Therapeutic exploitation of AUTAC in hematologic malignancies with cardiac protection

Background: Myeloid cell leukemia-1 (Mcl1) is a critical member of the Bcl 2 family that plays an essential role in regulating apoptosis and maintaining mitochondrial integrity, particularly within cardiac tissue. While Mcl1 overexpression is a major driver of resistance to multiple anticancer therapies, its pharmacologic inhibition has been hampered by dose-limiting cardiotoxicity, representing a significant and unresolved challenge in cardio-oncology. The development of safer therapeutic strategies that selectively target Mcl1 in cancer cells while sparing cardiac cells remains a high-priority goal. In this study, our group investigated a novel autophagy-targeting chimera (AUTAC) designed to selectively degrade Mcl1 via the autophagy-lysosomal pathway. We aimed to determine whether AUTAC could induce selective cytotoxicity in multiple myeloma cells while preserving cardiac cell viability and mitochondrial function. Methods: Multiple myeloma cell lines (U266B1, RPMI-8226) and cardiac cell lines (AC16, H9c2) were used as tumor and cardiac models, respectively. Cell viability was assessed using the CellTiter-Glo assay and IncuCyte live-cell imaging. Mitochondrial membrane potential was measured by TMRE fluorescence. Effects of AUTAC on Mcl1 and autophagy dynamics were evaluated by Western blotting, confocal microscopy, and inhibition studies with 3-methyladenine (3-MA) and chloroquine (CQ). To confirm autophagy dependence, ATG5 was knocked down using shRNA. Results: Treatment with AUTAC selectively induced lysosomal degradation of Mcl1, without detectable effects on other anti-apoptotic Bcl 2 family members, including Bcl 2 and Bcl xL . Pharmacological inhibition of autophagy with 3-MA (early-stage inhibitor) or CQ (late-stage inhibitor) effectively blocked AUTAC-mediated Mcl1 degradation, confirming that the process required an intact autophagy-lysosomal pathway. Furthermore, knockdown of ATG5 abolished Mcl1 reduction, reinforcing the autophagy dependence of this degradation mechanism. Functionally, AUTAC treatment led to a greater than 50% reduction in cell viability in U266B1 and RPMI-8226 multiple myeloma cells, accompanied by increased apoptotic signaling. In contrast, AUTAC caused minimal cytotoxicity in AC16 and H9c2 cells, maintaining mitochondrial function and morphology. Compared with conventional Mcl1 inhibitors, AUTAC exhibited substantially reduced cardiotoxicity, underscoring its favorable safety profile. Conclusions: Targeted lysosomal degradation of Mcl1 via AUTAC represents a promising therapeutic strategy that repurposes autophagy for selective oncogenic protein removal. AUTAC effectively degraded Mcl1 in malignant cells while sparing cardiac cells from apoptosis and mitochondrial injury, highlighting its potential as a next-generation, cardio-safe alternative to conventional Mcl1 inhibitors.