Population pharmacokinetic modelling and simulation of 5-fluorouracil incorporating a circadian rhythm in rats

Diurnal Changes in Capecitabine Clock-Controlled Metabolism Enzymes Are Responsible for Its Pharmacokinetics in Male Mice

The circadian timing system controls absorption, distribution, metabolism, and elimination processes of drug pharmacokinetics over a 24-h period. Exposure of target tissues to the active form of the drug and cytotoxicity display variations depending on the chronopharmacokinetics. For anticancer drugs with narrow therapeutic ranges and dose-limiting side effects, it is particularly important to know the temporal changes in pharmacokinetics. A previous study indicated that pharmacokinetic profile of capecitabine was different depending on dosing time in rat. However, it is not known how such difference is attributed with respect to diurnal rhythm. Therefore, in this study, we evaluated capecitabine-metabolizing enzymes in a diurnal rhythm-dependent manner. To this end, C57BL/6J male mice were orally treated with 500 mg/kg capecitabine at ZT1, ZT7, ZT13, or ZT19. We then determined pharmacokinetics of capecitabine and its metabolites, 5'-deoxy-5-fluorocytidine (5'DFCR), 5'-deoxy-5-fluorouridine (5'DFUR), 5-fluorouracil (5-FU), in plasma and liver. Results revealed that plasma C_max and AUC_0-6h (area under the plasma concentration-time curve from 0 to 6 h) values of capecitabine, 5'DFUR, and 5-FU were higher during the rest phase (ZT1 and ZT7) than the activity phase (ZT13 and ZT19) (p < 0.05). Similarly, C_max and AUC_0-6h values of 5'DFUR and 5-FU in liver were higher during the rest phase than activity phase (p < 0.05), while there was no significant difference in liver concentrations of capecitabine and 5'DFCR. We determined the level of the enzymes responsible for the conversion of capecitabine and its metabolites at each ZT. Results indicated the levels of carboxylesterase 1 and 2, cytidine deaminase, uridine phosphorylase 2, and dihydropyrimidine dehydrogenase (p < 0.05) are being rhythmically regulated and, in turn, attributed different pharmacokinetics profiles of capecitabine and its metabolism. This study highlights the importance of capecitabine administration time to increase the efficacy with minimum adverse effects.

Population Pharmacokinetic Model-Based Evaluation of Circadian Variations in Plasma 5-Fluorouracil Concentrations During Long-Term Infusion in Rats: A Comparison With Oral Anticancer Prodrugs

Circadian fluctuations in the plasma concentration of 5-fluorouracil impede the accurate estimation of target therapeutic concentrations in the long-term infusion or oral 5-fluorouracil-based prodrug regimen. We evaluated the circadian patterns of plasma 5-fluorouracil concentrations in rats using population pharmacokinetic model. Rats were divided into 2 groups, and a continuous infusion (50 mg/m²/h) for 48 h was initiated with or without a bolus injection of 60 mg/kg 5-fluorouracil. In the group not administered a loading dose, significant circadian variation of plasma 5-fluorouracil concentration was observed. In contrast, in the loading dose group, this circadian variation disappeared. Additionally, decreased hepatic dihydropyrimidine dehydrogenase activity was observed. Population model analysis revealed that the concentrations of 5-fluorouracil followed a 24-h cosine circadian curve, representing an overall 1.8-fold increase from a nadir to a peak, with a relative amplitude (% of mesor) of 28%. The circadian 5-fluorouracil clearance pattern in the infusion-regimen was consistent with previously reported pattern for capecitabine and uracil-tegafur. In the recently modified regimen omitting the bolus injection of 5-fluorouracil, the circadian variations should be considered for blood sampling time points in therapeutic drug monitoring. The chronomodulated chemotherapy using oral prodrug administration could be established based on accumulating evidence in the infusion-regimen.

Association between circadian and chemotherapeutic cycle effects on plasma concentration of 5-fluorouracil and the clinical outcome following definitive 5-fluorouracil/cisplatin-based chemoradiotherapy in patients with esophageal squamous cell carcinoma

Therapeutic drug monitoring (TDM) of 5-fluorouracil (5-FU) is believed to be a clinical option for improving clinical responses. Evaluating the potential factors contributing to plasma 5-FU concentration is important to develop TDM of 5-FU. Our aim was to evaluate the association of the circadian and treatment cycle effects on plasma 5-FU concentration with the clinical response. A post hoc population analysis was performed using the plasma concentration of 5-FU and clinical response data, including prognosis from 49 patients with esophageal squamous cell carcinoma after treatment with definitive 5-FU/cisplatin-based chemoradiotherapy, consisting of prolonged infusion of 5-FU at 400 mg/(m²·day) for 5 days. The circadian rhythm and treatment cycle were applied as covariates to the model equation. The plasma 5-FU concentration in the evening was 1.3-fold higher compared with the morning, and in the second cycle, it was 1.5-fold increased compared with the first cycle, with relatively small inter-individual variations (23.3 and 16.8%). Clinical efficacy depended on the plasma 5-FU concentration, excluding the covariate effects (P=0.025), which correlated with age and height but not body surface area. Circadian variation did not contribute to the clinical response, and the increase in 5-FU plasma concentration in the second cycle significantly correlated with leucocyte counts obtained before chemoradiotherapy. The higher plasma concentration of 5-FU in the early phase of treatment may be the key determinant of clinical efficacy, whereas the variations in the plasma concentration of 5-FU owing to the time of day and treatment cycle are small contributors.

Circadian variations in the pharmacokinetics of the oral anticancer agent tegafur-uracil (UFT) and its metabolites in rats

Uracil-tegafur (UFT) is an oral anticancer drug containing uracil and 5‑fluorouracil prodrug tegafur and is widely used for adjuvant chemotherapy of colorectal cancer. Although clinical data show circadian variations in plasma 5‑fluorouracil concentrations during its long-term infusion, and feasibility studies of chronomodulated administration have been previously reported, the circadian pattern in plasma 5‑fluorouracil concentration after UFT administrations remains unclear. The aim of this study was to identify factors causing circadian variations in UFT pharmacokinetics and estimate circadian patterns of plasma 5‑fluorouracil concentration corresponding to UFT dosing time in rats. Rats were orally administered UFT (15 mg/kg as tegafur) at three different times of the day: 07:00 (23 h after light onset, HALO), 13:00 (5 HALO), or 19:00 (11 HALO), and then plasma concentrations of tegafur, 5‑fluorouracil, and uracil were measured after UFT administration. We found that the area under the plasma concentration-time curves (AUC_0-∞) of 5‑fluorouracil depended on the UFT dosing time of day with a 2.4-fold difference between the peak (at 19:00: 13.7 ± 1.4 μmol·h/L) and trough (at 13:00: 5.6 ± 1.3 μmol·h/L). The simulated population mean clearance of 5‑fluorouracil followed a 24-h cosine circadian curve, with the highest value in the early light phase being 2.2-fold higher than the lowest value in the early dark phase, which was an inverse circadian pattern compared to the plasma 5‑fluorouracil concentration. The plasma tegafur levels suggested that circadian variation in tegafur absorption and conversion to 5‑fluorouracil are factors causing variations in plasma 5‑fluorouracil levels following UFT administration. In conclusion, the circadian pattern of 5‑fluorouracil clearance and circadian variations in tegafur pharmacokinetics are important determinants of plasma 5‑fluorouracil concentrations following UFT administration. This knowledge could help in developing a chronomodulated administration strategy of UFT for improving clinical outcomes.

Population Pharmacokinetic-Pharmacodynamic Modeling of 5-Fluorouracil for Toxicities in Rats

BACKGROUND AND OBJECTIVES

Myelosuppression is a dose-limiting toxicity of 5-fluorouracil (5-FU). Predicting the inter- and intra-patient variability in pharmacokinetics and toxicities of 5-FU may contribute to the individualized medicine. This study aimed to establish a population pharmacokinetic-pharmacodynamic model that could evaluate the inter- and intra-individual variability in the plasma 5-FU concentration, 5-FU-induced body weight loss and myelosuppression in rats.

METHOD

Plasma 5-FU concentrations, body weight loss, and blood cell counts in rats following the intravenous administration of various doses of 5-FU for 4 days were used to develop the population pharmacokinetic-pharmacodynamic model.

RESULTS

The population pharmacokinetic model consisting of a two-compartment model with Michaelis-Menten elimination kinetics successfully characterized the individual and population predictions of the plasma concentration of 5-FU and provided credible parameter estimates. The estimates of inter-individual variability in maximal rate of saturable metabolism and residual variability were 8.1 and 22.0%, respectively. The population pharmacokinetic-pharmacodynamic model adequately described the individual complete time-course of alterations in body weight loss, erythrocyte, leukocyte, and lymphocyte counts in rats treated with various doses of 5-FU. The inter-individual variability of the drug effects in the pharmacodynamic model for body weight loss was 82.6%, which was relatively high. The results of the present study suggest that not only individual fluctuations in the 5-FU concentration but also the cell sensitivity would affect the onset and degree of 5-FU-induced toxicity.

CONCLUSION

This population pharmacokinetic-pharmacodynamic model could evaluate the inter- and intra-individual variability in drug-induced toxicity and guide the assessments of novel anticancer agents in drug development.

Circadian variations in the pharmacokinetics of capecitabine and its metabolites in rats

Capecitabine, an orally available prodrug of 5-fluorouracil, is widely used to treat patients with colorectal cancer. Although various studies have shown circadian variations in plasma 5-fluorouracil concentrations during long-term infusion, it is still unknown whether circadian variations also exist following administration of capecitabine. The present study aimed to investigate whether the pharmacokinetics of capecitabine and its metabolites, including 5-fluorouracil, vary according to administration time in rats. Rats were orally administered capecitabine (180mg/kg) at 07:00 (23h after light onset, HALO), 13:00 (5 HALO), or 19:00h (11 HALO). Plasma concentrations of capecitabine and its metabolites, such as 5'-deoxy-5-fluorocytidine (5'-DFCR), 5'-deoxy-5-fluorouridine (5'-DFUR), and 5-fluorouracil, were determined after capecitabine administration. The results showed that the t_1/2 and AUC_0-∞ values of 5-fluorouracil differed as a function of the dosing time of capecitabine. The maximum and minimum mean t_1/2 values of 5-fluorouracil were obtained when the drug was administered at 07:00h (23 HALO: 3.1±1.2h) and 13:00h (5 HALO: 1.5±0.6h), respectively. The AUC_0-∞ value of 5-fluorouracil at 07:00h (23 HALO: 533.9±195.7μmol∙h/L) was 1.8-fold higher than the value at 13:00h (5 HALO: 302.5±157.1μmol∙h/L). The clearance of 5-fluorouracil followed a cosine circadian curve, and the simulated population mean clearance was highest at rest times and lowest during active times in rats. The results for the plasma 5'-DFCR and 5'-DFUR levels indicated that circadian variations in the sequential metabolism of capecitabine to 5-fluorouracil would also affect plasma 5-fluorouracil levels following capecitabine administration. In conclusion, the pharmacokinetics of capecitabine and its metabolites, including 5-fluorouracil, varied according to time of dosing, suggesting that the capecitabine administration time is an important factor in achieving sufficient efficacy and reducing toxicity in patients.