Supporting Evidence from In Vitro and Animal Studies
At the 2007 AHA, supporting evidence that torcetrapib raises levels of aldosterone came from 2 posters reporting studies in animal models including rodents (which do not have CETP) and in vitro investigations using an adrenal carcinoma cell line, showing that torcetrapib induced secretion of aldosterone in cell culture. Lee Morehouse, PhD (Pfizer Global Research and Development, Groton, Connecticut) presented part of the considerable research that has been carried out thus far, to try and understand the mechanism behind the blood pressure effect of torcetrapib.[12] Early preclinical screening did not identify a pressor effect of torcetrapib, but later studies with improved formulations (exposures) demonstrated pressor activity in rats, rabbits, and monkeys.
Dr. Morehouse and his colleagues also looked at 2 backup CETP-inhibitor compounds that were in development with Pfizer, CP-532,623, a structural analogue of torcetrapib (both are tetrahydroxyquinones), and CP-800,569, a compound structurally unrelated to torcetrapib. Both of these compounds were studied in phase I clinical trials, but because of the adverse effects seen with torcetrapib in the ILLUMINATE trial, all development on the backup compounds was stopped until the torcetrapib situation is re-evaluated, Dr. Morehouse said.
In phase I studies, CP-532,623 produced a dose-related inhibition of CETP activity with an increase in blood pressure. The blood pressure increases with this compound were higher than those seen with torcetrapib in clinical and preclinical studies. Use of the agent raised blood pressure in rats, which do not have CETP, as well as in rabbits which do. The drug concentrations needed to raise blood pressure in rats and rabbits were 10-15 fold greater than those needed to inhibit CETP.
On the other hand, in both animals and humans treated with CP-800,569, there were substantial elevations in HDL-C but no increase in blood pressure. Further studies with torcetrapib, the 2 backup compounds, and anacetrapib (MK-859), a CETP inhibitor in clinical development with Merck (see below) indicated that the blood pressure effect represents an off-target pharmacology unrelated to inhibition of CETP ( Table 7 ).
Further studies using CP-532,623 failed to identify any molecular basis for its pressor effect, although they did rule out numerous pathways. Pressor activity appeared to be associated with an increase in total peripheral resistance, although no specific regional vascular bed was identified as uniquely impacted, Dr. Morehouse reported.
Following the emergence of data from ILLUMINATE suggesting that chronic administration of torcetrapib activated the RAAS, additional nonclinical studies were carried out to examine this further. In vitro studies ruled out direct interaction with mineralocorticoid or glucocorticoid receptors. Torcetrapib and structurally related compounds induced aldosterone secretion from a human adrenocortical carcinoma cell line, whereas the structurally distinct compound, CP-800,569, did not ( Table 8 ).
Rats treated with torcetrapib 5 or 100 mg/kg for 14 days achieved mean trough concentrations of torcetrapib 6-30 fold higher than measured in the phase III program and showed a pressor effect at the highest dose that returned to baseline within 3 days. There were no differences in plasma electrolytes between the 2 groups and no evidence of RAAS activation by Day 14. Cynomolgus monkeys treated with torcetrapib 10 or 100 mg/kg/day showed a dose-related and sustained increase in blood pressure, and, in the 10 mg/kg/day group only, transient rises in aldosterone on Day 2. At Day 11 both aldosterone and plasma renin activity were suppressed.
Chronic animal models have been developed to explore the pressor effect and hormonal effects of torcetrapib and its analogues and additional mechanism of action studies are ongoing to provide a molecular basis for the pressor effects of torcetrapib, Dr. Morehouse said.
Researchers from Merck reported investigations into the hemodynamic effects of torcetrapib in a range of animal models.[13] Michael J. Forrest and Daniel Bloomfield, MD, MPhil (Merck Research Laboratories, Rahway, New Jersey) presented data indicating that the pressor effect of torcetrapib is independent of CETP inhibition, but is associated with adrenal steroid release. The effect appears to be dependent on the adrenal glands, but does not involve aldosterone. Torcetrapib, but not the CETP inhibitor anacetrapib, increased blood pressure in wild-type mice (which do not have CETP) and in CETP-transgenic mice. Torcetrapib did not directly contract vascular smooth muscle in rats. Increased plasma levels of aldosterone and corticosterone were found in plasma samples from rats treated with torcetrapib. A concentration-dependent increase in aldosterone release was seen in rat adrenocortical cells incubated in vitro with torcetrapib. This increase was similar to that produced by angiotensin II.
Anacetrapib did not stimulate aldosterone release. There was no blood pressure response in rats that had undergone acute adrenalectomy. The blood pressure response produced by torcetrapib 5 mg/kg administered over 30 minutes was not reduced in rats that had been treated with 30 mg/kg of eplerenone, a mineralocorticoid receptor antagonist. Acute administration of aldosterone did not increase blood pressure in rats in which torcetrapib had already produced an acute pressor response. Plasma levels of epinephrine or norepinephrine in adrenalectomized or sham-operated rats were unchanged in animals that had been treated with torcetrapib or anacetrapib.
According to Prof. Barter, based on all the preclinical and clinical evidence, "we have to accept that torcetrapib increases aldosterone and does it independently of its effects of CETP."
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Cite this: The Failure of Torcetrapib -- The Search for the Reason Why - Medscape - Feb 14, 2008.