(P 098) COMBINATION THERAPY WITH IMATINIB AND PAC-1 INDUCES TREATMENT SYNERGISM IN GASTROINTESTINAL STROMAL TUMORS

Objective: Imatinib is an effective treatment for gastrointestinal stromal tumors (GISTs). However, the majority of GISTs will at some point develop drug resistance. Combining imatinib with an additional treatment is a rational strategy to reduce dependence on c-Kit inhibition and overcome imatinib resistance. Our lab has previously reported synergistic increase in apoptosis with a BH-3 mimetic. PAC-1 is a small molecule that directly activates procaspase-3 and induces apoptosis of cancer cells. Thus, in order to mechanistically enhance apoptosis, we examined the antitumor effects of procaspase activating compound-1 (PAC-1), alone and in combination with imatinib.

Methods: GIST-T1 and GIST-882 cells were treated with PAC-1, imatinib, or combination therapy. We determined the anti‐proliferative and apoptotic effects by cellular morphology, cell viability assay, flow cytometric apoptosis and cell cycle analysis, and immunoblotting. The synergistic effects of PAC-1 and imatinib treatment were assessed over a dose–response matrix. The effects of the drug combination were estimated using SynergyFinder.

Results: PAC-1 inhibited proliferation and induced apoptosis of GIST-T1 and GIST-882 in a dose dependent manner, as evidenced by cell viability assays, apoptosis assays, caspase activation, PARP cleavage, and cell morphology. Treatment with both, imatinib and PAC-1, decreased cell viability by 71% in GIST-T1 compared to only 30% (p<.0001) and 46% (p<.01) with imatinib or PAC-1 treatment, respectively. In GIST-882, the combination decreased cell viability by 66% compared to only 39% by either imatinib or PAC-1 treatment (p<.01). Combination treatment of GIST-T1 and GIST-882 led to induction of apoptosis as evidenced by a significant increase of cells in sub-G1 phase (p<.001 in GIST-T1 and p<.05 in GIST-882) compared to single therapy. Combination therapy decreased the phosphorylation of c-Kit, Akt and Erk compared with imatinib alone, indicating that PAC-1 and imatinib work in synergy to inhibit pro-survival signaling in GIST cells. Synergy maps revealed the strongest synergistic effects (synergy scores >10) when imatinib and PAC-1 were combined at concentrations around their IC50s.

Conclusion: We show that imatinib combined with PAC-1 work synergistically to inhibit proliferation and induce apoptosis in the treatment of GIST-T1 and GIST-882 cells. These findings provide preclinical evidence for future clinical trials in GIST treatment.

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