Competitive binding of fluoroquinolone antibiotics and some other drugs to human serum albumin: a luminescence spectroscopic study.

Co-administration of several drugs in multidrug therapy may alter the binding of each to human serum albumin (HSA) and hence their pharmacological activity. Thirty-two frequently prescribed drug combinations, consisting of four fluoroquinolone antibiotics and eight competing drugs, have been studied using fluorescence and circular dichroism spectroscopic techniques. Competitive binding studies on the drug combinations are not available in the literature. In most cases, the presence of competing drug decreased the binding affinity of fluoroquinolone, resulting in an increase in the concentration of free pharmacologically active drug. The competitive binding mechanism involved could be interpreted in terms of the site specificity of the binding and competing drugs. For levofloxacin, the change in the binding affinity was small because in the presence of site II-specific competing drugs, levofloxacin mainly occupied site I. A competitive interference mechanism was operative for sparfloxacin, whereas competitive interference as well as site-to-site displacement of competing drugs was observed in the case of ciprofloxacin hydrochloride. For enrofloxacin, a different behavior was observed for different combinations; site-to-site displacement and conformational changes as well as independent binding has been observed for various drug combinations. Circular dichroism spectral studies showed that competitive binding did not cause any major structural changes in the HSA molecule.