ABSTRACT GS-9191 is a novel double prodrug of the nucleotide analog 9-(2-phosphonylmethoxyethyl)guanine (PMEG) designed as a topical agent to permeate skin and be metabolized to the active nucleoside triphosphate analog in the epithelial layer. The prodrug was shown to be metabolized intracellularly to 9-(2-phosphonylmethoxyethyl)-N6-cyclopropyl-2,6,diaminopurine (cPrPMEDAP) and subsequently deaminated to PMEG. The active form, PMEG diphosphate, was shown to be a potent inhibitor of DNA polymerase α and ß while showing weaker activity against mitochondrial DNA polymerase γ (50% enzyme inhibition observed at 2.5, 1.6, and 59.4 μM, respectively). GS-9191 was markedly more potent than PMEG or cPrPMEDAP in a series of human papillomavirus (HPV)-positive cell lines, with effective concentrations to inhibit 50% cell growth (EC50) as low as 0.03, 207, and 284 nM, respectively. In contrast, GS-9191 was generally less potent in non-HPV-infected cells and primary cells (EC50s between 1 and 15 nM). DNA synthesis was inhibited by GS-9191 within 24 h of treatment; cells were observed to be arrested in S phase by 48 h and to subsequently undergo apoptosis (between 3 and 7 days). In an animal model (cottontail rabbit papillomavirus), topical GS-9191 was shown to decrease the size of papillomas in a dose-related manner. At the highest dose (0.1%), cures were evident at the end of 5 weeks, and lesions did not recur in a 30-day follow-up period. These data suggest that GS-9191 may have utility in the treatment of HPV-induced lesions.

Purpose: GS-9219, a novel prodrug of the nucleotide analogue 9-(2-phosphonylmethoxyethyl)guanine (PMEG), was designed as a cytotoxic agent that preferentially targets lymphoid cells. Our objective was to characterize the antiproliferative activity, pharmacokinetics, pharmacodynamics, and safety of GS-9219.Experimental Design: GS-9219 was selected through screening in proliferation assays and through pharmacokinetic screening. The activation pathway of GS-9219 was characterized in lymphocytes, and its cytotoxic activity was evaluated against a panel of hematopoietic and nonhematopoietic cell types. To test whether the prodrug moieties present in GS-9219 confer an advantage over PMEG in vivo, the pharmacokinetics, pharmacodynamics (lymph node germinal center depletion), and toxicity of equimolar doses of GS-9219 and PMEG were evaluated after i.v. administration to normal beagle dogs. Finally, proof of concept of the antitumor efficacy of GS-9219 was evaluated in five pet dogs with spontaneous, advanced-stage non-Hodgkin's lymphoma (NHL) following a single i.v. administration of GS-9219 as monotherapy.Results: In lymphocytes, GS-9219 is converted to its active metabolite, PMEG diphosphate, via enzymatic hydrolysis, deamination, and phosphorylation. GS-9219 has substantial antiproliferative activity against activated lymphocytes and hematopoietic tumor cell lines. In contrast, resting lymphocytes and solid tumor lines were less sensitive to GS-9219. GS-9219, but not PMEG, depleted the germinal centers in lymphoid tissues of normal beagle dogs at doses that were tolerated. In addition, GS-9219 displayed significant in vivo efficacy in five dogs with spontaneous NHL after a single administration, with either no or low-grade adverse events.Conclusion: GS-9219 may have utility for the treatment of NHL.