A sensitive radioimmunoassay for vincristine and vinblastine

A rapid, sensitive and highly reproducible radioimmunoassay (RIA) has been devised for vincristine and vinblastine. These alkaloids bind to the antiserum very tightly with an association constant (Ka) of 5 x 10(9) M-1 as determined by Scatchard analysis. The kinetic association constant of the alkaloids, as determined by the 'on' and 'off' rates at 4 degrees C, is 30-fold higher than the Scatchard analysis value. At 4 degrees C, 80% of the alkaloid is bound to the antiserum within 20 min, and the remaining 20% reaches an equilibrium within 48-72 h. In the RIA procedure the amount of the alkaloid (1-2 ng) needed to compete for 50% binding remains unchanged when the reaction mixtures are incubated for 2, 4, or 20 h. By a modification of the RIA procedure, where the antiserum is first allowed to react with the unlabeled alkaloid followed by addition of the radiolabeled alkaloid, sensitivity of the assay has been increased five- to tenfold. Other drugs, such as adriamycin, CCNU, methyl CCNU, cyclophosphamide, 5-fluorouracil, heparin, hydrocortisone, prednisone, and nitrogen mustard do not interfere with the assay. This assay can thus be used to determine vincristine or vinblastine levels in 5 x 10(-10)--10(-9) M concentrations in biological fluids.