the pheno menon of
continu ous nuc leati on take :; pl ace w ithin the mi c roe mul sion aqueo us co re and the po lymer ch ain builds
up gradually , till most of the mo nomers and c rosslinking agents are used up . Furth er, f rom the
corre lati on of th e measure me nt of th e mol ecul ar
we ight and parti c le di a me ter of po lyacry lamide
produced by mi croemul s ion po lymerizati o n of ac rylamide in wate r-AOT-to luene syste m, it was
conc luded th at the numbe r of po lymer c hain per
50
parti c le was onl y o ne o r two w hi c h is a lso indi cati ve
of the building up of the po lyme r c ha in within a few
water d ro pl et nanoreactors.
Po lyv in ylpyrro lido ne cross- linked w ith MBA was
fo und to ent rap effec tive ly, comparati ve ly la rger
water-so lu ble mo lecul es like bov ine immun oglobulins and anti gen ic mo lec ul es of Aspergillus
f umiga tus o r marke r mo lec ul es like FITC-D x ( mo l.
mass 19.3 kDa) . Th ese e ntrapped nanopa rtic les
prod uced and processed by our meth ods were fo un d
to be non-toxic in mi ce and had e li c ited sati sfac to ry
53
anti gen ic res ponse . Our system of hydrophil ic
po lymeric nanoparti cles of po lyv inylpyrrol id one wi th
contro ll ed cross- linkin g by M B A ho lds pote nti a l fo r
entrap pin g d iffe rent kinds of bi o-ac ti ve wate r solu ble
mo lecul es of mo lecular mass of 5 kDa a nd above;
these e ntrapped parti c les have poss ib iliti es of
targeting bod y ti ssues spec ia ll y the tumour ti ss ues .
Smalle r mo lecu les cou ld be co nverted into prod rugs
by conjugatin g w ith appropri ate water-so lubl e nontox ic hapte ns o r no n-anti gen ic substances to in c rease
the ir mo lecul ar we ig hts for easing e ntrapme nt. These
nanopartic les a re not eas il y scavenged by the RES '"
in contrast w ith most of th e presentl y ava il abl e
nanoparticl es that are large ly hyd ro ph obi c in surface
characte ri stics. However, these po lyme ri c hydrophili c
nanoparticl es a re not suitabl e for e ntrappin g water
insolubl e bi oac ti ve substances .
Entrapment of water insoluble drug or tracer
molecules into nanopa rticles of hydrophilic
polymeric micelles
In orde r to entrap wate r-in so lub le bi oacti ve
substances, investi gation and ad vance me nt s ha ve
been made using outer surface hydrophili c nanoparti c les of poly meric mi ce ll es during the rece nt time .
277
The re are seve ra l repo rts w he re wate r-so lu ble , bioco mpatibl e po lyme ri c mi ce ll es have been used as the
d ru g d e 1Ive ry ve h IC Ies·W -6 1_ p o Iyme n c mi ce II es are
dy nami c aggre gates of severa l hundred mo lecul es of
dibl ock copolyme rs. They have di a mete rs of 2050 nm . They are formed as two co-centri c po lymeri c
region s w here the oute r s he ll is made up of
hydro phili c materi a ls and the inne r co re is of
hydro phobic substances. The inne i: hydro ph obi c core
is res ponsibl e fo r ho ldin g o r "solubili sing" the
hyd ro ph o bi c substances inc luding dru gs o r marke r
mo lec ul es. The o uter hydro phili c periph e ry e mpowe rs
th e co mpos ite to evade th e R ES of the body w hen
used in vivo . S uc h nano co mpos ites of po lymeric
mi cell es a re made up of se lf-a ssembl age of
amphi phili c bl ock o r graft-copo lyme rs w he re one part
of the po lymer is hi ghl y so lubl e in the so lvent used
(say wate r so lubili sing hyd rop hilic parts of the
d ibl ock co-po lyme r) w hil e the other part is in so lu bl e
in the same so lvent. F ig 3 illu st rates sc he maticall y the
mi celli sati on of a mphiphili c po lyme rs in water.
0
0
0
0
T he hyd roph ili c no n-tox ic vehi c les exte nsive ly
used are po ly (ethyle ne ox ide), and po ly (e th yle ne
glyco l); the hydrophob ic vehi c les often used as th e
copo lymer co nj ugates w ith the hydrophili c vehicl es
are po ly (propy le ne ox ide), po lysty re ne, po ly
(methy lme thacrylate) , po ly (L-asparti c ac id) , po ly
(L-Iysine) and po ly (L-g luta mi c ac id) . Ac ti ve d rugs
in sol ubl e in wate r have a lso bee n c o va lent ly lin ked
on to the hyd rophob ic part in suc h syste ms . The
62
concept
first in trodu ced in 1985 has been
.
d61--74 . T h e wate r
ex te ns .ive Iy stu d.Ie d an d re viewe
inso lubl e d rugs have a lso been phys icall y e ntrapped
in suc h hydroph obi c co re po lymeric micel le s. Dru gs
suc h as indomethaci n, a mphoteric in-B and doxoru bi c in have bee n used in suc h e ntrapment
75 78
compos ites - . U nimo lecul ar de nd riti c mi cell es with
a hydro ph obi c core surroundin g a hydroph ili c shell
was prepared on the above conce pt an d the
"co nta ine r" property of the m ice lle s was de mo nstrated by "solub ili sing" pyrene in aqueous sol ut ion of
79
such mi ce ll es . F rom these stud ies it ca n be argu ed
that by proper c ho ice of two or more mono mers
conta ini ng lipop hilic a nd hyd ro ph il ic groups, and by
main tenance of ap pro priate rati os of them, it should
be poss ible to produce copo lymers of d iffere nt cha in
length co nta ini ng hyd rophili c and li pophili c pockets.
S uc h amphiph ili c po lymers shou ld be use ful for
solubili sin g a lmost a ll kind s of wa te r in so lu bl e
substances of lowe r mo lecul ar we ig ht of say npto
278
INDI AN J BIOCHEM BIOPHYS, VOL. 37, OCTOB ER 2000
Indiv idual polymers or c opolymer molecules
..f\N\N\1\/'-
-----/VINV'N'
-IINV\IIN'
"
Hydrophilic portio n s
.fV\1\NW'~
-MIV\NV' \
Hydrophobic pc.rtion s
-IINV\IIN' /
.fV\1\NW'-
Fig. 3-Schcmati c illu stratio n o r micelli zati o n or di -block hydro phili c-h yd rophobic co po lymer co nj ugati o n
S kDa . The amphiphili c po lyme rs preformed by
appl yin g standa rd procedures of po lyme rization and
the rea ft e r d isso lved in wate r could be co ntac ted w ith
the target compound s so lubili sed in a suitable so lvent
to en ab le the ir e ntrapme nt into th e hydro ph o bi c core .
Suc h non-to xi c, hydro phili c nanoparti c les of po lymeri c mice ll es co ntainin g target drugs e ntrapped into
the hydroph obi c core would f ind appli catio ns in an
array of situ ation s for targeting spec ifi c ti ss ues a nd
these co mpos ite parti c les would large ly be ab le to
e vade the RES du e to the ir sma ll e r part ic le size
(bel ow I 00 nm) and the ir pe riphe ra l hydrophili c
surface c haracte ri sti cs.
Smart hydrophilic nanoparticles
Fun cti ona l po lyme rs e ithe r fo und in nature o r
synthes ised in the lab whi c h a re hi ghl y non-line ar to
e xtern a l stimuli can be converted into nanosize
parti c les. Sma ll c hanges in ex te rn a l stimuli suc h as
c hange in te mpe rature, pH, magneti c or e lectric fie ld
o r so me suc h para me te rs in the mi cro-environme nt
brin gs abo ut fas t reversibl e c hanges in the mi crostru ctures of suc h po lyme rs fro m a hyd ro phi Iic state
to th e hydroph o bi c state o r vice versa. These c hanges
can be pe rce ived at the mac roscopi c leve l such as in
the mani fes tation of o rde r-of-magnitude c hanges in
parti c le s ize or wate r-conte nt o r formati o n of
prec ipitate at the s ite of c hange of th e ex te rna l
stimuli . S uc h c hanges are however, reversibl e and th e
po lyme rs return to th e ir initi a l state, w he n the tri gger
. remove dso·sl . T l1ese po Iyme rs w 'ne re ve r b.tocomIS
patib le, when co nverted into nanos:ze parti c les and
loaded w ith dru gs a re ex pected to beco me useful as
vehic les fo r sys te mi c dru g de livery. V."e produ ced pH
54
and the rmo-sens iti ve hyd roge l nanoparti c les of copo lymers of N -vin y lpyrro lid one (VP) a nd ac ryli c ac id
(AA); the co-po lyme rs of di ffe re nt mo lar rati os
betwee n YP and AA were prepared . We showed th at
the co-po lyme r of VP and AA co nta inin g 20 % AA (in
mo les) pe r I 00 mo les of to ta l mono me rs of VP and
AA, that e ntrapped FITC-Dx (mol. mass 19.3 kDa) up
to I % w/w o f th e co-poly me r had shown d iffe re nt
re lease rates o f the marke r compo und, both at
diffe re nt pH as we ll as at di ffe re nt te mpe ratures. T he
average parti cle s ize o f the co- pol yme r was about
SO nm. The pe rcent hydrati on (swe ll ing) of th e co-.
po lyme r w ith time at di ffe re nt p H was a lso very
diffe rent , be ing hi ghes t at hi g he r p H (pH I 0) and
signifi cantl y lowe r at lower pH (pH 3). As these copo lyme rs are bioco mpatib le and as Lhe ir s izes can be
modulated, these are expec ted to be useful in va ri ous
drug de li very sys te ms. A pH respo nsive in sulin
loaded po lyme r matrix was produced by c ompress in g
a pH respons ive po lyme r, g lu cose ox idase and bov ine
serum a lbumin 82 . Wh e n the m atri x was exposed to
glucose so luti on, g lu cose ox id ase oxi di sed g lucose to
g luconi c ac id , whi ch resul ted in a dec rease in pH
ac ro ss th e mi c roenviro nme nt . T hi s res ulted in
GHOSH: HYDROPHILIC POLYMERI C NANOPARTICLES AS DR UG CARRIERS
protonation of the polymer and swe llin g; swellin g
resulted in the rel ease of insulin. The release of
insulin stopped in I 0 min of glucose removal and th e
process could be restimul ated by the addition of
glucose. Othe r designs for the deli very of insulin that
83 74
responds to glucose were also developed earli e r ' .
Te mperature sensiti ve polymers e ntrapping active
substances and made up of po ly N-isopropylacrylamide85'86 or po lyethyleneox ide de rivative of
87
po lymeth ylac ryli c acid from which rel ease of th e
entrapped active substan ces can be a lte red by alte ring
the te mpe rature have also bee n sy nthes ised. The
value of these smart polymers can be increased
considerabl y by converting these into nanoparticles or
nanocomposites, as these small composites can be
tran sported to inaccessible body ti ssues The
preparation and properti es of micro-capsules and
mi cro-sphe res of chitosan, a natural, non -tox ic, biodegradabl e/bi o-absorbabl e hydroge l polymer has also
88 89
been desc ribed ' . Cross-linked c hitosa n mi c rospheres loaded with 5-fluorouracil and its a mino acid
derivatives ha ve been prepared; these microspheres
were loaded ei the r w ith an ioni c polysaccharides or
with lipids 111 multil ayers, and the resultin g
composi tes had diameters of 250-300 nm w ith a
narrow size distribution . These products showed
effecti ve barriers to the release of 5-fluorouracil ; th e
lipid multi-l ayer coated micro-spheres showed nonlinear re lease of 5-flu orouracil in physiological
sa line, increasi ng signi f icantl y above the ph ase
transition temperature of 4 1.4°C and decreasing to
90
lower re lease rate at 37°C. Such on-off control
hydroge l pol ymer composi tes for the release of
effective dru gs via a change in te mpe rature at the s ite
of infliction can be made more effective, if the
diameter of the composites can be reduced further,
below I 00 nm .
Smart amphip hili c nanoparticl es were prepared 111
our laboratory from vin ylpy rrol icl one and N-isopropylacrylamide that we re sens iti ve to temperature 56 .
These parti cles were loaded with pacl itaxo l with a
view to use them as veh icl es for dru g delive ry.
Ani mal experime nt s on the utility of these nanoparticles ·are in progress and appl icati ons for
patenting the process have been filed . The preparative
method is outlined below .
279
Smart hydrophilic polymeric nanoparticles of
copolymers of vinylpyrrolidone and N-isopropylacrylamide
Freshly distilled vinylpyrrolidone (I 0 mg) and
freshly crystallised N-i sop ropylac rylamide were
di sso lved in double distill ed water (10 ml ). To this
so lution was added 28 f..LI of MBA (49 mg/m l) and
nitrogen gas was passed into the so lution for half an
hour. To thi s so lution was added 20 f..LI of TEMED
( 11 .2 % w/w in water) a nd 30 fll of ammonium
persulphate (20 % w/w) and nitrogen was bubbled for
24 hr. The polyme ri zation was ca rri ed out at 35 °C.
After polymerization, the so luti on was di alysed for
2 hr. To the dial ysed so luti on was added a saturated
alcoholic so lution of taxol (40 mg/ml ). The e ntrapped
pac litaxol within the po lymeri c nanopartic les was
lyophilised for furth e r experiments.
The Fig. 4 shows the spectra of th e parti cles take n
in our Brookh aven 9000 instrument with a BI200SM
go ni ometer. The quasi-elastic li ght scatte rin g spectra
(QELS) were taken in an air-coo l argon ion laser
operated at 488 nm as the li ght source. The time
dependence of the inte nsity autocorre lati on fu nction
of the scattered intensity was derived by usi ng a 128c hannel di gital corre lator. The s ize of the
nanoparticles was determined by the diffusion of th e
particles using th e Stokes-Ei nstein equati on and the
representative size distribution spectra was as in
Fig. 4. Fig. 5 shows th e respo nse of the void and taxol
loaded nanoparticles with variation in te mperature.
T he sizes of the nanoparti cles in creased sharply wi th
a small c hange in te mpe rature above a critical
so lution te mperature, which was around 40°C.
100
Ql
..
.~
Ill
c:
Ql
...u
Ql
0..
0-+-------,--~llULr-------.------~
1
s
so
hoo
5000
Diameter (nm)
Fig. 4-Quasi-elasti c-li ght-scattcring spectra of pacl itaxol lmdcd
po lymeri c nanop articles
INDIAN J BIOCHEM BIOPHYS, VOL. 37 , OCTOB ER 2000
280
~
-----------------------------------.
350
~250
E
.s
~2CX)
QJ
E
"'
i5 150
100
-
50
-.i
administration of these particles has re main ed large ly
by the injectable route and the admini stration through
oral , ocular or nasa l route is yet at the developmental
stage.
The future years are expec ted to witness better
understanding of the inte raction between the
nanoparticles and the phagocytic cell s. Improved
unde rstanding of de live rin g the part ic les at target
sites will a lso evo lve to provide a con veni e nt method
of site specific drug de livery . The th ree syste ms of
hydrophilic pol yme ric nanoparti c les reviewed hold
great promise of max imi sing dmg effec tiveness while
minimising dmg toxicity.
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Temperature ( °C )
Fig. 5-Si zc of void (- +-) ancltaxol-loadcd pol ymeri c
nanopani cles (-• -).
I
2
3
Concluding Remarks
4
Targeting of th e dm gs using hydrophilic nan opa rti c les or nanoparticl es of polyme ric micelles as
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