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Abstract

  • Caffeine (1,3,7-trimethylxanthine): Major active component in foods and beverages.

  • Theophylline (1,3-dimethylxanthine): Used to dilate and stimulate respiratory tract.

  • Study Aims:

    • Measure maximum velocity (Vmax) of cytochrome (CYP) 1A2-mediated caffeine biotransformation using NADPH-fortified human liver microsomes (HLMs).

    • Determine in vitro inhibitory potency (IC50) and inhibition constant (Ki) of theophylline on caffeine biotransformation.

    • Explore mechanistic explanation for caffeine/theophylline interaction.

  • Findings:

    • Vmax of caffeine 3-N-demethylation: 106.3 ± 3.4 ng paraxanthine/hour/mg protein.

    • IC50 of theophylline: 75.8 ± 5.2 µM; Ki: 0.41 ± 0.03 µM.

    • Theophylline acts as a competitive inhibitor of caffeine metabolism.

Introduction

  • Caffeine: A xanthine alkaloid stimulant; restores mental alertness, used for cold and headache remedies.

  • Biotransformation: By hepatic cytochrome P450 oxidases to metabolites: paraxanthine (84%), theobromine (12%), and theophylline (4%).

    • CYP1A2 isoenzyme primarily demethylates caffeine, others may also participate.

  • Theophylline: Used as a bronchodilator, effective for asthma and respiratory conditions, with a therapeutic blood range of 10-20 mg/l.

  • Study Objectives:

    1. Measure Vmax of caffeine 3-N-demethylation.

    2. Determine IC50 and Ki of theophylline on caffeine metabolism.

    3. Provide mechanistic explanation for the caffeine/theophylline interaction.

Materials and Methods

  • Chemicals:

    • Obtained from Sigma-Aldrich: caffeine, paraxanthine, theophylline, others.

  • Human Liver Microsomes (HLMs): Pooled, purchased with protein concentration of 10 mg/ml.

  • Procedure for In Vitro Biotransformation:

    • Incubation mixture: caffeine (0.128-32.78 mM), sodium phosphate buffer (pH 7.4), NADPH.

    • 2-hour incubation at 37°C, stopped by HCl.

  • High Performance Liquid Chromatography (HPLC):

    • Caffeine and paraxanthine analyzed using Agilent HPLC with a ZORBAX Eclipse Plus-C18 column.

    • Used isocratic elution; mobile phase: methanol:water:acetic acid (75:20:5).

Kinetic Parameters

  • Km and Vmax Determination:

    • Biotransformation rate plotted against caffeine concentrations for Michaelis-Menten kinetics.

  • IC50 and Ki Determination:

    • IC50 determined by incubating caffeine with varying theophylline concentrations.

    • Ki assessed through multiple concentration datasets.

Results and Discussion

  • Biotransformation Kinetics:

    • KM of caffeine 3-N-demethylation: 0.66 ± 0.06 mM; Vmax: 106.3 ± 3.4 ng paraxanthine/hour/mg protein, consistent with literature values.

  • Inhibition by Theophylline:

    • IC50 (75.8 ± 5.2 µM) determined via non-linear regression analysis of inhibition data.

    • Ki (0.41 ± 0.03 µM) suggests theophylline inhibits caffeine metabolism via competitive inhibition based on the large alpha value indicating decreased binding of caffeine when theophylline is present.

  • Table 1: Summary of Kinetic Parameters

    • KM: 0.66 ± 0.06 mM Caffeine

    • Vmax: 106.3 ± 3.4 ng paraxanthine/hour/mg

    • IC50: 75.8 ± 5.2 µM Theophylline

    • Ki: 0.41 ± 0.03 µM Theophylline

Conclusion

  • The study reveals that theophylline significantly inhibits caffeine metabolism, reinforcing the necessity of understanding interactions between drugs metabolized by CYP1A2.

  • Potential effects on metabolism of other caffeine metabolites, such as theobromine, warrant further research.