PB2102 A SAME DAY ASSAY USING CYTOCHROME C RELEASE DETERMINES SENSITIVITY TO COOPERATIVE BCL‐2 AND MCL‐1 TARGETING IN MULTIPLE MYELOMA CELL LINES AND PRIMARY SAMPLES

Background: Multiple myeloma is an incurable haematological malignancy with remarkable clonal and genetic heterogeneity. Although both MCL‐1 and BCL‐2 are necessary for myeloma survival, most myelomas are dependent on MCL‐1 for survival such that BCL‐2 inhibition alone only yields significant cytotoxicty in a minority and cooperative inhibition of both proteins is required. Knowing which patients will respond can be challenging since cytotoxicty assessment of primary patient samples is significantly hampered by the lack of longevity of those cells in culture ex vivo. Aims: We aimed to study whether a short term flow cytometric assay could be used to determine cytotoxicity in myeloma cell lines and primary samples after cooperative inhibition of BCL‐2 and MCL‐1 by the BH3 mimetics (venetoclax and S63845). Methods Myeloma cell lines and patient samples were treated with venetoclax and S63845 alone and in combinations. Sub‐toxic doses of drugs were determined for each cell line. Samples were fixed after 4 hours drug incubation thus precluding the need to keep those cells alive in culture for prolonged periods and then assayed for cytochrome c release using flow cytometry. The Bliss independence model was used to demonstrate synergy. Results: The cytochrome C release assay demonstrated that the combination of venetoclax and S63845 was synergistic as compared to controls and single agent treatment in all six cell lines tested (Combination index values 7.8‐80). All three cyclin D2 translocated cell lines were particularly sensitive to the combination and synergy was p53 independent. Furthermore, the same day assay was used on CD138‐selected primary myeloma cells also demonstrating effective cooperative targeting in patient samples. Summary/Conclusion: The attractiveness in this assay lies in its ability as a predictor of apoptotic response to chemotherapy and molecular targeting agents providing rapid results. Here, we have demonstrated for the first time in myeloma that using the cytochrome C release assay we can quickly predict apoptosis in cell lines. Importantly we are now demonstrating that this assay can also be used in primary samples. This can set the steps for an individualised assay to determine patient responses to targeted molecular therapies.