title: Population pharmacokinetics and effects of efavirenz in patients with human immunodeficiency virus infection.
abstract: OBJECTIVE
  The reverse transcriptase inhibitor efavirenz is currently used at a fixed dose of 600 mg/d. However, dosage individualization based on plasma concentration monitoring might be indicated. This study aimed to assess the efavirenz pharmacokinetic profile and interpatient versus intrapatient variability in patients who are positive for human immunodeficiency virus, to explore the relationship between drug exposure, efficacy, and central nervous system toxicity and to build up a Bayesian approach for dosage adaptation.
  
  
  METHODS
  The population pharmacokinetic analysis was performed by use of NONMEM based on plasma samples from a cohort of unselected patients receiving efavirenz. With the use of a 1-compartment model with first-order absorption, the influence of demographic and clinical characteristics on oral clearance and oral volume of distribution was examined. The average drug exposure during 1 dosing interval was estimated for each patient and correlated with markers of efficacy and toxicity. The population kinetic parameters and the variabilities were integrated into a Bayesian equation for dosage adaptation based on a single plasma sample.
  
  
  RESULTS
  Data from 235 patients with a total of 719 efavirenz concentrations were collected. Oral clearance was 9.4 L/h, oral volume of distribution was 252 L, and the absorption rate constant was 0.3 h(-1). Neither the demographic covariates evaluated nor the comedications showed a clinically significant influence on efavirenz pharmacokinetics. A large interpatient variability was found to affect efavirenz relative bioavailability (coefficient of variation, 54.6%), whereas the intrapatient variability was small (coefficient of variation, 26%). An inverse correlation between average drug exposure and viral load and a trend with central nervous system toxicity were detected. This enabled the derivation of a dosing adaptation strategy suitable to bring the average concentration into a therapeutic target from 1000 to 4000 microg/L to optimize viral load suppression and to minimize central nervous system toxicity.
  
  
  CONCLUSIONS
  The high interpatient and low intrapatient variability values, as well as the potential relationship with markers of efficacy and toxicity, support the therapeutic drug monitoring of efavirenz. However, further evaluation is needed before individualization of an efavirenz dosage regimen based on routine drug level monitoring should be recommended for optimal patient management.
authors.affiliation: Division of Clinical Pharmacology, University Hospital, Lausanne, Switzerland. chantal.csajka@chuv.hospvd.ch
authors.forename: Chantal
authors.initials: C
authors.lastname: Csajka
authors.forename: Catia
authors.initials: C
authors.lastname: Marzolini
authors.forename: Karin
authors.initials: K
authors.lastname: Fattinger
authors.forename: Laurent A
authors.initials: LA
authors.lastname: Décosterd
authors.forename: Jacques
authors.initials: J
authors.lastname: Fellay
authors.forename: Amalio
authors.initials: A
authors.lastname: Telenti
authors.forename: Jérôme
authors.initials: J
authors.lastname: Biollaz
authors.forename: Thierry
authors.initials: T
authors.lastname: Buclin
mesh_terms: D000328:Adult; D000368:Aged; D000480:Alkynes; D019380:Anti-HIV Agents / Q000008:administration & dosage* / Q000009:adverse effects / Q000097:blood / Q000493:pharmacokinetics*; D001499:Bayes Theorem; D048588:Benzoxazines; D003521:Cyclopropanes; D004334:Drug Administration Schedule; D004359:Drug Therapy, Combination; D005260:Female; D015658:HIV Infections / Q000188:drug therapy / Q000378:metabolism*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010078:Oxazines / Q000008:administration & dosage* / Q000009:adverse effects / Q000097:blood / Q000493:pharmacokinetics*; D018894:Reverse Transcriptase Inhibitors / Q000008:administration & dosage* / Q000009:adverse effects / Q000097:blood / Q000493:pharmacokinetics*; D017211:Treatment Failure
chemical_list: D000480:Alkynes; D019380:Anti-HIV Agents; D048588:Benzoxazines; D003521:Cyclopropanes; D010078:Oxazines; D018894:Reverse Transcriptase Inhibitors; C098320:efavirenz