Although it remains an unsettled issue evidence favors that
Although it remains an unsettled issue, evidence favors that the endothelin ETA Topotecan HCl may play a more important pathological role. While both endothelin ETA and ETB receptors are found in most tissues, endothelin ETA receptor is the predominant receptor responsible for the vasoconstricting and mitogenic/anti-apoptotic effects of endothelin-1 (Davenport et al., 1995; Maguire and Davenport, 1995; Opgenorth, 1995; Wu-Wong et al., 1997b). Although endothelin ETB receptor is also involved in vasoconstriction in some tissues (Pollock and Opgenorth, 1993; Davenport and Maguire, 1994), endothelin ETB receptor seems to play a pivotal role in endothelin-1-mediated vasodilation via nitric oxide release (Ishikawa et al., 1994), and is involved in clearing endothelin from the circulation (Fukuroda et al., 1994a). The endothelin ETB receptor has also been shown to mediate the inhibitory effect of endothelin-1 on platelet aggregation (McMurdo et al., 1993). More recently, the evidence suggests that blocking endothelin ETA receptor alone is as effective as blocking both endothelin ETA and ETB receptors in several disease models (Schiffrin et al., 1995; Burke et al., 1997; Chen et al., 1997; Borgeson et al., 1998). Taken together, these findings support the concept that a highly endothelin ETA receptor-selective antagonist may be the preferred modality for treating vascular diseases.
Previously, we have reported the characterization of A-127722 (2-(4-methoxyphenyl)-4-(1,3-benzodioxol-5-yl)-1-[[(dibutyl amino)carbonyl]methyl]-pyrrolidine-3-carboxylic acid), a potent, orally available, non-peptide antagonist that is ∼1000-fold more selective for endothelin ETA receptor (Opgenorth et al., 1996). Because of a concern that, under conditions of acute overdosing and/or chronic dosing, endothelin receptor antagonists with an ETA/ETB selectivity of ∼1000-fold may `cross-over\' to block endothelin ETB receptor and thus confound the interpretation of data from in vivo studies, we feel that there is a need to develop a highly endothelin ETA receptor-selective antagonist. Such an antagonist will be very useful for identifying the roles of endothelin ETA receptor in a more precise manner, without a concern for whether endothelin ETB receptor is affected. Based on the above rationale, we have developed A-216546. In this study, we report the in vitro and in vivo characterization of A-216546, a non-peptide antagonist that is >25,000-fold more selective for the endothelin ETA receptor than for the endothelin ETB receptor.
Materials and methods
Discussion The availability of potent antagonists for endothelin has sparked keen interest in further developing these agents for clinical utilization. However, it continues to be a matter of debate and great interest as to what kind of antagonists against which subtype of endothelin receptor may be more clinically beneficial. Emerging evidence suggests that a highly endothelin ETA receptor-selective antagonist may be the choice for therapeutic treatment. First, endothelin ETB receptor is known to mediate the release of NO, which results in a beneficial vasodilation effect (Parris and Webb, 1997). Secondly, endothelin ETB receptor is involved in the clearance of endothelin-1 from circulation, and chronic endothelin ETB receptor blockage will result in an elevation in the plasma endothelin-1 level (Fukuroda et al., 1994a), which may incur adverse effects. Recently at the Fifth International Conference on Endothelin, Willette et al. (1998)showed that systemic administration of equivalent doses of endothelin receptor antagonists with different potencies for endothelin ETB receptor produced elevations in the plasma endothelin-1 level, which correlated linearly with their affinities at the endothelin ETB receptor, further confirming the role of endothelin ETB receptor as a clearance receptor for circulating endothelin-1. Thirdly, endothelin ETA receptor-selective antagonists have been shown to be equally to or more efficacious than non-selective antagonists in various animal disease models (Opgenorth, 1995).