The selective PARP inhibitor, CEP-8983 exhibits significant chemosensitization in combination with TMZ and SN38 against chemoresistant tumor cell lines and xenografts

5109 Poly (ADP-ribose) polymerase 1 (PARP-1) a nuclear zinc finger DNA binding protein, is activated by and implicated in DNA repair. Inhibition of PARP by small molecules and through knockouts causes cells to become hypersensitive to various chemotherapeutic agents. The ability of CEP-8983, a potent and selective PARP-1 and 2 inhibitor (enzyme IC50 19 nM and 6 nM, respectively; cell assay for attenuation of H2O2-induced NAD+ depletion IC50 300 nM), to enhance the anti-tumor efficacy of temozolomide (TMZ), SN38 and/or camptothecin (CPT) against chemo-resistant tumor cell lines was evaluated in vitro. We also evaluated the ability of CEP-8983 delivered as a prodrug to potentiate the anti-tumor efficacy of TMZ against TMZ-resistant RG2 rat glioblastoma xenografts in nude mice. Co-incubation of CEP-8983 with TMZ or SN38 significantly potentiated tumor cell growth inhibition as compared to treatment with TMZ or SN38 alone in NB1691 human neuroblastoma or HT29 human colon carcinoma cells, respectively. CEP-8983 (1 μM) potentiated the fraction of and/or duration of time RH18 rhabdomyosarcoma or HT29 cells accumulated in TMZ and SN38-induced G2/M arrest, respectively. In addition, CEP-8983 (1 μM) potentiated CPT-induced DNA damage in HT29 cells over 72 hours. CEP-8983 was evaluated in a validated in vitro CFU-GM colony formation assay to determine if it potentiated the myelotoxicity associated with TMZ or TPT treatment2. Co-incubation of human peripheral blood mononuclear cells with CEP-8983 (1 or 3 μM) and TMZ or TPT did not result in potentiation of chemotherapy-associated toxicity, nor did CEP-8983 have intrinsic hematotoxic effects. Administration of a prodrug of CEP-8983 (100 mg/kg/dose s.c., QD x 5 days) one hour after administration of TMZ (68 mg/kg/dose p.o., QD x 5 days) inhibited significantly the growth of RG2 tumors compared to TMZ monotherapy with a maximum inhibition of tumor growth of 60% relative to TMZ monotherapy. Plasma analysis revealed sustained levels of CEP-8983 (40 μM) substantially above the cell-based IC50 throughout the dosing period. The combination of TMZ and CEP-8983 prodrug was well tolerated with only transient and reversible body weight loss observed. An ELISA using PAR-specific antibodies demonstrated CEP-8983 prodrug dose dependently attenuated in vivo TMZ-induced PAR accumulation in glioma xenografts. These combined data indicate that the selective PARP inhibitor, CEP-8983 prodrug, attenuates in vivo TMZ-induced PARP activity and results in sustained and significant chemosensitization of TMZ and SN38 in chemotherapy-resistant human and rodent tumor cell lines and xenografts. 2 Parchment R, et al. Annals of Oncology 9:357-364, 1998.