G protein-coupled bile acid receptor 1 stimulation mediates arterial vasodilation through a K(Ca)1.1 (BK(Ca))-dependent mechanism.

Bile acids (BAs) and BA receptors, including G protein-coupled bile acid receptor 1 (GPBAR1), represent novel targets for the treatment of metabolic and inflammatory disorders. However, BAs elicit myriad effects on cardiovascular function, although this has not been specifically ascribed to GPBAR1. This study was designed to test whether stimulation of GPBAR1 elicits effects on cardiovascular function that are mechanism based that can be identified in acute ex vivo and in vivo cardiovascular models, to delineate whether effects were due to pathways known to be modulated by BAs, and to establish whether a therapeutic window between in vivo cardiovascular liabilities and on-target efficacy could be defined. The results demonstrated that the infusion of three structurally diverse and selective GPBAR1 agonists produced marked reductions in vascular tone and blood pressure in dog, but not in rat, as well as reflex tachycardia and a positive inotropic response, effects that manifested in an enhanced cardiac output. Changes in cardiovascular function were unrelated to modulation of the levothyroxine/thyroxine axis and were nitric oxide independent. A direct effect on vascular tone was confirmed in dog isolated vascular rings, whereby concentration-dependent decreases in tension that were tightly correlated with reductions in vascular tone observed in vivo and were blocked by iberiotoxin. Compound concentrations in which cardiovascular effects occurred, both ex vivo and in vivo, could not be separated from those necessary for modulation of GPBAR1-mediated efficacy, resulting in project termination. These results are the first to clearly demonstrate direct and potent peripheral arterial vasodilation due to GPBAR1 stimulation in vivo through activation of large conductance Ca(2+) activated potassium channel K(Ca)1.1.

The clinically-tested S1P receptor agonists, FTY720 and BAF312, demonstrate subtype-specific bradycardia (S1P1) and hypertension (S1P3) in rat.

Sphingosine-1-phospate (S1P) and S1P receptor agonists elicit mechanism-based effects on cardiovascular function in vivo. Indeed, FTY720 (non-selective S1P(X) receptor agonist) produces modest hypertension in patients (2-3 mmHg in 1-yr trial) as well as acute bradycardia independent of changes in blood pressure. However, the precise receptor subtypes responsible is controversial, likely dependent upon the cardiovascular response in question (e.g. bradycardia, hypertension), and perhaps even species-dependent since functional differences in rodent, rabbit, and human have been suggested. Thus, we characterized the S1P receptor subtype specificity for each compound in vitro and, in vivo, the cardiovascular effects of FTY720 and the more selective S1P1,5 agonist, BAF312, were tested during acute i.v. infusion in anesthetized rats and after oral administration for 10 days in telemetry-instrumented conscious rats. Acute i.v. infusion of FTY720 (0.1, 0.3, 1.0 mg/kg/20 min) or BAF312 (0.5, 1.5, 5.0 mg/kg/20 min) elicited acute bradycardia in anesthetized rats demonstrating an S1P1 mediated mechanism-of-action. However, while FTY720 (0.5, 1.5, 5.0 mg/kg/d) elicited dose-dependent hypertension after multiple days of oral administration in rat at clinically relevant plasma concentrations (24-hr mean blood pressure = 8.4, 12.8, 16.2 mmHg above baseline vs. 3 mmHg in vehicle controls), BAF312 (0.3, 3.0, 30.0 mg/kg/d) had no significant effect on blood pressure at any dose tested suggesting that hypertension produced by FTY720 is mediated S1P3 receptors. In summary, in vitro selectivity results in combination with studies performed in anesthetized and conscious rats administered two clinically tested S1P agonists, FTY720 or BAF312, suggest that S1P1 receptors mediate bradycardia while hypertension is mediated by S1P3 receptor activation.

Strategic integration of in vivo cardiovascular models during lead optimization: predictive value of 4 models independent of species, route of administration, and influence of anesthesia.

The strategic integration of in vivo cardiovascular models is important during lead optimization to enable a wide therapeutic index for cardiovascular safety. However, under what conditions (eg, species, route of administration, anesthesia) studies should be performed to drive go/no-go is open to interpretation. Two compounds, torcetrapib and a novel steroid hormone mimetic (SHM-1121X), both with off-target cardiovascular liabilities, were profiled in 4 in vivo cardiovascular models. Overlapping plasma concentrations of torcetrapib were achieved in all models tested; values ranged from therapeutic to supratherapeutic. In anesthetized rats, intravenous torcetrapib elicited dose-dependent increases in mean arterial pressure (MAP; 2-18 mm Hg above vehicle during the low- and high-dose infusion), and in anesthetized dogs, torcetrapib increased MAP from 4 to 22 mm Hg. In conscious rats, a single oral dose of torcetrapib increased MAP from 10 to 18 mm Hg in the low-dose and high-dose groups, respectively, whereas in conscious dogs, MAP increased from 3 to 12 mm Hg. SHM-1121X produced marked hypotension in the same models. Pharmacokinetic-pharmacodynamic analysis demonstrated strong correlation across the models tested for both compounds. Results suggest that equivalency across models allows for flexibility to address key issues and enable go/no-go during lead optimization without concern for discordant results. The predictive value of each model was validated with torcetrapib and, when put into practice, led to a decisive no-go for SHM-1121X.