Developing New Therapeutic Approaches for BRAFV600E Colorectal Cancer

The presence of BRAFV600E mutations is associated with poor prognosis in CRC. In 2020, the FDA approved a combination therapy of encorafenib and cetuximab for metastatic BRAFV600E CRC. However, despite significant benefit of this therapy, only 22% of patients respond to this therapeutic approach, and resistance ultimately develops in the majority of patients. Therefore, there is a need for development of new combinational therapeutic approaches to improve the outcome for BRAFV600E CRC patients. We have established a panel of primary and established BRAFV600E CRC cell lines resistant to PLX8394 (plixorafenib), a second-generation BRAFi which selectively inhibits BRAF in cancer cells and prevents paradoxical MAPK pathway activation. PLX has been granted FDA Orphan Drug designation for treatment of primary brain/CNF malignancies and is on fast-track designation for the treatment of patients with BRAFV600E solid cancers who have exhausted prior therapies (Fore Biotherapeutics). We found that development of resistance to PLX is associated with an increase in cellular proliferation, metastatic potential, and upregulation of lipid metabolism. Moreover, we found that fatty acid synthase (FASN), a key enzyme of de novo lipid synthesis and therapeutic target in CRC, is highly upregulated in PLX resistant cells. Moreover, pharmacological inhibition of FASN postpones the development of resistance but is not sufficient to overcome the acquired resistance to PLX. To address this challenge, we utilized the APExBIO DiscoveryProbe™ FDA-approved drug library to screen for compounds that are efficacious in PLX resistant cells. The impetus for this pilot award is identification of histone deacetylase inhibitors (HDACi) as a class of compounds that has high efficacy in PLX resistant cells. We have showed higher efficacy of HDACi in BRAFV600E as compared to cells without BRAF mutation. Importantly, we show high synergy in inhibiting cell viability between HDACi and PLX in parental BRAFV600E cells and between HDACi and FASN-targeted therapy in cells with acquired resistance to PLX8394. Therefore, we hypothesized that addition of HDACi to the treatment regimen can improve the efficacy of PLX8394 and postpone and/or overcome resistance to PLX8394 in BRAFV600E CRC. First, we will test the effects of HDACi alone and in combination with PLX8394 in parental and resistant BRAFV600E CRC models; second, we will determine if inhibition of de novo lipid synthesis using TVB-3664 increases sensitivity to HDACi in CRC with acquired resistance to PLX8394. BRAFV600E primary cells, patient-derived xenografts, human tumor organoids, and cutting-edge technology such as RNAseq, ATACseq, multiplex imaging, and multiplex cytokine array will be utilized in the proposed studies. The overall significance of this proposal lies in its potential to develop innovative therapeutic strategies using FDA-approved drugs or drugs in clinical trials. Therefore, as these studies progress, they will have a high potential to be quickly translated to the clinic and improve outcomes for patients.