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Studies on the binding of [3H]vinblastine by rat blood platelets in vitro

1972; Elsevier BV; Volume: 21; Issue: 11 Linguagem: Inglês

10.1016/0006-2952(72)90311-5

1873-2968

C.J. Secret, John R. Hadfield, C. T. Beer,

Pharmacogenetics and Drug Metabolism

Tritiated vinblastine (3H-VLB) of high specific activity was prepared by proton exchange with tritio-trifluoroacetic acid. The compound was used to study the binding in vitro of vinblastine by rat blood platelets in citrated platelet rich plasma. At 37°, 3H-VLB was initially taken up very rapidly by platelets; after ca. 90 min the amount bound reached a plateau which was maintained for at least 4 hr. The compound was bound as such and was not metabolized to any significant extent by platelets. The binding process was apparently fully reversible. A plot of the amount of 3H-VLB bound at equilibrium vs. the concentration in the plasma gave a sigmoidal curve. A Scatchard plot of the data indicated that at vinblastine plasma concentrations up to 0.06 μgml, when the platelets were less than one-quarter saturated, there was a “co-operative” (possibly allosteric) interaction between the alkaloid and platelet receptors; at higher vinblastine concentrations, the binding followed the simple Langmuir isotherm. Association and rate constants for the simple mechanism— vinblastine + platelets⇌k−1K+1vinblastine-platelet complex —were derived from the Scatchard plot and from an analog computer analysis of the binding kinetics. The platelet saturation value at 37° was 0.3 μg vinblastine per 109 platelets. Non-radioactive vinblastine rapidly displaced (and equilibrated with) platelet-bound 3H-VLB. Vincristine, in terms of its ability to displace 3H-VLB, interacted much more slowly than vinblastine with platelets. Colchicine, even at high concentrations, did not displace 3H-VLB; instead it enhanced significantly the vinblastine-binding capacity of platelets. The results suggest that platelet preparations may be suitable model systems for studying the binding of certain antimitotic drugs to cell receptors. The marked ability of platelets to concentrate vinblastine from plasma and the ready reversibility of the process indicate that platelets could have an important role in the distribution of the drug in vivo.