Elucidating nonlinear pharmacokinetics of telmisartan: Integration of target-mediated drug disposition and OATP1B3-mediated hepatic uptake in a physiologically based model.

Telmisartan, a selective inhibitor of angiotensin II receptor type 1 (AT1), demonstrates nonlinear pharmacokinetics (PK) when orally administered in ascending doses to healthy volunteers, but the underlying mechanisms remain unclear. This study presents a physiologically based pharmacokinetic model integrated with target-mediated drug disposition (TMDD-PBPK model) to explore the mechanism of its nonlinear PK. We employed the Cluster-Gauss Newton method for top-down analysis, estimating the in vivo Km,OATP1B3 (Michaelis-Menten constant for telmisartan hepatic uptake via Organic Anion Transporting Polypeptide 1B3) to be 2.0-5.7 nM. This range is significantly lower than the reported in vitro value of 810 nM, obtained in 0.3% human serum albumin (HSA) conditions. Further validation was achieved through in vitro assessment in plated human hepatocytes with 4.5% HSA, showing a Km of 4.5 nM. These results underscore the importance of albumin-mediated uptake effect for the hepatic uptake of telmisartan. Our TMDD-PBPK model, developed through a "middle-out" approach, underwent sensitivity analysis to identify key factors in the nonlinear PK of telmisartan. We found that the nonlinearity in the area under the concentration-time curve (AUC) and/or maximum concentration (Cmax) of telmisartan is sensitive to Km,OATP1B3 across all dosages. Additionally, the dissociation constant (Kd) for telmisartan binding to the AT1 receptor, along with its receptor abundance, notably influences PK at lower doses (below 20 mg). In conclusion, the nonlinear PK of telmisartan appears primarily driven by hepatic uptake saturation across all dose ranges and by AT1-receptor binding saturation, notably at lower doses.

A Simple Decision Tree Suited for Identification of Early Oral Drug Candidates With Likely Pharmacokinetic Nonlinearity by Intestinal CYP3A Saturation.

To identify oral drugs that likely display nonlinear pharmacokinetics due to saturable metabolism by intestinal CYP3A, our previous report using CYP3A substrate drugs proposed an approach using thresholds for the linear index number (LIN3A = dose/Km; Km, Michaelis-Menten constant for CYP3A) and the intestinal availability (FaFg). Here, we aimed to extend the validity of the previous approach using both CYP3A substrate and non-substrate drugs and to devise a decision tree suited for early drug candidates using in vitro metabolic intrinsic clearance (CLint, vitro) instead of FaFg. Out of 152 oral drugs (including 136 drugs approved in Japan, US or both), type I nonlinearity (in which systemic drug exposure increases in a more than dose-proportional manner) was noted with 82 drugs (54%), among which 58 drugs were identified as CYP3A substrates based on public information. Based on practical feasibility, 41 drugs were selected from CYP3A substrates and subjected to in-house metabolic assessment. The results were used to determine the thresholds for CLint, vitro (0.45 µL/min/pmol CYP3A4) and LIN3A (1.0 L). For four drugs incorrectly predicted, potential mechanisms were looked up. Overall, our proposed decision tree may aid in the identification of early drug candidates with intestinal CYP3A-derived type I nonlinearity.

Microdosing clinical study to clarify pharmacokinetic and pharmacogenetic characteristics of atorvastatin in Japanese hypercholesterolemic patients.

The present study investigated whether the clinical pharmacokinetics of atorvastatin can be predicted from the results of microdosing study in Japanese patients with hypercholesterolemia whose SLCO1B1 and ABCG2 polymorphisms were analyzed. Forty seven statin-naive patients clinically indicated to LDL cholesterol-lowering therapy with atorvastatin were enrolled in a two-period crossover study. Microdose (100 µg) of atorvastatin was orally administered followed by therapeutic dose (10 mg) administration. Transport studies were performed with BCRP-expressing membrane vesicles. The dose-normalized plasma AUC following the therapeutic dose of atorvastatin was positively correlated with that following its microdose, but the AUC increased more than dose proportionally from microdose to therapeutic dose. The patients carrying SLCO1B1 c.521TC showed a significantly higher AUC compared with those carrying c.521TT following the microdose (175%) and therapeutic dose (139%). On the other hand, SLCO1B1 c.388G or ABCG2 c.421A variant alleles did not significantly affect the pharmacokinetics of atorvastatin. Atorvastatin showed ATP-dependent transport in BCRP-expressing membrane vesicles and it inhibited rosuvastatin transport with Ki of 6.3 ± 2.9 µM (mean ± SD). Microdosing study appears to be feasible to roughly estimate the pharmacokinetic and pharmacogenetic profiles of atorvastatin following the oral therapeutic dose in hypercholesterolemic patients.

Physiologically Based Pharmacokinetic Modeling of Bosentan Identifies the Saturable Hepatic Uptake As a Major Contributor to Its Nonlinear Pharmacokinetics.

Bosentan is a substrate of hepatic uptake transporter organic anion-transporting polypeptides (OATPs), and undergoes extensive hepatic metabolism by cytochrome P450 (P450), namely, CYP3A4 and CYP2C9. Several clinical investigations have reported a nonlinear relationship between bosentan doses and its systemic exposure, which likely involves the saturation of OATP-mediated uptake, P450-mediated metabolism, or both in the liver. Yet, the underlying causes for the nonlinear bosentan pharmacokinetics are not fully delineated. To address this, we performed physiologically based pharmacokinetic (PBPK) modeling analyses for bosentan after its intravenous administration at different doses. As a bottom-up approach, PBPK modeling analyses were performed using in vitro kinetic parameters, other relevant parameters, and scaling factors. As top-down approaches, three different types of PBPK models that incorporate the saturation of hepatic uptake, metabolism, or both were compared. The prediction from the bottom-up approach (models 1 and 2) yielded blood bosentan concentration-time profiles and their systemic clearance values that were not in good agreement with the clinically observed data. From top-down approaches (models 3, 4, 5-1, and 5-2), the prediction accuracy was best only with the incorporation of the saturable hepatic uptake for bosentan. Taken together, the PBPK models for bosentan were successfully established, and the comparison of different PBPK models identified the saturation of the hepatic uptake process as a major contributing factor for the nonlinear pharmacokinetics of bosentan.

Virtual Clinical Studies to Examine the Probability Distribution of the AUC at Target Tissues Using Physiologically-Based Pharmacokinetic Modeling: Application to Analyses of the Effect of Genetic Polymorphism of Enzymes and Transporters on Irinotecan Induced Side Effects.

PURPOSE: To establish a physiologically-based pharmacokinetic (PBPK) model for analyzing the factors associated with side effects of irinotecan by using a computer-based virtual clinical study (VCS) because many controversial associations between various genetic polymorphisms and side effects of irinotecan have been reported. METHODS: To optimize biochemical parameters of irinotecan and its metabolites in the PBPK modeling, a Cluster Newton method was introduced. In the VCS, virtual patients were generated considering the inter-individual variability and genetic polymorphisms of enzymes and transporters. RESULTS: Approximately 30 sets of parameters of the PBPK model gave good reproduction of the pharmacokinetics of irinotecan and its metabolites. Of these, 19 sets gave relatively good description of the effect of UGT1A1 *28 and SLCO1B1 c.521T>C polymorphism on the SN-38 plasma concentration, neutropenia, and diarrhea observed in clinical studies reported mainly by Teft et al. (Br J Cancer. 112(5):857-65, 20). VCS also indicated that the frequency of significant association of biliary index with diarrhea was higher than that of UGT1A1 *28 polymorphism. CONCLUSION: The VCS confirmed the importance of genetic polymorphisms of UGT1A1 *28 and SLCO1B1 c.521T>C in the irinotecan induced side effects. The VCS also indicated that biliary index is a better biomarker of diarrhea than UGT1A1 *28 polymorphism.

Effects of Cremophor EL on the absorption of orally administered saquinavir and fexofenadine in healthy subjects.

Modulation of CYP3A and/or P-gp function by several excipients has been reported. However, relatively few studies have investigated their effects in humans. Therefore, the aim of this clinical study was to clarify the effects of Cremophor EL on the inhibition of CYP3A and P-gp in the human small intestine. Eight healthy Japanese subjects received an oral dose of saquinavir (2 mg, substrate of P-gp/CYP3A) or fexofenadine (50 µg, substrate of P-gp) without or with Cremophor EL (720 mg and 1440 mg). Significant increases in Cmax (1.3-fold) and AUC0-24 (1.6-fold) were observed for fexofenadine when administered with 1440 mg of Cremophor EL. In contrast, a significant decrease was observed for saquinavir when administered with 720 mg of Cremophor EL. The equilibrium dialysis experiment was performed to investigate the micellar interaction between Cremophor EL and drugs. The equilibrium dialysis study showed that saquinavir was far extensively entrapped into the micelles. The reduced concentration of free saquinavir by entrapping in micelles was considered to cause the reduction of systemic exposure for saquinavir. In conclusion, this clinical study suggests that Cremophor EL at least inhibits P-gp in the human small intestine.

Analysis of the pharmacokinetic boosting effects of ritonavir on oral bioavailability of drugs in mice.

  Ritonavir dramatically increases the bioavailability of a variety of concurrently administered drugs by inhibition of metabolic enzymes and drug transporters. The purpose of this study was to investigate the extent to which ritonavir's inhibition of drug transporters and/or CYP3A contributes to the increased oral bioavailability in mice. The area under the plasma concentration-time curves (AUC) for orally administered saquinavir after coadministration with 50 mg/kg ritonavir dramatically increased (325-fold). As a result, the bioavailability, Fa·Fg and Fh increased 75-, 38- and twofold, respectively. In addition, the increase in the AUC predicted from the in vitro Ki value was ninefold, which was derived from the inhibition of metabolic enzymes by ritonavir in the liver. The remaining 36-fold increase in the AUC was considered to be derived from the inhibition in the small intestine. The AUCinf for probe substrate midazolam, fexofenadine, and pravastatin increased after the oral administration of ritonavir by only five-, 13-, and sevenfold, respectively. Moreover, the AUC0-12 for saquinavir was affected negligibly by itraconazole. These results indicate ritonavir mainly affects the first-pass effect of saquinavir in the small intestine, increasing the bioavailability of orally administered saquinavir. Furthermore, cyp isoforms other than CYP3A, which contribute to the metabolism of saquinavir in humans, are involved in the metabolism of saquinavir in mice.

Studies on the intestinal absorption characteristics of sulfasalazine, a breast cancer resistance protein (BCRP) substrate.

Oral sulfasalazine (SASP) is now used clinically as a probe substrate of a breast cancer resistance protein (BCRP) activity; however the intestinal absorption characteristics of SASP are not well understood. The purpose of this study was to clarify the characteristics of SASP transport in the mouse intestine. The everted ileum was incubated with SASP in the absence or presence of the Bcrp inhibitor Ko134. The steady-state intestinal absorptive clearance was 0.14 µL/min/cm in the absence of Ko134 and increased by 4.8-fold in the presence of Ko134. These results indicate that Bcrp mediates the efflux of SASP in the intestine. The absorptive clearance of SASP did not change in a concentration-dependent manner in the range of 0.1 to 50 µM in wild-type mice. By contrast, the absorptive clearance of SASP decreased significantly in a concentration-dependent manner in the presence of Ko134. Similar results were obtained in Bcrp(-/-) mice. These results suggest the possible involvement of some influx transporters in the intestinal absorption of SASP. In conclusion, both the influx and efflux transporters are involved in the intestinal absorption of SASP, which would explain why the absorptive clearance did not appear to change at various SASP concentrations in wild-type mice.