CELL KINETIC ALTERATIONS IN NORMAL AND NEOPLASTIC CELL POPULATIONS IN VITRO AND IN VIVO FOLLOWING VINCRISTINE *. A REPLY TO DR CAMPLEJOHN'S REVIEW ARTICLE †

It is shown that the lethal action of vincristine (VCR) is dose‐dependent and may occur at interphase and mitosis. In general, the VCR dose used to destroy cells must be approximately ten times higher than that used to arrest cells in mitosis at metaphase. There is strong evidence that cells can survive metaphase arrest by a sublethal dose of VCR either completing cytokinesis normally after metabolism of the drug or becoming polyploid because of an impaired mitotic spindle apparatus. These cells are not doomed to die, at least in some cell systems. Furthermore, there is strong evidence in three animal tumour systems (transplantable and autochthonous tumours) that VCR is able to induce in vivo partial synchronization of proliferating tumour cells and/or recruitment of resting cells into the proliferating compartment. Failures to induce partial synchrony in cell populations by VCR may be attributed to resistance to VCR or cytolysis or slow proliferation of cells in badly vascularized tumours. Chemotherapy after synchronization seems to be effective as shown by non‐randomized trials in bad‐risk patients with solid tumours and acute leukaemias. In a randomized co‐operative trial results of the two‐drug synchronization protocol in patients with non‐Hodgkin's lymphoma of high grade malignancy were statistically better than those of a four‐drug protocol (COPP) established empirically. The two‐drug protocol was equally effective as the four‐drug protocol in Hodgkin's disease. Side‐effects were less pronounced with the so‐called synchronization scheme.