Ritonavir Solution Structure and
Relationship to Crystal Polymorphs
C4X Discovery
Safer, Be3er, Faster
Wojciech Augustyniak*, Martin J. Watson, Charles D. Blundell
C4X Discovery Ltd., Manchester One, 53 Portland Street, Manchester, M1 3LD, UK
Contact email: [email protected]
Solution structure vs crystal polymorphs
Ritonavir is a potent inhibitor of HIV protease [1] that was introduced
in 1996. During development only polymorph form I was found, but in
1998 the more stable form II polymorph spontaneously appeared
[2,3]. This less soluble crystal form compromised oral bioavailability
and caused the temporary removal of the drug from the market.
Polymorphs I and II differ in both hydrogen-bond arrangements and
molecular conformation [2]. Here, the dynamic 3D-structure of
ritonavir in solution is reported and its relationship to the crystal
polymorph conformations is discussed.
Torsion angles
Solution
structure
(anti carbamate)
Form I
CSD YIGPIO02
Solution
structur
e (anti
carbam
ate)
Polymorph I
Polymorph II
Introduction
DoF 1
DoF 2
Ritonavir dynamic 3D-structure
The recently reported method for determining dynamic solution
structures [4] was used to measure the ritonavir structure. Data was
recorded in ethanol-d5 to mimic crystallisation conditions.
DoF 3
Carbamate
NMR analysis is complicated by slow
syn/anti conformational exchange at
the carbamate group.
DoF 4
Form I backbone matches
16% conformations in solution
The structure was solved for the
predominant anti-rotamer of the
carbamate (85%), which is present in
the form I polymorph.
Form II
CSD YIGPIO01
1H
(ppm)
DoF 5
DoF 6
DoF 7
DoF 8
Restraint type
Number of
restraints
NOEs
720
ROEs
626
Scalar Couplings
13
Total
1359
37 ± 3%
DoF 9
DoF 10
DoF 11
DoF 12
Form II backbone matches
6% conformations in solution
16 ± 4%
9 ± 4%
Dominant core backbone conformations
and extent of local libration
With 19 rotatable bonds, ritonavir
oscillates about a great variety of
conformations in solution,
ranging from largely extended to
considerably curled shapes.
Many bonds display multi-modal
conformational behaviour (see
right).
Although form I and form II have different
crystal conformations compared to each
other, when considered bond by bond all
but one torsion (DoF 1, form II) adopt
conformations that are represented in
solution.
Interestingly however, the specific set of
conformations for all bonds together is
not present in solution for either form I or
form II, indicating that a global level of
conformational strain is induced in the
molecule upon crystallisation even
though locally there is little strain.
DoF 13
DoF 14
DoF 15
DoF 16
DoF 17
DoF 18
DoF 19
Conclusions
Comparison between solution conformations and known crystal polymorphs is proposed to be of general use in assessing whether the set of
known polymorphs for a drug is likely to contain the most stable polymorph and also whether there are likely to be other metastable forms
that are as yet unknown.
[1] Kempf et al., Proc. Natl. Acad. Sci., 92, 2484-2488, 1995
[2] Bauer et al., J. Pharm. Res., 18, 859-866, 2001
[3] Morisette et al., Proc. Natl. Acad. Sci., 100, 2180-2184, 2003
[4] Blundell et al., Bioorgan. Med. Chem., 21, 4987–4976, 2013
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