Pharmacokinetics, pharmacodynamics and drug interaction potential of enfuvirtide

Prolonged release and antiviral efficacy of HIV fusion inhibitor LP-98-loaded microspheres in rhesus macaques

Non-adherence to antiretroviral treatment is a critical obstacle to effectively managing the progression of AIDS and reducing transmission and mortality rates. A promising strategy to address the clinical disadvantages of user-dependent dosing and decrease medication frequency is the development of long-acting antiretrovirals. In this study, we fabricated PLGA microspheres (MS) incorporating the lipopeptide LP-98 (LP-98-MS), which has previously exhibited potent anti-HIV efficacy. Our findings demonstrate that a single-dose injection of LP-98-MS in SHIV-infected rhesus macaques resulted in sustained and gradual release, maintaining antiviral effects at least 28 days. Notably, a single administration of LP-98-MS provided more than 28 days of sustained release, resulting in high-level pre-exposure prophylaxis (PrEP) for rhesus macaques, even providing complete protection when exposed to repeated intravaginal and intrarectal SHIV challenges. Overall, LP-98-MS holds significant potential in reducing medication frequency and shows promising prospects for further development.

Development of Anti-HIV Therapeutics: From Conventional Drug Discovery to Cutting-Edge Technology

The efforts to discover HIV therapeutics have continued since the first human immunodeficiency virus (HIV) infected patient was confirmed in the 1980s. Ten years later, the first HIV drug, zidovudine (AZT), targeting HIV reverse transcriptase, was developed. Meanwhile, scientists were enlightened to discover new drugs that target different HIV genes, like integrase, protease, and host receptors. Combination antiretroviral therapy (cART) is the most feasible medical intervention to suppress the virus in people with HIV (PWH) and control the epidemic. ART treatment has made HIV a chronic infection rather than a fatal disease, but ART does not eliminate latent reservoirs of HIV-1 from the host cells; strict and life-long adherence to ART is required for the therapy to be effective in patients. In this review, we first discussed the scientific history of conventional HIV drug discovery since scientists need to develop more and more drugs to solve drug-resistant issues and release the side effects. Then, we summarized the novel research technologies, like gene editing, applied to HIV treatment and their contributions to eliminating HIV as a complementary therapy.

Comparison of pharmacokinetics and safety of albuvirtide in healthy subjects after intravenous drip and bolus injection

Albuvirtide (ABT) is the first long-acting HIV fusion inhibitor developed in China, blocking the invasion of HIV-1 virus into target cells. This study aimed to compare the pharmacokinetics (PK), tolerability, and safety of ABT following a single intravenous (IV) bolus injection or intravenous drip in healthy Chinese subjects. A single-center, randomized, open-label, single-period, parallel phase I clinical trial was conducted. Thirty subjects were randomly divided into three groups in a ratio of 1:1:1. After an overnight fast, all subjects received a single dose of 320 mg ABT either by intravenous drip for 45 min (group A) or bolus injection for 0.5 min (group B), or bolus injection for 3 min (group C). ABT plasma concentrations were analyzed using a validated enzyme-linked immunosorbent assay (ELISA). Non-compartmental analysis was used to evaluate PK parameters. The median time to reach maximum concentration was 0.75 h in group A and 0.16 h in both groups B and C. Elimination half-life, mean residence time, apparent clearance, and apparent volume of distribution were similar among the three groups. The 90% confidence intervals (CI) of geometric mean ratios of PK parameters for groups B and C relative to group C were within 85-120%. All adverse events (AEs) reported in this study were mild, according to the CTCAE guidelines and the study investigator's judgement. ABT bolus injections for 0.5 min and 3 min are expected to be well tolerated and to exhibit similar PK characteristics as IV drip for 45 min, offering potential clinical benefits.

Enzyme Responsive Delivery of Anti-Retroviral Peptide via Smart Hydrogel

In response to an urgent need for advanced formulations for the delivery of anti-retrovirals, a stimuli-sensitive hydrogel formulation that intravaginally delivers HIV-1 entry inhibitor upon being exposed to a specific protease was developed. The hydrogel formulation consists of PEG-azide and PEG-DBCO covalently linked to the entry inhibitor peptide, enfuvirtide, via substrate linker that is designed to undergo proteolysis by prostate specific antigen (PSA) present in seminal fluid and release innate enfuvirtide. Of the tested PSA substrate linkers (HSSKLQYY, GISSFYSSK, AYLMYY, and AYLMGRR), HSSKLQ was found to be an optimal candidate for PEG-based hydrogel with k_cat/K_M of 2.2 M^-1 s^-1. The PEG-based hydrogel displayed a pseudoplastic, thixotropic behavior with overall viscosity varying between 1516 and 2.2 Pa.s, within the biologically relevant shear rates of 0.01-100 s^-1. It also exhibited viscoelastic properties appropriate for uniform spreading and being retained in vagina. PEG-based hydrogels were loaded with N3-HSSKLQ-enfuvirtide (HF42) that is customarily synthesized enfuvirtide prodrug with its N-terminus connected to HSSKLQ linker. The stimuli-sensitive PEG-based hydrogel formulations upon being exposed to PSA released 36.5 ± 4.8% of enfuvirtide over 24 h in human ejaculate mimic of vaginal simulant fluid and seminal simulant fluid mixed in 1:3 ratio, which is significantly greater than its IC₅₀. The PEG-based hydrogel was non-cytotoxic to both vaginal epithelial cells (VK2/E6E7) and murine macrophages (RAW 264.7) and did not significantly induce the production of nitric oxide, an inflammatory mediator. The PEG-based hydrogel is found to have suitable physicochemical properties for an intravaginal formulation of the PSA substrate-linked anti-retrovirals and is safe towards vaginal epithelium. It is capable of delivering enfuvirtide with effective concentrations to prevent women from HIV-1 infection.

A Protein-Based, Long-Acting HIV-1 Fusion Inhibitor with an Improved Pharmacokinetic Profile

Recently, a series of highly effective peptide- or protein-based HIV fusion inhibitors have been identified. However, due to their short half-life, their clinical application is limited. Therefore, the development of long-acting HIV fusion inhibitors is urgently needed. Here, we designed and constructed a protein-based, long-acting HIV fusion inhibitor, termed FLT (FN3-L35-T1144), consisting of a monobody, FN3, which contains an albumin-binding domain (ABD), a 35-mer linker (L35), and a peptide-based HIV fusion inhibitor, T1144. We found that FLT bound, via its FN3 component, with human serum albumin (HSA) in a reversible manner, thus maintaining the high efficiency of T1144 against infection by both HIV-1 IIIB (X4) and Bal (R5) strains with IC₅₀ of 11.6 nM and 15.3 nM, respectively, and remarkably prolonging the half-life of T1144 (~27 h in SD rats). This approach affords protein-based HIV fusion inhibitors with much longer half-life compared to enfuvirtide, a peptide-based HIV fusion inhibitor approved for use in clinics. Therefore, FLT is a promising candidate as a new protein-based anti-HIV drug with an improved pharmacokinetic profile.

Cyclic peptides with a distinct arginine-fork motif recognize the HIV trans-activation response RNA in vitro and in cells

RNA represents a potential target for new antiviral therapies, which are urgently needed to address public health threats such as the human immunodeficiency virus (HIV). We showed previously that the interaction between the viral Tat protein and the HIV-1 trans-activation response (TAR) RNA was blocked by the cyclic peptide TB-CP-6.9a. This peptide was derived from a TAR-binding loop that emerged during lab-evolution of a TAR-binding protein (TBP) family. Here we synthesized and characterized a next-generation, cyclic-peptide library based on the TBP scaffold. We sought to identify conserved RNA-binding interactions, and the influence of cyclization linkers on RNA binding and antiviral activity. A diverse group of cyclization linkers, encompassing disulfide bonds to bicyclic aromatic staples, was used to restrain the cyclic peptide geometry. Thermodynamic profiling revealed specific arginine-rich sequences with low to sub-micromolar affinity driven by enthalpic and entropic contributions. The best compounds exhibited no appreciable off-target binding to related molecules, such as BIV TAR and human 7SK RNAs. A specific arginine-to-lysine change in the highest affinity cyclic peptide reduced TAR binding by 10-fold, suggesting that TBP-derived cyclic peptides use an arginine-fork motif to recognize the TAR major-groove while differentiating the mode of binding from other TAR-targeting molecules. Finally, we showed that HIV infectivity in cell culture was reduced in the presence of cyclic peptides constrained by methylene or naphthalene-based linkers. Our findings provide insight into the molecular determinants required for HIV-1 TAR recognition and antiviral activity. These findings are broadly relevant to the development of antivirals that target RNA molecules.

The impact of age on antiretroviral drug pharmacokinetics in the treatment of adults living with HIV

INTRODUCTION

People living with HIV (PLWH) are aging and will receive life-long treatment: despite substantial improvement in drug efficacy and tolerability, side effects still occur and they can blunt antiretroviral treatment effectiveness. Since age may affect drug exposure and may be associated with side-effects we aimed at reviewing available data on the effect of age on antiretrovirals' pharmacokinetics in adult patients.

AREAS COVERED

We searched public databases and major conference proceedings for data on age and pharmacokinetics/pharmacodynamics in PLWH. We limited our review to currently used drugs and focused on population pharmacokinetics and physiologically-based pharmacokinetic modeling studies.

EXPERT OPINION

Available evidence of a potential detrimental effect in elderly PLWH is limited by study design and small sample sizes. Careful consideration of undoubtful benefits and potential harms is advised when prescribing ARVs to geriatric patients and the knowledge of pharmacokinetics changes need to be included in the process. With the 'greying' of the pandemic we need studies with a specific focus on geriatric patients living with HIV that will consider specific phenotypes and associated changes (including sarcopenia).

Efficacy and safety of the long-acting fusion inhibitor albuvirtide in antiretroviral-experienced adults with human immunodeficiency virus-1: interim analysis of the randomized, controlled, phase 3, non-inferiority TALENT study

BACKGROUND

Albuvirtide is a once-weekly injectable human immunodeficiency virus (HIV)-1 fusion inhibitor. We present interim data for a phase 3 trial assessing the safety and efficacy of albuvirtide plus lopinavir-ritonavir in HIV-1-infected adults already treated with antiretroviral drugs.

METHODS

We carried out a 48-week, randomized, controlled, open-label non-inferiority trial at 12 sites in China. Adults on the World Health Organization (WHO)-recommended first-line treatment for >6 months with a plasma viral load >1000 copies/mL were enrolled and randomly assigned (1:1) to receive albuvirtide (once weekly) plus ritonavir-boosted lopinavir (ABT group) or the WHO-recommended second-line treatment (NRTI group). The primary endpoint was the proportion of patients with a plasma viral load below 50 copies/mL at 48 weeks. Non-inferiority was prespecified with a margin of 12%.

RESULTS

At the time of analysis, week 24 data were available for 83 and 92 patients, and week 48 data were available for 46 and 50 patients in the albuvirtide and NRTI groups, respectively. At 48 weeks, 80.4% of patients in the ABT group and 66.0% of those in the NRTI group had HIV-1 RNA levels below 50 copies/mL, meeting the criteria for non-inferiority. For the per-protocol population, the superiority of albuvirtide over NRTI was demonstrated. The frequency of grade 3 to 4 adverse events was similar in the two groups; the most common adverse events were diarrhea, upper respiratory tract infections, and grade 3 to 4 increases in triglyceride concentration. Renal function was significantly more impaired at 12 weeks in the patients of the NRTI group who received tenofovir disoproxil fumarate than in those of the ABT group.

CONCLUSIONS

The TALENT study is the first phase 3 trial of an injectable long-acting HIV drug. This interim analysis indicates that once-weekly albuvirtide in combination with ritonavir-boosted lopinavir is well tolerated and non-inferior to the WHO-recommended second-line regimen in patients with first-line treatment failure.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02369965; https://www.clinicaltrials.gov.Chinese Clinical Trial Registry No. ChiCTR-TRC-14004276; http://www.chictr.org.cn/enindex.aspx.

Pharmacogenomics of Antiretroviral Drug Metabolism and Transport

Antiretroviral therapy has markedly reduced morbidity and mortality for persons living with human immunodeficiency virus (HIV). Individual tailoring of antiretroviral regimens has the potential to further improve the long-term management of HIV through the mitigation of treatment failure and drug-induced toxicities. While the mechanisms underlying anti-HIV drug adverse outcomes are multifactorial, the application of drug-specific pharmacogenomic knowledge is required in order to move toward the personalization of HIV therapy. Thus, detailed understanding of the metabolism and transport of antiretrovirals and the influence of genetics on these pathways is important. To this end, this review provides an up-to-date overview of the metabolism of anti-HIV therapeutics and the impact of genetic variation in drug metabolism and transport on the treatment of HIV. Future perspectives on and current challenges in pursuing personalized HIV treatment are also discussed.

A New Model to Describe the Single-dose Pharmacokinetics of Bevacizumab and Predict Its Multiple-Dose Pharmacokinetics in Beagle Dogs

Complex pharmacokinetic (PK) properties including nonlinear elimination were encountered by some monoclonal antibodies (mAbs), and classic compartment models sometimes failed to appropriately describe those properties. In this work, a new model was built on a comprehensive analysis of the complex elimination of mAbs. This new model was firstly utilized to fit with the single-dose plasma concentration data of bevacizumab in beagle dogs receiving an intravenous administration of 2.5 mg/kg bevacizumab. Then, the optimal PK parameters from fitting with the single-dose PK data were employed into the multiple-dose mathematical expressions to predict bevacizumab's multiple-dose PK profiles. One-compartment model recommended as the optimal classic model by DAS 2.0 software was set as a control. As a result, new model fitted better with the single-dose PK profiles of bevacizumab with smaller weighted residual sum of squares and higher fitting degree compared with the classic model. Importantly, new model also accurately predicted the multiple-dose PK profiles of bevacizumab and performed well at the single-to-multiple transition. In conclusion, the new model reasonably explained the complex elimination of bevacizumab, and it might play a big role in the PK studies of bevacizumab and other mAbs.

IgG Fc-binding motif-conjugated HIV-1 fusion inhibitor exhibits improved potency and in vivo half-life: Potential application in combination with broad neutralizing antibodies

The clinical application of conventional peptide drugs, such as the HIV-1 fusion inhibitor enfuvirtide, is limited by their short half-life in vivo. To overcome this limitation, we developed a new strategy to extend the in vivo half-life of a short HIV-1 fusion inhibitory peptide, CP24, by fusing it with the human IgG Fc-binding peptide (IBP). The newly engineered peptide IBP-CP24 exhibited potent and broad anti-HIV-1 activity with IC50 values ranging from 0.2 to 173.7 nM for inhibiting a broad spectrum of HIV-1 strains with different subtypes and tropisms, including those resistant to enfuvirtide. Most importantly, its half-life in the plasma of rhesus monkeys was 46.1 h, about 26- and 14-fold longer than that of CP24 (t1/2 = 1.7 h) and enfuvirtide (t1/2 = 3 h), respectively. IBP-CP24 intravenously administered in rhesus monkeys could not induce significant IBP-CP24-specific antibody response and it showed no obvious in vitro or in vivo toxicity. In the prophylactic study, humanized mice pretreated with IBP-CP24 were protected from HIV-1 infection. As a therapeutic treatment, coadministration of IBP-CP24 and normal human IgG to humanized mice with chronic HIV-1 infection resulted in a significant decrease of plasma viremia. Combining IBP-CP24 with a broad neutralizing antibody (bNAb) targeting CD4-binding site (CD4bs) in gp120 or a membrane proximal external region (MPER) in gp41 exhibited synergistic effect, resulting in significant dose-reduction of the bNAb and IBP-CP24. These results suggest that IBP-CP24 has the potential to be further developed as a new HIV-1 fusion inhibitor-based, long-acting anti-HIV drug that can be used alone or in combination with a bNAb for treatment and prevention of HIV-1 infection.

Evolved Proteins Inhibit Entry of Enfuvirtide-Resistant HIV-1

Drugs that block HIV-1 entry are relatively limited. Enfuvirtide is a 36-residue synthetic peptide that targets gp41 and blocks viral fusion. However, Enfuvirtide-resistant HIV has been reported, and this peptide drug requires daily injection. Previously, we have reported helix-grafted display proteins, consisting of HIV-1 gp41 C-peptide helix grafted onto Pleckstrin Homology domains. Some of these biologics inhibit HIV-1 entry with relatively modest and varied potency (IC₅₀ = 190 nM to >1 μM). Here, we report that gp41 C-peptide helix-grafted Sac7d (Sac7d-Cpep) potently suppresses HIV-1 entry in a live virus assay (IC₅₀ = 1.9-12.4 nM). Yeast display sequence optimization of solvent exposed helix residues led to new biologics with improved expression in E. coli (a common biosimilar expression host), with no appreciable change in entry inhibition. Evolved proteins inhibit the entry of a clinically relevant mutant of HIV-1 that is gp41 C-peptide sensitive and Enfuvirtide resistant. Fusion proteins designed for serum stability also potently suppress HIV-1 entry. Collectively, we report several evolved biologics that are functional against an Enfuvirtide-resistant strain and are designed for serum stability.

A Peptide-Based HIV-1 Fusion Inhibitor with Two Tail-Anchors and Palmitic Acid Exhibits Substantially Improved In Vitro and Ex Vivo Anti-HIV-1 Activity and Prolonged In Vivo Half-Life

Enfuvirtide (T20) is the first U.S. FDA-approved HIV fusion inhibitor-based anti-HIV drug. Its clinical application is limited because of its low potency and short half-life. We previously reported that peptide HP23-E6-IDL, containing both N- and C-terminal anchor-tails, exhibited stronger potency and a better resistance profile than T20. Here we designed an analogous peptide, YIK, by introducing a mutation, T639I, and then a lipopeptide, YIK-C16, by adding palmitic acid (C16) at the C-terminus of YIK. We found that YIK-C16 was 4.4- and 3.6-fold more potent than HP23-E6-IDL and YIK against HIV-1_IIIB infection and 13.3- and 10.5-fold more effective than HP23-E6-IDL and YIK against HIV-1_Bal infection, respectively. Consistently, the ex vivo anti-HIV-1_IIIB activity, as determined by the highest dilution-fold of the serum causing 50% inhibition of HIV-1 infection, of YIK-C16 in the sera of pretreated mice was remarkably higher than that of YIK or HP23-E6-IDL. The serum half-life (t_1/2 = 5.9 h) of YIK-C16 was also significantly longer than that of YIK (t_1/2 = 1.3 h) and HP23-E6-IDL (t_1/2 = 1.0 h). These results suggest that the lipopeptide YIK-C16 shows promise for further development as a new anti-HIV drug with improved anti-HIV-1 activity and a prolonged half-life.

A 30 kDa polyethylene glycol-enfuvirtide complex enhances the exposure of enfuvirtide in lymphatic viral reservoirs in rats

HIV therapy with anti-retroviral drugs is limited by the poor exposure of viral reservoirs, such as lymphoid tissue, to these small molecule drugs. We therefore investigated the effect of PEGylation on the anti-retroviral activity and subcutaneous lymphatic pharmacokinetics of the peptide-based fusion inhibitor enfuvirtide in thoracic lymph duct cannulated rats. Both the peptide and the PEG were quantified in plasma and lymph via ELISA. Conjugation to a single 5 kDa linear PEG decreased anti-HIV activity three-fold compared to enfuvirtide. Whilst plasma and lymphatic exposure to peptide mass was moderately increased, the loss of anti-viral activity led to an overall decrease in exposure to enfuvirtide activity. A 20 kDa 4-arm branched PEG conjugated with an average of two enfuvirtide peptides decreased peptide activity by six-fold. Plasma and lymph exposure to enfuvirtide, however, increased significantly such that anti-viral activity was increased two- and six-fold respectively. The results suggest that a multi-enfuvirtide-PEG complex may optimally enhance the anti-retroviral activity of the peptide in plasma and lymph.

Pharmacokinetic and Chemical Synthesis Optimization of a Potent d-Peptide HIV Entry Inhibitor Suitable for Extended-Release Delivery

Peptides often suffer from short in vivo half-lives due to proteolysis and renal clearance that limit their therapeutic potential in many indications, necessitating pharmacokinetic (PK) enhancement. d-Peptides, composed of mirror-image d-amino acids, overcome proteolytic degradation but are still vulnerable to renal filtration due to their small size. If renal filtration could be slowed, d-peptides would be promising therapeutic agents for infrequent dosing, such as in extended-release depots. Here, we tether a diverse set of PK-enhancing cargoes to our potent, protease-resistant d-peptide HIV entry inhibitor, PIE12-trimer. This inhibitor panel provides an opportunity to evaluate the PK impact of the cargoes independently of proteolysis. While all the PK-enhancing strategies (PEGylation, acylation, alkylation, and cholesterol conjugation) improved in vivo half-life, cholesterol conjugation of PIE12-trimer dramatically improves both antiviral potency and half-life in rats, making it our lead anti-HIV drug candidate. We designed its chemical synthesis for large-scale production (CPT31) and demonstrated that the PK profile in cynomolgous monkeys supports future development of monthly or less frequent depot dosing in humans. CPT31 could address an urgent need in both HIV prevention and treatment.

Precision medicine for HIV: where are we?

To date, antiretroviral therapy is highly effective in HIV-affected patients, but the individualization of such a life-long therapy may be advised. This review briefly summarizes the main factors involved in the potential personalization of antiretroviral treatment. Relevant articles in English were identified by PubMed and recent congresses’ abstracts. Foremost influences concerning pharmacodynamics, therapeutic drug monitoring, pharmacogenetics, comorbidities, immune recovery and viral characteristics affecting the healthcare of HIV-positive patients are listed here. Furthermore, pharmacoeconomic aspects are mentioned. Applying pharmacokinetic and pharmacogenetic knowledge may be informative and guide the better choice of treatment in order to achieve long-term efficacy and avoid adverse events. Randomized investigations of the clinical relevance of tailored antiretroviral regimens are needed in order to obtain a better management of HIV/AIDS-affected patients.

Helix-Grafted Pleckstrin Homology Domains Suppress HIV-1 Infection of CD4-Positive Cells

The size, functional group diversity and three-dimensional structure of proteins often allow these biomolecules to bind disease-relevant structures that challenge or evade small-molecule discovery. Additionally, folded proteins are often much more stable in biologically relevant environments compared to their peptide counterparts. We recently showed that helix-grafted display-extensive resurfacing and elongation of an existing solvent-exposed helix in a pleckstrin homology (PH) domain-led to a new protein that binds a surrogate of HIV-1 gp41, a validated target for inhibition of HIV-1 entry. Expanding on this work, we prepared a number of human-derived helix-grafted-display PH domains of varied helix length and measured properties relevant to therapeutic and basic research applications. In particular, we showed that some of these new reagents expressed well as recombinant proteins in Escherichia coli, were relatively stable in human serum, bound a mimic of pre-fusogenic HIV-1 gp41 in vitro and in complex biological environments, and significantly lowered the incidence of HIV-1 infection of CD4-positive cells.

Evaluation of pharmacokinetic interactions between long-acting HIV-1 fusion inhibitor albuvirtide and lopinavir/ritonavir, in HIV-infected subjects, combined with clinical study and simulation results

1. A clinical study to assess the interactions between albuvirtide (320 mg) and lopinavir/ritonavir (400/100 mg) was conducted in 10 HIV-1-infected subjects. Because albuvirtide requires a long period to achieve steady state, and extended monotherapy may lead to early resistance, it is unethical to take albuvirtide alone to achieve steady state. Therefore, a population pharmacokinetic model was developed to predict steady-state concentration-time curve of solely administered albuvirtide. 2. When albuvirtide and lopinavir/ritonavir were co-administered, the plasma concentration of albuvirtide when the infusion ended (C_end) increased by about 34%, but the geometric mean ratios and 90% confidence intervals (90% CIs) of AUC_(0-t) [1.09 (0.96-1.24)] and C_trough [1.00 (0.83-1.20)] were within the range of 0.8-1.25. For lopinavir, the ratios (90% CIs) of AUC_(0-t), C_max and C_trough were 0.63 (0.49-0.82), 0.67 (0.53-0.86) and 0.65 (0.46-0.91); for ritonavir, those ratios (90% CIs) were 0.62 (0.42-0.91), 0.61 (0.38-0.99) and 0.72 (0.40-1.26), respectively. 3. Co-administration of albuvirtide with lopinavir/ritonavir has little effect on albuvirtide exposure, but it decreases the plasma exposures of lopinavir/ritonavir. However, the drug-drug interactions may not reduce the effectiveness of this co-therapy, the trough concentration of lopinavir may be sufficient and this combination could achieve similar clinical efficacy with marketed drugs. So, a phase 3 clinical trial without dose adjustment is underway to validate their effectiveness and safety.

Nanoformulation of antiretroviral drugs enhances their penetration across the blood brain barrier in mice

Eradication of virus by sanctuary sites is a main goal in HIV management. The central nervous system (CNS) is a classic model of sanctuary where viral replication occurs despite a complete viral suppression in peripheral blood. In recent years, nanotechnologies have provided a great promise in the eradication of HIV from the CNS. We hereby demonstrate for the first time that the structurally complex antiretroviral drug enfuvirtide (Enf), which normally is unable to penetrate the cerebrospinal fluid, is allowed to cross the blood brain barrier (BBB) in mice by conjugation with a nanoconstruct. Iron oxide nanoparticles coated with an amphiphilic polymer increase Enf translocation across the BBB in both in vitro and in vivo models. The mechanism involves the uptake of nanoconjugated-Enf in the endothelial cells, the nanocomplex dissociation and the release of the peptide, which is eventually excreted by the cells in the brain parenchyma.

FROM THE CLINICAL EDITOR

Despite the success of cocktail therapy of antiretroviral drugs, the complete eradication of HIV remains elusive, due to existence of viral sanctuary sites. The authors showed in this study that an antiretroviral drug complexed with iron oxide nanoparticles and coated with PMA amphiphilic polymer crosses the blood brain barrier. Furthermore, there was significant anti-viral activity. The results would aid further drug designs to eradicate HIV.

Pharmacological interactions between rifampicin and antiretroviral drugs: challenges and research priorities for resource-limited settings

Coadministration of antituberculosis and antiretroviral therapy is often inevitable in high-burden countries where tuberculosis (TB) is the most common opportunistic infection associated with HIV/AIDS. Concurrent use of rifampicin and many antiretroviral drugs is complicated by pharmacokinetic drug-drug interactions. Rifampicin is a very potent enzyme inducer, which can result in subtherapeutic antiretroviral drug concentrations. In addition, TB drugs and antiretroviral drugs have additive (pharmacodynamic) interactions as reflected in overlapping adverse effect profiles. This review provides an overview of the pharmacological interactions between rifampicin-based TB treatment and antiretroviral drugs in adults living in resource-limited settings. Major progress has been made to evaluate the interactions between TB drugs and antiretroviral therapy; however, burning questions remain concerning nevirapine and efavirenz effectiveness during rifampicin-based TB treatment, treatment options for TB-HIV-coinfected patients with nonnucleoside reverse transcriptase inhibitor resistance or intolerance, and exact treatment or dosing schedules for vulnerable patients including children and pregnant women. The current research priorities can be addressed by maximizing the use of already existing data, creating new data by conducting clinical trials and prospective observational studies and to engage a lobby to make currently unavailable drugs available to those most in need.

Treatment optimization in patients co-infected with HIV and Mycobacterium tuberculosis infections: focus on drug-drug interactions with rifamycins

Tuberculosis (TB) and HIV continue to be two of the major causes of morbidity and mortality in the world, and together are responsible for the death of millions of people every year. There is overwhelming evidence to recommend that patients with TB and HIV co-infection should receive concomitant therapy of both conditions regardless of the CD4 cell count level. The principles for treatment of active TB disease in HIV-infected patients are the same as in HIV-uninfected patients. However, concomitant treatment of both conditions is complex, mainly due to significant drug-drug interactions between TB and HIV drugs. Rifamycins are potent inducers of the cytochrome P450 (CYP) pathway, leading to reduced (frequently sub-therapeutic) plasma concentrations of some classes of antiretrovirals. Rifampicin is also an inducer of the uridine diphosphate glucuronosyltransferase (UGT) 1A1 enzymes and interferes with drugs, such as integrase inhibitors, that are metabolized by this metabolic pathway. Rifampicin is also an inducer of the adenosine triphosphate (ATP) binding cassette transporter P-glycoprotein, which may also lead to decreased bioavailability of concomitantly administered antiretrovirals. On the other side, rifabutin concentrations are affected by the antiretrovirals that induce or inhibit CYP enzymes. In this review, the pharmacokinetic interactions, and the relevant clinical consequences, of the rifamycins-rifampicin, rifabutin, and rifapentine-with antiretroviral drugs are reviewed and discussed. A rifampicin-based antitubercular regimen and an efavirenz-based antiretroviral regimen is the first choice for treatment of TB/HIV co-infected patients. Rifabutin is the preferred rifamycin to use in HIV-infected patients on a protease inhibitor-based regimen; however, the dose of rifabutin needs to be reduced to 150 mg daily. More information is required to select optimal treatment regimens for TB/HIV co-infected patients whenever efavirenz cannot be used and rifabutin is not available. Despite significant pharmacokinetic interactions between antiretrovirals and antitubercular drugs, adequate clinical response of both infections can be achieved with an acceptable safety profile when the pharmacological characteristics of drugs are known, and appropriate combination regimens, dosing, and timing of initiation are used. However, more clinical research is needed for newer drugs, such as rifapentine and the recently introduced integrase inhibitor antiretrovirals, and for specific population groups, such as children, pregnant women, and patients affected by multidrug-resistant TB.

GLUE that sticks to HIV: a helix-grafted GLUE protein that selectively binds the HIV gp41 N-terminal helical region

Methods for the stabilization of well-defined helical peptide drugs and basic research tools have received considerable attention in the last decade. Here, we report the stable and functional display of an HIV gp41 C-peptide helix mimic on a GRAM-Like Ubiquitin-binding in EAP45 (GLUE) protein. C-peptide helix-grafted GLUE selectively binds a mimic of the N-terminal helical region of gp41, a well-established HIV drug target, in a complex cellular environment. Additionally, the helix-grafted GLUE is folded in solution, stable in human serum, and soluble in aqueous solutions, and thus overcomes challenges faced by a multitude of peptide drugs, including those derived from HIV gp41 C-peptide.

Pharmacokinetics of sifuvirtide in treatment-naive and treatment-experienced HIV-infected patients

The pharmacokinetics assessment in two clinical studies of sifuvirtide (a novel HIV fusion inhibitor) was first reported in Chinese HIV patients. Nineteen treatment-naive HIV patients were treated with s.c.(subcutaneous injection) sifuvirtide [10 or 20 mg q.d.(quaque die)] for 28 days in study 1, and eight treatment-experienced HIV patients were treated with s.c. sifuvirtide (20 mg q.d.) in combination with HAART drugs (lamivudine, didanosine, and Kaletra) for 168 days in study 2. In study 1, T1/2 was 17.8 ± 3.7 h for 10 mg group and 39.0 ± 3.5 h for 20 mg group; the mean Cmax of last dose was 498 ± 54 ng/mL for 10 mg group and 897 ± 136 ng/mL for 20 mg group. In study 2, T1/2 was 6.71 ± 2.17 h in treatment-experienced patients. Cmax was 765 ± 288 ng/mL after last 168th dosage. Sifuvirtide showed improved clinical pharmacokinetics characteristics compared with Enfuvirtide, and showed very different pharmacokinetic characteristics between treatment-naive and treatment-experienced patients. © 2014 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 103:4038-4047, 2014.

Clinically relevant drug-drug interactions between antiretrovirals and antifungals

INTRODUCTION

Complete delineation of the HIV-1 life cycle has resulted in the development of several antiretroviral drugs. Twenty-five therapeutic agents belonging to five different classes are currently available for the treatment of HIV-1 infections. Advent of triple combination antiretroviral therapy has significantly lowered the mortality rate in HIV patients. However, fungal infections still represent major opportunistic diseases in immunocompromised patients worldwide.

AREAS COVERED

Antiretroviral drugs that target enzymes and/or proteins indispensable for viral replication are discussed in this article. Fungal infections, causative organisms, epidemiology and preferred treatment modalities are also outlined. Finally, observed/predicted drug-drug interactions between antiretrovirals and antifungals are summarized along with clinical recommendations.

EXPERT OPINION

Concomitant use of amphotericin B and tenofovir must be closely monitored for renal functioning. Due to relatively weak interactive potential with the CYP450 system, fluconazole is the preferred antifungal drug. High itraconazole doses (> 200 mg/day) are not advised in patients receiving booster protease inhibitor (PI) regimen. Posaconazole is contraindicated in combination with either efavirenz or fosamprenavir. Moreover, voriconazole is contraindicated with high-dose ritonavir-boosted PI. Echinocandins may aid in overcoming the limitations of existing antifungal therapy. An increasing number of documented or predicted drug-drug interactions and therapeutic drug monitoring may aid in the management of HIV-associated opportunistic fungal infections.

Pharmacokinetic drug interactions between drugs of abuse and antiretroviral medications: implications and management for clinical practice

Substance abuse and HIV/AIDS are two of the most serious, yet treatable diseases worldwide. Global access to HIV treatment continues to expand. In settings where both active illicit drug use and HIV treatment are concurrent, potentional problematic pharmacokinetic drug interactions may arise and complicate therapy. Clinical case series and carefully controlled pharmacokinetic interaction studies have been conducted between only a few drugs of abuse and approved antiretroviral therapies. Important pharmacokinetic drug interactions have been described for benzodiazepines, 3,4-methylenedioxymethamphetamine, methadone and buprenorphine; however, most have not been studied and few well-controlled studies have been conducted to adequately address the clinical implications of these interactions. The metabolism of drugs of abuse, description of the known interactions, and clinical implications and management of these interactions are reviewed. Certain interactions between drugs of abuse and antiretroviral therapies are known and others are likely based upon shared metabolic pathways. These may result in important clinical consequences. To optimize care, clinicians must be alert, knowledgeable about known and possible interactions and equipped to clinically manage the medical consequences. Moreover, there is considerable need for carefully controlled studies in this important and emerging area.

Children with HIV are not small adults: what is different in pharmacology?

PURPOSE OF REVIEW

The pharmacokinetics of antiretroviral drugs are highly variable among HIV-infected children. This review describes pharmacokinetic processes in children and recent pharmacokinetic data in children with HIV. The general lack of pharmacokinetic data and the potential role of therapeutic drug monitoring are discussed.

RECENT FINDINGS

It was found unexpectedly that exposure to lopinavir is decreased in the first 6 months of life. Recent findings of subtherapeutic efavirenz concentrations in children suggest that pediatric dose recommendations should be re-evaluated. In addition, recommended dosing of lamivudine leads to lower exposure in children younger than 6 years of age. Preliminary results of pediatric fixed-dose combination tablets for HIV-infected children with a higher nevirapine to stavudine and lamivudine ratio than adult fixed-dose combinations suggest adequate drug exposure. As an alternative to plasma sampling, concentrations of nevirapine can be determined in saliva.

SUMMARY

There is a shortage of pharmacokinetic data in the highly variable population of HIV-infected children. Selected pharmacology studies should be undertaken to improve pediatric dose guidance of existing antiretroviral drugs. Therapeutic drug monitoring is a useful tool to optimize treatment in HIV-infected children. More data are needed, however, to establish child-specific reference values.

Biophysical property and broad anti-HIV activity of albuvirtide, a 3-maleimimidopropionic acid-modified peptide fusion inhibitor

Albuvirtide (ABT) is a 3-maleimimidopropionic acid (MPA)-modified peptide HIV fusion inhibitor that can irreversibly conjugate to serum albumin. Previous studies demonstrated its in vivo long half-life and potent anti-HIV activity. Here, we focused to characterize its biophysical properties and evaluate its antiviral spectrum. In contrast to T20 (Enfuvirtide, Fuzeon), ABT was able to form a stable α-helical conformation with the target sequence and block the fusion-active six-helix bundle (6-HB) formation in a dominant-negative manner. It efficiently inhibited HIV-1 Env-mediated cell membrane fusion and virus entry. A large panel of 42 HIV-1 pseudoviruses with different genotypes were constructed and used for the antiviral evaluation. The results showed that ABT had potent inhibitory activity against the subtypes A, B and C that predominate the worldwide AIDS epidemics, and subtype B', CRF07_BC and CRF01_AE recombinants that are currently circulating in China. Furthermore, ABT was also highly effective against HIV-1 variants resistant to T20. Taken together, our data indicate that the chemically modified peptide ABT can serve as an ideal HIV-1 fusion inhibitor.

Pediatric Dosing and Body Size in Biotherapeutics

Although pediatric doses for biotherapeutics are often based on patients' body weight (mg/kg) or body surface area (mg/m²), linear body size dose adjustment is highly empirical. Growth and maturity are also important factors that affect the absorption, distribution, metabolism and excretion (ADME) of biologics in pediatrics. The complexity of the factors involved in pediatric pharmacokinetics lends to the reconsideration of body size based dose adjustment. A proper dosing adjustment for pediatrics should also provide less intersubject variability in the pharmacokinetics and/or pharmacodynamics of the product compared with no dose adjustment. Biological proteins and peptides generally share the same pharmacokinetic principle with small molecules, but the underlying mechanism can be very different. Here, pediatric and adult pharmacokinetic parameters are compared and summarized for selected biotherapeutics. The effect of body size on the pediatric pharmacokinetics for these biological products is discussed in the current review.

[Diagnosis, treatment and prevention of renal diseases in HIV infected patients. Recommendations of the Spanish AIDS Study Group/National AIDS Plan]

The incidence of opportunistic infections and tumours in HIV-infected patients has sharply declined in the HAART era. At the same time there has been a growing increase of other diseases not directly linked to immunodeficiency. Renal diseases are an increasing cause of morbidity and mortality among HIV-infected patients. In the general population, chronic renal failure has considerable multiorgan repercussions that have particular implications in patients with HIV infection. The detection of occult or subclinical chronic kidney disease is crucial since effective measures for delaying progression exist. Furthermore, the deterioration in glomerular filtration should prompt clinicians to adjust doses of some antiretroviral agents and other drugs used for treating associated comorbidities. Suppression of viral replication, strict control of blood pressure, dyslipidemia and diabetes mellitus, and avoidance of nephrotoxic drugs in certain patients are fundamental components of programs aimed to prevent renal damage and delaying progression of chronic kidney disease in patients with HIV. Renal transplantation and dialysis have also special implications in HIV-infected patients. In this article, we summarise the updated clinical practice guidelines for the evaluation, management and prevention of renal diseases in HIV-infected patients from a panel of experts in HIV and nephrologists on behalf of the Spanish AIDS Study Group (GESIDA) and the National AIDS Plan.

Design of antiretroviral drug interaction studies

PURPOSE OF REVIEW

Pharmacokinetic and/or pharmacodynamic drug interactions can occur as a result of treatment with antiretrovirals. Elucidating the mechanism, direction, and magnitude of an interaction, as well as its clinical impact, can be facilitated by proper study design. Two approaches may be considered: assessment based on full pharmacokinetic profiles within a conventional drug interaction trial, or a population pharmacokinetic analysis as a part of a larger trial. Consideration should be given to the population (i.e. healthy volunteer or HIV infected), dosing and administration of medications, and interpretation of findings. Dosing recommendations should be based on the clinical relevance of the interaction.

RECENT FINDINGS

Guidance documents on the conduct of clinical drug interaction study designs are summarized and applied to the design of antiretroviral drug interaction trials.

SUMMARY

Designing an antiretroviral drug interaction trial poses unique challenges, associated with the conditions and the drug combinations of the intended population. A rational study design can provide insights into antiretroviral pharmacology and allows the clinician to treat more effectively.

An albumin-conjugated peptide exhibits potent anti-HIV activity and long in vivo half-life

The clinical application of conventional peptide drugs often is limited by their short in vivo half-life and potential immunogenicity. Frequent injection presents challenges to the treatment of chronic diseases, such as HIV infection. We chemically modified a peptide HIV fusion inhibitor with 3-maleimidopropionic acid (MPA), which allows rapid and irreversible conjugation with serum albumin at a 1:1 molar ratio. FB006M, with an MPA modification at the 13th amino acid, rapidly formed conjugate with albumin upon intravenous injection, and it exhibited a remarkably extended in vivo half-life. The albumin conjugate of FB006M displayed potent inhibitory activity against a number of laboratory and clinical isolates of HIV-1 in vitro and in vivo. No immunogenicity or antibody formation was detected after repeated dosing. The clinical application of FB006M may decrease the cost of treatment and improve treatment compliance and patient quality of life.

Antiviral drug disposition and natural health products: risk of therapeutic alteration and resistance

The HIV/AIDS patient population is known to use natural health products (NHPs) in addition to the several antiretroviral drugs that constitute the treatment regimen for this disease. This review focuses on NHPs and their potential for interactions with antiretroviral agents resulting in therapeutic alterations or resistance. There are conflicting published medical literature reports and very few well-documented human clinical studies that unequivocally demonstrate if this concomitant use increases the risk of interaction/adverse reaction with these therapeutic products. This article outlines some findings from the Canadian domestic adverse reaction case reports associated with the use of antiretrovirals and NHPs. These adverse reaction case reports were specifically examined for patients taking NHPs together with their highly active antiretroviral therapy during or around the time when the adverse reaction developed. Together, the case reports and limited human clinical studies suggest that the risk for therapeutic alterations and resistance can exist due to changes in pharmacokinetic parameters with concomitant use of these therapeutic products.

Reassessment of enfuvirtide's role in the management of HIV-1 infection

BACKGROUND

The development of new protease inhibitors, new non-nucleoside reverse transcriptase inhibitors and novel therapeutic drug classes has dramatically changed the approach to managing HIV-1 patients with multidrug resistant virus. This has led many clinicians to reevaluate the clinical utility of enfuvirtide.

OBJECTIVES

To summarize recent literature on enfuvirtide and to reassess enfuvirtide's role in the management of HIV-1 infection.

METHODS

MEDLINE (1990 to February Week 2 2008) and EMBASE (1990 to 2008 week 8) databases were searched using the following terms: 'enfuvirtide', 'Fuzeon', 'T20', 'HIV fusion inhibitors', and 'HIV entry inhibitor'; limits: English language. Reference lists of articles deemed relevant were hand searched for additional publications. Significant abstracts from recent international HIV conferences were also identified.

CONCLUSION

Enfuvirtide can optimize the response to new combinations of HIV-1 drug regimens in multiresistant patients. Its inclusion as an active agent is effective but use is impacted by its high cost, inconvenient route of administration and cosmetic side-effect profile.

Enfuvirtide Cerebrospinal Fluid (CSF) Pharmacokinetics and Potential use in Defining CSF HIV-1 Origin

Background

Enfuvirtide is a potent inhibitor of systemic HIV-1 replication, but its penetration into the human central nervous system (CNS) has not been analysed. Here, we define cerebrospinal fluid (CSF) enfuvirtide pharmacokinetics and present a case illustrating the use of enfuvirtide as a probe to trace the origins of CSF HIV-1 quasispecies.

Methods

Enfuvirtide CSF pharmacokinetics were assessed in 18 CSF and plasma sample pairs from four HIV-1-infected individuals. Enfuvirtide levels were measured by liquid chromatography tandem mass spectrometry using known standards and controls that included spiked CSF samples from untreated, HIV-negative individuals. A segment of the gp41 coding region encompassing the heptad repeat HR-1 and HR-2 domains was amplified from selected CSF and plasma samples and independent clones sequenced to assess resistance-associated mutations.

Results

CSF and plasma samples obtained between 2 and 20 h after enfuvirtide injection showed plasma concentrations similar to previous reports (mean 3.687 SD ±1.828 mg/ml) with prolonged decay. By contrast, enfuvirtide in all CSF samples was below the assay detection limit of 0.025 mg/ml. In one individual, who developed a transient increase in CSF HIV-1 RNA, seven of seven CSF and plasma clones had identical enfuvirtide resistance-associated V38A mutations, suggesting that the CSF quasispecies derived from that of blood.

Conclusions

Enfuvirtide penetration into CSF is negligible; thus, in clinical settings, where direct CNS drug exposure is crucial, this drug Is not likely to directly contribute to the local therapeutic effect. Enfuvirtide can be used as a tool to dissect the origin of the CNS virus.

Concurrent drug therapy for tuberculosis and HIV infection in resource-limited settings: present status and future prospects

Rapid scale-up of antiretroviral treatment (ART) in resource-limited settings where the burden of tuberculosis (TB) is high has resulted in the increasingly frequent need for patients to receive TB treatment and ART concurrently. This presents a major challenge to ART programs in these settings, where the therapeutic options and the healthcare infrastructure to effectively deliver and monitor overlapping treatment are limited. This article reviews the issues of pharmacokinetic interactions, drug cotoxicity and TB immune reconstitution disease. The currently available treatment options and the impact of concurrent treatment on patient outcomes are described. The use of ART in the treatment of HIV-associated multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) is also considered. Finally we discuss how new therapeutic agents currently in development may improve treatment options in the future.

A covalent inhibitor targeting an intermediate conformation of the fusogenic subunit of the HIV-1 envelope complex

Peptide inhibitors corresponding to sequences in the six helix bundle structure of the fusogenic portion (gp41) of the HIV envelope glycoprotein have been successfully implemented in preventing HIV entry. These peptides bind to regions in HIV gp41 transiently exposed during the fusion reaction. In an effort to improve upon these entry inhibitors, we have successfully designed and tested peptide analogs composed of chemical spacers and reactive moieties positioned strategically to facilitate covalent attachment. Using a temperature-arrested state prime wash in vitro assay we show evidence for the trapping of a pre-six helix bundle fusion intermediate by a covalent reaction with the specific anti-HIV-1 peptide. This is the first demonstration of the trapping of an intermediate conformation of a viral envelope glycoprotein during the fusion process that occurs in live cells. The permanent specific attachment of the covalent inhibitor is projected to improve the pharmacokinetics of administration in vivo and thereby improve the long-term sustainability of peptide entry inhibitor therapy and help to expand its applicability beyond salvage therapy.

Population pharmacokinetics of enfuvirtide in HIV-1-infected pediatric patients over 48 weeks of treatment

The objective of this study was to characterize the population pharmacokinetics of enfuvirtide in HIV-1-infected children and adolescents. HIV-infected patients received combination antiretroviral therapy, including enfuvirtide 2.0 mg/kg subcutaneously, twice daily. Serial and trough blood samples were collected up to 48 weeks. NONMEM was used for population pharmacokinetic analysis. Enfuvirtide exposure was calculated from individual parameter estimates derived from the final model. A total of 218 samples from 43 patients were included in the analysis. Enfuvirtide plasma concentration-time data were described by a 1-compartment model with first-order absorption and elimination. The addition of each subject's actual body weight as a covariate affected CL/F but not V/F or K(a). The population CL/F, V/F, and K(a) for a 33-kg reference patient was 1.31 L/h, 2.31 L, and 0.105 h(-1), respectively. The final model was CL/F (L/h) = 1.31 . (body weight/33 [kg])(0.721). Age did not affect enfuvirtide exposure. These results confirm the appropriateness of body weight-based pediatric enfuvirtide dosing.

Recomendaciones españolas sobre el uso adecuado de enfuvirtida

Enfuvirtide is a high-cost, parenterally administered drug commonly used in late phases of HIV infection, when its efficacy may be compromised. To optimize enfuvirtide use, consensus recommendations for this purpose have been formulated by 247 physicians attending patients with HIV infection in Spain. A literature review was performed in which grades of evidence and recommendations were defined according to the origin of the data (randomized clinical trials, non-randomized studies, expert opinion). Twenty-eight local consensus meetings were held between May and September 2005 to discuss the most important aspects related to the use of enfuvirtide, following a pre-established system used in all the meetings. The main conclusions were as follows: a) enfuvirtide use is often excessively delayed and is given to patients with little chance of treatment success; b) enfuvirtide is indicated in patients who require antiretroviral treatment and for whom an optimum treatment with three other fully effective drugs cannot be designed; c) the most important prognostic factor is the availability of at least one other completely active drug; d) there is no infallible method to avoid the development of local reactions, but measures are available to decrease their incidence and severity; and e) patient counseling and training for correct administration of the drug are essential to improve adherence, the repercussions of local reactions and, of course, the efficacy of the treatment.

Unexpected drug-drug interaction between tipranavir/ritonavir and enfuvirtide

Fifty-five patients placed on tipranavir/ritonavir 500/200 mg twice a day (27 with enfuvirtide and 28 without) underwent tipranavir and ritonavir plasma concentration measurements by high-pressure liquid chromatography. Markedly higher tipranavir and ritonavir trough concentrations were observed in enfuvirtide recipients. The modelling of sparse plasma samples using a first order absorption and elimination monocompartmental model without time lag predicted higher tipranavir elimination half-life and volume of distribution in enfuvirtide takers. This unexpected drug-drug interaction warrants further investigation.

Development and validation of a quantitative assay for the measurement of two HIV-fusion inhibitors, enfuvirtide and tifuvirtide, and one metabolite of enfuvirtide (M-20) in human plasma by liquid chromatography–tandem mass spectrometry

A method for the quantification of two peptide HIV-1 fusion inhibitors (enfuvirtide, T-20 and tifuvirtide, T-1249) and one metabolite of enfuvirtide (M-20) in human plasma has been developed and validated, using liquid chromatography coupled with electrospray tandem mass spectrometry (LC-MS/MS). The analytes were extracted from plasma by solid-phase extraction (SPE) on vinyl-copolymer cartridges. Chromatographic separation of the peptides was performed on a Symmetry 300 C(18) column (50mmx2.1mm I.D., particle size 3.5 microm), using a water-acetonitrile gradient containing 0.25% (v/v) formic acid. The triple quadrupole mass spectrometer was operated in the positive ion-mode and multiple reaction monitoring (MRM) was used for peak detection. Deuterated (d60) enfuvirtide and (d50) tifuvirtide were used as internal standards. The assay was linear over a concentration range of 20-10,000 ng/ml for enfuvirtide and tifuvirtide and of 20-2000 ng/ml for M-20. Intra- and inter-assay precisions and deviations from the nominal concentrations were </=13%. Stability of the analytes was tested under all relevant conditions for sample handling. The method was capable to measure concentrations of enfuvirtide and its metabolite in plasma samples of human immunodeficiency virus type-1 (HIV-1) infected patients treated with the drug.