Potent antitumor activity of the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine in choriocarcinoma-bearing rats

The acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine (PMEA) is a potent and selective antiretroviral agent which is currently evaluated in its oral prodrug form, bis(POM)PMEA (adefovir dipivoxil), in phase II and III clinical trials in human hepatitis B virus (HBV)- and human immunodeficiency virus (HIV)-infected individuals, respectively. We have now found that PMEA is also a potent inhibitor of growth of the highly aggressive choriocarcinoma tumor arising from rat choriocarcinoma RCHO cells grafted under the kidney capsule of syngeneic WKA/H rats. In untreated rats, massive invasive RCHO tumors, covering the whole surface of the kidney and resulting in a marked enlargement of the kidney, were observed at day 10 after tumor cell grafting. Daily treatment with PMEA at 25 mg/kg/day afforded a marked reduction in tumor size (i.e., smaller tumors and slight, if any, enlargement of the kidney). Increasing the PMEA dose to 50, 100 or 250 mg/kglday resulted in a gradual increase of the antitumor effect of the compound. At the highest dose tested, i.e., 250 mg/kg/day, PMEA completely suppressed tumor growth. The antitumor activity of PMEA persisted for at least 10 days after termination of drug treatment. In addition, delayed treatment with PMEA at a dose of 200 mg/kg/day, started at a time point where choriocarcinoma tumors had already developed, stopped further growth and even induced regression of the tumors. PMPA, a closely related structural analogue of PMEA, failed to inhibit choriocarcinoma tumor growth. This observation points to the specificity of PMEA as an antitumor agent. In view of our findings, the therapeutic potential of PMEA for the treatment of neoplastic diseases appears to merit further investigation.