9-(2-Phosphonylmethoxyethyl)-N6-cyclopropyl-2,6-diaminopurine (cpr-PMEDAP) as a prodrug of 9-(2-phosphonylmethoxyethyl)guanine (PMEG)

9-(2-Phosphonylmethoxyethyl)-N6-cyclopropyl-2,6-diaminopurine (cpr-PMEDAP) is an acyclic nucleotide analog of the [9-(2-phosphonylmethoxyethyl)-] (PME) series containing a cyclopropyl substituent on the N6 position of the 2,6-diaminopurine (DAP) base. Growth inhibition assays in a broad range of tumor cell lines demonstrated that this analog had potent antiproliferative activity with IC50 values similar to those of the structurally related guanine analog 9-(2-phosphonylmethoxyethyl)guanine (PMEG). A substantially lower growth inhibitory effect was observed for the 2,6-diaminopurine analog, PMEDAP. To dissect the basis for these varying potencies, the metabolism of the three analogs was examined in a human pancreatic carcinoma cell line, BxPC-3. HPLC analysis of the intracellular metabolites demonstrated that the cpr-PMEDAP was deaminated to PMEG and subsequently phosphorylated to PMEG mono- and diphosphates (PMEGp and PMEGpp). The level of PMEGpp generated from cpr-PMEDAP-treated cells was 50% greater than the level generated from cells incubated with PMEG. The presence of PMEG in the DNA of cells incubated with cpr-PMEDAP confirmed that the cpr-PMEDAP was converted to PMEG. In contrast, PMEDAP was not deaminated to PMEG, but directly phosphorylated to PMEDAPp and PMEDAPpp. The adenylate deaminase inhibitor 2'-deoxycoformycin (dCF) inhibited the conversion of cpr-PMEDAP in a rat liver cytosolic extract and increased the IC50 value for growth inhibition by 40-fold. The antiproliferative activities of PMEG and PMEDAP were unaffected by dCF. Thus, it appears that cpr-PMEDAP, but not PMEDAP, is converted by an adenylate deaminase-like Enzyme and functions as a prodrug of PMEG.