13,14-Dihydro-15-deoxy-δ7-prostaglandin A1methyl ester (TEI-9826), an antitumor prostaglandin analog, is a candidate for clinical trial. In the present study, we examined its biological stabilityin vitro, antitumor activityin vitroandin vivo, and pharmacokinetics. Although TEI-9826 was rapidly hydrolyzed to the carboxylic acid form (TOK-4528), TOK-4528 as well as δ12-prostaglandin J2(PGJ2) were found to be stable in rat, mouse and human serumin vitro. TEI-9826 exhibited nearly identical or greater potential antitumor activity compared to δ12-PGJ2and δ7-PGA1in vitroagainst Colon26 tumor cells. Further evaluation of TEI-9826 using the 38 human cancer cell lines panel and COMPARE analysis suggested that its mode of action is quite different from other anticancer agents that are currently used. TEI-9826 was integrated into lipid microspheres (Lipo TEI-9826) for dosing. Growth inhibition by Lipo TEI-9826 against Colon26 tumor inoculated s.c. in mice depended on administration route, i.e. at 80 mg/kg, no growth suppressive effect was observed for daily bolus i.v., but significant growth suppressive effect was observed for daily i.p., daily s.c. every other day s.c. and 4 times a day continuous (5 min) i.v. These tumor growth-suppressive effects were cytostatic and the tumor started to regrow at the end or a few days after the end of administration. The pharmacokinetic study suggested that maintaining the blood level of TEI-9826 and/or TOK-4528 was essential for their antitumor effects. These results show that continuous i.v. infusion might be the most suitable administration method of Lipo TEI-9826 for clinical trial.