Abstract 411: PHI-501, a novel pan-RAF/DDRs dual kinase inhibitor, overcomes BRAF or MEK inhibitor resistance in melanoma

Abstract Background: PHI-501 is a newly developed, highly effective, and orally accessible dual inhibitor for pan-RAF and discoidin domain receptor (DDR) that is a collagen-activated receptor tyrosine kinase. Mutations of the BRAF and NRAS are the most common genetic alterations in melanoma and consequently lead to activation of mitogen activated protein kinase (MAPK) signaling. The major drawback of melanoma treatment is the innate and acquired drug resistance to MAPK inhibitors. In this study, we investigated the inhibitory effects of a novel compound, PHI-501 on resistance to known BRAF and MEK inhibitors in melanoma. Methods: After compounds treatment, the growth-inhibiting activity was assessed using the CCK test. Inhibition of RAF/DDR1 pathway signaling was evaluated by western blotting. Using annexin-V/propidium iodide-stained cells and flow cytometry, apoptosis induction was assessed. To quantify cell mobility, monolayer cells were subjected to a wound-healing assay. Long-term therapy with dabrafenib (BRAF inhibitor), cobimetinib or trametinib (MEK inhibitor) led to the establishment of the drug-resistant SK-MEL-3 (BRAF V600E) and SK-MEL-30 (NRAS Q61K) melanoma cell line. Results: PHI-501 demonstrated potent growth inhibition (GI50 <1 µM) in seven melanoma cell lines harboring BRAF V600E or NRAS mutations. Western blot revealed that PHI-501 inhibited the phosphorylation of MEK, ERK, and AKT proteins in the downstream signaling of RAF/DDRs and decreased the levels of cyclin D1 and survivin. In SK-MEL-3, PHI-501 was 47-fold more effective than the class I BRAF inhibitor, dabrafenib (GI50: 0.08 µM vs 3.73 µM). Established dabrafenib- or trametinib-resistant SK-MEL-3 cells were extremely sensitive to PHI-501 (GI50: 0.20 µM and 0.13 µM), whereas their sensitivity to dabrafenib or trametinib was diminished (GI50: NA and 39.61 µM). PHI-501 triggered apoptosis more effectively than these inhibitors, as revealed by a larger proportion of annexin staining and extensive PARP1 cleavage. Comparing the potency of PHI-501 to cobimetinib in acquired resistant SK-MEL-30 cells showed that PHI-501 had 74-fold more inhibitory effect on cell proliferation (GI50: 0.55 µM to 40 µM). Moreover, treatment with PHI-501 strongly suppressed cell migration of cobimetinib-resistant SK-MEL-30 cells. PHI-501 inhibited the growth of drug-resistant melanoma cells as effectively as the original drug-naïve cells, and western blot analysis revealed considerable suppression of DDR1/2, ERK, and AKT phosphorylation in MAPK inhibitor-resistant cells. Conclusion: Melanoma cells resistant to RAF or MEK-targeted treatments or harboring NRAS mutation exhibited strong antiproliferative activity when treated with PHI-501, a highly potent pan-RAF/DDR dual inhibitor. The results of this study suggest that PHI-501, as a single agent, has the potential to overcome the restricted response in the treatment of melanoma. Citation Format: Sue Min Kim, Jung Hee Park, Ky-Youb Nam, June H-J Han, Kyu-Tae Kim, Jeong Hyeok Yoon, Sandip Sengupta, Taebo Sim, Sang Joon Shin. PHI-501, a novel pan-RAF/DDRs dual kinase inhibitor, overcomes BRAF or MEK inhibitor resistance in melanoma [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 411.