Robert S. Foti

ALTERED AZT (3′-AZIDO-3′-DEOXYTHYMIDINE) GLUCURONIDATION KINETICS IN LIVER MICROSOMES AS AN EXPLANATION FOR UNDERPREDICTION OF IN VIVO CLEARANCE: COMPARISON TO HEPATOCYTES AND EFFECT OF INCUBATION ENVIRONMENT

Human liver microsomes are a reagent commonly used to predict human hepatic clearance of new chemical entities via phase 1 metabolism. Another common metabolic pathway, glucuronidation, can also be observed in human liver microsomes, although the scalability of this process has not been validated. In fact, several groups have demonstrated that clearance estimated from liver microsomes with UDP-glucuronic acid typically underpredicts the actual in vivo clearance more than 10-fold for compounds that are predominantly glucuronidated. In contrast, clearance predicted using human hepatocytes, for these same compounds, provides a more accurate assessment of in vivo clearance. We sought to characterize the kinetics of glucuronidation of the selective UGT2B7 substrate AZT (3′-azido-3′-deoxythymidine), a selective UGT2B7 substrate, in human liver microsomes (HLMs), recombinant UGT2B7, and human hepatocytes. Apparent Km values in these three preparations were 760, 490, and 87 μM, with apparent Vmax values highest in hepatocytes. The IC50 for ibuprofen against AZT glucuronidation, when run at its Km concentration in HLMs and hepatocytes, was 975 and 170 μM, respectively. Since incubation conditions have been shown to modulate glucuronidation rates, AZT glucuronidation was performed in various physiological and nonphysiological buffer systems, namely Tris, phosphate, sulfate, carbonate, acetate, human plasma, deproteinized human liver cytosol, and Williams E medium. The data showed that carbonate and Williams E medium, more physiologically relevant buffers, yielded the highest rates of AZT glucuronidation. Km observed in HLM/carbonate was 240 μM, closer to that found in hepatocytes, suggesting that matrix differences might cause the kinetic differences observed between liver preparations. Caution should be exercised when extrapolating metabolic lability via glucuronidation or inhibition of UGT enzymes from human liver microsomes, since this system appears to underpredict the degree of lability or inhibition, respectively, due in part to an apparent decrease in substrate affinity.

CYP2C19 Inhibition: The Impact of Substrate Probe Selection on in Vitro Inhibition Profiles

Understanding the potential for cytochrome P450 (P450)-mediated drug-drug interactions is a critical part of the drug discovery process. Factors such as nonspecific binding, atypical kinetics, poor effector solubility, and varying ratios of accessory proteins may alter the kinetic behavior of an enzyme and subsequently confound the extrapolation of in vitro data to the human situation. The architecture of the P450 active site and the presence of multiple binding regions within the active site may also confound in vitro-in vivo extrapolation, as inhibition profiles may be dependent on a specific inhibitor-substrate interaction. In these studies, the inhibition profiles of a set of 24 inhibitors were paneled against the CYP2C19 substrate probes (S)-mephenytoin, (R)-omeprazole, (S)-omeprazole, and (S)-fluoxetine, on the basis of their inclusion in recent U.S. Food and Drug Administration guidance for in vitro drug-drug interactions with CYP2C19. (S)-Mephenytoin was inhibited an average of 5.6-fold more potently than (R)- or (S)-omeprazole and 9.2-fold more potently than (S)-fluoxetine. Hierarchical clustering of the inhibition data suggested three substrate probe groupings, with (S)-mephenytoin exhibiting the largest difference from the rest of the substrate probes, (S)-fluoxetine exhibiting less difference from (S)-mephenytoin and the omeprazoles and (R)- and (S)-omeprazole exhibiting minimal differences from each other. Predictions of in vivo inhibition potency based on the in vitro data suggest that most drug-drug interactions will be identified by either (S)-mephenytoin or omeprazole, although the expected magnitude of the interaction may vary depending on the chosen substrate probe.

Evaluation of CYP2C8 Inhibition In Vitro: Utility of Montelukast as a Selective CYP2C8 Probe Substrate

Understanding the potential for cytochrome P450 (P450)-mediated drug-drug interactions is a critical step in the drug discovery process. Although in vitro studies with CYP3A4, CYP2C9, and CYP2C19 have suggested the presence of multiple binding regions within the P450 active site based on probe substrate-dependent inhibition profiles, similar studies have not been performed with CYP2C8. The ability to understand CYP2C8 probe substrate sensitivity will enable appropriate in vitro and in vivo probe selection. To characterize the potential for probe substrate-dependent inhibition with CYP2C8, the inhibition potency of 22 known inhibitors of CYP2C8 were measured in vitro using four clinically relevant CYP2C8 probe substrates (montelukast, paclitaxel, repaglinide, and rosiglitazone) and amodiaquine. Repaglinide exhibited the highest sensitivity to inhibition in vitro. In vitro phenotyping indicated that montelukast is an appropriate probe for CYP2C8 inhibition studies. The in vivo sensitivities of the CYP2C8 probe substrates cerivastatin, fluvastatin, montelukast, pioglitazone, and rosiglitazone were determined in relation to repaglinide on the basis of clinical drug-drug interaction (DDI) data. Repaglinide exhibited the highest sensitivity in vivo, followed by cerivastatin, montelukast, and pioglitazone. Finally, the magnitude of in vivo CYP2C8 DDI caused by gemfibrozil-1-O-β-glucuronide was predicted. Comparisons of the predictions with clinical data coupled with the potential liabilities of other CYP2C8 probes suggest that montelukast is an appropriate CYP2C8 probe substrate to use for the in vivo situation.

Prediction of CYP2D6 Drug Interactions from In Vitro Data: Evidence for Substrate-Dependent Inhibition

Predicting the magnitude of potential drug-drug interactions is important for underwriting patient safety in the clinical setting. Substrate-dependent inhibition of cytochrome P450 enzymes may confound extrapolation of in vitro results to the in vivo situation. However, the potential for substrate-dependent inhibition with CYP2D6 has not been well characterized. The inhibition profiles of 20 known inhibitors of CYP2D6 were characterized in vitro against four clinically relevant CYP2D6 substrates (desipramine, dextromethorphan, metoprolol, and thioridazine) and bufuralol. Dextromethorphan exhibited the highest sensitivity to in vitro inhibition, whereas metoprolol was the least sensitive. In addition, when metoprolol was the substrate, inhibitors with structurally constrained amino moieties (clozapine, debrisoquine, harmine, quinidine, and yohimbine) exhibited at least a 5-fold decrease in inhibition potency when results were compared with those for dextromethorphan. Atypical inhibition kinetics were observed for these and other inhibitor-substrate pairings. In silico docking studies suggested that interactions with Glu216 and an adjacent hydrophobic binding pocket may influence substrate sensitivity and inhibition potency for CYP2D6. The in vivo sensitivities of the clinically relevant CYP2D6 substrates desipramine, dextromethorphan, and metoprolol were determined on the basis of literature drug-drug interaction (DDI) outcomes. Similar to the in vitro results, dextromethorphan exhibited the highest sensitivity to CYP2D6 inhibition in vivo. Finally, the magnitude of in vivo CYP2D6 DDIs caused by quinidine was predicted using desipramine, dextromethorphan, and metoprolol. Comparisons of the predictions with literature results indicated that the marked decrease in inhibition potency observed for the metoprolol-quinidine interaction in vitro translated to the in vivo situation.

Application of cytochrome P450 drug interaction screening in drug discovery.

Advances in drug interaction screening have resulted in reduced compound attrition rates due to unfavorable CYP-mediated drug interactions in clinical trials and improved patient safety. A major driver for the success in predicting drug interactions is a better understanding of the biological, chemical or mechanical factors that can impact the prediction of drug interactions in vitro. The enzyme source, probe substrate, accessory proteins and pharmacogenetics can all have profound effects upon the robustness and relevance of data generated with in vitro drug-drug interaction assays. Furthermore, the use of in silico techniques can potentially afford a priori knowledge of drug interaction potential, thus reducing the time and cost associated with drug interaction screening. This review will focus on recent advances in in vitro, in silico and bioanalytical techniques and demonstrate how these tools are currently used to provide effective CYP drug interaction screening in a discovery setting.

Sulfonamides as Selective NaV1.7 Inhibitors: Optimizing Potency and Pharmacokinetics While Mitigating Metabolic Liabilities

Several reports have recently emerged regarding the identification of heteroarylsulfonamides as NaV1.7 inhibitors that demonstrate high levels of selectivity over other NaV isoforms. The optimization of a series of internal NaV1.7 leads that address a number of metabolic liabilities including bioactivation, PXR activation, as well as CYP3A4 induction and inhibition led to the identification of potent and selective inhibitors that demonstrated favorable pharmacokinetic profiles and were devoid of the aforementioned liabilities. The key to achieving this within a series prone to transporter-mediated clearance was the identification of a small range of optimal cLogD values and the discovery of subtle PXR SAR that was not lipophilicity dependent. This enabled the identification of compound 20, which was advanced into a target engagement pharmacodynamic model where it exhibited robust reversal of histamine-induced scratching bouts in mice.

Sulfonamides as Selective NaV1.7 Inhibitors: Optimizing Potency, Pharmacokinetics, and Metabolic Properties to Obtain Atropisomeric Quinolinone (AM-0466) that Affords Robust in Vivo Activity

Because of its strong genetic validation, NaV1.7 has attracted significant interest as a target for the treatment of pain. We have previously reported on a number of structurally distinct bicyclic heteroarylsulfonamides as NaV1.7 inhibitors that demonstrate high levels of selectivity over other NaV isoforms. Herein, we report the discovery and optimization of a series of atropisomeric quinolinone sulfonamide inhibitors [ Bicyclic sulfonamide compounds as sodium channel inhibitors and their preparation . WO 2014201206, 2014 ] of NaV1.7, which demonstrate nanomolar inhibition of NaV1.7 and exhibit high levels of selectivity over other sodium channel isoforms. After optimization of metabolic and pharmacokinetic properties, including PXR activation, CYP2C9 inhibition, and CYP3A4 TDI, several compounds were advanced into in vivo target engagement and efficacy models. When tested in mice, compound 39 (AM-0466) demonstrated robust pharmacodynamic activity in a NaV1.7-dependent model of histamine-induced pruritus (itch) and additionally in a capsaicin-induced nociception model of pain without any confounding effect in open-field activity.

Discovery and in Vivo Evaluation of the Potent and Selective PI3Kδ Inhibitors 2-((1S)-1-((6-Amino-5-cyano-4-pyrimidinyl)amino)ethyl)-6-fluoro-N-methyl-3-(2-pyridinyl)-4-quinolinecarboxamide (AM-0687) and 2-((1S)-1-((6-Amino-5-cyano-4-pyrimidinyl)amino)ethyl)-5-fluoro-N-methyl-3-(2-pyridinyl)-4-quinolinecarboxamide (AM-1430)

Optimization of the potency and pharmacokinetic profile of 2,3,4-trisubstituted quinoline, 4, led to the discovery of two potent, selective, and orally bioavailable PI3Kδ inhibitors, 6a (AM-0687) and 7 (AM-1430). On the basis of their improved profile, these analogs were selected for in vivo pharmacodynamic (PD) and efficacy experiments in animal models of inflammation. The in vivo PD studies, which were carried out in a mouse pAKT inhibition animal model, confirmed the observed potency of 6a and 7 in biochemical and cellular assays. Efficacy experiments in a keyhole limpet hemocyanin model in rats demonstrated that administration of either 6a or 7 resulted in a strong dose-dependent reduction of IgG and IgM specific antibodies. The excellent in vitro and in vivo profiles of these analogs make them suitable for further development.

Ligand-based design of a potent and selective inhibitor of cytochrome P450 2C19.

A series of omeprazole-based analogues was synthesized and assessed for inhibitory activity against CYP2C19. The data was used to build a CYP2C19 inhibition pharmacophore model for the series. The model was employed to design additional analogues with inhibitory potency against CYP2C19. Upon identifying inhibitors of CYP2C19, ligand-based design shifted to attenuating the rapid clearance observed for many of the inhibitors. While most analogues underwent metabolism on their aliphatic side chain, metabolite identification indicated that for analogues such as compound 30 which contain a heterocycle adjacent to the sulfur moiety, metabolism primarily occurred on the benzimidazole moiety. Compound 30 exhibited improved metabolic stability (Cl(int) = 12.4 mL/min/nmol) and was selective in regard to inhibition of CYP2C19-catalyzed (S)-mephenytoin hydroxylation in human liver microsomes. Finally, representative compounds were docked into a homology model of CYP2C19 in an effort to understand the enzyme-ligand interactions that may lead to favorable inhibition or metabolism properties.

The Combination of Chemical and Antibody Inhibitors for Superior P450 3A Inhibition in Reaction Phenotyping Studies

Cytochrome P450 (P450) reaction phenotyping is a key process toward accurately determining the contribution of different P450s to the metabolism of new chemical entities. The significance of P450s to drug disposition has led to the identification of selective chemical and antibody inhibitors for individual P450 enzymes. Despite these advances, the maximal inhibition attainable is limited by the use of inhibitor concentrations that maintain selectivity for the individual P450s. Thus, most commercially available inhibitors produce a maximal inhibition of ∼80%. Herein, the combination of chemical plus antibody inhibitors is explored to find whether P450 3A could be selectively and completely (>99%) inhibited by using both inhibitors simultaneously.