Article pubs.acs.org/OPRD
Appearance of a New Hydrated Form during Development: A Case Study in Process and Solid-State Optimization Henry Morrison,† Bin P. Quan,§ Shawn D. Walker,§ Karl B. Hansen,§ Karthik Nagapudi,*,‡ and Sheng Cui*,§ §
Drug Substance Technologies, Amgen, 360 Binney Street, Cambridge, Massachusetts 02142, United States Gilead Sciences Inc., 333 Lakeside Drive, Foster City, California 94404, United States ‡ Genentech Inc., 1 DNA Way, South San Francisco, California 94080, United States †
ABSTRACT: The appearance of a new solid phase during the development of a small molecule presents significant challenges for optimizing form control during crystallization and mitigating potential delays in development timelines. Herein we present a case study regarding the appearance of a new hydrated form during a good manufacturing procedures production of a late-phase development molecule. Associated with this new form, we discuss the investigations required to overcome challenges associated with the development of this hydrated phase as well as strategies taken to minimize the risk and impact to the overall program timeline. The solid-state properties of the new hydrated phase were assessed against the original anhydrous phase to understand the potential development implications. The critical water activity required to form the hydrated phase was determined, and milling investigations were performed to understand potential improvement in the dissolution rate profile. On the basis of these investigations, it was decided to switch to the new hydrated phase as an acceptable physical form for long-term clinical development.
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temperature and pressure, the system becomes invariant.18 This means that the composition of the organic solvent/water mixture at which this equilibrium between the hydrated and anhydrous phases occurs is fixed for a given temperature and pressure. The water activity (aw) of the solvent system can be calculated from the activity coefficient (γw) and mole fraction of water (xw) in the system using
INTRODUCTION The development of a scalable crystallization process for an active pharmaceutical ingredient (API) is an important part of process optimization because it requires the isolation of the correct final form with acceptable quality attributes and adequate purity.1−4 Typically the solid form of the API is selected prior to Phase 1 trials, and the crystallization process is optimized to deliver this form.5−8 In order to accomplish form selection, a lot of effort is expended to screen, identify, and characterize as many solid phases of the compound as possible. In spite of this rigorous testing, the risk of appearance of a new stable polymorph during later stages of development cannot be completely eliminated. Appearance of a new form during the course of development can impose several challenges due to the affect this new form can have on physical-chemical properties such as melting point, hygroscopicity, chemical stability, dissolution rate, and bioavailability.5,9−15 This in turn can affect further downstream processes such as crystallization and the formulation of the drug product. These challenges are exacerbated when such solid form transformations occur in later phases of clinical development, which in many cases result in delay to the overall program timeline. Due to the ubiquitous presence of water in the atmosphere, process solvents and drug product excipients, i.e., hydrates, are commonly encountered during pharmaceutical development.16,17 Once a hydrated form of the API is identified, it is imperative to understand the phase transformation between the anhydrous and the hydrated phase. In a given organic solvent: water mixture, it is possible to produce either the anhydrous or the hydrated phase depending upon the amount of water used. On the basis of the Gibbs phase rule, it becomes evident that when both the anhydrous and hydrated phases are in equilibrium with the organic solvent/water mixture at a given © XXXX American Chemical Society
a w = γwx w Thus, at a given temperature and pressure, there is a defined water activity above which the hydrated phase would be more stable than the anhydrous phase. Once the water activity is determined, a process can be designed to selectively produce either the anhydrous or the hydrated phases depending upon which solid form is selected for development. The aw required to convert the anhydrous to the hydrated phase can be determined by measuring the solubility of the two solid forms in organic solvent/water mixtures of varying water composition.19−21 However, a problem with this approach is the possible conversion of one phase into another in the process of solubility determination. Slurry bridging experiments is another way to determine the critical aw by slurrying equal amounts of both the hydrated and anhydrous phases in organic solvent/ water mixtures of varying water composition.22−24 The advantage of this method is that the inferred thermodynamic stability can be applied to the stability of the solid phase as a function of relative humidity (RH) as aw is directly related to the RH. Special Issue: Polymorphism & Crystallisation 2015 Received: January 26, 2015
A
DOI: 10.1021/acs.oprd.5b00030 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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hemihydrate was collected, representing 62.4 g of static losses and a milling yield of 97.5%. The milled AMG A hemihydrate was assayed at 99.9A% purity, 97.3w% purity, and 2.2wt% water. The surface area was measured at 5.9 m2/g, and the particle size distribution (PSD) was measured, confirming d10 = 1 μm, d50 = 4 μm, and d90 =13 μm. Metals were assayed as Fe = 20 ppm, Cr = 3 ppm, all other assayed species 3 days and drying >1 week). After significant process development on the crystallization, the improved method produced larger primary particles with a needle-like crystal habit (Figure 3) that resulted in an improved filtration of 10 h (calculated) for a 40 kg batch. Discovery of a New Form. A new form was discovered during the seed preparation for the GMP campaign. This was a C
DOI: 10.1021/acs.oprd.5b00030 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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Figure 4. PLM images of seed (left) and final cake (right) produced during the seed preparation with sonication.
The exothermic event following desolvation with an extrapolated onset temperature of 145.1 °C in the DSC thermogram of AH is attributed to crystallization of an anhydrous phase following water removal from the sample. This was confirmed via variable-temperature X-ray powder diffraction (VT-XRPD, Figure 7). The VT- XRPD data for AH
Figure 5. Comparison of the XRPD patterns of the original anhydrous phase (AA) and new hemihydrate phase (AH).
Figure 7. Variable-temperature XRPD data of AH.
show no major changes in the pattern up to 120 °C; however, at 150 °C (above the onset temperature of the exotherm observed in the DSC trace) new peaks, labeled with “*” begin to appear in the XRPD pattern indicating transformation to a new anhydrous phase. By 175 °C, the sample has completely converted to a new (yet unidentified) anhydrous phase. The second endothermic event with an extrapolated onset of 184.2 °C in the DSC trace corresponds to the melting of this new anhydrous phase that is formed as a result of heating AH. This anhydrous phase appears to be a polymorph of AA as evidenced by the difference in melting temperatures. On the basis of the XRPD, DSC, TGA, and KF data, it was concluded that AH was a hydrated form of AMG A with a water stoichiometry of 0.5 mol (hemihydrate) per mole of active. This was further confirmed by single crystal analysis (data not shown). Determination of Critical Water Activity. The moisture sorption profile of AH (Figure 8) indicated that it is nonhygroscopic, with a percent weight change of less than 0.1%
Figure 6. Thermal analysis data of AA and AH.
77.6 °C in the DSC thermogram of AH is attributed to loss of water from the sample upon heating. D
DOI: 10.1021/acs.oprd.5b00030 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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samples for which the solid form in equilibrium with the supernatant is the hydrated phase AH as confirmed by XRPD. On the basis of the data, it was calculated that above 6.2 wt% water in n-propanol (water activity of 0.452) AH is favored. This indicates that AH will be the thermodynamically stable phase when aw > 0.452 (45.2% RH), while AA will be the thermodynamically stable phase when aw < 0.452 (45.2% RH). This is further indicated in Figure 9 whereby the region of the phase diagram where AA is stable is marked as a hatched area. Although the critical RH of 45.2% represents a value typical for ambient RH, based on the facts that AH could be produced in a broad range of water activities and that the form is kinetically stable for extended periods at low RH, it was decided to develop AH for long-term development. Process for the Production of AH. Having chosen AH as the final drug substance form, comprehensive studies were carried out to develop a scalable crystallization process. Key process parameters such as choice of solvent, temperature, seeding point, drying procedure, and impurities rejection limit were evaluated. Solvent. Water-miscible solvents were examined for the crystallization of AH. Examination of n-propanol, acetone, ethanol, methanol, isopropanol, acetonitrile, DMF, and DMSO as mixtures with water all yielded AH. Further experimentation demonstrated that n-propanol/water was the most suitable system for AH isolation due to better rejection of impurities, higher process efficiency (higher yield and lower volume), and a wider metastable zone (Figure 10). In addition, several stress
Figure 8. Gravimetric vapor sorption profile of AH.
from 5 to 95% RH in the absorption curve. The XRPD analysis of the sample post moisture sorption showed no phase change had occurred during the experiment. Kinetic solid-state stability of the hydrate was further evaluated for a period of 2 months from 1% to 22% RH using saturated salt solution chambers, and no form change was observed for AH during the investigation (even when AH was seeded with 10% AA material). This indicated that AH is physically stable in the solid state even at low RH values during the time frame of the experiment, although this does not imply thermodynamic stability of AH at low RH values. Slurry bridging experiments were conducted in n-propanol/ water mixtures at 25 °C, and the data are shown in Figure 9.
Figure 10. Metastable zone of AH measured in n-propanol/water system. Figure 9. Solubilities of AA (△) and AH (▲) as a function of water content in n-propanol. Water content of the supernatant was measured by KF and is plotted in the x-axis.
studies were carried out to better understand the scope and limitations of the process, and we determined the following: (1) On the basis of the critical water activity result, >15% water was selected as the starting point of crystallization. Eventually it was found that the process could be started with the range of 25−30% water/n-propanol without missing the seeding point and without negative impact on product quality. (2) The crude AH is stable (72 h without negative impact on product quality. Drying Process. Since DSC and TGA analysis indicated that dehydration commences at about 70 °C, stress tests were performed by drying the wet cake (49 wt% water, 9 wt% npropanol) at 45 °C in the oven for 16 h. No negative impact was observed on form or water content. During the GMP campaign, the cake was dried under vacuum with a wet nitrogen sweep (>75% RH) with the same results. Milling Assessment of AH. The PSDs of AA, unmilled AH, and milled AH are shown in Figure 11 (left panel), and the data are summarized in Table 1, along with the Brunauer− Emmett−Teller (BET) surface area (SA) values.
measurements, indicating that AH could withstand the milling process. Milling of AH was further investigated to see if the surface area could be increased further. AH was subjected to multiple passes through a pin-mill and jet-mill at different conditions. The PSD and surface area data are summarized in Table 2. The Table 2. PSD Metrics and Surface Area of Milled AH sample unmilled pin-milled, 1 pass pin-milled, 2 passes pin-milled, 4 passes jet-milled at 95% setting, 1 pass jet-milled at 95% setting, 2 passes jet-milled at highest setting, 1 pass
Table 1. Comparison of PSD Metrics and Surface Area of AA and AH sample
D10 (μm)
D50 (μm)
D90 (μm)
VMD (μm)
surface area (m2/g)
AA AH unmilled AH milled
1 8 3
5 27 12
26 77 26
10 35 14
6.9 0.5 0.8
D10 (μm)
D50 (μm)
D90 (μm)
surface area (m2/g)
8 3 3 2 4
27 12 11 8 11
77 26 23 18 28
0.5 0.8 2.8 3.3 3.3
4
10
22
4.2
1
6
18
3.1
data do not show a good correlation between PSD and surface area. For example, the samples pin-milled for one and two passes at 35,000 rpm show relatively minor differences in the PSD, while a 3 times improvement in surface area is observed. This lack of correlation between PSD and surface area is attributed to surface roughness that is generated as a part of the milling process.25−27 The powder dissolution profiles of the samples milled under different conditions are shown in Figure 12. The dissolution profiles show a clear trend with surface area, whereby samples with higher surface area show faster dissolution profiles. The dissolution data also show that, regardless of how the material is produced, as long as the surface area is >3 m2/g, the dissolution reaches equilibrium rapidly. The data presented here show the danger of solely using PSD as an indicator of how a material is going to behave in terms of dissolution profile. A better approach in assessing the materials performance would be to use PSD data in conjunction with surface area data, and then determine a process to deliver the required surface area. This result is consistent with the Noyes−Whitney equation, where surface area is directly proportional to dissolution rate. This is an important consideration as PSD is used widely in the industry as a surrogate for surface area owing to ease of measurement. However, in order to be meaningful, the PSD must be
The unmilled AH lot was found to have bigger particles than the AA lot with the volume mean distribution (VMD) of unmilled AH being about four times that of AA. The powder dissolution profiles of AA, unmilled AH, and milled AH in phosphate-buffered saline at pH 6.8 are shown in Figure 11 (right panel). The unmilled AA is able to reach its equilibrium solubility (0.027 mg/mL) within 30 min, while the unmilled AH is unable to reach its equilibrium solubility (0.017 mg/mL) even after 70 min. On the basis of this information, it was decided to pin-mill the sample. The PSD of the milled AH sample is now more comparable to that of the unmilled AA (Table 1); however, the unmilled AA sample has a larger proportion of fines attributed to the morphology being fine needles. The difference in fines between AA and milled AH is reflected in the measured surface area. The surface area of AH increases from 0.5 to 0.8 m2/g after milling, but is still much lower than the surface area of the unmilled AA. After milling, the dissolution profile of AH showed vast improvement with the sample reaching its equilibrium solubility in about 30 min. In addition, it was also verified that no form change had occurred in AH post milling via XRPD, DSC, TGA, and KF F
DOI: 10.1021/acs.oprd.5b00030 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS We thank Julie Calahan for help with the surface area measurements.
Figure 12. Dissolution profiles of AH from different milling processes: (1) unmilled AH, (2) AH pin-milled for one pass, (3) AH pin-milled for four passes, and (4) AH jet-milled for one pass.
correlated with surface area during development.3 Since multiple passes through the pin-mill was required to get a surface area >3 m2/g, for delivering milled AH, it was decided to use one pass through a jet-mill, which produced an AH product with desired surface area. It is also noteworthy that no amorphous content was detected in all the milled materials as determined by solid-state NMR and XRPD analysis.
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CONCLUSION Although great effort may be taken in an attempt to identify as many polymorphs as possible during a programs preclinical phase, the possibility of a new stable form showing up during late-stage development is always a risk. A new hydrated form of AMG A was discovered during a GMP production, which required the team to quickly characterize and understand its relationship to the historically produced anhydrous form. The critical water activity, particle size, surface area, and dissolution profile of the hydrate were investigated in order to understand the potential development implications and to determine which phase should be pursued for long-term development. These investigations were quickly conducted over a 6 week time frame, which enabled the team to move forward with the hydrated phase on the basis of its ability to be produced in a broad range of water activities and the fact that it was kinetically stable for extended periods at low RH. In addition to the ability to produce the desired form in a reproducible and scalable manner, it is also important to consider the solid-state attributes necessary to enable further downstream development of the drug product. In this context, relying solely on PSD was found to be inappropriate, and further experiments were developed to understand dissolution performance. The dissolution performance of the milled materials was found to correlate well with surface area, and this metric was used to guide milling development.
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The authors declare no competing financial interest. G
DOI: 10.1021/acs.oprd.5b00030 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
