Effects of Glass Bead Size on Dissolution Profiles in Flow-through Dissolution Systems (USP 4)

The effects of glass bead size in the conical space of flow-through cells on the dissolution profiles were investigated in a USP apparatus 4. Dissolution tests of disintegrating and non-disintegrating tablets in flow-through dissolution systems were performed using semi-high precision glass beads with diameters ranging from 0.5 mm to 1.5 mm. Computational fluid dynamics (CFD) was used to evaluate the effect of shear stress from the dissolution media flow. The use of smaller glass beads in a larger cell resulted in a faster dissolution of the model formulations under certain test conditions. The effect on the dissolution was highly dependent on the size of the beads in the top layer, including those in contact with the tablets. The absence of a bead-size effect on the dissolution of an orodispersible tablet in a small cell can be explained by the floating fragments during the test. CFD analysis showed that smaller bead diameters led to greater shear stress on the tablet, which was correlated with the dissolution rate. Hence, fluid flow through the narrow gaps between the small beads generated strong local flows, causing shear stress. The size of the glass beads used in flow-through cells affects the dissolution rate of tablets by altering the shear stress on the tablets in certain cases (e.g., direct deposition of the formulation on glass beads, large cells, and very low flow rates). Thus, glass bead size must be considered for a robust dissolution test in a flow-through cell system.

A novel physiologically based pharmacokinetic model of rectal absorption, evaluated and verified using clinical data on 10 rectally administered drugs

We present a physiologically based pharmacokinetic (PBPK) model simulating systemic drug concentrations following administration to the human rectum. Rectum physiology is parameterized based on literature data. The model utilizes in vitro release (IVRT) profiles from which drug mass transfer through the rectal fluid and tissue and into the systemic circulation are predicted. Due to a lack of data, rectal fluid and tissue absorption parameters are predicted either from colon absorption, with modifications relevant to rectal physiology, or optimized. The PBPK model is evaluated by simulating 29 clinical studies for 10 drugs. For 8 drugs (diazepam, diclofenac, indomethacin, naproxen, paracetamol, pentobarbital, phenobarbital and theophylline) the bias (average fold error, AFE) and precision (absolute average fold error, AAFE) of Cmax, AUC0-t and AUC0-inf simulations range from 0.87 to 2.22, indicating good agreement with observed values. For prochlorperazine and promethazine, the AFEs and AAFEs of Cmax predictions are 1.30 and 2.52, respectively. TheAUC0-t and AUC0-inf are overpredicted for both compounds(AFEs and AAFEs from 2.66 to 4.90). This results from a lack of reliable elimination data for prochlorperazine and the relevance of the IVRT profiles used in the promethazine model. The model paves the way for more mechanistic rectal drug absorption studies and virtual bioequivalence methods for rectal drug products.