Emtricitabine

Synthesis of Fluorinated Nucleosides/Nucleotides and Their Antiviral Properties

The FDA has approved several drugs based on the fluorinated nucleoside pharmacophore, and numerous drugs are currently in clinical trials. Fluorine-containing nucleos(t)ides offer significant antiviral and anticancer activity. The insertion of a fluorine atom, either in the base or sugar of nucleos(t)ides, alters its electronic and steric parameters and transforms the lipophilicity, pharmacodynamic, and pharmacokinetic properties of these moieties. The fluorine atom restricts the oxidative metabolism of drugs and provides enzymatic metabolic stability towards the glycosidic bond of the nucleos(t)ide. The incorporation of fluorine also demonstrates additional hydrogen bonding interactions in receptors with enhanced biological profiles. The present article discusses the synthetic methodology and antiviral activities of FDA-approved drugs and ongoing fluoro-containing nucleos(t)ide drug candidates in clinical trials.

Fluorine in the pharmaceutical industry: Synthetic approaches and application of clinically approved fluorine-enriched anti-infectious medications

The strategic integration of fluorine atoms into anti-infectious agents has become a cornerstone in the field of medicinal chemistry, owing to the unique influence of fluorine on the chemical and biological properties of pharmaceuticals. This review examines the synthetic methodologies that enable the incorporation of fluorine into anti-infectious drugs, and the resultant clinical applications of these fluorine-enriched compounds. With a focus on clinically approved medications, the discussion extends to the molecular mechanisms. It further outlines the specific effects of fluorination, which contribute to the heightened efficacy of anti-infective therapies. By presenting a comprehensive analysis of current drugs and their developmental pathways, this review underscores the continuing evolution and significance of fluorine in advancing anti-infectious treatment options. The insights offered extend valuable guidance for future drug design and the development of next-generation anti-infectious agents.

Cytosolic 5'-Nucleotidase III and Nucleoside Triphosphate Diphosphohydrolase 1 Dephosphorylate the Pharmacologically Active Metabolites of Gemcitabine and Emtricitabine

Gemcitabine (dFdC) and emtricitabine (FTC) are first-line drugs that are used for the treatment of pancreatic cancer and human immunodeficiency virus, respectively. The above drugs must undergo sequential phosphorylation to become pharmacologically active. Interindividual variability associated with the responses of the above drugs has been reported. The molecular mechanisms underlying the observed variability are yet to be elucidated. Although this could be multifactorial, nucleotidases may be involved in the dephosphorylation of drug metabolites due to their structural similarity to endogenous nucleosides. With these in mind, we performed in vitro assays using recombinant nucleotidases to assess their enzymatic activities toward the metabolites of dFdC and FTC. From the above in vitro experiments, we noticed the dephosphorylation of dFdC-monophosphate in the presence of two 5'-nucleotidases (5'-NTs), cytosolic 5'-nucleotidase IA (NT5C1A) and cytosolic 5'-nucleotidase III (NT5C3), individually. Interestingly, FTC monophosphate was dephosphorylated only in the presence of NT5C3 enzyme. Additionally, nucleoside triphosphate diphosphohydrolase 1 (NTPDase 1) exhibited enzymatic activity toward both triphosphate metabolites of dFdC and FTC. Enzyme kinetic analysis further revealed Michaelis-Menten kinetics for both NT5C3-mediated dephosphorylation of monophosphate metabolites, as well as NTPDase 1-mediated dephosphorylation of triphosphate metabolites. Immunoblotting results confirmed the presence of NT5C3 and NTPDase 1 in both pancreatic and colorectal tissue that are target sites for dFdC and FTC treatment, respectively. Furthermore, sex-specific expression patterns of NT5C3 and NTPDase 1 were determined using mass spectrometry-based proteomics approach. Based on the above results, NT5C3 and NTPDase 1 may function in the control of the levels of dFdC and FTC metabolites. SIGNIFICANCE STATEMENT: Emtricitabine and gemcitabine are commonly used drugs for the treatment of human immunodeficiency virus and pancreatic cancer. To become pharmacologically active, both the above drugs must be phosphorylated. The variability in the responses of the above drugs can lead to poor clinical outcomes. Although the sources of drug metabolite concentration variability are multifactorial, it is vital to understand the role of nucleotidases in the tissue disposition of the above drug metabolites due to their structural similarities to endogenous nucleosides.

Current insights and molecular docking studies of HIV-1 reverse transcriptase inhibitors

Human immunodeficiency virus (HIV) causes acquired immunodeficiency syndrome (AIDS), a lethal disease that is prevalent worldwide. According to the Joint United Nations Programme on HIV/AIDS (UNAIDS) data, 38.4 million people worldwide were living with HIV in 2021. Viral reverse transcriptase (RT) is an excellent target for drug intervention. Nucleoside reverse transcriptase inhibitors (NRTIs) were the first class of approved antiretroviral drugs. Later, a new type of non-nucleoside reverse transcriptase inhibitors (NNRTIs) were approved as anti-HIV drugs. Zidovudine, didanosine, and stavudine are FDA-approved NRTIs, while nevirapine, efavirenz, and delavirdine are FDA-approved NNRTIs. Several agents are in clinical trials, including apricitabine, racivir, elvucitabine, doravirine, dapivirine, and elsulfavirine. This review addresses HIV-1 structure, replication cycle, reverse transcription, and HIV drug targets. This study focuses on NRTIs and NNRTIs, their binding sites, mechanisms of action, FDA-approved drugs and drugs in clinical trials, their resistance and adverse effects, their molecular docking studies, and highly active antiretroviral therapy (HAART).

Review on fluorinated nucleoside/non-nucleoside FDA-approved antiviral drugs

FDA-approved antiviral agents represent an important class that has attracted attention in recent years to combat current and future threats of viral pandemics. Fluorine ameliorates the electronic, lipophilic and steric problems of drugs. Additionally, fluorine can prolong drug activity and improve metabolic stability, thereby, modifying their pharmacodynamic and pharmacokinetic character. Herein, we summarized the fluorinated FDA-approved antiviral agents, dealing with biological aspects, mechanisms of action, and synthetic pathways.

HIV nucleoside reverse transcriptase inhibitors

More than 40 years into the pandemic, HIV remains a global burden and as of now, there is no cure in sight. Fortunately, highly active antiretroviral therapy (HAART) has been developed to manage and suppress HIV infection. Combinations of two to three drugs targeting key viral proteins, including compounds inhibiting HIV reverse transcriptase (RT), have become the cornerstone of HIV treatment. This review discusses nucleoside reverse transcriptase inhibitors (NRTIs), including chain terminators, delayed chain terminators, nucleoside reverse transcriptase translocation inhibitors (NRTTIs), and nucleotide competing RT inhibitors (NcRTIs); focusing on their history, mechanism of action, resistance, and current clinical application, including long-acting regimens.

The best backbone for HIV prevention, treatment, and elimination: Emtricitabine+tenofovir

The advent of antiretroviral combination therapy has significantly impacted the HIV/AIDS epidemic. No longer a death sentence, HIV infection can be controlled and suppressed using cocktail therapies that contain two or more small molecule drugs. This review aims to highlight the discovery, development, and impact of one such molecule, namely, emtricitabine (FTC, emtriva), which is one of the most successful drugs in the fight against HIV/AIDS and has been taken by over 94% of individuals infected with HIV in the USA. We also pay tribute to Dr. John C. Martin, former CEO and Chairman of Gilead Sciences, who unexpectedly passed away in 2021. A true visionary, he was instrumental in delivering FTC, as part of combination therapy with TDF (tenofovir, viread) to the global stage. As the fight to eradicate HIV marches on, we honor Dr. Martin's legacy of collaboration, achievement, and hope.

Development of Human Immunodeficiency Virus Type 1 Resistance to 4'-Ethynyl-2-Fluoro-2'-Deoxyadenosine (EFdA) Starting with Wild-Type or Nucleoside Reverse Transcriptase Inhibitor Resistant-Strains

4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA, MK-8591, islatravir) is a nucleoside reverse transcriptase translocation inhibitor (NRTTI) with exceptional potency against WT and drug-resistant HIV-1, in Phase III clinical trials. EFdA resistance is not well characterized. To study EFdA-resistance patterns as it may emerge in naïve or tenofovir- (TFV), emtricitabine/lamivudine- (FTC/3TC), or zidovudine- (AZT) treated patients we performed viral passaging experiments starting with wild-type, K65R, M184V, or D67N/K70R/T215F/K219Q HIV-1. Regardless the starting viral sequence, all selected EFdA-resistant variants included the M184V RT mutation. Using recombinant viruses, we validated the role for M184V as the primary determinant of EFdA resistance; none of the observed connection subdomain (R358K and E399K) or RNase H domain (A502V) mutations significantly contributed to EFdA resistance. A novel EFdA resistance mutational pattern that included A114S was identified in the background of M184V. A114S/M184V exhibited higher EFdA resistance (∼24-fold) than M184V (∼8-fold) or A114S alone (∼2-fold). Remarkably, A114S/M184V and A114S/M184V/A502V resistance mutations were up to 50-fold more sensitive to tenofovir than WT HIV-1. These mutants also had significantly lower specific infectivity than WT. Biochemical experiments confirmed decreases in the enzymatic efficiency (k_cat/K_m) of WT vs. A114S (2.1-fold) and A114S/M184V/A502V (6.5-fold) RTs, with no effect of A502V on enzymatic efficiency or specific infectivity. The rather modest EFdA resistance of M184V or A114S/M184V (8- and 24-fold), their hypersusceptibility to tenofovir, and strong published in vitro and in vivo data, suggest that EFdA is an excellent therapeutic candidate for naïve, AZT-, FTC/3TC, and especially tenofovir-treated patients.

Potential Anti-SARS-CoV-2 Therapeutics That Target the Post-Entry Stages of the Viral Life Cycle: A Comprehensive Review

The coronavirus disease-2019 (COVID-19) pandemic continues to challenge health care systems around the world. Scientists and pharmaceutical companies have promptly responded by advancing potential therapeutics into clinical trials at an exponential rate. Initial encouraging results have been realized using remdesivir and dexamethasone. Yet, the research continues so as to identify better clinically relevant therapeutics that act either as prophylactics to prevent the infection or as treatments to limit the severity of COVID-19 and substantially decrease the mortality rate. Previously, we reviewed the potential therapeutics in clinical trials that block the early stage of the viral life cycle. In this review, we summarize potential anti-COVID-19 therapeutics that block/inhibit the post-entry stages of the viral life cycle. The review presents not only the chemical structures and mechanisms of the potential therapeutics under clinical investigation, i.e., listed in clinicaltrials.gov, but it also describes the relevant results of clinical trials. Their anti-inflammatory/immune-modulatory effects are also described. The reviewed therapeutics include small molecules, polypeptides, and monoclonal antibodies. At the molecular level, the therapeutics target viral proteins or processes that facilitate the post-entry stages of the viral infection. Frequent targets are the viral RNA-dependent RNA polymerase (RdRp) and the viral proteases such as papain-like protease (PLpro) and main protease (Mpro). Overall, we aim at presenting up-to-date details of anti-COVID-19 therapeutics so as to catalyze their potential effective use in fighting the pandemic.

Darunavir/Cobicistat/Emtricitabine/Tenofovir Alafenamide: A Review in HIV-1 Infection

Darunavir/cobicistat/emtricitabine/tenofovir AF (Symtuza^®) is the first protease inhibitor (PI)-based single-tablet regimen (STR) available for the treatment of adults and adolescents (aged ≥ 12 years) with HIV-1 infection. It combines the PI darunavir (which has a high genetic barrier to resistance) with the pharmacokinetic booster cobicistat and the nucleos(t)ide reverse transcriptase inhibitors emtricitabine and tenofovir alafenamide (tenofovir AF), the latter being associated with less off-target tenofovir exposure than its predecessor tenofovir disoproxil fumarate (tenofovir DF). Over 48 weeks in phase 3 trials, darunavir/cobicistat/emtricitabine/tenofovir AF was noninferior to darunavir/cobicistat plus emtricitabine/tenofovir DF in establishing virological suppression in antiretroviral therapy (ART)-naïve adults and, likewise, was noninferior to an ongoing boosted PI, emtricitabine plus tenofovir DF regimen in preventing virological rebound in virologically-suppressed, ART-experienced adults. Resistance did not emerge to the STR components, with the exception being an emtricitabine resistance-associated mutation (RAM) [M184I/V] in one of seven recipients who experienced virological failure (although M184V was a minority variant at screening in this patient). Darunavir/cobicistat/emtricitabine/tenofovir AF was generally well tolerated, with renal and bone profile improvements but less favourable effects on some lipids versus tenofovir DF-based regimens. Thus, although longer-term and cost-effectiveness data would be beneficial, darunavir/cobicistat/emtricitabine/tenofovir AF is a welcome addition to the STRs available for the treatment of adults and adolescents with HIV-1 infection, being the first to combine the high genetic resistance barrier of darunavir with the renal/bone profile of tenofovir AF, thus expanding the patient population for whom an STR may be suitable.

Fluorinated nucleosides as an important class of anticancer and antiviral agents

Fluorine-containing nucleoside analogs (NAs) represent a significant class of the US FDA-approved chemotherapeutics widely used in the clinic. The incorporation of fluorine into drug-like agents modulates lipophilic, electronic and steric parameters, thus influencing pharmacodynamic and pharmacokinetic properties of drugs. Fluorine can block oxidative metabolism of drugs and the formation of undesired metabolites by changing H-bonding interactions. In this review, we focus our attention on chemical fluorination reagents and methods used in the NAs field, including positron emission tomography radiochemistry. We briefly discuss both the cellular biology and clinical properties of FDA-approved and fluorine-containing nucleoside/nucleotide analogs in development as well as common resistance mechanisms associated with their use. Finally, we emphasize pronucleotide strategies used to improve therapeutic outcome of NAs in the clinic.

Reversible diabetes mellitus induced by use of, and improved after discontinuation of, the antiretroviral medication zidovudine: a case report

BACKGROUND

With the advent of effective antiretroviral therapy, the care of patients with human immunodeficiency virus infection became more like that of other chronic diseases. Diabetes mellitus can also occur as one of the chronic illnesses affecting patients with human immunodeficiency virus infection. We report a case of newly developed diabetes mellitus in a patient with human immunodeficiency virus infection, most likely caused by the nucleoside analogue zidovudine, and its improvement after discontinuation of zidovudine.

CASE PRESENTATION

A Chinese man in his 30s visited our outpatient clinic for routine follow-up of human immunodeficiency virus infection. Blood tests showed hyperglycemia with a glucose level of 31.8 mmol/L and hemoglobin A1c of 8.5%. He was diagnosed with diabetes mellitus and treated with oral diabetic medications. The use of zidovudine was suspected as the cause of his diabetes, and it was replaced by other antiretroviral medication. His hyperglycemia improved, and he now no longer requires diabetic medications.

CONCLUSIONS

Diabetes mellitus can develop with the use of antiretroviral medications, but its occurrence associated with use of zidovudine is quite rare. Healthcare personnel should be aware of this rare, yet important, side effect.

Determination of Phenotypic Resistance Cutoffs From Routine Clinical Data

Supplemental Digital Content is Available in the Text.

Background:

HIV-1 drug resistance can be measured with phenotypic drug-resistance tests. However, the output of these tests, the resistance factor (RF), requires interpretation with respect to the in vivo activity of the tested variant. Specifically, the dynamic range of the RF for each drug has to be divided into a suitable number of clinically meaningful intervals.

Methods:

We calculated a susceptible-to-intermediate and an intermediate-to-resistant cutoff per drug for RFs predicted by geno2pheno_[resistance]. Probability densities for therapeutic success and failure were estimated from 10,444 treatment episodes. The density estimation procedure corrects for the activity of the backbone drug compounds and for therapy failure without drug resistance. For estimating the probability of therapeutic success given an RF, we fit a sigmoid function. The cutoffs are given by the roots of the third derivative of the sigmoid function.

Results:

For performance assessment, we used geno2pheno_[resistance] RF predictions and the cutoffs for predicting therapeutic success in 2 independent sets of therapy episodes. HIVdb was used for performance comparison. On one test set (n = 807), our cutoffs and HIVdb performed equally well receiver operating characteristic curve [(ROC)–area under the curve (AUC): 0.68]. On the other test set (n = 917), our cutoffs (ROC–AUC: 0.63) and HIVdb (ROC–AUC: 0.65) performed comparatively well.

Conclusions:

Our method can be used for calculating clinically relevant cutoffs for (predicted) RFs. The method corrects for the activity of the backbone drug compounds and for therapy failure without drug resistance. Our method's performance is comparable with that of HIVdb. RF cutoffs for the latest version of geno2pheno_[resistance] have been estimated with this method.

Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide: A Review in HIV-1 Infection

Tenofovir alafenamide (tenofovir AF) is a novel oral prodrug of the nucleos(t)ide reverse transcriptase inhibitor (NRTI) tenofovir that has several pharmacological advantages over tenofovir disoproxil fumarate (tenofovir DF), including increased plasma stability and reduced tenofovir systemic exposure. Tenofovir AF has been coformulated with elvitegravir, cobicistat and emtricitabine as a once-daily, single-tablet regimen (elvitegravir/cobicistat/emtricitabine/tenofovir AF; Genvoya(®)) for the treatment of adults and adolescents with HIV-1 infection. With regard to establishing and/or maintaining virological suppression over 48 weeks in randomized, phase III trials, elvitegravir/cobicistat/emtricitabine/tenofovir AF was noninferior to elvitegravir/cobicistat/emtricitabine/tenofovir DF in antiretroviral therapy (ART)-naive adults, and statistically superior (subsequent to established noninferiority) to ongoing treatment with tenofovir DF-containing regimens in ART-experienced adults with virological suppression. In single-arm, phase III trials, elvitegravir/cobicistat/emtricitabine/tenofovir AF also provided high rates of virological suppression among ART-naive adolescents and ART-experienced adults with stable renal impairment. In general, elvitegravir/cobicistat/emtricitabine/tenofovir AF was well tolerated and associated with more favourable renal and bone parameters, but a less favourable lipid profile, than tenofovir DF-containing regimens. Thus, elvitegravir/cobicistat/emtricitabine/tenofovir AF is an alternative single-tablet regimen for adults and adolescents with HIV-1 infection, particularly those with an estimated creatinine clearance of ≥30 to <50 mL/min or an increased risk of tenofovir DF-related bone toxicity.

Tenofovir alafenamide, emtricitabine, elvitegravir, and cobicistat combination therapy for the treatment of HIV

INTRODUCTION

Tenofovir alafenamide (TAF) is a novel prodrug of the nucleotide analogue reverse-transcriptase inhibitor, tenofovir. TAF has been co-formulated with emtricitabine (FTC), elvitegravir (EVG) and the EVG metabolic enhancer, cobicistat (COBI) as a single-tablet regimen being the first TAF-containing antiretroviral combination available. Areas covered: This article summarizes the available information on the pharmacology of the novel compound TAF and overviews TAF/FTC/EVG/COBI use for HIV-1 infected patients, with specific focus on clinical efficacy and safety data. Information sources include peer-reviewed scientific literature, conference proceedings and publically available regulatory reports. Compared to tenofovir disoproxil fumarate, TAF results in higher concentrations of the active metabolite tenofovir diphosphate within lymphoid cells, whereas plasma tenofovir exposure is about 90% lower. The efficacy and safety of TAF/FTC/EVG/COBI in treatment-naïve HIV-infected patients has been assessed in phase-III randomized trials, showing non-inferior virological suppression in comparison with TDF/FTC/EVG/COBI, and significantly lower renal and bone toxicity. In addition, TAF/FTC/EVG/COBI has demonstrated efficacy and safety as a switching strategy in suppressed HIV-1 infected individuals, including those with mild or moderate renal impairment. Expert commentary: Approval of the single-tablet TAF/FTC/EVG/COBI regimen is an important advance in HIV therapy, as it is associated with very high efficacy and a better kidney and bone safety profile compared to TDF-containing regimens due to the incorporation of TAF.

Elvitegravir/cobicistat/emtricitabine/tenofovir disoproxil fumarate single-tablet regimen (Stribild®): a review of its use in the management of HIV-1 infection in adults

A new single-tablet, fixed-dose formulation consisting of elvitegravir, an HIV-1 integrase strand transfer inhibitor (INSTI); cobicistat, a pharmacokinetic enhancer; emtricitabine, a nucleoside reverse transcriptase inhibitor; and tenofovir disoproxil fumarate (tenofovir DF), a nucleotide reverse transcriptase inhibitor (elvitegravir/cobicistat/emtricitabine/tenofovir DF 150 mg/150 mg/200 mg/300 mg; Stribild®) is available in some countries for the once-daily treatment of HIV-1 infection in antiretroviral therapy-naïve adults. Elvitegravir/cobicistat/emtricitabine/tenofovir DF is the first INSTI-based single-tablet regimen available for the complete initial treatment of adults with HIV-1 infection. In two large, randomized, double-blind, phase III trials, once-daily treatment with elvitegravir/cobicistat/emtricitabine/tenofovir DF was effective in reducing plasma HIV-1 RNA levels to <50 copies/mL at the week 48 assessment and showed virological efficacy noninferior to that of the efavirenz/emtricitabine/tenofovir DF single-tablet regimen or a once-daily regimen of atazanavir plus ritonavir (ritonavir-boosted atazanavir) plus the fixed-dose combination of emtricitabine/tenofovir DF. Elvitegravir/cobicistat/emtricitabine/tenofovir DF also showed durable efficacy in terms of achieving sustained suppression of HIV-1 RNA levels to <50 copies/mL for up to 144 weeks in both of the phase III trials. Elvitegravir/cobicistat/emtricitabine/tenofovir DF is an important addition to the group of simplified once-daily single-tablet regimens currently available for the effective treatment of HIV-1 infection in antiretroviral therapy-naïve patients and is among the preferred regimens recommended for use as initial treatment. It offers advantages over more complex multiple-tablet regimens that may impair treatment adherence, which is fundamental to the successful management of HIV-1 infection.

Pharmacokinetics of single- and multiple-dose emtricitabine in healthy male Chinese volunteers

Emtricitabine (FTC) is used for the treatment of HIV infection and pre-exposure chemoprophylaxis. It is often used in combination with tenofovir disoproxil fumarate (TDF). This study was designed to evaluate FTC pharmacokinetics in healthy male Chinese volunteers. Sixty subjects were recruited into this single-centre, randomised, open-label study and randomly received single (groups A, B and C) or multiple oral doses (once daily for 6 days; groups D, E and F) of 200-mg FTC capsules alone (A and D), or combined with 300-mg TDF tablets (B and E), or 200 mg of FTC plus 300 mg of TDF with a high-fat diet (C and F), respectively. FTC was well-tolerated in all groups. After a single dose, there were no differences in the mean AUC0-∞ values; however, there were significant differences in the mean Tmax values (1.05, 1.40 and 2.10 h for groups A, B and C, respectively; p < 0.05). In the multiple-dose study, our results were significantly different from published t1/2 values following single-dose FTC.

Synthesis, anti-HIV and cytostatic evaluation of 3'-deoxy-3'-fluorothymidine (FLT) pro-nucleotides

A series of pro-nucleotide phosphoramidates and phosphorodiamidates of the antiviral lead compound 3'-deoxy-3'-fluorothymidine (FLT) have been designed and synthesized. In vitro antiretroviral and cytostatic studies revealed potent (sub-micromolar) inhibition of HIV-1 and HIV-2 replication, with retention of activity in thymidine kinase-negative cell models, as predicted by the ProTide concept.

Emtricitabine/tenofovir disoproxil fumarate: a review of its use in HIV-1 pre-exposure prophylaxis

The fixed-dose combination of emtricitabine (FTC) 200 mg and tenofovir disoproxil fumarate (TDF) 300 mg (Truvada(®)), administered orally once daily, is widely used as part of first-line regimens for the treatment of HIV-1 infection. Recently, once-daily administration of FTC/TDF was approved in the USA for pre-exposure prophylaxis in conjunction with safer sex practices to reduce the risk of sexually acquired HIV-1 in high-risk adults who are not infected. To date, results of four large, randomized, double-blind, placebo-controlled, multicentre trials with FTC/TDF as pre-exposure prophylaxis have been published. Three studies showed statistically significant reductions in the number of individuals with emergent HIV-1 infection when FTC/TDF was compared with placebo over the ≈1- to 2-year study periods. Efficacy (i.e. risk reduction relative to placebo) was 44 % in the iPrEx trial in men who have sex with men, 75 % in the Partners PrEP study in heterosexual HIV-1-serodiscordant couples and 62 % in the TDF2 trial in heterosexual men and women. The fourth study (FEM-PrEP) in heterosexual women did not show a statistically significant difference between FTC/TDF and placebo, although low adherence rates reported in this trial may have been a factor. No unexpected adverse events were reported in the trials. However, since pre-exposure prophylaxis involves long-term administration of drugs to healthy individuals, it is important to monitor the long-term safety of FTC/TDF (e.g. renal function, bone mineral density) in this setting. Other notable considerations include adherence, cost and the potential for development of drug resistance. Interim guidelines are available for prescribing FTC/TDF as pre-exposure prophylaxis. If used appropriately in selected high-risk individuals, pre-exposure prophylaxis with FTC/TDF represents an important additional strategy to reduce the spread of HIV-1 infection, which continues to be a significant global concern.

Clinical pharmacokinetics of antiretroviral drugs in older persons

INTRODUCTION

Combination antiretroviral therapy has enabled HIV-infected persons to reach older ages in high numbers. Hepatic and renal changes that normally occur with advancing age occur earlier and with higher incidence in HIV-infected individuals. A limited number of prospective controlled studies have demonstrated small reductions (17 to 41%) in lopinavir, atazanavir and lamivudine clearance in older versus younger adults. A much larger number of retrospective studies in adults (age range ∼ 20 to 60 years), including all antiretroviral drugs, have evaluated age as a covariate for pharmacokinetics. Most studies did not detect substantial associations between drug exposures and age.

AREAS COVERED

This review summarizes antiretroviral drug pharmacokinetics in older persons. The authors review articles from PubMed (search terms: elderly, antiretroviral, pharmacokinetics) in addition to the bibliographies of those selected.

EXPERT OPINION

The evidence to date does not support major pharmacokinetic changes in adults between ∼ 20 and 60 years of age. However, additional prospective, well-controlled studies are needed in more persons > 60 years, including those with frailty and comorbidities, with assessment of unbound drug clearance, and incorporation of adherence, pharmacogenetics and concomitant medications. Until then, guidelines for drug-drug interactions and dosing in renal and hepatic impairment should be followed in older HIV-infected individuals.

Emtricitabine/tenofovir in the treatment of HIV infection: current PK/PD evaluation

INTRODUCTION

Emtricitabine/tenofovir disoproxil fumarate fixed-dose combination (FTC/TDF FDC) is the co-formulation of a nucleoside and a nucleotide, respectively. After oral administration, both drugs exhibit plasma and intracellular half-lives suitable for once-daily dosing. Within the host cells, active metabolites FTC-TP and TFV-DP act as chain terminators to the newly synthesized proviral DNA, showing synergy at enzymatic level (viral reverse transcriptase). When given in HAART combinations, FTC/TDF FDC has a remarkable effectiveness in controlling HIV replication and securing a significant CD4(+) cell recovery. If patients treated with FTC/TDF FDC fail, a lower incidence of TDF-associated K65R resistance mutation seems to develop. Furthermore, cytidine analog-associated M184V is less likely to appear with FTC than with lamivudine when both are given with TDF. FTC and TFV are not metabolized by CYP450 enzymes and are eliminated by the renal route. TFV may accumulate in tubular cells and cause a decrease in GFR and a loss of phosphates. As a onsequence, patients treated with FTC/TDF FCD may experience varied degrees of renal impairment and osteopenia/osteoporosis.

AREAS COVERED

This paper has focused on the PK/PD features of FTC and TDF, when given as single agent or when administered as FDC. The interpretation of efficacy/toxicity was guided by PK/PD features. The review of the available literature included also conference presentations and recent guidelines (as of May 2012).

EXPERT OPINION

FTC/TDF FDC is a potent and reliable component of most HAART combinations due to its maintained activity across time, as demonstrated in many trials and studies. Toxicity issues (kidney, bone) are still to be entirely elucidated and the drug-induced component well separated from patient- and HIV-related ones. However, the clinical gain associated with the use of FTC/TDF FDC is fully acknowledged by its leading position in most current treatment guidelines.

An update on the use of Atripla in the treatment of HIV in the United States

Atripla^® (Gilead Sciences Inc, Foster City, CA, USA and Bristol-Myers Squibb, New York City, NY, USA) is a coformulated single pill composed of efavirenz, emtricitabine, and tenofovir disoproxil, intended as a once-daily potent combination antiretroviral therapeutic agent. Its efficacy is equivalent to the 3 component drugs taken in a combination as single medications. The coformulated antiretroviral regimen can be quite effective in patients whose human immunodeficiency virus is sensitive to all 3 components of Atripla. However, women at risk of pregnancy, already pregnant, or nursing mothers should not take Atripla, due to the teratogenic potential of the efavirenz moiety. Adverse effects are similar to those seen with the constituent medications, including potential central nervous system effects and renal toxicity. Since its US Food and Drug administration approval, prescriptions for Atripla have increased steadily.

Modelling imperfect adherence to HIV induction therapy

Background

Induction-maintenance therapy is a treatment regime where patients are prescribed an intense course of treatment for a short period of time (the induction phase), followed by a simplified long-term regimen (maintenance). Since induction therapy has a significantly higher chance of pill fatigue than maintenance therapy, patients might take drug holidays during this period. Without guidance, patients who choose to stop therapy will each be making individual decisions, with no scientific basis.

Methods

We use mathematical modelling to investigate the effect of imperfect adherence during the inductive phase. We address the following research questions: 1. Can we theoretically determine the maximal length of a possible drug holiday and the minimal number of doses that must subsequently be taken while still avoiding resistance? 2. How many drug holidays can be taken during the induction phase?

Results

For a 180 day therapeutic program, a patient can take several drug holidays, but then has to follow each drug holiday with a strict, but fairly straightforward, drug-taking regimen. Since the results are dependent upon the drug regimen, we calculated the length and number of drug holidays for all fifteen protease-sparing triple-drug cocktails that have been approved by the US Food and Drug Administration.

Conclusions

Induction therapy with partial adherence is tolerable, but the outcome depends on the drug cocktail. Our theoretical predictions are in line with recent results from pilot studies of short-cycle treatment interruption strategies and may be useful in guiding the design of future clinical trials.

Emtricitabine/tenofovir disoproxil fumarate: in combination with a protease inhibitor in HIV-1 infection

Emtricitabine, a nucleoside reverse transcriptase inhibitor (RTI), and tenofovir disoproxil fumarate (tenofovir DF), a nucleotide RTI, as a fixed-dose combination tablet (emtricitabine/tenofovir DF) for once-daily oral administration, are used as the nucleoside/nucleotide RTI backbone in combination with other antiretroviral agents, including ritonavir-boosted protease inhibitors (PIs), in the treatment of adults with HIV-1 infection. Emtricitabine and tenofovir DF show good activity against laboratory strains and clinical isolates of HIV-1 in vitro, although strains with resistance to emtricitabine or tenofovir have also been reported. Regimens consisting of once-daily emtricitabine/tenofovir DF 200 mg/300 mg plus lopinavir/ritonavir (in the randomized, double-blind, placebo-matched, multicentre HEAT study) or boosted atazanavir or efavirenz (in the randomized, partially-blind, multicentre ACTG 5202 trial) were effective in the initial treatment of patients with HIV-1 infection (with screening plasma HIV-1 RNA levels of >or=100,000 copies/mL in ACTG 5202). In other randomized studies, emtricitabine/tenofovir DF 200 mg/300 mg once daily was an effective backbone for boosted PI-based regimens in the initial treatment of HIV-1 infection. Treatment-experienced patients with HIV-1 infection also experienced beneficial virological effects when treated with similar regimens. Emtricitabine/tenofovir DF in combination with various boosted PIs was generally well tolerated by adults with HIV-1 infection.

"One pill, once daily": what clinicians need to know about Atriplatrade mark

As the number of persons chronically prescribed antiretrovirals has grown and the realization that antiretrovirals are required to be continued for life, pharmaceutical manufacturers have developed new classes of agents, improved the pharmacokinetics of marketed products through dosing reformulations, and in an effort to maximize success with respect to adherence, compiled into a single dosing unit all necessary elements for an antiretroviral regimen. Atriplatrade mark represents the first ever fixed-dose combination antiretroviral available. This article reviews currently available data on this agent, the impact of resistance on clinical use and implementation, as well as extensive descriptions of the pharmacokinetics, adverse effects and drug-interactions warranting consideration. Whether beginning in a naïve patient or switching from other regimens for tolerability issues, Atriplatrade mark represents a viable option. Its demonstrated advantages with respect to lipid and hematologic parameters and equivalent incidence of renal toxicity are tempered by the findings of bone mineral density decreases, however. Combining multiple mechanisms of action in a single dosing unit appears to improve efficacy, increase the likelihood for adherence and maintain viral suppression compared to administering these agents independently. It is suggested other pharmaceutical companies assess the potential to replicate this for the remaining antiretrovirals.

Syncope as a probable side effect to combination antiretroviral therapy initiated during primary HIV-1 infection

A previously healthy young man experienced several episodes of syncope while being treated with tenofovir, emtricitabine and nevirapine initiated during primary HIV-1 infection. Symptoms disappeared after discontinuation of antiretroviral therapy and we suggest that syncope may be a side effect to one of the three antiretroviral drugs that has not been described previously.

Nucleoside and nucleotide reverse transcriptase inhibitors in children

By the end of 2006, approximately 2.3 million children worldwide were living with HIV infection, representing about 15% of all HIV-infected individuals but only 5-7% of the total population of treated patients worldwide. Despite a general increase in the use of antiretroviral therapy (ART) in resource-limited settings, appropriate care and ART remain inaccessible for most of the world's HIV-infected children. ART of children is challenging because of a general lack of paediatric formulations (including tablets in paediatric strengths), limited options of drugs available for children (some have been approved only for use in adults), different viral and immunological responses, dependency on caregivers for administration of the therapy, and specific issues of toxicity in long-term therapy related to maturation and development. As in adults, nucleoside reverse transcriptase inhibitors (NRTIs) are a key component of any ART schedule in children, being the recommended 'backbone' treatment in US, European and WHO guidelines, and, indeed, NRTIs have been extensively studied in children. NRTIs are the class of antiretroviral drugs that have more drugs licensed for paediatric use and more paediatric formulations.Generally, the dual NRTI backbone treatment of combination with a non-NRTI (NNRTI) or protease inhibitor (PI) should comprise a cytidine analogue (lamivudine, emtricitabine) and a thymidine analogue (stavudine, zidovudine), guanosine analogue (i.e. abacavir), or nucleotide RTI (NtRTI; i.e. tenofovir). European and US guidelines recommend the use of triple NRTI therapy (abacavir/lamivudine/zidovudine) in children with anticipated poor adherence to other treatment regimens because of tablet burden. In conclusion, while use of ART in children needs to be dramatically increased, selecting and administering the best drug combination for children is still limited by a lack of paediatric formulations and knowledge of drug metabolism, safety and efficacy in children. NRTIs are already a key component of paediatric ART, but fixed-dose combinations and specific research in children are needed to optimise their use. In this article we review the available information to facilitate selection of the best NRTI for backbone treatment in combination ART for HIV-infected children.

Steady-state pharmacokinetics of emtricitabine and tenofovir disoproxil fumarate administered alone and in combination in healthy volunteers

The approved antiretroviral drugs, emtricitabine and tenofovir disoproxil fumarate, were considered good candidates for a fixed-dose combination product that could be administered as a single pill once daily (qd), thereby simplifying existing treatment regimens and promoting patient adherence. As both drugs are extensively renally eliminated, a randomized, 3-way crossover study was conducted in 19 healthy volunteers to formally evaluate the potential pharmacokinetic interaction when the drugs are administered alone and together (ie, 200 mg emtricitabine qd for 7 days, 300 mg tenofovir disoproxil fumarate qd for 7 days, and 200 mg emtricitabine plus 300 mg tenofovir disoproxil fumarate qd for 7 days) with no washout between treatments. Steady-state pharmacokinetic parameters (AUC(tau), C(max), and C(min)) of emtricitabine and tenofovir (as tenofovir disoproxil fumarate) in combination were essentially equivalent versus each drug alone, providing a pharmacokinetic rationale for combining these products in emtricitabine/tenofovir disoproxil fumarate fixed-dose tablets.

New antiretroviral drugs in clinical use

The advent of combination antiretroviral therapy for the treatment of human immunodeficiency virus (HIV) infection has dramatically changed the prognosis and quality of life of HIV-infected adults and children. To date, there are 21 antiretroviral agents available with only 11 agents being approved for the use in young children less than 6 years of age. The currently available antiretroviral agents belong to four different classes; nucleoside/nucleotide reverse transcriptase inhibitors (NRTI, NtRTI), non-nucleoside reverse transcriptase inhibitors (NNRTI), protease inhibitors (PI), and a new class of fusion inhibitors (FI). It is recommended that the treatment regimen should be a combination of at least 3 drugs from different drug classes as this has been shown to slow disease progression, improve survival, and result in better virologic and immunologic responses. Treatment with antiretroviral agents is frequently complicated by the issues of adherence, tolerability, long term toxicity and drug resistance. Many efforts have been made to develop new antiretroviral agents with greater potency, higher tolerability profiles and better convenience. Some new agents are also effective against drug-resistant strains of HIV. Since 2001, there were 7 new antiretroviral agents and 2 fixed-dose multidrug formulations being approved for the treatment of HIV infection, most are approved only for use in adults. In this article, we will review new antiretroviral agents including emtricitabine, tenofovir disoproxil fumarate, atazanavir, fosamprenavir, tipranavir and enfuvirtide. Pediatric information on these drugs will be provided when available.

An Update in the Management of Hepatitis B/HIV Coinfection

With growing numbers of hepatitis B virus (HBV)/ HIVcoinfected patients and the complexity of treating both diseases together, new treatment options and guidelines are available. This article reviews treatment and management options for HBV/HIV-coinfected patients.