l-2',3'-Didehydro-2',3'-dideoxy-3'-fluoronucleosides: synthesis, anti-HIV activity, chemical and enzymatic stability, and mechanism of resistance.

As antiviral nucleosides containing a 2',3'-unsaturated sugar moiety with 2'-fluoro substitution are endowed with increased stabilization of the glycosyl bond, it was of interest to investigate the influence of the fluorine atom at the 3'-position. Various pyrimidine and purine L-3'-fluoro-2',3'-unsaturated nucleosides were synthesized from their precursors, L-3',3'-difluoro-2',3'-dideoxy nucleosides, by elimination of hydrogen fluoride. In the L-3',3'-difluoro-2',3'-dideoxy nucleoside series, cytidine 16 and 5-fluorocytidine 18 analogues showed modest antiviral activity (EC(50) 11.5 and 8.8 microM, respectively) when evaluated against HIV-1 in human peripheral blood mononuclear (PBM) cells. In the 2',3'-unsaturated series, L-3'-fluoro-2',3'-didehydro-2',3'-dideoxycytidine 24 and 5-fluorocytidine 26 showed highly potent antiviral activity (EC(50) 0.089 and 0.018 microM, respectively) without significant cytotoxicity. The guanosine analogue 48 showed only marginal anti-HIV activity with some cytotoxicity (EC(50) 38.5 microM, and IC(50) 17.4, 58.4, 36.5 microM in PBM, CEM, and Vero cells, respectively). The cytidine 24 and 5-fluorocytidine 26 analogues, however, showed significantly decreased antiviral activity against the clinically important lamivudine-resistant variants (HIV-1(M184V)). Molecular modeling studies demonstrated that the 3'-fluoro atom of the L-3'-fluoro-2',3'-unsaturated nucleoside is within the hydrogen bonding distance with the amide backbone of Asp185, which favors the binding of the nucleoside triphosphate to the wild-type RT. This favorable binding mode, however, cannot be maintained when the triphosphate of 3'-fluoro 2',3'-unsaturated nucleoside binds to the active site of M184V RT because the bulky side chain of Val184 occupies the space needed for the nucleotide. The biological results suggest that, in addition to the sugar conformation, the base moiety may also play a role in their interaction with the M184V RT.

N4-acyl-modified D-2',3'-dideoxy-5-fluorocytidine nucleoside analogues with improved antiviral activity.

A series of 2',3'-dideoxy (D2) and 2',3'-didehydro-2',3'-dideoxy (D4) 5-fluorocytosine nucleosides modified with substituted benzoyl, heteroaromatic carbonyl, cycloalkylcarbonyl and alkanoyl at the N4-position were synthesized and evaluated for anti-human immunodeficiency virus type 1 (HIV-1) and anti-hepatitis B virus (HBV) activity in vitro. For most D2-nucleosides, N4-substitutions improved the anti-HIV-1 activity markedly without increasing the cytotoxicity. In the D4-nucleosides series, some of the substituents at the N4-position enhanced the anti-HIV-1 activity with a modest increase in the cytotoxicity. The most potent and selective N4-modified nucleoside for the D2-series was N4-p-iodobenzoyl-D2FC, which had a 46-fold increase in anti-HIV-1 potency in MT-2 cells compared to the parent nucleoside D-D2FC. In the D4-series, N4-p-bromobenzoyl-D4FC was 12-fold more potent in MT-2 cells compared to the parent nucleoside D-D4FC. All eight N4-p-halobenzoyl-substituted D2- and D4-nucleosides evaluated against HBV in HepAD38 cells demonstrated equal or greater potency than the two parental compounds, D-D2FC and D-D4FC. The N4-modification especially in the D2-nucleoside series containing the N4-nicotinoyl, o-nitrobenzoyl and n-butyryl showed a significant reduction in mitochondrial toxicity relative to the parent nucleoside analogue. Although the 5'-triphosphate of the parent compound (D-D4FC-TP) was formed from the N4-acyl-D4FC analogues in different cells, the levels of the 5'-triphosphate nucleotide did not correlate with the cell-derived 90% effective antiviral concentrations (EC90), suggesting that a direct interaction of the triphosphates of these N4-acyl nucleosides was involved in the antiviral activity.

Antiviral and cellular metabolism interactions between Dexelvucitabine and lamivudine.

Studies on cellular drug interactions with antiretroviral agents prior to clinical trials are critical to detect possible drug interactions. Herein, we demonstrated that two 2'-deoxycytidine antiretroviral agents, dexelvucitabine (known as beta-d-2',3'-didehydro-2',3'-dideoxy-5-fluorocytidine, DFC, d-d4FC, or RVT) and lamivudine (3TC), combined in primary human peripheral blood mononuclear (PBM) cells infected with human immunodeficiency virus 1 strain LAI (HIV-1(LAI)), resulted in additive-to-synergistic effects. The cellular metabolism of DFC and 3TC was studied in human T-cell lymphoma (CEM) and in primary human PBM cells to determine whether this combination caused any reduction in active nucleoside triphosphate (NTP) levels, which could decrease with their antiviral potency. Competition studies were conducted by coincubation of either radiolabeled DFC with different concentrations of 3TC or radiolabeled 3TC with different concentrations of DFC. Coincubation of radiolabeled 3TC with DFC at concentrations up to 33.3 microM did not cause any marked reduction in 3TC-triphosphate (TP) or any 3TC metabolites. However, a reduction in the level of DFC metabolites was noted at high concentrations of 3TC with radiolabeled DFC. DFC-TP levels in CEM and primary human PBM cells decreased by 88% and 94%, respectively, when high concentrations of 3TC (33.3 and 100 microM) were added, which may influence the effectiveness of DFC-5'-TP on the HIV-1 polymerase. The NTP levels remained well above the median (50%) inhibitory concentration for HIV-1 reverse transcriptase. These results suggest that both beta-d- and beta-l-2'-deoxycytidine analogs, DFC and 3TC, respectively, substrates of 2'-deoxycytidine kinase, could be used in a combined therapeutic modality. However, it may be necessary to decrease the dose of 3TC for this combination to prove effective.

Mechanism of anti-human immunodeficiency virus activity of beta-D-6-cyclopropylamino-2',3'-didehydro-2',3'-dideoxyguanosine.

To better understand the importance of the oxygen in the ribose ring of planar unsaturated nucleoside analogs that target human immunodeficiency virus (HIV), a 6-cyclopropyl-substituted prodrug of 2',3'-didehydro-2',3'-dideoxyguanosine (cyclo-d4G) was synthesized, and its cellular metabolism, antiviral activity, and pharmacokinetic behavior were studied. Cyclo-d4G had selective anti-HIV activity in primary blood mononuclear cells (PBMCs), effectively inhibiting the LAI strain of HIV-1 by 50% at 1.1 +/- 0.1 microM while showing 50% inhibition of cell viability at 84.5 microM. The antiviral activity in PBMCs was not markedly affected by mutations of methionine to valine at position 184 or by thymidine-associated mutations in the viral reverse transcriptase. Mutations of leucine 74 to valine and of lysine 65 to arginine had mild to moderate resistance (as high as fivefold). Studies to delineate the mechanism of cellular metabolism and activation of cyclo-d4G showed reduced potency in inhibiting viral replication in the presence of the adenosine/adenylate deaminase inhibitor 2'-deoxycoformycin, implying that the antiviral activity is due to its metabolism to the 2'-dGTP analog d4GTP. Intracellular formation of sugar catabolites illustrates the chemical and potentially enzymatic instability of the glycosidic linkage in d4G. Further studies suggest that cyclo-d4G has a novel intracellular phosphorylation pathway. Cyclo-d4G had a lower potential to cause mitochondrial toxicity than 2',3'-dideoxycytidine and 2',3'-didehydro-3'-deoxythymidine in neuronal cells. Also, cyclo-d4G had advantageous synergism with many currently used anti-HIV drugs. Poor oral bioavailability observed in rhesus monkeys may be due to the labile glycosidic bond, and special formulation may be necessary for oral delivery.

2('),3(')-didehydro-2('),3(')-dideoxynucleosides are degraded to furfuryl alcohol under acidic conditions.

2('),3(')-Didehydro-2('),3(')-dideoxynucleosides are clinically relevant antiviral agents. These nucleosides could be degraded under acidic conditions. Acidic stability studies showed the D4N had the following increasing stability order: D4G

Antiviral activities and cellular toxicities of modified 2',3'-dideoxy-2',3'-didehydrocytidine analogues.

The antiviral efficacies and cytotoxicities of 2',3'- and 4'-substituted 2',3'-didehydro-2',3'-dideoxycytidine analogs were evaluated. All compounds were tested (i) against a wild-type human immunodeficiency virus type 1 (HIV-1) isolate (strain xxBRU) and lamivudine-resistant HIV-1 isolates, (ii) for their abilities to inhibit hepatitis B virus (HBV) production in the inducible HepAD38 cell line, and (iii) for their abilities to inhibit bovine viral diarrhea virus (BVDV) production in acutely infected Madin-Darby bovine kidney cells. Some compounds demonstrated potent antiviral activities against the wild-type HIV-1 strain (range of 90% effective concentrations [EC(90)s], 0.14 to 5.2 micro M), but marked increases in EC(90)s were noted when the compounds were tested against the lamivudine-resistant HIV-1 strain (range of EC(90)s, 53 to >100 micro M). The beta-L-enantiomers of both classes of compounds were more potent than the corresponding beta-D-enantiomers. None of the compounds showed antiviral activity in the assay that determined their abilities to inhibit BVDV, while two compounds inhibited HBV production in HepAD38 cells (EC(90), 0.25 micro M). The compounds were essentially noncytotoxic in human peripheral blood mononuclear cells and HepG2 cells. No effect on mitochondrial DNA levels was observed after a 7-day incubation with the nucleoside analogs at 10 micro M. These studies demonstrate that (i) modification of the sugar ring of cytosine nucleoside analogs with a 4'-thia instead of an oxygen results in compounds with the ability to potently inhibit wild-type HIV-1 but with reduced potency against lamivudine-resistant virus and (ii) the antiviral activity of beta-D-2',3'-didehydro-2',3'-dideoxy-5-fluorocytidine against wild-type HIV-1 (EC(90), 0.08 micro M) and lamivudine-resistant HIV-1 (EC(90) = 0.15 micro M) is markedly reduced by introduction of a 3'-fluorine in the sugar (EC(90)s of compound 2a, 37.5 and 494 micro M, respectively).