A dual drug therapy for sunitinib resistant RCC: An in vitro analysis

340 Background: Over the last decade, medical treatment for metastatic renal cell carcinoma has made significant advances through the development of tyrosine kinase inhibitors (TKI) like sunitinib. However, of patients initiated on TKI therapy, 70% respond well while 30% are believed to be primarily resistant to treatment. Additionally, 30% of patients who initially respond to treatment gain secondary drug resistance and present with increased tumor burden. In this study, we seek to develop a combination therapy of Tipifarnib + Sunitinib to target exosome conferred drug resistance. Methods: 786-O, 786-0 Sunitinib Resistant (SR), and 293-T cells were cultured in RPMI 1640 medium supplemented with 10% fetal bovine serum, 2mM L-glutamine, and 1% penicillin/streptomycin (P/S). Exosomes were collected from conditioned media treated with Tipifarnib and isolated using differential ultracentrifugation. Exosomes were analyzed using the qNano IZON system. Colony forming units assay and Immunoblot analysis were used to further characterize or samples. Results: Exosomes collected from 786-O, 786-0 SR, and 293-T cells treated with 0.5 uM of Tipifarnib were compared using our qNANO IZON system. Exosome concentrations of all cell lines showed a decrease after Tipifarnib treatment. However, our 293-T cells showed a 16% decrease in exosome concentration while our 786-O and 786-0 SR lines displayed a 66% and 75% decrease respectively. To assess the pathway Tipifarnib used to decrease exosome concentrations, immunoblot assay was used after treating cells with 0, 0.1, 0.25, 0.5, 1 uM of tipifarnib. 293-T cells showed a dose dependent increase in ESCRT-dependent marker Alix and no change in either ESCRT-independent marker nSMase or trafficking marker Rab27a. Conversely, our 786-O and 786-0 SR cell lines showed a decrease in all 3 markers: Alix, nSMase, and Rab27a. Furthermore, a colony forming units assay was used to assess the drug combination of tipifarnib + sunitinib ability to alter cell growth. After 48hr, 293-T cell showed no decrease in colony forming units when compared to DMSO control. Our drug combination showed a synergistic ability to decrease colony forming units in the RCC 786-O cell line. 786-O SR cells were resistant to sunitinib treatment, showing comparable CFUs to DMSO control. When treated with the combination of sunitinib and tipifarnib, CTUs of 786-0 SR cells dropped significantly when compared to unaccompanied sunitinib and tipifarnib treatments. Conclusions: Tipifarnib has the ability to attenuate both the exosome ESCRT–dependent and –independent pathways. Our study also showed that when used injunction with sunitinib, tipifarnib is effective at decreasing cell proliferation. This drug combination is also pre-clinically useful in sunitinib resistance cancer cells. We believe this drug combination to be efficacious at decreasing tumor burden through blocking exosome biogenesis and secretion.