Object
A disturbed balance of the cerebrovascular nitric oxide (NO)-Endothelin (ET)-1 network, seems to play an important role in the development of cerebral vasospasm (CVS) after subarachnoid hemorrhage (SAH). The ET-1 effect mediated by the dominant smooth muscular ET(A)-receptor seems to represent the contractile part of this network. Activation of the endothelial ET(B)-receptor, however, results in a release of NO, which may imply a neuroprotective effect by vasodilatation. Accordingly, an ET(A)-receptor selective antagonist may be the most promising approach for treatment of CVS. Clazosentan is a putatively high selective ET(A)-receptor antagonist and the first drug that provides clinical prove of ET-1 being involved in the pathogenesis of CVS. Aim of the present investigation was, therefore, to provide the pharmacological properties of clazosentan including its actual selectivity to the ET-receptors in the cerebrovasculature in a functional approach.
Methods
Isometric force measurements were performed in rat basilar artery (BA) ring segments with (E+) and without (E−) endothelial function. Concentration effect curves (CEC) were constructed by cumulative application of ET-1, Sarafotoxin S6c (S6c), or bigET-1 in the presence or absence of clazosentan (10−9 M–10−6 M). To characterize the contractile, ET(A)-receptor segments under resting tone were used. To evaluate the vasodilatory component segments were pre-contracted with prostaglandin F2α(PGF F2α). The inhibitory potency of clazosentan was determined by the pA2-value.
Results
CECs for the contraction by ET-1 and bigET-1 were shifted to the right in the presence of clazosentan dose dependently and in a parallel manner, which indicates competitive antagonism. The pA2-values for ET-1 were 7.8 (E+) and 8.6 (E−) compared to the values for bigET-1 of 8.6 (E+) and 8.3 (E−), respectively. The relaxation by S6c, ET-1 and bigET-1 was also inhibited by clazosentan in a competitive manner, yielding pA2–values of 7.1, 6.7 and 6.5, respectively. The selectivity to the ET(A)-receptor in the cerebrovascular system was, therefore, approximately two logarithmic units.
Conclusion
The present data characterize clazosentan as a potent competitive antagonist on the ET(A)-receptor mediated constriction of the cerebrovasculature by ET-1 and its precursor bigET-1. A competitive inhibition of ET(B)-receptor mediated relaxation in cerebral vessels by clazosentan in therapeutically relevant concentrations, however, is suggested by our data. Therefore, evaluation of its concentration in the cerebrospinal fluid should be performed in further clinical trials. The present data may additionally be taken to describe the pharmacological properties for an ET-receptor antagonist specifically tailored for the treatment of pathological conditions of impaired cerebral blood flow.