Effects of chlorambucil on human chromosomes

No significant amount of chromosomal damage was found in the 48-h cultures of lymphocytes of 18 patients who had been treated with the bifunctional alkylating agent chlorambucil (CBC). However, there was suggestive evidence of chromatid damage (i.e. of types attributable to damage during or after DNA synthesis in the cell cycle). In marrow cells of 3 patients given a single large dose of chlorambucil (equivalent to 2 days' normal treatment) there was also suggestive evidence of induced chromatide-type damage.Extensive series of in vitro experiments yielded evidence that (a) exposure of human lymphocytes over the whole period of culture showed chromatid-type damage; (b) this damage increased sharply from concentrations of 0.5 μg/ml to3.0 μg/ml; (c) although chromatide-type damage always predominated, there was suggestive evidence also of chromosome-type aberrations attributable to damage occuring in the G₀/G₁ period, although some or all of this could be attributed to “derived” chromatid damage; (d) even if lymphocytes were only exposed during the G₀ or G₁ periods of the cycle, damage was found in the subsequent metaphases and it was almost entirely of the chromatid type; (e) much more damage occurred in lymphocytes exposed for varying periods to the drugs after stimulation by phytohaemagglutinins than in those exposed in whole blood, or in medium before stimulation; (f) damaged occurred in lymphocytes exposed to the drug while in S but not exposed only when in G₂; (g) no evidence was found that unschaduled DNA synthesis during G₀ or G₁ was induced by the drug; (h) there appeared to be no delay caused by the drug in the time at which cells reached the first “S” phase in culture but there was some evidence consistent with prolongation of “S” in cells exposed in culture; (i) there was evidence that CBC alone could stimulate lymphocyte tto DNA synthesis, and that a few cells proceeded in the cycle to prophase, or even metaphase. However, there was a considerable amount of cell-killing during CBC-stimulated DNA synthesis.