microolecular design can overcome the metabolic instability of Delta7-PGA1, while maintaining its antitumor potency. Saturation of the C(13)-C(14) double bond enhances the biological stability but decreases the antiproliferative activity. Configurational inversion of the isomerase-sensitive C(12) stereocenter from the natural S to the unnatural R geometry not only enhances biological stability but also significantly suppresses the growth of the tumor cells. The 12R derivatives markedly increase the induction of p21, a Cdk inhibitor, leading to sharp cell cycle arrest at the G1 phase at a dose level so low that at this dose Delta7-PGA1 methyl ester scarcely exerts an effect. These conspicuous biological properties lead to long-term suppression of tumor cell growth. The structure-stability relationship demonstrates that the stability of prostaglandins (PGs) is crucially controlled by the C(12) configuration and is unaffected by the geometry of the hydroxy-bearing C(15). The successful design of antitumor PGs resistant to enzymatic metabolism provides a new strategy applicable to creating a useful PG for cancer chemotherapy.