Process research and development of a MTP inhibitor: Another case of disappearing polymorphs upon scale-up

Article abstract of DOI:10.1021/op100115u

LAB687 is an inhibitor of microsomal triglyceride transfer protein (MTP) designed to lower triglycerides and LDL cholesterol. The discovery of its polymorphic forms closely intertwines with the synthesis development of the molecule. At the early developme

Full text of DOI:10.1021/op100115u

Organic Process Research & Development 2010, 14, 878–882
Process Research and Development of a MTP Inhibitor: Another Case of
Disappearing Polymorphs upon Scale-up
Mahavir Prashad, Paul Sutton, Raeann Wu, Bin Hu, James Vivelo, Joseph Carosi, Prasad Kapa, and Jessica Liang*
Chemical and Analytical DeVelopment, NoVartis Pharmaceuticals Corporation, One Health Plaza, East HanoVer,
New Jersey 07936, U.S.A.
Abstract:
number of functional groups that may form a number of
hydrogen bonds are predisposed to polymorphism. When a
3
LAB687 is an inhibitor of microsomal triglyceride transfer protein
(
MTP) designed to lower triglycerides and LDL cholesterol. The
compound exhibits polymorphism, cases of difficulties in
obtaining crystals of a particular known form or irreproducibility
discovery of its polymorphic forms closely intertwines with the
synthesis development of the molecule. At the early development
stage, LAB687 was known to crystallize in two modifications,
Forms A and B. Knowledge of the molecule’s polymorphic nature
prompted extensive polymorphic screening using drug substance
produced by the earlier synthesis routes. These studies revealed
the existence of a third polymorph, Form C. Subsequently, Form
C was selected for further development based on data from the
additional formulation and polymorphic studies. Surprisingly, a
new modification, Form D, appeared when the crystallization
process known to routinely produce Form C was scaled up in the
pilot plant. Once Form D was introduced to the laboratory, Forms
A and C could no longer be made. We hypothesize that a change
in drug substance impurity profile due to the changes in synthesis,
led to the emergence of the most stable Form D.
4-7
of the experimentation abound.
A celebrated tale of the
serendipitous nature of polymorphs is the protease inhibitor
ritonavir (Abbott Laboratories) where a given polymorphic form
could not be produced even though it had previously been
obtained routinely over long time periods, resulting in drug
4
product shortage. It is believed that once a particular polymorph
has been obtained, it is always possible to obtain it again given
5,6
the right conditions. Numerous factors, including the solvent,
solution concentration, degree of supersaturation, heating and
cooling rates, seeding and mixing, may affect the crystallization
8-14
process and result in the production of different polymorphs.
Quite often our ability to manipulate the kinetic processes of
nucleation and growth in polymorphic systems is poor, and the
consequence of this is that the level of process control is
15
limited.
To further explain why a process that has remained stable
and under control for years can suddenly go out of control, it
has been argued that the presence or absence of impurities or
byproduct could direct the polymorphic outcome of the crystal-
lization process. In the case of a polymorphic drug, sulfathiazole,
the polymorphic purity of the crystallized product could be
affected by the final hydrolysis byproduct, ethamidosulfathia-
Introduction
Polymorphism, the ability of a solid material to exist in more
than one crystal structure, was first discovered in minerals by
German chemist Martin Heinrich Klaproth in 1798. Since then,
this phenomenon has been encountered in many areas of drug
development. It is the variation in the properties of organic
compounds, such as the melting point, solid-state chemical
reactivity, and bioavailability that makes polymorphism such a
potentially important issue for the pharmaceutical industry.
Numerous drug substances, which are mostly small organic
molecules with molecular weights below 600, have been
discovered to exhibit polymorphism. McCrone even suggested
that the number of forms known for a given compound is
proportional to the time and money spent in research on the
16
zole, at concentrations as low as 1 mol %. Changes in the
supplier of an intermediate could cause a change in the impurity
4
profile, thus allowing another polymorph to appear. In the
context of process development and route selection, these
findings have profound implications. The continual process
improvement could give rise to increasingly selective chemistry,
(
4) Chemburkar, S. R.; Bauer, J.; Deming, K.; Spiwek, H.; Patel, K.;
Morris, J.; Henry, R.; Spanton, S.; Dziki, W.; Porter, W.; Quick, J.;
Bauer, P. Org. Process Res. DeV. 2000, 4, 413.
1
compound. Its occurrence introduces complications during
manufacturing and adds yet another challenge to the complexity
of drug development. Often process chemists find themselves
not only facing the complexity of achieving chemical purity,
but also the challenges of understanding and controlling the
crystal polymorph through crystallization.
As the search for new therapies intensifies, drug candidates
are becoming more conformationally flexible with greater
(5) Dunitz, J. D.; Bernstein, J. Acc. Chem. Res. 1995, 28, 193.
(
(
6) Bernstein, J.; Henck, J. O. Cryst. Eng. 1998, 1, 119.
7) Laird, T. Org. Process Res. DeV. 2004, 8, 301.
(8) Threlfall, T. Org. Process Res. DeV. 2004, 4, 384.
(
9) Lian, Y. J. Am. Chem. Soc. 2003, 125, 6380.
(
10) Beckmann, W.; Nickisch, K.; Budde, U. Org. Process Res. DeV. 1998,
2-4
2, 298.
(
11) Beckmann, W.; Nickisch, K.; Budde, U. Org. Process Res. DeV. 2001,
5
, 387.
(
(
12) Beckmann, W. Org. Process Res. DeV. 2000, 4, 372.
13) Liang, J. K.; Wilkinson, D.; White, G.; Roberts, K. J.; Ford, L.; Wood,
W. Ind. Eng. Chem. Res. 2004, 43, 1227.
*
Author to whom correspondence may be sent. E-mail: jessica.liang@
novartis.com.
(14) Liang, J. K.; Wilkinson, D.; White, G.; Roberts, K. J.; Ford, L.; Wood,
W. Cryst. Growth Des. 2003, 4, 1039.
(
1) McCrone, W. C. Physics and Chemistry of the Organic Solid State;
Interscience: New York, 1965.
(15) Davey, R. J.; Blagden, N.; Potts, G. D.; Docherty, R. J. Am. Chem.
Soc. 1997, 119, 1767.
(
(
2) Norman, L. Org. Process Res. DeV. 2000, 4, 407.
3) Lian, Y.; Reutzel-Edens, S. M.; Mitchell, C. A. Org. Process Res.
DeV. 2000, 4, 396.
(16) Blagden, N.; Davey, R. J.; Rowe, R.; Roberts, R. Int. J. Pharm. 1998,
172, 169.
8
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Vol. 14, No. 4, 2010 / Organic Process Research & Development
10.1021/op100115u  2010 American Chemical Society
Published on Web 05/27/2010

Table 1. Summary of polymorph screen study
temperature
method
solvent
[°C]
results
slurry
heptane
hexane
25
25
Form A
Form A
methyl tert-butyl ether
isopropyl ether
water
25
25
25
Form A
Form A
Form A
ethanol/water (1:1)
heptane
25
50
Form A
Form A
hexane
50
Form A
methyl tert-butyl ether
toluene
50
50
Form A
Form A
water
50
Form A
ethanol/water (1:1)
evaporation acetone
acetonitrile
50
25
25
25
25
25
Forms A and C
amorphous
amorphous
amorphous
amorphous
Form A
ethanol abs.
ethanol 95%
ethyl ether
Figure 1. Molecular structures of LAB687 and its key
ethyl acetate
methanol
methylene chloride
25
25
25
25
25
25
60-4
60-4
60-4
20
20
20
20
20
20
50
50
amorphous
amorphous
amorphous
amorphous
amorphous
Form A
intermediates.
whereby the decreasing level of an important reaction byproduct
could affect the polymorphic purity of the product. The
byproduct that directs the crystallization of a specific polymorph
during pilot scale or early manufacturing trials could be
eliminated at the full scale, again allowing a different polymorph
2
-propanol
tetrahydrofuran
toluene
ethanol/water (1:1)
toluene
methyl tert-butyl ether
acetone/water
tetrahydrofuran/water
ethanol/water
crash cool
antisolvent
Form B
Form S
A
Form A
oil
oil
Form A
Form A
Form A
Form A
Form A
Form A
17
to appear. It is therefore imperative that development chemists
are vigilant to these issues during the development of a synthetic
2
ethyl acetate/heptane
route.
2
-propanol/water
In this paper, we present a case study on the polymorphic
behavior of LAB687 where the discovery of its polymorphic
forms closely intertwines with the synthesis development of
the drug molecule.
tetrahydrofuran/heptane
tetrahydrofuran/heptane
ethanol/water
Table 2. Thermal properties of LAB687 polymorphs
analysis
Form A
Form C
Form D
Process Development of LAB687
melting point (onset) [°C]
heat of fusion [mJ/mg]
130.6
57.5
120.5
54.3
157.4
71.6
LAB687 is an inhibitor of microsomal triglyceride transfer
protein (MTP) designed to decrease the production of gut-
derived chylomicrometers and hepatic very low density lipo-
proteins (VLDL); thus, to lower triglycerides and LDL cho-
Table 3. Solubility of LAB687 polymorphs in common
solvents at 25°C
18
lesterol. LAB687 was synthesized by the coupling of the
amine (R)-1 and the biaryl acid chloride 2 (Figure 1). Significant
improvements were attained in the synthesis of the key
enantiomerically pure intermediate (R)-1 at various development
stages to develop an economical route. The first synthetic route
used for producing gram scale of LAB687 is referred to as the
early route (Figure 2). Major modifications have led to an
efficient and practical synthesis of this enantiomerically pure
compound (R)-1 for the phase I synthesis (Figure 3). This new
route consequently introduced a new dimeric urea impurity 6
to the drug substance, which was not present in the batches
produced by previous routes. The compound 6 originated from
solvent
Form C
Form A
Form D
water
heptane
0.0012
0.64
1.93
8.9
22.6
32.4
33.5
0.0011
0.59
1.8
0.0009
0.41
1.4
ethanol/water (1:1)
methanol/water (2:1)
ethanol/water (2:1)
methyl tert-butyl ether
toluene
7.8
6.8
21.3
17.8
26.4
11.2
15.6
15.4
exhibited polymorphism when the early route produced a new
crystalline form, which was named Form A. The research batch
was named Form B. To search for all potential polymorphs of
LAB687, a polymorphic screening study was performed using
Form A drug substance, (purity 98.9%). A series of approaches
(Table 1), namely slurry equilibration, evaporative crystalliza-
tion, cooling crystallization and antisolvent crystallization,
produced a new polymorph (Form C) and a toluene solvate.
Seeded crystallization processes were successfully developed
to routinely produce Forms A or C at larger scale (100 g).
19
byproduct 5 (Figure 4, ). that was formed in the conversion of
compounds 3 to 4 via Schiff base 5-P (Figure 3).
Polymorphism of LAB687
The first anhydrous crystalline LAB687 was isolated by a
research chemist. It became apparent that the compound
Figure 2. Early synthesis route for key intermediate (R)-1.
Vol. 14, No. 4, 2010 / Organic Process Research & Development
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879

Figure 3. Phase I synthesis route for key intermediate (R)-1.
Table 4. Results of dynamic vapor absorption studies of
LAB687 polymorphs
RH%
Form A
Form C
Form D
4
6
9
5
5
0
0.04
0.06
0.16
0.49
0.68
0.88
0.03
0.08
0.17
Additional polymorphic studies were carried out to inves-
tigate the thermodynamic relationship between Forms A and
C and to facilitate the form selection. Though the chemical and
physical stabilities and the intrinsic solubility of Forms A and
C are very similar, the trigonal Form C with superior filterability
and flowability was selected for further development. However,
when the Form C crystallization process was scaled up to
multikilogram scale in the pilot plant, a new polymorph, Form
D, was produced. Since the emergence of Form D, those seeded
crystallization processes that consistently produced Forms A
and C started to produce predominately Form D in the
laboratory. Shortly after the pilot plant campaign during which
one multikilogram batch of Form D was produced, LAB687
was terminated due to toxicity concerns.
Figure 4. Molecular structure of the new dimeric impurity (6)
and its origin.
During these development activities, two more solvates (heptane
and methylcyclohexane) were also identified. Interestingly, after
the point of the discovery of Forms A and C, Form B could no
longer be produced.
Clearly, the conclusions from the polymorphic investigations
that were performed early in the laboratory did not predict the
existence of the most stable Form D. We hypothesize that the
Figure 5. Solubility of Forms A, C, and D in 2-propanol/water (2:1) and ethanol/water (2:1).
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Vol. 14, No. 4, 2010 / Organic Process Research & Development

Figure 6. Scanning electron microscopic images of LAB687 polymorphs.
appearance of Form D could be attributed to the lack of certain
impurities due to the changes in the synthesis route, or presence
of the new dimeric urea impurity (6) unique to the phase I route.
Characterization of LAB687 Polymorphs. LAB687 pol-
ymorphs were characterized using several established tech-
niques. Data on Form B are limited due to insufficient material.
Thermal Properties. Thermal properties of the three forms
were determined by differential scanning calorimetry (DSC).
The results are listed in Table 2. Form D exhibits the highest
melting point (157.4 °C), and heat of fusion (71.6 mJ/mg). The
melting point and heat of fusion of Form A are significantly
Figure 7. Powder X-ray diffraction patterns of the LAB687
polymorphic forms.
20
greater than those of Form C. According to heat of fusion rule,
these thermal data suggest that Form D is the thermodynami-
cally most stable form and is monotropic with respect to Forms
A and C.
solubility curve of Form A seems to intercept with that of Form
C suggesting an enantiotropic relationship. However, the trend
was not observed in the 2-propanol/water system. Given the
small difference in their solubilities, the relationship between
Forms A and C is not conclusive.
Hygroscopicity. Dynamic vapor absorption studies show
that the LAB687 polymorphs are not hygroscopic. At room
temperature, Forms A and D pick up less than 0.2% of water
at 90% RH, while Form C is able to uptake 0.88% of water
Solubility of Forms A, C, and D. A comparison of the
equilibrium solubility of Forms A, C, and D in common organic
solvents at room temperature (Table 3) shows that Form D is
the least soluble polymorph. The solubility profiles of these
polymorphs in alcohol/water (Figure 5) also indicate that Form
D is significantly less soluble than Forms A and C over a wide
range of temperatures. The solubility of Forms A and C on the
other hand are relatively similar. In ethanol/water system, the
(Table 4.).
(
(
17) Rowe, R. C. Drug DiscoVery Today 2001, 6, 395.
18) Fink, C.; Ksander, G.; Kukkola, P.; Prashad, M. U.S. Patent 6,197,798,
March 6, 2001.
Crystal Morphology. As shown in Figure 6, the crystal
morphology of Form A is blade-like, Form B is acicular, Form
C is trigonal, and Form D is columnar. As shown in Figure 7,
the polymorphic forms have distinctively different powder X-ray
diffraction patterns.
(
19) Prashad, M.; Hu, B.; Har, D.; Repic, O.; Blacklock, T. J.; Acemoglu,
M. AdV. Synth. Catal. 2001, 5, 343.
(
20) Lohani, S.; Grant, D. Polymorphism in the Pharmaceutical Industry;
Wiley-VCH: Weinheim, 2006; Chapter 2.
Vol. 14, No. 4, 2010 / Organic Process Research & Development
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Table 5. Phase equilibration of mixtures of Forms D/A and
Forms D/C
in an oven at ∼75 °C, 35 mmHg for at least 16 h to obtain
3
.4 g of Form A.
D/A mixture
D/C mixture
Form C Process. Charge 30.0 g of LAB687 to a crystallizer,
solvent
5 °C 25 °C 50 °C 5 °C 25 °C 50 °C
then add 217.5 g of methanol and stir at room temperature until
a clear solution is obtained. Filter the solution and charge it to
a clean crystallizer. Heat the solution to ∼50 °C. Add 100 g of
distilled water over ∼30 min with vigorous agitation while
maintaining the temperature at 45 °C. Cool the solution to 40
heptane
D/A D/A
-
-
D
D
D
C/D C/D
-
-
methyl tert-butyl ether D/A
D
D
D
D
D
D
toluene
water
D
D
D/A D/A
C/D C/D C/D
ethanol/water (1:1)
D
D
D
D
D
°
C and add 30.0 mg of Form C. Stir the mixture for 15 min.
Cool the mixture to ∼33 °C over ∼1 h and stir at 33 °C for
Thermodynamic Relationships. Solubility data and thermal
properties of these polymorphs indicate that Form D is the
thermodynamically most stable form and is monotropic to
Forms A and C. This finding is further supported by a series of
equilibration experiments (Table 5) during which the mixtures
of Forms D/C and D/A were slurried in common solvents at
various temperatures for at least 24 h. Enrichment of Form D
was seen in all experiments. Previous equilibration experiments
of mixtures of Forms A and C indicate that they are equally
stable; however, when these experiments were repeated after
the discovery of Form D, the mixtures of Forms A and C
converted to Form D instead.
1
h. Add 100 g of distilled water over ∼40 min. Stir for 2 h at
30 °C. Isolate the solid by filtration and wash the cake twice
with 60 mL of methanol/water (3:2 v/v). Dry the cake in an
Fo vo er mn a Ct .∼75 °C, 35 mmHg for at least 16 h to obtain 28 g of
Prior to the discovery of Form D, the above processes were
developed to generate Forms A and C, respectively. However,
once Form D was introduced to the laboratory, these processes
no longer produced Forms A or C, but primarily Form D.
Summary
The development of LAB687 demonstrates that dealing with
polymorphs is an unpredictable process. Although we have
carried out extensive research early in the development to screen
potential polymorphs, Mother Nature surprised us upon scale-
up. In reflection, once the final route of synthesis has been
chosen, additional polymorph screening studies may be carried
out to seek the potential new polymorphs due to the changes
in the impurity profile of the drug substance. Nevertheless,
considering the precarious nature of the drug development,
process chemists should always be prepared for surprises.
Experimental Section
The typical processes to generate Forms A and C are
described here. However, these processes are of historical
significance only. Once Form D was obtained, they no longer
produced the intended polymorph.
Form A Process. Charge 4 g of LAB687 to a crystallizer,
then add 25 mL of 2-propanol. Heat the mixture to 50 °C to
yield a clear solution. Filter the solution and charge it to a clean
crystallizer. Cool the solution to 10 °C rapidly and add 10 mg
of Form A seeds. Stir for 2 h at 10 °C and cool to 3 °C over
∼
15 min. Add 25 mL of distilled water over ∼30 min. Stir for
Received for review April 22, 2010.
OP100115U
2
h at 3 °C. Isolate the solid by filtration and wash the cake
twice with 10 mL of 2-propanol/water (1:1 v/v). Dry the cake
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