Data from Phase IIb and III trials are often used by regulatory agencies to determine whether a particular product should be approved for widespread use. Pyrimidinedione [PYD; 1- 3-cyclopentenylmethyl ethyl 3,5-dimethylbenzoyl -2,4 1H,3H ] derivatives constitute a unique class of non-nucleoside reverse transcriptase inhibitors that also inhibit HIV entry through an unknown mechanism thought to target a conformational epitope formed prior to the fusion of the viral envelope with the host cellular membrane 5. As previously reported, results from the permeability measurements indicated no difference between fresh and frozen vaginal tissue Chemical stability of the active compound in the gel formulation.