Abstract
BackgroundThe current standard of care for Glioblastoma Multiforme (GBM) consists of fractionated focal irradiation with concomitant temozolomide (TMZ) chemotherapy. A promising strategy to increase the efficacy of TMZ is through interference with the DNA damage repair machinery, by poly(ADP-ribose) polymerase protein inhibition(PARPi). The objective of the present study was to investigate the therapeutic benefit of combination therapy in patient-derived glioma stem-like cells (GSC).MethodsCombination therapy feasibility was tested on established GBM cell lines U373 and T98. We developed an in vitro drug-screening assay based on GSC cultures derived from a panel of primary patient tissue samples (n = 20) to evaluate the effect of PARPi (ABT-888) monotherapy and combination therapy with TMZ. Therapeutic effect was assessed by viability, double stranded breaks, apoptosis and autophagy assays and longitudinal microscopic cell monitoring was performed. O-6-methylguanine-DNA methyltransferase (MGMT) status was determined by methylation assay and protein expression by western blots.ResultsPARPi monotherapy was found to decrease viability by more than 25% in 4 of the 20 GSCs (20%) at 10 μM. TMZ monotherapy at 50 μM and 100 μM was effective in 12 and 14 of the 20 GSCs, respectively. TMZ resistance to 100 μM was found in 7 of 8 MGMT protein positive cultures. Potentiation of TMZ therapy through PARPi was found in 90% (n = 20) of GSCs, of which 6 were initially resistant and 7 were sensitive to TMZ monotherapy. Increased induction of double stranded breaks and apoptosis were noted in responsive GSCs. There was a trend noted, albeit statistically insignificant, of increased autophagy both in western blots and accumulation of autophagosomes.ConclusionPARPi mediated potentiation of TMZ is independent of TMZ sensitivity and can override MGMT(-) mediated resistance when administered simultaneously. Response to combination therapy was associated with increased double strand breaks induction, and coincided by increased apoptosis and autophagy. PARPi addition potentiates TMZ treatment in primary GSCs. PARPi could potentially enhance the therapeutic efficacy of the standard of care in GBM.
Highlights
Glioblastoma Multiforme (GBM) is the most common primary brain tumor for which there is no curative therapeutic option [1]
In accordance with previous reports [18], U373 was sensitive to TMZ monotherapy, whereas T98 was resistant (p>0.05 for both dosages compared to non-treated control)
PARP protein inhibitors (PARPi) addition potentiates TMZ treatment in primary glioma stem-like cells (GSC) To determine the ability of the PARPi ABT-888 to potentiate TMZ efficacy, we evaluated the therapeutic effect of combination therapy in the aforementioned GSC cultures (n=20)
Summary
Glioblastoma Multiforme (GBM) is the most common primary brain tumor for which there is no curative therapeutic option [1]. ADP Ribose Polymerase (PARP) proteins share the ability to transfer an ADP-ribose moïety from nicotinamide adenine dinucleotide (NAD+) to an acceptor protein and facilitate the accumulation of multiple sequential (poly) ADP-ribose units to the preceding one(s), a process termed PARylation These proteins have been demonstrated to be essential in BER, after DNA damage induced by cytotoxic agents. PARP protein inhibitors (PARPi) were demonstrated to enhance therapeutic efficacy of several conventional cytotoxic agents, such as alkylating chemotherapy. One such agent, ABT-888 (Veliparib®), has been demonstrated to primarily bind to PARP-1and PARP-2, and inferiorly to PARP-3 and PARP-4 [8]. The objective of the present study was to investigate the therapeutic benefit of combination therapy in patient-derived glioma stem-like cells (GSC)
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