Water-soluble paclitaxel may cause less side effects and be less costly to administer In comparison to a taxol formulation using a cremophor EL/alcohol vehicle. In this study, polyethylene glycol (PEG; MW 5000) was conjugated to the 2' position of paclitaxel through a spacer succlnyl group. PEG paclitaxel as a non-Ionic paclitaxel prodrug was highly water soluble (>20 mg equlv. paclitaxel/ml). The release of paclitaxel from phosphate-buffered solution was pH dependent. The half-life of PEG paclitaxel was 7.6, 54 and 311 min at pH 9.0, 7.4 and 6.0, respectively. PEG paclitaxel inhibited the growth of B16 melanoma cells to an extent similar to that of paclitaxel. In MCA-4 mammary tumor-bearing mice, a single dose of PEG-paclitaxel (40 mg equiv. paclitaxel/kg body weight) significantly delayed tumor growth. The average number of days for the tumor to reach 12 from 8 mm in diameter increased from 6.5 days for control animals to 8.5 days for PEG paclitaxeltreated animals and 9.4 days for paclitaxel-treated animals. These studies demonstrated that PEG may be used as an effective solubillzing carrier for paclitaxel.