PH-dependent drug release from certain commercial tablets.

A dissolution-dialysis profile for each of several brands of nitrofurantoin, hydrochlorothiazide and sulfisoxazole tablets was developed at three pH levels. The study used a two-compartment diffusion cell with a semipermeable cellophane membrane. Nitrofuratoin tablets were tested at pH values of 7.2, 4.7 and 1.2; sulfisoxazole tablets at 7.2, 5.5 and less than 1. Studies with hydrochlorothiazide tablets were terminated because the pH of the dissolution medium had no effect of practical significance on dissolution or dialysis rates. The dissolution rates of the nitrofurantoin and sulfisoxazole tablets were highly brand-individualized and complex. The effects of pH upon a particular formulation were unpredictable. The results suggest that dissolution testing at one fixed pH, as commonly practiced, may be insufficient for some products. This pH-dependent release characteristic may not correspond to a change in bioavailability in all cases, but it represents a potential problem which should be considered by the formulator.