An inhibitor of heat shock protein 90 (HSP90) can include a coumermycin A1 analog having a structure that inhibits HSP90 greater than coumermycin A1. That is, the coumermycin A1 analog is not coumermycin A1. The coumermycin A1 analog can have an antiproliferative biological activity, which can be superior to coumermycin A1. The activity can include the coumermycin A1 analog inhibiting a C-terminus of HSP90.