The clinical application of gambogic acid (GA), a natural component with promising antitumor activity, was limited due to its extremely poor aqueous solubility, rapid elimination in vivo, and wide bio-distribution. To solve these problems, 30 poly(ethylene glycol)-amino acid (or dipeptide)-gambogic acid (PEG-spacer-GA) conjugates were synthesized. All polymeric prodrugs showed satisfactory aqueous solubility (1.2 x 10(3)-4.5 x 10(5) times of GA solubility). It was found that the molecular weight of PEG and the choice of spacers played important role in controlling the drug percentage, water solubility, and drug release properties of PEG-GA conjugates with and without spacers. Studies of pharmacokinetics, bio-distribution, and cell cytotoxicity revealed that, employing the polymeric conjugation strategy, the remarkably improved circulatory retention time and bioavailability, as well as reduced peripheral toxicity were obtained in comprising with GA and its Cremophor EL formulation. The liver target character of PEG-GA conjugates made them potential prodrugs for liver cancer treatment. (C) 2012 Elsevier Ltd. All rights reserved.