1468 Aldosterone and pressor responses to torcetrapib MJ Forrest et al Figure 1 Torcetrapib increased blood pressure in normal and CETP transgenic mice. Mean arterial blood pressure was measured in conscious B6 mice and B6-Tg(CETP) transgenic mice. Torcetrapib (10 mg kg 1) or anacetrapib (10 mg kg 1) was administered as an i.v. infusion of 30 min duration. Blood pressure was monitored continuously and is shown as average values at 1 min intervals. Each point represents the mean, and vertical lines represent the s.e. Numbers of animals per group are shown in parentheses. CETP, cholesteryl ester transfer protein. vehicle (data not shown) under the same dosing conditions was without effect on either blood pressure or heart rate. The lack of effect of anacetrapib on blood pressure cannot be explained on the basis of an insufficient dose of anacetrapib for the following reasons. In the study shown in Figure 1, the two compounds achieved comparable systemic exposure: in the wild-type mice, terminal compound levels were 23±4 mM of torcetrapib and 25±3 mM of anacetrapib. In the transgenic mice, the relevant values were 12±7 mM of torcetrapib and 9±3 mM of anacetrapib. Furthermore, the two compounds have similar intrinsic potencies against CETP. In an in vitro fluorogenic assay of CETP activity (Eveland et al., 2007), torcetrapib had an IC50 of 13±3 nM (n ¼ 9) and anacetrapib had an IC50 of 17±5 nM (n ¼ 10). The two compounds also have similar potencies in vivo. In a study using the same British Journal of Pharmacology (2008) 154 1465–1473 Figure 2 Torcetrapib did not increase perfusion pressure in the isolated perfused rat hindlimb. The effects of torcetrapib or vehicle on perfusion pressure were evaluated in urethane (800 mg kg 1, i.p.)- and a-chloralose (40 mg kg 1, i.p.)-anaesthetized rats. The femoral artery was cannulated and connected to a perfusion pump at a rate that produced a resting perfusion pressure of B65 mm Hg. Subsequently, animals were infused sequentially with torcetrapib (3–30 mM) followed by phenylephrine (5 mM). Each data point is the mean of determinations made in two animals. strain of B6-Tg(CETP) mice used in Figure 1, 2 weeks of dosing of each compound (10 mice per group) at approxi- mately 50 mg kg 1 (p.o.) yielded substantial increases in HDL compared to control animals: 214±54% for torcetrapib and 318±51% for anacetrapib. In this 2-week study, terminal compound levels were also comparable: 4.6±3.2 mM for torcetrapib and 9.4±3.1 mM for anacetrapib. The effects of torcetrapib on blood pressure and heart rate were also evaluated in rats, dogs and rhesus monkeys, whereas anacetrapib was evaluated only in dogs and rhesus monkeys (Supplementary Figures 1–3). Torcetrapib increased mean arterial pressure in anaesthetized rats (5 mg kg 1, i.v.), anaesthetized dogs (1–10 mg kg 1, i.v.) and in conscious rhesus monkeys (500 mg kg 1, p.o.). Interestingly, torcetra- pib had no effect on heart rate in either rats or rhesus monkeys but did produce increases in heart rate of approximately 35% in anaesthetized dogs. Anacetrapib was without effect on haemodynamic parameters in either dogs (1–10 mg kg 1, i.v.) or rhesus monkeys (500 mg kg 1, p.o.). Torcetrapib did not directly contract vascular smooth muscle A series of studies was performed to investigate the mechan- ism by which torcetrapib increases blood pressure. The ability of torcetrapib to directly contract vascular smooth muscle was evaluated using both isolated vascular smooth muscle preparations as well as a rat perfused hindlimb preparation. Torcetrapib (10 9–10 5 M) did not exhibit any contractile activity when added to isolated rat aortic rings or isolated rat mesenteric artery rings, whereas methoxamine (10 6 M) evoked a robust contractile response of either aortic or mesenteric artery rings (data not shown). Furthermore, torcetrapib neither enhanced the contractile response to a-adrenoceptor agonists or angiotensin II nor reduced the relaxant effect of ACh (data not shown). In a perfused rat hindlimb preparation (Figure 2), torcetrapib (3–30 mM) had no effect on perfusion pressure, whereas phenylephrine evoked a marked increase in perfusion pressure.