Multicompartmental Pharmacokinetic Model of Tenofovir Delivery to the Rectal Mucosa by an Enema

Rectal enemas that contain prophylactic levels of anti-HIV microbicides such as tenofovir have emerged as a promising dosage form to prevent sexually transmitted HIV infections. The enema vehicle is promising due to its likely ability to deliver a large amount of drug along the length of the rectal canal. Computational models of microbicide drug delivery by enemas can help their design process by determining key factors governing drug transport and, more specifically, the time history and degree of protection. They can also inform interpretations of experimental pharmacokinetic measures such as drug concentrations in biopsies. The present work begins rectal microbicide PK modeling, for enema vehicles. Results here show that a paramount factor in drug transport is the time of enema retention; direct connectivity between enema fluid and the fluid within rectal crypts is also important. Computations of the percentage of stromal volume protected by a single enema dose indicate that even with only a minute of enema retention, protection of 100% can be achieved after around 14 minutes post dose. Concentrations in biopsies are dependent on biopsy thickness; and control and/or knowledge of thickness could improve accuracy and decrease variability in biopsy measurements. Results here provide evidence that enemas are a promising dosage form for rectal microbicide delivery, and offer insights into their rational design.