Purpose: To evaluate dissolution enhancement by supersaturating dosage forms (SSDFs) of BCS Class-II compounds, standard USP dissolution tests are used. However, results of these tests do not always correlate with bio-availability, for two reasons. First, any effects of drug absorption, which will tend to lower drug concentration in the intestinal fluid and influence precipitation kinetics, are suppressed, thereby underestimating the efficacy of the formulation. Second, analytical techniques usually do not distinguish nano-sized aggregates and complexes, that are not available for absorption, from the free drug which is available, thereby overestimating the ability of these SSDFs to provide drug in a supersaturated, highly bio-available form. These deficiencies warrant improvement of in vitro-in vivo correlation by simultaneously testing dissolution and absorption.
Objective: The overall goal of this project is to develop and validate an ‘Artificial Gut Simulator (AGS)’ to simultaneously simulate dissolution and absorption of a drug released from an SSDF, and to obtain physiologically relevant absorption rate (ka) with the optimized set-up. The apparatus consists of an absorption module with a large surface area, that maintains absorptive sink conditions, placed in a small volume (3 ml) dissolution vessel.
Methods: AGS Development and Testing: As a model system, simultaneous supersaturation generation and absorption of a weakly basic drug, Ketoconazole (KTZ) is being carried out using the experimental setup shown in Figure 1. Absorbance of KTZ in the donor and absorption media is determined using a UV-Vis spectrophotometer equipped with a temperature-controlled and magnetically stirred automatic multi-cell changer. The AGS consists of a) a cuvette that serves as a donor of drug dissolved/suspended in an aqueous donor medium (D) and, b) an absorption module made of hollow fibers (HF) suspended from the lid of the cuvette to provide an absorptive sink. HF are arranged on a home-made laser-cut supporting framework. The HF ends are bundled into inlet and outlet ports for the lumenal absorption fluid (A), as shown in Figure 2. After absorption of KTZ from D by A in the HF lumen and flow out of the module, the intraluminal absorbate is continuously replaced by fresh A, thereby maintaining an absorptive sink. To optimize the set-up to obtain a physiologically relevant value of ka, KTZ was maintained at a sub-solubility concentration in D while A was pumped into the HF modules of various surface area-to-volume ratios (SA-to-V) and at various rates by a syringe pump. Spectrophotometric assay of D was conducted every two minutes. Next, the concentration of KTZ in A was analyzed when it was pumped to a flow-through cell placed in the cell slot adjacent to the donor cuvette. By this means, alternating absorbance measurements of D and A are made continuously in-line during the entire duration of the study. A mass transfer model has been developed to explain the transfer process of KTZ from D into the HF lumen.
Results: At sub-solubility, the concentration of KTZ in D was found to decrease with time as the drug was cleared out of the system by A. Furthermore, the drug concentration in D decreased with increasing rate of flow of A and SA-to-V ratio of the HF module. The corresponding concentration in A increased with time until a steady state was achieved beyond which the concentration decreased with decreasing concentration in D. Mass balance, agreement with mass transfer model and a physiologically relevant ka was obtained using the optimized apparatus. At supersaturating concentration, in the absence of an absorptive sink, KTZ concentration in D decreased due to precipitation. The presence of an absorptive sink resulted in a greater reduction in KTZ concentration in D compared to the control. This reduction also increased with an increasing flow rate of A (Figure 3). The reduction in drug concentration in D due to precipitation and drug absorption was predicted using a mass transfer model coupled with nucleation and growth kinetics model.
Conclusion: The apparatus developed in this study introduces a novel method of simultaneously testing dissolution, supersaturation, and absorption of BCS Class-II drug formulations. This set-up was designed to also overcome the shortcomings of the currently existing systems which were developed for the same purpose. Some of them are (a) a large SA-to-V ratio of the HF module helped obtain physiologically relevant permeability coefficient as well as concentrations of drug in A above its limit of quantification in very reasonable amount of time, (b) low molecular weight cut-off of HF ensured that drug in aggregates and complexes which are non-bioavailable are excluded from absorption, (c) in-line, continuous measurement of drug concentration in A and D ensured there were no errors due to sampling and that the measurements were done as frequently as possible (measurement was made every minute), (d) low D volume (3 ml) is especially desirable during early stage product development when drug samples are limited and expensive.