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BACKGROUND AND PURPOSE Recently identified antagonists of the urotensin–II (U-II) receptor (UT) are of limited utility for investigating the (patho)physiological role of U-II due to poor potency and limited selectivity and/or intrinsic activity. EXPERIMENTAL APPROACH The pharmacological properties of two novel UT antagonists, GSK1440115 and GSK1562590, were compared using multiple bioassays. KEY RESULTS GSK1440115 (pKi= 7.34–8.64 across species) and GSK1562590 (pKi= 9.14–9.66 across species) are high affinity ligands of mammalian recombinant (mouse, rat, cat, monkey, human) and native (SJRH30 cells) UT. Both compounds exhibited >100-fold selectivity for UT versus 87 distinct mammalian GPCR, enzyme, ion channel and neurotransmitter uptake targets. GSK1440115 showed competitive antagonism at UT in arteries from all species tested (pA2= 5.59–7.71). In contrast, GSK1562590 was an insurmountable UT antagonist in rat, cat and hUT transgenic mouse arteries (pKb= 8.93–10.12 across species), but a competitive antagonist in monkey arteries (pKb= 8.87–8.93). Likewise, GSK1562590 inhibited the hU-II-induced systemic pressor response in anaesthetized cats at a dose 10-fold lower than that of GSK1440115. The antagonistic effects of GSK1440115, but not GSK1562590, could be reversed by washout in rat isolated aorta. In ex vivo studies, GSK1562590 inhibited hU-II-induced contraction of rat aorta for at least 24 h following dosing. Dissociation of GSK1562590 binding was considerably slower at rat than monkey UT. CONCLUSIONS AND IMPLICATIONS Whereas both GSK1440115 and GSK1562590 represent high-affinity/selective UT antagonists suitable for assessing the (patho)physiological role of U-II, only GSK1562590 exhibited sustained UT residence time and improved preclinical efficacy in vivo.

Several peptidic urotensin-II (UT) receptor antagonists exert 'paradoxical' agonist activity in recombinant cell- and tissue-based bioassay systems, likely the result of differential urotensin-II receptor (UT receptor) signal transduction/coupling efficiency between assays. The present study has examined this phenomenon in mammalian arteries and recombinant UT-HEK (human embryonic kidney) cells.BacMam-mediated recombinant UT receptor upregulation in HEK cells augmented agonist activity for all four peptidic UT ligands studied. The nominal rank order of relative intrinsic efficacy was U-II>urantide ([Pen(5)-DTrp(7)-Orn(8)]hU-II(4-11))>SB-710411 (Cpa-c[DCys-Pal-DTrp-Lys-Val-Cys]-Cpa-amide)>>GSK248451 (Cin-c[DCys-Pal-DTrp-Orn-Val-Cys]-His-amide) (the relative coupling efficiency of recombinant HEK cells was cat>human>>rat UT receptor). The present study further demonstrated that the use of high signal transduction/coupling efficiency isolated blood vessel assays (primate>cat arteries) is required in order to characterize UT receptor antagonism thoroughly. This cannot be attained simply by using the rat isolated aorta, an artery with low signal transduction/coupling efficiency in which low-efficacy agonists appear to function as antagonists. In contrast to the 'low-efficacy agonists' urantide and SB-710411, GSK248451 functioned as a potent UT receptor antagonist in all native isolated tissues studied (UT receptor selectivity was confirmed in the rat aorta). Further, GSK248451 exhibited an extremely low level of relative intrinsic activity in recombinant HEK cells (4-5-fold less than seen with urantide). Since GSK248451 (1 mg kg(-1), i.v.) blocked the systemic pressor actions of exogenous U-II in the anaesthetized cat, it represents a suitable peptidic tool antagonist for delineating the role of U-II in the aetiology of mammalian cardiometabolic diseases.

Purpose To compare atherosclerotic plaque uptake of a first (ferumoxtran-10) and second generation (ferumoxytol) ultrasmall superparamagnetic iron oxide (USPIO) contrast agent with different pharmacokinetic/pharmacodynamic properties. Materials and Methods New Zealand White rabbits maintained on a high cholesterol/fat diet were subjected to balloon injury to the abdominal aorta. Ferumoxtran-10 or ferumoxytol (500 μmol/kg) was administered at 2, 4, and 8 weeks following injury. In vivo magnetic resonance imaging (MRI) was performed immediately prior to, immediately after, and 6 days post-contrast administration. Ex vivo MRI, histologic, and inductively coupled plasma-mass spectrometry (ICP-MS) iron analyses were performed on the excised vessels. Results The blood pool clearance of ferumoxytol (t½ ≤ 6 hours) was more rapid than that of ferumoxtran-10 (t½ ≤ 48 hours). Decreased in vivo MRI signal intensity in the abdominal aorta was observed at 2, 4, and 8 weeks following injury with ferumoxtran-10, but not with ferumoxytol. Consistent with these observations, ex vivo MRI signal intensity was decreased in the ferumoxtran-10 vessels, and to a lesser degree in the ferumoxytol vs. control vessels (− contrast agent). In contrast, in vitro macrophage phagocytosis of USPIO was four to six fold greater with ferumoxytol than with ferumoxtran-10. Additionally, the absolute iron content correlated with ex vivo MRI signal intensity in all vessels (r = −0.86, P < 0.0001). Conclusions These data suggest that the exposure period of atherosclerotic plaque to USPIO rather than the kinetics of the USPIO uptake by plaque alone is a critical criterion for experimental design of in vivo studies. J. Magn. Reson. Imaging 2005;21:432–442. © 2005 Wiley-Liss, Inc.

1 Urotensin-II (U-II) is among the most potent mammalian vasoconstrictors identified and may play a role in the aetiology of essential hypertension. Currently, only one mouse U-II receptor (UT) gene has been cloned. It is postulated that this protein is solely responsible for mediating U-II-induced vasoconstriction. 2 This hypothesis has been investigated in the present study, which assessed basal haemodynamics and vascular reactivity to hU-II in wild-type (UT((+/+))) and UT receptor knockout (UT((-/-))) mice. 3 Basal left ventricular end-diastolic and end-systolic volumes/pressures, stroke volumes, mean arterial blood pressures, heart rates, cardiac outputs and ejection fractions in UT((+/+)) mice and in UT((-/-)) mice were similar. 4 Relative to UT((+/+)) mouse isolated thoracic aorta, where hU-II was a potent spasmogen (pEC(50)=8.26+/-0.08) that evoked relatively little vasoconstriction (17+/-2% 60 mM KCl), vessels isolated from UT((-/-)) mice did not respond to hU-II. However, in contrast, the superior mesenteric artery isolated from both the genotypes did not contract in the presence of hU-II. Reactivity to unrelated vasoconstrictors (phenylephrine, endothelin-1, KCl) and endothelium-dependent/independent vasodilator agents (carbachol, sodium nitroprusside) was similar in the aorta and superior mesenteric arteries isolated from both the genotypes. 5 The present study is the first to directly link hU-II-induced vasoconstriction with the UT receptor. Deletion of the UT receptor gene results in loss of hU-II contractile action with no 'nonspecific' alterations in vascular reactivity. However, as might be predicted based on the limited contractile efficacy recorded in vitro, the contribution that hU-II and its receptor make to basal systemic haemodynamics appears to be negligible in this species.

1. Urotensin-II (U-II) and its G-protein-coupled receptor, GPR14, are expressed within mammalian cardiac and peripheral vascular tissue and, as such, may regulate mammalian cardiovascular function. The present study details the vasoconstrictor profile of this cyclic undecapeptide in different vascular tissues isolated from a diverse range of mammalian species (rats, mice, dogs, pigs, marmosets and cynomolgus monkeys). 2. The vasoconstrictor activity of human U-II was dependent upon the anatomical origin of the vessel studied and the species from which it was isolated. In the rat, constrictor responses were most pronounced in thoracic aortae and carotid arteries: -log[EC(50)]s 9.09+/-0.19 and 8.84+/-0.21, R(max)s 143+/-21 and 67+/-26% 60 mM KCl, respectively (compared, for example, to -log[EC(50)] 7.90+/-0.11 and R(max) 142+/-12% 60 mM KCl for endothelin-1 [ET-1] in thoracic aortae). Responses were, however, absent in mice aortae (-log[EC(50)]

A short duration of ischemia (i.e., ischemic preconditioning) was shown to result in significant tolerance to subsequent ischemic injury. Since previous reports suggest that interleukin-1beta (IL-1beta) may be involved in both ischemic damage and neuroprotection, the present work examined the expression of IL-1beta mRNA in cortical brain tissue after an established preconditioning (PC) stimulus known to produce significant brain tolerance to focal stroke after 1-7 days. Significant induction of IL-1beta mRNA was observed in the ipsilateral cortex at 6 hr (87+/-9 copies of the mRNA per microgram of brain tissue compared to 16+/-5 copies in sham-operated samples, P < 0.001, n = 4) and 8 hr (46+/-4 copies, P < 0.01, n = 4) after PC by means of real-time Taqman polymerase chain reaction (PCR). The peak expression of IL-1beta mRNA after PC was significantly (P < 0.01) lower than that after permanent occlusion of the middle cerebral artery (MCAO), i.e., 87+/-9 and 546+/-92 copies of RNA per microgram tissue at peak levels for PC and focal stroke, respectively. Immunohistochemistry studies revealed a parallel induction of IL-1beta in the ipsilateral cortex after PC. The maximal expression of IL-1beta was observed during the first week post-PC, showing marked parallelism with the duration of ischemic tolerance. These data suggest that the significant but low levels of IL-1beta induction after PC may contribute to ischemic brain tolerance.