Cisapride disposition in neonates and infants: in vivo reflection of cytochrome P450 3A4 ontogeny

Background: Lopinavir is a specific reversible inhibitor of the enzyme HIV protease with mean oral bioavailability of less than 20 % due to extensive hepatic metabolism by cytochrome P450 3A4. The reported half-life of Lopinavir is 5-6 hours and the maximum recommended daily dose is 400 mg/day. All the marketed tablet and capsule formulations of lopinavir are generally combined with Ritonavir, a potent inhibitor of cytochrome P450 3A4, to minimize presystemic metabolism of lopinavir. Hence, to overcome limitations associated with oral administration of lopinavir and to promote single drug administration, utilization of vesicular nanocarriers through topical route could prove to be effective, as the approach combines the inherent advantages of topical route and the drug-carrying potential of vesicular nanocarriers across the tough and otherwise impervious skin barrier layer, i.e., stratum corneum. Objective: The objective was to develop solid lipid nanoparticles (SLN) of lopinavir and formulate a topical gel for improved systemic bioavailability of lopinavir. Method: SLNs were prepared using high-pressure homogenization technique and optimized. The nanoparticles were characterized by SEM to confirm their spherical shape. Differential Scanning Calorimetry (DSC) analysis was carried out to ensure the entrapment of drug inside the SLNs. A comparative evaluation was done between SLN based gel and plain gel of drug by performing exvivo skin permeation studies using Franz diffusion cell. To explore the potential of topical route, invivo bioavailability study was conducted in male Wistar rats. Results: The optimized formulation composed of Compritol 888ATO (0.5 %) as a lipid, Poloxamer 407 (0.25 %) as a surfactant and Labrasol (0.25 %) as a co-surfactant gave the maximum entrapment of 69.78 % with mean particle size of 48.86nm. The plain gel of the drug gave a release of 98.406 ± 0.007 % at the end of 4hours whereas SLN based gel gave a more sustained release of 71.197 ±0.006 % at the end of 12hours ex-vivo. As observed from the results of in-vivo studies, highest Cmax was found with SLN based gel (20.3127 ± 0.6056) µg/ml as compared to plain gel (8.0655 ± 1.6369) µg/ml and oral suspension (4.2550 ± 16.380) µg/ml of the drug. Also, the AUC was higher in the case of SLN based gel indicating good bioavailability as compared to oral suspension and plain gel of drug. Conclusion: Lopinavir SLN based gel was found to have modified drug release pattern providing sustained release as compared to plain drug gel. This indicates that Lopinavir when given topically has a good potential to target the HIV as compared to when given orally.

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The Role of Cytochrome P450 3A4 in Alfentanil Clearance

Anesthesiology ◽

10.1097/00000542-199707000-00006 ◽

1997 ◽

Vol 87 (1) ◽

pp. 36-50 ◽

Cited By ~ 80

Author(s):

Evan D. Kharasch ◽

Michael Russell ◽

Douglas Mautz ◽

Kenneth E. Thummel ◽

Kent L. Kunze ◽

...

Keyword(s):

Cytochrome P450 ◽

Rate Constant ◽

Human Liver ◽

Plasma Concentrations ◽

Time Dependent ◽

Pharmacokinetic Parameters ◽

Cytochrome P450 3A4 ◽

Systemic Clearance

Background There is considerable unexplained variability in alfentanil pharmacokinetics, particularly systemic clearance. Alfentanil is extensively metabolized in vivo, and thus systemic clearance depends on hepatic biotransformation. Cytochrome P450 3A4 was previously shown to be the predominant P450 isoform responsible for human liver microsomal alfentanil metabolism in vitro. This investigation tested the hypothesis that P450 3A4 is responsible for human alfentanil metabolism and clearance in vivo. Methods Nine healthy male volunteers who provided institutionally approved written informed consent were studied in a three-way randomized crossover design. Each subject received alfentanil (20 micrograms/kg given intravenously) 30 min after midazolam (1 mg injected intravenously) on three occasions: control; high P450 3A4 activity (rifampin induction); and low P450 3A4 activity (selective inhibition by troleandomycin). Midazolam is a validated selective in vivo probe for P450 3A4 activity. Venous blood was sampled for 24 h and plasma concentrations of midazolam and alfentanil and their primary metabolites 1'-hydroxymidazolam and noralfentanil were measured by gas chromatography-mass spectrometry. Pharmacokinetic parameters were determined by two-stage analysis using both noncompartmental and three-compartment models. Results Plasma alfentanil concentration-time profiles depended significantly on P450 3A4 activity. Alfentanil noncompartmental clearance was 5.3 +/- 2.3, 14.6 +/- 3.8, and 1.1 +/- 0.5 ml.kg-1.min-1, and elimination half-life was 58 +/- 13, 35 +/- 7, and 630 +/- 374 min, respectively, in participants with normal (controls), high (rifampin), and low (troleandomycin) P450 3A4 activity (means +/- SD; P < 0.05 compared with controls). Multicompartmental modeling suggested a time-dependent inhibition-resynthesis model for troleandomycin effects on P450 3A4 activity, characterized as k10(t) = k10[1-phi e-alpha(t-tzero)], where k10(t) is the apparent time-dependent rate constant, k10 is the uninhibited rate constant, phi is the fraction of P450 3A4 inhibited, and alpha is the apparent P450 3A4 reactivation rate. Alfentanil clearance was calculated as V1 k10 for controls and men receiving rifampin, and as V1.average k10(t) for men receiving troleandomycin. This clearance was 4.9 +/- 2.1, 13.2 +/- 3.6, and 1.5 +/- 0.8 ml.kg-1.min-1, respectively, in controls and in men receiving rifampin or troleandomycin. There was a significant correlation (r = 0.97, P < 0.001) between alfentanil systemic clearance and P450 3A4 activity. Conclusions Modulation of P450 3A4 activity by rifampin and troleandomycin significantly altered alfentanil clearance and disposition. These results strongly suggest that P450 3A4 is the major isoform of P450 responsible for clinical alfentanil metabolism and clearance. This observation, combined with the known population variability in P450 3A4 activity, provides a mechanistic explanation for the interindividual variability in alfentanil disposition. Furthermore, known susceptibility of human P450 3A4 activity to induction and inhibition provides a conceptual framework for understanding and predicting clinical alfentanil drug interactions. Finally, human liver microsomal alfentanil metabolism in vitro is confirmed as an excellent model for human alfentanil metabolism in vivo.

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Structural basis for regiospecific midazolam oxidation by human cytochrome P450 3A4

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1616198114 ◽

2016 ◽

Vol 114 (3) ◽

pp. 486-491 ◽

Cited By ~ 43

Author(s):

Irina F. Sevrioukova ◽

Thomas L. Poulos

Keyword(s):

Crystal Structure ◽

Cytochrome P450 ◽

Active Site ◽

Structural Information ◽

Binding Mode ◽

Structural Basis ◽

Cytochrome P450 3A4 ◽

Human Cytochrome ◽

Cyp3a4 Substrate

Human cytochrome P450 3A4 (CYP3A4) is a major hepatic and intestinal enzyme that oxidizes more than 60% of administered therapeutics. Knowledge of how CYP3A4 adjusts and reshapes the active site to regioselectively oxidize chemically diverse compounds is critical for better understanding structure–function relations in this important enzyme, improving the outcomes for drug metabolism predictions, and developing pharmaceuticals that have a decreased ability to undergo metabolism and cause detrimental drug–drug interactions. However, there is very limited structural information on CYP3A4–substrate interactions available to date. Despite the vast variety of drugs undergoing metabolism, only the sedative midazolam (MDZ) serves as a marker substrate for the in vivo activity assessment because it is preferentially and regioselectively oxidized by CYP3A4. We solved the 2.7 Å crystal structure of the CYP3A4–MDZ complex, where the drug is well defined and oriented suitably for hydroxylation of the C1 atom, the major site of metabolism. This binding mode requires H-bonding to Ser119 and a dramatic conformational switch in the F–G fragment, which transmits to the adjacent D, E, H, and I helices, resulting in a collapse of the active site cavity and MDZ immobilization. In addition to providing insights on the substrate-triggered active site reshaping (an induced fit), the crystal structure explains the accumulated experimental results, identifies possible effector binding sites, and suggests why MDZ is predominantly metabolized by the CYP3A enzyme subfamily.

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