Purpose: To examine the role of G2-block abrogation, DNA repair inhibition and apoptosis in the enhancement of radiotoxicity by pentoxifylline.
Materials and methods: The influence of pentoxifylline on radiotoxicity was assessed by colony assay in TP53 wild-type Bell and mutant MeWo melanoma, and in TP53 wild-type 4197 and mutant 4451 squamous cell carcinoma (SCC) cell lines. G2-block abrogation was assessed by flow cytometry. Induction of DNA damage and repair was measured over a dose range of 0-100 Gy by constant field gel electrophoresis (CFGE). The Annexin-V binding assay was used to identify apoptotic cells.
Results: Pentoxifylline, when combined with irradiation, significantly increased radiotoxicity in the TP53 mutant MeWo and 4451 cell lines by radiotoxicity enhancement factors of 3 and 14.5 respectively. No radiosensitization was seen in the TP53 wild-type Be11 and 4197 cells. When the drug was added after irradiation at the time of maximum G2-block expression, no radiosensitization was seen in any of the four cell lines. CFGE analyses showed that pentoxifylline effectively suppressed DNA double-strand break (DSB) repair in all four cell lines, as indicated by 20 h repair inhibition factors of 1.4-2.4. Pentoxifylline did not increase apoptosis in any of the four cell lines.
Conclusion: These data suggest that radiosensitization by pentoxifylline is not a consequence of G2-block abrogation alone, but that inhibition of DSB repair plays a role in certain cell types.