LAUSR

Clear cell renal cell carcinoma (ccRCC) is a highly vascularized tumor type, primarily due to loss of the Von Hippel-Lindau (VHL) gene, which is observed in ~88% of ccRCC cases. Deletion of VHL stabilizes hypoxia-inducible factor alpha protein (HIFα) to drive the hypoxic transcriptional response, including induction of VEGF and PDGF2 that mediate tumor angiogenesis. Anti-angiogenic tyrosine kinase inhibitors (TKIs) such as sunitinib and axitinib have demonstrated therapeutic benefit in patients with ccRCC by exploiting the critical dependency of ccRCC tumors on the vasculature for oxygen, nutrients, and growth factors. However, resistance to TKIs is commonly observed, leading to clinical relapse. Here, we report that the combination of the farnesyltransferase inhibitor (FTI), tipifarnib, and any of several anti-angiogenic TKIs results in deeper and more durable responses in the VHL-mutant 786-O CDX model and an RCC PDX model, compared to TKI alone. While the anti-angiogenic TKIs or tipifarnib alone merely slowed or occasionally arrested tumor growth, the TKI-tipifarnib combination induced marked tumor regressions in all treated animals. To investigate the mechanism of action, VHL-mutant RCC cell lines were subjected to hypoxia (1% O2) in vitro to mimic the hypoxic conditions induced by TKIs in vivo and were treated with tipifarnib to evaluate tipifarnib’s impact on signaling pathways in hypoxia-exposed cells. In the Caki2 cell line, hypoxia initially reduced mTOR signaling, but it rebounded strongly after 24 hours in hypoxia, suggesting that mTOR pathway reactivation is a potential mechanism of resistance to TKIs. Addition of tipifarnib blocked hypoxia-induced mTOR reactivation. Mechanistically, tipifarnib potently inhibits the farnesylation, and hence the activity, of an obligate farnesylated protein RHEB, a positive regulator of mTOR, suggesting that the synergy may arise through RHEB inhibition in this model. The mechanistic data in cell lines suggest that tipifarnib’s ability to inhibit mTOR reactivation observed in ccRCC cell lines under hypoxic stress contributes to the enhanced treatment durability observed in vivo. Because FTIs are pleiotropic drugs, there are likely additional cell intrinsic mechanisms that may influence synergy with TKIs. Further in vitro and in vivo studies are underway to establish the scope and mechanistic underpinnings of TKI-tipifarnib effects and strengthen the therapeutic rationale for combining TKIs with tipifarnib in the treatment of patients with ccRCC. Citation Format: Jovylyn Gatchalian, Linda Kessler, Stacia Chan, Francis Burrows, Shivani Malik. Tipifarnib synergizes with TKIs in clear cell renal cell carcinoma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1071.