Discovery of DNL343: A Potent, Selective, and Brain-Penetrant eIF2B Activator Designed for the Treatment of Neurodegenerative Diseases.

Eukaryotic translation initiation factor 2B (eIF2B) is a key component of the integrated stress response (ISR), which regulates protein synthesis and stress granule formation in response to cellular insult. Modulation of the ISR has been proposed as a therapeutic strategy for treatment of neurodegenerative diseases such as vanishing white matter (VWM) disease and amyotrophic lateral sclerosis (ALS) based on its ability to improve cellular homeostasis and prevent neuronal degeneration. Herein, we report the small-molecule discovery campaign that identified potent, selective, and CNS-penetrant eIF2B activators using both structure- and ligand-based drug design. These discovery efforts culminated in the identification of DNL343, which demonstrated a desirable preclinical drug profile, including a long half-life and high oral bioavailability across preclinical species. DNL343 was progressed into clinical studies and is currently undergoing evaluation in late-stage clinical trials for ALS.

Biotransformation research advances - 2022 year in review.

This annual review is the eighth of its kind since 2016 (Baillie et al. 2016, Khojasteh et al. 2017, Khojasteh et al. 2018, Khojasteh et al. 2019, Khojasteh et al. 2020, Khojasteh et al. 2021, Khojasteh et al. 2022). Our objective is to explore and share articles which we deem influential and significant in the field of biotransformation.

Bioactivation and reactivity research advances - 2022 year in review‡.

With the 50th year mark since the launch of Drug Metabolism and Disposition journal, the field of drug metabolism and bioactivation has advanced exponentially in the past decades (Guengerich 2023).This has, in a major part, been due to the continued advances across the whole spectrum of applied technologies in hardware, software, machine learning (ML), and artificial intelligence (AI). LC-MS platforms continue to evolve to support key applications in the field, and automation is also improving the accuracy, precision, and throughput of these supporting assays. In addition, sample generation and processing is being aided by increased diversity and quality of reagents and bio-matrices so that what is being analyzed is more relevant and translatable. The application of in silico platforms (applied software, ML, and AI) is also making great strides, and in tandem with the more traditional approaches mentioned previously, is significantly advancing our understanding of bioactivation pathways and how these play a role in toxicity. All of this continues to allow the area of bioactivation to evolve in parallel with associated fields to help bring novel or improved medicines to patients with urgent or unmet needs.Shuai Wang and Cyrus Khojasteh, on behalf of the authors.

The Absolute Bioavailability and Absorption, Metabolism, and Excretion of Ipatasertib, a Potent and Highly Selective Protein Kinase B (Akt) Inhibitor.

Ipatasertib (GDC-0068) is a potent, highly selective, small-molecule inhibitor of protein kinase B (Akt) being developed by Genentech/Roche as a single agent and in combination with other therapies for the treatment of cancers. To fully understand the absorption, metabolism, and excretion of ipatasertib in humans, an open-label study using 14C-radiolabeled ipatasertib was completed to characterize the absolute bioavailability (period 1) and mass balance and metabolite profiling (period 2). In period 1, subjects were administered a 200 mg oral dose of ipatasertib followed by an 80 µg (800 nCi) intravenous dose of [14C]-ipatasertib. In period 2, subjects received a single oral dose containing approximately 200 mg (100 µCi) [14C]-ipatasertib. In an integrated analytical strategy, accelerator mass spectrometry was applied to measure the 14C microtracer intravenous pharmacokinetics in period 1 and fully profile plasma radioactivity in period 2. The systemic plasma clearance and steady-state volume of distribution were 98.8 L/h and 2530 L, respectively. The terminal half-lives after oral and intravenous administrations were similar (26.7 and 27.4 hours, respectively) and absolute bioavailability of ipatasertib was 34.0%. After a single oral dose of [14C]-ipatasertib, 88.3% of the administered radioactivity was recovered with approximately 69.0% and 19.3% in feces and urine, respectively. Radioactivity in feces and urine was predominantly metabolites with 24.4% and 8.26% of dose as unchanged parent, respectively; indicating that ipatasertib had been extensively absorbed and hepatic metabolism was the major route of clearance. The major metabolic pathway was N-dealkylation mediated by CYP3A, and minor pathways were oxidative by cytochromes P450 and aldehyde oxidase. SIGNIFICANCE STATEMENT: The study provided definitive information regarding the absolute bioavailability and the absorption, metabolism, and excretion pathways of ipatasertib, a potent, novel, and highly selective small-molecule inhibitor of protein kinase B (Akt). An ultrasensitive radioactive counting method, accelerator mass spectrometry was successfully applied for 14C-microtracer absolute bioavailability determination and plasma metabolite profiling.

Discovery of Potent and Selective Dual Leucine Zipper Kinase/Leucine Zipper-Bearing Kinase Inhibitors with Neuroprotective Properties in In Vitro and In Vivo Models of Amyotrophic Lateral Sclerosis.

Dual leucine zipper kinase (DLK) and leucine zipper-bearing kinase (LZK) are regulators of neuronal degeneration and axon growth. Therefore, there is a considerable interest in developing DLK/LZK inhibitors for neurodegenerative diseases. Herein, we use ligand- and structure-based drug design approaches for identifying novel amino-pyrazine inhibitors of DLK/LZK. DN-1289 (14), a potent and selective dual DLK/LZK inhibitor, demonstrated excellent in vivo plasma half-life across species and is anticipated to freely penetrate the central nervous system with no brain impairment based on in vivo rodent pharmacokinetic studies and human in vitro transporter data. Proximal target engagement and disease relevant pathway biomarkers were also favorably regulated in an in vivo model of amyotrophic lateral sclerosis.

Characterization of Hepatic UDP-Glucuronosyltransferase Enzyme Abundance-Activity Correlations and Population Variability Using a Proteomics Approach and Comparison with Cytochrome P450 Enzymes.

The expression of ten major drug-metabolizing UDP-glucuronosyltransferase (UGT) enzymes in a panel of 130 human hepatic microsomal samples was measured using a liquid chromatography-tandem mass spectrometry-based approach. Simultaneously, ten cytochromes P450 and P450 reductase were also measured, and activity-expression relationships were assessed for comparison. The resulting data sets demonstrated that, with the exception of UGT2B17, 10th to 90th percentiles of UGT expression spanned 3- to 8-fold ranges. These ranges were small relative to ranges of reported mean UGT enzyme expression across different laboratories. We tested correlation of UGT expression with enzymatic activities using selective probe substrates. A high degree of abundance-activity correlation (Spearman's rank correlation coefficient > 0.6) was observed for UGT1As (1A1, 3, 4, 6) and cytochromes P450. In contrast, protein abundance and activity did not correlate strongly for UGT1A9 and UGT2B enzymes (2B4, 7, 10, 15, and 17). Protein abundance was strongly correlated for UGTs 2B7, 2B10, and 2B15. We suggest a number of factors may contribute to these differences including incomplete selectivity of probe substrates, correlated expression of these UGT2B isoforms, and the impact of splice and polymorphic variants on the peptides used in proteomics analysis, and exemplify this in the case of UGT2B10. Extensive correlation analyses identified important criteria for validating the fidelity of proteomics and enzymatic activity approaches for assessing UGT variability, population differences, and ontogenetic changes. SIGNIFICANCE STATEMENT: Protein expression data allow detailed assessment of interindividual variability and enzyme ontogeny. This study has observed that expression and enzyme activity are well correlated for hepatic UGT1A enzymes and cytochromes P450. However, for the UGT2B family, caution is advised when assuming correlation of expression and activity as is often done in physiologically based pharmacokinetic modeling. This can be due to incomplete probe substrate specificities, but may also be related to presence of inactive UGT protein materials and the effect of splicing variations.

Characterization of Differential Tissue Abundance of Major Non-CYP Enzymes in Human.

The availability of assays that predict the contribution of cytochrome P450 (CYP) metabolism allows for the design of new chemical entities (NCEs) with minimal oxidative metabolism. These NCEs are often substrates of non-CYP drug-metabolizing enzymes (DMEs), such as UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), carboxylesterases (CESs), and aldehyde oxidase (AO). Nearly 30% of clinically approved drugs are metabolized by non-CYP enzymes. However, knowledge about the differential hepatic versus extrahepatic abundance of non-CYP DMEs is limited. In this study, we detected and quantified the protein abundance of eighteen non-CYP DMEs (AO, CES1 and 2, ten UGTs, and five SULTs) across five different human tissues. AO was most abundantly expressed in the liver and to a lesser extent in the kidney; however, it was not detected in the intestine, heart, or lung. CESs were ubiquitously expressed with CES1 being predominant in the liver, while CES2 was enriched in the small intestine. Consistent with the literature, UGT1A4, UGT2B4, and UGT2B15 demonstrated liver-specific expression, whereas UGT1A10 expression was specific to the intestine. UGT1A1 and UGT1A3 were expressed in both the liver and intestine; UGT1A9 was expressed in the liver and kidney; and UGT2B17 levels were significantly higher in the intestine than in the liver. All five SULTs were detected in the liver and intestine, and SULT1A1 and 1A3 were detected in the lung. Kidney abundance was the most variable among the studied tissues, and overall, high interindividual variability (>15-fold) was observed for UGT2B17, CES2 (intestine), SULT1A1 (liver), UGT1A9, UGT2B7, and CES1 (kidney). These differential tissue abundance data can be integrated into physiologically based pharmacokinetic (PBPK) models for the prediction of non-CYP drug metabolism and toxicity in hepatic and extrahepatic tissues.

Regional Proteomic Quantification of Clinically Relevant Non-Cytochrome P450 Enzymes along the Human Small Intestine.

Current challenges in accurately predicting intestinal metabolism arise from the complex nature of the intestine, leading to limited applicability of available in vitro tools as well as knowledge deficits in intestinal physiology, including enzyme abundance. In particular, information on regional enzyme abundance along the small intestine is lacking, especially for non-cytochrome P450 enzymes such as carboxylesterases (CESs), UDP-glucuronosyltransferases (UGTs), and sulfotransferases (SULTs). We used cryopreserved human intestinal mucosa samples from nine donors as an in vitro surrogate model for the small intestine and performed liquid chromatography tandem mass spectrometry-based quantitative proteomics for 17 non-cytochrome P450 enzymes using stable isotope-labeled peptides. Relative protein quantification was done by normalization with enterocyte marker proteins, i.e., villin-1, sucrase isomaltase, and fatty acid binding protein 2, and absolute protein quantification is reported as picomoles per milligram of protein. Activity assays in glucuronidations and sequential metabolisms were conducted to validate the proteomics findings. Relative or absolute quantifications are reported for CES1, CES2, five UGTs, and four SULTs along the small intestine: duodenum, jejunum, and ileum for six donors and in 10 segments along the entire small intestine (A-J) for three donors. Relative quantification using marker proteins may be beneficial in further controlling for technical variabilities. Absolute quantification data will allow for scaling factor generation and in vivo extrapolation of intestinal clearance using physiologically based pharmacokinetic modeling. SIGNIFICANCE STATEMENT: Current knowledge gaps exist in intestinal protein abundance of non-cytochrome P450 enzymes. Here, we employ quantitative proteomics to measure non-cytochrome P450 enzymes along the human small intestine in nine donors using cryopreserved human intestinal mucosa samples. Absolute and relative abundances reported here will allow better scaling of intestinal clearance.

Novel Homodimer Metabolites of GDC-0994 via Cytochrome P450-Catalyzed Radical Coupling.

Two novel homodimer metabolites were identified in rat samples collected during the in vivo study of GDC-0994. In this study, we investigated the mechanism of the formation of these metabolites. We generated and isolated the dimer metabolites using a biomimetic oxidation system for NMR structure elucidation to identify a symmetric dimer formed via carbon-carbon bond between two pyrazoles and an asymmetric dimer formed via an aminopyrazole-nitrogen to pyrazole-carbon bond. In vitro experiments demonstrated formation of these dimers was catalyzed by cytochrome P450 enzymes (P450s) with CYP3A4/5 being the most efficient. Using density functional theory, we determined these metabolites share a mechanism of formation, initiated by an N-H hydrogen atom abstraction by the catalytically active iron-oxo of P450s. Molecular modeling studies also show these dimer metabolites fit in the CYP3A4 binding site in low energy conformations with minimal protein rearrangement. Collectively, the results of these experiments suggest that formation of these two homodimer metabolites is mediated by CYP3A, likely involving activation of two GDC-0994 molecules by a single P450 enzyme and proceeding through a radical coupling mechanism. SIGNIFICANCE STATEMENT: These studies identified structures and enzymology for two distinct homodimer metabolites and indicate a novel biotransformation reaction mediated by CYP3A. In it, two molecules may bind within the active site and combine through radical coupling. The mechanism of dimerization was elucidated using density functional theory computations and supported by molecular modeling.

Coexpression of Human Hepatic Uridine Diphosphate Glucuronosyltransferase Proteins: Implications for Ontogenetic Mechanisms and Isoform Coregulation.

Uridine diphosphate glucuronosyltransferases (UGTs) catalyze glucuronidation to facilitate systemic and local clearance of numerous chemicals and drugs. To investigate whether UGT expression is coregulated in human liver, we analyzed the protein expression of UGTs 1A1, 1A3, 1A4, 1A6, 1A9, 2B7, 3A1, and 3A2 using western blots from 164 healthy human liver samples, comparing expression with age and sex. UGT1A6 levels were significantly higher in children than adults, and UGT3A1 and 3A2 expression significantly increased with age from childhood to age >65 yearas. In children aged <18 years, UGT1A4/1A9 protein expression was significantly correlated, but not for adults aged >18 years. UGT1A3 expression was always significantly correlated with other UGT1A isoforms in all adults aged >18 years. In individuals aged ≥12 years, expression of UGT1A1/1A4, UGT1A1/1A6, UGT1A1/1A9, and UGT1A4/1A6 significantly correlated, which was not observed in children aged <12 years. In contrast, UGT1A4/2B7 showed significant correlation in children aged <12 years, but not in individuals aged ≥12 years, and this was observed in female but not male individuals. Expression of UGT1A6/1A9 and UGT3A1/3A2 correlated in the entire sample population, but UGT3As did not correlate with other UGTs. These correlations were sex dependent, as UGT1A3/1A1, UGT1A4/2B7 and UGT3A1/3A2 correlated more highly in male than female individuals, while UGT1A4/1A6 protein correlated more significantly in female than male individuals. This is the first report on the ontogeny of UGT3A isoforms, showing maximal expression in the elderly, and is the first demonstration that UGT isoforms commonly coexpress in vivo, in both age-dependent and sex-dependent manners.

Human Cytochrome P450 1A1 Adapts Active Site for Atypical Nonplanar Substrate.

The human cytochrome P450 1A1 (CYP1A1) is well known for chemical activation of procarcinogens and often has a substrate scope of small and highly planar compounds. Substrates deviating from these characteristics are certainly known, but how these larger and nonplanar substrates are accommodated and oriented within the CYP1A1 active site is not understood. Herein a new X-ray structure of CYP1A1 bound to the pan-Pim kinase inhibitor GDC-0339 reveals how the CYP1A1 active site cavity is reconfigured to bind larger and nonplanar compounds. The shape and size of the cavity are controlled by structural elements in the active site roof, with major changes in the conformation of the F helix break and relocation of Phe224 from the active site to the protein surface. This altered CYP1A1 active site architecture is consistent with the proposed mechanism for CYP1A1 generation of an unusual aminoazepane-rearranged metabolite for this substrate. SIGNIFICANCE STATEMENT: Cytochrome P450 1A1 metabolizes drugs, procarcinogens, and toxins and although previous structures have revealed how its stereotypical planar, aromatic compounds are accommodated in the CYP1A1 active site, this is not the case for flexible and nonplanar compounds. The current work determines the X-ray structure of CYP1A1 with such a flexible, nonplanar Pim kinase inhibitor, revealing significant modification of the CYP1A1 roof that accommodate this preclinical candidate and support an unusual intramolecular rearrangement reaction.

Characterization of the Ontogeny of Hepatic UDP-Glucuronosyltransferase Enzymes Based on Glucuronidation Activity Measured in Human Liver Microsomes.

An understanding of the postnatal development of hepatic UDP-glucuronosyltransferase (UGT) enzymes is required for accurate prediction of the age-dependent changes in pharmacokinetics of many drugs used in children. However, the maturation rate of hepatic UGT isoforms remains a major knowledge gap. This study aimed to establish the age-associated changes in glucuronidation activity of 10 major hepatic UGT isoforms in humans, namely, UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, UGT2B15, and UGT2B17. Human liver microsomes from pediatric and adult donors were incubated under optimized incubation conditions to assess the activity rates of hepatic UGT isoforms using a panel of 19 in vitro UGT probe substrates and clinically used drugs. Statistically strong correlations of glucuronidation activities allowed the ontogeny of UGT1A1, UGT1A4, UGT2B7, UGT2B10, and UGT2B15 to be established using multiple selective UGT substrates and matched human liver microsome samples. The postnatal development of hepatic UGTs is isoform-dependent using either individual or cross-correlated selective isoform substrates. Maximal adult activity was reached at different times ranging from within a month (UGT1A1, UGT2B4, UGT2B7, UGT2B10, and UGT2B15), during infancy (UGT1A3, UGT1A4, and UGT1A9), to adolescence (UGT1A6 and UGT2B17). This study provides an extensive characterization of the postnatal ontogeny profiles of hepatic UGT enzymes that are instrumental for predicting drug disposition via in vitro-in vivo extrapolation algorithms and verifying pharmacokinetic predictions against in vivo observations via pediatric physiologically based pharmacokinetic modeling in pediatric patients.

Unequal Absorption of Radiolabeled and Nonradiolabeled Drug from the Oral Dose Leads to Incorrect Estimates of Drug Absorption and Circulating Metabolites in a Mass Balance Study.

BACKGROUND: Mass balance studies conducted using radiolabeled material (14C or 3H) definitively characterize the Absorption, Metabolism, and Excretion (AME) of a drug. A critical aspect of these studies is that the radiotracer maintains its proportion to total drug from its administration to its complete elimination from the body. In the study of GDC-0276 in beagle dogs, we observed that the 14C radiotracer proportion (specific activity) varied through the study. METHOD: High resolution-accurate mass spectrometric measurements of 12C and 14C isotopes of GDC- 0276 and its metabolites in plasma and excreta samples were used to determine the apparent specific activities, which were higher than the specific activity of the dosing formulation. Drug concentrations were adjusted to the observed specific activities to correct the readouts for GDC-0276 AME and PK. RESULTS: The enrichment of 14C, which resulted in higher specific activities, was consistent with faster and more extensive absorption of the radiotracer from the dosing formulation. This resulted in overestimating the dose absorbed, the extent of elimination in urine and bile, and the exposures to circulating metabolites. These biases were corrected by the specific activities determined for study samples by mass spectrometry. CONCLUSION: Assuming that the radiotracer was proportional to total drug throughout a radiolabeled study was not valid in a 14C study in beagle dogs. This presumably resulted from unequal absorption of the radiotracer and nonradiolabeled test articles from the oral dose due to inequivalent solid forms. We were able to provide a more accurate description of the AME of GDC-0276 in dogs by characterizing the differential absorption of the radiotracer.

Development of a mass spectrometry-based tryptophan 2, 3-dioxygenase assay using liver cytosol from multiple species.

A novel and rapid method to determine the potency of inhibitors for tryptophan 2, 3-dioxygenase (TDO2) activities in human and preclinical species was successfully developed and validated utilizing LC-MS/MS. Previously reported TDO2 activity assays are resource intensive, requiring cloning and overexpression of TDO2. Here, we demonstrated that liver cytosol contained sufficient active TDO2 for evaluating the potency of TDO2 inhibitors across multiple species. TDO2 expression in human cytosol was estimated by LC-MS/MS to be 41 pmoL/mg cytosolic protein, with similar levels in dogs and monkeys, whereas mice and rats had 9.6 and 5.0-fold greater expression, respectively. Reaction conditions for TDO2-mediated conversion of l-tryptophan to kynurenine were optimized. Marked differences in kinetic parameters and inhibition potency were observed in TDO2 across species, with different Km values in dog (0.055 mM), monkey (0.070 mM), human (0.19 mM), mouse (0.32 mM) and rat (0.36 mM). Subsequently, IC50 values were determined for a series of TDO2 inhibitors in liver cytosol of five species, and good agreement with the literature values was observed for human enzyme. Taken together, these data indicate that TDO2 inhibition can be rapidly determined in readily available hepatic cytosol to assess potential species differences in potency.

CYP1A1-Mediated Intramolecular Rearrangement of Aminoazepane in GDC-0339.

GDC-0339 is a novel small molecule pan-Pim kinase inhibitor that was discovered as a potential treatment of multiple myeloma. During the in vitro and in vivo metabolite profiling of GDC-0339, a metabolite was detected that had the same elemental composition as the parent but was distinct with respect to its chromatographic separation and mass spectrometric fragmentation pattern. High resolution tandem mass spectrometry data indicated the metabolite was modified at the aminoazepane moiety. The structure was solved by nuclear magnetic resonance analysis of the isolated metabolite and further confirmed by comparing it to a synthetic standard. These results indicated that the metabolite was formed by an intramolecular amine replacement reaction with the primary amine forming a new attachment to pyrazole without any change in stereochemistry. In vitro experiments showed cytochrome P450s catalyzed the reaction and demonstrated high isoform selectivity by CYP1A1. Results from kinetic experiments showed that the CYP1A1-mediated rearrangement of GDC-0339 was an efficient reaction with apparent turnover number (kcat) and Michaelis constant (Km) of 8.4 minutes-1 and 0.6 µM, respectively. The binding of GDC-0339 to the cytochrome P450 active site was examined by characterizing the direct inhibition of CYP1A1-mediated phenacetin O-deethylation, and GDC-0339 was a potent competitive inhibitor with Ki of 0.9 µM. This high affinity binding was unexpected given a narrow active site for CYP1A1 and GDC-0339 does not conform structurally to known CYP1A1 substrates, which are mostly polyaromatic planar molecules. Further, we explored some of the structural requirements for the rearrangement reaction and identified several analogs to GDC-0339 that undergo this biotransformation.

Mixed Matrix Method Provides A Reliable Metabolite Exposure Comparison for Assessment of Metabolites in Safety Testing (MIST).

BACKGROUND: The regulatory guidances on metabolites in safety testing (MIST) by US Food and Drug Administration (FDA) and International Conference on Harmonisation (ICH) describe the necessity to assess exposures of major circulating metabolites in humans at steady state relative to exposures achieved in nonclinical safety studies prior to the initiation of large scale clinical trials. This comparison can be accomplished by measuring metabolite concentrations in animals and humans with validated bioanalytical methods. However, bioanalysis of metabolites in multiple species and multiple studies is resource intensive and may impact the timelines of clinical studies. METHOD: A simple, reliable and accurate method has been developed for quantitative assessment of metabolite coverage in preclinical safety species by mixing equal volume of human plasma with blank plasma of animal species and vice versa followed by an analysis using LC-SRM or LC-HRMS. Here, we explored the reliability and accuracy of this method in several development projects at Genentech and compared the results to those obtained from validated bioanalytical methods. RESULTS: The mixed-matrix method provided comparable accuracy (within ±20%) to those obtained from validated bioanalysis but does not require authentic standards or radiolabeled compounds, which could translate to time and resource savings in drug development. CONCLUSION: Quantitative assessment of metabolite coverage in safety species can be made using mixed matrix method with similar accuracy and scientific rigor to those obtained from validated bioanalytical methods. Moving forward, we are encouraging the industry and regulators to consider accepting the mixed matrix method for assessing metabolite exposure comparisons between humans and animal species used in toxicology studies.

Utility of CYP3A4 and PXR-CAR-CYP3A4/3A7 Transgenic Mouse Models To Assess the Magnitude of CYP3A4 Mediated Drug-Drug Interactions.

Species differences in the expression, activity, regulation, and substrate specificity of metabolizing enzymes preclude the use of animal models to predict clinical drug-drug interactions (DDIs). The objective of this work is to determine if the transgenic (Tg) Cyp3a-/-Tg-3A4Hep/Int and Nr1i2/Nr1i3-/--Cyp3a-/-Tg-PXR-CAR-3A4/3A7Hep/Int (PXR-CAR-CYP3A4/3A7) mouse models could be used to predict in vivo DDI of 10 drugs; alprazolam, bosutinib, crizotinib, dasatinib, gefitinib, ibrutinib, regorafenib, sorafenib, triazolam, and vandetinib (as victims); with varying magnitudes of reported CYP3A4 clinical DDI. As an assessment of the effect of CYP3A4 inhibition, these drugs were coadministered to Cyp3a-/-Tg-3A4Hep/Int mice with the CYP3A inhibitor, itraconazole. For crizotinib, regorafenib, sorafenib, and vandetanib, there was no significant increase of AUC observed; with alprazolam, bosutinib, ibrutinib, dasatinib, and triazolam, pretreatment with itraconazole resulted in a 2-, 4-, 17-, 7-, and 15-fold increase in AUC, respectively. With the exception of gefinitib for which the DDI effect was overpredicted (12-fold in Tg-mice vs 2-fold in the clinic), the magnitude of AUC increase observed in this study was consistent (within 2-fold) with the clinical DDI observed following administration with itraconazole/ketoconazole. As an assessment of CYP3A4 induction, following rifampin pretreatment to PXR-CAR-3A4/3A7Hep/Int mice, an 8% decrease in vandetanib mean AUC was observed; 39-52% reduction in AUC were observed for dasatinib, ibrutinib, regorafenib, and sorafenib compared to vehicle treated mice. The greatest effect of rifampin induction was observed with alprazolam, bosutinib, crizotinib, gefitinib, and triazolam where 72-91% decrease in AUC were observed. With the exception of vandetanib for which rifampin induction was under-predicted, the magnitude of induction observed in this study was consistent (within 2-fold) with clinical observations. These data sets suggest that, with two exceptions, these transgenic mice models were able to exclude or capture the magnitude of CYP3A4 clinical inhibition and induction. Data generated in transgenic mice may be used to gain confidence and complement in vitro and in silico methods for assessing DDI potential/liability.

Applying Stable Isotope Labeled Amino Acids in Micropatterned Hepatocyte Coculture to Directly Determine the Degradation Rate Constant for CYP3A4.

The rate of enzyme degradation (kdeg) is an important input parameter for the prediction of clinical drug-drug interactions (DDIs) that result from mechanism-based inactivation or induction of cytochrome P450 (P450). Currently, a large range of reported estimates for CYP3A4 enzyme degradation exists, and consequently extensive uncertainty exists in steady-state predictions for DDIs. In the current investigations, the stable isotope labeled amino acids in culture technique was applied to a long-lived primary human hepatocyte culture, HepatoPac, to directly monitor the degradation of CYP3A4. This approach allowed selective isotope labeling of a population of de novo synthesized CYP3A4 and specific quantification of proteins with mass spectrometry to determine the CYP3A4 degradation within the hepatocytes. The kdeg estimate was 0.026 ± 0.005 hour-1 This value was reproduced by cultures derived across four individual donors. For these cultures, the data indicated that CYP3A4 mRNA and total protein expression (i.e., labeled and unlabeled P450s), and activity were stable over the period where degradation had been determined. This kdeg value for CYP3A4 was in good agreement with recently reported values that used alternate analytical approaches but also employed micropatterned primary human hepatocytes as the in vitro model.

Novel Mechanism of Decyanation of GDC-0425 by Cytochrome P450.

GDC-0425 [5-((1-ethylpiperidin-4-yl)oxy)-9H-pyrrolo[2,3-b:5,4-c']dipyridine-6-carbonitrile] is an orally bioavailable small-molecule inhibitor of checkpoint kinase 1 that was investigated as a novel cotherapy to potentiate chemotherapeutic drugs, such as gemcitabine. In a radiolabeled absorption, distribution, metabolism, and excretion study in Sprague-Dawley rats, trace-level but long-lived 14C-labeled thiocyanate was observed in circulation. This thiocyanate originated from metabolic decyanation of GDC-0425 and rapid conversion of cyanide to thiocyanate. Excretion studies indicated decyanation was a minor metabolic pathway, but placing 14C at nitrile magnified its observation. Cytochrome P450s catalyzed the oxidative decyanation reaction in vitro when tested with liver microsomes, and in the presence of 18O2, one atom of 18O was incorporated into the decyanated product. To translate this finding to a clinical risk assessment, the total circulating levels of thiocyanate (endogenous plus drug-derived) were measured following repeated administration of GDC-0425 to rats and cynomolgus monkeys. No overt increases were observed with thiocyanate concentrations of 121-154 µM in rats and 71-110 µM in monkeys receiving vehicle and all tested doses of GDC-0425. These findings were consistent with results from the radiolabel rat study where decyanation accounted for conversion of <1% of the administered GDC-0425 and contributed less than 1 µM thiocyanate to systemic levels. Further, in vitro studies showed only trace oxidative decyanation for humans. These data indicated that, although cyanide was metabolically released from GDC-0425 and formed low levels of thiocyanate, this pathway was a minor route of metabolism, and GDC-0425-related increases in systemic thiocyanate were unlikely to pose safety concerns for subjects of clinical studies.

Absorption, metabolism and excretion of cobimetinib, an oral MEK inhibitor, in rats and dogs.

1. The absorption, metabolism and excretion of cobimetinib, an allosteric inhibitor of MEK1/2, was characterized in mass balance studies following single oral administration of radiolabeled (14C) cobimetinib to Sprague-Dawley rats (30 mg/kg) and Beagle dogs (5 mg/kg). 2. The oral dose of cobimetinib was well absorbed (81% and 71% in rats and dogs, respectively). The maximal plasma concentrations for cobimetinib and total radioactivity were reached at 2-3 h post-dose. Drug-derived radioactivity was fully recovered (∼90% of the administered dose) with the majority eliminated in feces via biliary excretion (78% of the dose for rats and 65% for dogs). The recoveries were nearly complete after the first 48 h following dosing. 3. The metabolic profiles indicated extensive metabolism of cobimetinib prior to its elimination. For rats, the predominant metabolic pathway was hydroxylation at the aromatic core. Lower exposures for cobimetinib and total radioactivity were observed in male rats compared with female rats, which was consistent to in vitro higher clearance of cobimetinib for male rats. For dogs, sequential oxidative reactions occurred at the aliphatic portion of the molecule. Though rat metabolism was well-predicted in vitro with liver microsomes, dog metabolism was not. 4. Rats and dogs were exposed to the two major human circulating Phase II metabolites, which provided relevant metabolite safety assessment. In general, the extensive sequential oxidative metabolism in dogs, and not the aromatic hydroxylation in rats, was more indicative of the metabolism of cobimetinib in humans.