title: Determination of unbound antiretroviral drug concentrations by a modified ultrafiltration method reveals high variability in the free fraction.
abstract: Total plasma concentrations are used for therapeutic drug monitoring of antiretroviral drugs, whereas antiviral activity is expected to depend on unbound concentrations. The determination of free (unbound) concentrations by ultrafiltration may be flawed by the irreversible adsorption of many drugs onto the membrane filters and plastic components of the device. The authors describe a modified ultrafiltration method enabling the accurate measurement of unbound concentrations of 10 antiretroviral drugs by liquid chromatography-tandem mass spectroscopy, which circumvents the problem of loss by adsorption in the early ultrafiltration fractions. The method was applied to assess the variability of free fractions of antiretroviral drugs during routine therapeutic drug monitoring in 144 patients with HIV. In in vitro experiments, ultrafiltrate collected in four fractions (0-8, 8-16, 16-24, and 24-30 minutes) gave much lower and more variable free drug concentrations in the first ultrafiltrate fraction than in the last three fractions for lopinavir, nelfinavir, saquinavir, tipranavir, and efavirenz. In the last two fractions, free concentrations remained constant, indicating saturable adsorption. The adsorption was modest for indinavir, amprenavir, and ritonavir, and unnoticeable for atazanavir and nevirapine. Free fraction values obtained with this modified ultrafiltration method reveal substantial interindividual variability, suggesting that monitoring unbound antiretroviral drug concentrations may increase its clinical usefulness, especially for lopinavir, saquinavir, and efavirenz.
authors.affiliation: Division of Clinical Pharmacology, Centre Hospitalier Universataire Vaudois, Lausanne, Switzerland.
authors.forename: Aurélie
authors.initials: A
authors.lastname: Fayet
authors.forename: Alexandre
authors.initials: A
authors.lastname: Béguin
authors.forename: Begona Martinez
authors.initials: BM
authors.lastname: de Tejada
authors.forename: Sara
authors.initials: S
authors.lastname: Colombo
authors.forename: Matthias
authors.initials: M
authors.lastname: Cavassini
authors.forename: Stefan
authors.initials: S
authors.lastname: Gerber
authors.forename: Chin B
authors.initials: CB
authors.lastname: Eap
authors.forename: Amalio
authors.initials: A
authors.lastname: Telenti
authors.forename: Thierry
authors.initials: T
authors.lastname: Buclin
authors.forename: Jérôme
authors.initials: J
authors.lastname: Biollaz
authors.forename: Laurent A
authors.initials: LA
authors.lastname: Decosterd
mesh_terms: D019380:Anti-HIV Agents / Q000097:blood* / Q000627:therapeutic use; D023241:Antiretroviral Therapy, Highly Active; D002138:Calibration; D002851:Chromatography, High Pressure Liquid; D006442:Glycated Hemoglobin / Q000032:analysis; D015658:HIV Infections / Q000097:blood / Q000188:drug therapy; D006801:Humans; D007202:Indicators and Reagents; D011786:Quality Control; D012709:Serum Albumin / Q000032:analysis; D053719:Tandem Mass Spectrometry; D014462:Ultrafiltration
chemical_list: D019380:Anti-HIV Agents; D006442:Glycated Hemoglobin; D007202:Indicators and Reagents; D012709:Serum Albumin