Torcetrapib did not increase blood pressure via a central mechanism, via sympathetic nervous system activation or via activation of angiotensin type I or endothelin receptors The possible central effects of torcetrapib, for elevation of blood pressure in rats, were evaluated by performing studies in control animals or in pithed rats. The acute administra- tion of torcetrapib (5 mg kg 1, i.v.), infused over a 30-min time period, produced an equivalent increase in blood pressure in control and pithed rats (Supplementary Figure 4), indicating that torcetrapib was not increasing blood pressure via a central mechanism. In dogs, pretreatment with the a1-adrenoceptor antagonist (prazosin; 1 mg kg 1) or the b1-adrenoceptor antagonist (atenolol; 3 mg kg 1) did not attenuate the pressor response to torcetrapib (Supplementary Figures 5a and 6a). Atenolol partly reduced the torcetrapib- evoked increase in heart rate (Supplementary Figure 6b), whereas hexamethonium (3 mg kg 1) completely suppressed the increase in heart rate (Supplementary Figure 7b) but with only a minor reduction in the pressor response (Supplemen- Aldosterone and pressor responses to torcetrapib MJ Forrest et al 1469 tary Figure 7a). A similar lack of effect of prazosin or phentolamine, to reduce the pressor response to torcetrapib, was observed in rats (data not shown). The pressor response to torcetrapib (10 mg kg 1, i.v.) was not reduced in rats pretreated with either a mixed ETA/B receptor antagonist (compound A; 3 mg kg 1, i.v.; Figure 3a) or an angiotensin II receptor blocker (E-3174; 1 mg kg 1, i.v.; Figure 3b). Evaluation of adrenal steroid levels in rats following administration of CETP inhibitors To explore other potential mediators that could account for the hypertensive effects observed with torcetrapib, plasma levels of aldosterone prior to and after i.v. infusion of either torcetrapib or anacetrapib to conscious rats were measured. Administration of torcetrapib (20 mg kg 1) by i.v. infusion for 30 min evoked a significant (Po0.01) increase in plasma aldosterone (Figure 4a) from baseline values to those measured 20 min post initiation of compound infusion and remained elevated for 30 min after completion of the infusion. In contrast to torcetrapib, neither vehicle nor anacetrapib evoked significant changes in plasma aldo- sterone levels. Subsequent studies demonstrated that the Figure 3 Effects of endothelin or angiotensin II receptor antago- nists. Mean arterial blood pressure was measured in pentobarbital (55 mg kg 1, i.p.)-anaesthetized rats. (a) Separate groups of animals were given either the ETA/ETB antagonist, compound A (3 mg kg 1, i.v.) or vehicle (saline) 60 min prior to the i.v. infusion of torcetrapib (10 mg kg 1) in a volume of 1 mL per rat, over 30 min. Data are expressed as the change in pressure from the baseline period 20– 40 min prior to test compound infusion. Each point represents the mean, and vertical lines represent the s.e. for groups of five rats each. (b) Mean arterial blood pressure was measured in pentobarbital (55 mg kg 1, i.p.)-anaesthetized rats. Separate groups of animals were given either the AT1 receptor antagonist, E-3174 (1 mg kg 1, i.v.) or vehicle (saline) 60 min prior to the i.v. infusion of torcetrapib (10 mg kg 1) in a volume of 1 mL per rat over 30 min. Data are expressed as the change in blood pressure from the baseline period 20–40 min (D mean BP) prior to giving test compounds. Each point represents the mean, and vertical lines represent the s.e. for groups of four rats each. Figure 4 Measurement of adrenal steroids. (a) Plasma levels of aldosterone were measured in plasma from conscious rats prior to and at the indicated times following the i.v. infusion of compounds at 20 mg kg 1 or vehicle (50% dimethylacetamide:50% polyethy- lene glycol 200) for 20 min. Each data point is the mean and vertical lines represent the s.e. for the numbers of animals indicated in parentheses. (b) Plasma levels of corticosterone were measured in plasma from conscious rats 20 min following the i.v. infusion of compounds at 5 mg kg 1 or vehicle (50% dimethylacetamide:50% polyethylene glycol 200) for 20 min. Each data point is the mean and vertical lines represent the s.e. for the numbers of animals indicated in parentheses. British Journal of Pharmacology (2008) 154 1465–1473