Effect of late pregnancy on salicylate, diazepam, warfarin, and propranolol binding: use of fluorescent probes.

Serum protein binding was measured in six women 38 wk pregnant and in five control subjects. Three distinct binding sites for drugs on human serum albumin have been identified. To determine whether changes in binding during pregnancy occur for common drugs or only for drugs that bind to a specific binding site, serum protein binding of three drugs--diazepam (site I), warfarin (site III), and salicylate--and four fluorescent probes--dansylsarcosine (site I), 1-anilino-8-naphthalenesulfonate (site I), 7-anilinocoumarin-4-acetic acid (site II), and 5-dimethylaminonaphthalene-1-sulfonamide (DNSA) (site III)--were determined in control and pregnant sera. Unbound fractions of diazepam and salicylate in pregnant women increased but the unbound fraction of warfarin did not change. Dissociation constants (Kd1) of all fluorescent probes but DNSA were almost the same in control and pregnant sera, while the Kd1 of DNSA in pregnant serum was approximately 50% of control. Binding capacities of all probes decreased, which was attributed to decreased serum albumin concentration. We concluded that serum protein binding of drugs that bind to site I or site II on albumin decreased largely because of the reduced serum albumin concentration during pregnancy and that the binding of drugs that bind to site III changed little because of compensating effects of the decrease in serum albumin concentration and the increase in binding affinity to serum albumin. Serum concentration of alpha 1-acid glycoprotein and serum binding of propranolol did not change in pregnant women.