Pitfalls in cefepime titration from human plasma: plasma- and temperature-related drug degradation in vitro

Abstract

While developing a high-pressure liquid chromatography assay for cefepime in plasma, we observed significant drug degradation at 20 and 37 degrees C but not at 4 degrees C. This plasma-related degradation persisted after protein removal. This warrants caution regarding cefepime assays for pharmacokinetic and pharmacodynamic studies of cefepime in vitro and in vivo.

FIG. 1.

Cefepime degradation in deproteinized human plasma at 37°C. Starting concentrations ranged from 10 mg/liter (20.8 μmol/liter) to 500 mg/liter (1,040 μmol/liter). Depending on the initial concentration, the degradation rate was between 0.101 h⁻¹ (1,040 μmol/liter) and 0.189 h⁻¹ (20.8 μmol/liter).

FIG. 2.

Left panel, effect of diluting deproteinized plasma in water on cefepime degradation. Samples were serially diluted in water before addition of cefepime (100-mg/liter final concentration) and incubation at 37°C. Symbols represent no dilution (Δ) and fourfold (+) and 16-fold (×) dilutions. The resulting degradation rates for these conditions were 0.407, 0.068, and 0.008 h⁻¹, respectively. Further dilutions of plasma resulted in nonmeasurable degradation over the experimental time frame. Right panel, effect of temperature on the degradation of cefepime in deproteinized human plasma. Samples containing a final cefepime concentration of 100 mg/liter were incubated at 4°C (×), 20°C (+), and 37°C (Δ). The degradation rates for these kinetics were 0.0059, 0.062, and 0.31 h⁻¹, respectively. Inset, according to the Arrhenius law, the energy of activation computed from these constants was 20,520 cal/mol.