A semimechanistic population pharmacokinetic and pharmacodynamic model incorporating autoinduction for the dose justification of TAS-114.

TAS-114 is a dual deoxyuridine triphosphatase (dUTPase) and dihydropyrimidine dehydrogenase (DPD) inhibitor expected to widen the therapeutic index of capecitabine. Its maximum tolerated dose (MTD) was determined from a safety perspective in a combination study with capecitabine; however, its inhibitory effects on DPD activity were not assessed in the study. The dose justification to select its MTD as the recommended dose in terms of DPD inhibition has been required, but the autoinduction profile of TAS-114 made it difficult. To this end, an approach using a population pharmacokinetic (PPK)/pharmacodynamic (PD) model incorporating autoinduction was planned; however, the utility of this approach in the dose justification has not been reported. Thus, the aim of this study was to demonstrate the utility of a PPK/PD model incorporating autoinduction in the dose justification via a case study of TAS-114. Plasma concentrations of TAS-114 from 185 subjects and those of the endogenous DPD substrate uracil from 24 subjects were used. A two-compartment model with first-order absorption with lag time and an enzyme turnover model were selected for the pharmacokinetic (PK) model. Moreover, an indirect response model was selected for the PD model to capture the changes in plasma uracil concentrations. Model-based simulations provided the dose justification that DPD inhibition by TAS-114 reached a plateau level at the MTD, whereas exposures of TAS-114 increased dose dependently. Thus, the utility of a PPK/PD model incorporating autoinduction in the dose justification was demonstrated via this case study of TAS-114.

Pharmacokinetics of S-1 monotherapy in plasma and in tears for gastric cancer patients.

BACKGROUND: S-1 is an oral anticancer drug composed of tegafur (FT), which is a prodrug of 5-FU, 5-chloro-2,4-dihydroxypyridine (CDHP), and potassium oxonate. Recently, some studies have been reported on watering eyes caused by S-1. However, the mechanism of watering eyes caused by S-1 is still unclear. The aim of this study was to investigate the correlation between tears and plasma concentrations of FT, 5-FU, and CDHP, which are components and active modulator of S-1. METHODS: We prospectively investigated the pharmacokinetics (PK) of FT, 5-FU, and CDHP in plasma and in tears of gastric cancer patients who were treated with S-1 monotherapy at the dose of 80 mg/m2/day. Plasma and tears from both eyes were obtained 1, 2, 4, and 8 h after S-1 administration on day 1 and 14 of the first cycle. RESULTS: Total of eight patients were enrolled. All the FT, 5-FU and CDHP were detected both in plasma and in tears, and their PK parameters were measured. There was a positive correlation between the concentrations of FT, 5-FU and CDHP in the plasma and those in the tears on day 1 and day 14 (correlation coefficients r, right eye/left eye: r = 0.882/0.878, 0.877/0.890, and 0.885/0.878, respectively). CONCLUSION: There was a positive correlation between the concentrations of FT, 5-FU and CDHP in the plasma and those in the tears. The result is expected to facilitate the further investigation into the causes of watering eyes and the establishment of the effective methods for the prevention and the treatment.

TAS-114, a First-in-Class Dual dUTPase/DPD Inhibitor, Demonstrates Potential to Improve Therapeutic Efficacy of Fluoropyrimidine-Based Chemotherapy.

5-Fluorouracil (5-FU) is an antimetabolite and exerts antitumor activity via intracellularly and physiologically complicated metabolic pathways. In this study, we designed a novel small molecule inhibitor, TAS-114, which targets the intercellular metabolism of 5-FU to enhance antitumor activity and modulates catabolic pathway to improve the systemic availability of 5-FU. TAS-114 strongly and competitively inhibited deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase), a gatekeeper protein preventing aberrant base incorporation into DNA, and enhanced the cytotoxicity of fluoropyrimidines in cancer cells; however, it had little intrinsic activity. In addition, TAS-114 had moderate and reversible inhibitory activity on dihydropyrimidine dehydrogenase (DPD), a catabolizing enzyme of 5-FU. Thus, TAS-114 increased the bioavailability of 5-FU when coadministered with capecitabine in mice, and it significantly improved the therapeutic efficacy of capecitabine by reducing the required dose of the prodrug by dual enzyme inhibition. Enhancement of antitumor efficacy caused by the addition of TAS-114 was retained in the presence of a potent DPD inhibitor containing oral fluoropyrimidine (S-1), indicating that dUTPase inhibition plays a major role in enhancing the antitumor efficacy of fluoropyrimidine-based therapy. In conclusion, TAS-114, a dual dUTPase/DPD inhibitor, demonstrated the potential to improve the therapeutic efficacy of fluoropyrimidine. Dual inhibition of dUTPase and DPD is a novel strategy for the advancement of oral fluoropyrimidine-based chemotherapy for cancer treatment. Mol Cancer Ther; 17(8); 1683-93. ©2018 AACR.

Contribution of equilibrative nucleoside transporter(s) to intestinal basolateral and apical transports of anticancer trifluridine.

Trifluridine (FTD) exhibits anticancer activities after its oral administration despite its hydrophilic nature. It was previously reported that concentrative nucleoside transporter (CNT) 1 mediates the apical uptake of FTD in human small intestinal epithelial cells (HIECs). In the present study, FTD was also identified as a substrate for equilibrative nucleoside transporter (ENT) 1 and ENT2 in transporter gene-transfected cells. An immunocytochemical analysis revealed that ENT1 was expressed at the basolateral and apical membranes of HIECs. Cellular accumulation increased in the presence of S-(4-nitrobenzyl)-6-thioinosine (NBMPR), an ENT selective inhibitor. Cytotoxicity in HIEC monolayers at low FTD concentrations was increased by NBMPR, and this may have been due to inhibition of the ENT-mediated basolateral transport of FTD by NBMPR. These results suggest that ENTs reduce the intestinal cytotoxicity of FTD by facilitating its basolateral efflux. On the other hand, the intracellular accumulation and cytotoxicity of FTD in HIECs were decreased at higher concentrations of FTD by NBMPR, and this may have been due to the NBMPR inhibition of the apical uptake of FTD, which has been suggested to be mediated by CNTs and ENTs. In conclusion, ENTs were responsible for intestinal transepithelial permeation by mediating the basolateral efflux of FTD after its uptake by CNT1 from the apical side, resulting in decreases in its intracellular accumulation and intestinal toxicity in humans. Equilibrative nucleoside transporters may also partially contribute to the low-affinity uptake of FTD across the apical membrane along with high-affinity CNT1.

Pharmacokinetics of initial full and subsequent reduced doses of S-1 in patients with locally advanced head and neck cancer-effect of renal insufficiency.

BACKGROUND: S-1 is a combination of tegafur [metabolized to 5-fluorouracil (5-FU)] with the modulators gimeracil (5-chloro-2,4-dihydroxypyridine) and oteracil potassium. 5-Chloro-2,4-dihydroxypyridine maintains plasma 5-FU concentrations by inhibiting dihydropyrimidine dehydrogenase, a pyrimidine catabolism enzyme that degrades 5-FU. As 50% of 5-chloro-2,4-dihydroxypyridine is excreted in urine, renal insufficiency may increase its blood level, increasing 5-FU concentrations. We investigated whether special dose modification is needed in the presence of renal insufficiency. OBJECTIVE: We compared steady state pharmacokinetics of 5-FU for the initial S-1 dose and reduced doses in patients with head and neck cancer requiring dose reduction due to renal and non-renal toxicities. METHODS: Chemoradiotherapy with S-1 and cisplatin was administered every 5 weeks for two courses with a radiation dose totaling 70 Gy over 33-35 fractions. Two additional courses of adjuvant chemotherapy were administered in the case of an objective response. The S-1 and/or cisplatin dose was reduced in response to renal, hematologic or other toxicities. The primary endpoint was the change in area under the plasma concentration-versus-time curve from time 0-10 hours (5-FU AUCss 0-10) between the initial and reduced S-1 doses. RESULTS: Although the mean 5-FU levels in patients with non-renal toxicities significantly decreased between the full and reduced dose, the full-dose and reduced-dose mean maximum 5-FU plasma concentrations at steady state (Css max) and AUCss 0-10 in patients with renal insufficiency were similar. CONCLUSIONS: Standard S-1 dose reduction for renal toxicity did not result in a significant decrease in 5-FU levels at steady state. A greater reduction to lower plasma 5-chloro-2,4-dihydroxypyridine may be necessary in patients with renal insufficiency.

Phase I clinical and pharmacokinetic study of S-1 plus oral leucovorin in patients with metastatic colorectal cancer.

PURPOSE: S-1 has shown a response rate of 35% in chemonaïve patients with metastatic colorectal cancer (mCRC). Leucovorin enhances the antitumor activity of 5-fluorouracil, and concurrent oral administration of S-1 and leucovorin may represent a more active treatment option for mCRC. METHODS: S-1 (35 mg/m2) and leucovorin (25 mg/body) were orally administered twice daily to chemonaïve patients with mCRC. Predefined dose (schedule)-limiting toxicities (DLTs) during the first course and treatment continuity during the first two courses were evaluated during three periods of treatment with S-1 plus leucovorin (level 0, 2 weeks; level 1, 3 weeks; and level 2, 4 weeks), each followed by a 2-week rest. The pharmacokinetics (PK) of S-1 and leucovorin were studied on days 1 and 14 of the first course. RESULTS: Fifteen patients were enrolled. All three patients had DLTs at level 2, and this level was considered the maximum tolerated schedule. Level 0 was designated as the recommended schedule based on the incidences of DLTs and treatment continuity. The main toxic effects were gastrointestinal, such as diarrhea and stomatitis. There was no grade 4 adverse event or treatment-related death. The overall response rate was 67% (95% confidence interval, 38-88%). The PK profiles of S-1 plus leucovorin were similar to those in previous studies. CONCLUSIONS: The recommended schedule was 2 weeks of S-1 plus leucovorin followed by a 2-week rest. The increased response and gastrointestinal toxicities of S-1 plus leucovorin as compared with S-1 monotherapy suggest that co-administration of leucovorin enhanced the activity of S-1.

Exposure-dependent incorporation of trifluridine into DNA of tumors and white blood cells in tumor-bearing mouse.

PURPOSE: Trifluridine (TFT) is an antitumor component of a novel nucleoside antitumor agent, TAS-102, which consists of TFT and tipiracil hydrochloride (thymidine phosphorylase inhibitor). Incorporation of TFT into DNA is a probable mechanism of antitumor activity and hematological toxicity. The objective of this study was to examine the TFT incorporation into tumor- and white blood cell-DNA, and to elucidate the mechanism of TFT-related effect and toxicity. TFT effect on the colony formation of mouse bone marrow cells was also investigated. METHODS: Pharmacokinetics of TFT was determined in nude mice after single oral administration of TAS-102, while the antitumor activity and body weight change were evaluated in the tumor-bearing nude mice after multiple oral administrations for 2 weeks. TFT concentrations in the blood- and tumor-DNA were determined by LC/MS/MS. The colony formation was evaluated by CFU-GM assay. RESULTS: TFT systemic exposure in plasma increased dose-dependently. The tumor growth rate and body weight gain decreased dose-dependently, but TFT concentrations in the DNA of tumor tissues and white blood cells increased dose-dependently. TFT inhibited colony formation of bone marrow cells in a concentration-dependent manner. CONCLUSIONS: A significant relationship between systemic exposure of TFT and pharmacological effects including the antitumor activity and body weight change was well explained by the TFT incorporation into DNA. TFT inhibited proliferations of mouse bone marrow cells and human colorectal carcinoma cells implanted to nude mice dose-dependently. The highest tolerable TFT exposure provides the highest antitumor activity, and the hematological toxicity may serve as a potential surrogate indicator of TAS-102 efficacy.

Involvement of Concentrative Nucleoside Transporter 1 in Intestinal Absorption of Trifluridine Using Human Small Intestinal Epithelial Cells.

TAS-102, which is effective for refractory metastatic colorectal cancer, is a combination drug of anticancer trifluridine (FTD; which is derived from pyrimidine nucleoside) and FTD-metabolizing enzyme inhibitor tipiracil hydrochloride (TPI) at a molecular ratio of 1:0.5. To evaluate the intestinal absorption mechanism of FTD, the uptake and transcellular transport of FTD by human small intestinal epithelial cell (HIEC) monolayer as a model of human intestinal epithelial cells was investigated. The uptake and membrane permeability of FTD by HIEC monolayers were saturable, Na(+) -dependent, and inhibited by nucleosides. These transport characteristics are mostly comparable with those of concentrative nucleoside transporters (CNTs). Moreover, the uptake of FTD by CNT1-expressing Xenopus oocytes was the highest among human CNT transporters. The obtained Km and Vmax values of FTD by CNT1 were 69.0 µM and 516 pmol/oocyte/30 min, respectively. The transcellular transport of FTD by Caco-2 cells, where CNT1 is heterologously expressed, from apical to basolateral side was greater than that by Mock cells. In conclusion, these results demonstrated that FTD exhibits high oral absorption by the contribution of human CNT1.

Species variation in the enantioselective metabolism of tegafur to 5-fluorouracil among rats, dogs and monkeys.

OBJECTIVES: Tegafur (FT), a pro-drug of 5-fluorouracil (5-FU), is a racemate consisting of two enantiomers, R and S-FT. The aim of this study was to clarify interspecies variation in the enantioselective metabolism of FT. METHODS: Plasma concentrations of FT enantiomers were determined in rats, dogs and monkeys following intravenous and oral dosing of the racemate (5 mg/kg). In addition, the enzymatic conversion of FT enantiomers to 5-FU was assayed using hepatic preparations. KEY FINDINGS: Metabolic clearance of R-FT was higher than that of S-FT in rats and monkeys, but S-FT was the preferential substrate for dogs. An inhibition study revealed that cytochrome P450 is primarily responsible for the enantioselective metabolism of FT in rats and dogs. In contrast, in monkeys, thymidine phosphorylase was a determinant of the enantioselectivity in FT metabolism. Although oral bioavailability was not enantioselective, in-vitro and in-vivo kinetic studies suggested that the enantioselectivity in the hepatic intrinsic clearance of FT directly influences the body clearance in all animal species examined. CONCLUSIONS: The interspecies variations were observed in the enantioselective pharmacokinetics of FT, and the in-vivo enantioselectivity could be extrapolated from the in-vitro metabolic activities.

Effect of CYP2A6 genetic polymorphism on the metabolic conversion of tegafur to 5-fluorouracil and its enantioselectivity.

Tegafur (FT), a prodrug of 5-fluorouracil, is a chiral molecule, a racemate of R- and S-isomers, and CYP2A6 plays an important role in the enantioselective metabolism of FT in human liver microsomes (R-FT >> S-FT). This study examined the enantioselective metabolism of FT by microsomes prepared from Sf9 cells expressing wild-type CYP2A6 and its variants (CYP2A6*7, *8, *10, and *11) that are highly prevalent in the Asian population. We also investigated the metabolism of coumarin and nicotine, both CYP2A6 probe drugs, in these variants. Enzyme kinetic analyses showed that CYP2A6.7 (I471T) and CYP2A6.10 (I471T and R485L) had markedly lower Vmax values for both enantiomers than wild-type enzyme (CYP2A6.1) and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both enantiomers than CYP2A6.1. The ratios of Vmax and Km values for R-FT to corresponding values for S-FT (R/S ratio) were similar among enzymes, indicating little difference in enantioselectivity among the wild-type and variant enzymes. Similarly, both CYP2A6.7 and CYP2A6.10 had markedly lower Vmax values for coumarin 7-hydroxylase and nicotine C-oxidase activities than CYP2A6.1 and other variant enzymes, whereas Km values were higher in most of the variant enzymes for both activities than CYP2A6.1. In conclusion, the amino acid substitutions in CYP2A6 variants generally resulted in lower affinity for substrates, while Vmax values were selectively reduced in CYP2A6.7 and CYP2A6.10. Consistent R/S ratios among CYP2A6.1 and variant enzymes indicated that the amino acid substitutions had little effect on enantioselectivity in the metabolism of FT.

First-in-human, phase I dose-escalation study of single and multiple doses of a first-in-class enhancer of fluoropyrimidines, a dUTPase inhibitor (TAS-114) in healthy male volunteers.

PURPOSE: TAS-114 is a first-in-class oral deoxyuridine triphosphatase (dUTPase) inhibitor, which acts as a modulator of the pyrimidine nucleotide metabolic pathway. This was a first-in-human, phase 1 study that investigated the pharmacokinetics (PK) and safety of single-agent TAS-114 when it was given at single and multiple doses. METHODS: For the single-dose cohort (n = 25), healthy male volunteers received a single dose of TAS-114 at 6, 18, 60, 150, and 300 mg. The magnitude of dihydropyrimidine dehydrogenase (DPD) inhibition and the food effect on TAS-114 PK were also investigated. For the multiple-dose cohort (n = 10), subjects received TAS-114 for 14 days consecutively. RESULTS: In the dose-escalating single-dose cohort, the disposition of TAS-114 followed linear kinetics. The elimination half-life was approximately 2 h. The urine excretion rate and food effect were minimal. A significant increase in uracil Cmax was observed at administered doses of 150 mg or higher of TAS-114, suggesting that significant inhibition of DPD occurred at these doses. No apparent CYP3A4 auto-induction was observed in the multiple-dose cohort. No significant safety concerns at these dose levels were noted after single and multiple dosing. CONCLUSIONS: TAS-114 has shown both a favorable safety and pharmacokinetic profile after single and repeated doses. TAS-114 was considered to possess a moderate DPD inhibitory effect. These findings will facilitate clinical studies of the combination chemotherapies in cancer patients and may reduce the safety risk in the frail cancer patients.

Enantioselectivity in the cytochrome P450-dependent conversion of tegafur to 5-fluorouracil in human liver microsomes.

Tegafur (FT) is a prodrug of 5-fluorouracil (5-FU) used in cancer chemotherapy, and the bioactivation of FT to 5-FU is mainly catalyzed by cytochrome P450 (CYP) in hepatic microsomes. FT has a chiral center and is a racemate consisting of the enantiomers, R- and S-FT. In the present study, we clarified the enantioselectivity in the conversion of FT to 5-FU and identified human CYP isoforms involved in the metabolism of its enantiomers using human hepatic preparations and recombinant CYP isoforms. Although 5-FU was generated from both FT enantiomers, R-FT was a preferred substrate than S-FT, because of the considerably higher intrinsic clearance for 5-FU formation from R-FT in liver. Eadie-Hofstee plots in microsomes showed that the conversions of R- and S-FT to 5-FU followed biphasic and monophasic kinetics, respectively. Based on the evaluation using cDNA-expressed enzymes, CYP2A6 showed the highest activity for 5-FU formation from R-FT with the K m value similar to that of the high-affinity component in microsomes. Also, CYP2A6 was the most effective catalyst for S-FT. Inhibition studies using CYP-selective inhibitors and anti-CYP antibodies demonstrated that CYP2A6 mainly contributed to the enantioselective metabolism of FT, and were almost in accordance with the relative percentage contribution of each CYP isoform to the metabolism of FT estimated using relative activity factor methods. These results suggest that the enantioselectivity in the bioactivation of FT to 5-FU in humans is mainly due to the large difference of the catalytic activity of CYP2A6 between R- and S-FT.

Involvement of concentrative nucleoside transporter 1 in intestinal absorption of trifluorothymidine, a novel antitumor nucleoside, in rats.

ααα-Trifluorothymidine (TFT), an anticancer nucleoside analog, is a potent thymidylate synthase inhibitor. TFT exerts its antitumor activity primarily by inducing DNA fragmentation after incorporation of the triphosphate form of TFT into the DNA. Although an oral combination of TFT and a thymidine phosphorylase inhibitor has been clinically developed, there is little information regarding TFT absorption. Therefore, we investigated TFT absorption in the rat small intestine. After oral administration of TFT in rats, more than 75% of the TFT was absorbed. To identify the uptake transport system, uptake studies were conducted by using everted sacs prepared from rat small intestines. TFT uptake was saturable, significantly reduced under Na(+)-free conditions, and strongly inhibited by the addition of an endogenous pyrimidine nucleoside. From these results, we suggested the involvement of concentrative nucleoside transporters (CNTs) in TFT absorption into rat small intestine. In rat small intestines, the mRNAs coding for rat CNT1 (rCNT1) and rCNT2, but not for rCNT3, were predominantly expressed. To investigate the roles of rCNT1 and rCNT2 in TFT uptake, we conducted uptake assays by using Xenopus laevis oocytes injected with rCNT1 complementary RNA (cRNA) and rCNT2 cRNA. TFT uptake by X. laevis oocytes injected with rCNT1 cRNA, and not rCNT2 cRNA, was significantly greater than that by water-injected oocytes. In addition, in situ single-pass perfusion experiments performed using rat jejunum regions showed that thymidine, a substrate for CNT1, strongly inhibited TFT uptake. In conclusion, TFT is absorbed via rCNT1 in the intestinal lumen in rats.

Formation pathways of gamma-butyrolactone from the furan ring of tegafur during its conversion to 5-fluorouracil.

Tegafur (FT) is a 5-fluorouracil (5-FU) prodrug that has been clinically used for various cancer chemotherapies. The following metabolites of FT were identified in patients: 5-FU, fluoro-beta-alanine, and gamma-butyrolactone (GBL) and its acidic form, gamma-hydroxybutyrate (GHB). GBL/GHB, which is probably generated from the furan ring of FT, inhibits tumor cell angiogenesis, contributing to the antitumor effect of FT-based therapies. In the present study, we identified the metabolites formed from the furan ring of FT by CYP2A6 and thymidine phosphorylase (TPase) using 2,4-dinitrophenylhydrazine derivatization procedures and clarified the metabolic pathway of FT to GBL/GHB. Succinaldehyde (SA) and 4-hydroxybutanal (4-OH-BTL) were produced as the metabolites because of the cleavage of the furan ring of FT during its conversion to 5-FU in cDNA-expressed CYP2A6 and purified TPase, respectively; however, GBL/GHB was hardly detected in cDNA-expressed CYP2A6 and purified TPase. GBL/GHB was formed after human hepatic microsomes or cDNA-expressed CYP2A6 mixed with cytosol were incubated with FT. Furthermore, 4-OH-BTL was converted to GBL/GHB in the microsomes and cytosol. These results suggest that GBL/GHB is generated from FT through the formation of SA and 4-OH-BTL but not directly from FT. Furthermore, the amount of 5-FU and GBL/GHB formed in the hepatic S9 was markedly decreased in the presence of a CYP2A6 inhibitor, suggesting that GBL/GHB may be mainly generated through the CYP2A6-mediated formation of SA.

Effect of dimethylnitrosamine-induced liver dysfunction on the pharmacokinetics of 5-fluorouracil after administration of S-1, an antitumour drug, to rats.

OBJECTIVES: The anti-tumour agent S-1 comprises tegafur (a prodrug of 5-fluorouracil; 5-FU), gimeracil (2-chloro-2,4-dihydroxypyridine (CDHP); a competitive inhibitor of 5-FU metabolism) and oteracil potassium. The effect of hepatic dysfunction induced by dimethylnitrosamine (DMN) on the pharmacokinetics of 5-FU after administration of S-1 to rats was investigated. METHODS: S-1 (5 mg/kg) was administered intravenously and orally to rats with DMN-induced liver dysfunction. Plasma concentrations of S-1 components and 5-FU were measured by HPLC and LC/MS-MS. Blood tests and in-vitro enzymatic investigations were also conducted. KEY FINDINGS: DMN treatment induced hepatic dysfunction and decreased the conversion of tegafur to 5-FU in the liver without altering renal function or dihydropyrimidine dehydrogenase activity. Following intravenous administration of S-1, the blood concentration-time profiles of CDHP were similar between control rats and rats with hepatic dysfunction, but the half-life of tegafur was significantly prolonged. The maximum plasma concentration (C(max)) of 5-FU was significantly reduced and the area under the blood concentration-time curve (AUC) was reduced by 22%. Following oral administration, the C(max) of tegafur, 5-FU and CDHP were significantly decreased and half-lives significantly increased. Hepatic dysfunction had a less pronounced effect on the AUC of 5-FU (13.6% reduction). CONCLUSIONS: The pharmacokinetic profiles of tegafur, 5-FU and CDHP were altered by changes in the elimination rate of tegafur induced by a decrease in the conversion of tegafur to 5-FU. However, hepatic dysfunction had less of an effect on the AUC of 5-FU, which correlates with anti-tumour effect, after the oral administration of S-1.

The quantitative prediction of CYP-mediated drug interaction by physiologically based pharmacokinetic modeling.

PURPOSE: The objective is to confirm if the prediction of the drug-drug interaction using a physiologically based pharmacokinetic (PBPK) model is more accurate. In vivo Ki values were estimated using PBPK model to confirm whether in vitro Ki values are suitable. METHOD: The plasma concentration-time profiles for the substrate with coadministration of an inhibitor were collected from the literature and were fitted to the PBPK model to estimate the in vivo Ki values. The AUC ratios predicted by the PBPK model using in vivo Ki values were compared with those by the conventional method assuming constant inhibitor concentration. RESULTS: The in vivo Ki values of 11 inhibitors were estimated. When the in vivo Ki values became relatively lower, the in vitro Ki values were overestimated. This discrepancy between in vitro and in vivo Ki values became larger with an increase in lipophilicity. The prediction from the PBPK model involving the time profile of the inhibitor concentration was more accurate than the prediction by the conventional methods. CONCLUSION: A discrepancy between the in vivo and in vitro Ki values was observed. The prediction using in vivo Ki values and the PBPK model was more accurate than the conventional methods.