Abstract 4674: Vitamin D enhances erlotinib response in EGFR-mutant non-small cell lung cancer

Background. Despite remarkable initial therapeutic responses, most patients develop resistance to the first generation of EGFR tyrosine kinase inhibitors (TKIs), erlotinib and gefitinib, within one year of treatment initiation. Epithelial-mesenchymal transition (EMT) is one of the key mechanisms of resistance to EGFR TKIs. We recently demonstrated that vitamin D promotes epithelial phenotype in NSCLC cells. We hypothesized that we could prevent/overcome EMT-driven resistance by combining vitamin D with EGFR TKIs. Methods. EGFR-mutant, erlotinib sensitive HCC827 cells were treated with: vehicle control, 100nM 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), 1ng/ml TGF-β, or 1,25(OH)2D3+TGF-β for 14 days. Expression of EMT markers was determined by q-RT PCR and immunoblot. Sensitivity to erlotinib was analyzed by MTT assay. For the in-vivo study, HCC 827 tumor xenografts were established in vitamin-D deficient mice. Mice were stratified based on tumor volume to receive diets containing 25 IU vitamin D3/kg (deficient) or 10,000 IU vitamin D3/kg (supplemented). Erlotinib treatment (12.5 mg/kg) was initiated two weeks after the diet switch and administered 5 days per week for the duration of the study. Tumors were collected two weeks after the diet switch (baseline), 48 hours after the erlotinib initiation (acute response) and upon treatment failure (outgrowth). Results. Vitamin D attenuated expression of mesenchymal markers in the presence of TGF-β in-vitro and opposed TGF-β driven resistance to erlotinib. Erlotinib EC50 values were 9.85 nM, 122 nM and 3.83 nM for vehicle, TGF-β, and 1,25(OH)2D3+TGF-β treated cells, respectively. In-vivo, vitamin D supplementation enhanced acute response to erlotinib. We documented a greater decrease in tumor volume after 48h of erlotinib initiation (1.1% of baseline ±19.3% in vitamin D deficient diet and 25.9% ±29.5% in supplemented diet). Greater efficacy was accompanied by increased suppression of STAT3 phosphorylation and decreased vimentin expression in the vitamin D supplemented group. Although we observed a trend towards a longer progression-free survival in animals maintained on vitamin D supplemented diet (8 vs 5.5 weeks), statistical significance was not achieved. The outgrowth tumors in both groups maintained their epithelial phenotype but exhibited a significant increase in the expression of AXL tyrosine kinase. However, outgrowth tumors from the vitamin D supplemented group had lower AXL expression than those from the vitamin D deficient group. Conclusion. Vitamin D supports epithelial phenotype in-vitro and enhances acute response to erlotinib in-vivo, possibly, by suppressing STAT3 signaling. Vitamin D supplementation also attenuates overexpression of AXL tyrosine kinase in erlotinib resistant tumors. The clinical implication of our work is that patients diagnosed with EGFR mutant NSCLC who receive erlotinib may benefit from vitamin D supplementation. Citation Format: Tatiana Shaurova, Suzanne F. Shoemaker, Pamela A. Hershberger. Vitamin D enhances erlotinib response in EGFR-mutant non-small cell lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4674.