Baseline lipophilicity relationships in human cytochromes P450 associated with drug metabolism

Theranostic Fluorescent Probes

The ability to gain spatiotemporal information, and in some cases achieve spatiotemporal control, in the context of drug delivery makes theranostic fluorescent probes an attractive and intensely investigated research topic. This interest is reflected in the steep rise in publications on the topic that have appeared over the past decade. Theranostic fluorescent probes, in their various incarnations, generally comprise a fluorophore linked to a masked drug, in which the drug is released as the result of certain stimuli, with both intrinsic and extrinsic stimuli being reported. This release is then signaled by the emergence of a fluorescent signal. Importantly, the use of appropriate fluorophores has enabled not only this emerging fluorescence as a spatiotemporal marker for drug delivery but also has provided modalities useful in photodynamic, photothermal, and sonodynamic therapeutic applications. In this review we highlight recent work on theranostic fluorescent probes with a particular focus on probes that are activated in tumor microenvironments. We also summarize efforts to develop probes for other applications, such as neurodegenerative diseases and antibacterials. This review celebrates the diversity of designs reported to date, from discrete small-molecule systems to nanomaterials. Our aim is to provide insights into the potential clinical impact of this still-emerging research direction.

Optimization of 3-Cyano-7-cyclopropylamino-pyrazolo[1,5-a]pyrimidines toward the Development of an In Vivo Chemical Probe for CSNK2A

3-Cyano-7-cyclopropylamino-pyrazolo[1,5-a]pyrimidines, including the chemical probe SGC-CK2-1, are potent and selective inhibitors of CSNK2A in cells but have limited utility in animal models due to their poor pharmacokinetic properties. While developing analogues with reduced intrinsic clearance and the potential for sustained exposure in mice, we discovered that phase II conjugation by GST enzymes was a major metabolic transformation in hepatocytes. A protocol for codosing with ethacrynic acid, a covalent reversible GST inhibitor, was developed to improve the exposure of analogue 2h in mice. A double codosing protocol, using a combination of ethacrynic acid and irreversible P450 inhibitor 1-aminobenzotriazole, increased the blood level of 2h by 40-fold at a 5 h time point.

Optimization of 3-Cyano-7-cyclopropylamino-pyrazolo[1,5-a]pyrimidines Toward the Development of an In Vivo Chemical Probe for CSNK2A

3-cyano-7-cyclopropylamino-pyrazolo[1,5-a]pyrimidines, including the chemical probe SGC-CK2-1, are potent and selective inhibitors of CSNK2A in cells but have limited utility in animal models due to their poor pharmacokinetic properties. While developing analogs with reduced intrinsic clearance and the potential for sustained exposure in mice, we discovered that Phase II conjugation by GST enzymes was a major metabolic transformation in hepatocytes. A protocol for co-dosing with ethacrynic acid, a covalent reversible GST inhibitor, was developed to improve the exposure of analog 2h in mice. A double co-dosing protocol, using a combination of ethacrynic acid and irreversible P450 inhibitor 1-aminobenzotriazole increased the blood level of 2h by 40-fold at a 5 h time point.

Descriptors of Cytochrome Inhibitors and Useful Machine Learning Based Methods for the Design of Safer Drugs

Roughly 2.8% of annual hospitalizations are a result of adverse drug interactions in the United States, representing more than 245,000 hospitalizations. Drug-drug interactions commonly arise from major cytochrome P450 (CYP) inhibition. Various approaches are routinely employed in order to reduce the incidence of adverse interactions, such as altering drug dosing schemes and/or minimizing the number of drugs prescribed; however, often, a reduction in the number of medications cannot be achieved without impacting therapeutic outcomes. Nearly 80% of drugs fail in development due to pharmacokinetic issues, outlining the importance of examining cytochrome interactions during preclinical drug design. In this review, we examined the physiochemical and structural properties of small molecule inhibitors of CYPs 3A4, 2D6, 2C19, 2C9, and 1A2. Although CYP inhibitors tend to have distinct physiochemical properties and structural features, these descriptors alone are insufficient to predict major cytochrome inhibition probability and affinity. Machine learning based in silico approaches may be employed as a more robust and accurate way of predicting CYP inhibition. These various approaches are highlighted in the review.

Lipophilic Metabolic Efficiency (LipMetE) and Drug Efficiency Indices to Explore the Metabolic Properties of the Substrates of Selected Cytochrome P450 Isoforms

Cytochrome P450 (CYP450) enzymes belong to a superfamily of heme-containing proteins that are involved in the metabolism of structurally diverse endogenous and exogenous compounds. Various proof-of-concept studies indicate that metabolic stability and intrinsic clearance of CYP450 substrates are linked with the respective lipophilicity (log P or log D). This necessitates the normalization of lipophilicity (log P or log D) of a given CYP450 substrate with respect to its metabolic stability (LipMetE) and intrinsic clearance (log₁₀CL_int,u). Therefore, in this article, the LipMetE values of already known substrates of CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4, including some marketed drugs, have been calculated to elucidate the relationship between lipophilicity (log D _7.4) and in vitro clearance. Moreover, various drug efficiency metrics including lipophilic efficiency (LipE) and ligand efficiency (LE) have been evaluated, and the thresholds of these parameters have been defined for the CYP450 substrates exhibiting normalized LipMetE. Our results indicate that for a given range of LipMetE, greater the log D value of the substrate the more avidly it binds to a given CYP450 enzyme and shows more intrinsic clearance (log₁₀CL_int,u). Overall, the majority of the model substrates of CYP450 isoforms and already marketed drugs in our datasets exhibit log D _7.4 values of ∼2.5 with LipMetE values in the range of 0-2.5 and LipE values of ≤3. Overall, consideration of these parameters in ADME profiling could aid in reducing the drug failure rate in the later stages of clinical investigations.

Exploring the Chemical Space of Cytochrome P450 Inhibitors Using Integrated Physicochemical Parameters, Drug Efficiency Metrics and Decision Tree Models

The cytochrome P450s (CYPs) play a central role in the metabolism of various endogenous and exogenous compounds including drugs. CYPs are vulnerable to inhibition and induction which can lead to adverse drug reactions. Therefore, insights into the underlying mechanism of CYP450 inhibition and the estimation of overall CYP inhibitor properties might serve as valuable tools during the early phases of drug discovery. Herein, we present a large data set of inhibitors against five major metabolic CYPs (CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4) for the evaluation of important physicochemical properties and ligand efficiency metrics to define property trends across various activity levels (active, efficient and inactive). Decision tree models for CYP inhibition were developed with an accuracy >90% for both the training set and 10-folds cross validation. Overall, molecular weight (MW), hydrogen bond acceptors/donors (HBA/HBD) and lipophilicity (clogP/logPo/w) represent important physicochemical descriptors for CYP450 inhibitors. However, highly efficient CYP inhibitors show mean MW, HBA, HBD and logP values between 294.18–482.40,5.0–8.2,1–7.29 and 1.68–2.57, respectively. Our results might help in optimization of toxicological profiles associated with new chemical entities (NCEs), through a better understanding of inhibitor properties leading to CYP-mediated interactions.

Alleviating CYP and hERG liabilities by structure optimization of dihydrofuran-fused tricyclic benzo[d]imidazole series - Potent, selective and orally efficacious microsomal prostaglandin E synthase-1 (mPGES-1) inhibitors: Part-2

In an effort to identify CYP and hERG clean mPGES-1 inhibitors from the dihydrofuran-fused tricyclic benzo[d]imidazole series lead 7, an extensive structure-activity relationship (SAR) studies were performed. Optimization of A, D and E-rings in 7 afforded many potent compounds with human whole blood potency in the range of 160-950 nM. Selected inhibitors 21d, 21j, 21m, 21n, 21p and 22b provided selectivity against COX-enzymes and mPGES-1 isoforms (mPGES-2 and cPGES) along with sufficient selectivity against prostanoid synthases. Most of the tested analogs demonstrated required metabolic stability in liver microsomes, low hERG and CYP liability. Oral pharmacokinetics and bioavailability of lead compounds 21j, 21m and 21p are discussed in multiple species like rat, guinea pig, dog, and cynomolgus monkey. Besides, these compounds revealed low to moderate activity against human pregnane X receptor (hPXR). The selected lead 21j further demonstrated in vivo efficacy in acute hyperalgesia (ED₅₀: 39.6 mg/kg) and MIA-induced osteoarthritic pain models (ED₅₀: 106 mg/kg).

Drug metabolism in drug discovery and development

Drug metabolism as a discipline plays an important role in drug discovery and development and the effects of drug metabolism on pharmacokinetics (PK), pharmacodynamics (PD), and safety should be carefully considered. This communication provides an overview of common strategies in the area of drug metabolism for improving PK/PD and safety profiles of drug candidates; these include, but are not limited to, collaboration with medicinal chemists on structure–activity relationships (SAR) to overcome high clearance, using deuterium replacement to further optimize a lead, prodrug approaches to circumvent formulation and delivery difficulties, and addressing issues such as species differences in metabolism, drug–drug interactions (DDI) and formation of reactive metabolites.

The Cytochrome P450 3A4 has three Major Conformations: New Clues to Drug Recognition by this Promiscuous Enzyme

We computed the channels of the 3A4 isoform of the cytochrome P450 3A4 (CYP) on the basis of 24 crystal structures extracted from the Protein Data Bank (PDB). We identified three major conformations (denoted C, O1 and O2) using an enhanced version of the CCCPP software that we developed for the present work, while only two conformations (C and O² ) are considered in the literature. We established the flowchart of definition of these three conformations in function of the structural and physicochemical parameters of the ligand. The channels are characterized with qualitative and quantitative parameters, and not only with their surrounding secondary structures as it is usually done in the literature.

Characterization of the active site properties of CYP4F12

Cytochrome P450 4F12 is a drug-metabolizing enzyme that is primarily expressed in the liver, kidney, colon, small intestine, and heart. The properties of CYP4F12 that may impart an increased catalytic selectivity (decreased promiscuity) were explored through in vitro metabolite elucidation, kinetic isotope effect experiments, and computational modeling of the CYP4F12 active site. By using astemizole as a probe substrate for CYP4F12 and CYP3A4, it was observed that although CYP4F12 favored astemizole O-demethylation as the primary route of metabolism, CYP3A4 was capable of metabolizing astemizole at multiple sites on the molecule. Deuteration of astemizole at the site of O-demethylation resulted in an isotope effect of 7.1 as well as an 8.3-fold decrease in the rate of clearance for astemizole by CYP4F12. Conversely, although an isotope effect of 3.8 was observed for the formation of the O-desmethyl metabolite when deuterated astemizole was metabolized by CYP3A4, there was no decrease in the clearance of astemizole. Development of a homology model of CYP4F12 based on the crystal structure of cytochrome P450 BM3 predicted an active site volume for CYP4F12 that was approximately 76% of the active site volume of CYP3A4. As predicted, multiple favorable binding orientations were available for astemizole docked into the active site of CYP3A4, but only a single binding orientation with the site of O-demethylation oriented toward the heme was identified for CYP4F12. Overall, it appears that although CYP4F12 may be capable of binding similar ligands to other cytochrome P450 enzymes such as CYP3A4, the ability to achieve catalytically favorable orientations may be inherently more difficult because of the increased steric constraints of the CYP4F12 active site.

Quantitative Property-Property Relationship for Screening-Level Prediction of Intrinsic Clearance of Volatile Organic Chemicals in Rats and Its Integration within PBPK Models to Predict Inhalation Pharmacokinetics in Humans

The objectives of this study were (i) to develop a screening-level Quantitative property-property relationship (QPPR) for intrinsic clearance (CL_int) obtained from in vivo animal studies and (ii) to incorporate it with human physiology in a PBPK model for predicting the inhalation pharmacokinetics of VOCs. CL_int, calculated as the ratio of the in vivo V_max (μmol/h/kg bw rat) to the K_m (μM), was obtained for 26 VOCs from the literature. The QPPR model resulting from stepwise linear regression analysis passed the validation step (R² = 0.8; leave-one-out cross-validation Q² = 0.75) for CL_int normalized to the phospholipid (PL) affinity of the VOCs. The QPPR facilitated the calculation of CL_int (L PL/h/kg bw rat) from the input data on log P_ow, log blood: water PC and ionization potential. The predictions of the QPPR as lower and upper bounds of the 95% mean confidence intervals (LMCI and UMCI, resp.) were then integrated within a human PBPK model. The ratio of the maximum (using LMCI for CL_int) to minimum (using UMCI for CL_int) AUC predicted by the QPPR-PBPK model was 1.36 ± 0.4 and ranged from 1.06 (1,1-dichloroethylene) to 2.8 (isoprene). Overall, the integrated QPPR-PBPK modeling method developed in this study is a pragmatic way of characterizing the impact of the lack of knowledge of CL_int in predicting human pharmacokinetics of VOCs, as well as the impact of prediction uncertainty of CL_int on human pharmacokinetics of VOCs.

Compound lipophilicity as a descriptor to predict binding affinity (1/K(m)) in mammals

In bioaccumulation models, biotransformation is one of the processes decreasing the concentration of chemicals in an organism. In order to be metabolized, a compound needs to bind to an enzyme. In this study, we derived relationships between binding affinity and lipophilicity, expressed as Log (1/K(m)) and Log K(ow), respectively. We focused on oxidations in mammals catalyzed by alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), flavin-containing monooxygenase (FMO), and cytochrome P450 (CYP) enzymes. For all regressions, 1/K(m) increased with compound K(ow), which can be understood from the tendency to biotransform lipophilic compounds into more polar, thus more easily excretable metabolites. Lipophilicity was relevant to the binding of most of the substrate classes of ADH, ALDH, and CYP. The resulting slopes had 95% Confidence Intervals covering the value of 0.63, typically noted in protein-water distribution (Log K(pw)) and Log K(ow) regressions. A reduced slope (0.2-0.3) was found for FMO: this may be due to a different reaction mechanism involving a nucleophilic attack. The general patterns of metabolism were mechanistically interpreted in terms of partitioning theory. Information on the overall principles determining biotransformation may be helpful in predicting metabolic rates.

QSARs for PBPK modelling of environmental contaminants

Physiologically-based pharmacokinetic (PBPK) models are increasingly finding use in risk assessment applications of data-rich compounds. However, it is a challenge to determine the chemical-specific parameters for these models, particularly in time- and resource-limiting situations. In this regard, SARs, QSARs and QPPRs are potentially useful for computing the chemical-specific input parameters of PBPK models. Based on the frequency of occurrence of molecular fragments (CH(3), CH(2), CH, C, C=C, H, benzene ring and H in benzene ring structure) and exposure conditions, the available QSAR-PBPK models facilitate the simulation of tissue and blood concentrations for some inhaled volatile organic chemicals. The application domain of existing QSARs for developing PBPK models is limited, due to lack of relevant data for diverse chemicals and mechanisms. Even though this approach is conceptually applicable to non-volatile and high molecular weight organics as well, it is more challenging to predict the other PBPK model parameters required for modelling the kinetics of these chemicals (particularly tissue diffusion coefficients, association constants for binding and oral absorption rates). As the level of our understanding of the mechanistic basis of toxicokinetic processes improves, QSARs to provide a priori predictions of key chemical-specific PBPK parameters can be developed to expedite the internal dose-based health risk assessments in data-poor situations.

Increasing selectivity of CC chemokine receptor 8 antagonists by engineering nondesolvation related interactions with the intended and off-target binding sites

The metabolic stability and selectivity of a series of CCR8 antagonists against binding to the hERG ion channel and cytochrome Cyp2D6 are studied by principal component analysis. It is demonstrated that an efficient way of increasing metabolic stability and selectivity of this series is to decrease compound lipophilicity by engineering nondesolvation related attractive interactions with CCR8, as rationalized by three-dimensional receptor models. Although such polar interactions led to increased compound selectivity, such a strategy could also jeopardize the DMPK profile of compounds. However, once increased potency is found, the lipophilicity can be readjusted by engineering hydrophobic substituents that fit to CCR8 but do not fit to hERG. Several such lipophilic fragments are identified by two-dimensional fragment-based QSAR analysis. Electrophysiological measurements and site-directed mutagenesis studies indicated that the repulsive interactions of these fragments with hERG are caused by steric hindrances with residue F656.

Quantitative structure–activity relationships (QSARs) in CYP3A4 inhibitors: The importance of lipophilic character and hydrogen bonding

The results of Quantitative Structure-Activity Relationship (QSAR) analyses on three series of CYP3A4 inhibitors are reported for enzyme inhibition expressed as Ki values. These include a small series of structurally related statins and two larger groupings of structurally diverse compounds, some of which display competitive inhibition of CYP3A4 whereas others act via heme iron ligation. In all cases, however, it is apparent that there are lipophilicity relationships associated with CYP3A4 inhibitory activity in the total of 46 compounds investigated. This is evidenced by linear correlations between inhibition of CYP3A4 and the octanol-water partition coefficient (P value) when expressed logarithmically (ie., log P). In the case of the statins, however, the distribution coefficient (D) at pH 7 is used due to the effect of compound ionization. Conversion of equilibrium constants (ie. Ki and P) to the corresponding free energy changes (deltaG values) facilitates exploration of the likely intermolecular forces of interaction between the inhibitors and the active site region of CYP3A4. In this respect, there appears to be good agreement between QSAR analyses and molecular modelling of the CYP3A4 enzyme itself, and both are consistent with the known mechanisms of inhibition displayed.

Lipophilicity Relationships in Inhibitors of CYP2C9 and CYP2C19 Enzymes

Quantitative structure-activity relationships (QSARs) within a series of cytochrome P450 2C9 (CYP2C9) and cytochrome P450 2C19 (CYP2C19) inhibitors are reported. In particular, it is noted that compound lipophilicity, in the form of log P values (where P is the octanol/water partition coefficient), is an important factor in explaining the variation in inhibitory potency within these series of compounds, many of which also act as substrates for the respective enzymes. In addition, there is a role for hydrogen bonding and pi-pi stacking interactions within the P450 active site which represent secondary factors in the binding processes of these compounds.

Structure-activity relationships for the inhibition of recombinant human cytochromes P450 by curcumin analogues

Inhibition of cytochrome P450 (CYP) is a major cause of drug-drug interactions. In this work, inhibitory potentials of 33 curcumin analogues, i.e. 2,6-dibenzylidenecyclohexanone (A series), 2,5-dibenzylidenecyclopentanone (B series) and 1,4-pentadiene-3-one (C series) substituted analogues of curcumin towards recombinant human CYP1A2, CYP3A4, CYP2B6, CYP2C9 and CYP2D6, all important for drug metabolism, were studied in vitro. Fluorescence plate reader and high performance liquid chromatography (HPLC) assays were used to evaluate CYP-inhibitory activities. MOE-based Quantitative structure-activity relationship (QSAR) analysis suggested that electrostatic and hydrophobic interactions and lipophilicity are important factors for CYP inhibition. Apart from insights in important molecular properties for CYP inhibition, the present results may also guide further design of curcumin analogues with less susceptibility to drug-drug interactions.

Quantitative structure-activity relationships (QSARs) within cytochromes P450 2B (CYP2B) subfamily enzymes: the importance of lipophilicity for binding and metabolism

The results of qualitative structure-activity relationship (QSAR) analysis are reported for several series of compounds which act as substrates for mammalian CYP2B subfamily enzymes, together with a homologous series of aliphatic primary amines which are known to inhibit CYP2B enzymes. It is found that the compound lipophilicity in the form of the log P value (where P is the octanol/water partition coefficient) is related, either linearly or quadratically, to equilibrium constants of inhibition (Ki), binding (Ks) or metabolism (Km) depending on the series of compounds in question. In some instances, the difference between frontier orbital energy levels (deltaE) also features in several of the log P expressions with biological activity. Also present in a small number of correlations are parameters which are likely to be related to logP: namely, Rm, which is the partitioning factor derived from thin layer chromatography (TLC) retention times, and also the compound molecular weight (Mr). All of these three parameters ((log P, Rm and Mr) are thought to be related to the compound's ability to desolvate the P450 active site when they bind to the enzyme. Although the linear relationships between lipophilicity and CYP2B-related activity point to a major role for desolvation of the enzyme binding site in the overall interaction, it is noted that there may be an optimal log P value displayed by preferred substrates as shown by parabolic relationships with this lipophilic parameter. In addition, there is a remarkable similarity in the coefficients for the log P term of any QSAR expression, which suggests that the hydrophobicity of CYP2B active sites may be broadly equivalent between the various mammalian species.

Design and evaluation of new antipsoriatic antedrug candidates having 16-en-22-oxa-vitamin D3 structures

Design, synthesis, and in vitro and in vivo evaluation of a series of antipsoriatic antedrugs having 16-en-22-oxa-vitamin D3 are described. Among the seven compounds examined, two are promising: ester 5c and amide 5f, both of which exhibit greater potent antiproliferation activity with lessened calcemic activity than the presently prescribed maxacalcitol (2).

Calibration of the channel that determines the omega-hydroxylation regiospecificity of cytochrome P4504A1: catalytic oxidation of 12-HALODOdecanoic acids

The fatty acid omega-hydroxylation regiospecificity of CYP4 enzymes may result from presentation of the terminal carbon to the oxidizing species via a narrow channel that restricts access to the other carbon atoms. To test this hypothesis, the oxidation of 12-iodo-, 12-bromo-, and 12-chlorododecanoic acids by recombinant CYP4A1 has been examined. Although all three 12-halododecanoic acids bind to CYP4A1 with similar dissociation constants, the 12-chloro and 12-bromo fatty acids are oxidized to 12-hydroxydodecanoic acid and 12-oxododecanoic acid, whereas the 12-iodo analogue is very poorly oxidized. Incubations in H(2)(18)(2)O show that the 12-hydroxydodecanoic acid oxygen derives from water, whereas that in the aldehyde derives from O(2). The alcohol thus arises from oxidation of the halide to an oxohalonium species that is hydrolyzed by water, whereas the aldehyde arises by a conventional carbon hydroxylation-elimination mechanism. No irreversible inactivation of CYP4A1 is observed during 12-halododecanoic acid oxidation. Control experiments show that CYP2E1, which has an omega-1 regiospecificity, primarily oxidizes 12-halododecanoic acids to the omega-aldehyde rather than alcohol product. Incubation of CYP4A1 with 12,12-[(2)H](2)-12-chlorododecanoic acid causes a 2-3-fold increase in halogen versus carbon oxidation. The fact that the order of substrate oxidation (Br > Cl >> I) approximates the inverse of the intrinsic oxidizability of the halogen atoms is consistent with presentation of the halide terminus via a channel that accommodates the chloride and bromide but not iodide atoms, which implies an effective channel diameter greater than 3.90 Angstroms but smaller than 4.30 Angstroms.

Quantitative structure-activity relationships (QSARs) within the cytochrome P450 system: QSARs describing substrate binding, inhibition and induction of P450s

Quantitative structure-activity relationships (QSARs) within substrates, inducers and inhibitors of cytochromes P450 involved in xenobiotic metabolism are reported, together with QSARs associated with induction, inhibition and metabolic rate. The importance of frontier orbitals and shape descriptors, such as planarity (estimated by the area/depth(2) parameter) and rectangularity (estimated by the length/width parameter) is discussed, particularly in the context of the COMPACT system which discriminates between several P450 families associated with the activation and detoxication of xenobiotics. The use of parameters, particularly those derived from homology modelling of mammalian (especially human) P450s that are involved in exogenous metabolism, in generating QSARs for P450 substrates is discussed in the context of explaining differences in the binding affinities of human P450 substrates which are pharmacologically active.

Compound lipophilicity for substrate binding to human P450s in drug metabolism

Compound lipophilicity is of key importance to P450 binding affinity and enzyme selectivity. Here, lipophilicity is discussed with reference to the human drug-metabolizing P450 enzymes of families CYP1, CYP2 and CYP3. From an extensive compilation of log P values for P450 substrates, and by analysis of relationships between partitioning energy and substrate-binding free energy, the relevance of lipophilicity and other factors pertaining to P450 binding affinity is explained, leading to the formulation of lipophilicity relationships within substrates of each human P450 enzyme involved in drug metabolism. Furthermore, log P values for P450 substrates appear to represent markers for enzyme selectivity. Together with the important roles of hydrogen bonding and pi-pi stacking interaction energies, the desolvation of the P450 active site makes a major contribution to the overall substrate-binding energy and, consequently, a good agreement with experimental information is reported based on this analysis.

Molecular modelling of CYP2B6 based on homology with the CYP2C5 crystal structure: analysis of enzyme-substrate interactions

The results of homology modelling of CYP2B6 based on the CYP2C5 crystal structure is described in terms of substrates and inhibitors binding within the putative active site. In general these results are in agreement with currently available evidence from substrate metabolism, mode of inhibitor action and site-directed mutagenesis experiments within the CYP2B subfamily of enzymes. Consequently, the model based on the CYP2C5 template represents an advance on those models produced from bacterial P450s, such as CYP101 and CYP102. Quantitative Structure-Activity Relationships (QSARs) for substrates binding to CYP2B6 indicate a key role for hydrogen bonding, and lipophilic character, as determined by the log P parameter (where P is the octanol/water partition coefficient), is also of importance for explaining the variation in experimental binding affinity for CYP2B6 substrates. It is possible to estimate the binding energies for typical CYP2B6 substrates based on their properties and interactions with the enzyme, which show good concordance with experimental data in the form of apparent Km values.