Inhibition kinetics of monoclonal antibodies against cytochromes P450

Enzyme-mediated drug-drug interactions: a review of in vivo and in vitro methodologies, regulatory guidance, and translation to the clinic

Enzyme-mediated pharmacokinetic drug-drug interactions can be caused by altered activity of drug metabolizing enzymes in the presence of a perpetrator drug, mostly via inhibition or induction. We identified a gap in the literature for a state-of-the art detailed overview assessing this type of DDI risk in the context of drug development. This manuscript discusses in vitro and in vivo methodologies employed during the drug discovery and development process to predict clinical enzyme-mediated DDIs, including the determination of clearance pathways, metabolic enzyme contribution, and the mechanisms and kinetics of enzyme inhibition and induction. We discuss regulatory guidance and highlight the utility of in silico physiologically-based pharmacokinetic modeling, an approach that continues to gain application and traction in support of regulatory filings. Looking to the future, we consider DDI risk assessment for targeted protein degraders, an emerging small molecule modality, which does not have recommended guidelines for DDI evaluation. Our goal in writing this report was to provide early-career researchers with a comprehensive view of the enzyme-mediated pharmacokinetic DDI landscape to aid their drug development efforts.

Monoclonal Antibodies and Multifunctional Cytochrome P450: Drug Metabolism as Paradigm

Monoclonal antibodies are reagents par excellence for analyzing the role of individual cytochrome P450 isoforms in multifunctional biological activities catalyzed by cytochrome P450 enzymes. The precision and utility of the monoclonal antibodies have heretofore been applied primarily to studies of human drug metabolism. The unique and precise specificity and high inhibitory activity toward individual cytochrome P450s make the monoclonal antibodies extraordinary tools for identifying and quantifying the role of each P450 isoform in the metabolism of a drug or nondrug xenobiotic. The monoclonal antibodies identify drugs metabolized by individual, several, or polymorphic P450s. A comprehensive collection of monoclonal antibodies has been isolated to human P450s: 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C family, 2C19, 2D6, 2E1, 3A4/5, and 2J2. The monoclonal antibodies can also be used for identifying drugs and/or metabolites useful as markers for in vivo phenotyping. Clinical identification of a patient's phenotype, coupled with precise knowledge of a drug's metabolism, should lead to a reduction of adverse drug reactions and improved drug therapeutics, thereby promoting advances in drug discovery.

Antibodies as a probe in cytochrome P450 research

Cytochrome P450 (P450) is the superfamily of enzymes responsible for biotransformation of endobiotics and xenobiotics. However, their large isoform multiplicity, inducibility, diverse structure, widespread distribution, polymorphic expression, and broad overlapping substrate specificity make it difficult to measure the precise role of each individual P450 to the metabolism of drugs (or carcinogens) and hamper the understanding of the relationship between the genetic/environmental factors that regulate P450 phenotype and the responses of the individual P450s to drugs. The antibodies against P450s have been useful tools for the quantitative determination of expression level and contribution of the epitope-specific P450 to the metabolism of a drug or carcinogen substrate in tissues containing multiple P450 isoforms and for implications in pharmacogenetics and human risk assessment. In particular, the inhibitory antibodies are uniquely suited for reaction phenotyping that helps to predict human pharmacokinetics for clinical drug-drug interaction potential in drug discovery and development.

Cytochrome P450 3A-Dependent Metabolism of a Potent and Selective γ-Aminobutyric AcidAα2/3 Receptor Agonist in Vitro: Involvement of Cytochrome P450 3A5 Displaying Biphasic Kinetics

In vitro metabolism studies were conducted to determine the human cytochrome P450 enzyme(s) involved in the biotransformation of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3b]pyridazine (TPA023), a selective agonist of human gamma-aminobutyric acid(A) receptor alpha2 and alpha3 subunits. Incubation of TPA023 with NADPH-fortified human liver microsomes resulted in the formation of t-butyl hydroxy TPA023, N-desethyl TPA023, and three minor metabolites. Both t-butyl hydroxylation and N-deethylation reactions were greatly inhibited (>85%) in the presence of CYP3A-selective inhibitory antibodies and chemical inhibitors, indicating that members of the CYP3A subfamily play an important role in TPA023 metabolism. Eadie-Hofstee plots of t-butyl hydroxylation and N-deethylation in pooled CYP3A5-rich human liver microsomes revealed a low K(m) (3.4 and 4.5 microM, respectively) and a high K(m) (12.7 and 40.0 microM, respectively) component. For both metabolites, the high K(m) component was not observed with a pool of microsomal preparations containing minimal levels of CYP3A5. Preincubation of liver microsomes with mifepristone (selectivity for CYP3A4 > CYP3A5) greatly inhibited both t-butyl hydroxylation and N-deethylation (>75%); however, the residual activities were significantly higher in the pooled CYP3A5-rich liver microsomes (p < 0.0005). In addition, elevated levels of residual t-butyl hydroxylase and N-deethylase activities were observed in the presence of both CYP3A5-rich and CYP3A5-deficient preparations when the substrate concentration increased from 4 to 40 microM. In agreement, metabolite formation catalyzed by recombinant CYP3A5 was described by a biphasic model. It is concluded that CYP3A4 plays a major role in TPA023 metabolism, and CYP3A5 may also contribute at higher concentrations of the compound.

CYTOCHROME P450 2C8 (CYP2C8)-MEDIATED HYDROXYLATION OF AN ENDOTHELIN ETA RECEPTOR ANTAGONIST IN HUMAN LIVER MICROSOMES

In vitro studies were performed to identify the human cytochrome P450 enzyme(s) involved in the hydroxylation (isopropyl moiety) of a previously reported endothelin ET(A) receptor antagonist, compound A [(+)-(5S,6R,7R)-2-isopropylamino-7-(4-methoxy-2-[(2R)-3-methoxy-2-methylpropyl])-5-(3,4-methylenedioxyphenyl)cyclopenteno(1,2-b) pyridine 6-carboxylic acid]. Several lines of evidence indicated that the reaction was mainly catalyzed by CYP2C8. Of the 10 recombinant cytochrome P450 isoforms tested, only CYP2C8 exhibited hydroxylase activity. In agreement, inhibitory antibodies selective for CYP2C8 attenuated (>95%) the hydroxylase activity in human liver microsomes, whereas antibodies and chemical inhibitors selective for other cytochrome P450 isoforms had a minor or no effect on the reaction. In addition, the formation of the hydroxy metabolite correlated well with CYP2C8-selective paclitaxel 6alpha-hydroxylation (r(2) approximately 0.92; p < 0.0001) and amodiaquine N-de-ethylation (r(2) approximately 0.91; p < 0.0001) in a bank of human liver microsomes (n = 15 organ donors). Finally, compound A hydroxylase activity conformed to Michaelis-Menten kinetics, and the K(m) (Michaelis constant) in human liver microsomes was similar to that of CYP2C8 ( approximately 10 microM). It is concluded that the hydroxylation of compound A is mainly catalyzed by CYP2C8, and thus the reaction can possibly serve as an alternative marker assay for CYP2C8 in human liver microsomes.

Eletriptan metabolism by human hepatic CYP450 enzymes and transport by human P-glycoprotein

"Reaction phenotyping" studies were performed with eletriptan (ETT) to determine its propensity to interact with coadministered medications. Its ability to serve as a substrate for human P-glycoprotein (P-gp) was also investigated since a central mechanism of action has been proposed for this "triptan" class of drug. In studies with a characterized bank of human liver microsome preparations, a good correlation (r2 = 0.932) was obtained between formation of N-desmethyl eletriptan (DETT) and CYP3A4-catalyzed testosterone 6 beta-hydroxylation. DETT was selected to be monitored in our studies since it represents a significant ETT metabolite in humans, circulating at concentrations 10 to 20% of those observed for parent drug. ETT was metabolized to DETT by recombinant CYP2D6 (rCYP2D6) and rCYP3A4, and to a lesser extent by rCYP2C9 and rCYP2C19. The metabolism of ETT to DETT in human liver microsomes was markedly inhibited by troleandomycin, erythromycin, miconazole, and an inhibitory antibody to CYP3A4, but not by inhibitors of other major P450 enzymes. ETT had little inhibitory effect on any of the P450 enzymes investigated. ETT was determined to be a good substrate for human P-gp in vitro. In bidirectional transport studies across LLC-MDR1 and LLC-Mdr1a cell monolayers, ETT had a BA/AB transport ratio in the range 9 to 11. This finding had significance in vivo since brain exposure to ETT was reduced 40-fold in Mdr1a+/+ relative to Mdr1a-/- mice. ETT metabolism to DETT is therefore catalyzed primarily by CYP3A4, and plasma concentrations are expected to be increased when coadministered with inhibitors of CYP3A4 and P-gp activity.

Identification of epitopes on cytochrome P450 3A4/5 recognized by monoclonal antibodies

In this study we describe the mapping of epitopes on CYP3A4/5 recognized by a panel of monoclonal antibodies (MAbs). CYP3A4 and CYP3A5 cDNAs were cloned in GST expression vectors and the fusion proteins were subjected to Western blot. Eight MAbs reacted with the full-length GST-3A4 fusion protein as well as baculovirus cDNA-expressed CYP3A4, while six of these reacted with baculovirus cDNA-expressed CYP3A5. Five (MAb 347, 351, 352, 354, and 357) out of 8 MAbs were inhibitory in a metabolic assay using quinine as substrate. MAbs 352, 354, and 357 brought about a moderate inhibition of quinine metabolism (60-70%) while MAb 347 inhibited quinine 3- hydroxylation in human liver microsomes (n=6) by more than 70%. MAb 347 was a potent inhibitor of baculovirus-expressed CYP3A5-catalyzed metabolism of quinine (95%) at </=0.20 mg IgG/nmol P450 but only moderately inhibited CYP3A4 at much higher ratios of MAb to P450. This MAb was mapped to a region of 283 to 504 amino acids on CYP3A4 protein and to an identical region on CYP3A5 protein. The region that was identified on the CYP3A5 construct was further validated based on the ability of the construct harboring the epitope to reverse the inhibition of hydroxylation of quinine by MAb 347. Our experiments clearly demonstrate that a spatial antigenic determinant is responsible for the inhibitory potency of MAb 347.

Interaction with indinavir to enhance systemic exposure of an investigational HIV protease inhibitor in rats, dogs and monkeys

1. The use of a beneficial interaction between indinavir and compound A, a potent investigational HIV protease inhibitor to enhance systemic exposure of compound A, was investigated. 2. When administrated alone, compound A underwent extensive hepatic first-pass metabolism in rats and monkeys, resulting in low oral bioavailability. 3. In vitro studies with liver microsomes revealed that compound A metabolism was mediated exclusively by CYP3A enzymes in rats, dogs and monkeys. Indinavir, which also was metabolized predominantly by CYP3A enzymes, extensively inhibited compound A metabolism in microsomes, whereas compound A showed weak inhibitory potency on indinavir metabolism. 4. Consistent with in vitro observations, co-administration of the two compounds resulted in a 17-fold increase in oral AUC of compound A in rats owing to the inhibition of metabolism of compound A by indinavir, whereas compound A did not affect indinavir metabolism as indicated by the unchanged indinavir AUC. Similarly, the systemic exposure of compound A in dogs and monkeys was increased substantially following oral co-administration with indinavir by 7- and > 50-fold, respectively. 5. Enhancement in compound A systemic exposure by indinavir in humans, as predicted based on the in vivo animal and in vitro human liver microsomal data, was confirmed in subsequent clinical studies.