Enhanced radiosensitization of human glioma cells by combining inhibition of poly(ADP-ribose) polymerase with inhibition of heat shock protein 90.

Glioblastoma multiforme (GBM) are the most common primary brain tumor and are resistant to standard therapies. The nondividing nature of normal brain provides an opportunity to enhance the therapeutic ratio by combining radiation with inhibitors of replication-specific DNA repair pathways. Based on our previous findings that inhibition of poly(ADP-ribose) polymerase (PARP) increases radiosensitivity of human glioma cells in a replication-dependent manner and generates excess DNA breaks that are repaired by homologous recombination (HR), we hypothesized that inhibition of HR would amplify the replication-specific radiosensitizing effects of PARP inhibition. Specific inhibitors of HR are not available, but the heat shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) has been reported to inhibit HR function. The radiosensitizing effects of 17-AAG and the PARP inhibitor olaparib were assessed, and the underlying mechanisms explored. 17-AAG down-regulated Rad51 and BRCA2 protein levels, abrogated induction of Rad51 foci by radiation, and inhibited HR measured by the I-Sce1 assay. Individually, 17-AAG and olaparib had modest, replication-dependent radiosensitizing effects on T98G glioma cells. Additive radiosensitization was observed with combination treatment, mirrored by increases in gammaH2AX foci in G(2)-phase cells. Unlike olaparib, 17-AAG did not increase radiation sensitivity of Chinese hamster ovary cells, indicating tumor specificity. However, 17-AAG also enhanced radiosensitivity in HR-deficient cells, indicating that its effects were only partially mediated by HR inhibition. Additional mechanisms are likely to include destabilization of oncoproteins that are up-regulated in GBM. 17-AAG is therefore a tumor-specific, replication-dependent radiosensitizer that enhances the effects of PARP inhibition. This combination has therapeutic potential in the management of GBM.

Replication-dependent radiosensitization of human glioma cells by inhibition of poly(ADP-Ribose) polymerase: mechanisms and therapeutic potential.

PURPOSE: Current treatments for glioblastoma multiforme are inadequate and limited by the radiation sensitivity of normal brain. Because glioblastoma multiforme are rapidly proliferating tumors within nondividing normal tissue, the therapeutic ratio might be enhanced by combining radiotherapy with a replication-specific radiosensitizer. KU-0059436 (AZD2281) is a potent and nontoxic inhibitor of poly(ADP-ribose) polymerase-1 (PARP-1) undergoing a Phase II clinical trial as a single agent. METHODS AND MATERIALS: Based on previous observations that the radiosensitizing effects of PARP inhibition are more pronounced in dividing cells, we investigated the mechanisms underlying radiosensitization of human glioma cells by KU-0059436, evaluating the replication dependence of this effect and its therapeutic potential. RESULTS: KU-0059436 increased the radiosensitivity of four human glioma cell lines (T98G, U373-MG, UVW, and U87-MG). Radiosensitization was enhanced in populations synchronized in S phase and abrogated by concomitant exposure to aphidicolin. Sensitization was further enhanced when the inhibitor was combined with a fractionated radiation schedule. KU-0059436 delayed repair of radiation-induced DNA breaks and was associated with a replication-dependent increase in gammaH2AX and Rad51 foci. CONCLUSION: The results of our study have shown that KU-0059436 increases radiosensitivity in a replication-dependent manner that is enhanced by fractionation. A mechanism is proposed whereby PARP inhibition increases the incidence of collapsed replication forks after ionizing radiation, generating persistent DNA double-strand breaks. These observations indicate that KU-0059436 is likely to enhance the therapeutic ratio achieved by radiotherapy in the treatment of glioblastoma multiforme. A Phase I clinical trial is in development.

Anti-trypanosomal activities of DNA topoisomerase inhibitors.

Only four drugs are available for chemotherapy of human African sleeping sickness with undesirable toxic side effects. The development of new anti-trypanosomal drugs is, therefore, urgently required. In this study, 15 DNA topoisomerase inhibitors, including approved anti-cancer drugs, were tested for in vitro activity against bloodstream forms of Trypanosoma brucei and human leukaemia HL-60 cells. All compounds exhibited anti-trypanosomal activity, with ED50 values ranging between 3 nM and 30 microM, and MIC values between 100 nM and >100 microM. The trypanocidal activities of the most effective DNA topoisomerase inhibitors, aclarubicin, doxorubicin and mitoxantrone, were comparable with those of commercial anti-trypanosomal drugs. These data support the use of DNA topoisomerase inhibitors as lead compounds for anti-trypanosomal drug development.