The 90 kilo-Dalton heat shock protein (Hsp90) is a molecular
chaperone
that facilitates the maturation of nascent polypeptides into their
biologically active conformation. Because many of the >400 known
client
protein substrates are implicated in the development/progression of
cancer, it is hypothesized that Hsp90 inhibition will simultaneously
shut down numerous oncogenic pathways. Unfortunately, most of the
small molecule Hsp90 inhibitors that have undergone clinical evaluation
thus far have failed due to various toxicities. Therefore, the disruption
of Hsp90 protein–protein interactions with cochaperones and/or
client substrates has been proposed as an alternative way to achieve
Hsp90 inhibition without such adverse events. The hexadepsipeptide
Enniatin A (EnnA) has recently been reported to be one such inhibitor
that also manifests immunogenic activity. Herein, we report preliminary
structure–activity relationship (SAR) studies to determine
the structural features that confer this unprecedented activity for
an Hsp90 inhibitor. Our studies find that EnnA’s branching
moieties are necessary for its activity, but some structural modifications
are tolerated.