Abstract PO029: Development of a translatable targeted therapy-resistant melanoma model
Authors: Chacon Alexander , Jackson Katherine , Qin Shuyang , Williams Alyssa , Jewell Rachel , Prieto Peter 
Published: February 2021

Abstract
Background: Resistance to BRAF V600E inhibitors (dabrafenib) and MEK inhibitors (trametinib) occurs in 6-8 months in the majority of melanoma patients with the target V600E mutation. For this reason, combination targeted therapy (TT) offers reliable but often non-durable responses. A translatable model exhibiting de novo resistance to TT with response to checkpoint inhibition (ICB) may offer an avenue for exploring mechanisms of TT resistance, responses in combined TT/ICB, and modes of treatment salvage.
Methods: C57/BL6J mice flanks were injected with Yumm 1.7 melanoma. After 14 days, mice were subjected to TT with dabrafenib/trametinib chow. A select few tumors (3/40) exhibited resistance to TT. Tumors from resistant mice were harvested, morcellized, and cultured to isolate a Yumm 1.7 cell line (Y1.7 BrafR) with a resistant phenotype and passaged to ensure uniform clonality. Y1.7 BrafR was assessed for growth characteristics, morphology, and response to dabrafenib/trametinib in vitro. In a synchronous mouse melanoma model, Yumm 1.7 and Y1.7 BrafR were injected into opposing flanks, allowed 14 days growth, and mice were subjected to either TT, ICB (aPD-1/aCTLA-4 combined therapy), TT + ICB, or vehicle for 5 days.
Results: Y1.7 BrafR exhibited in vitro resistance to dabrafenib at 0.1, 1, and 10uM after 72 hr exposure (12%, 32%, and 47% growth depletion vs untreated cells, Yumm 1.7 with 67%, 61%, and 84%). This effect was not noted with trametinib except at 10uM (84% vs 92% growth depletion). Both lines succumbed to combination TT of varying doses by 72 hours. Y1.7 Braf R exhibits a doubling time of 18.14 hr vs. 12.35 hr for Yumm 1.7, with no appreciable differences in morphology on 40x light microscopy, as both lines extend dendritic processes while forming dense nests. Despite slower in vitro growth, Y1.7 BrafR exhibits markedly greater growth in vivo after 14 days incubation and 5 days treatment (1.16 v 0.44 cm^3 in vehicle mice), with less response to TT chow (51% tumor volume depletion from vehicle vs. 90% in Yumm1 .7) and TT/ICB (data not shown) and greater response to ICB (15% volume depletion vs 24% volume increase in Yumm 1.7). All data are statistically significant (2-tail t-test, p&lt;0.05).
Conclusions: Our results show the development of a TT-resistant mouse melanoma model. This model is clinically relevant for patients who have initially potent, but non-durable responses to TT. Future studies will focus on mechanisms of dabrafenib resistance, tumor nutrition and metabolism, modes of salvage, and efficacy of T-cell recruitment to the tumor microenvironment in this de novo TT-resistant line.
Citation Format: Alexander Chacon, Katherine Jackson, Shuyang Qin, Alyssa Williams, Rachel Jewell, Peter Prieto. Development of a translatable targeted therapy-resistant melanoma model [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr PO029.