The results suggested that these derivatives showed some desirable properties compared to the first-generation HSP90 inhibitors, including improved water solubility and lowered susceptibility.411 BIIB021 was the first purine-based HSP90 inhibitor to enter clinical trials (Table 2).412 Among them, a Phase II clinical study indicated that BIIB021 could alter the metabolic activity of gastrointestinal stromal tumors (GIST), without substantial hepatotoxicity observed. The gene discussed is HSP90AB1; the disease is gastrointestinal stromal tumor.