Population pharmacokinetics of flucytosine: comparison and validation of three models using STS, NPEM, and NONMEM.

The objective of this study is to compare and validate three models of flucytosine (5-FC) population pharmacokinetics using three methods of analysis to elucidate which model describes 5-FC pharmacokinetics most accurately and which method is the most suitable for this purpose. Retrospectively, demographic and clinical data of two similar sets of a total of 88 intensive care unit (ICU) patients were gathered for calculation and validation of 5-FC pharmacokinetics respectively. Three pharmacokinetic models were analyzed: a one-compartment with renal elimination (renal model), a one-compartment with renal and metabolic elimination (mixed model), and a two-compartment with renal elimination (two-compartment model). Population pharmacokinetic parameters were calculated using the standard two-stage method (STS), NONMEM, and NPEM. Furthermore, a covariate model was built by NONMEM. Validation of the 10 calculated pharmacokinetic models showed that NONMEM is most suitable for predicting 5-FC population pharmacokinetics. Based upon AIC values, bias and precision, the best results are obtained using a two-compartment model with renal elimination (k(elr) = 0.000858 +/- 0.000143 l/h per mL per min, k12 = 0.0313 +/- 0.0168 h(-1), k21 = 0.0353 +/- 0.0145 h(-1), and Vd = 0.541 +/- 0.084 L/kg; bias = -13.16; 95% CI = -16.77; -9.55; precision = 30.50; 95% CI = 27.47; 33.26) or a two-compartment covariate model as built by NONMEM [Vd (L) = 0.572 x WT, Cl(5FC) (L/h) = 1.69 + 0.0273 x (Cl(cr) (mL/min) - 52.5), k12 = 0.0235 +/- 0.0107 h(-1), and k21 = 0.0375 +/- 0.0147 h(-1); bias = -8.29; 95% CI = -11.63; -4.95; precision = 26.77; 95% CI = 24.24; 29.07]. In conclusion, this study shows that a two-compartment model with renal elimination best describes 5-FC population pharmacokinetics and NONMEM is able to build a two-compartment covariate model that predicts 5-FC levels equally well in our population of ICU patients. Furthermore, NONMEM appeared to be the most suitable method of population pharmacokinetics in our population and for this purpose it offers more reliable and accurate results than NPEM or the STS method.

Pharmacokinetics and pharmacodynamics of the novel marine-derived anticancer agent ecteinascidin 743 in a phase I dose-finding study.

Ecteinascidin (ET) 743 is an anticancer agent derived from the Caribbean tunicate Ecteinascidia turbinata. Preclinical studies revealed activity of ET-743 against different tumor types. A Phase I clinical trial was designed with ET-743 to identify the maximum tolerated dose and dose-limiting toxicities (DLTs). Furthermore, the pharmacokinetics of ET-743 and relationships with pharmacodynamics were evaluated. Adult patients with solid, resistant tumors received ET-743 as a 24-h i.v. infusion every 21 days. Blood samples were obtained during the first treatment course and in several consecutive courses. Noncompartmental pharmacokinetic analysis was performed. Relationships between pharmacokinetics and hepatic and hematological toxicities were explored. Fifty-two patients were treated at nine dose levels (50-1800 microg/m2). The DLTs, neutropenia and thrombocytopenia, were experienced at 1800 microg/m2. Twenty-five patients were treated at the recommended Phase II dose of 1500 microg/m2. At this dose, the mean value +/- SD for total body clearance was 59 +/- 31 liters/h, and the mean t(1/2) was 89 +/- 41 h. Pharmacokinetics were linear over the dose range tested. Prior exposure to ET-743 did not alter the pharmacokinetics in subsequent courses. The percentage of decrease in WBC count and absolute neutrophil count was correlated to the area under the plasma concentration versus time curve (AUC). Hepatic toxicity, defined as rise in alanine aminotransferase and aspartate aminotransferase, increased with dose and AUC but was reversible and not dose limiting. In conclusion, ET-743 administered as a 24-h i.v. infusion at a dose of 1500 microg/m2 is clinically feasible; severe thrombocytopenia and neutropenia are the DLTs.