Strong Hsp90α/β Protein Expression in Advanced Primary CRC Indicates Short Survival and Predicts Response to the Hsp90α/β-Specific Inhibitor Pimitespib

The prognosis of advanced (UICC IIb-IV) primary colorectal cancer (pCRC) remains poor. More effective targeted therapies are needed. Heat shock protein 90 alpha/beta (Hsp90α/β) expression was immunohistologically quantified in 89 pCRCs and multivariately correlated with survival. Pimitespib (Pim, TAS-116), a Hsp90α/β-specific inhibitor, was tested in pCRC cell lines and patient-derived cancer spheroids (PDCS) and referenced to the pan-Hsp90 inhibitor ganetespib (Gan, STA-9090) and standard-of-care therapies. A total of 26.97% pCRCs showed strong tumoral Hsp90α/β expression (Hsp90α/β > 40%), which correlated with reduced PFS (HR: 3.785, 95%CI: 1.578–9.078, p = 0.003) and OS (HR: 3.502, 95%CI: 1.292–9.494, p = 0.014). Co-expression of Hsp90α/β > 40% with its clients BRAF-V600E and Her2/neu aggravated the prognosis (BRAF-V600E mutated: PFS, p = 0.002; OS, p = 0.012; Her2/neu score3: PFS, p = 0.029). The prognostic cut-off Hsp90α/β > 40% was also a predictor for response to Pim-based therapy. Pim efficacy was increased in combination with 5-FU, 5-FU + oxaliplatin, and 5-FU + irinotecan (all p < 0.001). Pim induced sensitization to all chemotherapies in HT-29 (p < 0.001), Caco-2 (p < 0.01), and HCT116 (p < 0.05) cells. Pim combined with encorafenib in HT-29 and with trastuzumab in Caco-2 cells was most effective in dual-target inhibition approaches (HT-29: p < 0.005; Caco-2: p < 0.05). The anti-cancer effect and chemosensitization of Pim-based therapy were prospectively confirmed in PDCS directly generated from Hsp90α/β > 40% pCRCs. Protein profiling combined with functional drug testing stratifies Hsp90α/β > 40% pCRC patients diagnosed with UICC IIb-IV for effective Pim-based therapy.