Divergent Synthesis of a Pochonin Library Targeting HSP90 and In Vivo Efficacy of an Identified Inhibitor

Abstract

The heat-shock protein 90 (HSP90) has emerged as one of the most exciting therapeutic targets in recent years. [1,2] Despite the seemingly ubiquitous function of this constitutively expressed chaperone, its role in stabilizing conformationally labile proteins has implications in pathologies ranging from oncology to neurodegenerative diseases. Most endogenous clients [3] of HSP90s are key regulators of cell signaling which are destabilized and degraded in the absence of the chaperoning activity of HSP90. The dependence of transformed cells on HSP90 is further heightened by the fact that many oncogenic mutations, while increasing the activity of progrowth signaling pathways, are less stable than their wild-type counterparts and have an increased dependence on the chaperoning activity of HSP90.[4] A clinically relevant example is the heightened dependence of drug-resistant Bcr-Abl mutants on the activity of HSP90, and the fact that HSP90 inhibitors in combination with Abl inhibitors remain effective against such mutants.[5, 6] Accordingly, HSP90 inhibition provides a broad and effective target for the treatment of cancer. Furthermore, HSP90 inhibitors can act synergistically with a cytotoxic agent. [7] HSP90 is also implicated in regulatScheme 1. Structure of radicicol, geldanamycin, 17AAG, pochonin D, and general structure of the pochonin library (5). [*] Dr.

Inhibition of HSP90 with Pochoximes: SAR and Structure‐Based Insights

“…Preliminary experiments with the aryl moiety (fragment A) had shown that modifications of either phenol was detrimental; however, the presence or absence of a chlorine at R 1 did have a subtle impact on the activity and both alternatives were included in the library. We [27] and others [30] had previously found that in the lower part of the macrocycle (derived from fragment B) the saturated fragment B2 was tolerated. However, additional substitutions had not been investigated and three new combinations (B3-5) were included.…”

“…The last group (D) included the largest diversity to probe the optimal oxime substituent. [27,31,32] As shown in Scheme 2, deprotonation of the toluate fragment A with LDA followed by the addition of fragment B afforded a total of ten different combinations of intermediate 2 www.chembiochem.org in moderate to good yield (50-85 %), except for the coupling with fragment B2, which was afforded unacceptable yield (< 10 %); this is presumably due to competing enolization of the Weinreb amide. Alternatively, the same product could be obtained by conjugate reduction of the coupling product obtained with fragment B1 by using NaCNBH 3 (25-50 %).…”

“…To broaden the structure-activity relationship (SAR) data on the pochoximes, a library with four points of diversity was envisioned (1, Scheme 1); the library would stem from a divergent coupling of fragment A to B followed by oxime formation and introduction of fragment C and then D. Our choice of fragments A-D was based on our preliminary SAR data [27][28][29] and the objective of biasing the conformational profile of the macrocycle through different ring sizes and additional small substituents. Preliminary experiments with the aryl moiety (fragment A) had shown that modifications of either phenol was detrimental; however, the presence or absence of a chlorine at R 1 did have a subtle impact on the activity and both alternatives were included in the library.…”

“…The synthetic planning of the library was based on previously developed chemistry [27] and leveraged on the use of solidphase synthesis and polymerbound reagents. To broaden the structure-activity relationship (SAR) data on the pochoximes, a library with four points of diversity was envisioned (1, Scheme 1); the library would stem from a divergent coupling of fragment A to B followed by oxime formation and introduction of fragment C and then D. Our choice of fragments A-D was based on our preliminary SAR data [27][28][29] and the objective of biasing the conformational profile of the macrocycle through different ring sizes and additional small substituents.…”

“…Furthermore, significant improvements in cellular efficacy could be achieved through the formation of oximes. [27] In fact, pochoximes A-C are amongst the most potent HSP90 inhibitors reported to date; inducing client protein degradation in SKBR3 cell lines at low-nm concentration. Pochoxime A treatment leads to tumor regression in xenografts bearing BT474 breast tumor cells.…”