Abstract 4657: PHI-501 is a novel potent next-generation pan-RAF/DDRs inhibitor, and overcomes resistance to RAF or MEK inhibitor in melanoma via dual inhibition of RAF and DDR1/2 signaling
Authors: Kim Sue Min , Cho Sungmin , Sung Gi-Jun , Nam Ky-Youb , Han June , Shin Sang Joon 
Published: March 2024

Background: The prevalent mutation of BRAF (~50%) and NRAS (~30%) in malignant melanomas has led to the aggressive development of RAF and MEK inhibitors as anticancer drugs. However, the major hurdle of targeted therapies for melanoma is resistance to RAF or MEK inhibitors, either intrinsic- or acquired- resistance. PHI-501 is an orally bioavailable and highly effective pan-RAF inhibitor. PHI-501 has dual inhibition activity against pan-RAF and discoidin domain receptor (DDR), a collagen-activated receptor tyrosine kinase. In this study, we evaluated the ability of PHI-501 to overcome drug-resistance to RAF or MEK inhibitors in melanoma harboring NRAS or BRAF mutations.

Methods: Three drug-resistant cell lines were established by long-term treatment of belvarafenib in SK-MEL-2 (SK-MEL2BR) and each of dabrafenib and trametinib in SK-MEL-3 (SK-MEL3DR and SK-MEL3TR, respectively). RNA-seq of each resistant cell line and in-depth analysis of sequencing data was conducted, including GSEA. The anti-proliferative activity and signaling inhibition of PHI-501 were evaluated by CCK-8 assay and western blots. A long-term clonogenic assay was performed to confirm cell survival upon PHI-501 treatment. In vivo efficacy was evaluated in SK-MEL-3DR cell line xenograft models.

Results: PHI-501 demonstrated significant anti-proliferative effects (GI50 &lt; 1 µM, Mean GI50: 0.3 µM) in all 12 melanoma cell lines tested in this study. PHI-501 exerted cell growth inhibitory activities at least 3 to 50 times stronger than other RAF or MEK inhibitors in each of the resistance cell lines. Moreover, PHI-501 doses of less than 100 nM, significantly inhibited the colony formation of drug-resistant cells. Western blot revealed that PHI-501 retained strong inhibition of both ERK and AKT phosphorylation through downregulating pDDRs. In the SK-MER3DR xenograft mouse model, treatment of PHI-501 alone induced a rapid tumor reduction (TGI 72.1%), whereas the vehicle or dabrafenib did not display an anti-melanoma effect (TGI -7.4%). A comprehensive pathway analysis showed the TNFA-NFKB, IL6-JAK-STAT3, and KRAS signaling enriched in drug-resistant cells compared to the parental cells. PHI-501 treatment effectively downregulated those pathways in all three drug-resistant cells. In particular, PHI-501 downregulates the geneset in epithelial-mesenchymal transition (EMT) signaling more effectively in SK-MEL2BR, SK-MEL3DR, and SK-MEL3TR (Adjusted P=&lt;.0001, and NES: -2.01, -1.72, -2.30, respectively).

Conclusion: Novel pan-RAF/DDR dual inhibitor PHI-501 has greater anti-tumor activity in RAF or MEK inhibitor-resistant melanoma, which is through downregulation of MAPK signaling and EMT-related genesets and promoting apoptosis. These data support that the clinical development of PHI-501 for melanoma refractory to MAPK inhibitors.

Citation Format: Sue Min Kim, Sungmin Cho, Gi-Jun Sung, Ky-Youb Nam, June Han, Sang Joon Shin. PHI-501 is a novel potent next-generation pan-RAF/DDRs inhibitor, and overcomes resistance to RAF or MEK inhibitor in melanoma via dual inhibition of RAF and DDR1/2 signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4657.