Bioavailability of pyridoxal phosphate from enteric-coated tablets. I. Apparent critical dissolution pH and bioavailability of commercial products in humans.

The critical dissolution pH values for forty-eight enteric-coated tablets of pyridoxal phosphate (PAL-P) commercially available in Japan were determined by using an oscillating basket apparatus with buffers of various pH. The tablets were divided into three groups according to their critical dissolution pH (pH<4.5, 4.5-5.5 and 5.5-6.6). The bioavailabilities of five enteric-coated and a sugar-coated tablets of PAL-P were studied in humans by measuring 4-pyridoxic acid (PiA), one of the main metabolites of PAL-P, in the urine. Two enteric-coated tablets whose critical dissolution pH values were 4.5-5.5 and <4.5, and one enteric-coated tablet having a critical dissolution pH of 5.5-6.6 and showing a relatively short lag in dissolution at this pH were bioequivalent with the sugar-coated tablet, though the absorption of the drug from the enteric-coated tablets was significantly delayed. However, the other two enteric-coated tablets having long lags in dissolution at their critical dissolution pH of 5.5-6.6 were found to show inferior bioavailabilities in both rate and extent.