Peter J. Skrdla

Practical comparison of 2.7 μm fused-core silica particles and porous sub-2 μm particles for fast separations in pharmaceutical process development

Fused-core silica stationary phases represent a key technological advancement in the arena of fast HPLC separations. These phases are made by fusing a 0.5 microm porous silica layer onto 1.7 microm nonporous silica cores. The reduced intra-particle flow path of the fused particles provides superior mass transfer kinetics and better performance at high mobile phase velocities, while the fused-core particles provide lower pressure than sub-2 microm particles. In this work, chromatographic performance of the fused-core particles (Ascentis Express) was investigated and compared to that of sub-2 microm porous particles (1.8 microm Zorbax Eclipse Plus C18 and 1.7 microm Acquity BEH C18). Specifically, retention, selectivity, and loading capacity were systematically compared for these two types of columns. Other chromatographic parameters such as efficiency and pressure drop were also studied. Although the fused-core column was found to provide better analyte shape selectivity, both columns had similar hydrophobic, hydrogen bonding, total ion-exchange, and acidic ion-exchange selectivities. As expected, the retention factors and sample loading capacity on the fused-core particle column were slightly lower than those for the sub-2 microm particle column. However, the most dramatic observation was that similar efficiency separations to the sub-2 microm particles could be achieved using the fused-core particles, without the expense of high column back pressure. The low pressure of the fused-core column allows fast separations to be performed routinely on a conventional LC system without significant loss in efficiency or resolution. Applications to the HPLC impurity profiling of drug substance candidates were performed using both types of columns to validate this last point.

Use of a Quality-by-Design approach to justify removal of the HPLC weight % assay from routine API stability testing protocols

Due to the high method variability (typically > or = 0.5%, based on a literature survey and internal Merck experience) encountered in the HPLC weight percent (%) assays of various active pharmaceutical ingredients (APIs), it is proposed that the routine use of the test in stability studies should be discouraged on the basis that it is frequently not sufficiently precise to yield results that are stability-indicating. The high method variability of HPLC weight % methods is not consistent with the current ICH practice of reporting impurities/degradation products down to the 0.05% level, and it can lead to erroneous out-of-specification (OOS) results that are due to experimental error and are not attributable to API degradation. For the vast majority of cases, the HPLC impurity profile provides much better (earlier and more sensitive) detection of low-level degradation products. Based on these observations, a Quality-by-Design (QbD) approach is proposed to phase out the HPLC weight % assay from routine API stability testing protocols.

A simple quantitative FT-IR approach for the study of a polymorphic transformation under crystallization slurry conditions

The pharmaceutical compound (2R,3S)-2-([(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethyl]oxy)-3-(4-fluorophenyl)morpholine hydrochloride (denoted here as Compound X), has been found to crystallize in at least two polymorphic forms. Using only two frequencies (1009 and 1058 cm(-1)) in the infrared, a linear (R=0.998) calibration plot, consisting of the ratio of the peak absorbances plotted against polymorph concentration, was constructed. This plot allowed the quantification of binary mixtures of polymorphs ranging from <3 to approximately 100 wt% Form II in Form I. Spectra were acquired in transmission mode using mineral oil (Nujol) mull sample preparation, for reasons of compatibility with wet cake and slurry samples. The transformation of the less thermodynamically stable polymorph (Form II) to the more stable form (Form I), in stirred methyl isobutyl ketone (MIBK) slurries, was monitored spectroscopically as a function of time. Performing the experiment at various temperatures allowed the energy of activation for the process to be estimated (42 kJ/mol).

Disproportionation of a crystalline citrate salt of a developmental pharmaceutical compound: characterization of the kinetics using pH monitoring and online Raman spectroscopy plus quantitation of the crystalline free base form in binary physical mixtures using FT-Raman, XRPD and DSC.

The crystalline citrate salt (CS) of a developmental pharmaceutical compound, MK-Q, was investigated in this work from two different, but related, perspectives. In the first part of the paper, the apparent disproportionation kinetics were surveyed using two different slurry systems, one containing water and the other a pH 6.9 phosphate buffer, using time-dependent measurements of the solution pH or by acquiring online Raman spectra of the solids. While the CS is generally stable when stored as a solid under ambient conditions of temperature and humidity, its low pHmax (<3) facilitates rapid disproportionation in aqueous solution, particularly at higher pH values. The rate of disappearance of the CS was found to obey first-order (Noyes-Whitney/dissolution rate-limited) kinetics, however, the formation of the crystalline product form in the slurry system was observed to exhibit kinetics consistent with a heterogeneous nucleation-and-growth mechanism. In the second part of this paper, more sensitive offline measurements made using XRPD, DSC and FT-Raman spectroscopy were applied to the characterization of binary physical mixtures of the CS and free base (FB) crystalline forms of MK-Q to obtain a calibration curve for each technique. It was found that all calibration plots exhibited good linearity of response, with the limit of detection (LOD) for each technique estimated to be ≤7 wt% FB. While additional calibration curves would need to be constructed to allow for accurate quantitation in various slurry systems, the general feasibility of these techniques is demonstrated for detecting low levels of CS disproportionation.