Activities of masked 2',3'-dideoxynucleoside monophosphate derivatives against human immunodeficiency virus in resting macrophages

A long acting nanoformulated lamivudine ProTide

A long acting (LA) hydrophobic and lipophilic lamivudine (3TC) was created as a phosphoramidate pronucleotide (designated M23TC). M23TC improved intracellular delivery of active triphosphate metabolites and enhanced antiretroviral and pharmacokinetic (PK) profiles over the native drug. A single treatment of human monocyte derived macrophages (MDM) with nanoformulated M23TC (NM23TC) improved drug uptake, retention, intracellular 3TC triphosphates and antiretroviral activities in MDM and CD4⁺ T cells. PK tests of NM23TC administered to Sprague Dawley rats demonstrated sustained prodrug and drug triphosphate levels in blood and tissues for 30 days. The development of NM23TC remains a substantive step forward in producing LA slow effective release antiretrovirals for future clinical translation.

Phosphoramidates and phosphonamidates (ProTides) with antiviral activity

Following the first report on the nucleoside phosphoramidate (ProTide) prodrug approach in 1990 by Chris McGuigan, the extensive investigation of ProTide technology has begun in many laboratories. Designed with aim to overcome limitations and the key resistance mechanisms associated with nucleoside analogues used in the clinic (poor cellular uptake, poor conversion to the 5'-monophosphate form), the ProTide approach has been successfully applied to a vast number of nucleoside analogues with antiviral and anticancer activity. ProTides consist of a 5'-nucleoside monophosphate in which the two hydroxyl groups are masked with an amino acid ester and an aryloxy component which once in the cell is enzymatically metabolized to deliver free 5'-monophosphate, which is further transformed to the active 5'-triphosphate form of the nucleoside analogue. In this review, the seminal contribution of Chris McGuigan's research to this field is presented. His technology proved to be extremely successful in drug discovery and has led to two Food and Drug Administration-approved antiviral agents.

Facile synthesis of the NNRTI microbicide MC-1220 and synthesis of its phosphoramidate prodrugs

A facile and novel synthetic route to MC-1220 was achieved by condensation of 4,6-dichloro-N,N-5-trimethylpyrimidin-2-amine (1) with the sodium salt of 2,6-difluorophenylacetonitrile, followed by methylation and strong acidic hydrolysis. The prodrugs of MC-1220 were synthesized by reaction of chlorophosphoramidate derivatives (7a-e) or α-acetobromoglucose with the sodium salt of MC-1220. The stability and anti-HIV-1 activity of phosphoramidate prodrugs turned out to be comparable to those of the parent drug MC-1220.

Self-assembled drug delivery systems: Part 3. In vitro/in vivo studies of the self-assembled nanoparticulates of cholesteryl acyl didanosine

Self-assembled drug delivery systems (SADDS) are defined as the self-assemblies of amphiphilic prodrugs, integrating prodrugs, molecular self-assembly and nanotechnology for drug targeting and controlled release. Cholesteryl-succinyl didanosine (CSD) and cholesteryl-adipoyl didanosine (CAD) nanoparticulate systems in water were previously prepared and optimized. In this paper, the in vitro and in vivo behavior of them was investigated. Precipitation occurred when they were mixed with acid solutions due to rapid production of hypoxanthine and subsequent disruption of supramolecular structures. They showed pH-dependent degradation and kept relatively stable in the neutral pH range. CSD is more stable than CAD due to the shorter spacer and poloxamer protection. CSD showed different degradation rates in various plasma with the descending order of rat, mouse, rabbit, dog and human. The half-life (t(1/2)) of CSD is 9 days in rat plasma, and 5.9 days in rat liver homogenates. CAD has a faster degradation than CSD though the t(1/2) in rat liver homogenates is long to 23 h. CSD nanoparticulates showed no significant anti-HIV effect in MT4 cell model because of very slow degradation. CSD nanoparticulates showed the distribution t(1/2) of 7.6 min after bolus intravenous (i.v.) administration to rats, and the site-specific distribution in liver, lung and spleen with the high t(1/2) of 10 days in liver. The factors affecting achievement of successful SADDS are discussed.

Therapeutic strategies towards HIV-1 infection in macrophages

It is widely recognized that macrophages (M/M) represent a crucial target of HIV-1 in the body and play a pivotal role in the pathogenic progression of HIV-1 infection. This strongly supports the clinical relevance of therapeutic strategies able to interfere with HIV-1 replication in M/M. In vitro studies showed that nucleoside analogue inhibitors of HIV-1 reverse transcriptase have potent antiviral activity in M/M, although the limited penetration of these compounds in sequestered body compartments and low phosphorylation ability of M/M, suggest that a phosphonate group linked to NRTIs may confer greater anti-HIV-1 activity in M/M. Differently, the antiviral activity of non-nucleoside reverse transcriptase inhibitors in M/M is similar to that found in CD4+ lymphocytes. Interestingly, protease inhibitors, acting at a post-integrational stage of HIV-1 life-cycle are the only drugs active in chronically infected M/M. A careful analysis of the distribution of antiviral drugs, and the assessment of their activity in M/M, represent key factors in the development of therapeutic strategies aimed to the treatment of HIV-1-infected patients. Moreover, testing new and promising antiviral compounds in such cells may provide crucial hints about their efficacy in patients infected by HIV.

Polymer-bound oxathiaphospholane: a solid-phase reagent for regioselective monothiophosphorylation and monophosphorylation of unprotected nucleosides and carbohydrates

Two polymers bound to N,N-diisopropylamino-1,3,2-oxathiaphospholane were reacted with unprotected carbohydrates and nucleosides in the presence of 1H-tetrazole, followed by oxidation with tert-butyl hydroperoxide or sulfurization with Beaucage's reagent. The 1,3,2-oxathiaphospholane ring-opening with 3-hydroxypropionitrile, followed by treatment with DBU, afforded the corresponding monophosphate and monothiophosphate derivatives, respectively, through the elimination of polymer-bound ethylene episulfide. Reactions using this strategy offer the advantages of high regioselectivity, monosubstitution, and facile isolation and recovery of products.

Solid-Phase Reagents for Selective Monophosphorylation of Carbohydrates and Nucleosides

Two classes of aminomethyl polystyrene resin-bound linkers of p-acetoxybenzyl alcohol were subjected to reactions with 2-cyanoethyl N,N-diisopropylchlorophosphoramidite to produce the corresponding polymer-bound phosphitylating reagents. These were reacted with a number of unprotected nucleosides and carbohydrates in the presence of 1H-tetrazole. Oxidation with tert-butyl hydroperoxide followed by removal of the cyanoethoxy group with 1,8-diazabicyclo[5.4.0]undec-7-ene afforded the corresponding polymer-bound phosphate diesters. Acidic cleavage of the p-acetoxybenzyl alcohol linker yielded monophosphorylated products with high regioselectivity and trapped linkers on the resins that can be reused.

Inhibition of human immunodeficiency virus replication by a dual CCR5/CXCR4 antagonist

Here we report that the N-pyridinylmethyl cyclam analog AMD3451 has antiviral activity against a wide variety of R5, R5/X4, and X4 strains of human immunodeficiency virus type 1 (HIV-1) and HIV-2 (50% inhibitory concentration [IC(50)] ranging from 1.2 to 26.5 microM) in various T-cell lines, CCR5- or CXCR4-transfected cells, peripheral blood mononuclear cells (PBMCs), and monocytes/macrophages. AMD3451 also inhibited R5, R5/X4, and X4 HIV-1 primary clinical isolates in PBMCs (IC(50), 1.8 to 7.3 microM). A PCR-based viral entry assay revealed that AMD3451 blocks R5 and X4 HIV-1 infection at the virus entry stage. AMD3451 dose-dependently inhibited the intracellular Ca(2+) signaling induced by the CXCR4 ligand CXCL12 in T-lymphocytic cells and in CXCR4-transfected cells, as well as the Ca(2+) flux induced by the CCR5 ligands CCL5, CCL3, and CCL4 in CCR5-transfected cells. The compound did not interfere with chemokine-induced Ca(2+) signaling through CCR1, CCR2, CCR3, CCR4, CCR6, CCR9, or CXCR3 and did not induce intracellular Ca(2+) signaling by itself at concentrations up to 400 microM. In freshly isolated monocytes, AMD3451 inhibited the Ca(2+) flux induced by CXCL12 and CCL4 but not that induced by CCL2, CCL3, CCL5, and CCL7. The CXCL12- and CCL3-induced chemotaxis was also dose-dependently inhibited by AMD3451. Furthermore, AMD3451 inhibited CXCL12- and CCL3L1-induced endocytosis in CXCR4- and CCR5-transfected cells. AMD3451, in contrast to the specific CXCR4 antagonist AMD3100, did not inhibit but enhanced the binding of several anti-CXCR4 monoclonal antibodies (such as clone 12G5) at the cell surface, pointing to a different interaction with CXCR4. AMD3451 is the first low-molecular-weight anti-HIV agent with selective HIV coreceptor, CCR5 and CXCR4, interaction.

Improved antiviral activity of the aryloxymethoxyalaninyl phosphoramidate (APA) prodrug of abacavir (ABC) is due to the formation of markedly increased carbovir 5'-triphosphate metabolite levels

The anti-human immunodeficiency virus (HIV) activity of abacavir (ABC; 1-(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-methanol) could be markedly enhanced by administering the aryloxymethoxyalaninyl phosphoramidate prodrug derivative of ABC (pro-ABC-MP) to virus-infected cell cultures. Metabolic studies with radiolabeled ABC and pro-ABC-MP in human T-lymphocyte and primary macrophage cell cultures revealed a significantly increased delivery of the activated (phosphorylated) metabolite of ABC (ABC-MP) by pro-ABC-MP, and the concomittant appearance of markedly higher intracellular levels of carbovir 5'-triphosphate (CBV-TP), which represents the eventual antivirally active metabolite of ABC. The intracellular amounts of ABC-MP and appearance of CBV-TP closely correlated with the extracellular pro-ABC-MP concentrations that were administered to the cell cultures within a concentration range between 0.5 and 100 microM. The highest amounts of CBV-TP were observed within 6-24 h after drug administration. The improved delivery of ABC-MP and metabolic conversion to CBV-TP explain the markedly enhanced antiviral activity of the prodrug of ABC, and warrant further exploration of this prodrug technology on ABC and related compounds to further enhance and optimize their antiviral efficacy.

Potent antiviral activity of amprenavir in primary macrophages infected by human immunodeficiency virus

Objective of the present study was then to assess the antiviral activity of the protease inhibitor amprenavir in macrophages (M/M), and to compare it with its efficacy in peripheral blood lymphocytes (PBL). M/M were obtained from blood of sero-negative healthy donors and infected with M-tropic HIV-1 strain (HIV-1(Ba-L)). The stabilized infection was assessed by monitoring the HIV-1 p24 gag antigen production in the supernatants of M/M cultures. In the setting of acute infection (treatment before HIV-1 challenge), amprenavir showed substantial activity both in M/M and PBL at similar concentrations (EC(50): 0.011 and 0.031 microM, respectively); complete inhibition of HIV-1 replication was achieved in both cell types at concentration of about 2 microM. In the setting of chronical infection (i.e. antiviral treatment several days after established infection), an antiviral effect of amprenavir was achieved in M/M, but at concentrations higher than those active in acutely infected M/M (EC(50): 0.72 microM, EC(90): 18.2 microM). The antiviral effect in chronically infected M/M was sustained for at least 2 weeks of continuous treatment. These findings suggest that amprenavir (at relatively high concentrations) has a clinically relevant antiviral effect in persistently infected reservoirs of HIV.

New anti-HIV agents and targets

Virtually all the compounds that are currently used or are subject of advanced clinical trials for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside reverse transcriptase inhibitors (NRTIs): i.e., zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, emtricitabine and nucleotide reverse transcriptase inhibitors (NtRTIs) (i.e., tenofovir disoproxil fumarate); (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e., nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e., saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, and lopinavir. In addition to the reverse transcriptase and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 (i.e., bicyclam (AMD3100) derivatives) and CCR5 (i.e., TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs, and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e., phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs ( i.e., TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii), as in the case of PIs, a different, modified peptidic (i.e., azapeptidic (atazanavir)) or non-peptidic scaffold (i.e., cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)). Non-peptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs.

Macrophages and HIV infection: therapeutical approaches toward this strategic virus reservoir

Cells of macrophage lineage represent a key target of human immunodeficiency virus (HIV) in addition to CD4-lymphocytes. The absolute number of infected macrophages in the body is relatively low compared to CD4-lymphocytes. Nevertheless, the peculiar dynamics of HIV replication in macrophages, their long-term survival after HIV infection, and their ability to spread virus particles to bystander CD4-lymphocytes, make evident their substantial contribution to the pathogenesis of HIV infection. In addition, infected macrophages are able to recruit and activate CD4-lymphocytes through the production of both chemokines and virus proteins (such as nef). In addition, the activation of the oxidative pathway in HIV-infected macrophages may lead to apoptotic death of bystander, not-infected cells. Finally, macrophages are the most important target of HIV in the central nervous system. The alteration of neuronal metabolism induced by infected macrophages plays a crucial role in the pathogenesis of HIV-related encephalopathy. Taken together, these results strongly support the clinical relevance of therapeutic strategies able to interfere with HIV replication in macrophages. In vitro data show the potent efficacy of all nucleoside analogues inhibitors of HIV-reverse transcriptase in macrophages. Nevertheless, the limited penetration of some of these compounds in sequestered districts, coupled with the scarce phosphorylation ability of macrophages, suggests that nucleoside analogues carrying preformed phosphate groups may have a potential role against HIV replication in macrophages. This hypothesis is supported by the great anti-HIV activity of tenofovir and other acyclic nucleoside phosphonates in macrophages that may provide a rationale for the remarkable efficacy of tenofovir in HIV-infected patients. Non-nucleoside reverse transcriptase inhibitors (NNRTI) do not affect HIV-DNA chain termination, and for this reason their antiviral activity in macrophages is similar to that found in CD4-lymphocytes. Interestingly, protease inhibitors (PIs), acting at post-integrational stages of virus replication, are the only drugs able to interfere with virus production and release from macrophages with established and persistent HIV infection (chronically-infected cells). Since this effect is achieved at concentrations and doses higher than those effective in de-novo infected CD4-lymphocytes, it is possible that lack of adherence to therapy, and/or suboptimal dosage leading to insufficient concentrations of PIs may cause a resumption of virus replication from chronically-infected macrophages, ultimately resulting in therapeutic failure. For all these reasons, therapeutic strategies aimed to achieve the greatest and longest control of HIV replication should inhibit HIV not only in CD4-lymphocytes, but also in macrophages. Testing new and promising antiviral compounds in such cells may provide crucial hints about their efficacy in patients infected by HIV.

New developments in anti-HIV chemotherapy

Virtually all the compounds that are currently used, or are subject of advanced clinical trials, for the treatment of human immunodeficiency virus (HIV) infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): i.e. zidovudine (AZT), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), lamivudine (3TC), abacavir (ABC), emtricitabine [(-)FTC], tenofovir disoproxil fumarate; (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e. nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e. saquinavir, ritonavir, indinavir, nelfinavir, amprenavir and lopinavir. In addition to the reverse transcriptase (RT) and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 [bicyclam (AMD3100) derivatives] and CCR5 (TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e. phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs (i.e. TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii) as in the case of PIs, a different, nonpeptidic scaffold [i.e. cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)]. Nonpeptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating the mode of action of these agents from cell-free enzymatic assays to intact cells. Two examples in point are L-chicoric acid and the nonapeptoid CGP64222, which were initially described as an integrase inhibitor or Tat antagonist, respectively, but later shown to primarily act as virus adsorption/entry inhibitors, the latter through blockade of CXCR4.

Levofloxacin penetration into epithelial lining fluid as determined by population pharmacokinetic modeling and monte carlo simulation

Levofloxacin was administered orally to steady state to volunteers randomly in doses of 500 and 750 mg. Plasma and epithelial lining fluid (ELF) samples were obtained at 4, 12, and 24 h after the final dose. All data were comodeled in a population pharmacokinetic analysis employing BigNPEM. Penetration was evaluated from the population mean parameter vector values and from the results of a 1,000-subject Monte Carlo simulation. Evaluation from the population mean values demonstrated a penetration ratio (ELF/plasma) of 1.16. The Monte Carlo simulation provided a measure of dispersion, demonstrating a mean ratio of 3.18, with a median of 1.43 and a 95% confidence interval of 0.14 to 19.1. Population analysis with Monte Carlo simulation provides the best and least-biased estimate of penetration. It also demonstrates clearly that we can expect differences in penetration between patients. This analysis did not deal with inflammation, as it was performed in volunteers. The influence of lung pathology on penetration needs to be examined.

Intracellular pharmacology of nucleoside analogues and protease inhibitors: role of transporter molecules

Antiretroviral agents target HIV replication within infected cells. It is therefore important to focus on the pharmacology of these drugs at their site of action rather than just in plasma. Activation of nucleoside analogues to a triphosphate is essential for antiretroviral activity. Following activation, by intracellular kinases, drug triphosphates compete with endogenous triphosphates for HIV reverse transcriptase. Methodologies to measure triphosphates in peripheral blood mononuclear cells from HIV patients have been described. This has allowed investigation of once-daily dosing regimens, drug interactions, modulation of intracellular activation and the bypassing of initial phosphorylation steps. Drug accumulation within a cell is a balance between influx and efflux. There is a growing body of evidence indicating that transport proteins are vitally important in regulating intracellular concentrations of antiretroviral drugs. Allelic variants, inhibition (or induction) are all potentially critical determinants of active drug present in the cell. It is hoped that understanding the intracellular pharmacology will improve long-term therapy and reduce the likelihood of cellular resistance in therapeutic failure.

An Introduction to Nucleoside and Nucleotide Analogues

The introduction of newer and more potent agents has diverted attention away from the importance of nucleoside analogue reverse transcriptase inhibitors (NRTIs) in modern antiretroviral drug regimens. As a class, these proviral chain terminators lack the virological potency of either non-nucleoside reverse transcriptase inhibitor (NNRTI) or protease inhibitor (PI) drugs, due largely to their competitive mode of inhibition and requirement for metabolic activation. However, neither NNRTIs nor PIs alone can maintain the complete suppression of HIV replication required for extended therapy, and both suffer from serious class cross-resistance on therapeutic failure. Thus, the NRTIs will remain essential components of highly active antiretroviral therapy (HAART) for the foreseeable future, both for their contribution to a regimen's virological potency and the subsequent preservation of the more potent drug classes used with them. However, it has become apparent in recent years that the current NRTIs exhibit duration-dependent adverse events as a class, which may limit the length of time for which they can be safely used. An independent contribution to peripheral fat wasting in lipodystrophy syndrome has been established for the use of NRTI drugs. Of greater clinical concern is their established association with potentially fatal lactic acidaemia and hepatic steatosis. Both these class events, as well as several individual drug events, such as peripheral neuropathy, can be linked to progressive mitochondrial destruction with a greater or lesser degree of confidence. Mitochondrial toxicity, due in large part to the high affinity of several NRTI agents for uptake by mitochondrial DNA polymerase γ, has been demonstrated both in vitro and in vivo. New chain-terminating agents are urgently needed that address issues of improved virological potency, greater efficacy in NRTI-experienced individuals, and greater long-term safety. The nucleotide class of reverse transcriptase inhibitor (NtRTI), currently under clinical development, addresses improved potency by abbreviating the intracellular activation pathway to allow a more rapid and complete conversion to the active agent. These nucleoside monophosphate analogues are taken as masked prodrugs bearing labile lipophilic groups to facilitate penetration of target cell membranes. Subsequent unmasking by endogenous chemolytic enzymes releases a partially activated nucleoside analogue metabolite. The NtRTI furthest along the developmental process is tenofovir disoproxil fumarate (TDF), an orally available acyclic adenine phosphonate analogue, currently in Phase III clinical trials. This agent has shown high potency and an unusually durable response in trials of single-agent therapy intensification in highly treatment-experienced individuals, and its active metabolite, tenofovir diphosphate, exhibits a long intracellular half-life in both resting and activated peripheral blood mononuclear cells that permits once daily dosing. Tenofovir diphosphate also exhibits a very low affinity for DNA polymerase γ in vitro, suggesting a low degree of in vivo mitochondrial toxicity may be observed on long-term follow-up, although clinical data to support this inference are not yet available. The introduction of TDF and other NtRTIs as ‘second-generation’ nucleoside analogues carefully evaluated for potential long-term toxicity, can be expected to significantly improve the therapeutic options for both those currently on HAART and those yet to begin.