Effects of single or split exposure of leukemic cells to temozolomide, combined with poly(ADP-ribose) polymerase inhibitors on cell growth, chromosomal aberrations and base excision repair components

Purpose: To evaluate the antitumor activity of single versus split exposure of neoplastic cells to temozolomide (TZM) and poly(ADP-ribose) polymerase (PARP) inhibitor. Methods: A leukemic Jurkat cell line and freshly isolated leukemic blasts were used. Jurkat cells are resistant to O-6-methylguanine damage induced by TZM due to high levels of O-6-alkylguanine-DNA alkyltran sferase and to a functional defect in the mismatch repair system. Cells wer e treated with 3-aminobenzamide or with NU1025 to inhibit PARP activity. TZ M was added to cell cultures immediately after PARP inhibitors. The concent rations of TZM used were 62.5 CIM (corresponding to the peak plasma concent ration in patients) or 125 muM. Treatment design: Cells were treated with 1 25 muM TZM plus PARP inhibitors (single exposure), or twice with 62.5 muM T ZM plus PARP inhibitors with an interval of 24 h between treatments (split exposure). Tumor cell growth, clastogenicity and base excision repair gene transcripts or enzymatic activity were evaluated. Results: The split exposu re of Jurkat cells to TZM induced more pronounced and persistent growth inh ibition and comparable chromosome damage in comparison with the single expo sure. In addition, PARP inhibitors potentiated the cytotoxic effects induce d by repeated treatment with TZM in fresh leukemic blasts. A marked decreas e in X-ray repair cross-complementing I transcript and methylpurine glycosy lase (MPG) transcript was detected in Jurkat cells subjected to the split e xposure. In this case, a significant reduction in the corresponding enzymat ic activity was also observed. Conclusions: Cytotoxicity induced by TZM and PARP inhibitors can be improved by a fractionated modality of drug treatme nt. The reduction in MPG transcript and function would presumably contribut e to an increase in cell susceptibility to DNA damage induced by the methyl ating agent and PARP inhibitors.