Category: Formulation and Quality
Purpose: Amorphous solid dispersions often improve the oral bioavailability of poorly soluble drugs; however, they have poor physical stability and are susceptible to crystallization either during manufacturing or storage. The resultant residual crystallinity may diminish the solubility and bioavailability advantage offered by these formulations. Therefore, an accurate in vitro evaluation of such formulations is of paramount importance at several levels of drug product development such as bioavailability assessment, process development, process validation, quality control or bioequivalence testing. Dissolution testing is a common formulation development tool. However, due its closed-compartmental setup, large dissolution volume and lack of absorptive sink conditions, it typically has poor discriminatory power to assess solution phase behavior of complex formulations. An improved approach integrates dissolution and absorption measurements. Recently, we have developed a novel high surface area, flow-through absorptive dissolution testing apparatus that simultaneously measures dissolution and absorption of formulations to provide in vivo relevant information, in biologically relevant time frames. The apparatus has proven to be highly sensitive to understand solution phase behavior. In this study, the apparatus was employed to evaluate dissolution and absorption behavior of generic amorphous formulation of tacrolimus with varied % residual crystallinity to determine the impact of residual crystallinity on absorption profiles. In addition, dissolution was performed in different volumes to understand the influence of sink and non-sink conditions on their dissolution-absorption performance. Inter-product and intra-product virtual bioequivalence were performed to assess the generic formulation with residual crystallinity. Furthermore, systematic studies were carried out to delineate the competition between the rate of dissolution, absorption and desupersaturation of drug in the solution.
Methods: Generation and characterization of residual crystallinity: The residual crystallinity was instilled in a generic formulation of tacrolimus (manufactured by Accord Healthcare) by exposure to 40 °C and 75% RH. Powder X-ray diffraction (PXRD) was used to determine the % crystallinity in the formulation, using calibration curve generated with samples of known amount of crystallinity.
Dissolution and absorption measurements: Dissolution and absorption measurements of brand (Prograf) and unexposed/exposed Accord formulations were carried out in the absorptive dissolution testing apparatus. The apparatus consists of a donor container with drug dissolved or suspended in the aqueous medium, a hollow fiber membrane module, a buffer reservoir containing the absorption medium, a receiver container to collect drug following absorption across the membrane and a peristaltic pump to pump fluids in the apparatus, at the flow rate of 2 mL/min. The apparatus was maintained at 37 ?C. Dissolution volumes were sink (240 and 100 mL) and non-sink (40 mL) with respect to the amorphous solubility (50 µg/mL). The donor concentration was analyzed using HPLC. The receiver/ absorption concentration was measured non-cumulatively using flow-through UV probe (correlates proportionally to donor concentration).
Results: Tacrolimus crystals obtained after exposure of Accord formulation to accelerated storage conditions displayed similar characteristic peaks and morphology in PXRD and SEM. Dissolution and absorption measurements under different dissolution conditions revealed overall slower dissolution rate of Prograf as compared to Accord formulation, with a small enhancement in dissolution rate under non-sink condition. The resultant absorption profiles hence varied under different dissolution conditions, indicating better bioequivalence between Prograf and Accord at smaller dissolution volume. High amount of crystallinity (50% and 100%) in Accord formulations influenced the maximum concentration generated, thus displayed lower dissolution-absorption profiles and failed to be bioequivalent. Interestingly, 20% crystallinity in Accord showed no differences in dissolution-absorption profiles as compared to the unexposed Accord under sink conditions, however, the drug crystallized over time under non-sink conditions due to the presence of seed crystals in the solution. To delineate the interplay between dissolution, absorption and desupersaturation, absorption and desupersaturation rates were evaluated from measurements at different supersaturation level. The study revealed that for a given dissolution rate, desupersaturation would supersede the absorption rate if high supersaturation is generated. Therefore, for low supersaturation generated under sink condition, absorption rate was large enough to further deplete the supersaturation over time, lowering the risk of crystallization. However, for high supersaturation in non-sink condition, formulations underwent crystallization, as in the case of 20% crystallinity.
Conclusion: This study demonstrates the importance and impact of discriminatory in vitro tool for formulation development on bioavailability assessment and bioequivalence outcomes. The absorptive dissolution testing enabled effective evaluation of amorphous formulations with residual crystallinity. The subsequent removal of drug from the dissolution container due to absorption altered the desupersaturation tendency, thus influencing the absorption advantage of these formulations. The robustness and sensitivity of the apparatus displays its potential as a methodology that could address quality control and clinically-relevant dissolution model, a model of great utility to the pharmaceutical industry.
Lynne Taylor– Professor, Department of Industrial and Physical Pharmacy,College of Pharmacy, Purdue University, West Lafayette, Indiana
Susan Reutzel-Edens– Senior Research Advisor, Small Molecule Design & Development, Eli Lilly and Company, Indianapolis, Indiana