Biological availability of sulfamethoxypyridazine and sulfisomidine polymers

Dissolution rates and apparent solubilities of forms I, II and III of Sulfamethoxypyridazine (1) and forms I and II of sulfisomidine (2) were determined in water at 37 degrees C. The ratios of apparent solubilities of I:II:III for 1 forms and I:II for 2 forms were 1:1.18:1.25 and 1:1.32 respectively. Upon long contact of 2 with water the ratio of II:I decreased. This has been attributed to gradual transformation of 2 from form II to I. Gastrointestinal absorption of form III of 1, in human volunteers, was studied in comparison with the more stable form I. The same study was carried out on forms I and II of 2. Data were correlated and expressed in availability rate constants (K1), applying the one compartment open: model. This and other parameters show that form III of 1 is 1.4 times as much absorbed as form I, and that the availability of the metastable form II of 2 is 1.2 times as much absorbed as the stable form I.