Dennis O Scott

Pfizer Inc., New York City, New York, United States

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Publications (26)75.97 Total impact

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    ABSTRACT: In this work, we leverage a mathematical model of the underlying physiochemical properties of tissues and physicochemical properties of molecules in order to support the development of hepatoselective glucokinase activators. Passive distribution is modeled via a Fick-Nernst-Planck approach, using in vitro experimental data to estimate the permeability of both ionized and neutral species. The model accounts for pH and electrochemical potential across cellular membranes, ionization according to Henderson-Hasselbalch, passive permeation of the neutral species using Fick's law, and passive permeation of the ionized species using the Nernst-Planck equation. The mathematical model of the physiochemical system allows derivation of a single set of parameters governing the distribution of drug molecules across multiple conditions both in vitro and in vivo. A case study using this approach in the development of hepatoselective glucokinase activators (GKA) via Organic Anion Transporting Polypeptide (OATP) mediated hepatic uptake and impaired passive distribution to the pancreas is described. The results for these molecules indicate the permeability penalty of the ionized form is offset by its relative abundance, leading to passive pancreatic exclusion according to the Nernst-Planck extension of Fickian passive permeation. Generally, this model serves as a useful construct for drug discovery scientists to understand subcellular exposure of acids or bases using specific physiochemical properties.
    Drug metabolism and disposition: the biological fate of chemicals 07/2014; · 3.74 Impact Factor
  • Avijit Ghosh, Dennis O Scott, Tristan S Maurer
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    ABSTRACT: In this work, we provide a unified theoretical framework describing how drug molecules can permeate across membranes in neutral and ionized forms for unstirred in vitro systems. The analysis provides a self-consistent basis for the origin of the unstirred water layer (UWL) within the Nernst-Planck framework in the fully unstirred limit and further provides an accounting mechanism based simply on the bulk aqueous solvent diffusion constant of the drug molecule. Our framework makes no new assumptions about the underlying physics of molecular permeation. We hold simply that Nernst-Planck is a reasonable approximation at low concentrations and all physical systems must conserve mass. The applicability of the derived framework has been examined both with respect to the effect of stirring and externally applied voltages to measured permeability. The analysis contains data for 9 compounds extracted from the literature representing a range of permeabilities and aqueous diffusion coefficients. Applicability with respect to ionized permeation is examined using literature data for the permanently charged cation, crystal violet, providing a basis for the underlying mechanism for ionized drug permeation for this molecule as being due to mobile counter-current flow.
    European journal of pharmaceutical sciences: official journal of the European Federation for Pharmaceutical Sciences 11/2013; · 2.61 Impact Factor
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    ABSTRACT: Metabolic stability of drug candidates are often determined in both liver microsome and hepatocyte assays. Comparison of intrinsic clearance values between the two assays provides additional information to guide drug design. Intrinsic clearance values from human liver microsomes and hepatocytes were compared for a set of commercial drugs with known metabolic pathways and transporter characteristics. The results showed that for compounds that were predominately metabolized by CYP mediated mechanisms, the intrinsic clearance values from the two assays were comparable. For compounds with non-CYP pathways, such as UGT and AO, intrinsic clearance was faster in hepatocytes than in microsomes. Substrates of uptake or efflux transporters in this study did not have significant differences of intrinsic clearance between microsomes and hepatocytes, when uptake into the hepatocytes was not the rate-limiting step. When hepatic uptake was rate limiting, intrinsic clearance in microsomes was faster than that in hepatocytes, which was more prevalent for compounds with rapid metabolism. Low passive permeability can limit the exposure to drug molecules to the metabolizing enzymes in the hepatocytes in relationship to the rate of metabolism. The faster the rate of metabolism, the higher permeability is needed for molecule to enter the cells and not becoming rate-limiting. The findings are very useful for drug discovery programs to gain additional insights on mechanistic information to help drug design without added experiments. Follow-up studies can then be designed to address specific questions.
    European journal of medicinal chemistry 07/2012; · 3.27 Impact Factor
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    ABSTRACT: Antagonism of cannabinoid-1 (CB1) receptor signaling has been demonstrated to inhibit feeding behaviors in humans, but CB1-mediated central nervous system (CNS) side effects have halted the marketing and further development of the lead drugs against this target. However, peripherally restricted CB1 receptor antagonists may hold potential for providing the desired efficacy with reduced CNS side effect profiles. In this report we detail the discovery and structure-activity-relationship analysis of a novel bicyclic scaffold (3) that exhibits potent CB1 receptor antagonism and oral activity in preclinical feeding models. Optimization of physical properties has led to the identification of analogues which are predicted to have reduced CNS exposure and could serve as a starting point for the design of peripherally targeted CB1 receptor antagonists.
    ACS Medicinal Chemistry Letters 05/2012; 3(5):397-401. · 3.31 Impact Factor
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    ABSTRACT: The importance of mitigating drug-drug interaction (DDI) risks, which arise from inhibition of major human cytochrome P450 enzymes is a well-established component of the lead optimization process in drug discovery. More recently, there has been much interest in clinical DDIs potentially arising via time- and concentration-dependent cytochrome P450 inhibition, a phenomenon consistent with mechanism-based inactivation. Inactivated P450 is catalytically incompetent and must be replenished by newly synthesized protein. Consequently, time-dependent inhibition of P450s presents a greater safety concern compared to reversible inhibition because of the increased propensity for pharmacokinetic interactions upon multiple dosing and the sustained duration of these interactions after discontinuation of the mechanism-based inactivator. Mechanism-based or time-dependent P450 inhibitors pose an additional risk of idiosyncratic drug toxicity since the mechanism of time-dependency often involves the formation of reactive metabolites, which can react with proteins other than the P450 isozyme responsible for catalysis. in vitro time-dependent inhibition (TDI) of P450 enzymes is now routinely assessed as part of lead optimization efforts in preclinical drug discovery. However, identification of an in vitro TDI liability can raise several questions such as: What is the mechanism of TDI? Does it involve the formation of reactive metabolites? Is there a 1:1 correlation between P450 TDI and RM formation (as measured from reactive metabolite trapping studies)? What is the likelihood that a P450 time-dependent inhibitor will also cause toxicity? What are the DDI risk mitigation options when dealing with P450 inactivators in drug discovery - compound progression or termination? Several drugs exhibit in vitro TDI of P450 enzymes, but only a fraction thereof causes clinical DDIs. Hence, when do we initiate labor-intensive medicinal chemistry efforts to design compounds devoid of P450 TDI liability? What are the best methods to precisely predict the likelihood of occurrence of clinical DDIs with drug candidates that inactivate P450 enzymes? What are (if any) the qualifying considerations for clinical progression of a P450 time-dependent inactivator with projected clinical DDI risks? In an effort to address these questions and hopefully provide answers to some of them, we embarked on the present venture wherein we highlight the current state-of-the-art knowledge in this field with a special emphasis on (a) available biochemical and mechanistic approaches in drug discovery to examine TDI of P450 isozymes with new chemical entities, (b) structure-activity relationship studies with marketed drugs associated with DDIs via P450 inactivation, (c) case studies of medicinal chemistry tactics to abrogate P450 inactivation liability, (d) strategies for progression of P450 TDI-positive drug candidates, and (e) the utility of in silico methodology, including the use of physiologically-based pharmacokinetic simulators, in drug discovery to predict the magnitude of clinical DDIs risks anticipated with new clinical candidates.
    Journal of Medicinal Chemistry 03/2012; 55(11):4896-933. · 5.61 Impact Factor
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    ABSTRACT: New cholecystokinin-1 receptor (CCK1R) agonist 'triggers' were identified using iterative library synthesis. Structural activity relationship studies led to the discovery of compound 10e, a potent CCK1R agonist that demonstrated robust weight loss in a diet-induced obese rat model with very low systemic exposure. Pharmacokinetic data suggest that efficacy is primarily driven through activation of CCK1R's located within the intestinal wall.
    Bioorganic & medicinal chemistry letters 02/2012; 22(8):2943-7. · 2.65 Impact Factor
  • Hao Sun, Christopher E Keefer, Dennis O Scott
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    ABSTRACT: Fluorine- and chlorine-containing moieties have been strategically integrated into chemical structures to optimize the pharmacokinetic and metabolic properties of therapeutic agents, based partly on the concept that the addition of these substituents may lower microsomal clearance. A large-scale systematic mechanistic study of drug metabolic alteration by aromatic halogenation has hitherto not been possible due to the lack of either large clearance databases or adequate data mining tools. To address this, we systematically searched compound pairs in Pfizer's human liver microsomal clearance database of over 220,000 unique compounds to assess the effects of fluoro-, chloro- and trifluoromethyl-substitution on phenyl derivatives. Although the para-position fluorination and chlorination lowered the microsomal clearance statistically, the substitution at the ortho and meta positions for the studied fluorine- and chlorine-containing moieties dramatically increased the microsomal clearance. More importantly, we found that changes in physicochemical properties, electronic properties, and specific binding of substrates to drug metabolizing enzymes, for instance, cytochrome P450s, are all determining factors that drive the direction of microsomal clearance when a specific series of compounds are studied.
    Drug metabolism letters. 09/2011; 5(4):232-42.
  • Hao Sun, Dennis O Scott
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    ABSTRACT: CYP2C enzymes are responsible for the oxidative metabolism of a diverse number of drugs for the treatment of type 2 diabetes mellitus, a severe metabolic disorder with high prevalence. Various clinical studies found the close association between CYP2C polymorphisms and altered pharmacokinetics, toxicological profiles, and drug-drug interactions of antidiabetic drugs. In this brief review, we discussed the impact of CYP2C polymorphisms on the metabolic fate of small-molecule antidiabetics including sulfonylureas, meglitinides, thiazolidinediones, gliptins, and gliflozins, with the key drug-protein molecular interactions highlighted.
    Chemico-biological interactions 09/2011; 194(2-3):159-67. · 2.46 Impact Factor
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    ABSTRACT: 1-(8-(2-Chlorophenyl)-9-(4-chlorophenyl)-9H-purin-6-yl)-4-(ethylamino)piperidine-4-carboxamide (CP-945,598) is an orally active antagonist of the cannabinoid CB-1 receptor that progressed into phase 3 human clinical trials for the treatment of obesity. In this study, we investigated the metabolic fate and disposition of CP-945,598 in rats, Tg-RasH2 mice, and dogs after oral administration of a single dose of [(14)C]CP-945,598. Total mean recoveries of the radioactive dose were 97.7, 97.8, and 99.3% from mice, rats, and dogs, respectively. The major route of excretion in all three species was via the feces, but on the basis of separate studies in bile duct-cannulated rats and dogs, this probably reflects excretion in bile rather than incomplete absorption. CP-945,598 underwent extensive metabolism in all three species, because no unchanged parent compound was detected in the urine across species. The primary metabolic pathway of CP-945,598 involved N-deethylation to form an N-desethyl metabolite (M1). M1 was subsequently metabolized by amide hydrolysis, oxidation, and ribose conjugation to numerous novel and unusual metabolites. The major circulating and excretory metabolites were species-dependent; however, several common metabolites were observed in more than one species. In addition to parent compound, M1, M3, M4, and M5 in rats, M1, M3, and M4 in mice, and M1 and M2 in dogs were identified as the major circulating metabolites. Gender-related differences were also apparent in the quantitative and qualitative nature of the metabolites in rats. An unprecedented metabolite, M4, formed by deamidation of M1 or M3 (N-hydroxy-M1), but not by decarboxylation of M2, was identified in all species. M4 was nonenzymatically converted to M5.
    Drug metabolism and disposition: the biological fate of chemicals 08/2011; 39(12):2191-208. · 3.74 Impact Factor
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    Hao Sun, Dennis O Scott
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    ABSTRACT: 4-Aminopiperidines are a variety of therapeutic agents that are extensively metabolized by cytochrome P450s with CYP3A4 as a major isoform catalyzing their N-dealkylation reaction. However, its catalytic mechanism has not been fully elucidated in a molecular interaction level. Here, we applied theoretical approaches including the molecular mechanics-based docking to study the binding patterns and quantum mechanics-based reactivity calculations. They were supported by the experimental human liver microsomal clearance and P450 isoform phenotyping data. Our results herein suggested that the molecular interactions between substrates and CYP3A4 active site residues are essential for the N-dealkylation of 4-aminopiperidines. We also found that the serine 119 residue of CYP3A4 may serve as a key hydrogen-bonding partner to interact with the 4-amino groups of the studied drugs. The reactivity of the side chain α-carbon hydrogens drives the direction of catalysis as well. As a result, structure-based drug design approaches look promising to guide drug discovery programs into the optimized drug metabolism space.
    ACS Medicinal Chemistry Letters 08/2011; 2(8):638-643. · 3.31 Impact Factor
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    ABSTRACT: Species independence of brain tissue binding was assessed with a large number of structurally diverse compounds using equilibrium dialysis with brain homogenates of seven species and strains (Wistar Han rat, Sprague-Dawley rat, CD-1 mouse, Hartley guinea pig, beagle dog, cynomolgus monkey, and human). The results showed that the fractions unbound of the seven species and strains were strongly correlated with correlation coefficients ranging from 0.93 to 0.99. The cross-species/strain correlations were not significantly different from the interassay correlation with the same species. The linear correlation between Wistar Han and other species had a slope close to 1 and an intercept near 0. Based on orthogonal statistical analysis, no correction is needed for extrapolation of fraction unbound from Wistar Han rat to the other species or strains. Hence, brain tissue binding of Wistar Han rat can be used to obtain binding of other species and strains in drug discovery.
    Drug metabolism and disposition: the biological fate of chemicals 07/2011; 39(7):1270-7. · 3.74 Impact Factor
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    ABSTRACT: We describe the design, synthesis, and structure-activity relationships of triazolobenzodiazepinone CCK1 receptor agonists. Analogs in this series demonstrate potent agonist activity as measured by in vitro and in vivo assays for CCK1 agonism. Our efforts resulted in the identification of compound 4a which significantly reduced food intake with minimal systemic exposure in rodents.
    Bioorganic & medicinal chemistry letters 11/2010; 20(22):6797-801. · 2.65 Impact Factor
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    ABSTRACT: Cannabinoid CB(1) receptor antagonists exhibit pharmacologic properties favorable for the treatment of metabolic disease. CP-945,598 (1-[9-(4-chlorophenyl)-8-(2-chlorophenyl)-9H-purin-6-yl]-4-ethylamino piperidine-4-carboxylic acid amide hydrochloride) is a recently discovered selective, high affinity, competitive CB(1) receptor antagonist that inhibits both basal and cannabinoid agonist-mediated CB(1) receptor signaling in vitro and in vivo. CP-945,598 exhibits sub-nanomolar potency at human CB(1) receptors in both binding (K(i)=0.7 nM) and functional assays (K(i)=0.2 nM). The compound has low affinity (K(i)=7600 nM) for human CB(2) receptors. In vivo, CP-945,598 reverses four cannabinoid agonist-mediated CNS-driven responses (hypo-locomotion, hypothermia, analgesia, and catalepsy) to a synthetic cannabinoid receptor agonist. CP-945,598 exhibits dose and concentration-dependent anorectic activity in two models of acute food intake in rodents, fast-induced re-feeding and spontaneous, nocturnal feeding. CP-945,598 also acutely stimulates energy expenditure in rats and decreases the respiratory quotient indicating a metabolic switch to increased fat oxidation. CP-945,598 at 10mg/kg promoted a 9%, vehicle adjusted weight loss in a 10 day weight loss study in diet-induced obese mice. Concentration/effect relationships combined with ex vivo brain CB(1) receptor occupancy data were used to evaluate efficacy in behavioral, food intake, and energy expenditure studies. Together, these in vitro, ex vivo, and in vivo data indicate that CP-945,598 is a novel CB(1) receptor competitive antagonist that may further our understanding of the endocannabinoid system.
    Biochemical and Biophysical Research Communications 03/2010; 394(2):366-71. · 2.28 Impact Factor
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    ABSTRACT: Apparent intrinsic clearance (CLia) determined from microsomal stability assays is a cornerstone in drug discovery. Categorical bins are routinely applied to this end point to facilitate analysis. However, such bins ignore the interdependent nature of apparent intrinsic microsome clearance on several ADME parameters. Considering CLia as a determinant for both metabolic stability and potential dose is more appropriate. In this context with proper accounting for nonspecific binding to microsomes and plasma, consideration of compounds with higher CLia may be warranted. The underlying benefit is the potential increase in the number of hits or chemical diversity for evaluation during the early stages of programs.Keywords: Plasma binding; microsome binding; fraction unbound; clearance; dose; ADME parameters
    Acs Medicinal Chemistry Letters. 02/2010; 1(2).
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    ABSTRACT: Cannabinoid 1 (CB1) receptor antagonists exhibit pharmacological properties favorable for the treatment of obesity and other related metabolic disorders. CE-178253 (1-[7-(2-Chlorophenyl)-8-(4-chlorophenyl)-2-methylpyrazolo[1,5-a]-[1,3,5]triazin-4-yl]-3-ethylaminoazetidine-3-carboxylic acid hydrochloride) is a recently discovered selective centrally-acting CB1 receptor antagonist. Despite a large body of knowledge on cannabinoid receptor antagonists little data exist on the quantitative pharmacology of this therapeutic class of drugs. The purpose of the current studies was to evaluate the quantitative pharmacology and concentration/effect relationships of CE-178253 based on unbound plasma concentration and in vitro pharmacology data in different in vivo preclinical models of FI and energy expenditure. In vitro, CE-178253 exhibits sub-nanomolar potency at human CB1 receptors in both binding (Ki = 0.33 nM) and functional assays (Ki = 0.07 nM). CE-178253 has low affinity (Ki > 10,000 nM) for human CB2 receptors. In vivo, CE-178253 exhibits concentration-dependent anorectic activity in both fast-induced re-feeding and spontaneous nocturnal feeding FI models. As measured by indirect calorimetry, CE-178253 acutely stimulates energy expenditure by greater than 30% in rats and shifts substrate oxidation from carbohydrate to fat as indicated by a decrease the respiratory quotient from 0.85 to 0.75. Determination of the concentration-effect relationships and ex vivo receptor occupancy in efficacy models of energy intake and expenditure suggest that a greater than a 2-fold coverage of the Ki (50-75% receptor occupancy) is required for maximum efficacy. Finally, in two preclinical models of obesity, CE-178253 dose-dependently promotes weight loss in diet-induced obese rats and mice. We have combined quantitative pharmacology and ex vivo CB1 receptor occupancy data to assess concentration/effect relationships in food intake, energy expenditure and weight loss studies. Quantitative pharmacology studies provide a strong a foundation for establishing and improving confidence in mechanism as well as aiding in the progression of compounds from preclinical pharmacology to clinical development.
    BMC Pharmacology 01/2010; 10:9.
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    Hao Sun, Dennis O Scott
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    ABSTRACT: Significant progress has been made in structure-based drug design by pharmaceutical companies at different stages of drug discovery such as identifying new hits, enhancing molecule binding affinity in hit-to-lead, and reducing toxicities in lead optimization. Drug metabolism is a major consideration for modifying drug clearance and also a primary source for drug metabolite-induced toxicity. With major cytochrome P450 structures identified and characterized recently, structure-based drug metabolism prediction becomes increasingly attractive. In silico methods based on molecular and quantum mechanics such as docking, molecular dynamics and ab initio chemical reactivity calculations bring us closer to understand drug metabolism and predict drug-drug interactions. In this study, we review important progress in drug metabolism and common in silico techniques adopted to predict drug regioselectivity, stereoselectivity, reactive metabolites, induction, inhibition and mechanism-based inactivation, as well as their implementation in hit-to-lead drug discovery.
    Chemical Biology &amp Drug Design 10/2009; 75(1):3-17. · 2.47 Impact Factor
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    ABSTRACT: A new series of CB(1) receptor antagonists incorporating an imidazole-based isosteric replacement for the hydrazide moiety of rimonabant (SR141716) is disclosed. Members of this imidazole series possess potent/selective binding to the rCB(1) receptor and exhibit potent hCB(1) functional activity. Isopropyl analog 9a demonstrated activity in the tetrad assay and was orally-active in a food intake model.
    Bioorganic & medicinal chemistry letters 09/2009; 19(18):5351-4. · 2.65 Impact Factor
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    ABSTRACT: We report the design, synthesis, and structure-activity relationships of novel bicyclic lactam-based cannabinoid type 1 (CB(1)) receptor antagonists. Members of these series are potent, selective antagonists in in vitro/in vivo efficacy models of CB(1) antagonism and exhibit robust oral activity in rodent models of food intake. These efforts led to the identification of 19d, which has been advanced to human clinical trials for weight management.
    Journal of Medicinal Chemistry 05/2009; 52(9):2652-5. · 5.61 Impact Factor
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    ABSTRACT: We report the structure-activity relationships, design, and synthesis of the novel cannabinoid type 1 (CB1) receptor antagonist 3a (CP-945,598). Compound 3a showed subnanomolar potency at human CB1 receptors in binding (Ki = 0.7 nM) and functional assays (Ki = 0.12 nM). In vivo, compound 3a reversed cannabinoid agonist-mediated responses, reduced food intake, and increased energy expenditure and fat oxidation in rodents.
    Journal of Medicinal Chemistry 01/2009; 52(2):234-7. · 5.61 Impact Factor
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    ABSTRACT: Estimation of unbound fraction of substrate in microsomal incubation media is important in accurately predicting hepatic intrinsic clearance and drug-drug interactions. In this study, the unbound fraction of 1223 drug-like molecules in human liver microsomal incubation media has been determined using equilibrium dialysis. These compounds, which include 27 marketed drug molecules, cover a much broader range of physiochemical properties such as hydrophobicity, molecular weight, ionization state, and degree of binding than those examined in previous work. In developing the in silico model, we have used two-dimensional molecular descriptors including cLogP, Kier connectivity, shape, and E-state indices, a subset of MOE descriptors, and a set of absorption, disposition, metabolism, and excretion structural keys used for our in-house absorption, disposition, metabolism, excretion, and toxicity modeling. Hydrophobicity is the most important molecular property contributing to the nonspecific binding of substrate to microsomes. The prediction accuracy of the model is validated using a subset of 100 compounds, and 92% of the variance is accounted for by the model with a root mean square error (RMSE) of 0.10. For the training set of compounds, 99% of variance is accounted for by the model with a RMSE of 0.02. The performance of the developed model has been further tested using the 27 marketed drug molecules with a RMSE of 0.10 between the observed and the predicted unbound fraction values.
    Drug metabolism and disposition: the biological fate of chemicals 08/2008; 36(10):2130-5. · 3.74 Impact Factor