Limited sampling strategies to estimate the area under the concentration-time curve. Biases and a proposed more accurate method

Analysis of glutathione S-transferase and cytochrome P450 gene polymorphism in recipients of dose-adjusted busulfan-cyclophosphamide conditioning

Sporadic incidence of veno-occlusive disease (VOD) continues to occur, despite achievement of recommended busulfan (BU) concentrations after real-time BU dose adjustment. To explore the potential influence of glutathione S-transferase (GST) and cytochrome P450 (CYP) genotypes on plasma BU concentration, subsequent VOD, and transplant outcome, we assessed the polymorphisms of multiple GST and CYP genes. Fifty-five patients were included (median age 38 years; range 21-67). Of these, 49 received dose-adjusted BU/CY therapy. Twenty-six patients received transplants from human leukocyte antigen-identical siblings, 26 from unrelated donors. The GSTA1*A/*A genotype was significantly associated with lower BU first-dose area under curve (AUC1st). We found that patients with higher AUC1st showed a significantly higher serum total bilirubin during the first month after transplantation, but this was not necessarily associated with subsequent development of VOD. We further analyzed a possible association of GST and CYP polymorphisms and VOD development, and found none of the polymorphisms investigated was associated with VOD incidence. Regarding transplant outcomes, GSTM1-positive patients showed lower relapse rates and better overall survival in multivariate analyses. These results suggest that a GSTM1-positive genotype in patients receiving BU/CY conditioning protects against relapse of hematological malignancies after allogeneic hematopoietic stem cell transplantation.

Limited sampling strategies to predict the area under the concentration-time curve for rifampicin

BACKGROUND

Rifampicin (RMP) is the most effective first-line antituberculosis drug. One of the most critical aspects of using it in fixed-drug combination formulations is to ensure it reaches therapeutic levels in blood. The determination of the area under the concentration-time curve (AUC) and appropriate dose adjustment of this drug may contribute to optimization of therapy. Even when the maximal concentration (Cmax) of RMP also predicts its sterilizing effect, the time to reach it (Tmax) takes 40 minutes to 6 hours. The aim of this study was to develop a limited sampling strategy (LSS) for therapeutic drug monitoring assistance for RMP.

METHODS

Full concentration-time curves were obtained from 58 patients with tuberculosis (TB) after the oral administration of RMP in fixed-drug combination formulation. A validated high-performance liquid chromatographic method was used. Pharmacokinetic parameters were estimated with a noncompartmental model. Generalized linear models were obtained by forward steps, and bootstrapping was performed to develop LSS to predict AUC curve from time 0 to the last measured at 24 hours postdose (AUC0-24). The predictive performance of the proposed models was assessed using RMP profiles from 25 other TB patients by comparing predicted and observed AUC0-24.

RESULTS

The mean AUC0-24 in the current study was 91.46 ± 36.7 mg·h·L, and the most convenient sampling time points to predict it were 2, 4 and 12 hours postdose (slope [m] = 0.955 ± 0.06; r = 0.92). The mean prediction error was -0.355%, and the root mean square error was 5.6% in the validation group. Alternate LSSs are proposed with 2 of these sampling time points, which also provide good predictions when the 3 most convenient are not feasible.

CONCLUSIONS

The AUC0-24 for RMP in TB patients can be predicted with acceptable precision through a 2- or 3-point sampling strategy, despite wide interindividual variability. These LSSs could be applied in clinical practice to optimize anti-TB therapy based on therapeutic drug monitoring.

Comparison of algorithms for oral busulphan area under the concentration-time curve limited sampling estimate

BACKGROUND AND OBJECTIVES

Therapeutic drug monitoring (TDM) of the first dose of busulphan during conditioning prior to allogeneic stem cell transplantation provides the possibility of improving the clinical outcome via dose adjustment of subsequent doses. The plasma area under the concentration-time curve (AUC) for busulphan is generally accepted as the parameter that gives the best exposure estimate; however, the sampling frequency needed for reliable AUC calculation remains controversial. The aim of the present investigation was to develop and evaluate a limited sampling model for oral busulphan.

METHODS

We have compared models using three to four samples with standard WinNonlin(®) adaptive compartment modeling based on eight samples as reference. The evaluated study population included both adult and pediatric patients, but the linear model was devised using analysis of only pediatric patient plasma concentrations. The present model was developed using data from 23 patients with a mean age of 38 years (range 13-59 years) and was evaluated in 20 pediatric patients with a mean age of 6 years (range 0.1-13 years) as well as 23 adult patients (mean age 43 years; range 18-67 years).

RESULTS

In 23 patients, the mean AUC from a curve fitting model (Purves method) and a single compartment model had an intraclass correlation coefficient (ICC) of 0.947. From a log-log plot of AUC values it was evident that using this estimate of the AUC would affect dose adjustment decisions for very few of the patients. Applying the linear model using three samples resulted in an ICC of 0.932, mostly due to worse performance in the adult population.

CONCLUSIONS

The present results support the use of limited sampling in clinical TDM for oral busulphan provided adequate algorithms and sampling times are used. Moreover, they also demonstrate the caution that is needed when transferring a pharmacokinetic model from a pediatric population to an adult population.