4. Discussion
In the present study, we have determined the effects of evolocumab on carotid IMT and the factors associated with the change in carotid IMT, in patients taking a statin. Evolocumab ameliorated the progression of carotid mean and maximum IMT in patients undergoing statin therapy, without any inducing serious adverse effects. It also had a favorable effect on serum lipid profile. Multiple linear regression analysis revealed that the change in the serum HDL-cholesterol concentration and the baseline carotid mean IMT independently correlated with the change in carotid mean IMT during the administration of evolocumab, whereas the change in serum HDL-cholesterol and triglyceride concentration independently correlated with the change in carotid maximum IMT. These results suggest that evolocumab protects against carotid atherosclerosis, including by increasing the concentration of HDL-cholesterol and decreasing the concentration of triglycerides, in patients undergoing statin therapy.
Carotid IMT is a well-established surrogate marker of atherosclerosis [
3], and is associated with various atherosclerotic risk factors, such as hypertension, diabetes mellitus, and dyslipidemia [
14]. A previous study has reported that baseline carotid mean IMT thickness was negatively correlated with the change in carotid mean IMT [
15], which is compatible with the findings of our study. Atherosclerosis is a chronic vascular inflammatory disease in which lipid-loaded and activated macrophages play a pivotal role [
16]. Injury to the vascular endothelium by various mechanisms, including high blood pressure, hyperglycemia, and dyslipidemia, leads to the infiltration and retention of monocytes in the subendothelial space [
16]. These monocytes differentiate into macrophages, engulf oxidized LDL-cholesterol through macrophage scavenger receptors, and finally transform into foam cells [
16]. This process induces intimal thickening and the formation of a lipid core that contains lipid-loaded foam cells covered with a layer of connective tissue [
16].
PCSK9 is a proprotein convertase that is produced in hepatocytes, vascular smooth muscle cells, endothelial cells, and macrophages [
17]. It increases the clearance of LDL-cholesterol receptors by hepatocytes by facilitating their lysosomal degradation, which contributes to an increase in serum LDL-cholesterol concentration [
18]. Several studies have shown that evolocumab reduced LDL-cholesterol concentration by 60–70%, as well as reducing triglyceride and lipoprotein (a) concentrations and increasing HDL-cholesterol concentration [
7,
8]. The results of these previous studies are consistent with the findings of the present study. PCSK9 also inhibits adenosine triphosphate (ATP)-binding cassette transporter A1 expression in macrophages, secondary to LDL receptor depletion, thereby reducing cholesterol efflux from lipid-loaded foam cells [
19]. Evolocumab has been reported to increase the uptake of serum LDL-cholesterol by hepatocytes through the inhibition of LDL-cholesterol receptor clearance [
20] and to increase ATP-binding cassette transporter A1 expression in macrophages by upregulating LDL receptor expression [
19]. These findings suggest that evolocumab attenuates the progression of carotid atherosclerosis through a reduction in serum LDL-cholesterol concentration and an increase in cholesterol efflux from lipid-loaded foam cells.
However, in the present study, neither LDL-cholesterol, nor the change in LDL-cholesterol concentration, correlated with the change in carotid IMT during the administration of evolocumab. It has been reported that there are no associations between serum LDL-cholesterol, the change in LDL-cholesterol, and the change in coronary atherosclerotic plaque size in patients who achieve an LDL-cholesterol concentration <70 mg/dL while on statin therapy [
21]. In the present study, the mean LDL-cholesterol concentration at baseline was well-controlled by statin therapy (69.4 ± 24.1 mg/dL). This may explain why neither serum LDL-cholesterol nor the change in LDL-cholesterol concentration were associated with the change in carotid IMT during the administration of evolocumab in the present study.
HDL is a lipoprotein that is principally composed of phospholipids and apolipoproteins (ApoA-I, ApoA-II, and ApoC) and promotes cellular cholesterol efflux and reverse cholesterol transport [
22]. HDL collects free cholesterol from cell surfaces via the ATP-binding cassette transporter and transports it to the liver for excretion in the bile [
22]. Statins increase the synthesis of ApoA-Ⅰ and HDL in the liver and increase the expression of the ATP-binding cassette transporter in peripheral cells [
23]. Similarly, evolocumab increases the circulating concentrations of HDL and ApoA-Ⅰ [
20] and increases the expression of ATP-binding cassette transporter in macrophages [
19]. It has been reported that the change in serum HDL-cholesterol concentration negatively correlated with the change in carotid IMT in patients taking statins [
24]. In the present study, the change in HDL-cholesterol concentration negatively correlated with the change in carotid mean and maximum IMT during the administration of evolocumab in patients who were already taking statins. These findings would tend to support the premise that reverse cholesterol transport was most likely activated due to the use of evolocumab in our study, but an increase in the rate of reverse cholesterol transport was not specifically quantified in the present study.
Triglycerides are the major component of triglyceride-rich lipoproteins such as chylomicron and very low-density lipoprotein. In circulating blood, chylomicrons and very low-density lipoprotein are catabolized rapidly by lipoprotein lipase on blood vessel walls, producing chylomicron remnants and very low-density lipoprotein remnants [
25]. Chylomicron remnants and very low-density lipoprotein remnants can directly penetrate the arterial intima and be taken up by macrophages without undergoing oxidative modifications observed with LDL particles [
26]. It has been reported that the change in serum triglyceride concentration positively correlated with the change in carotid IMT in patients taking statins [
24]. Several studies have shown that evolocumab reduced triglyceride concentration by 15–30%, alongside decreases in LDL-cholesterol concentration and an increase in HDL-cholesterol [
7,
8], which are consistent with the findings of our study. In the present study, the change in triglyceride concentration positively correlated with the change in carotid maximum IMT during the administration of evolocumab in patients who were already taking statins. These results suggest that the effects of evolocumab on carotid IMT might be partially explained by the reduction in triglycerides. Further studies are required to clarify the factors mediating the effect of evolocumab on carotid atherosclerosis in patients undergoing statin therapy.
This is the first study to show the beneficial effects of evolocumab on carotid IMT in patients undergoing statin therapy. An observational study reported that carotid maximum IMT was 2.2 ± 0.8 mm and the change in carotid maximum IMT was −0.012 mm/year in patients with coronary artery disease and carotid plaques [
4]. In the present study, baseline carotid maximum IMT was 2.5 ± 0.7 mm and the change in carotid maximum IMT before the initiation of evolocumab was 0.17 mm/year. Therefore, the patients in the present study had more advanced carotid atherosclerosis and were at high risk of cardiovascular diseases. It has been reported that lipid-lowering therapy with atorvastatin did not reduce the increase in carotid maximum IMT in patients with carotid atherosclerotic plaques [
27]. In the present study, evolocumab reduced the increase in carotid maximum IMT in patients taking a statin. These results suggest that evolocumab attenuates the progression of carotid atherosclerosis in patients with advanced carotid atherosclerosis and at high risk of cardiovascular diseases. In the present study, a mild increase in HbA1c concentration was observed. An animal study reported that PCSK9-knockout mice exhibited decreased insulin secretion and increased glucose intolerance [
28]. However, a recent randomized control trial showed that evolocumab did not affect HbA1c level, fasting glucose level, and insulin resistance [
29]. Further studies are needed to investigate the effect of evolocumab on glucose metabolism in patients taking statins.
The present study had several limitations. First, it was a retrospective observational study that might have been subject to patient selection bias. Second, it featured a before-after study design, with no control group. Therefore, we cannot exclude the possibility that other drugs, including the statins, may have affected the study results. Third, all the study participants were recruited at a single center, which might restrict the generalizability of our findings. Fourth, the percentage of familial hypercholesteremia was very low (0.9%) in this study, which influenced the results of this study because the proportion of familial hypercholesterolemia was higher in other clinical studies [
7,
8]. Fifth, the manual measurement of carotid maximum IMT might have caused variation and affected the results of this study. The inter- and intrasonographer reproducibility was not assessed in this study. Therefore, further multicenter, prospective studies with an appropriate control group are required to confirm the efficacy of evolocumab for the attenuation of carotid atherosclerosis progression.
In conclusion, evolocumab attenuates the progression of carotid mean and maximum IMT in patients on statin therapy, without causing any serious adverse effects. Serum HDL-cholesterol, triglycerides, and carotid IMT were associated with the change in carotid IMT during the administration of evolocumab. These results suggest that evolocumab protects against carotid atherosclerosis in these patients.