In vivo genotoxkinetics: an alternative to study pharmacokinetics and pharmacodynamics of antineoplastic agent busulfan

P Morales-Ramirez, PhD

Instituto Nacional de Investigaciones Nucleares, Mexico D.F., Mexico

AIM. The kinetics of the genotoxic activity (genotoxkinetics) was used to infer some pharmacokinetic and pharmacodynamic data of busulfan in mice in vivo. METHODS. The kinetics of induction of micronuclei in polychromatic erythrocytes (MN-PCE) and of DNA-damaged cells determined by the single cell gel electrophoresis assay (comet frequency) in leukocytes was used with this aim. RESULTS. MN-PCE induction indicates that busulfan produces micronuclei by two different dose dependent mechanisms, one of early, another more efficient one of late (48 h) expression. The late expression mechanism correlates with the dramatic increase in cytotoxicity. The latency periods (LP), the times of effective activity (TEA) and the half lives (HL) were 5.7 and 21.4 h; 5.2 and 4.0 h; 2.6 and 2.0 h for the early and the late mechanisms, respectively; only the LP exhibits an important difference. The DNA-damaged-cells induction indicates that doses between 16-30 Ýmoles/ g of body weight cause a response proportional to dose in leukocytes, which could be detected a few minutes after exposure, suggesting a rapid increase in the busulfan levels in the blood stream. Busulfan doses of 45 Ýmoles/ g of body weight or higher completely inhibit comet production. This indicates that dose of 30 Ýmoles/g mainly induce alkali labile sites, and that dose of 45 Ýmoles/g increases the frequency of DNA-protein or interstrand crosslinks, which inhibit the formation of comets. CONCLUSIONS. The results suggest that dramatic late MN induction, comet inhibition and cytotoxicity, are caused by a critical dose effect on DNA sensitization or lesion transformation, which in turn could explain the narrow therapeutic window of busulfan.

KEY WORDS: micronucleus, comet assay.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Novel Therapies, Part 1.