Three phosphonomethoxyalkyl purine analogues, i.e. HPMPA (S)-9-[(3-hydroxy-2-phosphonylmethoxy)propyl]adenine,

PMEA, and PMEG proved modestly active against intraperitoneal injected P388 murine leukemia cells in mice, PMEG being the most active and most potent of the three compounds (Rose et al., 1990). In this study, PMEG was also evaluated against subcutaneously implanted B16 melanoma in mice, affording increased life span and delay in primary tumor growth. When the PME analogues PMEA, PMEDAP and PMEG were evaluated for their in vitro antitumor efficacy against human HDAC inhibitor leukemia cells ( Franek et al., 1999), they caused reversible slowdown of growth at low concentrations due to continuous repairing of damaged DNA, while high concentrations induced apoptosis and a reduction of the proportion Imatinib clinical trial of cells in the G1 phase of the cell cycle. The antitumor properties of these analogues increased in the order PMEA < PMEDAP < PMEG. PMEG, PMEA, and

PMEDAP were also investigated in a model of spontaneous T-cell lymphoma in inbred SD/cub rats (Otova et al., 1999). Treatment with 16 daily doses of PMEDAP at 5 mg/kg applied to the vicinity of the growing lymphoma resulted in significant therapeutic effects while daily PMEA or PMEG administration (although at lower doses than those of PMEDAP) did not affect survival of lymphoma-bearing mice. PMEDAP was shown to induce apoptosis in this in vivo model of haematological malignancies. Because the utility of PMEG as an anticancer agent is limited by poor cellular

Mirabegron permeability and toxicity (especially for the kidney and gastrointestinal tract), prodrugs such as N6-cyclopropyl-PMEDAP (cPr-PMEDAP), GS-9191 and GS-9219 (Fig. 2) have been designed to increase permeability and accumulation of PMEGpp intracellularly (Kreider et al., 1990, Compton et al., 1999, Vail et al., 2009 and Wolfgang et al., 2009). cPr-PMEDAP is converted to PMEG and can be considered as an intracellular prodrug of PMEG, limiting plasma exposure to the toxic agent PMEG. cPr-PMEDAP showed higher antitumor efficacy and selectivity in choriocarcinoma-bearing rats compared to PMEDAP or PMEG (Naesens et al., 1999) and was reported to have 8- to 20-fold more pronounced cystostatic activity than PMEDAP and equivalent activity as PMEG against a variety of tumor cell lines (Hatse et al., 1999a). GS-9191, a double prodrug of PMEG, was specifically designed as a topical agent to permeate the skin and to be metabolized to the active form in the epithelial layer. The conversion of GS-9191 to cPr-PMEDAP was shown to occur in lysosomes via carboxypeptidase cathepsin A-mediated ester cleavage, being cPr-PMEDAP subsequently translocated to the cytosol where it undergoes deamination and phosphorylation, yielding the active metabolite PMEGpp (Birkus et al., 2011).