Despite significant advances in the understanding and managing gastroenteropancreatic neuroendocrine tumors (GEPNETs), the therapeutic success of treatments remains relatively poor. Recent FDA approval of Peptide Receptor Radionuclide Therapy (PRRT) has paved the way for radiation-based treatment for GEPNETs. PRRT delivers high doses of radiation to tumor cells in the body to destroy cells or slow their growth. The DNA-dependent protein kinase (DNA-PK) complex plays a pivotal role in non-homologous end-joining (NHEJ) repair after radiation therapy. A novel, clinical-stage DNA-PK inhibitor, M3814, potently and selectively blocks the NHEJ repair pathway for DNA double strand breaks (DSB). Here, we investigated the feasibility of radiosensitizing neuroendocrine (NET) cells with the DNA-PK inhibitor M3814, both in vitro and in preclinical NET models.

Methods: 96-well clonogenic assays were used to evaluate efficacy of M3814 in combination with radiation therapy (XRT) in vitro in QGP-1 and BON pancreatic NET cell lines. DSB-induced DNA repair was visualized with confocal laser scanning microscopy using γ-H2AX immunofluorescence after M3814 treatment. Western blot was used to confirm activation of DNA-PKcs (pDNA-PKcs S2056) after M3814 treatment. The efficacy of M3814 in combination with XRT was evaluated in QGP-1 and BON human xenograft models in athymic nude mice. Tumor cells were injected subcutaneously into athymic nude mice, and treatment started when palpable tumors (∼200 mm3) were established. M3814 was given orally (100 mg/kg) 30 min prior to XRT, which was applied using a radiation therapy device calibrated to deliver 2 Gy daily for 4 d.

Results: Clonogenic assays demonstrated lack of cytotoxicity of M3814 alone at doses below 1000 nM and a strong radiosensitizing effect at 500 nM in both QGP-1 and BON cell lines. Immunofluorescence analysis revealed high numbers of γH2AX foci, marking unrepaired DSBs inside the micronuclei. Western blot showed that activation of DNA-PKcs, induced by XRT treatment, was quickly suppressed by M3814 treatment.

In combination with XRT, M3814 showed efficacy in both QGP-1 and BON xenografts as demonstrated by a strong potentiation of the effect of XRT. In all models, a dose of 2 Gy administered daily for 4 d, in combination with M3814, induced a significant tumor growth inhibition compared to XRT alone. M3814, alone or in combination with XRT, did not induce significant weight loss or visual signs of toxicity in the mice in any of the studies.

Conclusions: Our results demonstrate a benefit of adding M3814 to XRT in NET treatment. Selective DNA-PK inhibition by M3814 provides a potent therapeutic strategy for disruption of NHEJ DSB repair and may offers a novel therapeutic approach in advanced NET.

Citation Format: Piotr Rychahou, Aman Chauhan, Zeta Chow, Tadahidi Izumi, Quan Chen, Eun Y. Lee, Dana Napier, Lowell B. Anthony, Michael J. Cavnar, Charles Kunos, B. Mark Evers. DNA-PK inhibitor, M3814, as a radiation sensitizer in the treatment of neuroendocrine tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6402.