The heme-containing Cytochrome P450s (CYPs) are a major enzymatic determinant of drug clearance and drug-drug interactions. The CYP3A4 isoform is inhibited by antifungal imidazoles or triazoles, which form low spin heme iron complexes via formation of a nitrogen-ferric iron coordinate bond. However, CYP3A4 also slowly oxidizes the antifungal itraconazole (ITZ) at a site that is ∼ 25 Å from the triazole nitrogens, suggesting that large antifungal azoles can adopt multiple orientations within the CYP3A4 active site. Here, we report a surface plasmon resonance (SPR) analysis with kinetic resolution of two binding modes of ITZ, and the related drug ketoconazole (KTZ). SPR reveals a very slow off-rate for one binding orientation. Multiphasic binding kinetics are observed and one of the two binding components resolved by curve-fitting exhibits 'equilibrium overshoot'. Pre-loading of CYP3A4 with the heme ligand imidazole abolishes this component of the antifungal azole binding trajectories, and it eliminates the conspicuously slow off-rate. The fractional populations of CYP3A4 complexes corresponding to different drug orientations can be manipulated by altering the duration of the pulse of drug exposure. UV-vis difference absorbance titrations yield low spin spectra and K D values that are consistent with the high affinity complex resolved by SPR. These results demonstrate that ITZ and KTZ bind in multiple orientations, including a catalytically productive mode and a slowly-dissociating inhibitory mode. Most importantly, they provide the first example of an SPR-based method for the kinetic characterization of drug binding to any human CYP, including mechanistic insight not available from other methods. Keywords protein-ligand interactions; tight binding inhibitors; drug metabolism; drug interactionsThe hepatic and intestinal heme-containing Cytochrome P450s (CYPs) 1 oxidize most drugs, and thus play a critical role in their metabolism and disposition [1][2][3]. Among the various human CYP isoforms, CYP3A4 contributes most to drug metabolism. CYP3A4 exhibits complex allosteric kinetics, which may result from drug-drug interactions on a single CYP molecule through multiple drug binding, as suggested by steady state kinetic behavior [4][5][6], large active sites observed in crystal structures [7][8][9][10] and spectroscopic studies [11]. Complex kinetics † This work was supported by NIH grants GM32165 (WMA, NI, KLK) and GM07750 (JTP). * Corresponding author: Tel: (206) FAX: (206) [12][13][14][15], and equilibrium optical titrations that rely on the inhibitor-or substrate-dependent changes in ferric spin state equilibrium [16][17][18]. The complexity of CYP kinetics severely impedes prediction of drug clearance and drug-drug interactions [12][13][14][15]19]. Clearly, new methods are required to elucidate the molecular basis of CYP allosterism and complex inhibitor or substrate binding.The antifungal azoles, including ketoconazole (KTZ) and itraconazole (ITZ), historically have been considered to be potent inh...

Interleukin-2 (IL-2) controls the homeostasis and function of regulatory T (Treg) cells, and defects in the IL-2 pathway contribute to multiple autoimmune diseases. Although recombinant IL-2 therapy has been efficacious in certain inflammatory conditions, the capacity for IL-2 to also activate inflammatory effector responses highlights the need for IL-2–based therapeutics with improved Treg cell specificity. From a panel of rationally designed murine IL-2 variants, we identified IL-2 muteins with reduced potency and enhanced Treg cell selectivity due to increased dependence on the IL-2 receptor component CD25. As an Fc-fused homodimer, the optimal Fc.IL-2 mutein induced selective Treg cell enrichment and reduced agonism of effector cells across a wide dose range. Furthermore, despite being a weaker agonist, overall Treg cell growth was greater and more sustained due to reduced receptor-mediated clearance of the Fc.IL-2 mutein compared with Fc-fused wild-type IL-2. Preferential Treg cell enrichment was also observed in the presence of activated pathogenic T cells in the pancreas of nonobese diabetic (NOD) mice, despite a loss of Treg cell selectivity in an IL-2R proximal response. These properties facilitated potent and extended resolution of NOD diabetes with infrequent dosing schedules.

The role of C239 as the active-site residue responsible for forming the covalent linkage with raloxifene during P450 3A4 time-dependent inactivation (TDI) was recently identified. The corresponding residue in CYP3A5 is S239, and when the potential for TDI in P450 3A5 was investigated, only reversible inhibition was observed against midazolam and testosterone, with median inhibitory concentration (IC50) values of 2.4 and 2.9 microM, respectively. In a similar fashion, when C239 was replaced with alanine in P450 3A4, TDI was successfully engineered out, and the reversible inhibition was characterized by IC50 values of 3.7 and 3.5 microM against midazolam and testosterone, respectively. Metabolism studies confirmed that the reactive diquinone methide intermediate required for P450 3A4 inactivation formed in all of the P450 3A enzymes investigated. Furthermore, the absence of TDI in P450 3A5 led to an increase in the formation of GSH-related adducts of raloxifene compared with that for P450 3A4. Consequently, the absence of the nucleophilic cysteine leads to differential TDI and generation of reactive metabolites in the P450 3A enzyme, providing the foundation for pharmacogenetics that contributes to individual differences in susceptibility to adverse drug reactions.

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One goal in drug design is to decrease clearance due to metabolism. It has been suggested that a compound's metabolic stability can be increased by incorporation of a sp 2 nitrogen into an aromatic ring. Nitrogen incorporation is hypothesized to increase metabolic stability by coordination of nitrogen to the heme iron (termed type II binding). However, questions regarding binding affinity, metabolic stability, and how metabolism of type II binders occurs remain unanswered. Herein, we use pyridinyl quinoline-4-carboxamide analogs to answer these questions. We show that type II binding can have a profound influence on binding affinity for CYP3A4, and the difference in binding affinity can be as high as 1,200 fold. We also find that type II binding compounds can be extensively metabolized, which is not consistent with the dead-end complex kinetic model assumed for type II binders. Two alternate kinetic mechanisms are presented to explain the results. The first involves a rapid equilibrium between the type II bound substrate and a metabolically oriented binding mode. The second involves direct reduction of the nitrogen-coordinated heme followed by oxygen binding.

Cytochrome P450's (P450's) catalyze the oxidative metabolism of most drugs and toxins. Although extensive studies have proven that some P450's demonstrate both homotropic and heterotropic cooperativity toward a number of substrates, the mechanistic and molecular details of P450 allostery are still not well-established. Here, we use UV/vis and heteronuclear nuclear magnetic resonance (NMR) spectroscopic techniques to study the mechanism and thermodynamics of the binding of two 9-aminophenanthrene (9-AP) and testosterone (TST) molecules to the erythromycin-metabolizing bacterial P450(eryF). UV/vis absorbance spectra of P450(eryF) demonstrated that binding occurs with apparent negative homotropic cooperativity for TST and positive homotropic cooperativity for 9-AP with Hill-equation-derived dissociation constants of K(S) = 4 and 200 microM, respectively. The broadening and shifting observed in the 2D-{1H,15N}-HSQC-monitored titrations of 15N-Phe-labeled P450(eryF) with 9-AP and TST indicated binding on intermediate and fast chemical exchange time scales, respectively, which was consistent with the Hill-equation-derived K(S) values for these two ligands. Regardless of the type of spectral perturbation observed (broadening for 9-AP and shifting for TST), the 15N-Phe NMR resonances most affected were the same in each titration, suggesting that the two ligands "contact" the same phenylalanines within the active site of P450(eryF). This finding is in agreement with X-ray crystal structures of bound P450(eryF) showing different ligands occupying similar active-site niches. Complex spectral behavior was additionally observed for a small collection of resonances in the TST titration, interpreted as multiple binding modes for the low-affinity TST molecule or multiple TST-bound P450(eryF) conformational substates. A structural and energetic model is presented that combines the energetics and structural aspects of 9-AP and TST binding derived from these observations.

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ABSTRACT:The in vitro characterization of the inhibition potential of four representative maytansinoid species observed upon hepatic and/or tumor in vivo processing of antibody-maytansine conjugates (AMCs) with cleavable and noncleavable linkers is reported. We investigated the free maytansinoid species , respectively. Because of instability in plasma, further characterization of the DM1 and DM4 intramolecular and intermolecular disulfide conjugates observed in vivo is required before an accurate drug-drug interaction (DDI) prediction can be made. AMCs with noncleavable thioether-linked DM1 as the cytotoxic agent are predicted to have no potential for a DDI with any of the major human P450s studied.