AT13387, discovered by a high-throughput x-ray craystallography fragment-based platform, is a non-geldanamycin heat shock proterin 90 (HPS90), a molecule chaperone regulating signal transduction pathways for cell growth and survival, inhibitor reportedly resulting in client protein degradation, suppression of signaling, and inducing cell cycle arrest and apoptosis. Since structurally unrelated to geldanamycin, it has high affinity for binding HSP90 (Kd = 0.5nM) and exhibited an in vitro cytotoxicity showing a median EC₅₀ value of 41 nM. A long tumor-specific retention was observed in AT13387 during xenograft  studies, which indicated the possibility of less frequent dosing. However, preliminary results showed that the antitumor activity of AT13387 against solid tumor and leukemia models is similar to alvespimycin.

Reference

Min H. Kang, C. Patrick Reynolds, Peter J. Houghton, Denise Alexander, Christopher L. Morton, E. Anders Kolb, Richard Gorlick, Stephen T. Keir, Hernan Carol, Richard Lock, John M. Maris, Amy Wozniak, and Malcolm A. Smith. Initial Testing (Stage 1) of AT13387, an HSP90 Inhibitor, by the Pediatric Preclinical Testing Program. Pediatr Blood Cancer. 2012; 59(1): 185-188