HIV Infection, Antiretroviral Drugs, and the Vascular Endothelium

Endothelial cell activation, injury, and dysfunction underlies the pathophysiology of vascular diseases and infections associated with vascular dysfunction, including human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome. Despite viral suppression with combination antiretroviral therapy (ART), people living with HIV (PLWH) are prone to many comorbidities, including neurological and neuropsychiatric complications, cardiovascular and metabolic diseases, premature aging, and malignancies. HIV and viral proteins can directly contribute to the development of these comorbidities. However, with the continued high prevalence of these comorbidities despite viral suppression, it is likely that ART or some antiretroviral (ARVs) drugs contribute to the development and persistence of comorbid diseases in PLWH. These comorbid diseases often involve vascular activation, injury, and dysfunction. The purpose of this manuscript is to review the current literature on ARVs and the vascular endothelium in PLWH, animal models, and in vitro studies. I also summarize evidence of an association or lack thereof between ARV drugs or drug classes and the protection or injury/dysfunction of the vascular endothelium and vascular diseases.

Selected Milestones in Antiviral Drug Development

This review article will describe the (wide) variety of approaches that I envisaged to develop a specific therapy for viral infections: (i) interferon and its inducers, (ii) HSV, VZV and CMV inhibitors, (iii) NRTIs (nucleoside reverse transcriptase inhibitors), NtRTIs (nucleotide reverse transcriptase inhibitors) and NNRTIs (non-nucleoside reverse transcriptase inhibitors) as HIV inhibitors, (iv) NtRTIs as HBV inhibitors, and finally, (v) the transition of an HIV inhibitor to a stem cell mobilizer, as exemplified by AMD-3100 (Mozobil®).

Repurposing nucleoside reverse transcriptase inhibitors (NRTIs) to slow aging

Repurposing drugs already approved in the clinic to be used off-label as geroprotectors, compounds that combat mechanisms of aging, are a promising way to rapidly reduce age-related disease incidence in society. Several recent studies have found that a class of drugs-nucleoside reverse transcriptase inhibitors (NRTIs)-originally developed as treatments for cancers and human immunodeficiency virus (HIV) infection, could be repurposed to slow the aging process. Interestingly, these studies propose complementary mechanisms that target multiple hallmarks of aging. At the molecular level, NRTIs repress LINE-1 elements, reducing DNA damage, benefiting the hallmark of aging of 'Genomic Instability'. At the organellar level, NRTIs inhibit mitochondrial translation, activate ATF-4, suppress cytosolic translation, and extend lifespan in worms in a manner related to the 'Loss of Proteostasis' hallmark of aging. Meanwhile, at the cellular level, NRTIs inhibit the P2X7-mediated activation of the inflammasome, reducing inflammation and improving the hallmark of aging of 'Altered Intercellular Communication'. Future development of NRTIs for human aging health will need to balance out toxic side effects with the beneficial effects, which may occur in part through hormesis.

The Influence of Nucleoside Reverse Transcriptase Inhibitors on Mitochondrial Activity, Lipid Content, and Fatty-Acid-Binding Protein Levels in Microglial HMC3 Cells

Despite the availability of a wide range of preventive measures and comprehensive treatment options following infection, the development of acquired immunodeficiency syndrome (AIDS) remains a persistent challenge. Nucleoside reverse transcriptase inhibitors (NRTIs) represent the most commonly utilized therapeutic approach, despite being on the pharmaceutical market for nearly four decades. During this time, a spectrum of side effects ranging from mild discomfort and hypersensitivity reactions to the more prevalent nephrotoxicity and hepatotoxicity has been documented. In light of these considerations, our study aimed to investigate the impacts of two NRTIs, lamivudine and zidovudine, on lipid metabolism in HMC3 microglial cells. Our findings revealed statistically significant reductions in the ATP levels (nearly 8%) and increased mitochondrial superoxide levels (around 10%) after 24 h of treatment with the maximum therapeutic concentration of zidovudine compared to the untreated microglial cells. Furthermore, the concentrations of fatty-acid-binding proteins 4 and 5 were significantly lower (approximately 40%) in the microglial cells that were exposed to NRTIs than in the untreated cells. Notably, the total lipid concentration within the microglial cells markedly increased following NRTI administration with a 13% rise after treatment with 10 µM lamivudine and a remarkable 70% surge following the administration of 6 µM zidovudine. These results suggest that the prolonged administration of NRTIs may potentially lead to lipid accumulation, posing a significant risk to the delicate homeostasis of the neuronal system and potentially triggering a pro-inflammatory response in microglial cells.

HIV and Drug-Resistant Subtypes

Acquired Immunodeficiency Syndrome (AIDS) is a human viral infectious disease caused by the positive-sense single-stranded (ss) RNA Human Immunodeficiency Virus (HIV) (Retroviridae family, Ortervirales order). HIV-1 can be distinguished into various worldwide spread groups and subtypes. HIV-2 also causes human immunodeficiency, which develops slowly and tends to be less aggressive. HIV-2 only partially homologates to HIV-1 despite the similar derivation. Antiretroviral therapy (ART) is the treatment approved to control HIV infection, based on multiple antiretroviral drugs that belong to different classes: (i) NNRTIs, (ii) NRTIs, (iii) PIs, (iv) INSTIs, and (v) entry inhibitors. These drugs, acting on different stages of the HIV life cycle, decrease the patient's total burden of HIV, maintain the function of the immune system, and prevent opportunistic infections. The appearance of several strains resistant to these drugs, however, represents a problem today that needs to be addressed as best as we can. New outbreaks of strains show a widespread geographic distribution and a highly variable mortality rate, even affecting treated patients significantly. Therefore, novel treatment approaches should be explored. The present review discusses updated information on HIV-1- and HIV-2-resistant strains, including details on different mutations responsible for drug resistance.

Novel Therapies of Hepatitis B and D

Hepatitis B virus (HBV) infection is a global public health issue and is a major cause of cirrhosis and hepatocellular carcinoma (HCC). Hepatitis D virus (HDV) requires the hepatitis B surface antigen (HBsAg) to replicate. The eradication of HBV, therefore, can also cure HDV. The current therapies for chronic hepatitis B and D are suboptimal and cannot definitely cure the viruses. In order to achieve functional or complete cure of these infections, novel therapeutic agents that target the various sites of the viral replicative cycle are necessary. Furthermore, novel immunomodulatory agents are also essential to achieve viral clearance. Many of these new promising compounds such as entry inhibitors, covalently closed circular DNA (cccDNA) inhibitors, small interfering RNAs (siRNAs), capsid assembly modulators and nucleic acid polymers are in various stages of clinical developments. In this review article, we provided a comprehensive overview of the structure and lifecycle of HBV, the limitations of the current therapies and a summary of the novel therapeutic agents for both HDV and HBV infection.

Human PrimPol Discrimination against Dideoxynucleotides during Primer Synthesis

PrimPol is required to re-prime DNA replication at both nucleus and mitochondria, thus facilitating fork progression during replicative stress. ddC is a chain-terminating nucleotide that has been widely used to block mitochondrial DNA replication because it is efficiently incorporated by the replicative polymerase Polγ. Here, we show that human PrimPol discriminates against dideoxynucleotides (ddNTP) when elongating a primer across 8oxoG lesions in the template, but also when starting de novo synthesis of DNA primers, and especially when selecting the 3′nucleotide of the initial dimer. PrimPol incorporates ddNTPs with a very low efficiency compared to dNTPs even in the presence of activating manganese ions, and only a 40-fold excess of ddNTP would significantly disturb PrimPol primase activity. This discrimination against ddNTPs prevents premature termination of the primers, warranting their use for elongation. The crystal structure of human PrimPol highlights Arg²⁹¹ residue as responsible for the strong dNTP/ddNTP selectivity, since it interacts with the 3′-OH group of the incoming deoxynucleotide, absent in ddNTPs. Arg²⁹¹, shown here to be critical for both primase and polymerase activities of human PrimPol, would contribute to the preferred binding of dNTPs versus ddNTPs at the 3′elongation site, thus avoiding synthesis of abortive primers.

Nucleoside Analogues Are Potent Inducers of Pol V-mediated Mutagenesis

Drugs targeting DNA and RNA in mammalian cells or viruses can also affect bacteria present in the host and thereby induce the bacterial SOS system. This has the potential to increase mutagenesis and the development of antimicrobial resistance (AMR). Here, we have examined nucleoside analogues (NAs) commonly used in anti-viral and anti-cancer therapies for potential effects on mutagenesis in Escherichia coli, using the rifampicin mutagenicity assay. To further explore the mode of action of the NAs, we applied E. coli deletion mutants, a peptide inhibiting Pol V (APIM-peptide) and metabolome and proteome analyses. Five out of the thirteen NAs examined, including three nucleoside reverse transcriptase inhibitors (NRTIs) and two anti-cancer drugs, increased the mutation frequency in E. coli by more than 25-fold at doses that were within reported plasma concentration range (Pl.CR), but that did not affect bacterial growth. We show that the SOS response is induced and that the increase in mutation frequency is mediated by the TLS polymerase Pol V. Quantitative mass spectrometry-based metabolite profiling did not reveal large changes in nucleoside phosphate or other central carbon metabolite pools, which suggests that the SOS induction is an effect of increased replicative stress. Our results suggest that NAs/NRTIs can contribute to the development of AMR and that drugs inhibiting Pol V can reverse this mutagenesis.

Development and Validation of an Up-to-Date Highly Sensitive UHPLC-MS/MS Method for the Simultaneous Quantification of Current Anti-HIV Nucleoside Analogues in Human Plasma

Therapeutic options to treat HIV infection have widened in the past years, improving both effectiveness and tolerability, but nucleoside reverse transcriptase inhibitors (NRTIs) are still considered the standard backbone of the combination regimens. Therapeutic drug monitoring (TDM) can be useful for these drugs, due to concentration–effect relationship, with risk of ineffectiveness, toxicity or adherence concerns: in this scenario, robust and multiplexed methods are needed for an effective TDM activity. In this work, the first validated ultra-high spectrometry liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS) method is described for the high-sensitive simultaneous quantification of all the currently used NRTIs in human plasma, including tenofovir alafenamide (TAF), following FDA and EMA guidelines. The automated sample preparation consisted in the addition of an internal standard (IS) working solution, containing stable-isotope-linked drugs, protein precipitation and drying. Dry extracts were reconstituted with water, then, these underwent reversed phase chromatographic separation: compounds were detected through electrospray ionization and multiple reaction monitoring. Accuracy, precision, recovery and IS-normalized matrix effect fulfilled guidelines’ requirements. The application of this method on samples from people living with HIV (PLWH) showed satisfactory performance, being capable of quantifying the very low concentrations of tenofovir (TFV) in patients treated with TAF.

Trends and strategies to combat viral infections: A review on FDA approved antiviral drugs

The infectious microscopic viruses invade living cells to reproduce themselves, and causes chronic infections such as HIV/AIDS, hepatitis B and C, flu, etc. in humans which may lead to death if not treated. Different strategies have been utilized to develop new and superior antiviral drugs to counter the viral infections. The FDA approval of HIV nucleoside reverse transcriptase inhibitor, zidovudine in 1987 boosted the development of antiviral agents against different viruses. Currently, there are a number of combination drugs developed against various viral infections to arrest the activity of same or different viral macromolecules at multiple stages of its life cycle; among which majority are targeted to interfere with the replication of viral genome. Besides these, other type of antiviral molecules includes entry inhibitors, integrase inhibitors, protease inhibitors, interferons, immunomodulators, etc. The antiviral drugs can be toxic to human cells, particularly in case of administration of combination drugs, and on the other hand viruses can grow resistant to the antiviral drugs. Furthermore, emergence of new viruses like Ebola, coronaviruses (SARS-CoV, SARS-CoV-2) emphasizes the need for more innovative strategies to develop better antiviral drugs to fight the existing and the emerging viral infections. Hence, we reviewed the strategic enhancements in developing antiviral drugs for the treatment of different viral infections over the years.

Molecular Genetics and the Incidence of Transmitted Drug Resistance Among Pre-Treatment HIV-1 Infected Patients in the Eastern Cape, South Africa

BACKGROUND

Transmitted drug resistance (TDR) remains a significant threat to Human immunodeficiency virus (HIV) infected patients that are not exposed to antiretroviral treatment. Although, combined antiretroviral therapy (cART) has reduced deaths among infected individuals, emergence of drug resistance is gradually on rise.

OBJECTIVE

To determine the drug resistance mutations and subtypes of HIV-1 among pre-treatment patients in the Eastern Cape of South Africa.

METHODS

Viral RNA was extracted from blood samples of 70 pre-treatment HIV-1 patients while partial pol gene fragment amplification was achieved with specific primers by RT-PCR followed by nested PCR and positive amplicons were sequenced utilizing ABI Prism 316 genetic sequencer. Drug resistance mutations (DRMs) analysis was performed by submitting the generated sequences to Stanford HIV drug resistance database.

RESULTS

Viral DNA was successful for 66 (94.3%) samples of which 52 edited sequences were obtained from the protease and 44 reverse transcriptase sequences were also fully edited. Four major protease inhibitor (PI) related mutations (I54V, V82A/L, L76V and L90M) were observed in seven patients while several other minor and accessory PIs were also identified. A total of 11(25.0%) patients had NRTIs related mutations while NNRTIs were observed among 14(31.8%) patients. K103N/S, V106M and M184V were the most common mutations identified among the viral sequences. Phylogenetic analysis of the partial pol gene indicated all sequences clustered with subtype C.

CONCLUSION

This study indicates that HIV-1 subtype C still predominates and responsible for driving the epidemic in the Eastern Cape of South Africa with slow rise in the occurrence of transmitted drug resistance.

Comparative Antiviral Activity of Remdesivir and Anti-HIV Nucleoside Analogs Against Human Coronavirus 229E (HCoV-229E)

Remdesivir is a nucleotide prodrug that is currently undergoing extensive clinical trials for the treatment of COVID-19. The prodrug is metabolized to its active triphosphate form and interferes with the action of RNA-dependent RNA polymerase of SARS-COV-2. Herein, we report the antiviral activity of remdesivir against human coronavirus 229E (HCoV-229E) compared to known anti-HIV agents. These agents included tenofovir (TFV), 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), alovudine (FLT), lamivudine (3TC), and emtricitabine (FTC), known as nucleoside reverse-transcriptase inhibitors (NRTIs), and a number of 5'-O-fatty acylated anti-HIV nucleoside conjugates. The anti-HIV nucleosides interfere with HIV RNA-dependent DNA polymerase and/or act as chain terminators. Normal human fibroblast lung cells (MRC-5) were used to determine the cytotoxicity of the compounds. The study revealed that remdesivir exhibited an EC₅₀ value of 0.07 µM against HCoV-229E with TC₅₀ of > 2.00 µM against MRC-5 cells. Parent NRTIs were found to be inactive against (HCoV-229E) at tested concentrations. Among all the NRTIs and 5'-O-fatty acyl conjugates of NRTIs, 5'-O-tetradecanoyl ester conjugate of FTC showed modest activity with EC₅₀ and TC₅₀ values of 72.8 µM and 87.5 µM, respectively. These data can be used for the design of potential compounds against other coronaviruses.

HIV drug resistance in patients in China's national HIV treatment programme who have been on first-line ART for at least 9 months

BACKGROUND

The aim of this study was to assess trends in drug resistance and associated clinical and programmatic factors at a national level during the rapid scale up of ART.

METHODS

Logistic regression was used to identify the factors associated with HIVDR. Variables associated with drug resistance in multivariable logistic regression were included in the Cochran-Armitage test for trend.

RESULTS

A total of 11,976 patients were enrolled in the study. The prevalence of HIVDR among patients who received ART for 9-24 months during 2003-2008, 2009-2012, and 2013-2015 significantly decreased (15.5%, 6.3%, and 2.3%, respectively, P < 0.01). With respect to the class of antiretroviral, there were substantial increases in resistance to both non-nucleoside reverse transcriptase inhibitors (NNRTIs) and nucleoside reverse transcriptase inhibitors (NRTIs) (2003-2008, 2009-2012, and 2013-2015: 49.7%, 58.9%, and 73.0%, respectively, P < 0.01). The prevalence of DR to protease inhibitors (PIs) was low, which supported their continued use as second-line therapy in China.

CONCLUSIONS

Our results provide evidence for the effectiveness of China's "Treat All" approach to guide policy makers to improve training for healthcare providers and education on ART adherence among patients.

DNA polymerase-γ hypothesis in nucleoside reverse transcriptase-induced mitochondrial toxicity revisited: A potentially protective role for citrus fruit-derived naringenin?

Nucleoside reverse transcriptase inhibitors (NRTIs) form the backbone in combination antiretroviral therapy (cARVs). They halt chain elongation of the viral cDNA by acting as false substrates in counterfeit incorporation mechanism to viral RNA-dependent DNA polymerase. In the process genomic DNA polymerase as well as mitochondrial DNA (mtDNA) polymerase-γ (which has a much higher affinity for these drugs at therapeutic doses) are also impaired. This leads to mitochondrial toxicity that manifests clinically as mitochondrial myopathy, peripheral neuropathy, hyperlactatemia or lactic acidosis and lipoatrophy. This has led to the revision of clinical guidelines by World Health Organization to remove stavudine from first-line listing in the treatment of HIV infections. Recent reports have implicated oxidative stress besides mtDNA polymerase-γ hypothesis in NRTI-induced metabolic complications. Reduced plasma antioxidant concentrations have been reported in HIV positive patients on cARVs but clinical intervention with antioxidant supplements have not been successful either due to low efficacy or poor experimental designs. Citrus fruit-derived naringenin has previously been demonstrated to possess antioxidant and free radical scavenging properties which could prevent mitochondrial toxicity associated with these drugs. This review revisits the controversy surrounding mtDNA polymerase-γ hypothesis and evaluates the potential benefits of naringenin as a potent anti-oxidant and free radical scavenger which as a nutritional supplement or therapeutic adjunct could mitigate the development of mitochondrial toxicity associated with these drugs.

Nucleotide Reverse Transcriptase Inhibitors: A Thorough Review, Present Status and Future Perspective as HIV Therapeutics

BACKGROUND

Human immunodeficiency virus type-1 (HIV-1) infection leads to acquired immunodeficiency syndrome (AIDS), a severe viral infection that has claimed approximately 658,507 lives in the US between the years 2010-2014. Antiretroviral (ARV) therapy has proven to inhibit HIV-1, but unlike other viral illness, not cure the infection.

OBJECTIVE

Among various Food and Drug Administration (FDA)-approved ARVs, nucleoside/ nucleotide reverse transcriptase inhibitors (NRTIs) are most effective in limiting HIV-1 infection. This review focuses on NRTIs mechanism of action and metabolism.

METHODS

A search of PubMed (1982-2016) was performed to capture relevant articles regarding NRTI pharmacology.

RESULTS

The current classical NRTIs pharmacology for HIV-1 prevention and treatment are presented. Finally, various novel strategies are proposed to improve the efficacy of NRTIs, which will increase therapeutic efficiency of present-day HIV-1 prevention/treatment regimen.

CONCLUSION

Use of NRTIs will continue to be critical for successful treatment and prevention of HIV-1.

Incidence and types of HIV-1 drug resistance mutation among patients failing first-line antiretroviral therapy

OBJECTIVE

This study aims to investigate the prevalence and types of drug resistance mutations among patients failing first-line antiretroviral therapy (ART).

METHODS

Plasma samples from 112 patients with human immunodeficiency virus-1 (HIV-1) were collected for virus RNA extract and gene amplification. The mutations related to drug resistance were detected and the incidence was statistically analyzed, and the drug resistance rate against common drugs was also evaluated.

RESULTS

103 cases were successfully amplified, and the main drug resistance mutations in the reverse transcriptase (RT) region were M184V (50.49%), K103N (28.16%), Y181C (25.24%), and K65R (27.18%), while no drug main resistance mutation was found in the protease (PR) region. The incidence of drug resistance mutations was significantly different among patients with different ages, routes of infection, duration of treatment, initial ART regimens and viral load. The drug resistance rate to the common drugs was assessed, including Efavirenz (EFV, 71.84%), Nevirapine (NVP, 74.76%), Lamivudine (3TC, 66.02%), Zidovudine (AZT, 4.85%), Stavudine (D4T, 16.51%), and Tenofovir (TDF, 21.36%).

CONCLUSION

The drug resistance mutations to NRTIs and NNRTIs are complex and highly prevalent, which was the leading cause of first-line ART failure. This study provides significant theoretical support for developing the second-line and third-line therapeutic schemes.

Nucleoside reverse transcriptase inhibitor-induced rat oocyte dysfunction and low fertility mediated by autophagy

Low fertility is one of the most common side effects caused by nucleoside reverse transcriptase inhibitors (NRTIs), whereas the molecular mechanism underlying this process were largely unclear. This study was conducted to investigate whether autophagy plays a role in NRTIs-induced oocyte dysfunction and low fertility in female rat. Both in vivo and in vitro experiments were conducted. For the in vivo experiment, female adult Sprague-Dawley rats were subjected to zidovudine (AZT) and lamivudine (3TC) intragastric treatment for 3, 6, 9, and 12 weeks; a control was also set. Oocytes were collected for maturation evaluation, in vitro fertilization and mitochondrial function assays, and apoptosis and autophagy analysis. For the in vitro experiment, oocytes were collected and assigned to the control, 3-methyladenine (3-MA, an effective autophagy inhibitor), AZT, AZT+3-MA, 3TC, and 3TC+3-MA groups. The oocytes were cultured with the abovementioned drugs for 24, 48, and 72 h and then, subjected to the same assays as in the in vivo study. The results showed a significant time-dependent decrease in oocyte maturation-related maker levels, oocyte cleavage rate, blastocyst formation rate, mitochondrial DNA copy number and adenosine triphosphate level, and apoptosis, and a significant increase in the reactive oxygen species levels (all P-values < 0.05), in both the in vivo and the in vitro experiments. These changes, except for the changes in the oocyte maturation-related markers, were partially attenuated by 3-MA. In conclusion, we demonstrated that NRTIs can cause rat oocyte dysfunction and low fertility, and this damage was, at least partially, mediated by autophagy.

Susceptibility of Human Endogenous Retrovirus Type K to Reverse Transcriptase Inhibitors

Human endogenous retroviruses (HERVs) make up 8% of the human genome. The HERV type K (HERV-K) HML-2 (HK2) family contains proviruses that are the most recent entrants into the human germ line and are transcriptionally active. In HIV-1 infection and cancer, HK2 genes produce retroviral particles that appear to be infectious, yet the replication capacity of these viruses and potential pathogenicity has been difficult to ascertain. In this report, we screened the efficacy of commercially available reverse transcriptase inhibitors (RTIs) at inhibiting the enzymatic activity of HK2 RT and HK2 genomic replication. Interestingly, only one provirus, K103, was found to encode a functional RT among those examined. Several nucleoside analogue RTIs (NRTIs) blocked K103 RT activity and consistently inhibited the replication of HK2 genomes. The NRTIs zidovudine (AZT), stavudine (d4T), didanosine (ddI), and lamivudine (3TC), and the nucleotide RTI inhibitor tenofovir (TDF), show efficacy in blocking K103 RT. HIV-1-specific nonnucleoside RTIs (NNRTIs), protease inhibitors (PIs), and integrase inhibitors (IIs) did not affect HK2, except for the NNRTI etravirine (ETV). The inhibition of HK2 infectivity by NRTIs appears to take place at either the reverse transcription step of the viral genome prior to HK2 viral particle formation and/or in the infected cells. Inhibition of HK2 by these drugs will be useful in suppressing HK2 infectivity if these viruses prove to be pathogenic in cancer, neurological disorders, or other diseases associated with HK2. The present studies also elucidate a key aspect of the life cycle of HK2, specifically addressing how they do, and/or did, replicate.IMPORTANCE Endogenous retroviruses are relics of ancestral virus infections in the human genome. The most recent of these infections was caused by HK2. While HK2 often remains silent in the genome, this group of viruses is activated in HIV-1-infected and cancer cells. Recent evidence suggests that these viruses are infectious, and the potential exists for HK2 to contribute to disease. We show that HK2, and specifically the enzyme that mediates virus replication, can be inhibited by a panel of drugs that are commercially available. We show that several drugs block HK2 with different efficacies. The inhibition of HK2 replication by antiretroviral drugs appears to occur in the virus itself as well as after infection of cells. Therefore, these drugs might prove to be an effective treatment by suppressing HK2 infectivity in diseases where these viruses have been implicated, such as cancer and neurological syndromes.

Characterization of Nucleoside Reverse Transcriptase Inhibitor-Associated Mutations in the RNase H Region of HIV-1 Subtype C Infected Individuals

The South African national treatment programme includes nucleoside reverse transcriptase inhibitors (NRTIs) in both first and second line highly active antiretroviral therapy regimens. Mutations in the RNase H domain have been associated with resistance to NRTIs but primarily in HIV-1 subtype B studies. Here, we investigated the prevalence and association of RNase H mutations with NRTI resistance in sequences from HIV-1 subtype C infected individuals. RNase H sequences from 112 NRTI treated but virologically failing individuals and 28 antiretroviral therapy (ART)-naive individuals were generated and analysed. In addition, sequences from 359 subtype C ART-naive sequences were downloaded from Los Alamos database to give a total of 387 sequences from ART-naive individuals for the analysis. Fisher’s exact test was used to identify mutations and Bayesian network learning was applied to identify novel NRTI resistance mutation pathways in RNase H domain. The mutations A435L, S468A, T470S, L484I, A508S, Q509L, L517I, Q524E and E529D were more prevalent in sequences from treatment-experienced compared to antiretroviral treatment naive individuals, however, only the E529D mutation remained significant after correction for multiple comparison. Our findings suggest a potential interaction between E529D and NRTI-treatment; however, site-directed mutagenesis is needed to understand the impact of this RNase H mutation.

SYBR Green II Dye-Based Real-Time Assay for Measuring Inhibitor Activity Against HIV-1 Reverse Transcriptase

There are arrays of in vitro assays to quantify the activity of HIV-1 reverse transcriptase (HIV-1 RT). These assays utilize either chemically customized/labelled nucleotides, or TaqMan probes, or radiolabeled nucleotides/primers. Although several real-time PCR assays exist commercially for measuring the RT activity, which are usually used for quantifying the viral titres, these assays are not optimized for measuring the inhibitory concentrations (IC50) of HIV-1 RT inhibitors. Moreover, a recently established inorganic pyrophosphate-coupled enzyme assay cannot be employed for studying nonphosphorylated nucleoside reverse transcriptase inhibitors (NRTIs). In the present study, we have developed a novel one-step assay with native nucleotide substrates and SYBR Green II dye to determine IC50 values of triphosphorylated NRTIs against HIV-1 RT. Using exact batches of wild-type and mutant RT, and triphosphorylated NRTIs, we showed that our method gave IC50 values for inhibitors similar to that of an earlier published colorimetric assay with BrdUTP substrate (CABS). Our assay should be suitable for high-throughput screening of antiretroviral drugs and could also be suitable for studying drug resistance profiles. Additionally, we also used our assay to study inhibition by AZT in its nonphosphorylated form by supplementing the reaction mixture with necessary kinases and ATP.

Abacavir induces platelet-endothelium interactions by interfering with purinergic signalling: A step from inflammation to thrombosis

The controversy connecting Abacavir (ABC) with cardiovascular disease has been fuelled by the lack of a credible mechanism of action. ABC shares structural similarities with endogenous purines, signalling molecules capable of triggering prothrombotic/proinflammatory programmes. Platelets are leading actors in the process of thrombosis. Our study addresses the effects of ABC on interactions between platelets and other vascular cells, while exploring the adhesion molecules implicated and the potential interference with the purinergic signalling pathway. The effects of ABC on platelet aggregation and platelet-endothelium interactions were evaluated, respectively, with an aggregometer and a flow chamber system that reproduced conditions in vivo. The role of adhesion molecules and purinergic receptors in endothelial and platelet populations was assessed by selective pre-incubation with specific antagonists and antibodies. ABC and carbovir triphosphate (CBT) levels were evaluated by HPLC. The results showed that ABC promoted the adherence of platelets to endothelial cells, a crucial step for the formation of thrombi. This was not a consequence of a direct effect of ABC on platelets, but resulted from activation of the endothelium via purinergic ATP-P2X₇ receptors, which subsequently triggered an interplay between P-selectin and ICAM-1 on endothelial cells with constitutively expressed GPIIb/IIIa and GPIbα on platelets. ABC did not induce platelet activation (P-selectin expression or Ca²⁺ mobilization) or aggregation, even at high concentrations. CBT levels in endothelial cells were lower than those required to induce platelet-endothelium interactions. Thus, ABC interference with endothelial purinergic signalling leads to platelet recruitment. This highlights the endothelium as the main cell target of ABC in this interaction, which is in line with previous experimental evidence that ABC induces manifestations of vascular inflammation.

Safety of zidovudine/lamivudine scored tablets in children with HIV infection in Europe and Thailand

Background

Zidovudine (ZDV) has been associated with risk of haematological toxicity. Safety data from clinical trials is generally limited to 48 weeks. We assessed the short- and mid-term toxicity of ZDV/lamivudine (3TC) fixed-dose combination scored tablets in HIV-infected children followed in the European Pregnancy and Paediatric HIV Cohort Collaboration (EPPICC) network.

Methods

Fourteen cohorts provided data on patients <18 years of age taking ZDV/3TC scored tablets between 2008 and 2012. Rates of Division of AIDS (DAIDS) grade ≥3 laboratory adverse events (AEs) for hepatobiliary and haematological disorders were estimated by duration on drug (<12, 12–24, >24 months). Clinical adverse events and reasons for tablet discontinuation were described.

Results

Of 541 patients on ZDV/3TC, 388 (72%) had weight and dose data available, of whom 350 (90%) weighed ≥14 kg and were eligible for tablet use; 161 (41%) were aged <10 years on an approved dose, 189 (49%) aged ≥10 years on an approved dose, and 30 (8%) were on an unapproved dose. Median age at ZDV/3TC start was 10 years, and 79% had taken ART previously (60% had prior exposure to ZDV/3TC). Overall rates of grade ≥3 AEs for absolute neutrophil counts, bilirubin, haemoglobin, platelet counts, white blood cell counts (WBC), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were ≤2/100 person years (PY) for patients taking approved doses. Two hundred thirty-three (43%) patients were not on ZDV/3TC tablets at most recent follow-up; a small number (17 (7%)) discontinued due to AEs (17 (7%)), and the most common reason for discontinuation was treatment simplification (73 (31%)).

Conclusions

Scored ZDV/3TC tablets, both approved and taken off-label, appear to be well tolerated with few side effects. Few patients discontinued treatment due to toxicity. As ZDV/3TC tablets are taken with other antiretrovirals, it is difficult to infer association between toxicities and specific agents, highlighting the importance of widening long-term pharmacovigilance to a broader spectrum of drug combinations.

Evaluation of nucleoside reverse transcriptase inhibitor dosing during continuous veno-venous hemofiltration

Background Unpredictable drug concentrations may lead to suboptimal exposure to nucleoside reverse transcriptase inhibitors (NRTIs) due to inadequate doses administered during continuous veno-venous hemofiltration (CVVH), which in turn may lead to decreased antiretroviral efficacy and possibly further HIV disease progression. Objective To compare administered doses of NRTIs to calculated doses of NRTIs to evaluate if patients were expected to have a favorable pharmacokinetic exposure profile while receiving CVVH. Methods The NRTI dose was compared to a table of recommendations based on a mathematical formula that estimates the amount of drug expected to be removed during CVVH. Results Twelve patients were on 27 NRTIs. Eleven (41%) NRTI doses were expected to provide a favorable pharmacokinetic profile based on pharmacokinetic mathematical calculations. Conclusion The majority of NRTIs were potentially not optimally dosed based on proposed pharmacokinetic calculations.

Structural basis of HIV inhibition by translocation-defective RT inhibitor 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA)

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) is the most potent nucleoside analog inhibitor of HIV reverse transcriptase (RT). It retains a 3'-OH yet acts as a chain-terminating agent by diminishing translocation from the pretranslocation nucleotide-binding site (N site) to the posttranslocation primer-binding site (P site). Also, facile misincorporation of EFdA-monophosphate (MP) results in difficult-to-extend mismatched primers. To understand the high potency and unusual inhibition mechanism of EFdA, we solved RT crystal structures (resolutions from 2.4 to 2.9 Å) that include inhibition intermediates (i) before inhibitor incorporation (catalytic complex, RT/DNA/EFdA-triphosphate), (ii) after incorporation of EFdA-MP followed by dT-MP (RT/DNAEFdA-MP(P)• dT-MP(N) ), or (iii) after incorporation of two EFdA-MPs (RT/DNAEFdA-MP(P)• EFdA-MP(N) ); (iv) the latter was also solved with EFdA-MP mismatched at the N site (RT/DNAEFdA-MP(P)• EFdA-MP(*N) ). We report that the inhibition mechanism and potency of EFdA stem from interactions of its 4'-ethynyl at a previously unexploited conserved hydrophobic pocket in the polymerase active site. The high resolution of the catalytic complex structure revealed a network of ordered water molecules at the polymerase active site that stabilize enzyme interactions with nucleotide and DNA substrates. Finally, decreased translocation results from favorable interactions of primer-terminating EFdA-MP at the pretranslocation site and unfavorable posttranslocation interactions that lead to observed localized primer distortions.

Naringin improves zidovudine- and stavudine-induced skeletal muscle complications in rats

Chronic use of nucleoside reverse transcriptase inhibitors (NRTIs) in managing human immunodeficiency virus (HIV) infection has been associated with several complications. Available management options for these complications have yielded controversial results, thus the need to urgently find newer alternatives. Naringin, a plant-derived flavonoid, has been shown to possess antioxidant and antiapoptotic properties which can be exploited in managing NRTI-induced complications. This study therefore investigated the effects of naringin on some NRTI-induced complications. Forty-nine rats (200-250 g) were divided into seven groups and were orally treated with stavudine (d4T)-only, d4T + naringin, d4T + vitamin E, zidovudine (AZT)-only, AZT + naringin, AZT + vitamin E, and distilled water, respectively. Drugs were administered once daily for 56 days, and oral glucose tolerance tests conducted on day 54 of the experiments and rats were thereafter sacrificed on day 56 by halothane overdose. Plasma samples and the left gastrocnemius muscles were stored at -80°C for further analysis. There was significant glucose intolerance, insulin resistance, oxidative stress, and apoptosis in the skeletal muscles of AZT- or d4T-only-treated rats. Naringin, however, significantly reduced fasting blood glucose and fasting plasma insulin concentrations, mitigated glucose intolerance, and insulin resistance in addition to reducing malondialdehyde and carbonyl protein concentrations when coadministered with either NRTIs. Furthermore, naringin improved antioxidant enzyme activities, reduced skeletal muscle BCL-2-associated X protein expression, and improved B-cell lymphoma-2 protein expression compared to AZT- or d4T-only-treated rats. Naringin ameliorated AZT- and d4T-induced complications and therefore should be further investigated as a possible nutritional supplement in managing HIV infection.

Naringin Reverses Hepatocyte Apoptosis and Oxidative Stress Associated with HIV-1 Nucleotide Reverse Transcriptase Inhibitors-Induced Metabolic Complications

Nucleoside Reverse Transcriptase Inhibitors (NRTIs) have not only improved therapeutic outcomes in the treatment of HIV infection but have also led to an increase in associated metabolic complications of NRTIs. Naringin’s effects in mitigating NRTI-induced complications were investigated in this study. Wistar rats, randomly allotted into seven groups (n = 7) were orally treated daily for 56 days with 100 mg/kg zidovudine (AZT) (groups I, II III), 50 mg/kg stavudine (d4T) (groups IV, V, VI) and 3 mL/kg of distilled water (group VII). Additionally, rats in groups II and V were similarly treated with 50 mg/kg naringin, while groups III and VI were treated with 45 mg/kg vitamin E. AZT or d4T treatment significantly reduced body weight and plasma high density lipoprotein concentrations but increased liver weights, plasma triglycerides and total cholesterol compared to controls, respectively. Furthermore, AZT or d4T treatment significantly increased oxidative stress, adiposity index and expression of Bax protein, but reduced Bcl-2 protein expression compared to controls, respectively. However, either naringin or vitamin E significantly mitigated AZT- or d4T-induced weight loss, dyslipidemia, oxidative stress and hepatocyte apoptosis compared to AZT- or d4T-only treated rats. Our results suggest that naringin reverses metabolic complications associated with NRTIs by ameliorating oxidative stress and apoptosis. This implies that naringin supplements could mitigate lipodystrophy and dyslipidemia associated with NRTI therapy.

Inverse association between microRNA-124a and ABCC4 in HepG2 cells treated with antiretroviral drugs

The ATP-binding cassette (ABC) super-family of drug transporters regulates efflux of xenobiotic compounds. The subfamily, multi-drug resistance proteins (MRPs) transports cyclic nucleotides and xenobiotics. Epigenetic modulation of drug transporters is scarcely described. The regulatory role of microRNA (miR)-124a on drug transporter gene ABCC4 was only recently reported. Our study investigated the differential regulation of miR-124a by nucleoside reverse transcriptase inhibitors (NRTIs): Zidovudine (AZT), Stavudine (d4T) and Tenofovir (TFV); at 24 h and 120 h treatments in HepG2 cells. ABCC4 mRNA (qPCR) and ABCC4 protein (western blot) were quantified. Cytotoxicity was evaluated by lactate dehydrogenase (LDH) levels. All NRTIs elevated miR-124a levels at 24 h, with a concomitant decline in ABCC4 mRNA levels (p<0.05). At 120 h, d4T and TFV elevated miR-124a and depleted ABCC4 mRNA levels (p<0.0001), while the inverse was observed with AZT (p<0.005). ABCC4 protein was increased by d4T and TFV at 24h. A significant reduction in protein levels was observed at 120 h in all three treatments (p<0.005). The disjoint in mRNA and protein levels is likely due to ABCC4 being a membrane bound protein. Following prolonged exposure, membrane integrity was compromised as evidenced by increased LDH leakage (p<0.005). We conclude antiretroviral drugs have varying effects on miR-124a and ABCC4.

A new hypothesis on HIV cure

In this opinion article, I provide the rationale for my hypothesis that nucleoside reverse transcriptase inhibitors (NRTIs) may prevent human immunodeficiency virus (HIV) cure by promoting the survival of cells with integrated provirus. If correct, we may be closer to a cure than we realize.

Maintenance of virologic efficacy and decrease in levels of β2-microglobulin, soluble CD40L and soluble CD14 after switching previously treated HIV-infected patients to an NRTI-sparing dual therapy

Novel strategies are necessary to decrease inflammatory parameters in successfully treated HIV-infected patients. Our aim was to evaluate the maintenance of viral suppression and potential changes in inflammatory, immune-activation and coagulation biomarkers in virologically suppressed HIV-infected patients switched to a nucleoside reverse transcriptase inhibitor-sparing (NRTI) and maraviroc (MVC)-containing combined antiretroviral therapy (cART). Fifty-eight HIV-infected patients were observed after their treatment regimens were changed to MVC 150mg/once daily plus ritonavir-boosted protease inhibitor therapy. Activation-, inflammation- and coagulation-associated biomarkers and mitochondrial (mt)DNA were analyzed after a median of 24weeks of follow-up. We observed that after changing to an NRTI-sparing regimen, 96.6% of HIV-patients on viral suppressive cART maintained viral suppression and their CD4+ T cell counts did not change significantly (median of 31weeks of follow-up). This cART switch reduced soluble CD40 ligand (p=0.002), beta-2 microglobulin (p=0.025), and soluble CD14 (p=0.009) in patients with higher baseline levels of these inflammation biomarkers after a median of 24weeks of follow-up. The results of our study show that changing to NRTI-sparing dual therapy decreased the levels of inflammatory biomarkers and maintained the immune-virologic efficacy. The potential benefits of this regimen warrant further investigation to uncover the association of this therapy with the potential decrease in the morbidity and mortality of HIV-infected patients from non-AIDS-defining illnesses.

Rapid plasma viral suppression in naive HIV-infected patients with high CD4 cells and low viraemia initiating a dual nucleoside reverse transcriptase inhibitor strategy: a proof-of-concept study

OBJECTIVES

To evaluate whether a dual nucleoside reverse transcriptase inhibitor (NRTI) strategy can control HIV replication in antiviral therapy (ART)-naive HIV-infected patients with a high CD4 cell count and a low viral load (VL).

METHODS

This observational study included all HIV-infected treatment-naive patients with a CD4 cell count >300 cells/mm(3), a plasma HIV RNA between 1000 copies/mL and 30,000 copies/mL and wild-type virus who initiated dual NRTI ART between January 2008 and December 2012. HIV RNA and CD4 cell count were assessed at Day 0, Week (W) 4, W12, W24 and W48. The primary endpoint was the proportion of patients with a plasma VL (pVL) <50 copies/mL at W24.

RESULTS

Twenty patients were included. The median (IQR) baseline characteristics were: time since HIV diagnosis, 25 months (8-66 months); CD4 cell count, 592 cells/mm(3) (405-798 cells/mm(3)); HIV RNA, 10,395 copies/mL (4106-16,566 copies/mL); and HIV DNA, 464 copies/10(6) peripheral blood mononuclear cells (195-1168 copies/10(6) PBMC). Nineteen patients received tenofovir/emtricitabine and one patient received abacavir/lamivudine. At W12, 88% of the patients with available data (n = 16/18, 95% CI 0.65-0.99) had a pVL <50 copies/mL. Overall, the proportion of patients with a pVL <50 copies/mL was 100% (n = 20/20, 95% CI 0.83-1.0) at W24 and 95% (n = 18/19, 95% CI 0.74-0.99) at W48 (with one patient lost to follow-up and one patient with poor treatment compliance). The median increase in CD4 cells was 83 cells/mm(3) (40-310 cells/mm(3)). There was no discontinuation of antiretroviral therapy for any reason such as lack of efficacy or toxicity.

CONCLUSIONS

This pilot study suggests that, in patients with a high CD4 cell count and a low VL, a dual NRTI strategy may represent a potentially effective treatment strategy to control HIV replication. This needs to be confirmed in larger controlled clinical studies.

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) inhibits HIV-1 reverse transcriptase with multiple mechanisms

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) is a nucleoside analog that, unlike approved anti-human immunodeficiency virus type 1 (HIV-1) nucleoside reverse transcriptase inhibitors, has a 3'-OH and exhibits remarkable potency against wild-type and drug-resistant HIVs. EFdA triphosphate (EFdA-TP) is unique among nucleoside reverse transcriptase inhibitors because it inhibits HIV-1 reverse transcriptase (RT) with multiple mechanisms. (a) EFdA-TP can block RT as a translocation-defective RT inhibitor that dramatically slows DNA synthesis, acting as a de facto immediate chain terminator. Although non-translocated EFdA-MP-terminated primers can be unblocked, they can be efficiently converted back to the EFdA-MP-terminated form. (b) EFdA-TP can function as a delayed chain terminator, allowing incorporation of an additional dNTP before blocking DNA synthesis. In such cases, EFdA-MP-terminated primers are protected from excision. (c) EFdA-MP can be efficiently misincorporated by RT, leading to mismatched primers that are extremely hard to extend and are also protected from excision. The context of template sequence defines the relative contribution of each mechanism and affects the affinity of EFdA-MP for potential incorporation sites, explaining in part the lack of antagonism between EFdA and tenofovir. Changes in the type of nucleotide before EFdA-MP incorporation can alter its mechanism of inhibition from delayed chain terminator to immediate chain terminator. The versatility of EFdA in inhibiting HIV replication by multiple mechanisms may explain why resistance to EFdA is more difficult to emerge.

An update on the pharmacological strategies in the treatment of HIV-1-associated adipose redistribution syndromes

INTRODUCTION

With the introduction of combination antiretroviral therapy (ART) for HIV infection in the mid-1990s, descriptions of morphological changes and metabolic disturbances in treated patients began to emerge. HIV-1/highly active ART-associated lipodystrophy syndrome (HALS) involves metabolic abnormalities and diverse forms of anomalous fat distribution. The current review focuses on the pathophysiological basis and the clinical evidence for the use of several medical strategies in the management of HALS.

AREAS COVERED

We have covered the most relevant studies related to the pharmacological strategies in the treatment of HALS, with attention to the current and novel antiretroviral agents.

EXPERT OPINION

The most commonly used strategies for HALS reversion have included modification of host-dependent factors, including those related to HIV-1 infection and those associated with ART. Preventive and medical strategies have been associated with moderate success. The only intervention that offers an immediate aesthetical improvement for patients with HALS so far has been plastic surgery.

Phase I/II study of the pharmacokinetics, safety and antiretroviral activity of tenofovir alafenamide, a new prodrug of the HIV reverse transcriptase inhibitor tenofovir, in HIV-infected adults

BACKGROUND

Tenofovir alafenamide (formerly GS-7340) is a new oral prodrug of tenofovir, a nucleotide analogue that inhibits HIV-1 reverse transcription. Unlike the currently marketed tenofovir prodrug, tenofovir disoproxil fumarate, tenofovir alafenamide is stable in plasma and then rapidly converted into tenofovir once inside cells.

METHODS

The pharmacokinetics, safety and antiviral activity of 40 or 120 mg of tenofovir alafenamide compared with 300 mg of tenofovir disoproxil fumarate when administered as monotherapy once daily for 14 days in HIV-1-infected, treatment-naive subjects was studied.

RESULTS

Administration of 40 mg of tenofovir alafenamide for 14 days resulted in lower tenofovir Cmax (13 versus 207 ng/mL) and lower systemic exposures (AUC0-t, 383 versus 1810 ng · h/mL) compared with subjects who received tenofovir disoproxil fumarate. There were higher intracellular tenofovir concentrations within peripheral blood mononuclear cells with both 40 mg of tenofovir alafenamide (8.2 μM) and 120 mg of tenofovir alafenamide (16.9 μM) compared with 300 mg of tenofovir disoproxil fumarate (0.9 μM). The most commonly observed adverse events were headache, nausea and flatulence, which occurred similarly across the three groups. After 14 days, the mean changes in HIV-1 RNA were -0.94 log₁₀copies/mL for the tenofovir disoproxil fumarate group, -1.57 log₁₀ copies/mL for the 40 mg of tenofovir alafenamide group and -1.71 log₁₀ copies/mL for the 120 mg of tenofovir alafenamide group. The mean first-phase HIV-1 RNA decay slopes were -0.36, -0.63 and -0.64 for the tenofovir disoproxil fumarate group, the 40 mg of tenofovir alafenamide group and the 120 mg of tenofovir alafenamide group, respectively. No resistance mutations to either tenofovir alafenamide or tenofovir disoproxil fumarate were detected.

CONCLUSIONS

Tenofovir alafenamide, a new once-daily oral prodrug of tenofovir, showed more potent anti-HIV-1 activity and higher intracellular tenofovir levels compared with tenofovir disoproxil fumarate, while maintaining lower plasma tenofovir exposure at 40 mg with good tolerability over 14 days of monotherapy.

Molecular mechanism of HIV-1 resistance to 3'-azido-2',3'-dideoxyguanosine

We reported that 3'-azido-2',3'-dideoxyguanosine (3'-azido-ddG) selected for the L74V, F77L, and L214F mutations in the polymerase domain and K476N and V518I mutations in the RNase H domain of HIV-1 reverse transcriptase (RT). In this study, we have defined the molecular mechanisms of 3'-azido-ddG resistance by performing in-depth biochemical analyses of HIV-1 RT containing mutations L74V, F77L, V106I, L214F, R277K, and K476N (SGS3). The SGS3 HIV-1 RT was from a single-genome-derived full-length RT sequence obtained from 3'-azido-ddG resistant HIV-1 selected in vitro. We also analyzed two additional constructs that either lacked the L74V mutation (SGS3-L74V) or the K476N mutation (SGS3-K476N). Pre-steady-state kinetic experiments revealed that the L74V mutation allows RT to effectively discriminate between the natural nucleotide (dGTP) and 3'-azido-ddG-triphosphate (3'-azido-ddGTP). 3'-azido-ddGTP discrimination was primarily driven by a decrease in 3'-azido-ddGTP binding affinity (Kd) and not by a decreased rate of incorporation (kpol). The L74V mutation was found to severely impair RT's ability to excise the chain-terminating 3'-azido-ddG-monophosphate (3'-azido-ddGMP) moiety. However, the K476N mutation partially restored the enzyme's ability to excise 3'-azido-ddGMP on an RNA/DNA, but not on a DNA/DNA, template/primer by selectively decreasing the frequency of secondary RNase H cleavage events. Collectively, these data provide strong additional evidence that the nucleoside base structure is major determinant of HIV-1 resistance to the 3'-azido-2',3'-dideoxynucleosides.

Reversal of multidrug resistance by the inhibition of ATP-binding cassette pumps employing "Generally Recognized As Safe" (GRAS) nanopharmaceuticals: A review

Pumps of the ATP-binding cassette superfamily (ABCs) regulate the access of drugs to the intracellular space. In this context, the overexpression of ABCs is a well-known mechanism of multidrug resistance (MDR) in cancer and infectious diseases (e.g., viral hepatitis and the human immunodeficiency virus) and is associated with therapeutic failure. Since their discovery, ABCs have emerged as attractive therapeutic targets and the search of compounds that inhibit their genetic expression and/or their functional activity has gained growing interest. Different generations of pharmacological ABC inhibitors have been explored over the last four decades to address resistance in cancer, though clinical results have been somehow disappointing. "Generally Recognized As Safe" (GRAS) is a U.S. Food and Drug Administration designation for substances that are accepted as safe for addition in food. Far from being "inert", some amphiphilic excipients used in the production of pharmaceutical products have been shown to inhibit the activity of ABCs in MDR tumors, emerging as a clinically translatable approach to overcome resistance. The present article initially overviews the classification, structure and function of the different ABCs, with emphasis on those pumps related to drug resistance. Then, the different attempts to capitalize on the activity of GRAS nanopharmaceuticals as ABC inhibitors are discussed.

The nucleoside reverse transcriptase inhibitors, nonnucleoside reverse transcriptase inhibitors, and protease inhibitors in the treatment of HIV infections (AIDS)

The majority of the drugs currently used for the treatment of HIV infections (AIDS) belong to either of the following three classes: nucleoside reverse transcriptase inhibitors (NRTIs), nonnucleoside reverse transcriptase inhibitors (NNRTIs), and protease inhibitors (PIs). At present, there are 7 NRTIs, 5 NNRTIs, and 10 PIs approved for clinical use. They are discussed from the following viewpoints: (i) chemical formulae; (ii) mechanism of action; (iii) drug combinations; (iv) clinical aspects; (v) preexposure prophylaxis; (vi) prevention of mother-to-child transmission; (vii) their use in children; (viii) toxicity; (ix) adherence (compliance); (x) resistance; (xi) new NRTIs, NNRTIs, or PIs in (pre)clinical development; and (xii) the prospects for a "cure" of the disease.