Assessing the relative contribution of CYP3A-and P-gp-mediated pathways to the overall disposition and drug-drug interaction of dabigatran etexilate using a comprehensive mechanistic physiological-based pharmacokinetic model

Dabigatran etexilate (DABE) is a clinical probe substrate for studying drug-drug interaction (DDI) through an intestinal P-glycoprotein (P-gp). A recent in vitro study, however, has suggested a potentially significant involvement of CYP3A-mediated oxidative metabolism of DABE and its intermediate monoester BIBR0951 in DDI following microdose administration of DABE. In this study, the relative significance of CYP3A- and P-gp-mediated pathways to the overall disposition of DABE has been explored using mechanistic physiologically based pharmacokinetic (PBPK) modeling approach. The developed PBPK model linked DABE with its 2 intermediate (BIBR0951 and BIBR1087) and active (dabigatran, DAB) metabolites, and with all relevant drug-specific properties known to date included. The model was successfully qualified against several datasets of DABE single/multiple dose pharmacokinetics and DDIs with CYP3A/P-gp inhibitors. Simulations using the qualified model supported that the intestinal CYP3A-mediated oxidation of BIBR0951, and not the gut P-gp-mediated efflux of DABE, was a key contributing factor to an observed difference in the DDI magnitude following the micro-versus therapeutic doses of DABE with clarithromycin. Both the saturable CYP3A-mediated metabolism of BIBR0951 and the solubility-limited DABE absorption contributed to the relatively modest nonlinearity in DAB exposure observed with increasing doses of DABE. Furthermore, the results suggested a limited role of the gut P-gp, but an appreciable, albeit small, contribution of gut CYP3A in mediating the DDIs following the therapeutic dose of DABE with dual CYP3A/P-gp inhibitors. Thus, a possibility exists for a varying extent of CYP3A involvement when using DABE as a clinical probe in the DDI assessment, across DABE dose levels.

Binding characteristics of pyrrole-scaffold hepatitis B virus capsid inhibitors and identification of novel potent compounds

Hepatitis B virus (HBV) capsid assembly modulators (CAMs) are currently being evaluated in clinical trials as potential curative therapies for HBV. This study used in silico computational modeling to provide insights into the binding characteristics between the HBV core protein and two pyrrole-scaffold inhibitors, JNJ-6379 and GLP-26, both in the CAM-Normal (CAM-N) series. Molecular dynamics simulations showed that the pyrrole inhibitors displayed similar general binding-interaction patterns to NVR 3-778, another CAM-N, with hydrophobic interactions serving as the major driving force. However, consistent with their higher potency, the pyrrole inhibitors exhibited stronger nonpolar interactions with key residues in a solvent-accessible region as compared to NVR 3-778. This feature was facilitated by distinct hydrogen bond interactions of the pyrrole scaffold inhibitors with the residue 140 in chain B of the HBV core protein (L140B). Based on these findings, novel CAM-N compounds were designed to mimic the interaction with L140B residue while maximizing nonpolar interactions in the solvent-accessible region. Several 1H-pyrrole-2-carbonyl substituted pyrrolidine-based compounds with various hydrophobic side chains were synthesized and evaluated. Through analyses of the structure-activity and structure-druggability relations of a series of compounds, CU15 emerged as the most promising lead CAM-N compound, exhibiting sub-nanomolar potency and good pharmacokinetic profiles. Overall, the study demonstrated a practical approach to leverage computational methods for understanding key target binding features for rationale-based design, and for guiding the identification of novel compounds.

The Potentially Significant Role of CYP3A-Mediated Oxidative Metabolism of Dabigatran Etexilate and Its Intermediate Metabolites in Drug-Drug Interaction Assessments Using Microdose Dabigatran Etexilate

Dabigatran etexilate (DABE), a double ester prodrug of dabigatran, is a probe substrate of intestinal P-glycoprotein (P-gp) commonly used in clinical drug-drug interaction (DDI) studies. When compared with its therapeutic dose at 150 mg, microdose DABE (375 µg) showed approximately 2-fold higher in DDI magnitudes with CYP3A/P-gp inhibitors. In this study, we conducted several in vitro metabolism studies to demonstrate that DABE, at a theoretical gut concentration after microdosing, significantly underwent NADPH-dependent oxidation (~40%-50%) in parallel to carboxylesterase-mediated hydrolysis in human intestinal microsomes. Furthermore, NADPH-dependent metabolism of its intermediate monoester, BIBR0951, was also observed in both human intestinal and liver microsomes, accounting for 100% and 50% of total metabolism, respectively. Metabolite profiling using high resolution mass spectrometry confirmed the presence of several novel oxidative metabolites of DABE and of BIBR0951 in the NADPH-fortified incubations. CYP3A was identified as the major enzyme catalyzing the oxidation of both compounds. The metabolism of DABE and BIBR0951 was well described by Michaelis-Menten kinetics, with Km ranging 1-3 µM, significantly below the expected concentrations following the therapeutic dose of DABE. Overall, the present results suggested that CYP3A played a significant role in the presystemic metabolism of DABE and BIBR0951 following microdose DABE administration, thus attributing partly to the apparent overestimation in the DDI magnitude observed with the CYP3A/P-gp inhibitors. Therefore, DABE at the microdose, unlike the therapeutic dose, would likely be a less predictive tool and should be considered as a clinical dual substrate for P-gp and CYP3A when assessing potential P-gp-mediated impacts by dual CYP3A/P-gp inhibitors. SIGNIFICANT STATEMENT: This is the first study demonstrating a potentially significant role of cytochrome P450-mediated metabolism of the prodrug DABE following a microdose but not a therapeutic dose. This additional pathway, coupled with its susceptibility to P-glycoprotein (P-gp), may make DABE a clinical dual substrate for both P-gp and CYP3A at a microdose. The study also highlights the need for better characterization of the pharmacokinetics and metabolism of a clinical drug-drug interaction probe substrate over the intended study dose range for proper result interpretations.

Quantification of CYP3A and Drug Transporters Activity in Healthy Young, Healthy Elderly and Chronic Kidney Disease Elderly Patients by a Microdose Cocktail Approach

Background: Ageing and chronic kidney disease (CKD) affect pharmacokinetic (PK) parameters. Since mechanisms are related and remain unclear, cytochrome P450 (CYP) 3A and drug transporter activities were investigated in the elderly with or without CKD and compared to healthy adults using a microdose cocktail. Methods: Healthy young participants (n = 20), healthy elderly participants (n = 16) and elderly patients with CKD (n = 17) received, in study period 1, a single dose of microdose cocktail probe containing 30 µg midazolam, 750 µg dabigatran etexilate, 100 µg atorvastatin, 10 µg pitavastatin, and 50 µg rosuvastatin. After a 14-day wash-out period, healthy young participants continued to study period 2 with the microdose cocktail plus rifampicin. PK parameters including area under the plasma concentration-time curve (AUC), maximum plasma drug concentration (C_max), and half-life were estimated before making pairwise comparisons of geometric mean ratios (GMR) between groups. Results: AUC and C_max GMR (95% confidence interval; CI) of midazolam, a CYP3A probe substrate, were increased 2.30 (1.70-3.09) and 2.90 (2.16-3.88) fold in healthy elderly and elderly patients with CKD, respectively, together with a prolonged half-life. AUC and C_max GMR (95%CI) of atorvastatin, another CYP3A substrate, was increased 2.14 (1.52-3.02) fold in healthy elderly and 4.15 (2.98-5.79) fold in elderly patients with CKD, indicating decreased CYP3A activity related to ageing. Associated AUC changes in the probe drug whose activity could be modified by intestinal P-glycoprotein (P-gp) activity, dabigatran etexilate, were observed in patients with CKD. However, whether the activity of pitavastatin and rosuvastatin is modified by organic anion transporting polypeptide 1B (OATP1B) and of breast cancer resistance protein (BCRP), respectively, in elderly participants with or without CKD was inconclusive. Conclusions: CYP3A activity is reduced in ageing. Intestinal P-gp function might be affected by CKD, but further confirmation appears warranted. Clinical Trial Registration:http://www.thaiclinicaltrials.org/ (TCTR 20180312002 registered on March 07, 2018).

Evaluation of Pharmacokinetic Drug Interactions of the Direct-Acting Antiviral Agents Elbasvir and Grazoprevir with Pitavastatin, Rosuvastatin, Pravastatin, and Atorvastatin in Healthy Adults

BACKGROUND

Many people infected with hepatitis C virus have comorbidities, including hypercholesterolemia, that are treated with statins. In this study, we evaluated the drug-drug interaction potential of the hepatitis C virus inhibitors elbasvir (EBR) and grazoprevir (GZR) with statins. Pitavastatin, rosuvastatin, pravastatin, and atorvastatin are substrates of organic anion-transporting polypeptide 1B, whereas rosuvastatin and atorvastatin are also breast cancer resistance protein substrates.

METHODS

Three open-label, phase I clinical trials in healthy adults were conducted with multiple daily doses of oral GZR or EBR/GZR and single oral doses of statins. Trial 1: GZR 200 mg plus pitavastatin 10 mg. Trial 2: Part 1, GZR 200 mg plus rosuvastatin 10 mg, then EBR 50 mg/GZR 200 mg plus rosuvastatin 10 mg; Part 2, EBR 50 mg/GZR 200 mg plus pravastatin 40 mg. Trial 3: EBR 50 mg/GZR 200 mg plus atorvastatin 10 mg.

RESULTS

Neither GZR nor EBR pharmacokinetics were meaningfully affected by statins. Coadministration of EBR/GZR did not result in clinically relevant changes in the exposure of pitavastatin or pravastatin. However, EBR/GZR increased exposure to rosuvastatin (126%) and atorvastatin (94%). Coadministration of statins plus GZR or EBR/GZR was generally well tolerated.

CONCLUSIONS

Although statins do not appreciably affect EBR or GZR pharmacokinetics, EBR/GZR can impact the pharmacokinetics of certain statins, likely via inhibition of breast cancer resistance protein but not organic anion-transporting polypeptide 1B. Coadministration of EBR/GZR with pitavastatin or pravastatin does not require adjustment of either dose of statin, whereas the dose of rosuvastatin and atorvastatin should be decreased when coadministered with EBR/GZR.

Development and Qualification of a Physiologically Based Pharmacokinetic Model of Finasteride and Minoxidil Following Scalp Application

In this study, we aimed to develop and qualify a PBPK model for scalp application using two drugs with marked differences in physicochemical properties and PK profiles. The parameters related to scalp physiology, drug PK, and formulations were incorporated into a Multi-Phase and Multi-Layer (MPML) Mechanistic Dermal Absorption (MechDermA) model within the Simcyp® Simulator V17. The finasteride PBPK model was linked to its effect on dihydrotestosterone (DHT) levels in plasma and scalp using an indirect response model. Predicted PK (and PD for finasteride) profiles and parameters were compared against the clinically reported data and justified by visual predictive checks and two-fold error criteria for model verification. The PBPK/PD model for finasteride reasonably demonstrated an ability to predict its respective PK and PD profiles, and parameters following scalp application under various clinical scenarios. Using the same scalp physiological input parameters, the minoxidil PBPK model was then developed and satisfactorily qualified with independent clinical datasets. Collectively, these results suggested that the established PBPK model may have broader utility for other topical formulations intended for scalp application.

SLCO1B1 and ABCG2 Gene Polymorphisms in a Thai Population

Introduction

Genetic polymorphisms of drug transporters influence drug transporter activity and alter pharmacokinetic profiles of the drugs. Organic anion transporting polypeptide 1B1 (OATP1B1) and breast cancer resistance protein (BCRP) are important transporters encoded by solute carrier organic anion transporter family member 1B1 (SLCO1B1) gene and ATP-binding cassette subfamily G member 2 (ABCG2) gene, respectively. Polymorphisms in these genes are associated with increased plasma statins concentrations, statin-induced myopathy and poor response to allopurinol treatment.

Purpose

We explored allele and genotype frequencies of SLCO1B1 and ABCG2 genes including their predicted phenotypes in 53 Thai participants. Of these, 17 had chronic kidney disease and were on statins.

Materials and Methods

Genotyping analysis for SLCO1B1 c.521T>C (rs4149056), c.388A>G (rs2306283), g.-11187G>A (rs4149015), and ABCG2 c.421C>A (rs2231142) was done by using TaqMan^® Real time PCR. All were tested for Hardy–Weinberg Equilibrium.

Results

Most of the participants (80%) had normal function haplotypes SLCO1B1 (*1A and *1B) while decreased (*5, *15, and *17) and unknown (*21) function haplotypes were less observed. Four phenotypes of SLCO1B1 were observed: 69.81% had normal function (*1A/*1A,*1A/*1B, and *1B/*1B), 13.21% had intermediate function (*1A/*17, *1B/*15 and *1B/*17), 9.43% had indeterminate function (*1A/*21 and *1B/*21) and 7.55% had low function (*5/*15, *15/*15, and *15/*17). ABCG2 c.421A allele frequency was 25%. The frequency of ABCG2 c.421CA and AA phenotypes were 37.7% and 5.7%, respectively. The allele and genotype frequencies observed are consistent with reports in Asians. However, there were differences in major allele distributions between Asians and Caucasians for SLCO1B1 c.388A>G; SLCO1B1 c.388G were highly found in Asians, but c.388A were more observed in Caucasians.

Conclusion

This study showed that in the Thai population, there were 4 SNPs of SLCO1B1 and ABCG2 genes. This finding may be clinically applied to minimize inter-individual variability of drugs such as statins and allopurinol. Further study with a larger sample size is needed to assess the drug profiles and responses to treatment.

Pharmacokinetic de-risking tools for selection of monoclonal antibody lead candidates

Pharmacokinetic studies play an important role in all stages of drug discovery and development. Recent advancements in the tools for discovery and optimization of therapeutic proteins have created an abundance of candidates that may fulfill target product profile criteria. Implementing a set of in silico, small scale in vitro and in vivo tools can help to identify a clinical lead molecule with promising properties at the early stages of drug discovery, thus reducing the labor and cost in advancing multiple candidates toward clinical development. In this review, we describe tools that should be considered during drug discovery, and discuss approaches that could be included in the pharmacokinetic screening part of the lead candidate generation process to de-risk unexpected pharmacokinetic behaviors of Fc-based therapeutic proteins, with an emphasis on monoclonal antibodies.

In vitro and in vivo characterisation of the metabolism and disposition of suvorexant in humans

1. Suvorexant (MK-4305, Belsomra®) is a first-in-class dual orexin receptor antagonist approved in the USA and Japan for the treatment of insomnia. The current studies describe suvorexant's absorption, disposition and potential for CYP-mediated drug interactions in humans. 2. Following single oral administration of [(14)C]suvorexant to healthy human subjects, 90% of the radioactivity was recovered (66% in faeces, 23% in urine), primarily as oxidative metabolites. 3. In plasma, suvorexant and M9 were predominant, accounting for 30 and 37% of the total radioactivity, respectively. Metabolite M17 became more prominent (approaching 10%) following multiple daily doses of unlabelled suvorexant. M9 and M17 are not expected to contribute to the pharmacological activity of suvorexant due to reduced orexin receptor binding affinity and limited brain penetration. 4. CYP3A was determined to be the predominant enzyme mediating suvorexant oxidation. In vitro, suvorexant demonstrated reversible inhibition of CYP3A4 and 2C19 (IC50 ∼ 4-5 μM), and weak time-dependent inhibition of CYP3A4 (KI = 12 μM, kinact = 0.14 min(-1)). Suvorexant was also a weak inducer of CYP3A4, 1A2 and 2B6. Given the low plasma concentrations at clinical doses, suvorexant was not anticipated to cause significant drug interactions via inhibition and/or induction of major CYPs in vivo.

Pitavastatin is a more sensitive and selective organic anion-transporting polypeptide 1B clinical probe than rosuvastatin

AIMS

Rosuvastatin and pitavastatin have been proposed as probe substrates for the organic anion-transporting polypeptide (OATP) 1B, but clinical data on their relative sensitivity and selectivity to OATP1B inhibitors are lacking. A clinical study was therefore conducted to determine their relative suitability as OATP1B probes using single oral (PO) and intravenous (IV) doses of the OATP1B inhibitor rifampicin, accompanied by a comprehensive in vitro assessment of rifampicin inhibitory potential on statin transporters.

METHODS

The clinical study comprised of two separate panels of eight healthy subjects. In each panel, subjects were randomized to receive a single oral dose of rosuvastatin (5 mg) or pitavastatin (1 mg) administered alone, concomitantly with rifampicin (600 mg) PO or IV. The in vitro transporter studies were performed using hepatocytes and recombinant expression systems.

RESULTS

Rifampicin markedly increased exposures of both statins, with greater differential increases after PO vs. IV rifampicin only for rosuvastatin. The magnitudes of the increases in area under the plasma concentration-time curve were 5.7- and 7.6-fold for pitavastatin and 4.4- and 3.3-fold for rosuvastatin, after PO and IV rifampicin, respectively. In vitro studies showed that rifampicin was an inhibitor of OATP1B1 and OATP1B3, breast cancer resistance protein and multidrug resistance protein 2, but not of organic anion transporter 3.

CONCLUSIONS

The results indicate that pitavastatin is a more sensitive and selective and thus preferred clinical OATP1B probe substrate than rosuvastatin, and that a single IV dose of rifampicin is a more selective OATP1B inhibitor than a PO dose.

Discovery of MK-3697: a selective orexin 2 receptor antagonist (2-SORA) for the treatment of insomnia

Orexin receptor antagonists have demonstrated clinical utility for the treatment of insomnia. The majority of clinical efforts to date have focused on the development of dual orexin receptor antagonists (DORAs), small molecules that antagonize both the orexin 1 and orexin 2 receptors. Our group has recently disclosed medicinal chemistry efforts to identify highly potent, orally bioavailable selective orexin 2 receptor antagonists (2-SORAs) that possess acceptable profiles for clinical development. Herein we report additional SAR studies within the 'triaryl' amide 2-SORA series focused on improvements in compound stability in acidic media and time-dependent inhibition of CYP3A4. These studies resulted in the discovery of 2,5-disubstituted isonicotinamide 2-SORAs such as compound 24 that demonstrated improved stability and TDI profiles as well as excellent sleep efficacy across species.

The effect of the neonatal Fc receptor on human IgG biodistribution in mice

The neonatal Fc receptor (FcRn) plays a pivotal role in IgG homeostasis, i.e., it salvages IgG antibodies from lysosomal degradation following fluid-phase pinocytosis, thus preventing rapid systemic elimination of IgG. Recombinant therapeutic antibodies are typically composed of human or humanized sequences, and their biodistribution, or tissue distribution, is often studied in murine models, although, the effect of FcRn on tissue distribution of human IgG in rodents has not been investigated. In this report, an (125)I-labeled human IgG1 antibody was studied in both wild type C57BL/6 (WT) and FcRn knockout (KO) mice. Total radioactivity in both plasma and tissues (0-96hr post-dose) was measured by gamma-counting. Plasma exposure of human IgG1 were significantly lower in FcRn KO mice, which is consistent with the primary function of FcRn. Differences in biodistribution of human IgG to selected tissues were also observed. Among the tissue examined, the fat, skin and muscle showed a decrease in tissue-to-blood (T/B) exposure ratio of human IgG1 in FcRn KO mice comparing to the WT mice, while the liver, spleen, kidney, and lung showed an increase in the T/B exposure ratio in FcRn KO mice. A time-dependent change in the T/B ratios of human IgG1 was also observed for many tissues in FcRn KO mice. These results suggest that, in addition to its role in IgG elimination, FcRn may also play a role in antibody biodistribution.

Discovery of 5''-chloro-N-[(5,6-dimethoxypyridin-2-yl)methyl]-2,2':5',3''-terpyridine-3'-carboxamide (MK-1064): a selective orexin 2 receptor antagonist (2-SORA) for the treatment of insomnia

The field of small-molecule orexin antagonist research has evolved rapidly in the last 15 years from the discovery of the orexin peptides to clinical proof-of-concept for the treatment of insomnia. Clinical programs have focused on the development of antagonists that reversibly block the action of endogenous peptides at both the orexin 1 and orexin 2 receptors (OX1 R and OX2 R), termed dual orexin receptor antagonists (DORAs), affording late-stage development candidates including Merck's suvorexant (new drug application filed 2012). Full characterization of the pharmacology associated with antagonism of either OX1 R or OX2 R alone has been hampered by the dearth of suitable subtype-selective, orally bioavailable ligands. Herein, we report the development of a selective orexin 2 antagonist (2-SORA) series to afford a potent, orally bioavailable 2-SORA ligand. Several challenging medicinal chemistry issues were identified and overcome during the development of these 2,5-disubstituted nicotinamides, including reversible CYP inhibition, physiochemical properties, P-glycoprotein efflux and bioactivation. This article highlights structural modifications the team utilized to drive compound design, as well as in vivo characterization of our 2-SORA clinical candidate, 5''-chloro-N-[(5,6-dimethoxypyridin-2-yl)methyl]-2,2':5',3''-terpyridine-3'-carboxamide (MK-1064), in mouse, rat, dog, and rhesus sleep models.

Discovery of 2,5-diarylnicotinamides as selective orexin-2 receptor antagonists (2-SORAs)

The orexin (or hypocretin) system has been identified as a novel target for the treatment of insomnia due to the wealth of biological and genetic data discovered over the past decade. Recently, clinical proof-of-concept was achieved for the treatment of primary insomnia using dual (OX1R/OX2R) orexin receptor antagonists. However, elucidation of the pharmacology associated with selective orexin-2 receptor antagonists (2-SORAs) has been hampered by the lack of orally bioavailable, highly selective small molecule probes. Herein, the discovery and optimization of a novel series of 2,5-diarylnicotinamides as potent and orally bioavailable orexin-2 receptor selective antagonists is described. A compound from this series demonstrated potent sleep promotion when dosed orally to EEG telemetrized rats.

Drug-drug interaction studies: regulatory guidance and an industry perspective

Recently, the US Food and Drug Administration and European Medicines Agency have issued new guidance for industry on drug interaction studies, which outline comprehensive recommendations on a broad range of in vitro and in vivo studies to evaluate drug-drug interaction (DDI) potential. This paper aims to provide an overview of these new recommendations and an in-depth scientifically based perspective on issues surrounding some of the recommended approaches in emerging areas, particularly, transporters and complex DDIs. We present a number of theoretical considerations and several case examples to demonstrate complexities in applying (1) the proposed transporter decision trees and associated criteria for studying a broad spectrum of transporters to derive actionable information and (2) the recommended model-based approaches at an early stage of drug development to prospectively predict DDIs involving time-dependent inhibition and mixed inhibition/induction of drug metabolizing enzymes. We hope to convey the need for conducting DDI studies on a case-by-case basis using a holistic scientifically based interrogative approach and to communicate the need for additional research to fill in knowledge gaps in these areas where the science is rapidly evolving to better ensure the safety and efficacy of new therapeutic agents.

In vitro assessment of drug-drug interaction potential of boceprevir associated with drug metabolizing enzymes and transporters

The inhibitory effect of boceprevir (BOC), an inhibitor of hepatitis C virus nonstructural protein 3 protease was evaluated in vitro against a panel of drug-metabolizing enzymes and transporters. BOC, a known substrate for cytochrome P450 (P450) CYP3A and aldo-ketoreductases, was a reversible time-dependent inhibitor (k(inact) = 0.12 minute(-1), K(I) = 6.1 µM) of CYP3A4/5 but not an inhibitor of other major P450s, nor of UDP-glucuronosyltransferases 1A1 and 2B7. BOC showed weak to no inhibition of breast cancer resistance protein (BCRP), P-glycoprotein (Pgp), or multidrug resistance protein 2. It was a moderate inhibitor of organic anion transporting polypeptide (OATP) 1B1 and 1B3, with an IC(50) of 18 and 4.9 µM, respectively. In human hepatocytes, BOC inhibited CYP3A-mediated metabolism of midazolam, OATP1B-mediated hepatic uptake of pitavastatin, and both the uptake and metabolism of atorvastatin. The inhibitory potency of BOC was lower than known inhibitors of CYP3A (ketoconazole), OATP1B (rifampin), or both (telaprevir). BOC was a substrate for Pgp and BCRP but not for OATP1B1, OATP1B3, OATP2B1, organic cation transporter, or sodium/taurocholate cotransporting peptide. Overall, our data suggest that BOC has the potential to cause pharmacokinetic interactions via inhibition of CYP3A and CYP3A/OATP1B interplay, with the interaction magnitude lower than those observed with known potent inhibitors. Conversely, pharmacokinetic interactions of BOC, either as a perpetrator or victim, via other major P450s and transporters tested are less likely to be of clinical significance. The results from clinical drug-drug interaction studies conducted thus far are generally supportive of these conclusions.

The impact of sialic acids on the pharmacokinetics of a PEGylated erythropoietin

Erythropoietin (EPO) is an important molecule in the erythropoiesis and various forms of EPO have been marketed in managing anemia in humans. Long acting EPOs for less frequent dosing have been generated either by increasing the number of glycosylation sites of the EPO molecule or by linking it to a polyethylene glycol (PEG). We have generated recombinant human EPO (rhEPO) using glycoengineered Pichia pastoris strains and evaluated the pharmacokinetics (PK) in rats of this molecule linked to a 40 kDa PEG (PEGylated rhEPO), in relation to its glycosylation patterns. As expected, the PEGylated rhEPO exhibited a significant improvement in half-life of serum when compared with the non-PEGylated version. Interestingly, the PK properties of the PEGylated rhEPO molecule were also significantly influenced by the glycosylation profile. Specifically, PEGylated rhEPO with a significantly higher sialic acid content in the biantennary structure (high A2) exhibited lower systemic clearance and higher systemic exposure than those with a lower sialic acid content (low A2) following either intravenous or subcutaneous administrations. These results suggest that A2 content may be one of the important criteria for release in manufacturing PEGylated rhEPO to ensure consistent PK.

UGT2B17 genetic polymorphisms dramatically affect the pharmacokinetics of MK-7246 in healthy subjects in a first-in-human study

MK-7246, an antagonist of the chemoattractant receptor on T helper type 2 (Th2) cells, is being developed for the treatment of respiratory diseases. In a first-in-human study, we investigated whether genetic polymorphisms contributed to the marked intersubject variability in the pharmacokinetics of MK-7246 and its glucuronide metabolite M3. Results from in vitro enzyme kinetic studies suggested that UGT2B17 is probably the major enzyme responsible for MK-7246 metabolism in both the liver and the intestine. As compared with those with the UGT2B17*1/*1 wild-type genotype, UGT2B17*2/*2 carriers, who possess no UGT2B17 protein, had 25- and 82-fold greater mean dose-normalized values of area under the plasma concentration–time curve (AUC) and peak concentration of MK-7246, respectively, and a 24-fold lower M3-to-MK-7246 AUC ratio. The apparent half-life of MK-7246 was not as variable between these two genotypes. Therefore, the highly variable pharmacokinetics of MK-7246 is attributable primarily to the impact of UGT2B17 genetic polymorphisms and extensive first-pass metabolism of MK-7246.

ADME of biologics-what have we learned from small molecules?

Thorough characterization and in-depth understanding of absorption, distribution, metabolism, and elimination (ADME) properties of a drug candidate have been well recognized as an important element in small molecule (SM) drug discovery and development. This has been the area of focus for drug metabolism and pharmacokinetics (DMPK) scientists, whose role has been evolving over the past few decades from primarily being involved in the development space after a preclinical candidate was selected to extending their involvement into the discovery stage prior to candidate selection. This paradigm shift has ensured the entry into development of the best candidates with optimal ADME properties, and thus has greatly impacted SM drug development through significant reduction of the failure rate for pharmacokinetics related reasons. In contrast, the sciences of ADME and DMPK have not been fully integrated into the discovery and development processes for large molecule (LM) drugs. In this mini-review, we reflect on the journey of DMPK support of SM drug discovery and development and highlight the key enablers that have allowed DMPK scientists to make such impacts, with the aim to provide a perspective on relevant lessons learned from SM drugs that are applicable to DMPK support strategies for LMs.

Pharmacokinetics of IgG1 monoclonal antibodies produced in humanized Pichia pastoris with specific glycoforms: a comparative study with CHO produced materials

A glycoengineered Pichia pastoris host was used to produce an IgG1 with either afucosylated N-glycosylation (afucosylated biantennary complex) or without N-glycosylation (N297A) while a wild type P. pastoris host was used to produce an IgG1 containing fungal-type N- and O-linked glycosylation. The PK properties of these antibodies were compared to a commercial IgG1 produced in CHO cells following intravenous administration in wild type C57B6, FcγR-/- or hFcRn transgenic mice. MAbs produced in glycoengineered yeast exhibited similar PK properties in wild type mice or FcγR-/- mice with respect to clearance (CL), volume of distribution at steady-state (Vss) and half-life (t(1/2)) to that produced in mammalian (CHO) cells, while the mAb produced in wild type yeast exhibited ∼2-3-fold faster CL, which might be due to the high mannose content interacting with mannose receptors. Furthermore, in vitro binding affinity to human FcRn or mouse FcRn was similar between the reference mAb and mAbs produced in humanized yeast, and the glycovariants produced in humanized yeast exhibited similar PK patterns in human FcRn transgenic mice and in wild type mice. These results suggest the potential application of P. pastoris as a production platform for clinically viable mAbs.

Monoclonal antibodies with identical Fc sequences can bind to FcRn differentially with pharmacokinetic consequences

The neonatal Fc receptor (FcRn) is a key determinant of IgG homeostasis. It binds to the Fc domain of IgG in a strictly pH-dependent manner and protects IgG from lysosomal degradation. The impact of FcRn salvage pathway on IgG monoclonal antibody (mAb) pharmacokinetics (PK) has been well established. In this report, a set of mAbs with wild-type human Fc sequences but different Fab domains were used to examine the potential impact of Fab domain on in vitro FcRn binding and in vivo PK. We were surprised to find that mAbs with the same wild-type human Fc sequences but different Fab domains were shown to bind FcRn with considerable differences in both the binding at acidic pH and the dissociation at neutral pH, suggesting that the Fab domain may also have an impact on FcRn interaction. For these mAbs, no relationship between the FcRn binding affinity at acidic pH and in vivo PK was found. Instead, an apparent correlation between the in vitro FcRn dissociation at neutral pH and the in vivo PK in human FcRn mice, nonhuman primates and humans was observed. Our results suggested that the Fab domain of mAbs can affect their interaction with FcRn and thus their pharmacokinetic properties and that in vitro FcRn binding/dissociation assays can be a useful screening tool for pharmacokinetic assessment of mAbs with wild-type Fc sequences.

Pyridyl amides as potent inhibitors of T-type calcium channels

A novel series of amide T-type calcium channel antagonists were prepared and evaluated using in vitro and in vivo assays. Optimization of the screening hit 3 led to identification of the potent and selective T-type antagonist 37 that displayed in vivo efficacy in rodent models of epilepsy and sleep.

Impact of methionine oxidation in human IgG1 Fc on serum half-life of monoclonal antibodies

IgG monoclonal antibodies (mAbs) consist of two Fab fragments and one Fc fragment. The Fab fragments contain the variable regions and are responsible for drug specificity (via antigen binding); the Fc fragment contains constant regions and is responsible for effector functions (via interactions with Fcγ receptors) and extended serum half-life (via interaction with the neonatal Fc receptor, FcRn). There are two conserved methionine (Met) residues located in the FcRn binding site of the Fc fragment. It has been shown previously that oxidation of these two Met residues decreases the binding affinity to FcRn. We have further evaluated the impact of Met oxidation on serum half-lives of two humanized IgG1 mAbs in transgenic mice with human FcRn. Variable oxidation levels were obtained by several procedures: exposure to an oxidizing agent, accumulation during extended refrigerated storage, or chromatographic separation. Our results show that Met oxidation can result in a significant reduction of the serum circulation half-life and the magnitude of the change correlates well with the extent of Met oxidation and changes in FcRn binding affinities. The relatively low levels of Met oxidation accumulated during 3 years of refrigerated storage had minimal impact on FcRn binding and no detectable impact on the serum half-life.

Short-acting T-type calcium channel antagonists significantly modify sleep architecture in rodents

A novel phenyl acetamide series of short-acting T-type calcium channel antagonists has been identified and evaluated using in vitro and in vivo assays. Heterocycle substitutions of the 4-position of the phenyl acetamides afforded potent and selective antagonists that exhibited desired short plasma half-lives across preclinical species. Lead compound TTA-A8 emerged as a compound with excellent in vivo efficacy as indicated by its significant modulation of rat sleep architecture in an EEG telemetry model, favorable pharmacokinetic properties, and excellent preclinical safety. TTA-A8 recently progressed into human clinical trials, and in line with our predictions, preliminary studies (n = 12) with a 20 mg oral dose afforded a high C max of 1.82 ± 0.274 μM with an apparent terminal half-life of 3.0 ± 1.1 h.

Indazole derivatives as novel bradykinin B1 receptor antagonists

A new class of indazole-derived bradykinin B(1) antagonists and their structure-activity relationships (SAR) is reported. A number of compounds were found to have low-nanomolar affinity for the human B(1) receptor and possess acceptable P-gp and pharmacokinetics properties.

Discovery of the dual orexin receptor antagonist [(7R)-4-(5-chloro-1,3-benzoxazol-2-yl)-7-methyl-1,4-diazepan-1-yl][5-methyl-2-(2H-1,2,3-triazol-2-yl)phenyl]methanone (MK-4305) for the treatment of insomnia

Despite increased understanding of the biological basis for sleep control in the brain, few novel mechanisms for the treatment of insomnia have been identified in recent years. One notable exception is inhibition of the excitatory neuropeptides orexins A and B by design of orexin receptor antagonists. Herein, we describe how efforts to understand the origin of poor oral pharmacokinetics in a leading HTS-derived diazepane orexin receptor antagonist led to the identification of compound 10 with a 7-methyl substitution on the diazepane core. Though 10 displayed good potency, improved pharmacokinetics, and excellent in vivo efficacy, it formed reactive metabolites in microsomal incubations. A mechanistic hypothesis coupled with an in vitro assay to assess bioactivation led to replacement of the fluoroquinazoline ring of 10 with a chlorobenzoxazole to provide 3 (MK-4305), a potent dual orexin receptor antagonist that is currently being tested in phase III clinical trials for the treatment of primary insomnia.

Prediction of human clearance of therapeutic proteins: simple allometric scaling method revisited

In this report, the utility of a commonly used interspecies scaling method to predict the systemic clearance (CL) of therapeutic proteins in humans was evaluated. Based on analysis of a pharmacokinetic data set of 34 therapeutic proteins, including 12 monoclonal antibodies (mAbs) and Fc fusion proteins, human CL can generally be predicted reasonably well with simple allometric scaling and a fixed exponent of 0.8: approximately 95% of the cases predicted values within 2-fold of the observed values when using CL data from multiple species, or approximately 90% simply using CL from monkeys. Specific to mAbs/Fc fusion proteins, scaling from monkey CL using a fixed exponent of 0.8 gave an excellent prediction; all predicted CL values were within 2-fold of the corresponding observed values. Compared with the simple allometric scaling method that uses a fitted exponent from CL data of > or =3 preclinical species, the fixed exponent approach with 1-2 preclinical species is simple, resource-saving and minimizes systematic bias. Together with its overall satisfactory prediction accuracy, especially in the absence of non-linear pharmacokinetics and species-specific clearance mechanisms, this fixed exponent method affords a viable alternative to other published allometric methods, including the Rule of Exponents (ROE).

Discovery of 4,4-Disubstituted Quinazolin-2-ones as T-Type Calcium Channel Antagonists

A novel series of quinazolinone T-type calcium channel antagonists have been prepared and evaluated using in vitro and in vivo assays. Optimization of the screening hit 3 by modifications of the 3- and 4-positions of the quinazolinone ring afforded potent and selective antagonists that displayed in vivo central nervous system efficacy in epilepsy and tremor models, as well as significant effects on rat active wake as measured by electrocorticogram.

Use of in vivo animal models to assess pharmacokinetic drug-drug interactions

Animal models are used commonly in various stages of drug discovery and development to aid in the prospective assessment of drug-drug interaction (DDI) potential and the understanding of the underlying mechanism for DDI of a drug candidate. In vivo assessments in an appropriate animal model can be very valuable, when used in combination with in vitro systems, to help verify in vivo relevance of the in vitro animal-based results, and thus substantiate the extrapolation of in vitro human data to clinical outcomes. From a pharmacokinetic standpoint, a key consideration for rational selection of an animal model is based on broad similarities to humans in important physiological and biochemical parameters governing drug absorption, distribution, metabolism or excretion (ADME) processes in question for both the perpetrator and victim drugs. Equally critical are specific in vitro and/or in vivo experiments to demonstrate those similarities, usually both qualitative and quantitative, in the ADME properties/processes under investigation. In this review, theoretical basis and specific examples are presented to illustrate the utility of the animal models in assessing the potential and understanding the mechanisms of DDIs.

Discovery of MK-3207: A Highly Potent, Orally Bioavailable CGRP Receptor Antagonist

Incorporation of polar functionality into a series of highly potent calcitonin gene-related peptide (CGRP) receptor antagonists was explored in an effort to improve pharmacokinetics. This strategy identified piperazinone analogues that possessed improved solubility at acidic pH and increased oral bioavailability in monkeys. Further optimization led to the discovery of the clinical candidate 2-[(8R)-8-(3,5-difluorophenyl)-10-oxo-6,9-diazaspiro[4.5]dec-9-yl]-N-[(2R)-2'-oxo-1,1',2',3-tetrahydrospiro[indene-2,3'-pyrrolo[2,3-b]pyridin]-5-yl]acetamide (MK-3207) (4), the most potent orally active CGRP receptor antagonist described to date.

An allosteric Akt inhibitor effectively blocks Akt signaling and tumor growth with only transient effects on glucose and insulin levels in vivo

The PI3K-Akt pathway is dysregulated in the majority of solid tumors. Pharmacological inhibition of Akt is a promising strategy for treating tumors resistant to growth factor receptor antagonists due to mutations in PI3K or PTEN. We have developed allosteric, isozyme-specific inhibitors of Akt activity and activation, as well as ex vivo kinase assays to measure inhibition of individual Akt isozymes in tissues. Here we describe the relationship between PK, Akt inhibition, hyperglycemia and tumor efficacy for a selective inhibitor of Akt1 and Akt2 (AKTi). In nude mice, AKTi treatment caused transient insulin resistance and reversible, dose-dependent hyperglycemia and hyperinsulinemia. Akt1 and Akt2 phosphorylation was inhibited in mouse lung with EC50 values of 1.6 and 7 μM, respectively, and with similar potency in other tissues and xenograft tumors. Weekly subcutaneous dosing of AKTi resulted in dose-dependent inhibition of LNCaP prostate cancer xenografts, an AR-dependent tumor with PTEN deletion and constitutively activated Akt. Complete tumor growth inhibition was achieved at 200 mpk, a dose that maintained inhibition of Akt1 and Akt2 of greater than 80% and 50%, respectively, for at least 12 hours in xenograft tumor and mouse lung. Hyperglycemia could be controlled by reducing C(max), while maintaining efficacy in the LNCaP model, but not by insulin administration. AKTi treatment was well tolerated, without weight loss or gross toxicities. These studies supported the rationale for clinical development of allosteric Akt inhibitors and provide the basis for further refining of pharmacokinetic properties and dosing regimens of this class of inhibitors.

Discovery of the selective androgen receptor modulator MK-0773 using a rational development strategy based on differential transcriptional requirements for androgenic anabolism versus reproductive physiology

Selective androgen receptor modulators (SARMs) are androgen receptor (AR) ligands that induce anabolism while having reduced effects in reproductive tissues. In various experimental contexts SARMs fully activate, partially activate, or even antagonize the AR, but how these complex activities translate into tissue selectivity is not known. Here, we probed receptor function using >1000 synthetic AR ligands. These compounds produced a spectrum of activities in each assay ranging from 0 to 100% of maximal response. By testing different classes of compounds in ovariectomized rats, we established that ligands that transactivated a model promoter 40-80% of an agonist, recruited the coactivator GRIP-1 <15%, and stabilized the N-/C-terminal interdomain interaction <7% induced bone formation with reduced effects in the uterus and in sebaceous glands. Using these criteria, multiple SARMs were synthesized including MK-0773, a 4-aza-steroid that exhibited tissue selectivity in humans. Thus, AR activated to moderate levels due to reduced cofactor recruitment, and N-/C-terminal interactions produce a fully anabolic response, whereas more complete receptor activation is required for reproductive effects. This bimodal activation provides a molecular basis for the development of SARMs.

Stereospecific reduction of a potent kinesin spindle protein (KSP) inhibitor in human tissues

Compound A, 1-{(3R,3aR)-3-[3-(4-acetylpiperazin-1-yl)propyl]-7-fluoro-3-phenyl-3a,4-dihydro-3H-pyrazolo[5,1-c][1,4]benzoxazin-2-yl}ethanone, is a novel and potent inhibitor of the mitotic kinesin spindle protein. Metabolism studies with human hepatocytes showed that Compound A underwent significant ketone reduction to its biologically active metabolite M1. Here, we describe the studies that characterized the metabolic interconversion between Compound A and M1 in vitro in human tissues. LC-MS/MS analysis showed that the ketone reduction was stereospecific for M1 with no diastereomer of M1 detected in incubations with human hepatocytes. Interestingly, such stereospecific ketone reduction was not observed with Compound B, the enantiomer of Compound A. No reductive products were observed when Compound B was incubated with human hepatocytes. Studies with human liver subcellular fractions showed that Compound A was reduced to M1 primarily by human liver cytosol with little contribution from human liver microsomes and mitochondria. NADPH was the preferred cofactor for the reduction reaction. Reverse oxidation of M1 back to Compound A was also observed, preferentially in human liver cytosol with NADP(+) as the cofactor. The interconversion between Compound A and M1 in human liver cytosol was inhibited significantly by flufenamic acid and phenolphthalein (potent inhibitors for aldo-keto reductase 1Cs, p<0.05), but not by menadione, a selective inhibitor for carbonyl reductase. In addition to the liver, S9 from human lung and kidney was also capable of catalyzing this interconversion. Collectively, the results implicated the aldo-keto reductase 1Cs as the most likely enzymes responsible for the metabolic interconversion of Compound A and its active metabolite M1.

Discovery of a potent, CNS-penetrant orexin receptor antagonist based on an n,n-disubstituted-1,4-diazepane scaffold that promotes sleep in rats

Silent Night: Antagonism of the orexin (or hypocretin) system has recently been identified as a novel mechanism for the treatment of insomnia. Herein, we describe discovery of a dual (OX(1)R/OX(2)R) orexin receptor antagonist featuring a 1,4-diazepane central constraint that blocks orexin signaling in vivo. In telemetry-implanted rats, oral administration of this antagonist produced a decrease in wakefulness, while increasing REM and non-REM sleep.

Design, synthesis, and biological evaluation of 16-substituted 4-azasteroids as tissue-selective androgen receptor modulators (SARMs)

A novel series of 16-substituted-4-azasteroids has been identified as potential tissue-selective androgen receptor modulators. These ligands display potent hAR binding and agonist activity, low virilizing potential, and good pharmacokinetic profiles in dogs. On the basis of its in vitro profile, 21 was evaluated in the OVX and ORX rat models and exhibited an osteoanabolic, tissue-selective profile.

Discovery of 1,4-substituted piperidines as potent and selective inhibitors of T-type calcium channels

The discovery of a novel series of potent and selective T-type calcium channel antagonists is reported. Initial optimization of high-throughput screening leads afforded a 1,4-substituted piperidine amide 6 with good potency and limited selectivity over hERG and L-type channels and other off-target activities. Further SAR on reducing the basicity of the piperidine and introducing polarity led to the discovery of 3-axial fluoropiperidine 30 with a significantly improved selectivity profile. Compound 30 showed good oral bioavailability and brain penetration across species. In a rat genetic model of absence epilepsy, compound 30 demonstrated a robust reduction in the number and duration of seizures at 33 nM plasma concentration, with no cardiovascular effects at up to 5.6 microM. Compound 30 also showed good efficacy in rodent models of essential tremor and Parkinson's disease. Compound 30 thus demonstrates a wide margin between CNS and peripheral effects and is a useful tool for probing the effects of T-type calcium channel inhibition.

Renal elimination of a novel and potent αvβ3integrin antagonist in animals

Compound A (3-{2-oxo-3-[3-(5,6,7,8-tetrahydro-[1,8]napthyridin-2-yl)propyl]-imidazolidin-1-yl}-3(S)-(6-methoxy-pyridin-3-yl)propionic acid), a hydrophilic zwitter-ion, is a potent and selective alphavbeta3 integrin antagonist currently under clinical development for the treatment of osteoporosis. The mechanism of renal excretion of compound A was investigated using a combination of in vivo and in vitro approaches. In rats, renal excretion of compound A involved tubular secretion; ratios between renal clearance, corrected for unbound fraction in plasma (CLr,u) and glomerular filtration rate (GFR) were greater than unity (2-5). The tubular secretion of compound A was saturable at high plasma levels (> 26 microM), and was inhibited significantly, although modestly (about twofold) by relatively high plasma concentrations of the organic anion PAH (160 microM) and the cation cimetidine (about 400 microM), but not by the P-gp inhibitor quinidine (about 50 microM). However, compound A (about 100 microM) had a minimal effect on CLr/GFRs for cimetidine and PAH. In rhesus monkeys, renal elimination of compound A also involved tubular secretion, with a CLr,u/GFR ratio of about 30. The renal secretion of compound A was not affected by either cimetidine (about 120 microM) or PAH (about 80 microM). Similarly, compound A (about 40 microM) had a minimal effect on the renal tubular secretion of both cimetidine and PAH. At the doses studied, neither rat nor monkey plasma protein binding of compound A, cimetidine or PAH was affected in the presence of each other. In vitro transport studies showed that compound A was not a substrate for P-gp in the Caco-2, human MDR1 and mouse mdr1a transfected LLC-PK1 cell lines. In an uptake study using rOAT1 and rOAT3 transfected HEK cell lines, compound A was shown to be a substrate for rat OAT3 (Km= 15 microM), but not rat OAT1. The results suggest that the tubular secretion of compound A is not mediated by P-gp, but rather is mediated, at least in part, via the organic anion transporter OAT3, the renal transporter shown to be capable of transporting both the organic anion PAH and the organic cation cimetidine. Although there is a possibility for pharmacokinetic interactions between compound A and substrates or inhibitors of OAT3, at the renal excretion level, the magnitude of interaction would likely be modest in humans at clinically relevant doses.