Careful Navigation of the Crystallographic Landscape of MK-8970: A

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Careful Navigation of the Crystallographic Landscape of MK-8970: A Racemic Acetal Carbonate Prodrug of Raltegravir Nancy Tsou,† C. Scott Shultz,*,† Teresa Andreani,† Richard G. Ball,† Andrew Brunskill,† Jaume Balsells,‡ Ryan D. Cohen,† Jimmy DaSilva,† Jing Li,‡ Robert A. Reamer,† Manuel de Lera Ruiz,‡ Narayan Variankaval,† Richard J. Varsolona,† Nobuyoshi Yasuda,† and Gregory York† †

Department of Process & Analytical Chemistry, Merck Research Laboratories, 126 East Lincoln Avenue, Rahway, New Jersey 07065, United States ‡ Department of Process & Analytical Chemistry, Merck Research Laboratories, 770 Sumneytown Pike, West Point, Pennsylvania 19486, United States S Supporting Information *

ABSTRACT: MK-8970 is an acetal carbonate prodrug of raltegravir (Isentress). This work presents the Merck team’s investigations into the polymorphism of MK-8970, the thermodynamic relationship between the discovered crystalline forms, and implementation of that knowledge toward solving key processing challenges. MK-8970 was found to exist in two enantiotropic polymorphs, with a crossover temperature of approximately 117 °C, as determined from solubility data. Form 2 of MK-8970, the stable form at ambient temperature, was confirmed to be a true racemic crystal form and not a conglomerate on the basis of single-crystal X-ray structure data. In preparation for scale-up of MK-8970, form control was established by mapping out solubility curves for the relevant crystalline forms in ethyl acetate as a function of temperature. Lastly an investigation of the relative solubilities of MK-8970 and a troublesome imidate impurity identified improved solvent systems for maximizing rejection of this impurity while avoiding significant yield losses.

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INTRODUCTION In 2007, Merck & Co. launched raltegravir (trade name Isentress), which was the first HIV-1 Integrase strand transfer inhibitor (InSTi) to receive U.S. FDA approval1 and is prescribed as part of highly active anti-retroviral therapy (HAART) for the treatment of the HIV-AIDS infection.2−4 Raltegravir is formulated commercially as the potassium salt, and HIV-infected adults are prescribed 400 mg twice daily.5 It has established robust efficacy in both treatment-experienced ̈ patients.6−9 From the period of 2000− and treatment-naive 2007, combination anti-retroviral therapy markedly increased. As a result, survival rates among infected adults in the U.S. and Canada likewise grew, making HIV infection a chronic yet manageable condition.10 In a 2013 article by Srivastava et al., the authors found higher patient adherence rates associated with once daily dosing relative to higher dosing frequencies across multiple disease areas, and per-patient costs and health care resource utilization likewise were consistently lower with once daily regimens.11 MK-8970 (eq 1), an optimized acetal carbonate prodrug of raltegravir, possesses enhanced colonic absorption and suitable physicochemical properties to support development of an immediate release/controlled release (IR/ CR) formulation for once daily dosing.12 Like the majority of pharmaceutical compounds, MK-8970 is chiral; however, it can be formulated as the racemate since both enantiomers are efficiently converted in vivo to the active achiral raltegravir molecule. 12 In order to develop a robust manufacturing process and consistently performing formulation, a thorough understanding of a compound’s crystallization behavior and the associated thermodynamics is critical. Racemic © XXXX American Chemical Society

compounds like MK-8970 crystallize typically in one of three modes. Each enantiomer can crystallize independently as either (+) or (−) in a physical mixture of individual crystals of each antipode. Compounds crystallizing in this way are referred to as conglomerate systems. The melting point of a conglomerate will be significantly lower than the melting point of pure (+) or Special Issue: Polymorphism & Crystallisation 2015 Received: April 20, 2015

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DOI: 10.1021/acs.oprd.5b00129 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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Figure 1. DSC and TGA traces of forms 1 and 2.

Since MK-8970 is a racemic compound, it was not clear if forms 1 and 2 are racemic crystals or conglomerates. It was necessary to make this determination in order to understand the thermodynamic relationship between these crystal forms and select a phase for development. Crystals of form 2 suitable for structure determination were grown from EtOH/water via slow evaporation. Form 2 crystallizes in the monoclinic centrosymmetric space group, P21/n, with unit cell parameters of a = 11.410(4) Å, b = 9.438(4) Å, c = 26.152(8) Å, and β = 102.66(4)° at 273 K. Figure 2 shows the molecular conformation of MK-8970. Since there is no molecular site positional disorder, this space group requires there be an equal number of enantiomers in the lattice; i.e., the crystal is a true racemate. Figure 2 shows the packing arrangement in form 2. The molecule arranges itself via an extended hydrogen bond network wherein the amide carbonyl of the p-F-benzyl amide group participates in a hydrogen bond with the amide N−H from the oxadiazole arm of a neighboring molecule, forming hydrogen-bonded chains (catemers). The catemers are formed between translationally related molecules with chains proceeding parallel to the b-axis. Thus, each catemer chain is of a single hand of the molecule, and four chains pass through the cell. Chains of the same hand are 21 screw-related, and chains of opposite hand are centrosymetrically related. Pure Enantiomer Evaluation. Numerous failed attempts were made to grow form 1 crystals suitable for single-crystal structure determination. Without a crystal structure of form 1, the team remained unsure if form 1 was also a true racemic crystal. Samples of the pure (R) and (S) enantiomers of MK8970 became available and subsequent crystallization provided key data for investigating the possibility of form 1 being a conglomerate. As expected, crystallization of pure (R)- and (S)MK-8970 resulted in identical XRPD patterns (Figure 3) and nearly identical DSC traces (Figure 4; onset of melting and enthalpy of fusion: for (R), 167.3 °C and 75.2 J/g; for (S), 166.6 °C and 76.1 J/g). Additional polymorph screening studies on the pure enantiomers uncovered no additional crystal forms. Crystals of the (S) enantiomer suitable for singlecrystal structure analysis were grown from ethanol/water. The enantiomer crystallizes in the orthorhombic P212121 space group with unit cell parameters of a = 9.493(1) Å, b = 11.246(1) Å, and c = 24.486(4) Å at 100 K. The XRPD

(−) crystals. A second mode of crystallization for racemic molecules is that where both enantiomers crystallize in the unit cell with a regular order. If the unit cell always has both enantiomers in an equal ratio, then this is truly a racemic crystal. The melting point of a racemic crystal may be higher or lower than the crystalline enantiomer. The third scenario occurs if there is no significant difference in the affinity between the same or opposite enantiomers. When this occurs, both enantiomers will occur in equal proportions in the crystal, albeit in an unordered manner. This system is called a pseudoracemate (or solid solution). Addition of a small amount of one enantiomer may change the melting point only slightly or not at all.13 This work presents the Merck team’s investigations into the polymorphism of MK-8970, the thermodynamic relationship between the discovered crystalline forms, and the implementation of that knowledge toward solving key early processing challenges. Acquiring a fundamental understanding of the system’s thermodynamics aided in rapidly navigating the laboratory-scale development and positioned the team for quick implementation at larger scale.

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RESULTS AND DISCUSSION Polymorphism. MK-8970 was initially received as a crystalline sample and characterized as an anhydrous crystalline phase (hereafter referred to as form 1) with a melting onset of 170.7 °C and an enthalpy of fusion of 85.2 J/g (Figure 1). A polymorph screen on form 1 was carried out in which the compound was suspended in a variety of solvents and held isothermally for a period of 13 days, allowing for a slurrymediated phase transition. Characterization of the recovered solids indicated that most conditions resulted in a form change. Thermal characterization of air-dried solids revealed an anhydrous phase (hereafter referred to as form 2) with a melting endotherm of 90.7 J/g and an onset temperature of 165.1 °C (∼6 °C lower than form 1) (Figure 1). Analysis of XRPD patterns of wet slurry samples of form 2, covered with kapton film to prevent solvent evaporation, did not show any differences between wet and subsequently air-dried samples, suggesting that form 2 is not generated via desolvation of a transient solvate. B



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Figure 2. (a) ORTEP representation of MK-8970 in crystal form 2. Ellipsoids are set at the 50% probability level. The molecule is shown arbitrarily in the S configuration. (b) Packing diagram of MK-8970 form 2 showing translationally related catemers passing through the cell. Chains shown in red and yellow are S, with chains shown in green and blue being R.

phase under process relevant conditions. However, a more thorough understanding of the thermodynamic relationship between forms 1 and 2 was required in order to develop a robust crystallization and isolation process. Form 1 was observed to have a higher melting point and a lower enthalpy of fusion than form 2 (see Figure 1). According to the “heat of fusion rule”, if the higher melting form has the lower heat of fusion, then the forms are enantiotropes; otherwise, they are monotropes.14 However, given the possibility of decomposition events contributing to the measured heats of fusion, other methods were required to establish the relationship. Solubility measurements for forms 1 and 2 were taken from 55 to 85 °C in toluene, and a van’t Hoff plot was constructed (see Figure 5). Form 2 was observed to exhibit a lower solubility than form 1 at each of these four temperatures, and a

patterns of the crystalline enantiomers were, however, different from forms 1 and 2 of MK-8970 (Figure 3) indicating that form 1 is not a conglomerate of the pure enantiomer crystal form. The melting endotherms for pure (R)- and (S)-MK-8970 are similar to forms 1 and 2; however, a simple 1:1 physical mixture of (R)- and (S)-MK-8970 showed a characteristic melting point depression that would be expected for a conglomerate of this composition with melting onset of 150.3 °C (Figure 4). Enantiotropic vs Monotropic FormsSelecting the Lead Crystalline Phase. In order to support formulation development of MK-8970, the team needed to select which of the two crystalline forms should be isolated in the final chemical step. Conversion of form 1 to form 2 at room temperature and 50 °C slurry condition had been observed in the polymorph study, suggesting form 2 is the more stable C



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Figure 3. XRPD of crystalline (R)- and (S)-MK-8970 compared to forms 1 and 2.

Figure 4. DSC traces of pure crystalline (S), pure (R), and 1:1 simple mixture.

crossover point of equal solubility was extrapolated at ∼117 °C, confirming the enantiotropic relationship between forms 1 and 2. From this analysis, form 2 is predicted to be more stable than form 1 at all temperatures below 117 °C, and form 1 likewise at all temperatures above 117 °C up to the melt. It is interesting to note from the DSC traces (Figure 1), however, that a thermal solid-to-solid conversion of form 2 to form 1 was not observed. Initial cooling crystallizations from EtOAc resulted in mixtures of forms 1 and 2, despite intentional seeding with form 2. Hence, a plot of the solubility in EtOAc as a function of temperature was constructed (Figure 6) to improve form control. The solubility curves of forms 1 and 2 show that cooling the system too rapidly can readily enable crystallization

of the metastable form 1. Isolation of pure form 2 can be achieved by maintaining the system concentration below the supersaturation curve for form 1. Purity Control in Preparation for Scale-Up. The XRPD of some laboratory runs showed an unexpected peak at 7.2° 2θ which had not been previously observed for form 2 samples. This peak was also not observed in the form 1 pattern, and it was not an allowed peak in the calculated form 2 XRPD pattern generated from the single-crystal structure (Figure 7). A table of allowed 2θ peak positions for form 2 is provided in the Supporting Information. Chromatographic analysis indicated that batches containing this extra XRPD peak were associated with an elevated amount (1−2% area from achiral HPLC) of an imidate impurity with structure shown in Figure 8, as later D



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electron density was apparent that resided in cavities within the structure near crystallographic inversion centers. The level of the residual electron density and the size of the cavities were consistent with a disordered ethyl acetate molecule residing across the inversion centers, thus yielding a disordered hemiEtOAc solvate. The calculated XRPD pattern of this solvated form based on cell dimensions at 293 K matches the experimentally collected imidate form 1 powder pattern. Furthermore, the broad endotherm (typical for a desolvation event) observed below 150 °C in the DSC of form 1 would be expected for an EtOAc solvate. To lay the groundwork for further process development of MK-8970, a brief study was undertaken to identify candidate solvent systems for improving the imidate rejection in the MK8970 form 2 crystallization. In this study, the imidate and MK8970 solubility was interrogated across a range of potential processing solvents (Table 1). This was accomplished by suspending excess MK-8970, containing an elevated amount of the imidate, overnight at 20 °C in a specific solvent and then seeding with imidate form 1. After an additional overnight age, the supernatant was assayed for MK-8970 and imidate concentrations. In the interest of time and efficiency, the imidate solubility values reported were calculated using the UV response of MK-8970 as a surrogate. This provided access to the solubility trend across a range of solvents, which was the key information needed to quickly guide process development. Interestingly, in EtOAc, the solubility of the imidate was observed to be substantially lower than that of MK-8970 form 2 at 20 °C, explaining the poor rejection of the imidate in the first GMP manufacture. The poor imidate solubility is perhaps not surprising, considering the existence of the hemi-EtOAc solvate that was observed in this system. As an illustration of the process efficiency of each solvent tested, the volume productivity (amount of solvent required to fully dissolve 2 wt% of the imidate impurity) and maximum theoretical yield of MK-8970 were calculated (Table 1). These solubility data suggest that the most desirable scenario for removing low levels of imidate while maintaining good MK-8970 recovery and minimizing solvent usage would be obtained using either pure IPAc or IPAc saturated with water. These data were generated

Figure 5. Van’t Hoff plot of MK-8970 forms 1 and 2 in toluene.

confirmed by NMR and SC-XRD analysis. While it appeared unlikely that an impurity at this level would be observable via XRPD, there were no other immediately obvious explanations. Pure crystalline imidate (imidate form 1) was serendipitously crystallized from mother liquors of an MK-8970 form 2 crystallization that were set aside for several days. XRPD analysis of the crystalline imidate form 1 (Figure 9) showed that it possessed a very intense peak at 7.0° 2θ. It was further observed in the DSC of imidate form 1 (Figure 10) that a second crystalline form of the imidate existed (imidate form 2). A broad endotherm from 50 to 150 °C (likely from loss of volatiles) is observed, followed by a small endotherm at 157.8 °C, and finally a larger endotherm with onset of 217.6 °C. Imidate form 2 was subsequently isolated by annealing a sample of imidate form 1 at 166 °C. The XRPD of imidate form 2 (Figure 9) possessed a very intense peak at 7.2° 2θ, making this crystalline form a better candidate to account for the extra peak in the first GMP batch pattern. A suitable crystal of the imidate form 1 was obtained from recrystallization of the imidate from DCM/EtOAc for SC-XRD analysis. The unit cell was determined to be monoclinic in space group P21/c. As refinement proceeded, high residual

Figure 6. Solubility of forms 1 and 2 of MK-8970 in EtOAc as a function of temperature. E



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Figure 7. XRPD of the first GMP scale-up batch compared to calculated form 2 pattern and form 1 reference.

to provide a general guide for process development, and further work is required to determine the true potential of any of these alternative solvent systems.

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CONCLUSIONS MK-8970 was found to crystallize in one of two enantiotropic polymorphs, with a crossover temperature of approximately 117 °C. Single-crystal structure investigation of form 2 uncovered that it crystallizes in the P21/n space group, thereby confirming it to be a true racemic crystal. Crystallization work on the pure enantiomers of MK-8970 resulted in a single crystalline form (distinct from MK-8970 forms 1 and 2). Conclusive evidence that form 1 is a racemate was not achieved via single-crystal structure or alternative methods. However, the inability to crystallize the pure enantiomer as another polymorph (or more specifically as a polymorph with XRPD pattern matching that of the racemic MK-8970 form 1 sample) leads the authors to conclude that MK-8970 form 1 is likely to

Figure 8. Structure of imidate impurity (left) and ORTEP representation of MK-8970 imidate impurity (right). Ellipsoids are set at the 50% probability level. The ethyl acetate molecule of this solvate form was disordered and was not discretely modeled.

Figure 9. XRPD overlay of the imidate crystalline forms with the first GMP batch of MK-8970. F



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Figure 10. DSC of the imidate impurity as crystallized from recovered mother liquors.

Table 1. Solubility of MK-8970 and Imidate in Various Processing Solvents at 20 °C MK-8970 solubility (mg/g solvent)

imidate solubility (mg/g solvent)a

volume productivityb,e

max theor yield (%)c,e

IPAc IPAc sat. w/H2O EtOH acetone/heptane 1:1 (v/v) toluene

3.03 7.94 3.49 7.09

2.51 4.83 1.42 1.02

9 5 18 27

98 97 95 86

best choice

0.43

0.400

58

98

good yield, poor volume productivity, and imidate rejection

MTBE 2-PrOH acetone THF ACN EtOAc EtOAc sat. w/H2O

0.08 0.74 63.2d 39.1 128d 9.81 32.3

0.097 0.46 6.49 4.06 6.22 0.48 1.34

279 55 4 6 4 46 17

98 97 80d 80 58d 58 51

solvent system

comment

poor yield

a All imidate solution compositions are calculated versus a MK-8970 standard. The actual values may be different depending upon the response factor ratio of imidate to MK-8970. bVolume productivity = mL of solvent necessary to dissolve 2 wt% imidate in 1 g of crude MK-8970. cMax theor yield is theoretical recovery of MK-8970 after dissolving all imidate. dAll solids dissolved; actual solubility > reported values; actual yields < reported values. eA sample calculation is provided in the Supporting Information.

be a true racemic crystal, although definitive proof has remained elusive. Early laboratory samples of MK-8970 were observed to be mixtures of forms 1 and 2, and solubility data in EtOAc were subsequently generated to establish conditions whereby form 2 could be crystallized exclusively. Lastly, an elevated imidate impurity was observed in the first GMP manufacture of MK8970. Investigation of the relative solubility of this impurity versus MK-8970 form 2 established several potential alternative solvent systems for maximizing rejection while avoiding significant yield losses.

approximately 5 mg of compound. Nitrogen purge gas at a constant rate of 25 mL/min was maintained during each run. X-ray Powder Diffraction (XRPD). XRPD measurements were carried out on an X’Pert Pro instrument (Panalytical Inc., Natick, MA) with a Cu-LFF source of wavelength 1.5418 Å, operating at 40 kV and 50 mA. Data were collected in the 2− 40° 2θ range. Single-Crystal X-ray Diffraction (SC-XRD). SC-XRD was carried out on a Bruker diffractometer equipped with an Apex II CCD area detector for data collection at 100 K. Cu Kα radiation was used. The intensity data were corrected for absorption and decay (SADABS). The structure was solved using Bruker SHELXTL software and refined using Bruker SHELXTL. A direct-methods solution was calculated to locate most non-hydrogen atoms from the E-map. Full-matrix leastsquares/difference Fourier cycles were performed to locate the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located either from the difference Fourier map or at calculated positions and allowed to ride on their parent atoms in the refinement cycles. Solubility Measurements for van’t Hoff Plot. Portions of 50−70 mg of each phase (forms 1 and 2) were weighed into four different clear screw-cap tubes (caps with Teflon liner were

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EXPERIMENTAL SECTION Materials. Raltegravir potassium salt was synthesized as described in ref 15. Methods. Thermal Analysis. Standard differential scanning calorimetry (DSC, Q2000, TA Instruments, USA) analysis was performed at heating rates of 5 °C/min. Pinhole hermetically sealed and open pans (pinhole pans without lid) were used as stated. Nitrogen purge was used for all the DSC experiments with a flow rate of 50 mL/min. Calibration was performed using indium under the same conditions. Thermogravimetric analysis (TGA, Q5000, TA Instruments, USA) was performed in aluminum open pans by weighing G



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and H19), 6.98−7.02 (2H, m, H20 and H22), 6.47 (1H, q, J = 5.3 Hz, H33), 4.57 (2H, d, J = 6.0 Hz, H17), 4.01−4.11 (2H, om, H39), 3.62 (3H, s, H8), 2.60 (3H, s, H31), 1.88 (3H, s, H11), 1.87 (3H, s, H10), 1.74 (3H, d, J = 5.3 Hz, H35), 1.22 (3H, t, J = 7.1 Hz, H40). 13C NMR (CDCl3, 150 MHz): δC 166.5 (s, C28), 162.2 (d, JF,C = 244.7 Hz, C21), 162.1 (s, C14), 161.3 (s, C6), 158.8 (s, C26), 154.9 (s, C2), 154.6 (s, C36), 152.0 (s, C25), 141.8 (s, C5), 137.7 (s, C4), 134.0 (d, JF,C = 3.0 Hz, C18), 129.7 (d, JF,C = 8.1 Hz, C19 and C23), 115.5 (d, JF,C = 21.4 Hz, C20 and C22), 99.5 (s, C33), 64.2 (s, C39), 58.3 (s, C9), 42.8 (s, C17), 33.4 (s, C8), 26.6 (s, C11), 26.5 (s, C10), 20.6 (s, C35), 14.2 (s, C40), 11.3 (s, C31). Isolation of Imidate (Z)-N-(2-(4-((4-Fluorobenzyl)imino)2,7-dimethyl-8-oxo-7,8-dihydro-4H-[1,3]dioxino[5,4-d]pyrimidin-6-yl)propan-2-yl)-5-methyl-1,3,4-oxadiazole-2carboxamide. The mother liquor from the crystallization of MK-8970 (containing ∼7.9 g of MK-8970) was saved and set aside. After several days it was noticed that a solid precipitated out from the solution. The solid was filtered, dried and characterized by NMR and HPLC-MS. 1H NMR (DMSO-d6, 600 MHz): δH 9.89 (1H, s, H24), 7.43−7.47 (2H, m, H15 and H19), 7.13−7.19 (2H, m, H16 and H18), 5.80 (1H, q, J = 5.1 Hz, H9), 4.69 (1H, d, J = 16.2 Hz, H13), 4.57 (1H, d, J = 16.2 Hz, H13), 3.50 (3H, s, H22), 2.57 (3H, s, H34), 1.72 (6H, s, H25 and H26), 1.67 (3H, d, J = 5.1 Hz, H11). 13C NMR (DMSO-d6, 150 MHz): δC 165.7 (s, C31), 161.0 (d, JF,C = 241.8 Hz, C17), 158.0 (s, C28), 156.9 (s, C6), 155.3 (s, C2), 152.5 (s, C27), 148.0 (s, C7), 141.4 (s, C5), 136.1 (d, JF,C = 3.0 Hz, C14), 130.0 (s, C4), 129.5 (d, JF,C = 8.0 Hz, C15 and C19), 114.9 (d, JF,C = 21.2 Hz, C16 and C18), 98.0 (s, C9), 57.9 (s, C23), 49.0 (s, C13), 32.8 (s, C22), 26.9 (s, C25), 26.7 (s, C26), 19.4 (s, C11), 10.7 (s, C34). Crystallization of (R)- and (S)-MK-8970. In a 100 mL glass vessel fitted with overhead stirring were added 4.1 g of amorphous (S)-MK-8970 and 60.6 g of toluene. The mixture was heated to 100 °C (solid did not completely dissolve) with stirring. The slurry was cooled quickly to 85 °C and then to 5 °C linearly over 8 h. After being stirred at 5 °C overnight, the crystalline solid was isolated by filtration and dried by sucking ambient air through the cake to constant weight. (R)-MK-8970 was crystallized in an analogous manner. Crystallization of MK-8970 Form 2 for SC-XRD. In a small vial, 5.0 mg of MK-8970 form 2 was dissolved completely in excess EtOH/H2O (5.3 wt% H2O, aw = 0.3). The vial was covered with foil and pierced to allow for slow evaporation. Crystals of form 2 suitable for single-crystal structure determination were recovered. Crystallization of (S)-Enantiomer for SC-XRD. In a small vial, 5.0 mg of the (S)-enantiomer was dissolved completely in excess EtOH/H2O (5.3 wt% water, aw = 0.3). The vial was covered with foil and pierced to allow for slow evaporation. Crystals suitable for single-crystal structure determination were recovered.

utilized). Screw threads were wrapped with additional Teflon tape. Next, 2.0 mL of toluene was added into each tube, and the tubes were capped. Additional Teflon tape was wrapped around the outside of the caps to avoid solvent leakage. Tubes were placed into a sonication bath, pre-equilibrated at the desired temperature for 2 h. At the end of the 2 h period, approximately 0.5 mL samples were taken and filtered through a pipet containing a cotton plug into a 10 mL volumetric flask. The weight was recorded. The volumetric flasks were placed into a 60 °C vacuum oven for about 10−15 min to remove excess toluene. Dilutions were prepared appropriately, and concentrations were analyzed by chromatography and, for solids, by XRD. Polymorph Study. Approximately 70 mg of MK-8970 form 1 was added to a 4 mL amber vial along with a Teflon-coated stirbar and suspended in a minimal amount of solvent (varied from 0.25 to 0.75 mL) in order to establish a suspension. Vials were capped and wrapped with parafilm to reduce potential for solvent loss. Each vial was then stirred isothermally at a specific temperature for 13 days. Any condition where it was not possible to establish a suspension was stirred for 3 days, and if precipitation did not occur, then the vial was moved to ambient temperature, and the cap was removed and replaced with aluminum foil pierced with a pushpin to allow for slow evaporation. After 13 days, samples were centrifuged, and the wet solids were sampled for XRPD analysis. The wet solids were mounted on XRPD sample holders and covered with Kapton tape. After initial analysis, the tape was removed, and XRPD samples were allowed to air-dry overnight under ambient conditions. Dry samples were re-analyzed by XRPD. See Supporting Information for further details. Synthesis of MK-8970, Ethyl (1-((4-((4-Fluorobenzyl)carbamoyl)-1-methyl-2-(2-(5-methyl-1,3,4-oxadiazole-2carboxamido)propan-2-yl)-6-oxo-1,6-dihydropyrimidin-5yl)oxy)ethyl)carbonate. Into a 3 L, three-neck RBF equipped with overhead stirrer, thermocouple, and N2 line were charged raltegravir potassium salt (95.0 g, 197 mmol), tetrabutylammonium bromide (69.8 g, 217 mmol), N,N-dimethylacetamide (475 mL), and Hünig’s base (37.8 mL, 217 mmol). Last, 1chloroethyl ethyl carbonate (29.1 mL, 217 mmol) was charged. The reaction mixture was heated to an internal temperature of 70(±2) °C and aged for 4 h. The heterogeneous reaction mixture was allowed to cool to room temperature. Ethyl acetate (1.1 L) and 5 wt % NaCl/H2O (950 mL) were charged. The aqueous layer was isolated and back-extracted with ethyl acetate (475 mL). The combined organic layers were washed with water (950 mL) and concentrated to a volume of ∼500−600 mL. The resulting mixture was distilled at constant volume (using 475 mL of ethyl acetate) until the KF < 1500 ppm of H2O. The heterogeneous mixture was heated to an internal temperature of 50 °C and seeded with 2.7 g of MK-8970 form 2. The mixture was aged at 50−60 °C for 12 h. XRPD analysis confirmed complete conversion to form 2. To the resulting slurry was added heptane (180 mL) via addition funnel over a period of 100 min. (XRPD analysis indicated that the desired crystalline form remained by the end of addition.) The slurry was then cooled to 26 °C over a period of 1.5 h and filtered. The cake was displacement-washed with 30% heptane in ethyl acetate (200 mL) and then heptane (200 mL). Drying at room temperature under house vacuum with N2 sweep overnight afforded 86.1 g (78% isolated yield) of solid as the desired crystalline form 2. 1H NMR (CDCl3, 600 MHz): δH 8.29 (1H, s, H12), 7.81 (1H, t, J = 6.0 Hz, H16), 7.33−7.37 (2H, m, H23

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ASSOCIATED CONTENT

S Supporting Information *

Polymorph screen on MK-8970 form 1, NMR spectra for MK8970, NMR spectra and HPLC-MS traces of imidate, chiral preparative separation of MK-8970 enantiomers, DSC of MK8970 pure enantiomers, MK-8970 solubility data, and CIF files of MK-8970 form 2, (S)-MK-8970, and the imidate form 1. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.5b00129. H



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 732-594-5268. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the following individuals from Merck Research Laboratories for their support during this work: Scott T. Trzaska, Zhihong Ge, David Tschaen, Bing Mao, Cameron Cowden, Joseph Armstrong, Ingrid Mergelsberg, and Michael Kress.

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REFERENCES

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