Synthesis and antiviral activity of rigid acyclonucleotide analogs

Anti-HIV Activity of MDL 74968, a Novel Acyclonucleotide Derivative of Guanine: Drug Resistance and Drug Combination Effects in Vitro

MDL 74968 (9-[2-methylidene-3-(phosphonomethoxy)-propyl]guanine), a novel acyclonucleotide derivative of guanine, inhibited human immunodeficiency virus type 1 (HIV-1) replication in vitro with activity comparable to that of adefovir (PMEA; 9-(2-phosphonomethoxyethyl)adenine). MDL 74968 was investigated in combination with two licensed nucleoside analogues, zidovudine and didanosine, using a cell viability assay, and drug interactions were evaluated by the isobologram technique, by calculating combination indices and by the MacSynergy™ program. Inhibition of HIV-1 replication was only additive in both cases. MDL 74968 had equivalent antiviral activity against strains of HIV-1 HXB2 engineered to have mutations which conferred resistance to the nucleoside analogues lamivudine, didanosine and zidovudine and the non-nucleoside inhibitor of reverse transcriptase (RT) nevirapine, as against the wild type strain. Continued passage of HIV-1 RF in C8166 cells in the presence of MDL 74968 for 5 months (30 passages) failed to select drug resistant mutants. Continued passage of virus in the presence of the same concentration of adefovir for the same length of time selected a virus in a single culture, which was 3-fold resistant to adefovir and cross-resistant to MDL 74968. Genotypic characterization of this virus revealed a lysine to arginine exchange (AAA to AGA) at position 65 in the RT gene. This virus was not cross-resistant to either zidovudine or nevirapine but showed reduced sensitivity to zalcitabine, didanosine and lamivudine. Continued passage of HIV-1 RF in the presence of nevirapine or zidovudine, using similar experimental protocols selected drug resistant viruses after eight and 17 passages, respectively, but these viruses remained sensitive to adefovir and MDL 74968.

Synthesis and biological evaluation of acyclic phosphonic acid nucleoside derivatives

As part of a project to generate a library of nucleosides as potential antiviral agents, a small subset of novel acyclic phosphonic acid nucleosides was prepared. Practical synthetic routes are described for three targets, which were then tested against HIV, hepatitis C virus (HCV), and Dengue virus.

Short synthesis and antiviral activity of acyclic phosphonic acid nucleoside analogues

An efficient route for synthesizing novel allylic and cyclopropanoid phosphonic acid nucleoside analogues is described. The condensation of the bromine derivatives 6 and 18 with nucleoside bases (A, U, T, C, G) under standard nucleophilic substitution and deprotection conditions, afforded the target phosphonic acid nucleoside analogues. These compounds were evaluated for their antiviral properties against various viruses. Cyclopropanoid phosphonic adenine nucleoside analogue 23 showed significant anti-HIV activity.

Synthesis of unsaturated carboacyclic nucleoside analogues via Mitsunobu reactions

2-Substituted allyl alcohols 9 and 14 were prepared starting from butane-1,2,4-triol and glycerol, respectively. Mitsunobu condensations of 9 and 14 with purine and pyrimidine bases, followed by deprotection, afforded a number of acyclonucleosides having 4-hydroxy-2-methylenebutyl or 3,3-bis(hydroxymethyl)-2-methylenepropyl chain.

Enhanced 9-(2-phosphonylmethoxyethyl)adenine secretion by a specific, indomethacin-sensitive efflux pump in a mutant 9-(2-phosphonylmethoxyethyl)adenine-resistant human erythroleukemia K562 cell line

We have investigated the molecular basis of the 100-fold resistance of mutant human erythroleukemia K562/PMEA-1 cells to the antiproliferative potential of 9-(2-phosphonylmethoxyethyl)adenine (PMEA). Upon exposure to high PMEA concentrations, comparable intracellular PMEA levels were initially observed in mutant K562/PMEA-1 and wild-type K562/0 cells, indicating that PMEA influx was unaltered. However, after 4 hr of exposure to 0.2 microM [3H]bis(pivaloyloxymethyl)-PMEA [bis(POM)-PMEA], the total intracellular level of unphosphorylated and mono- and diphosphorylated PMEA was 2.8-fold lower in K562/PMEA-1 than in K562/0 cells. Increased PMEA secretion from K562/PMEA-1 cells (compared with K562/0 cells) became more pronounced upon prolonged exposure to bis(POM)-PMEA; after 24 hr, K562/PMEA-1 cells showed 65-fold lower total intracellular PMEA levels than K562/0 cells and at 48 hr, >400-fold less total PMEA was detected in K562/PMEA-1 cells. In addition, PMEA phosphorylation was 25- to 50-fold less efficient in K562/PMEA-1 than in K562/0 cells, pointing to an additional defect at the level of the metabolism of PMEA. The PMEA efflux mechanism was shown to be temperature- and azide-dependent, was markedly inhibited by indomethacin, and did not recognize adenine nucleotides or the phosphorylated metabolites of 3'-azido-3'-deoxythymidine. Also, over a 28-hr period, PMEA efflux was not affected by an inhibitor of RNA synthesis (actinomycin D) or protein synthesis (cycloheximide). Our studies revealed that resistance of K562/PMEA-1 cells to PMEA is the combined result of a severely impaired PMEA phosphorylation on the one hand, and an enhanced PMEA secretion by a highly specific, indomethacin-sensitive efflux pump, different from the classical P-glycoprotein- and multidrug resistance protein-mediated resistance mechanisms, on the other hand.

Antiretroviral activities of acyclic nucleoside phosphonates [9-(2-phosphonylmethoxyethyl)adenine, 9-(2-phosphonylmethoxyethyl)guanine, (R)-9-(2-phosphonylmethoxypropyl)adenine, and MDL 74,968] in cell cultures and murine sarcoma virus-infected newborn NMRI mice

From a side-by-side comparative study, the acyclic nucleoside phosphonates (R)-9-(2-phosphonylmethoxypropyl)adenine [(R)-PMPA] and 9-(2-methylidene-3-phosphonomethoxypropyl)guanine (MDL 74,968) proved more selective in their inhibitory effect on human immunodeficiency virus types 1 and 2, feline immunodeficiency virus, and Moloney murine sarcoma virus (MSV) in cell cultures than the 9-(2-phosphonylmethoxyethyl) derivatives of adenine (PMEA) and guanine (PMEG). In particular, PMEG proved quite toxic. PMEA, (R)-PMPA, and MDL 74,968 afforded a marked delay in MSV-induced tumor initiation in MSV-infected newborn NMRI mice and substantially delayed associated animal death at doses as low as 4 to 10 mg/kg of body weight. Treatment of the NMRI mice with PMEA, (R)-PMPA, and MDL 74,968 at 25 or 50 mg/kg resulted in a high percentage of long-term survivors.

MDL 74,968, a new acyclonucleotide analog: activity against human immunodeficiency virus in vitro and in the hu-PBL-SCID.beige mouse model of infection

The novel acyclonucleotide derivative of guanine, 9-[2-methylidene-3-(phosphonomethoxy)propyl] guanine (MDL 74,968), had antiviral activity comparable to those of 9-(2-phosphonomethoxyethyl) adenine (PMEA) and 2',3'-dideoxyinosine against laboratory strains of both human immunodeficiency virus (HIV) types 1 and 2 cultured in MT-4 cells and several clinical HIV isolates cultured in human peripheral blood mononuclear cells (PBMCs). MDL 74,968 was at least fourfold less toxic than PMEA to MT-4 cells or PBMCs, thereby producing a more favorable in vitro selectivity index for the former compound. Studies of acute toxicity in CD-1 mice showed that MDL 74,968 was not toxic at doses of 1,600 mg/kg of body weight via the intraperitoneal route or at doses of 500 mg/kg via the intravenous route. Furthermore, no adverse effects of MDL 74,968 were apparent when mice were treated at doses of 200 mg/kg twice daily for 5 days. Treatment by continuous subcutaneous infusion of MDL 74,968 or PMEA at the daily dose of 20 mg/kg in the hu-PBL-SCID.beige murine model of HIV infection significantly reduced the severity of infection compared with that in placebo-treated controls. Quantitation of virus recovery by endpoint titration of spleen cells in coculture with mitogen-activated PBMCs demonstrated that MDL 74,968 as well as PMEA significantly reduced the amount of virus (P < 0.02). Moreover, by using DNA extracted from spleens, the mean HIV:HLA PCR product ratio, which takes into account individual variation in immune system reconstitution, were 0.50 and 0.40 for MDL 74,968 and PMEA treatments, respectively, whereas animals receiving the placebo control had significantly higher levels of HIV proviral DNA (mean 0.78; P < 0.02). Taken together, these promising findings suggest that an orally bioavailable prodrug of MDL 74,968 should be developed for the treatment of HIV infection.