The 5-hydroxytryptamine (5-HT1B/1D) receptor agonists have been shown to be effective in the short-term treatment of migraine headache (1-9). These agents have provided marked benefit to patients, but the exact mechanism of the therapeutic effects of sumatriptan and other 5-HT1B/1D receptor agonists is unknown. Some hypotheses suggest an involvement of constriction of cerebrovascular smooth muscle or a direct inhibitory effect on release of neuropeptide from primary afferent nerve terminals or both (10).
Sumatriptan produces potent contractile actions in pial arterial preparations from a variety of species including rabbit (11), dog (12), monkey (12), and humans (13,14). The actions of sumatriptan are not confined to the pial vasculature, as it also contracts isolated meningeal arteries (14) and increases perfusion pressure in the dural bed (15). The effects of sumatriptan also are not limited to the cerebral circulation, as it was initially identified by its ability to constrict dog isolated saphenous vein (16) and has been shown to produce constriction of human coronary artery (17,18). In animal models, sumatriptan has been shown to increase carotid vascular resistance (19-21) and to decrease trigeminal nerve ganglion-stimulated extravasation of protein in the meningeal tissue (22).
An essentially similar pharmacologic profile of activity also was observed for other 5-HT1B/1D-receptor agonists such as zolmitriptan (23), naratriptan (24), avitriptan (25), and alniditan (26). However, some differences have been observed. For example, it has been suggested that rizatriptan produces a reduced maximal degree of contraction in human isolated coronary arteries compared with sumatriptan (27,28). In contrast, other groups have found broadly similar contractile profiles for sumatriptan, naratriptan, rizatriptan, and zolmitriptan in the human isolated coronary artery (29).
Frovatriptan is a novel 5-HT1B/1D-receptor ligand with high affinity for the human cloned 5-HT1B (pki, 8.6) and 5-HT1D (pki, 8.4) receptors and selectivity over human cloned 5-HT2A (pki, <5.3) and 5-HT2C receptors (pki, <5.3; 30).
In view of the peripheral constrictor activity of 5-HT1B/1D agonists, the aim of this series of experiments was to compare the smooth-muscle contractile effects of frovatriptan, a novel 5-HT1B/1D ligand discovered in our laboratories (30) in human isolated basilar and coronary arteries.
Collection of arteries
Human basilar arteries were obtained <72 h post mortem from local hospitals. Vessels were dissected from brains and placed in oxygenated ice-cold modified Krebs' solution and then transported to the laboratory. Human coronary arteries were obtained from recipient hearts during heart transplant operations or from donor hearts when the heart was not suitable for transplant. The circumflex or left descending arteries or both were dissected from the hearts immediately and placed in ice-cold modified Kreb's solution and transported to the laboratory. Basilar and coronary arteries were therefore obtained from separate individuals. Both coronary and basilar arteries were stored at 4°C in modified Kreb's solution until use. All tissues were used in experiments within 24 h.
The effects of frovatriptan, sumatriptan, and 5-HT were investigated in basilar and coronary arteries by using protocols similar to those in previous studies (13,18). A metal wire was rubbed inside the lumen of the artery to remove the endothelial cells. The artery was then cut into 3- to 5-mm ring sections, which were suspended in tissue baths and bathed in a modified Krebs' solution bubbled with 95% O2/5% CO2 and placed under a resting isometric (Swema) force of 10 mN. After an equilibration period, tissues were contracted with a reference KCl (90 μM) solution. After washout, the bathing solution was supplemented with cocaine (6 μM), indomethacin (2.8 μM), ascorbate (0.2 μM), prazosin (1 μM), and ketanserin (1 μM), the latter to ensure blockade of the 5-HT2 receptor. The vessel rings were then allowed to equilibrate for ≥30 min.
Cumulative concentration-effect curves to 5-HT (2 nM to 2 μM) were then constructed in basilar arteries. In coronary arteries, the response to a single concentration of 5-HT (10 μM) was determined. These procedures have been found to produce reproducible responses (13,18).
Once a sustained effect of 5-HT had been achieved, the functional integrity of the endothelium was assessed. Only vessels in which carbachol (3 μM) was a weak (<10%) or ineffective relaxant were used in this study. After washout for ≤60 min, the tissues were rechallenged with KCl (90 μM) to assess the consistency of response. After a further 60-min wash period and supplementation of the modified Kreb's solution, the tissues were challenged with cumulative concentrations of 5-HT, sumatriptan, or frovatriptan.
All data were expressed as a percentage of the maximal effects defined by the response to the initial challenge with 5-HT. The contractile response component for each agonist concentration-effect curve was fitted to the following equation by using Microsoft Excel, which provided estimates of the asymptote (α), midpoint location (EC50), and midpoint slope (n) parameters. Equation 1
Drugs and materials
5-Hydroxytryptamine hydrochloride, indomethacin, prazosin hydrochloride, and cocaine hydrochloride were obtained from Sigma Chemical Co (Dorset, U.K.). Frovatriptan (S)-6-carboxamido-3-methylamino-1,2,3,4-tetrahydrocarbazole (succinate or camphor succinate salts), was synthesised at SmithKline Beecham Pharmaceuticals (Harlow, Essex, U.K.). Sumatriptan was extracted and purified from commercially available tablets. Ketanserin was obtained from Semat (U.K.). Indomethacin was dissolved in sodium bicarbonate (10% wt/vol) solution, whereas all other agents were dissolved in distilled water.
The composition of the modified Kreb's solution used in this study was (in mM): Na+ (140), K+ (5), Mg2+ (0.5), Ca2+ (2.25), Cl− (98.5), HCO3− (29), PO42− (1), EDTA (0.04), and supplemented with fumarate (10); pyruvate (5); L-glutamate (5) and glucose (10).
The 5-HT, sumatriptan, and frovatriptan produced concentration-related contractions of both isolated coronary and basilar arteries. Under conditions of blockade of the 5-HT2 receptor-mediated contractions with ketanserin, the mean maximal effects (±SEM) of 5-HT were 2.0 ± 0.3 mN (n = 10) in basilar arteries, 8.7 ± 2.6 mN (n = 18) in coronary arteries from recipient hearts, and 8.7 ± 4.2 mN (n = 10) in coronary arteries from donor hearts.
The calculated −log EC50 values and maximal effect (relative to initial 5-HT response) for frovatriptan, sumatriptan, and time-matched control 5-HT groups are shown in Table 1. There was no evidence for marked changes in maximal contractile response to 5-HT occurring with time in any tissue.
Frovatriptan was 8.5 times more potent than sumatriptan in producing contraction of human isolated basilar artery and produced contractions similar to those observed with sumatriptan and 5-HT (Fig. 1).
Frovatriptan was 2.9- and 6.5-fold more potent than sumatriptan as a contractile agent in donor and recipient coronary arteries, respectively. Maximal responses to frovatriptan were consistently smaller in magnitude than those observed with 5-HT (see Fig. 1), and there was a marked bell-shaped dose relation for frovatriptan in human isolated coronary arteries. Maximal effects, relative to 5-HT, were between 0.42 to 0.40 in recipient or donor hearts, respectively.
A different profile of activity was apparent with arteries exposed to sumatriptan. Sumatriptan produced a more variable response (see Fig. 1), which on average produced a magnitude of contraction similar to that produced by 5-HT. Maximal responses to sumatriptan were between 0.8 to 1.4 of the 5-HT response (1.0) in recipient or donor arteries, respectively.
In terms of the contractile potency, little difference was apparent in the EC50 value of frovatriptan in basilar or coronary artery (Table 1). However, there was a marked separation of the threshold concentration to elicit contractile activity in basilar and coronary arteries (Fig. 1). Concentrations of frovatriptan >2 nM produced contraction in isolated basilar artery, whereas concentrations >20 nM were required to produce contraction in isolated coronary arteries. This represents a functional selectivity of frovatriptan for the cerebral circulation. In contrast, no equivalent separation of effect was observed with sumatriptan or 5-HT in these arteries.
Frovatriptan has been shown to be a high-affinity 5-HT1B/1D-receptor ligand with potent contractile activity in rabbit isolated basilar artery (30) and is effective in aborting migraine attacks (9). Frovatriptan has a >1,000-fold selectivity over 5-HT2 receptors (30) and has selectivity for 5-HT1B and 5-HT1D receptors over 5-HT1F(30). Frovatriptan does not show any selectivity between 5-HT1B and 5-HT1D receptors.
In terms of physiologic roles of the known vascular 5-HT-receptor subtypes, 5-HT2 receptors mediate contractile effects of 5-HT in a variety of vascular beds. However, in a number of arteries, a mixed population of 5-HT receptors mediate contraction. This is particularly apparent in the human isolated coronary artery, where variable proportions of 5-HT1-like and 5-HT2 receptors have been demonstrated (18). The role of 5-HT2 receptors is particularly evident at high concentrations of 5-HT (18), whereas at lower, more physiologic, concentrations, contraction is primarily mediated by 5-HT1-like receptors (31). The nature of the 5-HT1-like receptor mediating the contractile response in the vasculature appears to be of the 5-HT1B subtype, based on a review of available pharmacologic studies (18,32) and on expression of messenger RNA (33).
Our study confirms the contractile effects of sumatriptan in coronary and isolated pial arteries. Sumatriptan was approximately threefold more potent in basilar artery as compared with previous studies (13,32). Similarly, sumatriptan was a full agonist in the basilar artery in our study, whereas in previous studies, it was shown to be a partial agonist producing ∼70% of the contractile maximal response to 5-HT (13) but a full agonist in other pial arteries (14). These small discrepancies between studies are consistent with differences in receptor density and coupling, inhibition of metabolism, or uptake of compound. Frovatriptan was 8.5-fold more potent than sumatriptan in producing contraction of the human isolated basilar artery and was a full agonist with respect to 5-HT. This potency difference is consistent with the relative potencies of each agent at human 5-HT1B receptors (30).
Frovatriptan also produced contraction of human isolated coronary artery and was 2.9- and 6.5-fold more potent than sumatriptan in arteries from donor or recipient hearts, respectively. It is also interesting to note that in contrast to the finding in basilar artery, the response to frovatriptan was consistently lower than that observed to 5-HT. Furthermore, frovatriptan exhibited a bell-shaped concentration-response relationship, with smaller responses observed at higher concentrations in a number of isolated rings. This profile of activity has not been identified with sumatriptan (17,18,27-32), rizatriptan (27-29), naratriptan (24,29), avitriptan (25,29), or zolmitriptan (29). The implications of this bell-shaped dose response are not fully clear. However, evidence suggests that 5-HT1B receptors are present in the cerebral and coronary vascular smooth muscle (18,32,33). Therefore if receptor density were similar in these tissues, it would be expected that 5-HT1B-receptor agonists would have similar potency and efficacy in each tissue. In the presence of ketanserin (1 μM) to block the effects of 5-HT2A-receptor activation, it may be expected that 5-HT would also produce a similar bell-shaped response. Some tissue did show evidence of a deflection in the concentration-response curve, although full reversal of contraction was not observed. It is interesting to note that ketanserin has a relatively high affinity for rodent 5-HT7 receptors (pki, 6.7; 35), but a lower affinity at human cloned 5-HT7 receptors (pki, 5.9; 36).
However, despite good selectivity over other 5-HT receptors for frovatriptan, it has appreciable affinity for the human cloned 5-HT7 receptor (pki, 6.7; 30). Activation of the 5-HT7 receptor in the coronary vasculature has been shown to produce relaxation in dog coronary arteries (34), although the functional significance of 5-HT7-receptor activation in human coronary arteries is not clear. The relaxant response at high concentrations of frovatriptan may also act to limit contraction within the coronary vascular bed at lower concentrations. This would appear to be reflected in the threshold concentrations for contraction of isolated coronary arteries, which was ∼10-fold greater than those in the cerebral artery. No clear separation was apparent for sumatriptan.
Evidence for the bell-shaped concentration response also was evident in vivo, where frovatriptan reduced coronary resistance at high doses in the dog (37). Understanding of these effects in human isolated cerebral and coronary arteries should be facilitated by the use of recently available selective 5-HT1B- and 5-HT1D-receptor antagonists and the development of selective agents at other receptor sites.
Collectively these data show that frovatriptan (>2 nM) produces contraction of cerebral arteries. Frovatriptan also has a unique profile of response in isolated coronary arteries compared with sumatriptan and other 5-HT1B/1D agonists, having a bell-shaped concentration-response relation in coronary arteries with contraction >20 nM and relaxation occurring >2 μM in some tissues. The mechanisms of these effects are not known but may convey a functional selectivity of this novel antimigraine agent to the cerebral circulation and a beneficial profile for patients at risk for coronary artery disease.
Acknowledgment: We are grateful for the help of the Surgeons of Papworth Hospital, Cambridgeshire, and the Staff of University College London. We thank Dr. T.C. Hamilton and Dr. A.M. Brown for helpful comments concerning the manuscript.
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