Interaction of homopyrimidazole derivatives with biopolymers. I. Binding of MZ 144, a new potent analgesic to human serum albumin.

The binding of 1,6-dimethyl-3-carbethoxy-4-oxo-6,7,8,9-tetrahydro-homopyrimidazolium-methyl-sulfate (MZ 144, Probon) to human serum albumin (HSA) has been studied in vitro. The measurements have been performed mostly by means of the classical equilibrium dialysis method. Initially several controls were run to estimate the binding of drug to Visking membrane, time required for equilibrium and to check the stability of the drug at different pH. In addition binding was studied spectrophotometrically, essentially following the method of Klotz. Treatment of binding data was based on the equation developed by Scatchard. The data obtained do not fit the single-site binding equation but could be resolved by a computer program into two sets of binding sites. At pH 7,35,4 degrees C and using a 1% HSA solution values found for binding parameters were: N1 = 0.4359, k1 = 4077 M(-1), N2 = 1.17, k2 = 424 M(-1). Protein binding is considered to have a strong effect on drug distribution only if the affinity constant for the drug, k, has a value greater than 1 times 10(4). The HSA-MZ 144 interaction is temperature dependent and pH dependent. The percentage bound as a function of total drug concentration was calculated at pH 4.96, 7.35 and 8.5; a considerable increase was observed at pH 8.5.