The PIK3CA/AKT pathway drives therapy resistance in rhabdomyosarcoma.

Olaparib and temozolomide (OT) combination therapy is in clinical trial evaluation for rhabdomyosarcoma (RMS). Unfortunately, OT resistance has been reported in other cancers. Using preclinical mouse xenograft experiments, we show that OT effectively suppresses RMS growth, yet over half of RMS tumors develop resistance associated with transcriptomic changes that occur in the absence of recurrent genomic mutation. Importantly, most resistant RMS models upregulate the PIK3CA/AKT pathway, activating NRF2 phosphorylation and subsequent transcriptional expression of multidrug resistance ABC transporters. PIK3CA inhibitor alpelisib re-sensitizes resistant cells to OT by suppressing expression of ABC transporters. The combination of OT + alpelisib also kills RMS cells which are resistant to standard-of-care combination chemotherapy and was effective in preclinical xenograft mouse models at curbing tumor growth. Our work defines a common resistance pathway in RMS and has credentialled PIK3CA/AKT inhibition as a preclinical strategy to kill therapy resistant RMS.