Preclinical In Vivo Evaluation Of A Novel Treatment Strategy Combining A Wee1 Inhibitor With Radiotherapy

10596 Background: Wee1 regulates G2 checkpoint signaling by phosphorylating and inactivating cyclin-dependent kinase1 (Cdc2). Interference with this checkpoint is hypothesized to render p53 deficient cells, which lack a G1 checkpoint, more susceptible to DNA damaging agents by not allowing for sufficient DNA repair before entering mitosis. A novel Wee1 inhibitor (MK-1775) sensitized p53 deficient cells to in vitro cytotoxicity of gemcitabine, cisplatin, and carboplatin and increased in vivo antitumor effects (Mol Cancer Ther, 8:2992, 2009). The present study examined whether Wee1 inhibition improves radioresponse of human lung cancer models. Methods: Xenografts (p53 wild type [wt] A549, p53 mutant H1299 and p53 null Calu6) were generated in the leg of nude mice by i.m. injection of 1- 5 X 106 tumor cells grown in vitro. Tumors were measured twice weekly and treatments started at 8mm tumor diameter. Irradiation (IR) was given 24h before Wee1 inhibitor (gavage BID separated by 3h for 5 days). When fractionated (Fx) IR (2 or 5Gy for 5 days) was used, the 2nd-5th exposures were given 1h following the first dose of Wee1 inhibitor. Treatment endpoints included tumor growth delay (TGD), tumor cure (TCD50), and immunohistochemistry (IHC). Statisticalanalyses included Student's t-test for TGD and IHC and likelihood ratio test for TCD50. Results: Wee1 inhibition combined with single or Fx IR increased TGD of p53 deficient (but not p53 wt) tumors supra-additively. The best combination consisted of Fx IR combined with Wee1 inhibitor (Calu6 TGD enhancement factor [EF] 2.04, P 0.03). This schedule also improved curability of Calu6 by EF of 1.18 (Fx IR TCD50 41.2Gy reduced to 35Gy, P 0.05). Treatment was associated with rapid transient downregulation of pCDC2, upregulation of pChk1, pHH3 and γH2AX, and increased mitoses followed by mitotic catastrophe. Mechanistically, both cell cycle deregulation and inhibition of DNA repair likely contributed to the observed synergy. Conclusions: This novel Wee1 inhibitor (MK-1775) significantly enhanced IR response of p53 deficient human tumor xenografts in preclinical studies and shows high potential for translation to clinical trial(s) in combination with radiotherapy. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Merck Merck