Protein binding characteristics of a new bronchodilator, 1-methyl-3-propylxanthine (MPX), in different species.

The protein binding of a new bronchodilator, 1-methyl-3-propylxanthine (MPX), in different animal species sera (human, dog, rabbit, rat and mouse) was investigated in vitro by ultrafiltration method. Interspecies differences in the binding affinity and binding capacity for MPX were observed. The dissociation constant (Kd1) for the high affinity binding site of human serum was approximately 17 times higher than that of rat serum. Those of rat serum and rabbit serum were nearly equal. On the other hand, the binding indices (n1p1/kd1) of human serum and rat serum were comparable. The binding of MPX to various proteins such as human serum albumin (HSA), alpha 1-acid glycoprotein (AGP) and isolated human lipoproteins was also investigated. The binding of MPX to HSA was lower than that of human serum protein but the number of specific binding sites (n1) in human serum was in agreement with that in HSA. The binding curves of MPX to AGP and lipoproteins were almost linear and the binding percentages were less than 7% in both cases. These data suggest that MPX is exclusively bound to albumin and that AGP and lipoproteins are of little importance in the protein binding of MPX. The present data appear to be useful for predicting the role of protein binding on the pharmacokinetics of MPX in different animal species.