The aim of the present study was to determine in vivo the kinetics of micronucleated polychromatic erythrocyte (MN-PCE) induction in mice, as an approach for studying the mechanism of micronuclei induction by mitomycin C, cis-diamine dichloroplatinum, busulfan and bis-chloroethylnitrosourea, bifuctional alkylating antineoplastic agents having different patterns of crosslink induction. The kinetics of MN-PCE induction was established by scoring the frequency of MN-PCE in 2000 PCE in peripheral blood, for periods of 8 or 10 h after acute treatment and up to 80 h, with different doses of the agent. The kinetics of MN-PCE induction and particularly the times of maximal induction by different bifunctional alkylating agents were compared with the kinetics previously obtained for ethylnitrosourea, methylnitrosourea and 6-mercaptopurine, agents that cause MN-PCE mainly in the first, second and third divisions after exposure, respectively. The results obtained in the present study allow us to conclude that: (i) bifunctional alkylating agents have very different efficiencies of genotoxic and cytotoxic action; (ii) all assayed bifunctional alkylating agents induced micronuclei during the first cell division, owing to the mistaken repair of primary lesions, e.g. excision; (iii) busulfan and bis-chloroethylnitrosourea showed an additional late mechanism of micronuclei induction, which is expressed at the third division and seems to be related to the mismatch repair process.

The aim of the present study was to correlate the time-response curves of micronucleated polychromatic erythrocyte (MN-PCE) induction by 5-azacytidine (azaC) with the possible processes involved in DNA break production; this is based on the results previously published by other authors. The MN-PCE induction at two different doses of azaC was determined by sampling blood from the tails of mice before the acute treatment and over nine periods of 8 h each afterwards. Both doses caused two peaks of MN-PCE induction, one at 32 h and another at 48 h, approximately; a shoulder was detected that remained high from 56 h up to the end of the study (72 h). These results suggest that azaC induced DNA breaks and subsequently MN (micronucleus) by three different mechanisms, and in agreement with data in the literature, these could be successively the following: (i) during excision of the large adduct comprising the DNA methyl transferase covalently linked to DNA; (ii) failure of recombination repair or mismatch repair; and (iii) persistent chromosome fragility in G-C rich sites due to DNA demethylation and chromatin decondensation.

A correlation analysis indicated that dFdC appears to induce MN-PCEs through only one mechanism or mechanisms that occur concomitantly, which could be explained by the previously reported concurrent inhibitory effects of dFdC on DNA polymerase alpha, polymerase epsilon, and/or topoisomerase. The timing of maximal cytotoxicity was correlated with the timing of maximal genotoxicity; however, an early cytotoxic effect that appeared to occur prior to the incorporation of dFdC into DNA was likely related to a previously reported inhibitory effect of dFdC on thymidylate synthase and/or ribonucleotide reductase.

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