The potent antitumor natural drug camptothecin (1, a topoisomerase I inhibitor) and several of its more water soluble synthetic congeners are currently in various phases of clinical trials. We sought to solubilize 1 as a nonionic R-alkoxy ester transport form (prodrug) and demonstrate increased circulatory retention as well as continuous therapeutic release of native drug. This was achieved by condensation of 1 with polyethylene glycol (PEG) 40 kDa dicarboxylic acid in the presence of diisopropylcarbodiimide, resulting in a mixture of camptothecin mono- and disubstituted esters (3a + 3b = 3) in a ratio of approximately 2:1. Structural proof was provided by controlled hydrolysis of 3, which released camptothecin, PEG dicarboxylic acid, and PEG acid-urea. 3 had an approximate aqueous solubility of 125 mg/mL and was stable at 4 °C for over 6 months with less than 5% loss. Hydrolysis rate studies showed that 3 had a half-life approximately 5 times longer than that of PEG-taxol in pH 7.4 phosphate buffer and rat plasma. In vitro P388 cell toxicity of 3 gave an IC50 of 27 nM (compared to 7 nM for 1). In vivo studies in P388-treated mice showed that 3 (16 mg/kg camptothecin equivalents, ip) produced a 194% increase in life expectancy (ILS) and an 80% cure rate, comparable to native 1 (70% cure rate) but superior to camptothecin 20-glycinate·TFA (64% ILS, no cures). Plasma studies revealed that 3 was completely hydrolyzed to 1 in rat plasma (t1/2 = 2 h), but only 20-30% of 1 was released in human plasma due to binding to plasma proteins, which could be reversed by protein denaturation or dilution.