PARP Inhibitor as Adjuvant Therapy for Glioblastoma

Yongchang (Ryan) Jang

Introduction. Glioblastoma Multiforme (GBM) is the deadliest form of brain cancer in adults which is also the most common form of malignant CNS tumor.7,11 Though the therapeutic advances, prognosis for GBM is still poor with 15-month median overall survival.6,10 Temozolomide, a common anti-neoplastic agent for GBM, induces DNA damage including O6-Methlguanine, N7-Methylguanine, and N3-Methylguanine.4 Poly (ADP-Ribose) Polymerase (PARP), a family of nuclear proteins, repairs these induced damages by serving as a scaffold to recruit base excision repair components such as XRCC1 and DNA polymerase β; PARP is also known to ADP-ribosylate other DNA repair proteins.1,4,8,9 PARP-1 gene is expressed at a higher rate in many malignancies including CNS cancers including GBM.4,5 To prevent this excessive DNA repair machinery, PARP inhibitors (PARPi) are under investigation as adjuvant therapies to traditional antineoplastic agents.3,9 The molecular mechanism of PARPi is to sequester PARP on chromatin preventing it from being disassociated.2,4,8 Talazoparib was the PARP inhibitor of choice to assess its ability to sensitize the therapeutic effect of temozolomide in GBM. Methods. U251 and T98G glioma cell lines were studied by administering variable doses of Temozolomide/Talazoparib and Western blot analyses and Ki-67-specific immunofluorescence were used. Then, MGMT promoter hypermethylated GBM12 line was studied focusing on neurosphere formation. In vivo efficacy of talazoparib was studied using MGMT promoter hypermethylated GBM12 line in heterotopic and orthotopic xenografts. Finally, Mdr1a/b-/-, Bcrp1-/-, and Mdr1a/b-/-Bcrp1-/- triple knockout mice were used to assess the efflux liability in the blood-brain barrier. Results. High concentrations of talazoparib were required to sensitize the effect of temozolomide in GBM cell lines; U251 cell lines required higher concentrations of talazoparib for enhanced temozolomide cytotoxicity than T98G.4 In vitro GBM12 line study showed decrease in neurosphere formation with temozolomide monotherapy and the addition of talazoparib further reduced neurosphere formation by 55-70%.4 Heterotopic GBM12 xenograft and orthotopic GBM12 xenograft models treated with talazoparib/temozolomide showed different results: flank xenografts showed significant increase in survival; however, intracranial xenografts did not show significant difference.4 MKO and TKO mice models showed significantly higher concentration of talazoparib in brain tissues.4 Conclusions. The relatively high dose of talazoparib can be clinically problematic. Also, the poor sensitizing efficacy of talazoparib in orthotopic xenografts is an issue. Mdr1-driven efflux pump is crucial for talazoparib resistance based on data from MKO, BKO, and TKO mice.4 Its low penetration into CNS may be further explained by increased intracellular accumulation talazoparib administration of Mdr1 inhibitor.

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