f Setting the adequate dose for voriconazole is challenging due to its variable pharmacokinetics. We investigated the impact of hypoalbuminemia (<35 g/liter) on voriconazole pharmacokinetics in adult intensive care unit (ICU) patients treated with voriconazole (20 samples in 13 patients) as well as in plasma samples from ICU patients that had been spiked with voriconazole at concentrations of 1.5 mg/liter, 2.9 mg/liter, and 9.0 mg/liter (66 samples from 22 patients). Plasma albumin concentrations ranged from 13.8 to 38.7 g/liter. Total voriconazole concentrations in adult ICU patients treated with voriconazole ranged from 0.5 to 8.7 mg/liter. Unbound and bound voriconazole concentrations were separated using high-throughput equilibrium dialysis followed by liquid chromatography-tandem mass spectrometry (LC-MSMS). Multivariate analysis revealed a positive relationship between voriconazole plasma protein binding and plasma albumin concentrations (P < 0.001), indicating higher unbound voriconazole concentrations with decreasing albumin concentrations. The correlation is more pronounced in the presence of elevated bilirubin concentrations (P ‫؍‬ 0.05). We therefore propose to adjust the measured total voriconazole concentrations in patients with abnormal plasma albumin and total serum bilirubin plasma concentrations who show adverse events potentially related to voriconazole via a formula that we developed. Assuming 50% protein binding on average and an upper limit of 5.5 mg/liter for total voriconazole concentrations, the upper limit for unbound voriconazole concentrations is 2.75 mg/liter. Alterations in voriconazole unbound concentrations caused by hypoalbuminemia and/or elevated bilirubin plasma concentrations cannot be countered immediately, due to the adult saturated hepatic metabolism. Consequently, increased unbound voriconazole concentrations can possibly cause adverse events, even when total voriconazole concentrations are within the reference range.

Plasma protein binding (PPB) can possibly alter the already variable pharmacokinetics of voriconazole. Voriconazole PPB was determined only once, being 58%, according to equilibrium dialysis (ED). We investigated voriconazole PPB more in detail, with a convenient and newer high-throughput ED assay (HT-ED), in human blank plasma spiked with voriconazole and in plasma from intensive care unit (ICU) patients treated with voriconazole. HT-ED was conducted in a 96-well plate, setup against phosphate-buffered saline. Voriconazole concentrations were measured by liquid chromatography-tandem mass spectrometry. The median PPB was 47.6% [interquartile range (IQR) 45.3%-50%] in vitro, and 49.6% (IQR 42.5%-52.5%) in ICU samples (p = 0.35), and is not depending on total voriconazole concentration (0.7-11.2 mg/L, p = 0.65). The drug mainly binds to albumin (25.5 ± 5.1%), and to a lesser extent to α-1-acid glycoprotein (AAG; 4.8 ± 1.2%). The HT-ED assay can be performed at 37 °C or 25 °C (p = 0.44) and in batch: PPB variations during freeze-thaw cycles (p = 0.13) and during frozen storage up to 12 months (p = 0.10) were not clinically relevant. Voriconazole PPB is approximately 50%, according to HT-ED. As albumin and AAG only account for approximately 30% of total voriconazole PPB, other plasma components could influence PPB and therefore efficacy or toxicity because of variations in unbound fractions.