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A solvent and Cremephor® free formulation of the anticancer chemotherapeutic geldanamycin was prepared using amphiphilic block co-polymer micelles of poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-b-PCL). Although geldanamycin was not solubilized by PEG-b-PCL micelles, fatty acid prodrugs of geldanamycin were encapsulated in PEG-b-PCL micelles by a co-solvent extraction technique. Resulting PEG-b-PCL micelles were < 120 nm in diameter and solubilized > 20% w/w geldanamycin prodrugs increasing aqueous solubility to > 2 mg/mL. PEG-b-PCL micelles released the geldanamycin prodrugs over several days, t1/2 2.2 to 9.6 days. The free prodrugs hydrolyzed rapidly, t1/2 < 6 h, into the geldanamycin analogue 17-β-hydroxyethylamino-17-demethoxygeldanamycin, which has high activity against MCF-7 breast cancer cells, IC50 240 nM.