Application of the model-informed drug development paradigm to datopotamab deruxtecan dose selection for late-stage development.

To replace the conventional maximum tolerated dose (MTD) approach, a paradigm for dose optimization and dose selection that relies on model-informed drug development (MIDD) approaches has been proposed in oncology. Here, we report our application of an MIDD approach during phase I to inform dose selection for the late-stage development of datopotamab deruxtecan (Dato-DXd). Dato-DXd is a TROP2-directed antibody-drug conjugate being developed for advanced/metastatic non-small cell lung cancer (NSCLC) and other tumors. Data on pharmacokinetics (PKs), efficacy, and safety in NSCLC were collected in the TROPION-PanTumor01 phase I dose-expansion and -escalation study over a wide dose range of 0.27-10 mg/kg administered every 3 weeks. Population PK and exposure-response analyses were performed iteratively at three data cutoffs to inform dose selection. The 6 mg/kg dose was identified as the optimal dose by the second data cutoff analysis and confirmed by the subsequent third data cutoff analysis. The 6 mg/kg dose was more tolerable (i.e., lower rates of interstitial lung disease, stomatitis, and mucosal inflammation) than the MTD (8 mg/kg) and was more efficacious than 4 mg/kg (simulated mean objective response rate: 23.8% vs. 18.6%; mean hazard ratio of progression-free survival: 0.74) - a candidate dose studied just below 6 mg/kg. Therefore, the 6 mg/kg dose was judged to afford the optimal benefit-risk balance. This case study demonstrated the utility of an MIDD approach for dose optimization and dose selection.

A First-in-Human Phase I Study of Milademetan, an MDM2 Inhibitor, in Patients With Advanced Liposarcoma, Solid Tumors, or Lymphomas.

PURPOSE: This study evaluated the safety, pharmacokinetics, pharmacodynamics, and preliminary efficacy of milademetan, a small-molecule murine double minute-2 (MDM2) inhibitor, in patients with advanced cancers. PATIENTS AND METHODS: In this first-in-human phase I study, patients with advanced solid tumors or lymphomas received milademetan orally once daily as extended/continuous (days 1-21 or 1-28 every 28 days) or intermittent (days 1-7, or days 1-3 and 15-17 every 28 days) schedules. The primary objective was to determine the recommended phase II dose and schedule. Secondary objectives included tumor response according to standard evaluation criteria. Predefined analyses by tumor type were performed. Safety and efficacy analyses included all patients who received milademetan. RESULTS: Between July 2013 and August 2018, 107 patients were enrolled and received milademetan. The most common grade 3/4 drug-related adverse events were thrombocytopenia (29.0%), neutropenia (15.0%), and anemia (13.1%). Respective rates at the recommended dose and schedule (260 mg once daily on days 1-3 and 15-17 every 28 days, ie, 3/14 days) were 15.0%, 5.0%, and 0%. Across all cohorts (N = 107), the disease control rate was 45.8% (95% CI, 36.1 to 55.7) and median progression-free survival was 4.0 months (95% CI, 3.4 to 5.7). In the subgroup with dedifferentiated liposarcomas, the disease control rate and median progression-free survival were 58.5% (95% CI, 44.1 to 71.9) and 7.2 months overall (n = 53), and 62.0% (95% CI, 35.4 to 84.8) and 7.4 months with the recommended intermittent schedule (n = 16), respectively. CONCLUSION: An intermittent dosing schedule of 3/14 days of milademetan mitigates dose-limiting hematologic abnormalities while maintaining efficacy. Notable single-agent activity with milademetan in dedifferentiated liposarcomas has prompted a randomized phase III trial (MANTRA).

Population Pharmacokinetics of Patritumab Deruxtecan in Patients With Solid Tumors.

Patritumab deruxtecan is an antibody-drug conjugate consisting of a fully human monoclonal antibody against human epidermal growth factor receptor 3 (HER3) attached to a topoisomerase I inhibitor payload via a tetrapeptide-based cleavable linker. As part of the pharmacometric analysis informing dose selection for later-stage development, population pharmacokinetics (PK) analysis of patritumab deruxtecan was conducted with pooled serum PK data from patients with HER3-expressing solid tumors (from 3 phase 1/2 studies in breast, lung, and colorectal cancer; N = 425) treated over the dose range of 1.6 to 8.0 mg/kg intravenously every 3 weeks. Population PK modeling for deruxtecan (DXd)-conjugated antibody (representing patritumab deruxtecan) and unconjugated MAAA-1181a (DXd, payload) was carried out sequentially. DXd-conjugated antibody PK was described using a 2-compartment model with parallel linear and nonlinear clearance. Unconjugated DXd PK was described using a 1-compartment model with linear clearance and release of DXd as a first-order, time-dependent function of the level of DXd-conjugated antibody in the central compartment. Preliminary covariate evaluation was conducted for prespecified covariates of pharmacological plausibility and clinical interest. The final model retained weight (on linear clearance and central volume) and albumin level, sex, and tumor type (on linear clearance) for DXd-conjugated antibody, and weight (on release rate constant) and hepatic function (on clearance) for unconjugated DXd. Effects of these covariates on the exposure metrics were generally mild and did not require dose adjustment for subpopulations in subsequent development. Further PK characterization for patritumab deruxtecan will evolve with emerging data.

Ribociclib Population Pharmacokinetics and Pharmacokinetic/Pharmacodynamic Analysis of Neutrophils in Cancer Patients.

The population pharmacokinetics (popPK) of ribociclib and population pharmacokinetic/pharmacodynamic (PK/PD) relationship between ribociclib and absolute neutrophil count (ANC) were characterized in patients with cancer. PopPK and ANC PK/PD modeling were both conducted in 2 rounds per data availability. Initial models were developed based on data sets from early-phase trials and qualified using external data from the phase III MONALEESA-2 trial. The second round of analyses was performed using updated data sets that included 2 more phase III trials (MONALEESA-3 and -7). The popPK and ANC PK/PD models adequately described the data and demonstrated reasonable predictive ability. Covariate analysis showed that ribociclib PK were not affected by age, sex, race, baseline Eastern Cooperative Oncology Group (ECOG) status (grade 1), mild/moderate renal impairment, mild hepatic impairment, or concomitant use of combination partners, including aromatase inhibitors (letrozole, anastrozole) or fulvestrant, proton-pump inhibitors, or weak cytochrome P450 3A4/5 inhibitors. Body weight had no impact on ribociclib clearance to warrant dose adjustment. The ANC PK/PD relationship was not affected by age, weight, sex, race, baseline ECOG status (grade 1), or concomitant use of letrozole, anastrozole, or fulvestrant. The PK/PD analysis confirmed reversibility of ribociclib's effect on ANC; it also suggested that lowering the dose of ribociclib would mitigate ANC decrease and neutropenia risk. The popPK and ANC PK/PD analyses support the use of ribociclib in combination with an aromatase inhibitor or fulvestrant in patients with hormone receptor-positive, human epidermal growth factor receptor-2-negative advanced or metastatic breast cancer without dose adjustment in subpopulations, and the use of dose interruption/reduction to mitigate potential treatment-emergent neutropenia.

Model-Based Meta-Analysis Compares DAS28 Rheumatoid Arthritis Treatment Effects and Suggests an Expedited Trial Design for Early Clinical Development.

A nonlinear mixed effects modeling approach was used to conduct a model-based meta-analysis (MBMA) of longitudinal, summary-level, baseline-corrected 28-joint Disease Activity Score (ΔDAS28) clinical trial data from seven approved rheumatoid arthritis (RA) drugs (abatacept, adalimumab, certolizumab, etanercept, rituximab, tocilizumab, and tofacitinib), representing 130 randomized clinical trials in 27,355 patients. All of the drugs except tocilizumab were found to have relatively similar ΔDAS28 time courses and efficacy (baseline-corrected and placebo-corrected) at 24 weeks and beyond of approximately 0.87-1.3 units in the typical RA patient population. Tocilizumab was estimated to have a differentially greater response of 1.99 at 24 weeks, likely due to its disproportionate effect on the acute-phase cytokine interleukin-6. Baseline DAS28, disease duration, percentage of male participants, and the year of conduct of the trial were found to have statistically significant effects on the timing and/or magnitude of ΔDAS28 in the control arms. Clinical trial simulations using the present MBMA indicated that abatacept, certolizumab, etanercept, tocilizumab, and tofacitinib would be expected to have a greater than 70% probability of showing a statistically significant difference vs. control at Week 6 with a sample size of ~ 30 patients per arm. In future RA clinical trials, an interim analysis conducted as early as 6 weeks after treatment initiation, with relatively small sample sizes, should be sufficient to detect the ΔDAS28 treatment effect vs. placebo.

Ribociclib Bioavailability Is Not Affected by Gastric pH Changes or Food Intake: In Silico and Clinical Evaluations.

Ribociclib (KISQALI), a cyclin-dependent kinase 4/6 inhibitor approved for the first-line treatment of HR+/HER2- advanced breast cancer with an aromatase inhibitor, is administered with no restrictions on concomitant gastric pH-elevating agents or food intake. The influence of proton pump inhibitors (PPIs) on ribociclib bioavailability was assessed using 1) biorelevant media solubility, 2) physiologically based pharmacokinetic (PBPK) modeling, 3) noncompartmental analysis (NCA) of clinical trial data, and 4) population PK (PopPK) analysis. This multipronged approach indicated no effect of gastric pH changes on ribociclib PK and served as a platform for supporting ribociclib labeling language, stating no impact of gastric pH-altering agents on the absorption of ribociclib, without a dedicated drug-drug interaction trial. The bioequivalence of ribociclib exposure with or without a high-fat meal was demonstrated in a clinical trial. Lack of restrictions on ribociclib dosing may facilitate better patient compliance and therefore clinical benefit.

Use of systems pharmacology modeling to elucidate the operating characteristics of SGLT1 and SGLT2 in renal glucose reabsorption in humans.

In the kidney, glucose in glomerular filtrate is reabsorbed primarily by sodium-glucose cotransporters 1 (SGLT1) and 2 (SGLT2) along the proximal tubules. SGLT2 has been characterized as a high capacity, low affinity pathway responsible for reabsorption of the majority of filtered glucose in the early part of proximal tubules, and SGLT1 reabsorbs the residual glucose in the distal part. Inhibition of SGLT2 is a viable mechanism for removing glucose from the body and improving glycemic control in patients with diabetes. Despite demonstrating high levels (in excess of 80%) of inhibition of glucose transport by SGLT2 in vitro, potent SGLT2 inhibitors, e.g., dapagliflozin and canagliflozin, inhibit renal glucose reabsorption by only 30-50% in clinical studies. Hypotheses for this apparent paradox are mostly focused on the compensatory effect of SGLT1. The paradox has been explained and the role of SGLT1 demonstrated in the mouse, but direct data in humans are lacking. To further explore the roles of SGLT1/2 in renal glucose reabsorption in humans, we developed a systems pharmacology model with emphasis on SGLT1/2 mediated glucose reabsorption and the effects of SGLT2 inhibition. The model was calibrated using robust clinical data in the absence or presence of dapagliflozin (DeFronzo et al., 2013), and evaluated against clinical data from the literature (Mogensen, 1971; Wolf et al., 2009; Polidori et al., 2013). The model adequately described all four data sets. Simulations using the model clarified the operating characteristics of SGLT1/2 in humans in the healthy and diabetic state with or without SGLT2 inhibition. The modeling and simulations support our proposition that the apparent moderate, 30-50% inhibition of renal glucose reabsorption observed with potent SGLT2 inhibitors is a combined result of two physiological determinants: SGLT1 compensation and residual SGLT2 activity. This model will enable in silico inferences and predictions related to SGLT1/2 modulation.

Quantitative prediction of renal transporter-mediated clinical drug-drug interactions.

Kidney plays a critical role in the elimination of xenobiotics. Drug-drug interactions (DDIs) via inhibition of renal organic anion (OAT) and organic cation (OCT) transporters have been observed in the clinic. This study examined the quantitative predictability of renal transporter-mediated clinical DDIs based on basic and mechanistic models. In vitro transport and clinical pharmacokinetics parameters were used to quantitatively predict DDIs of victim drugs when coadministrated with OAT or OCT inhibitors, probenecid and cimetidine, respectively. The predicted changes in renal clearance (CLr) and area under the plasma concentration-time curve (AUC) were comparable to that observed in clinical studies. With probenecid, basic modeling predicted 61% cases within 25% and 94% cases within 50% of the observed CLr changes in clinic. With cimetidine, basic modeling predicted 61% cases within 25% and 92% cases within 50% of the observed CLr changes in clinic. Additionally, the mechanistic model predicted 54% cases within 25% and 92% cases within 50% of the observed AUC changes with probenecid. Notably, the magnitude of AUC changes attributable to the renal DDIs is generally less than 2-fold, unlike the DDIs associated with inhibition of CYPs and/or hepatic uptake transporters. The models were further used to evaluate the renal DDIs of Pfizer clinical candidates/drugs, and the overall predictability demonstrates their utility in the drug discovery and development settings.

Cerebrospinal fluid ß-Amyloid turnover in the mouse, dog, monkey and human evaluated by systematic quantitative analyses.

BACKGROUND: Reducing brain ß-amyloid (Aß) via inhibition of ß-secretase, or inhibition/modulation of γ-secretase, has been widely pursued as a potential disease-modifying treatment for Alzheimer's disease. Compounds that act through these mechanisms have been screened and characterized with Aß lowering in the brain and/or cerebrospinal fluid (CSF) as the primary pharmacological end point. Interpretation and translation of the pharmacokinetic (PK)/pharmacodynamic (PD) relationship for these compounds is complicated by the relatively slow Aß turnover process in these compartments. OBJECTIVE: To understand Aß turnover kinetics in preclinical species and humans. METHODS: We collected CSF Aß dynamic data after ß- or γ-secretase inhibitor treatment from in-house experiments and the public domain, and analyzed the data using PK/PD modeling to obtain CSF Aß turnover rates (kout) in the mouse, dog, monkey and human. RESULTS: The kout for CSF Aß40 follows allometry (kout = 0.395 × body weight(-0.351)). The kout for CSF Aß40 is approximately 2-fold higher than the turnover of CSF in rodents, but in higher species, the two are comparable. CONCLUSION: The turnover of CSF Aß40 was systematically examined, for the first time, in multiple species through quantitative modeling of multiple data sets. Our result suggests that the clearance mechanisms for CSF Aß in rodents may be different from those in the higher species. The understanding of Aß turnover has considerable implications for the discovery and development of Aß-lowering therapeutics, as illustrated from the perspectives of preclinical PK/PD characterization and preclinical-to-clinical translation.

Herb-drug pharmacokinetic interaction between radix astragali and pioglitazone in rats.

ETHNOPHARMACOLOGICAL RELEVANCE: Radix astragali (RA) was the most frequently used traditional Chinese medicine (TCM) according to the statistics on 52 anti-diabetic formulas recorded in New National Traditional Chinese Medicine; it was employed in 34 out of the 52 formulas. The aim of this study was to elucidate potential pharmacokinetic interaction between RA and pioglitazone, and to provide guidance for clinical medicine safety. MATERIALS AND METHODS: A specific and rapid UPLC-MS/MS method was established to quantify pioglitazone in rat plasma. Then healthy and type 2 diabetes mellitus (T2DM) rats were each divided into control and RA decoction (RAD) administration groups-healthy, healthy-RAD, T2DM, T2DM-RAD; pharmacokinetics of pioglitazone was carried out after RAD was administrated to rats for 7 days. RESULTS: The established UPLC-MS/MS method was rapid, specific, and precise. Between healthy and healthy-RAD groups, half-life (T(1/2)), area under the curve (AUC (0-t)), Vz/F, and Cl/F showed mild yet statistically significant differences; no significant difference for any above parameter was detected between T2DM and T2DM-RAD groups. CONCLUSION: RAD co-administration did not affect the pharmacokinetics of pioglitazone especially in diabetic groups; RA and pioglitazone might be feasible herb-drug co-effectiveness.

Integrating experimentation and quantitative modeling to enhance discovery of Beta amyloid lowering therapeutics for Alzheimer's disease.

Drug discovery can benefit from a proactive-knowledge-attainment philosophy which strategically integrates experimentation and pharmacokinetic/pharmacodynamic (PK/PD) modeling. Our programs for Alzheimer's disease (AD) illustrate such an approach. Compounds that inhibit the generation of brain beta amyloid (Aß), especially Aß42, are being pursued as potential disease-modifying therapeutics. Complexities in the PK/Aß relationship for these compounds have been observed and the data require an advanced approach for analysis. We established a semimechanistic PK/PD model that can describe the PK/Aß data by accounting for Aß generation and clearance. The modeling characterizes the in vivo PD (i.e., Aß lowering) properties of compounds and generates insights about the salient biological systems. The learning from the modeling enables us to establish a framework for predicting in vivo Aß lowering from in vitro parameters.

A novel UPLC-MS/MS method for simultaneous quantification of rhein, emodin, berberine and baicalin in rat plasma and its application in a pharmacokinetic study.

BACKGROUND: Dahuang Huanglian Xiexin Decoction (DHXD) is a classical formula in traditional Chinese medicines. In this study, a novel UPLC-MS/MS method was developed for the simultaneous quantification of rhein, emodin, berberine and baicalin, the major bioactive compounds of DHXD in rat plasma. RESULTS: The method possessed high sensitivity and ultrashort analysis time (7 min). Linearity, accuracy, precision and extraction recovery of four analytes were all satisfactory. The method was then successfully applied in a pharmacokinetic study of four bioactive components after a single oral administration of DHXD extract to rats. CONCLUSION: The method was applicable for simultaneous bioanalysis of rhein, emodin, berberine and baicalin.

Cerebrospinal fluid amyloid-ß (Aß) as an effect biomarker for brain Aß lowering verified by quantitative preclinical analyses.

Reducing the generation of amyloid-ß (Aß) in the brain via inhibition of ß-secretase or inhibition/modulation of γ-secretase has been pursued as a potential disease-modifying treatment for Alzheimer's disease. For the discovery and development of ß-secretase inhibitors (BACEi), γ-secretase inhibitors (GSI), and γ-secretase modulators (GSM), Aß in cerebrospinal fluid (CSF) has been presumed to be an effect biomarker for Aß lowering in the brain. However, this presumption is challenged by the lack of quantitative understanding of the relationship between brain and CSF Aß lowering. In this study, we strived to elucidate how the intrinsic pharmacokinetic (PK)/pharmacodynamic (PD) relationship for CSF Aß lowering is related to that for brain Aß through quantitative modeling of preclinical data for numerous BACEi, GSI, and GSM across multiple species. Our results indicate that the intrinsic PK/PD relationship in CSF is predictive of that in brain, at least in the postulated pharmacologically relevant range, with excellent consistency across mechanisms and species. As such, the validity of CSF Aß as an effect biomarker for brain Aß lowering is confirmed preclinically. Meanwhile, we have been able to reproduce the dose-dependent separation between brain and CSF effect profiles using simulations. We further discuss the implications of our findings to drug discovery and development with regard to preclinical PK/PD characterization and clinical prediction of Aß lowering in the brain.

Application of the bicyclo[1.1.1]pentane motif as a nonclassical phenyl ring bioisostere in the design of a potent and orally active γ-secretase inhibitor.

Replacement of the central, para-substituted fluorophenyl ring in the γ-secretase inhibitor 1 (BMS-708,163) with the bicyclo[1.1.1]pentane motif led to the discovery of compound 3, an equipotent enzyme inhibitor with significant improvements in passive permeability and aqueous solubility. The modified biopharmaceutical properties of 3 translated into excellent oral absorption characteristics (~4-fold ↑ C(max) and AUC values relative to 1) in a mouse model of γ-secretase inhibition. In addition, SAR studies into other fluorophenyl replacements indicate the intrinsic advantages of the bicyclo[1.1.1]pentane moiety over conventional phenyl ring replacements with respect to achieving an optimal balance of properties (e.g., γ-secretase inhibition, aqueous solubility/permeability, in vitro metabolic stability). Overall, this work enhances the scope of the [1.1.1]-bicycle beyond that of a mere "spacer" unit and presents a compelling case for its broader application as a phenyl group replacement in scenarios where the aromatic ring count impacts physicochemical parameters and overall drug-likeness.

Development of a rapid UPLC-MS/MS method for quantification of saxagliptin in rat plasma and application to pharmacokinetic study.

A novel, simple and rapid ultraperformance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) assay was established for quantification of saxagliptin in rat plasma. Plasma samples were processed by liquid-liquid extraction with ethyl acetate and chromatographed on a C18 column (2.1 × 50 mm i.d., 1.7 µm). The mobile phase consisted of methanol and 0.1% formic acid (40:60, v/v). Multiple reaction monitoring transitions were performed for detection in positive-ion mode with an electrospray ionization source. The calibration curve was linear over the concentration range of 0.5-100 ng/mL (R² > 0.99). All accuracy values were between 90.62 and 105.60% relative error and the intra- and inter-day precisions were less than 9.66% relative standard deviation. Extraction recovery was more than 81.01% and the matrix effect ranged from 90.27 to 109.15%. After validation, the method was applied to a pharmacokinetic study where healthy rats were orally given 0.5 mg/kg saxagliptin.

Metabolism-directed design of oxetane-containing arylsulfonamide derivatives as γ-secretase inhibitors.

A metabolism-based approach toward the optimization of a series of N-arylsulfonamide-based γ-secretase inhibitors is reported. The lead cyclohexyl analogue 6 suffered from extensive oxidation on the cycloalkyl motif by cytochrome P450 3A4, translating into poor human liver microsomal stability. Knowledge of the metabolic pathways of 6 triggered a structure-activity relationship study aimed at lowering lipophilicity through the introduction of polarity. This effort led to several tetrahydropyran and tetrahydrofuran analogues, wherein the 3- and 4-substituted variants exhibited greater microsomal stability relative to their 2-substituted counterparts. Further reduction in lipophilicity led to the potent γ-secretase inhibitor and 3-substituted oxetane 1 with a reduced propensity toward oxidative metabolism, relative to its 2-substituted isomer. The slower rates of metabolism with 3-substituted cyclic ethers most likely originate from reductions in lipophilicity and/or unfavorable CYP active site interactions with the heteroatom. Preliminary animal pharmacology studies with a representative oxetane indicate that the series is generally capable of lowering Aß in vivo. As such, the study also illustrates the improvement in druglikeness of molecules through the use of the oxetane motif.

Quantitative pharmacokinetic/pharmacodynamic analyses suggest that the 129/SVE mouse is a suitable preclinical pharmacology model for identifying small-molecule γ-secretase inhibitors.

Alzheimer's disease (AD) poses a serious public health threat to the United States. Disease-modifying drugs slowing AD progression are in urgent need, but they are still unavailable. According to the amyloid cascade hypothesis, inhibition of ß- or γ-secretase, key enzymes for the production of amyloid ß (Aß), may be viable mechanisms for the treatment of AD. For the discovery of γ-secretase inhibitors (GSIs), the APP-overexpressing Tg2576 mouse has been the preclinical model of choice, in part because of the ease of detection of Aß species in its brain, plasma, and cerebrospinal fluid (CSF). Some biological observations and practical considerations, however, argue against the use of the Tg2576 mouse. We reasoned that an animal model would be suitable for GSI discovery if the pharmacokinetic (PK)/pharmacodynamic (PD) relationship of a compound for Aß lowering in this model is predictive of that in human. In this study, we assessed whether the background 129/SVE strain is a suitable preclinical pharmacology model for identifying new GSIs by evaluating the translatability of the intrinsic PK/PD relationships for brain and CSF Aß across the Tg2576 and 129/SVE mouse and human. Using semimechanistically based PK/PD modeling, our analyses indicated that the intrinsic PK/PD relationship for brain Aßx-42 and CSF Aßx-40 in the 129/SVE mouse is indicative of that for human CSF Aß. This result, in conjunction with practical considerations, strongly suggests that the 129/SVE mouse is a suitable model for GSI discovery. Concurrently, the necessity and utilities of PK/PD modeling for rational interpretation of Aß data are established.

A possible role of multidrug resistance-associated protein 2 (Mrp2) in hepatic excretion of PCB126, an environmental contaminant: PBPK/PD modeling.

3,3',4,4',5'-Pentachlorobiphenyl (PCB126) is a carcinogenic environmental pollutant and its toxicity is mediated through binding with aryl hydrocarbon receptor (AhR). Earlier, we found that PCB126 treated F344 rats had 110-400 times higher PCB126 concentration in the liver than in the fat. Protein binding was suspected to be a major factor for the high liver concentration of PCB126 despite its high lipophilicity. In this research, we conducted a combined pharmacokinetic/pharmacodynamic study in male F344 rats. In addition to blood and tissue pharmacokinetics, we use the development of hepatic preneoplastic foci (glutathione-S-transferase placental form [GSTP]) as a pharmacodynamic endpoint. Experimental data were utilized for building a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model. PBPK/PD modeling was consistent with the experimental PK and PD data. Salient features of this model include: (1) bindings between PCB126 and hepatic proteins, particularly the multidrug resistance-associated protein (Mrp2), a protein transporter; (2) Mrp2-mediated excretion; and (3) a relationship between area under the curve of PCB126 in the livers and % volume of GSTP foci. Mrp2 involvement in PCB126 pharmacokinetics is supported by computational chemistry calculation using a three-dimensional quantitative structure-activity relationship model of Mrp2 developed by S. Hirono et al. (2005, Pharm. Res. 22, 260-269). This work, for the first time, provided a plausible role of a versatile hepatic transporter for drugs, Mrp2, in the disposition of an important environmental pollutant, PCB126.

Application of PBPK modeling in support of the derivation of toxicity reference values for 1,1,1-trichloroethane.

PBPK modeling has been increasingly applied in chemical risk assessment for dose, route, and species extrapolation. The use of PBPK modeling was explored in deriving toxicity reference values for 1,1,1-trichloroethane (1,1,1-TCE). This effort involved a 5-step process: (i) reconstruction of several published PBPK models for 1,1,1-TCE in the rat and human; (ii) selection of appropriate pharmacokinetic datasets for model comparison; (iii) determination of the most suitable PBPK model for supporting reference value derivation; (iv) PBPK model simulation of two critical studies to estimate internal dose metrics; and (v) calculation of internal dose metrics for human exposure scenarios for reference value derivation. The published model by Reitz et al. [Reitz, R.H., McDougal, J.N., Himmelstein, M.W., Nolan, R.J., Schumann, A.M., 1988. Physiologically based pharmacokinetic modeling with methylchloroform: implications for interspecies, high dose/low dose, and dose route extrapolations. Toxicol. Appl. Pharmacol. 95, 185-199] was judged the most suitable. This model has liver, fat, and rapidly and slowly perfused compartments, contains a saturable process for 1,1,1-TCE hepatic metabolism, and accommodates multiple exposure pathways in three species. Data from a human volunteer study involving acute inhalation exposure [Mackay, C.J., Campbell, L., Samuel, A.M., Alderman, K.J., Idzikowski, C., Wilson, H.K., Gompertz, D., 1987. Behavioral changes during exposure to 1,1,1-trichloroethane: time-course and relationship to blood solvent levels. Am. J. Ind. Med. 11, 223-239] and a chronic rat inhalation study [Quast, J.F., Calhoun, L.L., Frauson, L.E., 1988. 1,1,1-Trichloroethane formulation: a chronic inhalation toxicity and oncogenicity study in Fischer 344 rats and B6C3F1 mice. Fundam. Appl. Toxicol. 11, 611-625] were selected to simulate appropriate internal dosimetry data from which to derive reference value points of departure. Duration, route, and species extrapolations were performed based on internal dose metrics.

Quantitative analysis of liver GST-P foci promoted by a chemical mixture of hexachlorobenzene and PCB 126: implication of size-dependent cellular growth kinetics.

The objectives of this study were twofold: (1) evaluating the carcinogenic potential of the mixture of two persistent environmental pollutants, hexachlorobenzene (HCB) and 3,3',4,4',5-pentachlorobiphenyl (PCB 126), in an initiation-promotion bioassay involving the development of pi glutathione S-transferase (GST-P) liver foci, and (2) analyzing the GST-P foci data using a biologically-based computer model (i.e., clonal growth model) with an emphasis on the effect of focal size on the growth kinetics of initiated cells. The 8-week bioassay involved a series of treatments of initiator, two-thirds partial hepatectomy, and daily oral gavage of the mixture of two doses in male F344 rats. The mixture treatment significantly increased liver GST-P foci development, indicating carcinogenic potential of this mixture. Our clonal growth model was developed to simulate the appearance and development of initiated GST-P cells in the liver over time. In the model, the initiated cells were partitioned into two subpopulations with the same division rate but different death rates. Each subpopulation was further categorized into single cells, mini- (2-11 cells), medium- (12-399 cells), and large-foci (>399 cells) with different growth kinetics. Our modeling suggested that the growth of GST-P foci is size-dependent; in general, the larger the foci, the higher the rate constants of division and death. In addition, the modeling implied that the two doses promoted foci development in different manners even though the experimental foci data appeared to be similar between the two doses. This study further illustrated how clonal growth modeling may facilitate our understanding in chemical carcinogenic process.