Randomized, double-blind, placebo (PL)-controlled, phase III trial of pimitespib (TAS-116), an oral inhibitor of heat shock protein 90 (HSP90), in patients (pts) with advanced gastrointestinal stromal tumor (GIST) refractory to imatinib (IM), sunitinib (SU) and regorafenib (REG).
Authors: Honma Yoshitaka , Kurokawa Yukinori , Sawaki Akira , Naito Yoichi , Iwagami Shiro , Baba Hideo , Komatsu Yoshito , Nishida Toshirou , Doi Toshihiko 
Published: June 2021

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Background: Pimitespib (PIM) is a novel class of orally active selective HSP90 inhibitors. KIT and PDGFRA are clients of HSP90 for their functional stability; therefore, HSP90 is a rational therapeutic target on GIST in pts with acquired resistance, such as secondary mutation in KIT, to approved tyrosine kinase inhibitors. A phase II trial showed clinical activity of PIM in pts with advanced GIST refractory to standard treatments whose medical need remains unmet. This phase III trial evaluated the efficacy and safety of PIM for this unmet clinical need. Methods: Eligible pts had histologically confirmed advanced GIST refractory to IM, SU, and REG, ≥1 measurable lesion, and ECOG performance status 0 or 1. Pts were randomized 2:1 to receive either PIM 160 mg once daily on a 5-days-on/ 2-days-off schedule or PL. Pts eligible for unblinding at the time of progressive disease were allowed to crossover to open-label PIM. The primary endpoint was progression-free survival (PFS) by blinded central radiological review based on modified RECIST 1.1. Secondary endpoints included overall survival (OS), PFS in the pts crossed over to PIM (secondary PFS), and safety. Crossover-adjusted OS was derived using the rank preserving structural failure time (RPSFT) model. Exploratory endpoints included pharmacogenomics (PGx). Results: From Oct 2018 to Apr 2020, 86 pts were randomized to receive either PIM (n = 58) or PL (n = 28). Baseline characteristics were well balanced between the two arms. Median PFS was 2.8 months (mo) (95% CI: 1.6–2.9) for PIM vs. 1.4 mo (95% CI: 0.9–1.8) for PL. The hazard ratio (HR) for PFS was 0.51 (95% CI: 0.30–0.87) ( p = 0.006, stratified log-rank test). Median OS was 13.8 mo (95% CI: 9.2–not reached) for PIM vs. 9.6 mo (95% CI: 5.5–not reached) for PL (HR for OS 0.63; p = 0.081), with 60.7 % of PL pts crossed over to PIM; secondary PFS was 2.7 mo (95% CI: 0.7–4.1). The RPSFT-adjusted median OS of PL was 7.6 mo (adjusted HR for OS 0.42; p = 0.007). Furthermore, the results of PGx analysis suggested that PIM was also effective in pts with secondary KIT mutation detected from blood samples. The most common ( > 5%) grade 3 or higher adverse events (AEs) in PIM/PL were diarrhea (13.8%/0%), anemia (6.9%/10.7%), decreased appetite (6.9%/0%), and tumor hemorrhage (5.2%/0%). AEs leading to PIM/PL study discontinuation were observed in 4/2 pts (6.9%/7.1%), respectively. Conclusions: This randomized trial demonstrated that PIM significantly improved PFS with OS prolongation in pts with advanced GIST refractory to IM, SU, and REG, as a HSP90 inhibitor for the first time. PIM was tolerated and AEs were manageable. With a mechanism of action different from that of standard therapies, PIM has the potential to be a new standard treatment in GIST. Clinical trial information: JapicCTI-184094.