Gene-specific formation and repair of DNA monoadducts and interstrand cross-links after therapeutic exposure to nitrogen mustards.

Abstract

To investigate the possibility of measuring the gene-specific DNA damage after therapeutic exposure to nitrogen mustards and to examine its relationship with the clinical response. The kinetics of gene-specific monoadducts and interstrand cross-link formation/repair were measured in the p53 and N-ras genes. DNA extracted from human peripheral lymphocytes following in vitro exposure to melphalan or therapeutic exposure to melphalan or cyclophosphamide was used. When lymphocytes were treated in vitro with biologically relevant doses of melphalan, monoadducts accumulated rapidly in both p53 and N-ras genes, reaching maximal levels within 2 h, whereas the highest interstrand cross-link levels were found within 8 h. Thereafter, the adducts were repaired with half-lives of 14.5 +/- 0.3 h (p53) or 18.8 +/- 1.5 h (N-ras) for monoadducts and 12.4 +/- 0.8 h (p53) or 14.1 +/- 2.2 h (N-ras) for interstrand cross-links. Moreover, peak levels of monoadducts in both genes were observed 2 h after treatment in peripheral leukocytes from patients with multiple myeloma treated with high-dose i.v. melphalan, supported by autologous stem cell transplantation, whereas interstrand cross-links were maximal within 8 h. Of seven patients examined, the three who showed the least levels of DNA damage did not respond to the high-dose melphalan. This is the first report showing that it is feasible to measure gene-specific DNA damage in a readily accessible tissue of humans exposed to bifunctional alkylating drugs and to examine, at the level of the individual patient, the relationships between the induction/repair of cytotoxic DNA damage and clinical response or long-term complications.