KF25706, a novel oxime derivative of radicicol, exhibits in vivo antitumoractivity via selective depletion of Hsp90 binding signaling molecules

Radicicol, a macrocyclic antifungal antibiotic, has been shown to bind to t he heat shock protein 90 (Hsp90) chaperone, interfering with its function. Hsp90 family chaperones have been shown to associate with several signaling molecules and play. an essential role in signal transduction, which is imp ortant for tumor cell growth, Because radicicol lacks antitumor activity in vivo in experimental animal models, we examined the antitumor activity of a novel radicicol oxime derivative, radicicol 6-oxime (KF25706), on human t umor cell growth both in vitro and in vivo. KF25706 showed potent antiproli ferative activities against various human tumor cell lines in vitro and inh ibited v-src- and g-ras-activated signaling as well as radicicol. In additi on, Hsp90 family chaperone-associated proteins, such as p185(erbB2), Raf-1, cyclin-dependent kinase ?, and mutant p53, were depleted by KF25706 at a d ose comparable to that required for antiproliferative activity. KF25706 was also shown to compete with geldanamycin for binding to Hsp90, KF29163, whi ch is an inactive derivative of radicicol, was less potent both in p185(erb B2) depletion and Hsp90 binding. More importantly, KF25706 showed significa nt growth-inhibitory activity against human breast carcinoma MX-1 cells tra nsplanted into nude mice at a dose of 100 mg/kg twice daily for five consec utive i.v. injections, KF15706 was also shown to possess antitumor activity against human breast carcinoma MCF-7, colon carcinoma DLD-1, and vulval ca rcinoma A431 cell lines ill vivo in an animal model. Finally, we confirmed the depletion of Hsp90-associated signaling molecules (Raf-1 and cyclin-dep endent kinase 4) with ex vivo Western blotting analysis using MX-1 xenograf ts, In agreement with in vivo antitumor activity, KF25706 depleted Hsp90-as sociated molecules in vivo, whereas KF29163 and radicicol did not show this activity bl vivo. Taken together, these results suggest that antitumor act ivity of KF25706 may be mediated, at least in part, by binding to Hsp90 fam ily proteins and destabilization of Hsp90-associated signaling molecules.