Compared to ET all agonists tested showed a

Compared to ET-1, all agonists tested showed a 2–4 fold bias for the G protein constrictor assay compared to the β-arrestin assay (Table 1). This preliminary analysis indicated that at least modest pathway bias for endogenous ET peptides is possible, however the physiological significance of this, if any, requires more comprehensive analysis of data for ET-1, ET-2 and ET-3 in a broader range of relevant pathway specific assays.
There are currently no published data exploring biased agonism at the ETB receptor. There are a number of ETB agonists available for study including the endogenous peptide ET-3 and related sarafotoxin 6c (S6c) in addition to peptide agonists such as BQ3020 and IRL 1620. IRL-1620 is of particular significance as it is under investigation in a number of therapeutic areas with efficacy demonstrated in animal models of stroke [12] and as an adjunct for improved delivery of chemotherapy targeting solid tumours [13]. It would therefore be of interest to determine the relative effect of these agonists in a number of disease relevant pathways, with comparison to ET-3 responses to determine evidence of bias. These types of studies may highlight any differences between the agonists investigated that could be used either to further understand the signalling of importance to disease progression or to refine clinical efficacy of drugs by reducing on target detrimental effects through defined pathway activation.
Do ET receptor antagonists show pathway bias? Of perhaps more consequence for the ET system is the possibility that antagonists exhibit pathway bias. This has been reported for the dual ETA/ETB antagonist bosentan. In human cloned receptors bosentan exhibits a modest 20 fold selectivity for the ETA receptor [14] and in human L189 that expresses both receptor subtypes bosentan competes for the binding of [125I]ET-1 with a single affinity (KD: 78nM) indicating that it does not distinguish between the native receptors in this tissue [15]. In human blood vessels that express predominantly ETA receptors bosentan exhibited, as expected, 2–20 fold higher affinity than in heart with KD of 32nM in saphenous vein [16] and 3nM in coronary artery [17]. In contrast bosentan was a much less effective antagonist than would be predicted from its binding affinity in both ETA mediated vasoconstriction in human saphenous vein and coronary artery [15], in ETB mediated smooth muscle contraction [14] and ETB β-arrestin recruitment experiments [10] with a functional affinity of about 2μM in all these assays. Unexpectedly, in the ETA mediated β-arrestin assay bosentan was 200 fold more effective an antagonist with KB of 10nM [10] suggesting that bosentan is an ETA β-arrestin biased antagonist. It is interesting to speculate that the relative effectiveness of bosentan in treatment of pulmonary arterial hypertension compared to its generally low potency as an antagonist in vitro may in part be explained by the greater antagonism of detrimental ETA linked β-arrestin mediated ERK1/2 signalling [18] that could contribute to smooth muscle cell proliferation in this disease.
Alternative strategies: cell penetrating peptides as biased antagonists Cell penetrating peptides (CPPs) are a superfamily of peptides that interact with an intracellular segment of a G protein coupled receptor and interfere with signalling [19]. Pepducins-lipidated CPPs, have been developed for over 20 GPCRs including proteinase activated (PAR1, PAR2 and PAR4) and chemokine (CXCR1, CXCR2 and CXCR4) receptors. There has been one report of a CPP, IC2B, targeting the second intracellular loop of the ETB receptor which has been shown to attenuate pulmonary Akt and ERK signalling and to blunt the development of hypoxic pulmonary hypertension in a rat in vivo model [20] that is thought to contribute to disease progression. What was most interesting is that blockade of the ETB receptor has previously been reported to enhance development of pulmonary arterial hypertension in rodents suggesting that in the study by Green and colleagues [20] IC2B may be selectively targeting those ETB pathways contributing to muscularisation of the pulmonary arterial smooth muscle. This would leave unopposed beneficial ETB receptor functions such as release of endothelial derived dilators, although this is yet to be confirmed.