Pre-clinical pharmacology of zolmitriptan (Zomig™;formerly 311C90),a centrally and peripherally acting 5HT1B/1D agonist for migraine

Zolmitriptan (Zomig™ formerly 311C90) is a novel 5-hydroxytryptamine (5HT)1B/1D receptor agonist with proven efficacy in the acute treatment of migraine with or without preceding aura. The drug differs from presently available members of this drug class in that it combines 5HT1B/1D receptor partial agonist activity with robust oral pharmacokinetics and an ability to inhibit trigeminovascular activation centrally as well as peripherally in preclinical studies. Consistent with its selectivity for 5HT1B/1D receptors, zolmitriptan produces constriction of various isolated blood vessels, most notably cranial arteries. In anaesthetized animals, these vascular effects manifest as a selective constriction of cranial arterio-venous anastomoses resulting in a redistribution of carotid arterial blood flow. This effect is produced without significant effects on heart rate, blood pressure or blood flow to the brain, heart or lungs. Zolmitriptan also inhibits trigeminal-evoked increases in cerebral blood flow in anaesthetized cats and blocks trigeminal-evoked plasma protein extravasation in the dura of guinea-pigs. These actions are consistent with a pre-junctional inhibition of neuropeptide release from perivascular afferents of the trigeminal nerve, as confirmed by independent studies showing that zolmitriptan blocks elevations of calcitonin-gene-related peptide in jugular venous blood during electrical stimulation of the trigeminal ganglion. In all of these effects, zolmitriptan is three to four times more potent than sumatriptan, but produces the same maximum response. Zolmitriptan crosses the intact blood-brain barrier to inhibit trigeminovascular activation in the brainstem. This was shown initially by the ability of the drug to block a brainstem reflex provoking vasoactive intestinal peptide release from the VIIth cranial (facial) nerve during trigeminal stimulation. Subsequent ex vivo autoradiography confirmed that intravenously injected [3H]zolmitriptan labels a discrete population of cells in the trigeminal nucleus caudalis (TNC) and nucleus tractus solitarius. Direct evidence for a central neuromodulatory effect of zolmitriptan was provided by electrophysiological experiments which clearly demonstrated that the drug inhibits the excitability of cells in the TNC after systemic administration. This novel preclinical profile not only distinguishes zolmitriptan from sumatriptan, but raises intriguing questions about the clinical relevance of a dual action. Studies to date show that zolmitriptan indeed modulates cranial sensory processing in humans, yet central side-effects are no different from sumatriptan. This property may account for the remarkable consistency in clinical efficacy observed in clinical trials.