Mechanism of Action and Pharmacological Effects
Aminophylline exerts complex effects on the respiratory and excretory systems.
- Effects on Bronchi: The primary clinical effect is the relaxation of bronchial smooth muscle. This bronchodilator action is the basis for its use in obstructive airway diseases.
- Renal Effects: The drug blocks $A_1$ adenosine receptors in the renal tubules. This inhibits the reabsorption of sodium and chloride ions and slightly increases potassium excretion.
- Hemodynamic Effects: Aminophylline increases renal blood flow and enhances glomerular filtration rate (GFR).
Note: Despite its proven diuretic effects, in modern clinical practice, it is utilized primarily as a bronchodilator.
Pharmacokinetics and Chronopharmacology
Absorption and Distribution: Upon oral administration, the active moiety (theophylline) is absorbed rapidly and completely, with bioavailability exceeding 90%. The peak plasma concentration ($C_{max}$) for conventional formulations is reached in 2 hours. The duration of action varies among patients but averages approximately 6 hours.
Chronopharmacokinetics: According to Reinberg and Smolensky, the pharmacokinetics of theophylline depend on the time of day. The highest peak plasma and saliva concentrations are achieved when the drug is administered in the morning at 07:00.
Metabolism: Hepatic metabolism converts the drug into inactive metabolites via two pathways:
- Microsomal Oxidation: Occurs in the endoplasmic reticulum via cytochrome P-450 enzymes involving dealkylation. The key isoenzyme for theophylline is CYP1A2.
- Non-microsomal Oxidation: Occurs in the hepatocyte cytosol via xanthine oxidase (hydroxylation of purine derivatives).
Drug Interactions
Aminophylline has a narrow therapeutic index; therefore, alterations in hepatic enzyme activity critically affect patient safety.
Metabolic Inhibition (Risk of Toxicity): Agents that inhibit cytochrome P-450 activity slow down theophylline metabolism, elevating blood concentrations to toxic levels.
- Potent CYP1A2 Inhibitor: Ciprofloxacin (fluoroquinolone antibiotic). Co-administration leads to a marked increase in theophylline toxicity.
- Other Inhibitors: Macrolides (erythromycin, clarithromycin), cimetidine, verapamil.
Metabolic Induction (Reduced Efficacy): Inducers accelerate xanthine metabolism, diminishing therapeutic efficacy.
- Drug Inducers: Phenobarbital (Phenobarbitalum), phenytoin, rifampin, omeprazole.
- Exogenous and Dietary Factors: CYP1A2 activity is heavily influenced by environmental factors. Components of tobacco smoke, charred foods (polycyclic aromatic hydrocarbons, methylcholanthrenes), and cruciferous vegetables (broccoli, Brussels sprouts) upregulate its activity.
Use During Lactation
Theophylline crosses the blood-milk barrier. Its concentration in breast milk is ~70% of the maternal plasma concentration.
Drug transfer into milk depends on physicochemical properties: lipophilicity, plasma protein binding, and the pH gradient (weak bases accumulate in milk via ion trapping). Even trace amounts of foreign compounds in breast milk can provoke allergic reactions in the nursing infant, requiring caution when prescribing the drug to nursing mothers.
Formulations and Administration Guidelines
Standard pharmaceutical formulations include:
- Tablets: 0.15 g;
- Suppositories: 0.15 g;
- Solution in ampoules: 2.4% (5–10 mL); 24% (1 mL).
Dosing Regimen:
- Oral: 1 tablet 3 times daily after meals. Maximum single dose (MSD) is 0.5 g; maximum daily dose (MDD) is 1.5 g.
- Intravenous: 10 mL of 2.4% solution must be diluted in 10 mL of 0.9% sodium chloride (NaCl) solution. Administration must be performed slowly (over 4–6 minutes).