2620 Background: Melanoma is the deadliest skin cancer, and more than 100,000 people in the United States were diagnosed in 2024. While the 5-year survival rate is 99% for localized disease, it is just 10-25% in metastasis. Treatment for metastatic melanoma often involves powerful immunotherapies (e.g. checkpoint inhibitors). However, approximately 55% of patients have innate resistance and 25% of responders develop resistance within two years. Immunotherapy resistance is due to continued checkpoint protein expression and antagonization of T-cell activation – processes controlled by the phosphatidyl inositol 3-kinase (PI3K) system. Of the four PI3K kinases (α, β, δ and γ), recent work from the Sheng lab suggests that PI3Kβ has the largest role in the growth of BRAF V600E /PTEN null (PI3K inhibitor) melanoma and is a prime drug target. There is an 18-residue motif found only in PI3Kβ that has close interactions with p85 - a key regulatory protein responsible for kinase activation. The Sheng lab harnessed this motif and developed a novel peptide (CPP-β18) that selectively blocks PI3Kβ with high affinity and decreases viability in cells that hyper-express PI3Kβ. Given the vital role of PI3K in immunosuppression, further investigation into CPP-β18’s sensitizing effects on checkpoint inhibitors is urgently needed to lead to new therapies. Methods: PI3K inhibition in BRAF V600E /PTEN null YUMM1.7 and BRAF V600E /PTEN WT YUMM5.2 murine cell lines was validated by exposing cells to various commercial PI3K inhibitors and CPP-β18. Cell viability was measured via MTS colorimetric response. Advanced live-cell imaging techniques were employed to assess apoptosis when melanoma cells were treated with these selective inhibitors. Both cell lines were fluorescently labeled to differentiate cells from cytotoxic T-cells in future co-culture work. Results: MTS colorimetric assays reveal decreased YUMM1.7 cell viability with various PI3Kβ inhibitors (IC 50 values = 6-10 µM) in comparison to increased YUMM5.2 cell viability (IC 50 values = 21-30 µM). PI3Kδ and PI3Kγ inhibition had negligible effect on cell survival. YUMM1.7 cell death (>70%) was also demonstrated when treated with the novel CPP-β18 peptide inhibitor. Live cell imaging studies confirm feasibility to investigate interactions between drug therapy and melanoma cells and to appreciate early and late stages of apoptosis on a cellular level. Both cell lines are successfully fluorescently labeled to undergo further drug challenge experiments. Conclusions: Our data strongly supports CPP-β18 as a potential drug therapy to augment checkpoint inhibitor efficacy and mitigate the development of resistance. Cell line validation demonstrates increased PI3K activity with deleted PTEN and decreased viability when challenged with PI3Kβ inhibitors. Selectively blocking PI3Kβ via CPP-β18 may augment checkpoint inhibitors and mitigate resistance, giving patients improved treatment options.
Gabby et al. (Wed,) studied this question.