Researchers at Mayo Clinic have identified a BCL-XL-dependent survival mechanism that contributes to PARP inhibitor resistance in ovarian cancer and shown preclinically that selective BCL-XL inhibition can restore treatment sensitivity. In a study published in Signal Transduction and Targeted Therapy, the central finding is that PARPi-resistant HGSOC cells can persist in a therapy-induced senescent state supported by BCL-XL, and that adding a selective BCL-XL inhibitor synergistically drives those cells into apoptosis — an effect not replicated by inhibitors targeting the related proteins BCL-2 or MCL-1.
The resistance mechanism
When PARPi treatment fails to kill tumor cells outright, a substantial fraction enter a senescent state — becoming multinucleated, metabolically active, and capable of persisting long-term. The paper reports that BCL-XL sequesters pro-apoptotic proteins, including BIM and PUMA, preventing them from activating the cell death machinery. In PARPi-resistant cell lines and patient-derived xenografts (PDXs), BCL-XL expression was further elevated relative to sensitive parental lines, deepening this apoptotic block. The authors describe this as a dual cell-fate problem: PARPi treatment primes cells with pro-death signals, but BCL-XL prevents those signals from being executed in both non-senescent and senescent populations.
Preclinical evidence
The researchers showed that BCL-XL inhibition with A-1155463 resensitized multiple PARPi-resistant HGSOC cell models, including models resistant to olaparib and veliparib. By contrast, venetoclax (BCL-2 selective) and S63845 (MCL-1 selective) did not produce meaningful sensitization, pointing to BCL-XL as the specific dependency. CRISPR knockout experiments confirmed the combination acts through the intrinsic mitochondrial apoptosis pathway: loss of BAX/BAK significantly reduced cell death, while loss of FADD did not.
In vivo, the combination showed activity in the aggressive niraparib-resistant PH039R patient-derived xenograft model, slowing tumor progression and significantly improving survival versus niraparib alone. However, the tumor-growth difference did not reach statistical significance, and the combination was ineffective in the more indolent PH077R resistant model. The findings therefore provide proof-of-concept for BCL-XL inhibition in PARPi-resistant disease but also indicate that the dependency is not universal across resistant tumors.