Effect of vincristine on the bone marrow cells of patients with multiple myeloma: A cytomorphologic study

The cytologic changes induced by vincristine (VCR) on the erythroblasts, the myeloid cells and the neoplastic plasma cells were studied on the bone marrow of 5 patients with plasma cell malignancies. Nine hours after the administration of the drug, the cytocidal effect on the 3 cell types was proportional to the magnitude of the sathmokinetic effect induced in them: marked on the erythroblasts (whose percentage incidence was sharply reduced), more modest on the myeloid cells, and still lower on the plasma cells. Nine days later the plasmocytomatous infiltrate was reduced as compared to before therapy, while the aliquot of hemopoietic cells was restored. At this time the mitotic index of plasma cells, but not that of the hemopoietic cells, was higher than before VCR administration. These findings suggest that the tumor mass reduction by VCR is followed by plasma cell recruitment, which is in progress 9 days after the drug administration. On the contrary, the regeneration of the hemopoietic cells has repopulated the bone marrow and is already exhausted in this lag time. It is hypothesized that VCR administrations given at about 9 day intervals are more and more effective on the recruited plasma cells, owing to the phase S-specificity of the drug. The regeneration of the hemopoietic cells is protected by this time interval.