Arbaprostil, a synthetic analog of prostaglandin E2 (PGE2) known as 15(R)-15-methyl prostaglandin E2, was developed for its cytoprotective activity. In the commercial production of bulk arbaprostil, one potential impurity is 15(R)-15-methyl Prostaglandin A2 (15(R)-15-methyl PGA2). The pharmacology and EP receptor binding affinity of 15(R)-15-methyl PGA2 have not been reported.