Hepatic Dihydropyrimidine Dehydrogenase Activity Research Articles

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.

A predictive biomarker for altered 5‐fluorouracil pharmacokinetics following repeated administration in a rat model of colorectal cancer

The relationship between the plasma ratio of dihydrouracil/uracil (UH2/Ura) and hepatic dihydropyrimidine dehydrogenase (DPD) activity after repeated 5-fluorouracil (5-FU) treatment in rats with colorectal cancer (CRC) was investigated. Repeated intravenous 5-FU bolus injections resulted in a significant decrease in the total clearance (CLtot ) and an increased area under the curve (AUC0-∞ ) in CRC rats. Furthermore, the hepatic DPD levels and the plasma ratio of UH2/Ura decreased significantly and lost their circadian rhythms in CRC rats treated repeatedly with 5-FU, although significant circadian variation in the two parameters was observed in the control CRC rats. Moreover, a significant correlation was found between the plasma ratio of UH2/Ura and hepatic DPD activity in CRC rats untreated and treated with single or repeated 5-FU administration (r(2) = 0.865, p < 0.01). The ratio of UH2/Ura in plasma could be a predictive biomarker of the suppression of hepatic DPD levels during repeated 5-FU-based treatment. Furthermore, by plotting the observed pharmacokinetic parameters of 5-FU against hepatic DPD activity levels predicted by the ratio of UH2/Ura in plasma, AUC0-∞ , CLtot and half-life (t1/2 ) were closely linked to predicted hepatic DPD activity levels. These observations suggest that the factor that significantly influences the AUC0-∞ , CLtot and t1/2 of 5-FU after single or repeated administration of 5-FU is the hepatic DPD activity and it could be assessed by the ratio of UH2/Ura in plasma.

Pharmacokinetic/Pharmacodynamic Modeling of 5-Fluorouracil by Using a Biomarker to Predict Tumor Growth in a Rat Model of Colorectal Cancer

We developed a pharmacokinetic/pharmacodynamic (PK/PD) model with the value of the plasma ratio of dihydrouracil (UH2)/uracil (Ura), which is a possible surrogate biomarker of hepatic dihydropyrimidine dehydrogenase activity, determined before 5-fluorouracil (5-FU) treatment to simulate the growth of tumors after 5-FU treatment in rats with colorectal cancer (CRC). In the PK/PD model, the value of the elimination rate constant of 5-FU-ke -was estimated using the plasma UH2/Ura ratio observed before 5-FU treatment for simulating PKs of 5-FU and tumor growth. The PK/PD model with plasma UH2/Ura ratio effectively captured the features of tumor growth and the anticancer effect of 5-FU treatment, which provided reliable parameter estimates. In addition to an appropriate dosing regimen, pretherapeutic assessment of the UH2/Ura ratio in the plasma of CRC patients and PK/PD analysis with the plasma UH2/Ura ratio could enable the development of an optimal therapeutic scheme for each patient.

Pharmacokinetics of 5-fluorouracil and increased hepatic dihydropyrimidine dehydrogenase activity levels in 1,2-dimethylhydrazine-induced colorectal cancer model rats

To investigate the hepatic dihydropyrimidine dehydrogenase (DPD) activity in colorectal cancer (CRC), which is critically important to create a patient-specific dosing regimen, we performed 5-FU pharmacokinetic studies in 1,2-dimethylhydrazine-induced CRC model rats (CRC rats). After rats received 5-FU intravenous (IV) bolus injections, the area under the plasma concentration-time curve (AUC) and elimination half-life (t 1/2) in CRC rats (10.02±0.37μg h mL(-1), 0.30±0.02h, respectively) were significantly lower than that in control rats (13.46±1.20μg h mL(-1), 0.52±0.05h, respectively), whereas total plasma clearance (CLtot) in CRC rats (2.01±0.07L h(-1) kg(-1)) was significantly increased compared with that in control rats (1.54±0.14L h(-1) kg(-1)). Conversely, the avoidance ratio of the hepatic first-pass effect was approximately 20% lower than that in control rats. Of interest is that hepatic DPD activity levels and the dihydrouracil-uracil ratio (UH2/Ura ratio) in plasma, which may act as a potential biomarker to evaluate hepatic DPD activity levels, were significantly increased in CRC rats. These results suggest that the decrease of hepatic availability in CRC rats is brought about by the increase in intrinsic clearance induced by the increase in DPD activity, resulting in a decrease in AUC and t 1/2 and an increase in CLtot after 5-FU IV bolus injection. Along with a proper dosing regimen for patients with CRC, a hepatic DPD activity monitoring system, such as the determination of UH2/Ura ratio in plasma, is desirable.

RELATIONSHIPS AMONG PLASMA [2-13C]URACIL CONCENTRATIONS, BREATH13CO2EXPIRATION, AND DIHYDROPYRIMIDINE DEHYDROGENASE (DPD) ACTIVITY IN THE LIVER IN NORMAL AND DPD-DEFICIENT DOGS

Dihydropyrimidine dehydrogenase (DPD), the first enzyme in the sequential metabolism of pyrimidine, regulates blood concentrations of 5-fluorouracil and is deeply involved in its toxicity. This study was designed to examine the effects of a DPD inhibitor on blood concentrations of [2-(13)C]uracil ([(13)C]uracil) and (13)CO(2) concentration (Delta(13)C) expired in breath after oral or intravenous administration of [(13)C]uracil to DPD-suppressed dogs prepared by pretreatment with 5-(trans-2-bromovinyl)uracil (BVU), a DPD inhibitor. Area under the curve (AUC(t)) of [(13) C]uracil after oral administration at 20 micromol/kg to dogs pretreated with BVU at 2, 5, and 40 mmol/kg were 37-, 88- and 120-fold higher than those of the control dogs, respectively. In contrast, breath AUC(t) values of Delta(13)C were reduced to 0.88-, 0.47- and 0.13-fold the control values, respectively. Upon intravenous administration of [(13)C]uracil at 20 micromol/kg to dogs pretreated with BVU at 0.5, 2, and 40 micromol/kg, blood AUC(t) values of [(13)C]uracil were 1.4-, 4.2-, and 13-fold higher than those of the control group, respectively, whereas breath AUC(t) values were reduced to 1.0-, 0.83-, and 0.07-fold the respective control values. DPD activities in the liver cytosol of dogs pretreated with BVU at 0.5, 2, 5, and 40 micromol/kg were decreased to 0.71-, 0.12-, 0.06-, and 0.04-fold those of the control dogs, respectively. These findings demonstrate that breath output (Delta(13)C) is a good marker of hepatic DPD activity in vivo.

P.39 Parenteral nutrition decreased hepatic dihydropyrimidinedehydrogenase activity and catabolism of 5-fluorouracil in rats

P.39 Parenteral nutrition decreased hepatic dihydropyrimidinedehydrogenase activity and catabolism of 5-fluorouracil in rats

Liver Dihydropyrimidine Dehydrogenase Activity in Human, Cynomolgus Monkey, Rhesus Monkey, Dog, Rat and Mouse

Interspecies differences in dihydropyrimidine dehydrogenase (DPD), the initial and rate-limiting enzyme in pyrimidine degradation, were assessed in cytosol from livers isolated from human, monkey, dog, rat, and mouse. Hepatic DPD activity was measured by an HPLC assay with on-line radioactivity detection, using ¹⁴C-5-fluorouracil as a substrate. Activity was highly variable within each species and significant interspecies differences in liver DPD activity were observed. The order of activity was mouse > rat > human > dog ≥ cynomolgus monkey > rhesus monkey. These data suggest that careful selection must be made when choosing in vivo models of human DPD for the preclinical development of novel fluoropyrimidine anticancer agents and DPD inhibitors.

Depressed hepatic dihydropyrimidine dehydrogenase activity and fluorouracil-related toxicities

Depressed hepatic dihydropyrimidine dehydrogenase activity and fluorouracil-related toxicities

The effect of dietary protein depletion on hepatic 5-fluorouracil metabolism.

This study investigated the mechanism of protein depletion and its effect on host toxicity to 5‐fluorouracil. The effects of protein malnutrition on the hepatic metabolism of 5‐fluorouracil were examined using 19F‐nuclear magnetic resonance spectroscopy, pharmacokinetic disposition of 5‐fluorouracil, and hepatic dihydropyrimidine dehydrogenase activity. Harlan Sprague‐Dawley rats were given 5‐fluorouracil (150 mg/kg) by intraperitoneal injection after receiving normal (21.5%) or low (2.5%) protein diets for 19 to 30 days (mean, 25 days). Toxicity was assessed by weight loss, lymphocyte count, and survival. Hepatic fluorine spectra were used to evaluate the time to achieve maximum 5‐fluorouracil concentration, apparent hepatic 5‐fluorouracil half‐life, duration of detectable 5‐fluorouracil concentrations, and time to reach peak fluoro‐β‐alanine concentration (the main 5‐fluorouracil degradation product). Parallel experiments determined the pharmacokinetic disposition of 5‐fluorouracil and the hepatic dihydropyrimidine dehydrogenase activity.In the low‐protein group, the initial detection of the degradation product fluoro‐β‐alanine and the mean duration of 5‐fluorouracil signal in the liver were significantly prolonged: time to reach peak fluoro‐β‐alanine (minutes) 62 ± 39 vs 24 ± 12 (p &lt;.05) and apparent 5‐fluorouracil half‐life (minutes) 96 ± 63 vs 42 ± 22 (p &lt;.001). The 5‐fluorouracil clearance and the hepatic dihydropyrimidine dehydrogenase activity were also significantly lower in the low‐protein group: 1.8 ± 0.8 L/h/kg vs 7.3 ± 2.1 Llhlkg (p &lt;.05) and 0.29 ± 0.04 U/mg of protein vs 0.37 ± 0.04 U/mg of protein (p &lt;.05), respectively. Compared with animals fed a normal diet, animals in the low‐protein group sustained greater weight loss (g) 36 ± 17 vs 16 ± 17 (p &lt;.05), lower lymphocyte count (mm3) 1767 ± 993 vs 5575 ± 2239 (p &lt;.05), and greater mortality 7 days after 5‐fluorouracil (8/9 vs 1/10) (p &lt;.01).

The effect of dietary protein depletion on hepatic 5-fluorouracil metabolism.

Protein calorie malnutrition, which is highly prevalent in tumor-bearing hosts, increases toxicity to 5-fluorouracil (5-FU), but the mechanisms are unclear. This study investigated the effects of protein depletion on 5-FU in vivo hepatic metabolism using F19-nuclear magnetic resonance spectroscopy (19F-NMRS). Rats received normal (21.5%) or low (2.5%) protein diet for 25 days. 5-FU was injected intraperitoneally, and hepatic fluorine spectra were obtained. Parallel experiments were conducted to determine serum 5-FU pharmacokinetics using high-performance liquid chromatography (HPLC) and to measure hepatic dihydropyrimidine dehydrogenase (DPD) activity. The mean time of initial detection of fluoro-beta-alanine and the mean duration of the 5-FU signal in the liver were significantly prolonged in the low-protein group. 5-FU clearance and hepatic DPD activity were significantly lower in the low-protein group. Low-protein animals demonstrated increased toxicity, with diarrhea, weight loss, leukopenia (P < 0.001), and an 85% mortality, compared with regular diet animals, who had mild diarrhea and weight loss but no leukopenia and a 12% mortality. Protein depletion results in increased toxicity to 5-FU, which is associated with a significantly decreased rate of hepatic metabolism and clearance of 5-FU and a significant decrease in hepatic DPD activity. 19F-NMRS can noninvasively identify these alterations of 5-FU metabolism in vivo and may serve as a useful guide to determining chemotherapy dosage adjustments to reduce toxicity.