Population pharmacokinetics of 5-fluorouracil in colorectal cancer patients

Aims. The pharmacokinetics of 5-fluorouracil (5-FU) after intravenous administration in color- ectal cancer patients were examined using population analysis. The relevant covariates and the extent of inter- and intraindividual variability were evaluated.

Methods. Data from 27 patients with diagnosis of nonmetastatic colorectal adenocarcinoma receiving weekly 5-FU (450 mg/m2), plus levamisol 50 mg/8 hours by oral route for 3 days every 15 days, were pooled with data from 17 patients with diagnosis of metastatic colorectal adenocarcinoma, receiving daily 5-FU (425 mg/m2) and intravenous folinic acid (20 mg/m2) over five consecutive days (daily times five), every four weeks. In both groups 5-FU was administered as a 60-minute infusion and blood samples were collected at 10, 30 and 60 minutes from the end of the infusion, and analysed using a validated high-performance liquid chromatography assay. An open two-compartmental pharmacokinetic model with first-order elimination from central compartment was fitted to the plasma concentration data using nonlinear mixed effect modelling (NONMEM). The potential effect of patient covariates was evaluated using a stepwise method. Model evaluation was performed using bootstrap method.

Results. The pharmacokinetic model was successfully fitted to the data. None of the covariates tested were significantly correlated to the pharmacokinetic parameters. The mean parameters’ estimates (%CV) and the percent coefficient of variation of the central tendency parameters, interindividual (IIV), interoccasion (IOV) and residual variability (s) for the final model were: CL (L/h), 65.3 (13.2); Vc (L), 14.7 (11.8); Vp (L), 334.0 (31.4); Q (L/h), 19.6 (25.5); IIVCL (%), 76.5 (34.6); IIVVc (%), 82.3 (31.0); IIVVp (%), 137.5 (35.1); IIVQ (%), 117.5 (38.5); IOVCL (%), 66.1 (45.3); IOVVc (%), 70.8 (39.5); IOVQ (%), 81.1 (27.8) and s (%), 3.0 (25.4). The bootstrap resampling method confirmed the stability of the final model. The estimates of the central tendency parameters, IIV, IOV and residual variability were essentially equal to those generated with the original dataset (0% to 18% deviation) and the 95% confidence intervals included the mean parameters’ estimates obtained from the former set.

Conclusions. The two-compartmental model accurately described the pharmacokinetics of 5-FU administered by short-term infusion. A population pharmacokinetic approach is a useful tool to integrate the knowledge gathered in the clinical setting. The model developed may help in dose adaptation and will further be used in PK/PD modelling of therapeutic outcomes and adverse events.

[Stability study of oxaliplatin and doxorubicin for intraperitoneal administration with hyperthermia]

OBJECTIVE

To evaluate the in vitro physicochemical stability of oxaliplatin and doxorubicin when the in vivo hyperthermic intraperitoneal conditions are reproduced.

METHODS

Three solutions were prepared, A (oxaliplatin 200 mg/L), B(doxorubicin 15 mg/L) and C (oxaliplatin 200 mg/L with doxorubicin 15mg/L) in glucose 5%. The three solutions were subjected to the maximum temperature reached in vivo (49° C) for two hours. Physical stability was focused on visual control of particles or precipitates in solutions, discharge of gases, odor and color. Samples were taken every 15 minutes and the chemical stability was evaluated by determining the concentration of oxaliplatin and doxorubicin remaining in the samples. Oxaliplatin concentrations were determined by atomic absorption graphite chamber while doxorubicin was determined by high performance liquid chromatography.The chemical stability criteria selected was the one described by the American Pharmacopoeia, which sets a permissible variation range between the 90-110% of the initial concentration.

RESULTS

During the assay there was no appearance of particles, precipitates in the samples, discharge of gases, nor colour changes in the solutions. The samples showed a remaining concentration of oxaliplatin and doxorubicin within the 90-110% limit. The stability of the samples that follow to two cycles of freeze-thaw after hyperthermia was also found within the specified limits.

CONCLUSION

A, B and c solutions in 5% glucose, are physically and chemically stable at 49° C for two hours. Under these conditions, these solutions could be used with guarantees of stability in patients with peritoneal carcinomatosis subsidiary of intraperitoneal hyperthermic chemotherapy based in these antineoplastic agents.

[Population pharmacokinetics of gemcitabine applied to personalize the dosage used in cancer patients]

OBJECTIVE

To develop and internally validate a population pharmacokinetic model for gemcitabine and its metabolite 2',2'-difluorodeoxyuridine (dFdU); and to evaluate its predictive perfomance for personalizing the dosage used in cancer patients.

METHODS

Gemcitabine and dFdU plasma concentrations were determined in 18 cancer patients. A 2-compartment pharmacokinetic model was implemented in the NONMEN VI program to determine the appropriate pharmacokinetic parameters. The power to identify the parameters was assessed by parametric bootstrap, and the internal model validation was performed using nonparametric bootstrap and visual and numerical predictive check methods. The final predictive performance of the model was assessed for accuracy and precision during the first (a priori) and second (a posteriori) chemotherapy cycles.

RESULTS

The mean and interpatient variability of gemcitabine and dFdU clearance was 2.70 L/min (31.0%) and 0.0515 L/min (35.8%), respectively. The estimated distribution volume at steady state was 30 L for gemcitabine and 238 L for dFdU. Internal validation confirmed that the population pharmacokinetic model was appropriate for describing the plasma concentrations of gemcitabine and dFdU over time, as well as its variability in the study population. The accuracy and precision of a posteriori gemcitabine plasma concentrations improved by 67% and 46%, respectively, compared to the a priori prediction.

CONCLUSION

The population pharmacokinetic model adequately characterised the gemcitabine and dFdU plasma concentrations in the study population over time, and can be used to accurately and precisely optimise gemcitabine dosing regimens in cancer patients.

[Populational pharmacokinetics of doxorubicin applied to personalised its dosing in cancer patients]

OBJECTIVE

To develop and internally validate a population pharmacokinetic model for doxorubicin and to evaluate its predictive performance for dose individualization in cancer patients.

METHODS

Doxorubicin plasma concentrations were determined in thirty-three cancer patients treated with intravenous doxorubicin. A three-compartment pharmacokinetic model was implemented in the NONMEN VI programme to determine the doxorubicin pharmacokinetic parameters. The identifiability of the parameters was assessed by parametric bootstrap and model validation was performed using nonparametric bootstrap, visual predictive check, and numerical predictive check. The final model's predictive performance was evaluated in terms of accuracy and precision of plasma concentration predictions during the first and second cycles of chemotherapy.

RESULTS

Doxorubicin clearance was 58.8 L/h, with interpatient variability of 29.2% and intrapatient variability of 18.9%. The estimated volume of distribution at steady state was 2294 L, with inter-and intrapatient variability of 7.3% and 26.1%, respectively. Internal validation confirmed that the population pharmacokinetic model is appropriate to describe the time course of the doxorubicin plasma concentrations and its variability in this population. The accuracy and precision of an a posteriori prediction of doxorubicin plasma concentrations improved by 63% and 41% compared to the a priori prediction.

CONCLUSION

The Bayesian population pharmacokinetic model characterised the time course of doxorubicine plasma concentrations and can be accurately and precisely used to optimise doxorubicine dosing regimens in cancer patients.

Modelling intestinal absorption of salbutamol sulphate in rats

The objective was to develop a semiphysiological population pharmacokinetic model that describes the complex salbutamol sulphate absorption in rat small intestine. In situ techniques were used to characterize the salbutamol sulphate absorption at different concentrations (range: 0.15-18 mM). Salbutamol sulphate at concentration of 0.29 mM was administered in presence of verapamil (10 and 20 mM), grapefruit juice and sodium azide (NaN3) (0.3, 3 and 6 mM). Different pharmacokinetic models were fitted to the dataset using NONMEM. Parametric and non-parametric bootstrap analyses were employed as internal model evaluation techniques. The validated model suggested instantaneous equilibrium between salbutamol sulphate concentrations in lumen and enterocyte, and the salbutamol sulphate absorption was best described by a simultaneous passive diffusion (ka = 0.636 h(-1)) and active absorption (VMax = 0.726 mM/h, Km = 0.540 mM) processes from intestinal lumen to enterocyte, together with an active capacity-limited P-gp efflux (V'max = 0.678 mM/h, K'm = 0.357 mM) from enterocyte to intestinal lumen. The extent of salbutamol sulphate absorption in rat small intestine can be improved by NaN3, grapefruit juice and verapamil.