In vitro activity of ABT-888 (A), 5-FU (F), and dacarbazine (D) in neuroendocrine tumors (NET).
236 Background: Low-grade neuroendocrine tumors (NET) have few cytotoxic chemotherapy options. Data suggests that a combination of temozolomide and a fluoropyrimidine has clinical efficacy. ABT-888 is a novel poly-ADP ribose polymerase (PARP) inhibitor that has been tested in a phase 1 setting with temozolomide, with synergy demonstrated in preclinical models of other tumors. We proposed an in-vitro study of ABT-888 with varying concentrations of dacarbazine (D), and 5-FU (F) on human neuroendocrine cells (BON). Methods: BON cells were incubated with varying concentrations and combinations of ABT-888 (2.5, 5 and 10uM), F (25-100uM), and D (25-100uM) for 96 hours. After incubation, cell growth was measured by MTT rapid colorimetric assay with absorbance reported as mean % control. Western blot analysis was performed for chromogranin A, PARP, γH2AXand XIAP to assess for cell death and on-target effects for the population treated with 5-FU. Combination indices (CI) were calculated using the Chou-Talalay method using CompuSyn. CI’s below 1 signified synergy. Results: ABT-888 alone did not demonstrate any antitumor effect (103%). D alone had antitumor effect (74% at 50uM, 67% at 100uM) which was improved by adding ABT-888 (70% at 50uM D; CI 0.73; p=0.06, 60% at 100 uM D; CI 0.88; p=0.0003). F alone had antitumor effect (82% at 50uM and 71% at 100uM) which was improved by adding 2.5uM ABT-888 (71% at 50uM of F; CI=0.59, and 58% at 100uM of F; CI=0.74), and further enhanced with 5uM of concomitant ABT-888 (56% at 50uM of F; CI=0.68, and 49% at 100uM of F; CI=0.76). Western analysis of lysates showed markers of increased apoptosis, decreased PARP, and decreased expression of CgA. The combination of F+D did not demonstrate increased cytotoxicity with the addition of ABT-888 (58% with/without ABT-888 p=0.61). Conclusions: ABT-888 demonstrated in vitro synergy against BON cells with F or D. The combination of all three compounds (A+F+D) did not demonstrate synergy above F+D. Synergy was statistically significant with increasing doses of cytotoxic compounds which are achievable in vivo with current doses of A, F, and D. The combination of ABT-888 with temozolomide or a fluoropyrimidine merits further study in human clinical trials.