Early nucleoside reverse transcriptase inhibitors for the treatment of HIV: a brief history of stavudine (D4T) and its comparison with other dideoxynucleosides

Polymer-Based Nanoparticles as Drug Delivery Systems for Purines of Established Importance in Medicine

Many purine derivatives are active pharmaceutical ingredients of significant importance in the therapy of autoimmune diseases, cancers, and viral infections. In many cases, their medical use is limited due to unfavorable physicochemical and pharmacokinetic properties. These problems can be overcome by the preparation of the prodrugs of purines or by combining these compounds with nanoparticles. Herein, we aim to review the scientific progress and perspectives for polymer-based nanoparticles as drug delivery systems for purines. Polymeric nanoparticles turned out to have the potential to augment antiviral and antiproliferative effects of purine derivatives by specific binding to receptors (ASGR1-liver, macrophage mannose receptor), increase in drug retention (in eye, intestines, and vagina), and permeation (intranasal to brain delivery, PEPT1 transport of acyclovir). The most significant achievements of polymer-based nanoparticles as drug delivery systems for purines were found for tenofovir disoproxil in protection against HIV, for acyclovir against HSV, for 6-mercaptopurine in prolongation of mice ALL model life, as well as for 6-thioguanine for increased efficacy of adoptively transferred T cells. Moreover, nanocarriers were able to diminish the toxic effects of acyclovir, didanosine, cladribine, tenofovir, 6-mercaptopurine, and 6-thioguanine.

Nucleoside Analog Reverse-Transcriptase Inhibitors in Membrane Environment: Molecular Dynamics Simulations

The behavior of four drugs from the family of nucleoside analog reverse-transcriptase inhibitors (zalcitabine, stavudine, didanosine, and apricitabine) in a membrane environment was traced using molecular dynamics simulations. The simulation models included bilayers and monolayers composed of POPC and POPG phospholipids. It was demonstrated that the drugs have a higher affinity towards POPG membranes than POPC membranes due to attractive long-range electrostatic interactions. The results obtained for monolayers were consistent with those obtained for bilayers. The drugs accumulated in the phospholipid polar headgroup region. Two adsorption modes were distinguished. They differed in the degree of penetration of the hydrophilic headgroup region. Hydrogen bonds between drug molecules and phospholipid heads were responsible for adsorption. It was shown that apricitabine penetrated the hydrophilic part of the POPC and POPG membranes more effectively than the other drugs. Van der Waals interactions between S atoms and lipids were responsible for this.

Physiologically Based Pharmacokinetic (PBPK) Model of Gold Nanoparticle-Based Drug Delivery System for Stavudine Biodistribution

Computational modelling has gained attention for evaluating nanoparticle-based drug delivery systems. Physiologically based pharmacokinetic (PBPK) modelling provides a mechanistic approach for evaluating drug biodistribution. The aim of this work is to develop a specific PBPK model to simulate stavudine biodistribution after the administration of a 40 nm gold nanoparticle-based drug delivery system in rats. The model parameters used have been obtained from literature, in vitro and in vivo studies, and computer optimization. Based on these, the PBPK model was built, and the compartments included were considered as permeability rate-limited tissues. In comparison with stavudine solution, a higher biodistribution of stavudine into HIV reservoirs and the modification of pharmacokinetic parameters such as the mean residence time (MRT) have been observed. These changes are particularly noteworthy in the liver, which presents a higher partition coefficient (from 0.27 to 0.55) and higher MRT (from 1.28 to 5.67 h). Simulated stavudine concentrations successfully describe these changes in the in vivo study results. The average fold error of predicted concentrations after the administration of stavudine-gold nanoparticles was within the 0.5–2-fold error in all of the tissues. Thus, this PBPK model approach may help with the pre-clinical extrapolation to other administration routes or the species of stavudine gold nanoparticles.

Tribute to John C. Martin at the Twentieth Anniversary of the Breakthrough of Tenofovir in the Treatment of HIV Infections

At Bristol-Myers (BM) (1985–1990), John C. Martin started his HIV career with directing the clinical development of didanosine (ddI) and stavudine (d4T). During this period, he became aware of the acyclic nucleoside phosphonates (ANPs), such as (S)-HPMPA and PMEA, as potential antiviral drugs. Under his impulse, BM got involved in the evaluation of these ANPs, but the merger of BM with Squibb (to become BMS) incited John to leave BM and join Gilead Sciences, and the portfolio of the ANPs followed the transition. At Gilead, John succeeded in obtaining the approval from the US FDA for the use of cidofovir in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients, which was reminiscent of John’s first experience with ganciclovir (at Syntex) as an anti-CMV agent. At Gilead, John would then engineer the development of tenofovir, first as TDF (tenofovir disoproxil fumarate) and then as TAF (tenofovir alafenamide) and various combinations thereof, for the treatment of HIV infections (i), TDF and TAF for the treatment of hepatitis B (HBV) infections (ii), and TDF and TAF in combination with emtricitabine for the prophylaxis of HIV infections (iii).

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Selected Thoughts on Hydrophobicity in Drug Design

The fundamental aim of drug design in research and development is to invent molecules with selective affinity towards desired disease-associated targets. At the atomic loci of binding surfaces, systematic structural variations can define affinities between drug candidates and biomolecules, and thereby guide the optimization of safety, efficacy and pharmacologic properties. Hydrophobic interaction between biomolecules and drugs is integral to binding affinity and specificity. Examples of antiviral drug discovery are discussed.

Modifications on the heterocyclic base of ganciclovir, penciclovir, acyclovir - syntheses and antiviral properties

АBSTRACTEsters of the antiherpetic drugs ganciclovir, penciclovir with the bile acids (cholic, chenodeoxycholic and deoxycholic) and amino acid esters of acyclovir were generated and evaluated for their in vitro antiviral activity against herpes simplex viruses type 1 and type 2 (HSV-1, HSV-2). The antiviral assays demonstrated that modified analogs of ACV and PCV are less active compared to the initial substances against HSV-1and HSV-2. CC50 for ganciclovir-deoxycholate corresponded to the CC50 of the other analogs and its activity is lower than ganciclovir. Obtained results show that tested modification do not improve bioavailability of nucleoside analogs in cells.

Pyrimidine 2,4-Diones in the Design of New HIV RT Inhibitors

The pyrimidine nucleus is a versatile core in the development of antiretroviral agents. On this basis, a series of pyrimidine-2,4-diones linked to an isoxazolidine nucleus have been synthesized and tested as nucleoside analogs, endowed with potential anti-HIV (human immunodeficiency virus) activity. Compounds 6a-c, characterized by the presence of an ethereal group at C-3, show HIV reverse transcriptase (RT) inhibitor activity in the nanomolar range as well as HIV-infection inhibitor activity in the low micromolar with no toxicity. In the same context, compound 7b shows only a negligible inhibition of RT HIV.

The evolution of nucleoside analogue antivirals: A review for chemists and non-chemists. Part 1: Early structural modifications to the nucleoside scaffold

This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs, written for a target audience of virologists and other non-chemists, as well as chemists who may not be familiar with the field. Rather than providing a simple chronological account, we have examined and attempted to explain the thought processes, advances in synthetic chemistry and lessons learned from antiviral testing that led to a few molecules being moved forward to eventual approval for human therapies, while others were discarded. The present paper focuses on early, relatively simplistic changes made to the nucleoside scaffold, beginning with modifications of the nucleoside sugars of Ara-C and other arabinose-derived nucleoside analogues in the 1960's. A future paper will review more recent developments, focusing especially on more complex modifications, particularly those involving multiple changes to the nucleoside scaffold. We hope that these articles will help virologists and others outside the field of medicinal chemistry to understand why certain drugs were successfully developed, while the majority of candidate compounds encountered barriers due to low-yielding synthetic routes, toxicity or other problems that led to their abandonment.

Biomedical Applications of DNA-Conjugated Gold Nanoparticles

Gold nanoparticles (AuNPs) are useful for diagnostic and biomedical applications, mainly because of their ease in preparation and conjugation, biocompatibility, and size-dependent optical properties. However, bare AuNPs do not possess specificity for targets. AuNPs conjugated with DNA aptamers offer specificity for various analytes, such as proteins and small molecules/ions. Although DNA aptamers themselves have therapeutic and target-recognizing properties, they are susceptible to degradation in vivo. When DNA aptamers are conjugated to AuNPs, their stability and cell uptake efficiency both increase, making aptamer-AuNPs suitable for biomedical applications. Additionally, drugs can be efficiently conjugated with DNA aptamer-AuNPs to further enhance their therapeutic efficiency. This review focuses on the applications of DNA aptamer-based AuNPs in several biomedical areas, including anticoagulation, anticancer, antibacterial, and antiviral applications.

Viral protein R of HIV type-1 induces retrotransposition and upregulates glutamate synthesis by the signal transducer and activator of transcription 1 signaling pathway

Viral protein R (Vpr) of HIV-1 plays an important role in viral replication in macrophages. Various lines of evidence suggest that expression of Vpr in macrophages causes immunopathogenesis; however, the underlying mechanism is not yet fully understood. In this study, it was shown that recombinant Vpr (rVpr) induces retrotransposition of long interspersed element-1 in RAW264.7, a macrophage-like cell line, and activates reverse transcriptase-dependent immunotoxic cascades including production of IFN-β and phosphorylation of signal transducer and activator of transcription 1 (STAT1). Knockout experiments based on the CRISPR/Cas9 nickase system further demonstrated that cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) and stimulator of interferon gene (STING) are responsible for IFN-β production and STAT1 phosphorylation, respectively. Moreover, rVpr was found to increase production of glutaminase C, a regulator of glutamate synthesis, which is also dependent on the cGAS-STING pathway. Taken together with reports that glutaminase C is involved in the pathogenesis of HIV-associated neurocognitive disorder (HAND) and that Vpr is detectable in the cerebrospinal fluid of HIV-1-positive patients, a possible role of Vpr-induced L1-RTP and immunotoxic cascades in the development of HAND is discussed.

Efficacy of Antiviral Drugs against Feline Immunodeficiency Virus

Feline immunodeficiency virus (FIV) is one of the most common infectious agents affecting cats worldwide .FIV and human immunodeficiency virus (HIV) share many properties: both are lifelong persistent lentiviruses that are similar genetically and morphologically and both viruses propagate in T-lymphocytes, macrophages, and neural cells. Experimentally infected cats have measurable immune suppression, which sometimes progresses to an acquired immunodeficiency syndrome. A transient initial state of infection is followed by a long latent stage with low virus replication and absence of clinical signs. In the terminal stage, both viruses can cause severe immunosuppression. Thus, FIV infection in cats has become an important natural model for studying HIV infection in humans, especially for evaluation of antiviral compounds. Of particular importance for chemotherapeutic studies is the close similarity between the reverse transcriptase (RT) of FIV and HIV, which results in high in vitro susceptibility of FIV to many RT-targeted antiviral compounds used in the treatment of HIV-infected patients. Thus, the aim of this article is to provide an up-to-date review of studies on antiviral treatment of FIV, focusing on commercially available compounds for human or animal use.

Myristoylated derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine) bi-functional prodrugs with potent anti-HIV-1 activity and low cytotoxicity

BACKGROUND

To improve in vitro antiviral activity and selectivity of stavudine (d4T), a range of its bi-functional prodrugs, 5'-O-myristoylated derivatives, have been synthesized.

METHODS

Stavudine 5'-O-myristoylated esters were synthesized using modified Parang's procedure. The cytotoxicity and anti-HIV activity was evaluated in the established MT-4 cell line. The level of p24 protein in culture medium was assayed, and EC50 and EC90 values were determined.

RESULTS

Excellent anti-HIV activity was obtained for stavudine derivatives 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine, 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine with C10 and C11 alkyl chains bearing thioethyl- and azido- substituents. These prodrugs were more potent than the parent stavudine, as is clear from their EC50 values: 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine (R=CO(CH2)10SC2H5, EC50 0.06 μM), 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine (R=CO(CH2)11SC2H5, EC50 0.09 μM) and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine (R=CO(CH2)11N3, EC50 0.06 μM), while 50% cytotoxic concentration was >16.65 μM, >7.5 μM and >18.53 μM, respectively.

CONCLUSIONS

Overall data demonstrate that compounds 2',3'-didehydro-2',3'-dideoxy-5'-O-(11-thioethylundecanoyl) thymidine, 2',3'-didehydro-2',3'-dideoxy-5'-O-(12-thioethyldodecanoyl) thymidine and 5'-O-(12-azidododecanoyl)-2',3'-didehydro-2',3'-dideoxythymidine are very potent and selective anti-HIV agents and could be useful in treatment of HIV infections of the central nervous system.

Hydroxy fatty acids for the delivery of dideoxynucleosides as anti-HIV agents

A series of α- and β-carboxylated phospholipid prodrugs of dideoxy nucleosides have been synthesized and evaluated against HIV. An increase in biological effect with a factor of 500 has only been observed for the adenine nucleoside, which suggests that this prodrug approach is base specific.

Synthesis and properties of 1-(3'-dihydroxyboryl-2',3'-dideoxyribosyl)pyrimidines

Nucleoside analogues having a boronic acid in place of the 3-hydroxyl group of deoxyribose have been synthesized. The synthesis of 3'-dihydroxyboryl-2',3'-dideoxyribose was based on asymmetric homologation of boronic esters with (dihalomethyl)lithium, beginning from a (silyloxymethyl)boronic ester. A change of chiral director is required before introduction of the second stereocenter, and the direct displacement of (S,S)-1,2-dicyclohexyl-1,2-ethanediol by (1S,2S,3R,5S)-pinanediol was used for this purpose. Coupling of the pinanediol ester of the 1-acetoxy-3-dioxyboryl-5-tert-butylsilyloxy deoxyribose analogue with silylated pyrimidine bases was accomplished with trimethylsilyl bromide. The boronic acid nucleoside analogues were not cytotoxic toward Hep G2 (human hepatocarcinoma) cells. Decomposition occurred over a period of several hours at 37 °C, pH 7.4, with liberation of free pyrimidine base.

Borononucleotides as substrates/binders for human NMP kinases: enzymatic and spectroscopic evaluation

Borononucleotides are a family of natural nucleotide monophosphate analogues with a 5'-boronic acid function. As B-O-P linkages are known to be unstable in solution, we evaluated the ability of borononucleotides to be recognized by nucleoside monophosphate kinases and eventually foil the phosphorylation process. In this context, and with the idea of probing the influence of their size, shape, and flexibility, a library of borononucleotides were synthetized starting from the borononucleotide analogue of thymidine, which was shown to behave as a slow substrate of human TMP kinase. This study thus constitutes a good starting point for the development of new monophosphate mimics as potential substrates or ligands for NMP kinases.

NNRTIs: pharmacological data

One of the choice criteria for antiretroviral therapy, once the viral load is controlled, is long-term treatment safety. Safety, despite similarities in each therapeutic class, can differ significantly from one agent to another, according to their respective pharmacokinetic and pharmacodynamic properties. We reviewed data on two very well-known NNRTIs, efavirenz and nevirapine, in this context. The pharmacokinetic properties of both agents are presented along with their impact on residual viremia and viral reservoirs, as well as their clinical consequences. The implications for the penetration of these antiretroviral drugs in the CNS and in female and male genital tracts are also discussed. Pharmacogenetics could become an interesting tool. Finally, the availability of new NNRTIs has recently boosted this therapeutic class, even if their long-term properties remain to be assessed. The consideration of all this data stresses the importance of communication among clinicians, virologists, and pharmacologists before choosing a treatment.

Excision of HIV-1 proviral DNA by recombinant cell permeable tre-recombinase

Over the previous years, comprehensive studies on antiretroviral drugs resulted in the successful introduction of highly active antiretroviral therapy (HAART) into clinical practice for treatment of HIV/AIDS. However, there is still need for new therapeutic approaches, since HAART cannot eradicate HIV-1 from the infected organism and, unfortunately, can be associated with long-term toxicity and the development of drug resistance. In contrast, novel gene therapy strategies may have the potential to reverse the infection by eradicating HIV-1. For example, expression of long terminal repeat (LTR)-specific recombinase (Tre-recombinase) has been shown to result in chromosomal excision of proviral DNA and, in consequence, in the eradication of HIV-1 from infected cell cultures. However, the delivery of Tre-recombinase currently depends on the genetic manipulation of target cells, a process that is complicating such therapeutic approaches and, thus, might be undesirable in a clinical setting. In this report we demonstrate that E.coli expressed Tre-recombinases, tagged either with the protein transduction domain (PTD) from the HIV-1 Tat trans-activator or the translocation motif (TLM) of the Hepatitis B virus PreS2 protein, were able to translocate efficiently into cells and showed significant recombination activity on HIV-1 LTR sequences. Tre activity was observed using episomal and stable integrated reporter constructs in transfected HeLa cells. Furthermore, the TLM-tagged enzyme was able to excise the full-length proviral DNA from chromosomal integration sites of HIV-1-infected HeLa and CEM-SS cells. The presented data confirm Tre-recombinase activity on integrated HIV-1 and provide the basis for the non-genetic transient application of engineered recombinases, which may be a valuable component of future HIV eradication strategies.

Application of kinase bypass strategies to nucleoside antivirals

Nucleoside and nucleotide analogs have served as the cornerstones of antiviral therapy for many viruses. However, the requirement for intracellular activation and side-effects caused by distribution to off-target sites of toxicity still limit the efficacy of the current generation of drugs. Kinase bypass strategies, where phosphorylated nucleosides are delivered directly into cells, thereby, removing the requirement for enzyme catalyzed phosphorylation steps, have already changed the face of antiviral therapy in the form of the acyclic nucleoside phosphonates, cidofovir, adefovir (given orally as its dipivoxil prodrug) and tenofovir (given orally as its disoproxil prodrug), currently used clinically. These strategies hold further promise to advance the field of antiviral therapy with at least 10 kinase bypass and tissue targeted prodrugs, representing seven distinct prodrug classes, currently in clinical trials. This article reviews the history of kinase bypass strategies applied to nucleoside antivirals and the evolution of different tissue targeted prodrug strategies, highlighting clinically relevant examples.

The development of antiretroviral therapy and its impact on the HIV-1/AIDS pandemic

In the last 25 years, HIV-1, the retrovirus responsible for the acquired immunodeficiency syndrome (AIDS), has gone from being an "inherently untreatable" infectious agent to one eminently susceptible to a range of approved therapies. During a five-year period, starting in the mid-1980s, my group at the National Cancer Institute played a role in the discovery and development of the first generation of antiretroviral agents, starting in 1985 with Retrovir (zidovudine, AZT) in a collaboration with scientists at the Burroughs-Wellcome Company (now GlaxoSmithKline). We focused on AZT and related congeners in the dideoxynucleoside family of nucleoside reverse transcriptase inhibitors (NRTIs), taking them from the laboratory to the clinic in response to the pandemic of AIDS, then a terrifying and lethal disease. These drugs proved, above all else, that HIV-1 infection is treatable, and such proof provided momentum for new therapies from many sources, directed at a range of viral targets, at a pace that has rarely if ever been matched in modern drug development. Antiretroviral therapy has brought about a substantial decrease in the death rate due to HIV-1 infection, changing it from a rapidly lethal disease into a chronic manageable condition, compatible with very long survival. This has special implications within the classic boundaries of public health around the world, but at the same time in certain regions may also affect a cycle of economic and civil instability in which HIV-1/AIDS is both cause and consequence. Many challenges remain, including (1) the life-long duration of therapy; (2) the ultimate role of pre-exposure prophylaxis (PrEP); (3) the cardiometabolic side-effects or other toxicities of long-term therapy; (4) the emergence of drug-resistance and viral genetic diversity (non-B subtypes); (5) the specter of new cross-species transmissions from established retroviral reservoirs in apes and Old World monkeys; and (6) the continued pace of new HIV-1 infections in many parts of the world. All of these factors make refining current therapies and developing new therapeutic paradigms essential priorities, topics covered in articles within this special issue of Antiviral Research. Fortunately, there are exciting new insights into the biology of HIV-1, its interaction with cellular resistance factors, and novel points of attack for future therapies. Moreover, it is a short journey from basic research to public health benefit around the world. The current science will lead to new therapeutic strategies with far-reaching implications in the HIV-1/AIDS pandemic. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol. 85, issue 1, 2010.

Nucleoside and nucleotide HIV reverse transcriptase inhibitors: 25 years after zidovudine

Twenty-five years ago, nucleoside analog 3'-azidothymidine (AZT) was shown to efficiently block the replication of HIV in cell culture. Subsequent studies demonstrated that AZT acts via the selective inhibition of HIV reverse transcriptase (RT) by its triphosphate metabolite. These discoveries have established the first class of antiretroviral agents: nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs). Over the years that followed, NRTIs evolved into the main component of antiretroviral drug combinations that are now used for the treatment of all populations of HIV infected patients. A total of thirteen NRTI drug products are now available for clinical application: eight individual NRTIs, four fixed-dose combinations of two or three NRTIs, and one complete fixed-dose regimen containing two NRTIs and one non-nucleoside RT inhibitor. Multiple NRTIs or their prodrugs are in various stages of clinical development and new potent NRTIs are still being identified through drug discovery efforts. This article will review basic principles of the in vitro and in vivo pharmacology of NRTIs, discuss their clinical use including limitations associated with long-term NRTI therapy, and describe newly identified NRTIs with promising pharmacological profiles highlighting those in the development pipeline. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, volume 85, issue 1, 2010.

In search of a selective therapy of viral infections

This article is meant as an introductory chapter to the special issue of Antiviral Research on "Twenty-five years of antiretroviral drug development: progress and prospects", commemorating the first description of azidothymidine (AZT) as an antiretroviral agent. This has prompted me to highlight some of the hallmarks that played an important role in my own search of a selective therapy of viral infections: i.e., the induction of interferon by double-stranded RNA [such as poly(I).poly(C)], allowing the cloning and expression of beta-interferon; the discovery of the reverse transcriptase (RT) (and HIV as a retrovirus depending for its replication on RT), allowing the identification and development of a wide variety of RT inhibitors, nowadays used for the treatment of AIDS; the specificity of herpesvirus inhibitors such as acyclovir and BVDU, in the treatment of HSV and VZV infections; the role of acyclic nucleoside phosphonates (tenofovir, adefovir and cidofovir) in the treatment of HIV, HBV and DNA virus infections; and that of the NNRTIs (leading from TIBO to rilpivirine) as an essential part of the current anti-HIV drug cocktails. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, vol. 85, issue 1, 2010.