Development of an Efficient Pd-Catalyzed Coupling Process for Axitinib

Article pubs.acs.org/OPRD

Development of an Efficient Pd-Catalyzed Coupling Process for Axitinib Brian P. Chekal,‡ Steven M. Guinness,‡ Brett M. Lillie,‡ Robert W. McLaughlin,‡ Charles W. Palmer,† Ronald J. Post,‡ Janice E. Sieser,‡ Robert A. Singer,*,‡ Gregory W. Sluggett,† Rajappa Vaidyanathan,‡ and Gregory J. Withbroe‡ ‡

Chemical Research and Development, Pfizer Worldwide Research and Development, Groton Laboratories, Eastern Point Road, Groton, Connecticut 06340, United States † Analytical Research and Development, Pfizer Worldwide Research and Development, Groton Laboratories, Eastern Point Road, Groton, Connecticut 06340, United States ABSTRACT: The manufacturing process of axitinib (1) involves two Pd-catalyzed coupling reactions, a Migita coupling and a Heck reaction. Optimization of both of these pivotal bond-formation steps is discussed as well as the approach to control impurities in axitinib. Essential to the control strategy was the optimization of the Heck reaction to minimize formation of impurities, in addition to the development of an efficient isolation of crude axitinib to purge impurities.

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an (THP) to provide intermediate 5. The first C−C bondforming step involves a Heck reaction between 5 and 2vinylpyridine (6) to afford 7. To set the stage for the final bond formation, the nitro group is reduced, and the amine undergoes a Sandmeyer reaction7 to provide 6-iodoindazole 9. After coupling 9 with thiol 10 using Pd(dppf)Cl2 as catalyst,5 THPprotected axitinib (11) is obtained. Deprotection of 11 with pTsOH in methanol affords crude axitinib (1a) as an ethyl acetate solvated form. Crude axitinib is converted to the desired solid form through dissolution in methanol and acetic acid followed by treatment with activated carbon or Florisil and displacement of solvent with xylenes to crystallize axitinib’s anhydrous form 4 (1b). One disadvantage of this route is that the potentially genotoxic intermediates 7 and 8 may require stringent control in the drug substance. In addition, the Sandmeyer reaction is relatively low yielding and not an operation that is desirable to conduct toward the end of a manufacturing route due to the energetic nature and potential toxicity of the intermediates involved. A number of important elements were learned regarding the Heck reaction for this key bond formation which controls the trans-olefin geometry. The THP protecting group on 5 blocks the indazole nitrogen from forming a Michael adduct with 6. The Heck reaction involving 5 is facilitated by the electrondeficient nitro group present, which otherwise is challenged at undergoing insertion due to the electron-rich character at the 3position of indazole. The main disadvantage of carrying out the Heck reaction early in the process is that the olefin moiety is potentially prone to degradation. This being the case, it is a more prudent strategy to carry out the Heck reaction toward

INTRODUCTION Vascular endothelial growth factor (VEGF) antagonists have been recognized as an important class of pharmaceutical agents for development due to their efficiency in controlling growth and proliferation of cancer cells.1 Axitinib (1) is a VEGF inhibitor in Pfizer’s oncology development program currently undergoing late-stage clinical trials for the treatment of solid tumors, including metastatic renal cell carcinoma (RCC).2,3 During the review of axitinib by the Food and Drug Administration (FDA), the oncology drug advisory committee unanimously endorsed approval of this drug and cited the different toxicity profile as beneficial in providing more options to patients. On January 27, 2012, axitinib was approved with the trade name INLYTA for treatment of patients in the United States with advanced renal cell carcinoma after failure of one prior systemic therapy. When considering a manufacturing process for axitinib (Figure 1), the vinylpyridine moiety could be installed through

Figure 1. Structure of axitinib (1) and bond-forming strategy.

a Heck reaction,4 and the diaryl thioether could be constructed through a Migita5 cross-coupling or SNAr strategy. To enable manufacture of axitinib for early clinical trials, a scalable route was identified (Scheme 1) using these approaches.6 The sequence begins with iodination of 6-nitroindazole (2) and a subsequent protection of 3-iodoindazole 3 with tetrahydropyr© XXXX American Chemical Society

Special Issue: Transition Metal-Mediated Carbon-Heteroatom Coupling Reactions Received: April 5, 2013

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dx.doi.org/10.1021/op400088k | Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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Scheme 1. First-generation synthesis of axitinib

Scheme 2. Second-generation synthesis of axitinib

the end of the route rather than rely on processing and engineering controls to mitigate potential degradation products through a lengthy synthetic route. The final recrystallization and polymorph control possess undesirable unit operations involving displacement of acetic acid and methanol with xylenes as well as treatment with activated charcoal or another processing agent (such as Florisil) for Pd removal. For this reason an essential goal of development was focused on designing a milder approach for accessing the desired solid form, while controlling levels of Pd without processing agents. The first-generation synthesis for axitinib (Scheme 1) was sufficient for preparing early clinical supplies and had identified key bond-formation steps. The use of the Sandmeyer reaction (conversion of 8 to 9) late in the process was highly undesirable from a manufacturing perspective, and this

prompted a revision of the route. A second-generation route was developed which reversed the order of the key bondformation steps to allow for a shorter process (Scheme 2)8 while also addressing the Pd control strategy.9 In this approach, 6-iodoindazole (12) is prepared by commercial suppliers from 6-aminoindazole through a Sandmeyer reaction.7 The Migita coupling of 12 with 10 is accomplished using Xantphos as a supporting ligand for Pd.10 After formation of 13 is complete, 14 is formed in situ with addition of iodine and base. A Heck reaction between 14 and 6 at 110 °C is carried out using 1,8-bis(dimethylamino)naphthalene (Proton-Sponge) as an additive to suppress the indazole nitrogen undergoing a Michael addition with 6. After a workup, crude axitinib (1a) is further purified to control Pd and organic impurities. Crude axitinib is first recrystallized from NMP and methanol in the presence of 1,2-diaminopropane B



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found to form 15 readily in solution if oxygen is not excluded by purging with inert gas. In the absence of 12, mercapto dimer 15 is likely to undergo oxidative addition with Pd to afford a stable complex 16 as proposed below in Scheme 3.12 It was found that this problem of inhibition by 15 is overcome by addition of zinc metal13 or a related reducing agent.14 Under ideal manufacturing conditions with efficient inertion of the headspace, the use of zinc has been unnecessary, since it is predominantly dissolved oxygen that promotes formation of 15, potentially leading to catalyst inhibition. Additional optimization of the coupling reaction includes the use of a milder base, sodium bicarbonate, which allows for lower reaction temperatures (50 °C) as shown in Scheme 4. With use of a weaker base, the amount of thiolate (deprotonated 10) is minimized. To further minimize potential inhibition of the Pd catalyst, 10 is slowly added to the reaction mixture after all other components have been charged. While steps 1−2 are highly efficient, it was recognized that some batches of 6-iodoindazole (12) appeared to inhibit the Pd catalyst for the Migita coupling. The problem was traced to low levels of sulfur being present in 12 as a result of the means of its preparation. The root cause of this was found to be the quench of the Sandmeyer reaction used to convert 6-aminoindazole to 12, and that by avoiding sodium thiosulfate during the quench, sulfur is no longer formed in the process.15 The conditions for iodination at step 2 remained unchanged from those originally developed. During development, it was observed that excess base and higher reaction temperatures led to the formation of sulfoxide impurity 18 (Figure 2). In addition, premixing of the iodine and base gave poor conversion of the iodination reaction. To achieve complete conversion while minimizing sulfoxide formation, 2 equiv of iodine is charged followed by 2 equiv of aqueous potassium hydroxide. The reaction mixture is transferred into aqueous, methanolic ascorbic acid to quench the excess iodine during crystallization of crude 14. In carrying out the telescoped process, crude 14 is isolated as a yellow-to-brown colored crystalline material in about 80% yield. The impurities tracked across steps 1−2 are shown in Figure 2. Compound 17 arises from unreacted 12 in step 1 undergoing iodination in step 2. The step 1 product, 13, is a common impurity found in 14 due to incomplete conversion in step 2. As mentioned earlier, 18 arises from competitive oxidation of 14 under the reaction conditions of step 2. The Xantphos used in step 1 gives rise to a family of impurities. One set of impurities is derived from oxidation, which provides mono- and bis-oxides of Xantphos. Another set of impurities results from exchange of the aryl groups of Xantphos with indazole during the coupling reaction, as proposed in Scheme 5.16 The initial product of the exchange reaction is a derivative of Xantphos with an indazole incorporated into the phosphine ligand (compound 23) as well as a byproduct resulting from coupling

(1,2-DAP) and 1,2-bis(diphenylphosphino)ethane (DIPHOS) for Pd removal.9 Following the recrystallization, reslurries from THF and methanol control organic impurities. As in the prior route, a final recrystallization from acetic acid, methanol, and xylenes is necessary to obtain form 4 of axitinib (1b). Development of this second-generation process was undertaken to ensure control of process impurities and to consistently deliver high-quality drug substance for commercial manufacturing.

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RESULTS AND DISCUSSION Overall, the second-generation route to axitinib (Scheme 2) is extremely efficient with a well-conceived sequence of steps; therefore, this route was deemed appropriate for commercial development. The first two telescoped steps required minimal optimization and provided 14 as a reliable intermediate for early control of impurities in the route. The Heck reaction was recognized as a step that required substantial development due to the need to control various process impurities resulting from the relatively high reaction temperatures used. To limit formation of a homocoupled dimer of 14, a large amount of LiBr was originally included in this step,11 but it would be ideal to avoid or limit the use of this additive. The subsequent reslurries of 1a, while effective in purity control, could be unreliable due to the heterogeneous nature of the process. Therefore, better control of the formation and purge of impurities within the Heck reaction became a primary goal, in addition to control of residual Pd and solid form to enable robust production of axitinib. Development of the Migita Coupling and Iodination. The initial work on the commercial manufacturing route for step 1 demonstrated that a Pd catalyst derived from Xantphos provided the optimal reactivity in the Migita coupling. A primary concern focused on thiol 10 acting as a Pd catalyst inhibitor. This point was best exemplified by combining the Pd catalyst and 10 in the absence of 12, which upon subsequent addition of 12 and base led to no reaction. This is most likely due to the Pd catalyst coordinating irreversibly with at least 2 equivalents of 10. Related to this problem is inhibition caused by the mercapto dimer, 15, an impurity formed through oxidation (shown in Scheme 3). Samples of 10 have been Scheme 3. Proposed inhibition of Pd catalyst by 15 undergoing oxidative addition

Scheme 4. Optimized process for steps 1, 2, and 2R

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Figure 2. Process impurities formed in steps 1−2.

Scheme 5. Proposed mechanism for exchange of aryl groups on Xantphos in step 1 coupling based on observed side products

catalysis.17 To further facilitate oxidative insertion of Pd, electron-withdrawing protecting groups for the indazole nitrogen were examined. From this study, the acyl protecting group emerged as the best option since it could be introduced in situ with acetic anhydride and Hunig’s base. With inclusion of the acyl protecting group, the undesired side reaction of the indazole moiety undergoing Michael addition with 2-vinylpyridine (6) is suppressed, while also lowering the temperature of the Heck reaction from 110 °C down to 90 °C. Equally important as the acyl protecting group is the choice of catalyst. A catalyst derived from Pd(OAc)2 and Xantphos was found to enable highly efficient coupling while minimizing formation of saturated impurity 26 (Figure 3) which is challenging to purge due to its structural similarity to 1. In the absence of Xantphos, 4 mol % of Pd(OAc)2 typically enables only 10−20% conversion to 30 over 24 h at 90 °C, while the optimal conditions (4 mol % of Pd and Xantphos) achieve complete conversion during this time. The relative stoichiometry of the Pd and Xantphos not only influences the rate of the reaction, but also determines the extent to which 26 is formed. Excess Xantphos inhibits the Pd catalyst, most likely through blocking coordination of the desired substrates with a second equivalent of Xantphos on the Pd catalyst. In contrast, using an excess of Pd relative to Xantphos leads to increased levels of 26, since unligated Pd promotes formation of 26. For example, using a 2:1 molar ratio of Pd to Xantphos results in levels of about 1% of 26 in solution, while using a 1:1 stoichiometry limits levels to only 0.2%. For these reasons, the process is typically operated near a 1:1 stoichiometry of Xantphos and Pd. While the reaction temperature and the composition of the catalyst significantly impact the levels of side products, another key element in the process is the stoichiometry of 2vinylpyridine, 6. Using 4.5−8 equivalents of 6 was found to lower the levels of a number of impurities (Figure 3), including those arising from dehalogenation (13), rearrangement of the acyl group (27), and inadvertent coupling with thiol (28). The trends in Table 2 show that the level of 26 is generally unaffected by the amount of 6; however, the formation of these other tracked impurities are well controlled with excess 6. In the case of 28, the reversibility of C−S bond formation is at work.18 During the Heck reaction, the Pd catalyst is capable of breaking the C−S bond that was formed in the first step of the

of a phenyl group with 10 (compound 24). More than one phenyl group can undergo exchange on Xantphos, leading to a complex mixture of byproducts. Control of Impurities through Recrystallization at Step 2R. Originally, reslurries of 14 were used to control impurities in the process at step 2, but it became apparent that a recrystallization of 14 would be necessary to control the Xantphos-related impurities in addition to the other processrelated impurities. To efficiently control impurities and ensure reliable performance of the downstream Heck reaction, a recrystallization at step 2R was developed. Dissolution of crude 14 is initially achieved in a blend of NMP and 1,2-DAP. To this solution is added methanol followed by water as the antisolvents. The 1,2-DAP is included to aid in color removal and to help solubilize Pd during isolation. In general, the recrystallization controls all of the organic impurities well as shown in Table 1, with the most prevalent impurities being 13 Table 1. Ranges of impurities in commercial scale batches of 14 impurity 13 17 18 total impurities

observed range in crude 14 (%)

observed range in recrystallized 14 (%)

0.52−2.4 0.13−1.4 ≤0.1 1.7−5.0

0.19−0.79 0.08−0.32 ≤0.05 0.45−1.2

and 17, typically, each at a level below 1%, while all other impurities are at levels below 0.2%. The recrystallization step 2R yields 14 in about a 85−90% recovery and ≥98% overall purity. This recrystallization ensures that none of the impurities introduced early in the process impact the purity profile of the drug substance. Development of the Heck Reaction. The further development of the Heck reaction was essential to streamline the overall process. The use of Proton-Sponge (Scheme 2) was an intriguing way of minimizing formation of 25 (Figure 3); however, its stoichiometric use was not practical and challenged the control strategy with the difficulty of its purge due to its highly crystalline nature. The oxidative addition of 14 onto Pd is challenged by the relatively electron-rich indazole system, as proven by the need for a 3-iodo moiety to achieve efficient D



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Figure 3. Typical impurities generated during the Heck reaction.

Table 2. Levels of impurities monitored in step 5 (deprotection) reaction mixture on lab scale which arise at step 4 (Heck reaction) based on the amount of 2-vinylpyridine (6) 6 (equiv)

% 26 (saturated impurity)

% 13 (dehalogenated impurity)

% 27 (rearranged impurity)

% 28 (C−S coupling)

% total impurities

1.5 3.0 6.0 8.0

0.19 0.33 0.29 0.41

5.48 3.72 2.40 2.10

14.83 7.20 3.27 2.98

2.72 1.06 0.31 0.28

41 30 23 20

Scheme 6. Optimized conditions for the Heck reaction, steps 3−5

process between 10 and 12. In doing so, the liberated 10 is able to undergo coupling with acylated 14 (intermediate 29) to afford 28 after deprotection. While some reports describe using tertiary amines as solvents to alleviate the C−S bond scrambling,18b,c in this case it is sufficient to increase the stoichiometry of 6. The structure of the amine base has a profound effect on the efficiency of the catalyst in the Heck reaction. When substituting tributylamine for N,N-diisopropylethylamine (Hunig’s base) in step 4, the conversion is only about 75% after 24 h, over which time the reaction is normally >98% complete. Even more striking, is that using N-ethylmorpholine or 4dimethylaminopyridine enables only 34% conversion after 24 h. Using tertiary amine bases with structures similar to N,Ndiisopropylethylamine provide optimal results, which is consistent with prior reports.19 For example, N,N-dicyclohexylmethylamine and N,N-diisopropylmethylamine both have roughly the same level of reactivity as N,N-diisopropylethylamine, enabling reaction completion within 24 h at 90 °C. To streamline operations, the protection (step 3), Heck reaction (step 4), and deprotection (step 5) are executed in a telescoped manner (Scheme 6). Typically, the Pd(OAc)2 and Xantphos are charged to NMP at the start of step 3 to enable formation of the catalyst, which is followed by the addition of

14, Hunig’s base, and acetic anhydride. Once the acylation to afford intermediate 29 is complete, 6 (6 equiv) is charged, and the reaction is heated for 24 h at 90 °C. After completing the Heck reaction at step 4, acylated product 30 is deprotected in situ (step 5) with 1,2-diaminopropane (1,2-DAP) in a solution of THF, followed by crystallization with water to yield 1c as a THF solvated form in about 75% for the three steps in ≥99% chemical purity (Table 3). Prior work has demonstrated the effectiveness of 1,2-DAP in combination with chelating phosphine ligands for maintaining Pd in solution while crystallizing 1.9 For the optimized process, 1,2-DAP was chosen not only for solvating Pd but also for enabling deprotection of the acyl group prior to crystallization Table 3. Levels of impurities in commercial-scale batches of isolated crude axitinib (1c)

E

impurity

observed range (%)

13 25 26 28 total impurities residual Pd

0.21−0.30 ≤0.05 0.13−0.17 0.11−0.13 0.51−0.82 ≤20−42 ppm



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Scheme 7. Conversion of crude axitinib to form 4 via an isopropanol solvate

of crude axitinib. While the 1,2-DAP and Xantphos in the process were found to be effective in maintaining the Pd in solution during the crystallization of 1c, it was discovered that the stoichiometry of 6 impacts the effectiveness of the Pd removal. Remarkably, when using 6 equiv of 6 in combination with 4 equiv of 1,2-DAP, 1c is consistently isolated with Pd levels below 50 ppm. Using less of 6 (such as 3 equiv) or less of 1,2-DAP (such as only 2−3 equiv) results in Pd levels that are typically an order of magnitude higher. On production scale, the Pd levels have ranged from 42 ppm to below 20 ppm. Control of Solid Form. While the purity and Pd level are well controlled during the isolation of 1c (crude axitinib) at step 5, additional processing controls are required for solid form. 1c is isolated as a THF solvated form and requires conversion to an anhydrous form suitable for formulation. Earlier in development, anhydrous form 4 (1b) was manufactured for use in the clinic. Prior to the polymorph conversion, a recrystallization from NMP, 1,2-DAP, and methanol was implemented to ensure that Pd was sufficiently purged (Scheme 2). After the recrystallization, axitinib was dissolved in a combination of acetic acid and methanol followed by distillation from xylenes to convert axitinib from an acetic acid solvate to 1b. Unfortunately, the conditions required for distillation of acetic acid also result in acylation at the 1-position of the indazole ring of axitinib to provide intermediate 30 as an impurity (structure shown in Scheme 6). During studies of solvated forms, it was found that an isopropanol solvated form (1d) will readily undergo conversion to 1b by heating in heptane. Consequently, this more preferable solid form conversion was incorporated into the process at step 5R following the recrystallization from NMP, 1,2-DAP, and methanol. The recrystallized product at step 5R is reslurried in isopropanol and dried to remove excess isopropanol. The solids are subsequently reslurried in heptane at 90 °C for conversion to 1b (step 6 in Scheme 7). The implementation of this improved process facilitated access and discovery of other anhydrous forms. Soon after this process was implemented, forms 6, 25, and 41 were prepared and characterized. All of these forms were found to be more stable than form 4. In all, there are five anhydrous solid forms known for axitinib, forms 1, 4, 6, 25, and 41.20 Form 41 (1e) is the most stable anhydrous form and therefore was selected for commercial development. To access 1e, the same recrystallization was implemented prior to the polymorph conversion. After recrystallization from NMP, 1,2-DAP, and methanol, the isopropanol solvate (1d) was accessed and converted to 1e by a reslurry in refluxing ethanol with seed crystals. Ethanol was found to be an ideal solvent since the anhydrous forms are thermodynamically favored as compared to the ethanol solvate near room temperature. Most other solvents convert axitinib to a solvated form, for which over 70 solvates have been characterized to date.20a,21

Using this process, recrystallization of crude axitinib with subsequent reslurries in isopropanol and ethanol afford 1e in 80% yield and ≥99.9% purity with Pd levels below 10 ppm. Typically, all impurities are below 0.10% due to the efficient control offered by the recrystallization and polymorph conversion. An advantage of isolating such a highly crystalline form (mp 225 °C) is that impurities are more readily purged from the process, including Pd. Spiking studies demonstrated that processing 1c through step 6 with an ingoing Pd level of up to 300 ppm consistently yields 1e with Pd levels below 10 ppm. While the process to prepare 1e as a slurry in ethanol performed suitably for development purposes, it was highly desirable to design a homogeneous process that would be more robust and would not be as reliant on stirring rate or vessel configuration. A process involving the seeding of a saturated solution of axitinib was developed and is shown in Scheme 8. Scheme 8. Integrating the solid form conversion with the final recrystallization at step 6

For this process, the polymorph conversion is integrated with the recrystallization step by replacing methanol with ethanol as the antisolvent. After adding a solution of axitinib in NMP/ THF to hot ethanol, seed crystals of 1e are introduced to the super-saturated solution to enable formation of the desired polymorph in a highly robust manner. A relatively slow cooling rate is used to avoid nucleation of any solvated forms. The overall solvent composition is vital to produce the desired form, as too much THF or NMP will also favor solvated forms. By directly preparing the desired solid form (1e) in the recrystallization, there is no longer any need for isolations of intermediate solvated forms after that of 1c at step 5. With this streamlined process for step 6, 1e is isolated in 80−85% yield based on ingoing 1c and is obtained in ≥99.9% purity (Table 4).

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CONCLUSIONS The second-generation route (Scheme 2) was deemed to be an optimal manufacturing route due to the short length and late introduction of the potentially unstable olefin. This route was further developed and optimized to about 50% overall yield for the process (Scheme 9). Essential to the efficient assembly in the route is the implementation of two Pd-catalyzed couplings. F



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column (2.7 μm; 100 mm × 3.0 mm) (Advanced Materials Technology, Wilmington, DE), running an acetonitrile/0.05% trifluoroacetic acid gradient was used for assessment of steps 1 and 2. Mobile phase solution A was 0.05% trifluoroacetic acid in water (v/v), and mobile phase solution B was 0.05% trifluoroacetic acid in acetonitrile (v/v). Gradient elution was performed as shown in Table 5 below with a run time of 20 min

Table 4. Maximum levels of impurities in commercial scale batches of 1e impurity

max. % in manufacturing batches

13 25 26 28 total impurities residual Pd

≤0.05 ≤0.05 ≤0.05 ≤0.05 ≤0.05% 7 ppm

Table 5. Gradient program for steps 1−2

For the Migita coupling at step 1 it is important to use a mild base with efficient inertion of the headspace to minimize inhibition of the Pd catalyst. In the Heck reaction, the electronrich iodoindazole 14 suffers from inefficient oxidative addition which is overcome in part with in situ acylation. The acyl group not only facilitates reactivity with the Pd catalyst but also blocks formation of 25 as a side product. The choice of Xantphos in the Heck reaction limits the formation of impurity 26, which is difficult to purge. Ultimately, Xantphos contributes to the control of Pd in a synergistic manner with 1,2-diaminopropane and 6 during isolation of 1c at step 5. Control of solid form is critical for axitinib as there are five known anhydrous forms. As the synthesis was optimized and the drug substance was routinely prepared in high purity, these solid forms were identified and characterized. From these studies, form 41 (1e) emerged as the most stable form which was later nominated for commercial development. As a benefit, the relatively high crystallinity of 1e facilitates control of impurities, including the purge of Pd. By incorporating the polymorph conversion into the recrystallization at step 6, the conversion of 1c to 1e ensures consistent purge of organic impurities as well as Pd.

time (min)

mobile phase A (%)

mobile phase B (%)

0 15 20

90 50 10

10 50 90

and a re-equilibration time of approximately 5 min. The flow rate was 0.7 mL/min and the column temperature was 50 °C. The injection volume was 5 μL, and UV detection was performed at 220 nm. Sample and standard solutions were prepared in water/acetonitrile/trifluoroacetic acid (500:500:0.5, v/v/v) at a nominal concentration of 0.1 mg/mL. For steps 4 and 5 and axitinib purity and assay, the HPLC column was a Symmetry C18 150 mm × 4.6 mm, 5 μm (Waters Corp., Milford, MA). Mobile phase solution A was composed of water/acetonitrile/trifluoroacetic acid (900:100:0.5, v/v/v), and mobile phase solution B was water/acetonitrile/trifluoroacetic acid (200:800:0.5, v/v/v). Gradient elution was performed as shown in Table 6 below Table 6. Gradient program for steps 4 and 5 and axitinib assay/purity

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EXPERIMENTAL SECTION General. All reactions were carried out in dry reaction vessels under an atmosphere of dry nitrogen unless otherwise specified. All reagents and solvents were used as received without further purification unless otherwise specified. For most steps, reactions were monitored, and purity was assessed by HPLC, using an Agilent 1100 series instrument. A Halo C18

time (min)

mobile phase A (%)

mobile phase B (%)

0 1 18 40 41

100 100 68.5 0 0

0 0 31.5 100 100

with a run time of 41 min and a 4-min re-equilibration time. The flow rate was 1.0 mL/min, and the column temperature was 30 °C. The injection volume was 10 μL, and UV detection

Scheme 9. Optimized manufacturing route for axitinib

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reaction is complete (typically not more than 2% of 29 remaining), the reaction mixture is cooled to a target of 50 °C. The reaction mixture is diluted with THF (121 kg) and passed through a polishing filter. The batch is further diluted with THF (40 kg) and 1,2-diaminopropane (33 kg, 4 equiv) at a target of 50 °C. After stirring the solution at a target of 50 °C for at least 30 min, water (496 L) is slowly charged while maintaining the temperature near a target of 50 °C. The mixture is stirred at a target of 50 °C for about 12 h and then cooled to a target of 15 °C. The product slurry is filtered. The cake is washed with water (136 L). The crude material is reslurried warm in THF (121 kg) with 1,2-diaminopropane (2.2 kg, 0.25 equiv) for ∼4 h and then cooled to ∼15 °C for filtration. The filtered solids are washed with THF (121 L), dried, and isolated to afford crude axitinib 1c (THF solvate), 34.0 kg (80% yield uncorrected by assay) in 99.3% purity. Residual Pd content was determined to be not more than 20 ppm by atomic absorption spectroscopy. Mp 216−217 °C. Recrystallization and Polymorph Conversion to Form 41 of (E)-N-Methyl-2-(3-(2-(pyridin-2-yl)vinyl)-1H-indazol-6-ylthio)benzamide (1e) (Step 6). To a purged reactor vessel is added 1c (34.0 kg, 88.0 mol) followed by NMP (87.6 kg) and THF (30.3 kg). The reactor is purged, and the mixture is heated to 68 °C and stirred to allow for dissolution of the ingoing material. In a separate vessel, ethanol (631 kg) is added (process can use either absolute ethanol or methanol denatured ethanol), and the reactor is heated to about 73 °C and stirred. Once axitinib has completely dissolved in NMP/THF in the first vessel, this solution is filtered to be speck-free and is transferred into the second vessel containing the ethanol. To this hot supersaturated solution is added an amount of form 41 axitinib seed crystals (1.70 kg, 0.05 equiv). The heating is increased to a reflux, and the mixture is held for about 7 h. The batch is slowly cooled to 20 °C at a rate of about 0.07 °C/min. After holding at the final isolation temperature, the slurry is filtered and washed with ethanol (102 L) to displace the mother liquor and remove residual NMP. After drying, 1e is isolated as a white crystalline solid (28.0 kg, 82% yield uncorrected by assay) in 99.9% purity. The Pd level was determined to be of not more than 6 ppm by atomic absorption spectroscopy. 1H NMR (DMSO-d6, 300 MHz) δ (ppm) 13.35 (1H, s), 8.61 (1H, d, J = 3.8 Hz), 8.39 (1H, q, J = 4.4 Hz), 8.21 (1H, d, J = 8.8 Hz), 7.96 (1H, d, J = 16.4 Hz), 7.85−7.76 (1H, m), 7.66 (1H, d, J = 7.8 Hz), 7.61 (1H, s), 7.58 (1H, d, J = 16.5 Hz), 7.50 (1H, dd, J = 5.7 Hz), 7.36−7.23 (3H, m), 7.19 (1H, dd, J = 8.4 Hz, 1.2 Hz), 7.05 (1H, dd, J = 7.5, 1.5 Hz), 2.78 (3H, d, J = 4.5 Hz). 13C NMR (DMSO-d6, 75 MHz) δ (ppm) 168.2, 155.2, 149.8, 142.4, 142.2, 137.3, 136.0, 132.9, 130.6, 129.5, 128.1, 126.5, 25.9, 124.1, 123.0, 122.9, 122.1, 120.6, 115.1, 26.5. Mp 224−225 °C.

was performed at 220 nm. Axitinib sample and standard solutions were prepared in water/acetonitrile/acetic acid (700:300:30, v/v/v) at a nominal concentration of 0.2 mg/mL. Crude 2-(3-Iodo-1H-indazol-6-ylthio)-N-methylbenzamide (14) (Steps 1−2). To a nitrogen-purged reactor vessel at ambient conditions is charged NMP (82.4 kg) followed by 12 (40 kg, 164 mol, 1.0 equiv), Xantphos (0.96 kg, 0.010 equiv), tris(dibenzylideneacetone)dipalladium [Pd2(dba)3] (0.76 kg, 0.005 equiv), and sodium bicarbonate (15.16 kg, 1.1 equiv). The reactor is purged, and the mixture is held at a target of 25 °C and stirred for about 30 min. A solution of 10 (28.78 kg, 1.05 equiv) in NMP (61.8 kg) is added over about 1 h. The batch mixture is heated to a target of 50 °C. The reaction is held for at least 2 h and then sampled for step 1 reaction completion. The mixture is cooled to a target of 25 °C after passing the step 1 reaction completion (typically not more than 5% of 12 remaining). A solution of iodine (87.5 kg, 2.0 equiv) in NMP (47.4 kg) is added at a rate that allows maintaining the temperature near a target of 25 °C. A solution of potassium hydroxide (43.0 kg, 2.0 equiv) in water (40 L) is added over at least 30 min with a target of 25 °C. The reaction mixture is held for at least 5 h, and then sampled for step 2 reaction completion. After passing the reaction completion (typically not more than 7% of 13 remaining), the reaction mixture is quenched into a solution of ascorbic acid (25.36 kg, 1.75 equiv) in water (200 L) over about 1 h at a target of 45 °C. Methanol (120 L) is added to the slurry. After stirring the suspension for about 3 h at 45 °C, the slurry is filtered warm. The filtercake is washed with water (120 L) followed by methanol (120 L), and then dried to give crude 14 as a brown crystalline solid (62 kg, 92% yield uncorrected by assay). Recrystallization of 2-(3-Iodo-1H-indazol-6-ylthio)-Nmethylbenzamide (14) (Step 2R). To a purged reactor vessel is added crude 14 (62 kg, 151.5 mol) followed by NMP (207 kg) and 1,2-diaminopropane (1.2 kg) at ambient conditions. This solution is heated to about 60 °C and stirred to ensure dissolution of the solids. Methanol (338 L) is charged while maintaining temperature at a target of 55 °C. Water (335 L) is then added while maintaining a target of 55 °C and precipitating the product from solution. The resulting slurry is held at temperature, then cooled to approximately 20 °C and granulated for about 3 h. The slurry is filtered. The filter cake is reslurried with water (200 L), reslurried with methanol (135 L), and blown dry with nitrogen. The solids were dried and isolated to give recrystallized 14 as a pale-yellow crystalline solid (45.3 kg, 73% yield uncorrected by assay) in 99.5% purity. 1 H NMR (DMSO-d6, 300 MHz) δ (ppm) 13.53 (s, 1H), 8.35 (q, J = 4.7 Hz, 1H), 7.56 (s, 1H), 7.51−7.40 (m, 2H), 7.36− 7.23 (m, 3H), 7.13 (dd, J = 8.5, 1.3 Hz, 1H), 7.06−7.01 (m, 1H), 2.76 (d, J = 4.7 Hz, 3H). 13C NMR (DMSO-d6, 75 MHz) δ (ppm) 168.2, 141.3, 137.6, 135.6, 134.4, 130.7, 128.2, 126.7, 126.7, 125.9, 122.0, 114.7, 94.1, 26.5; mp 219−221 °C. Crude (E)-N-Methyl-2-(3-(2-(pyridin-2-yl)vinyl)-1H-indazol-6-ylthio)benzamide (1c) (Steps 3−5). Pd(OAc)2 (1.00 kg. 0.04 equiv), Xantphos (2.56 kg, 0.04 equiv), and NMP (107 kg) are charged to a nitrogen-purged reaction vessel to form the reaction catalyst. After this, Hunig’s base (43 kg, 3 equiv), and 14 (45.3 kg, 110.6 mol, 1 equiv) are added under ambient conditions. The reaction is heated to a target of 50 °C, and acetic anhydride (23 kg, 2 equiv) is added to effect acylation. Once acylation is complete (not more than 2% of 14 remaining), 2-vinylpyridine (70 kg, 6 equiv) is added. The heating is further increased to a target of 90 °C. Once the Heck

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AUTHOR INFORMATION

Corresponding Author

*E-mail: robert.a.singer@pfizer.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Paul Glynn for overseeing the scale-up of this technology in the Pilot Plant. We thank John O’Connell and Philomena Enright for oversight of the scale-up in commercial manufacture. We are grateful for the work of the Structure H



Article

of the coupling reaction. Alternatives to zinc were also found, including ascorbic acid and poly(methylhydrosiloxane). (15) Xiang, Y.; Caron, P.-Y.; Lillie, B. M.; Vaidyanathan, R. Org. Process Res. Dev. 2008, 12, 116−119. (16) Exchange of aryl groups on Xantphos and related triarylphosphines has been observed, see: (a) Yin, J.; Zhao, M. M.; Huffman, M. A.; McNamara, J. M. Org. Lett. 2002, 4, 3481−3484. (b) Yin, J.; Buchwald, S. L. Org. Lett. 2000, 2, 1101−1104. (c) Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1998, 120, 3694−3703. (d) Segelstein, B. E.; Butler, T. W.; Chenard, B. L. J. Org. Chem. 1995, 60, 12−13. (17) The 3-bromoindazole derivative of 14 fails to react with the Pd catalyst and remains unchanged under the reaction conditions. (18) (a) Yamamoto, T.; Sekine, Y. Inorg. Chim. Acta 1984, 83, 47− 53. (b) Murata, M.; Buchwald, S. L. Tetrahedron 2004, 60, 7397−7403. (c) Kreis, M.; Braese, S. Adv. Synth. Catal. 2004, 347, 313−319. (19) (a) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989− 7000. (b) Hills, I. D.; Fu, G. C. J. Am. Chem. Soc. 2004, 126, 13178− 13179. (20) (a) Campeta, A. M.; Chekal, B. P.; Abramov, Y. A.; Meenan, P. A.; Henson, M. J.; Shi, B.; Singer, R. A.; Horspool, K. R. J. Pharm. Sci. 2010, 99, 3874−3886. (b) Chekal, B. P.; Campeta, A. M.; Abramov, Y. A.; Feeder, N.; Glynn, P. P.; McLaughlin, R. W.; Meenan, P. A.; Singer, R. A. Org. Process Res. Dev. 2009, 13, 1327−1337. (21) Samas, B.; Seadeek, C.; Campeta, A. M.; Chekal, B. P. J. Pharm. Sci. 2011, 100, 186−194.

Elucidation Group who identified many of the impurities encountered during development. We thank Dave Damon, Barbara Sitter, Carrie Wager, and John Lucas for carrying out screening of both metal-catalyzed coupling reactions to verify the optimal catalysts were selected for development. We acknowledge Gregg Tavares, Yanqiao Xiang, and Matthew Frierson for their assistance in the analytical research and development. We thank John Barry and Jerome McCormick for providing the analytical data from the axitinib commercial manufacturing campaigns.

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REFERENCES

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Development of an Efficient Pd-Catalyzed Coupling Process for Axitinib

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