Cyclophosphamide, the precursor to the active 4-hydroxycyclophosphamide, is used in active glomerulonephritis despite limited pharmacokinetics data. The pharmacokinetics of cyclophosphamide and 4-hydroxycyclophosphamide were evaluated. The influence of laboratory and pharmacogenomic covariates on pharmacokinetics was evaluated as a secondary aim.
Glomerulonephritis patients (n = 23) participated in a pharmacokinetic evaluation. Blood was serially collected and assayed for cyclophosphamide and 4-hydroxycyclophosphamide by LC/MS methods. Kidney function, serum albumin and polymorphisms in drug metabolism or transport genes were evaluated. Analyses included non-compartmental pharmacokinetics and parametric and non-parametric statistics.
The mean area under the plasma concentration–time curve (AUC(0,∞)) data were 110 100 ± 42 900 ng ml−1 h and 5388 ± 2841 ng ml−1 h for cyclophosphamide and 4-hydroxycyclophosphamide, respectively. The mean metabolic ratio was 0.06 ± 0.04. A statistically significant relationship was found between increased serum albumin and increased half-life (0.584, P = 0.007, 95% CI 0.176, 0.820) and a borderline relationship with AUC(0,∞) (0.402, P = 0.079, 95% CI –0.064, 0.724) for 4-hydroxycyclophosphamide. Covariate relationships that trended toward significance for cyclophosphamide included decreased serum albumin and increased elimination rate constant (–0.427, P = 0.061, 95% CI 0.738, 0.034), increased urinary protein excretion and increased AUC(0,∞) (–0.392, P = 0.064, 95% CI –0.699 to 0.037), decreased Cmax (0.367, P = 0.085, 95% CI –0.067, 0.684) and decreased plasma clearance (–0.392, P = 0.064, 95% CI –0.699, 0.037). CYP2B6*9 variants vs. wildtype were found to have decreased elimination rate constant (P = 0.0005, 95% CI 0.033, 0.103), increased Vd (P = 0.0271, 95% CI −57.5, −4.2) and decreased Cmax (P = 0.0176, 95% CI 0.696, 6179) for cyclophosphamide. ABCB1 C3435T variants had a borderline decrease in cyclophosphamide elimination rate constant (P = 0.0858; 95% CI –0.005, 0.102).
Pharmacokinetics of cyclophosphamide and 4-hydroxycyclophosphamide in patients with lupus nephritis and small vessel vasculitis are similar. Clinical and pharmacogenetic covariates alter disposition of cyclophosphamide and 4-hydroxycyclophosphamide. Clinical findings of worsened glomerulonephritis lead to increased exposure to cyclophosphamide vs. the active 4-hydroxycyclophosphamide, which could have relevance in terms of clinical efficacy. The CYP2B6*9 and ABCB1 C3435T polymorphisms alter the pharmacokinetics of cyclophosphamide and 4-hydroxycyclophosphamide in glomerulonephritis.