Vinblastine pharmacokinetics measured by a sensitive enzyme-linked immunosorbent assay

Radioimmunoassays have been developed for pharmacokinetic studies of vinblastine. Although highly sensitive, radioimmunoassays are both expensive and potentially biohazardous. This paper describes a new immunoassay procedure, enzyme-linked immunosorbent assay, which uses enzymatic activity rather than radioactivity as an index of drug concentration. Antiserum to vinblastine was attached to the plastic wells of microtiter plates and incubated in the presence of varying amounts of vinblastine which had been conjugated to alkaline phosphatase. After brisk washing of the wells, p-nitrophenylphosphate was added to each well. The amount of enzyme present in the well was quantitated by the production of the chromophore, p-nitrophenol. The concentration of free vinblastine present in a given sample was inversely proportional to the enzymatic activity. The enzyme-linked immunosorbent assay is capable of detecting as little as 5 pg of either vinblastine or vincristine and is not cross-reactive with other commonly used oncolytic agents. A pharmacokinetic study was performed in rats administered vinblastine i.v., and a triphasic elimination curve was obtained. These results indicate that the enzyme-linked immunosorbent assay provides a nonradioactive, inexpensive, and sensitive method to monitor plasma levels of vinblastine. Further, since the antibody is cross-reactive with vincristine, it would appear that similar data could be generated for vincristine.