Effects of PCSK9 Inhibition With Alirocumab on Lipoprotein Metabolism in Healthy Humans

Abstract

Background: Alirocumab, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9), lowers plasma low-density lipoprotein (LDL) cholesterol and apolipoprotein B100 (apoB). Although studies in mice and cells have identified increased hepatic LDL receptors as the basis for LDL lowering by PCSK9 inhibitors, there have been no human studies characterizing the effects of PCSK9 inhibitors on lipoprotein metabolism. In particular, it is not known whether inhibition of PCSK9 has any effects on very low-density lipoprotein or intermediate-density lipoprotein (IDL) metabolism. Inhibition of PCSK9 also results in reductions of plasma lipoprotein (a) levels. The regulation of plasma Lp(a) levels, including the role of LDL receptors in the clearance of Lp(a), is poorly defined, and no mechanistic studies of the Lp(a) lowering by alirocumab in humans have been published to date.

Methods: Eighteen (10 F, 8 mol/L) participants completed a placebo-controlled, 2-period study. They received 2 doses of placebo, 2 weeks apart, followed by 5 doses of 150 mg of alirocumab, 2 weeks apart. At the end of each period, fractional clearance rates (FCRs) and production rates (PRs) of apoB and apo(a) were determined. In 10 participants, postprandial triglycerides and apoB48 levels were measured.

Results: Alirocumab reduced ultracentrifugally isolated LDL-C by 55.1%, LDL-apoB by 56.3%, and plasma Lp(a) by 18.7%. The fall in LDL-apoB was caused by an 80.4% increase in LDL-apoB FCR and a 23.9% reduction in LDL-apoB PR. The latter was due to a 46.1% increase in IDL-apoB FCR coupled with a 27.2% decrease in conversion of IDL to LDL. The FCR of apo(a) tended to increase (24.6%) without any change in apo(a) PR. Alirocumab had no effects on FCRs or PRs of very low-density lipoproteins-apoB and very low-density lipoproteins triglycerides or on postprandial plasma triglycerides or apoB48 concentrations.

Conclusions: Alirocumab decreased LDL-C and LDL-apoB by increasing IDL- and LDL-apoB FCRs and decreasing LDL-apoB PR. These results are consistent with increases in LDL receptors available to clear IDL and LDL from blood during PCSK9 inhibition. The increase in apo(a) FCR during alirocumab treatment suggests that increased LDL receptors may also play a role in the reduction of plasma Lp(a).

Clinical trial registration: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01959971.

Keywords: LDL receptor; Lp(a); PCSK9; apoliprotein; lipoproteins; low-density lipoprotein.

Figure 1.

Effects of alirocumab on VLDL-, IDL-, and LDL–apoB fractional clearance rates (A) and production rates (B). FCRs of VLDL, IDL, and LDL-apoB (mean±SE) were determined by stable isotopic enrichment of apoB in each lipoproteins. PRs (mean±SE) were calculated using the FCRs and plasma pool size of apoB in each lipoprotein. Alirocumab treatment significantly increased the FCRs of IDL and LDL apoB and reduced the PR of LDL apoB compared with placebo. No significant effects of alirocumab on VLDL apoB metabolism were found. apoB indicates apolipoprotein B; FCR, fractional clearance rate; IDL, intermediate-density lipoprotein; LDL, low-density lipoprotein; ns, not significant; PR, production rate; SE, standard error; and VLDL, very low-density lipoprotein.

Figure 2.

Effect of alirocumab on VLDL-TG fractional clearance rates (A) and production rates (B). FCR of VLDL-TG (mean±SE) was determined by stable isotopic enrichment of glycerol in each sample of isolated VLDL over 48 hours. VLDL-TG PR (mean±SE) was calculated using the FCR and the plasma pool size of TG in VLDL. Alirocumab treatment did not affect either the FCR or the PR of VLDL-TG compared with placebo. FCR indicates fractional clearance rate; PR, production rate; TG, triglycerides; and VLDL, very low-density lipoprotein.

Figure 3.

Correlation between baseline serum-free PCSK9 levels and changes in (A) LDL-C and (B) LDL-apoB levels. Baseline levels of serum-free PCSK9 were examined for correlation with alirocumab-induced changes in LDL-C and LDL-apoB in all 18 subjects. Changes in both LDL-C and LDL-apoB were related to PCSK9 levels at baseline. apoB indicates apolipoprotein B; LDL, low-density lipoprotein; LDL-C, low-density lipoprotein cholesterol; and PCSK9, proprotein convertase subtilisin/kexin type 9.

Figure 4.

Postprandial levels of TG (A) and ApoB48 (B) after a high-fat meal. (Left) Mean concentrations over time. (Right) Incremental area-under-the-curve (IAUC). Ten participants had blood samples just before and at 1, 2, 3, 5, and 8 hours after consuming a high-fat liquid meal providing 1237 Kcal per 2 m² body surface area from 75% fat, 10% protein, and 15% carbohydrate The data are presented as individual timepoints (mean±SE) and area under the curve above baseline (IAUC) (mean±SE) of plasma TG and apoB48 concentrations. Alirocumab had no effects on postprandial levels of TG and apoB48 compared with placebo. apoB indicates apolipoprotein B; and TG, triglycerides.

Figure 5.

Role of PCSK9 in LDL metabolism and impact of PCSK9 monoclonal antibody. (A) (1) LDLR binds to LDL particle at the liver cell surface. PCSK9 can also bind to the LDLR. (2) The LDL particle-LDLR complexes with or without PCSK9 bound are internalized in the liver cell by endocytosis. (3) LDLR not bound to PCSK9 release the LDL particle, which goes to a lysosome for digestion, whereas the LDLR is recycled to the cell surface. (4) LDLR bound to PCSK9 is digested in the lysosome along with the LDL particle. (B) (1) PCSK9 mAb binds PCSK9 in the circulation, preventing it from binding the LDLR. (2) The LDL particle-LDLR complexes are internalized in the liver cell. (3) In the absence of PCSK9 binding, increased LDLR receptor recycling and more LDLR on the liver cell surface to bind result. (4) Circulating LDL particle levels are reduced. LDL indicates low-density lipoprotein; LDLR, low-density lipoprotein receptor; mAb, monoclonal antibody; and PCSK9, proprotein convertase subtilisin/kexin type 9.