Naratriptan clinical pharmacology
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Clinical Pharmacology
Mechanism of Action
Naratriptan binds with high affinity to 5-HT1D and 5-HT1B receptors and has no significant affinity or pharmacological activity at 5-HT2-4 receptor subtypes or at adrenergic α1, α2, or β; dopaminergic D1 or D2; muscarinic; or benzodiazepine receptors.
The therapeutic activity of naratriptan in migraine is generally attributed to its agonist activity at 5-HT1D/1B receptors. Two current theories have been proposed to explain the efficacy of 5-HT1D/1B receptor agonists in migraine. One theory suggests that activation of 5-HT1D/1B receptors located on intracranial blood vessels, including those on the arteriovenous anastomoses, leads to vasoconstriction, which is correlated with the relief of migraine headache. The other hypothesis suggests that activation of 5-HT1D/1B receptors on sensory nerve endings in the trigeminal system results in the inhibition of pro-inflammatory neuropeptide release.
In the anesthetized dog, naratriptan has been shown to reduce the carotid arterial blood flow with little or no effect on arterial blood pressure or total peripheral resistance. While the effect on blood flow was selective for the carotid arterial bed, increases in vascular resistance of up to 30% were seen in the coronary arterial bed. Naratriptan has also been shown to inhibit trigeminal nerve activity in rat and cat. In 10 human subjects with suspected coronary artery disease (CAD) undergoing coronary artery catheterization, there was a 1% to 10% reduction in coronary artery diameter following subcutaneous injection of 1.5 mg of naratriptan.
Pharmacokinetics
Naratriptan tablets are well absorbed, with about 70% oral bioavailability. Following administration of a 2.5 mg tablet orally, the peak concentrations are obtained in 2 to 3 hours. After administration of 1- or 2.5-mg tablets, the Cmax is somewhat (about 50%) higher in women (not corrected for milligram-per-kilogram dose) than in men. During a migraine attack, absorption was slower, with a Tmax of 3 to 4 hours. Food does not affect the pharmacokinetics of naratriptan. Naratriptan displays linear kinetics over the therapeutic dose range.
The steady-state volume of distribution of naratriptan is 170 L. Plasma protein binding is 28% to 31% over the concentration range of 50 to 1,000 ng/mL.
Naratriptan is predominantly eliminated in urine, with 50% of the dose recovered unchanged and 30% as metabolites in urine. In vitro, naratriptan is metabolized by a wide range of cytochrome P450 isoenzymes into a number of inactive metabolites.
The mean elimination half-life of naratriptan is 6 hours. The systemic clearance of naratriptan is 6.6 mL/min/kg. The renal clearance (220 mL/min) exceeds glomerular filtration rate, indicating active tubular secretion. Repeat administration of naratriptan tablets does not result in drug accumulation.[1]
References
Adapted from the FDA Package Insert.