In a recent study published in npj Vaccines, researchers evaluated the effectiveness of virus-like particle (VLP)-based vaccines targeting epitopes in the low-density lipoprotein (LDL) receptor (LDL-R) domain of proprotein convertase subtilisin/kexin type 9 (PCSK9).
Study: A virus-like particle-based bivalent PCSK9 vaccine lowers LDL-cholesterol levels in non-human primates. Image Credit: BaLL LunLa/Shutterstock.com
Background
Increased LDL cholesterol (LDL-C) in plasma is a risk factor for atherosclerotic cardiovascular disease (ASCVD), and reducing LDL-C levels can decrease the risk of ASCVD.
Statins are the most commonly prescribed drugs to reduce LDL-C; although well-tolerated, statins can be associated with adverse events, such as liver toxicity and myopathy. These limitations have led to the development of non-statin therapies targeting other pathways involved in LDL-C metabolism.
LDL-R removes LDL-C from circulation and is recycled to the plasma membrane once LDL-C is metabolized. PCSK9 is a secretory protein that inhibits and mediates the lysosomal degradation of LDL-R.
Thus, PCSK9 can elevate LDL-C by preventing LDL-R recycling and is a significant therapeutic target to decrease LDL-C and prevent ASCVD. Multiple strategies have been described to inhibit PCSK9, such as vaccines, monoclonal antibodies, macrocyclic peptides, small interfering RNA, and base editing.
The study and findings
In the present study, researchers assessed the efficacy of VLP-based vaccines displaying linear peptides from PCSK9 in reducing cholesterol levels in animal models.
They engineered VLPs displaying linear peptides (i.e., amino acids 153-163 and 207-223) from mouse PCSK9 (mPCSK9). These peptides were conjugated onto Qβ bacteriophage VLPs by a crosslinker.
Mice, heterozygous for Ldlrtm1Her mutation, that express high serum cholesterol but low function LDL-R were used to study vaccine effects. A small subset of LDLR+/- mice received three doses of 5 μg mPCSK9153-163 VLPs, mPCSK9207-223 VLPs, or wild-type Qβ VLPs (controls). Additionally, a large subset of mice received the bivalent vaccine comprising 5 μg each of mPCSK9153-163 and mPCSK9207-223 VLPs.
Antibody titers against target peptides of PCSK9 and full-length PCSK9 were measured. Individual VLPs and the bivalent vaccine elicited high titers against target peptides and the full-length protein, but the bivalent vaccine induced lower titers than individual VLPs against target epitopes. Nevertheless, the bivalent vaccine elicited robust anti-mPCSK9 immunoglobulin G (IgG) titers.
Total cholesterol was determined before vaccination and three weeks post-third dose. Recipients of mPCSK9207-223 VLPs or bivalent vaccine showed significantly lower total cholesterol levels than controls. However, the reduction in total cholesterol in mPCSK9153-163 recipients was not statistically significant.
Further, serum levels of PCSK9 were unchanged in mPCSK9153-163 recipients but increased in mPCSK9207-223 recipients relative to controls, whereas recipients of the bivalent vaccine had 40% lower PCSK9 in serum.
Livers were harvested from bivalent vaccine recipients and controls to measure LDL-R. LDL-R expression increased by about 50% in bivalent vaccine recipients compared to controls.
A subset of bivalent vaccine recipients was followed for more than a year post-first dose, and sera were collected regularly.
IgG titers against PCSK9 peaked around five weeks after the third dose. Antibody decay was substantially slower after an initial decline. After one year, the antibody titers were 10-fold lower than the peak but were still high. The half-life of anti-PCSK9 antibodies was approximately 20 weeks.
Next, the team investigated the effects of the VLP-based PCSK9 vaccine in rhesus macaques. Twenty-four healthy macaques aged 7-12 were screened for triglyceride and total cholesterol levels and assigned to receive the 1) bivalent vaccine comprising rhesus macaque PCSK9153-163 (rhPCSK9153-163) and rhPCSK9207-223 VLPs, 2) rhPCSK9207-223 VLPs alone, or 3) wild-type Qβ VLPs (controls) at days 0, 28, and 56.
The macaques also received simvastatin from days 77 to 105 to evaluate if stains had synergistic effects. Additionally, they were boosted four weeks before the necropsy.
Robust anti-PCSK8 IgG responses were observed against individual peptides and full-length PCSK9 after rhPCSK9207-223 or bivalent vaccination. The antibody titers were high over four months post-first immunization.
LDL-C levels at all time points in controls were similar to baseline levels, even after statin administration. However, after two doses of the bivalent vaccine, LDL-C levels were significantly lower and unaffected by statin treatment.
In rhPCSK9207-223 recipients, LDL-C levels did not decline until statins were administered. Moreover, bivalent vaccine recipients showed lower levels of apolipoprotein B after two doses, which was not observed in rhPCSK9207-223 recipients.
High-density lipoprotein cholesterol (HDL-C) levels were unaffected in all macaques. rhPCSK9207-223 recipients showed higher plasma PCSK9 levels before statin treatment. On the other hand, bivalent vaccinees had lower PCSK9 before statin administration.
Nevertheless, plasma PCSK9 levels were nearly identical at necropsy. Thus, unlike in mice, the bivalent booster did not significantly decrease plasma PCSK9 in macaques.
Although boosting increased anti-PCSK9 antibody titers by five- to 10-fold in vaccinated animals, it significantly reduced LDL-C only in bivalent vaccine recipients.
Liver LDL-R expression was significantly elevated in bivalent vaccinees. rhPCSK9207-223 recipients had higher LDL-R expression, albeit statistically insignificant. The expression of genes associated with cholesterol metabolism was unaffected in vaccinated animals.
Conclusions
To conclude, the study demonstrated that a bivalent vaccine with two different peptides of PCSK9 could elicit anti-PCSK9 antibodies, reduce serum levels of PCKS9, increase liver LDL-R expression, and decrease cholesterol in macaques and mice.
Overall, the findings highlight the efficacy of vaccine-based approaches to inhibit PCSK9 activity and reduce LDL-C, supporting further development.
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