CNS-Targeted Antiretroviral Strategies: When Are They Needed and What to Choose.

PURPOSE OF REVIEW: Neurocognitive disorders are not uncommon in HIV-positive patients but their pathogenesis is multifactorial and incompletely understood. After excluding contributing comorbidities, several factors may impair neurocognition including severe immune suppression, incomplete antiviral efficacy, drugs' persistent immune activation, vascular abnormalities, and drugs' neurotoxicity. The effectiveness of targeted antiretroviral strategies on these risk factors is unknown. RECENT FINDINGS: Recent studies support the idea that residual cerebrospinal fluid HIV RNA in the setting of plasma viral suppression is associated with compartmental immune activation but the link to neuronal damage is debated. Some authors have reported an incomplete antiviral efficacy in macrophage-derived cells but targeted antiretroviral regimen switches have not been performed. Additionally, improvements in neurocognition using drugs with better central nervous system penetration or maraviroc (associated with favorable immunological properties) have been observed in pilot studies. Trials evaluating specific interventions for cardiovascular health (including brain white matter abnormalities) and neurotoxicity of antiretrovirals are warranted. Central nervous system-targeted antiretroviral strategies are needed in patients with uncontrolled cerebrospinal HIV replication, and they may be suggested in subjects with low CD4 nadir, individuals carrying drug-resistant viruses, and those with compartmental immune activation.

Efavirenz Is Predicted To Accumulate in Brain Tissue: an In Silico, In Vitro, and In Vivo Investigation.

Adequate concentrations of efavirenz in the central nervous system (CNS) are necessary to suppress viral replication, but high concentrations may increase the likelihood of CNS adverse drug reactions. The aim of this investigation was to evaluate the efavirenz distribution in the cerebrospinal fluid (CSF) and the brain by using a physiologically based pharmacokinetic (PBPK) simulation for comparison with rodent and human data. The efavirenz CNS distribution was calculated using a permeability-limited model on a virtual cohort of 100 patients receiving efavirenz (600 mg once daily). Simulation data were then compared with human data from the literature and with rodent data. Wistar rats were administered efavirenz (10 mg kg of body weight-1) once daily over 5 weeks. Plasma and brain tissue were collected for analysis via liquid chromatography-tandem mass spectrometry (LC-MS/MS). The median maximum concentrations of drug (Cmax) were predicted to be 3,184 ng ml-1 (interquartile range [IQR], 2,219 to 4,851 ng ml-1), 49.9 ng ml-1 (IQR, 36.6 to 69.7 ng ml-1), and 50,343 ng ml-1 (IQR, 38,351 to 65,799 ng ml-1) in plasma, CSF, and brain tissue, respectively, giving a tissue-to-plasma ratio of 15.8. Following 5 weeks of oral dosing of efavirenz (10 mg kg-1), the median plasma and brain tissue concentrations in rats were 69.7 ng ml-1 (IQR, 44.9 to 130.6 ng ml-1) and 702.9 ng ml-1 (IQR, 475.5 to 1,018.0 ng ml-1), respectively, and the median tissue-to-plasma ratio was 9.5 (IQR, 7.0 to 10.9). Although it is useful, measurement of CSF concentrations may give an underestimation of the penetration of antiretrovirals into the brain. The limitations associated with obtaining tissue biopsy specimens and paired plasma and CSF samples from patients make PBPK modeling an attractive tool for probing drug distribution.

In Vivo Profiling and Distribution of Known and Novel Phase I and Phase II Metabolites of Efavirenz in Plasma, Urine, and Cerebrospinal Fluid.

Efavirenz (EFV) is principally metabolized by CYP2B6 to 8-hydroxy-efavirenz (8OH-EFV) and to a lesser extent by CYP2A6 to 7-hydroxy-efavirenz (7OH-EFV). So far, most metabolite profile analyses have been restricted to 8OH-EFV, 7OH-EFV, and EFV-N-glucuronide, even though these metabolites represent a minor percentage of EFV metabolites present in vivo. We have performed a quantitative phase I and II metabolite profile analysis by tandem mass spectrometry of plasma, cerebrospinal fluid (CSF), and urine samples in 71 human immunodeficiency virus patients taking efavirenz, prior to and after enzymatic (glucuronidase and sulfatase) hydrolysis. We have shown that phase II metabolites constitute the major part of the known circulating efavirenz species in humans. The 8OH-EFV-glucuronide (gln) and 8OH-EFV-sulfate (identified for the first time) in humans were found to be 64- and 7-fold higher than the parent 8OH-EFV, respectively. In individuals (n = 67) genotyped for CYP2B6, 2A6, and CYP3A metabolic pathways, 8OH-EFV/EFV ratios in plasma were an index of CYP2B6 phenotypic activity (P < 0.0001), which was also reflected by phase II metabolites 8OH-EFV-glucuronide/EFV and 8OH-EFV-sulfate/EFV ratios. Neither EFV nor 8OH-EFV, nor any other considered metabolites in plasma were associated with an increased risk of central nervous system (CNS) toxicity. In CSF, 8OH-EFV levels were not influenced by CYP2B6 genotypes and did not predict CNS toxicity. The phase II metabolites 8OH-EFV-gln, 8OH-EFV-sulfate, and 7OH-EFV-gln were present in CSF at 2- to 9-fold higher concentrations than 8OH-EFV. The potential contribution of known and previously unreported EFV metabolites in CSF to the neuropsychological effects of efavirenz needs to be further examined in larger cohort studies.

Protein-free efavirenz concentrations in cerebrospinal fluid and blood plasma are equivalent: applying the law of mass action to predict protein-free drug concentration.

Efavirenz (EFV) is one of the most commonly prescribed antiretroviral drugs (ARVs) for the treatment of HIV. Highly protein-bound drugs, like EFV, have limited central nervous system (CNS) penetration when measured using total drug concentration gradients between blood plasma (BP) and cerebrospinal fluid (CSF). However, the more relevant pharmacologically active protein-free drug concentrations are rarely assessed directly in clinical studies. Using paired BP and CSF samples obtained from 13 subjects on an EFV-containing regimen, both the protein-free and total concentrations of EFV were determined. Despite a median (interquartile range [IQR]) total EFV BP/CSF concentration ratio of 134 (116 to 198), the protein-free EFV BP/CSF concentration ratio was 1.20 (0.97 to 2.12). EFV median (IQR) protein binding was 99.78% (99.74 to 99.80%) in BP and 76.19% (74.47 to 77.15%) in CSF. In addition, using the law of mass action and an in vitro-derived EFV-human serum albumin dissociation constant, we have demonstrated that the predicted median (IQR) protein-free concentration in BP, 4.59 ng/ml (4.02 to 9.44 ng/ml), compared well to that observed in BP, 4.77 ng/ml (3.68 to 6.75 ng/ml). Similar results were also observed in CSF and seminal plasma. This method provides a useful predictive tool for estimating protein binding in varied anatomic compartments. Our results of equivalent protein-free EFV concentrations in BP and CSF do not support prior concerns of the CNS as a pharmacological sanctuary from EFV. As CSF penetration of ARVs may increase our understanding of HIV-associated neurological dysfunction and antiretroviral effect, assessment of protein-free CSF concentrations of other highly protein-bound ARVs is warranted.

Compartmentalization and antiviral effect of efavirenz metabolites in blood plasma, seminal plasma, and cerebrospinal fluid.

Efavirenz (EFV) is one of the most commonly prescribed antiretrovirals for use in the treatment of human immunodeficiency virus (HIV) infection. EFV is extensively metabolized by cytochrome P450 to a number of oxygenated products; however, the pharmacologic activity and distribution of these metabolites in anatomic compartments have yet to be explored. The systemic distribution of EFV oxidative metabolites was examined in blood plasma, seminal plasma, and cerebrospinal fluid from subjects on an EFV-based regimen. The 8-hydroxy EFV metabolite was detected in blood plasma, seminal plasma, and cerebrospinal fluid, with median concentrations of 314.5 ng/ml, 358.5 ng/ml, and 3.37 ng/ml, respectively. In contrast, 7-hydroxy and 8,14-hydroxy EFV were only detected in blood plasma and seminal plasma with median concentrations of 8.84 ng/ml and 10.23 ng/ml, and 5.63 ng/ml and 5.43 ng/ml, respectively. Interestingly, protein-free concentrations of metabolites were only detectable in seminal plasma, where a novel dihdyroxylated metabolite of EFV was also detected. This accumulation of protein-free EFV metabolites was demonstrated to be the result of differential protein binding in seminal plasma compared with that of blood plasma. In addition, the oxidative metabolites of EFV did not present with any significant pharmacologic activity toward HIV-1 as measured using an HIV green fluorescent protein single-round infectivity assay. This study is the first to report the physiologic distribution of metabolites of an antiretroviral into biologic compartments that the virus is known to distribute and to examine their anti-HIV activity. These data suggest that the male genital tract may be a novel compartment that should be considered in the evaluation of drug metabolite exposure.

Efavirenz pharmacokinetics in cerebrospinal fluid and plasma over a 24-hour dosing interval.

We determined the pharmacokinetics of efavirenz in plasma and cerebrospinal fluid (CSF) over a 24-h dosing interval in a patient who had undergone a lumbar drain because of cryptococcal meningitis. Drug concentrations were determined by high-performance liquid chromatography-tandem mass spectrometry in paired CSF (n = 24) and plasma (n = 25) samples. The median plasma efavirenz concentration was 3,718 ng/ml (range, 2,439 to 4,952), and the median CSF concentration was 16.3 ng/ml (range, 7.3 to 22.3). The CSF/plasma area-under-the-curve ratio was 0.0044 corresponding to a CSF penetration of 0.44% of plasma.

Dendritic spine injury induced by the 8-hydroxy metabolite of efavirenz.

Despite combination antiretroviral therapies (cARTs), a significant proportion of HIV-infected patients develop HIV-associated neurocognitive disorders (HAND). Ongoing viral replication in the central nervous system (CNS) caused by poor brain penetration of cART may contribute to HAND. However, it has also been proposed that the toxic effects of long-term cART may contribute to HAND. A better understanding of the neurotoxic potential of cART is critically needed in light of the use of CNS-penetrating cARTs to contend with the virus reservoir in the brain. The efavirenz (EFV) metabolites 7-hydroxyefavirenz (7-OH-EFV) and 8-hydroxyefavirenz (8-OH-EFV) were synthesized and purified, and their chemical structures were confirmed by mass spectrometry and NMR. The effects of EFV, 7-OH-EFV, and 8-OH-EFV on calcium, dendritic spine morphology, and survival were determined in primary neurons. EFV, 7-OH-EFV, and 8-OH-EFV each induced neuronal damage in a dose-dependent manner. However, 8-OH-EFV was at least an order of magnitude more toxic than EFV or 7-OH-EFV, inducing considerable damage to dendritic spines at a 10 nM concentration. The 8-OH-EFV metabolite evoked calcium flux in neurons, which was mediated primarily by L-type voltage-operated calcium channels (VOCCs). Blockade of L-type VOCCs protected dendritic spines from 8-OH-EFV-induced damage. Concentrations of EFV and 8-OH-EFV in the cerebral spinal fluid of HIV-infected subjects taking EFV were within the range that damaged neurons in culture. These findings demonstrate that the 8-OH metabolite of EFV is a potent neurotoxin and highlight the importance of directly determining the effects of antiretroviral drugs and drug metabolites on neurons and other brain cells.

Efavirenz concentrations in CSF exceed IC50 for wild-type HIV.

OBJECTIVES: HIV-associated neurocognitive disorders remain common despite use of potent antiretroviral therapy (ART). Ongoing viral replication due to poor distribution of antivirals into the CNS may increase risk for HIV-associated neurocognitive disorders. This study's objective was to determine penetration of a commonly prescribed antiretroviral drug, efavirenz, into CSF. METHODS: CHARTER is an ongoing, North American, multicentre, observational study to determine the effects of ART on HIV-associated neurological disease. Single random plasma and CSF samples were drawn within 1 h of each other from subjects taking efavirenz between September 2003 and July 2007. Samples were assayed by HPLC or HPLC/mass spectrometry with detection limits of 39 ng/mL (plasma) and <0.1 ng/mL (CSF). RESULTS: Eighty participants (age 44 ± 8 years; 79 ± 15 kg; 20 females) had samples drawn 12.5 ± 5.4 h post-dose. The median efavirenz concentrations after a median of 7 months [interquartile range (IQR) 2-17] of therapy were 2145 ng/mL in plasma (IQR 1384-4423) and 13.9 ng/mL in CSF (IQR 4.1-21.2). The CSF/plasma concentration ratio from paired samples drawn within 1 h of each other was 0.005 (IQR 0.0026-0.0076; n = 69). The CSF/IC(50) ratio was 26 (IQR 8-41) using the published IC(50) for wild-type HIV (0.51 ng/mL). Two CSF samples had concentrations below the efavirenz IC(50) for wild-type HIV. CONCLUSIONS: Efavirenz concentrations in the CSF are only 0.5% of plasma concentrations but exceed the wild-type IC(50) in nearly all individuals. Since CSF drug concentrations reflect those in brain interstitial fluids, efavirenz reaches therapeutic concentrations in brain tissue.

Cerebrospinal Fluid and Brain Tissue Penetration of Tenofovir, Lamivudine, and Efavirenz in Postmortem Tissues with Cryptococcal Meningitis.

The central nervous system (CNS) is a known HIV reservoir, yet little is known about drug exposure in the brain. Our primary objective was to quantify exposure of three common antiretrovirals in brain tissue and compare exposures to plasma and cerebrospinal fluid (CSF). We also sought to identify pockets of brain most vulnerable to inadequate drug exposures and examine the role of meningitis in drug penetration into the CNS. Tenofovir, lamivudine, and efavirenz concentrations were measured using liquid chromatography and tandem mass spectrometry in plasma and CSF from 14 individuals with HIV, 7 with cryptococcal meningitis. In four individuals (three with meningitis) drug concentrations were also measured in 13 distinct brain tissue regions. In subjects with meningitis, geometric mean ratio (95% confidence interval) of tenofovir CSF to plasma was 66% (7-598%) and 14% (6-31%) in subjects without meningitis. Lamivudine CSF penetration was 100% (25-409%) in subjects with meningitis and 30% (24-37%) in subjects without meningitis. Tenofovir brain tissue concentrations were 36% (14-124%) of plasma and 49% (1-572%) of CSF. Lamivudine brain concentrations were 37% (23-64%) of plasma and 27% (1-104%) of CSF. Efavirenz brain tissue concentrations were 128% (108-179%) of plasma. Tissues collected postmortem provide a unique opportunity to assess drug distribution in tissues difficult to sample in living subjects. CSF is a poor surrogate for drug exposure throughout the CNS. Antiretrovirals differentially penetrate into the CNS and penetration may be enhanced by meningitis.

Challenges regarding analysis of unbound fraction of highly bound protein antiretroviral drugs in several biological matrices: lack of harmonisation and guidelines.

The unbound drug concentration in plasma is usually considered the only active fraction; thus the binding of a drug to a protein limits its pharmacological actions. This is of special importance for those highly bound drugs. Therefore, binding studies can be of great utility for those drugs where relationship between free and total drug concentration is variable among patients, or it can be altered by some condition or disease, or even by interactions with other drugs. However, there is a lack of validation guidelines for the determination of unbound concentrations. Antiretroviral drugs (ARVs), protease inhibitors (PIs), efavirenz and nevirapine are highly bound to proteins. Here, we present a review on the overall methods for the study of unbound fractions of highly bound plasma protein ARVs. We also provide a critical evaluation of the methods applied, their differences and the main points to be controlled and validated.