Is ezetimibe/simvastatin no better than simvastatin alone? Lessons learned and clinical implications

Theory 2: Intima-media thickness does not reflect the true benefits of lowering LDL-C

The carotid intima-media thickness is a surrogate end point that predicts coronary events and the rate of progression of coronary atherosclerosis.²³ In trials of lovastatin (Mevacor),²⁴ pravastatin (Pravachol),²⁵ and rosuvastatin (Crestor),²⁶ the carotid intima-media was thinner at 24 months with the active drug than with placebo. In two relatively small trials—ARBITER 1 (n = 161),²⁷ which was open-label, and ASAP (n = 325)^28,29—aggressive lipid-lowering reduced the progression of intima-media thickness better than less-aggressive therapy. However, this measure has been used to evaluate the effects of differing degrees of LDL-C reduction between active treatments in fewer than 500 research participants.

Furthermore, what part or parts of the carotid system are we talking about? In recent trials led by Dr. Kastelein, the intima-media thickness of the common carotid arteries increased with pactimibe (an acyl-coenzyme A:cholesterol O-acyltransferase, or ACAT, inhibitor)³⁰ and torcetrapib,³¹ but the six-site composite measure (which was the primary end point in these trials, as in ENHANCE) did not increase more than in the control groups. Pactimibe was also shown to increase atheroma volume as measured by intravascular ultrasonography in the ACTIVATE trial.³² Therefore, the thickness of the common carotid arteries has been shown to be a better predictor of harm from a therapy than the composite measurement.

The advantage of measuring the common carotid artery is that it is easier to visualize and measure, and therefore the measurements vary less. In the METEOR trial,²⁶ the six-site measurement increased significantly less with rosuvastatin than with placebo, but the common carotid measurement alone was more strongly associated with a difference in progression. In the ENHANCE trial, the thickness of the common carotid arteries increased by 0.0024 mm with simvastatin alone vs 0.0019 mm with simvastatin/ezetimibe, a difference of 0.005 mm that was not statistically significant (P = .93).¹

Although the six-site measurement appears to be good for predicting coronary events and evaluating therapies, the measurement in the common carotid arteries appears to be a more reliable surrogate end point for predicting both benefit and harm from antiatherogenic agents. However, trials of statins and other lipid-lowering therapies that assessed clinical events have shown that the reduction in risk associated with a given reduction in cholesterol is similar regardless of the mechanism by which cholesterol is lowered.³³ Therefore, the LDL-C level is far superior as a marker of clinical benefit.

Theory 3: Previous statin treatment affected the ENHANCE results

By far the most likely explanation for the neutral findings in ENHANCE is that the patients were so well treated before entry that it was impossible to detect a difference between the two treatment groups in carotid intima-media thickness at the end of the study. Eighty percent of the patients had received statins previously, and at baseline the mean intima-media thickness of the common carotid arteries was only 0.68 mm.¹ In contrast, most other trials required a thickness greater than 0.7 mm for entry.

The two main reasons for selecting a population with familial hypercholesterolemia were the assumptions that these participants would have a greater-than-average carotid intima-media thickness at baseline and that they would show an above-average progression rate, even on high-dose statin therapy.⁴ Both of these assumptions were incorrect: the baseline thickness was normal and the progression rate was negligible in both groups.

Accordingly, the high prevalence of statin pretreatment and the near-normal carotid intima-media thickness at baseline may have prevented the 16.5% greater reduction in LDL-C due to ezetimibe from producing a difference in progression over 24 months of treatment. This conclusion is supported by the long-term follow-up results from ASAP, RADIANCE 1, and CAPTIVATE, all of which showed that in patients with familial hypercholesterolemia well treated with statins, progression of carotid intima-media thickness is negligible.^30,31

Further supporting this view, in a previous trial by Dr. Kastelein’s group in patients with familial hypercholesterolemia,³⁴ giving simvastatin 80 mg for 2 years decreased the intima-medial thickness by .081 mm (P < .001), compared with 0.0058 mm in ENHANCE (a 14-fold difference). In the previous trial, the baseline measurement was 1.07 mm (vs 0.68 mm in ENHANCE), and the extent of the change was significantly associated with the baseline measurement (r = .53, P < .001) but not with the change in LDL-C levels.

This is powerful evidence that, in two similar studies that used the same methodology and the same drug, the thinner arteries in the ENHANCE trial are by far the most likely explanation for the lack of change with the addition of ezetimibe to high-dose simvastatin. The METEOR trial enrolled only patients who had never received statins and whose carotid intima-media was thicker than 1.2 mm. In retrospect, a similar design would have been preferable for ENHANCE.³⁵