Synergistic Inhibition of PI3K and HSP90 Enhanced Antitumorigenic Efficacy in Adrenocortical Carcinoma

Adrenocortical cancer (ACC) is a rare and aggressive malignancy with poor survival due to a lack of effective treatments; therefore, it is important to identify therapies to be readily studied in clinical trials. Quantitative high-throughput drug combination screening identified potent synergy between phosphatidylinositol-3-kinase (PI3K) inhibitor, PIK75 and heat shock protein 90 (HSP90) inhibitors, Ganetespib (STA9090), HSP990, or Luminespib (NVP-AUY922). Preclinical in-vitro and in-vivo studies were performed to validate the synergistic efficacy of the most effective HSP90 inhibitor and PI3K inhibitor combination in ACC cell lines, human ACC xenografts and patient-derived organoids (PDOs). Combination of PIK75 and STA9090, synergistically inhibited cell proliferation (monolayer and 3-dimensional), cell migration/invasion and epithelial-to-mesenchymal transition with decreased phosphorylated proteins in PI3K/mTOR signaling pathway. Due to the unavailabilityof PIK75 for clinical trial, another PI3K inhibitor, BGT226, which was clinically available and demonstrated a comparable synergistic efficacy with STA9090, was validated in the ACC cell lines. RNA sequencing analysis and phenotypic studies revealed that the BGT226-STA9090 combination induced autophagy-related cell death in ACC cells, unlike the PIK75-STA9090 combination which induced caspase-dependent apoptosis and G2/M cell cycle arrest. Further antitumor efficacy was confirmed by the BGT226-STA9090 combination in human ACC xenograft model and five PDOs with different pathogenic mutations. Conclusively, the combinations of PI3K and HSP90 inhibitors were highly effective in preclinical studies, warranting a clinical trial in patients with advanced ACC.