Definition and Pathophysiology
Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a hepatic protein that regulates the availability of the low-density lipoprotein receptor (LDL receptor) on hepatocytes. By directing the receptor toward intracellular degradation rather than recycling it to the hepatocyte surface, PCSK9 reduces hepatic clearance of circulating LDL cholesterol (LDL-C). Increased PCSK9 activity therefore contributes to higher LDL-C concentrations.
The genetic evidence provides the rationale for therapeutic inhibition. Gain-of-function PCSK9 mutations are associated with familial hypercholesterolaemia (FH) and premature coronary heart disease, whereas loss-of-function variants are associated with protection from cardiovascular disease. Pharmacological PCSK9 inhibition increases the effective availability of hepatic LDL receptors and thereby enhances LDL-C removal from plasma.
Two principal therapeutic strategies have emerged:
Inhibition of circulating PCSK9 activity, using the monoclonal antibodies evolocumab and alirocumab.
Inhibition of hepatic PCSK9 synthesis, using the small interfering RNA (siRNA) agent inclisiran.
The magnitude of LDL-C lowering depends partly on residual LDL-receptor expression. PCSK9 monoclonal antibodies are less effective in homozygous FH when residual LDL-receptor function is limited.
Clinical Indications
PCSK9 inhibitors are used as additional lipid-lowering therapy in patients whose LDL-C remains above the recommended goal despite maximally tolerated statin therapy, generally with ezetimibe where appropriate. Established clinical atherosclerotic cardiovascular disease (ASCVD) and FH are principal settings in which these drugs have received approval.
In chronic coronary syndrome, the recommended LDL-C objective is:
LDL-C <1.4 mmol/L (<55 mg/dL), and
A reduction of at least 50% from baseline.
The treatment sequence is generally:
High-intensity statin therapy at the highest tolerated dose.
Addition of ezetimibe if the LDL-C target is not achieved.
Addition of a PCSK9 inhibitor when the target remains unmet despite maximally tolerated statin therapy and ezetimibe.
For patients with a recurrent atherothrombotic event while receiving maximally tolerated statin therapy, an LDL-C goal below 1.0 mmol/L (<40 mg/dL) may be considered in the circumstances specified by the guideline.
PCSK9 inhibitors are also relevant in statin-intolerant patients who remain above goal despite alternative lipid-lowering therapy. Evolocumab and alirocumab have demonstrated substantially greater LDL-C reductions than ezetimibe in statin-intolerant populations.
In symptomatic atherosclerotic disease, the international guideline identifies benefit from adding PCSK9 inhibition to statin therapy when LDL-C is ≥1.8 mmol/L. In peripheral arterial disease, addition to statins further reduces major adverse cardiovascular events (MACE) and major adverse limb events (MALE), and improves walking distance. The role of PCSK9 inhibition in thoracic or abdominal aortic aneurysmal disease remains an emerging area of research.
Pharmacological Classes and Mechanisms
Monoclonal Antibodies
Evolocumab and alirocumab are fully human monoclonal antibodies that bind circulating PCSK9 and prevent its interaction with the hepatic LDL receptor. They lower LDL-C by approximately 50–60%, including in patients receiving background statin therapy and in those unable to tolerate statins, provided that hepatic LDL-receptor expression is present.
Additional lipid effects include:
Approximately 25% reduction in triglycerides.
A possible increase in HDL cholesterol.
Approximately 25–30% reduction in lipoprotein(a), an effect not shared to the same extent by statins.
Inclisiran
Inclisiran is a liver-targeted small interfering RNA. It promotes degradation of the messenger RNA encoding PCSK9, thereby suppressing hepatic PCSK9 synthesis. The resulting increase in LDL-receptor availability lowers circulating LDL-C.
Its principal practical feature is prolonged activity permitting administration twice yearly. It reduces LDL-C and non-HDL cholesterol by up to approximately 50%, lowers apolipoprotein B by approximately 20–40%, and has variable effects on triglycerides and lipoprotein(a).
Investigational Approaches
Several additional approaches to PCSK9 inhibition are under development:
| Approach | Example or status | Administration or development stage |
|---|---|---|
| Monoclonal antibody against circulating PCSK9 | Evolocumab, alirocumab | Subcutaneous injection; approved and associated with reduced cardiovascular events |
| Small binding protein | Lerodalcibep | Monthly injection; phase III development |
| Oral peptide | Oral PCSK9-binding peptide | Daily oral administration; phase III development |
| Vaccine | PCSK9-directed vaccine | Yearly injection; phase I; efficacy and safety remain to be demonstrated |
| siRNA-mediated inhibition of PCSK9 synthesis | Inclisiran | Twice-yearly injection; phase III development and approved lipid-lowering use |
| Antisense oligonucleotide | PCSK9 expression inhibition | Oral or subcutaneous administration; phase I–II development |
| CRISPR base editing | Permanent suppression of PCSK9 expression | Single injection; effective in primates, with human studies ongoing |
Efficacy
Effects on LDL-C and Other Lipids
Evolocumab and alirocumab reduce LDL-C by approximately 40–60% when added to statin therapy, with reductions of approximately 50–60% reported across a broad range of background treatments. The response is generally similar in people with and without diabetes.
Inclisiran produces an additional LDL-C reduction of approximately 50% in patients with ASCVD or FH who are already receiving statin therapy or other lipid-lowering treatment. In pooled phase III data involving patients with heterozygous FH or ASCVD, twice-yearly inclisiran lowered LDL-C by approximately 50.5% when added to maximally tolerated statin therapy.
Effects on Cardiovascular Events
In patients with clinically evident ASCVD, evolocumab reduced the composite of cardiovascular death, myocardial infarction, stroke, hospitalisation for unstable angina, or coronary revascularisation by 15% relative to placebo. The benefit was consistent in patients with and without diabetes. During long-term follow-up, the reduction in a broader composite of cardiovascular death, myocardial infarction, or stroke was approximately 20%, and the effect on cardiovascular mortality became statistically significant.
In patients who had recently experienced acute coronary syndrome, alirocumab reduced the composite of cardiovascular death, myocardial infarction, stroke, or hospitalisation for unstable angina, with a hazard ratio of 0.85 compared with placebo. In the diabetes subgroup, the relative reduction was similar across glycaemic categories, although the absolute benefit was greater in patients with diabetes than in those with prediabetes or normoglycaemia.
A meta-analysis of randomized trials evaluating alirocumab or evolocumab found an approximately 20% reduction in myocardial infarction and ischaemic stroke. It did not demonstrate a significant reduction in all-cause or cardiovascular mortality over the reported mean follow-up of 2.3 years.
Inclisiran has produced a notable reduction in MACE in pooled phase III analysis, but its definitive role in peripheral arterial disease and its cardiovascular-outcome evidence remain less firmly established. A cardiovascular outcomes trial is ongoing.
Diabetes and Neurocognitive Safety
PCSK9 monoclonal antibodies did not increase the incidence of new-onset diabetes in the reported trial populations, including individuals with prediabetes. Meta-analytic data likewise showed no significant association between PCSK9 inhibitors and new-onset diabetes.
Formal cognitive testing with evolocumab and alirocumab did not show increased cognitive impairment compared with placebo, including among patients achieving very low LDL-C concentrations. Long-term evolocumab follow-up also did not show excess neurocognitive events, haemorrhagic stroke, muscle-related events, serious adverse events, or new-onset diabetes compared with placebo.
Drug Treatment and Practical Use
Evolocumab
The reported regimens are:
140 mg subcutaneously every 2 weeks, or
420 mg subcutaneously once monthly.
In the cardiovascular outcomes study, evolocumab was used in patients with established cardiovascular disease and additional risk factors who were receiving maximal statin therapy, with or without ezetimibe, and had LDL-C ≥1.8 mmol/L or non-HDL cholesterol ≥2.6 mmol/L at baseline.
Alirocumab
The source material establishes alirocumab as an approved fully human monoclonal antibody used in addition to maximally tolerated statin therapy, with or without ezetimibe, particularly in established ASCVD and FH. A specific dose is not provided in the source material.
Alirocumab was evaluated after acute coronary syndrome in patients receiving background statin therapy whose LDL-C was ≥1.8 mmol/L, non-HDL cholesterol was ≥2.6 mmol/L, or apolipoprotein B was ≥80 mg/dL.
Inclisiran
Inclisiran is administered subcutaneously twice yearly. The source material does not provide a numerical dose.
It is approved as an adjunct to diet and maximally tolerated statin therapy in patients with heterozygous FH or ASCVD who require further LDL-C reduction. Its infrequent administration may help address adherence barriers associated with more frequent injections.
Tolerability and Adverse Effects
PCSK9 monoclonal antibodies are generally well tolerated. Reported adverse effects include:
Injection-site reactions.
Flu-like symptoms.
These occur in approximately 5% of patients in the cited material. Rare autoantibodies have been reported with the fully human monoclonal antibodies.
Inclisiran is also generally well tolerated, although injection-site reactions occur more frequently than with placebo. The source material does not provide further laboratory-monitoring requirements or detailed management protocols for these reactions.
Guideline Recommendations
Chronic Coronary Syndrome
| Recommendation | Class | Level |
|---|---|---|
| Use high-intensity statin therapy up to the highest tolerated dose to achieve LDL-C targets | I | A |
| Add ezetimibe when the target is not achieved with the maximum tolerated statin dose | I | B |
| Add a PCSK9 inhibitor when the target is not achieved with maximally tolerated statin therapy and ezetimibe | I | A |
| Aim for LDL-C <1.4 mmol/L (<55 mg/dL) and a reduction of ≥50% from baseline | I | A |
| Consider an LDL-C goal <1.0 mmol/L (<40 mg/dL) after a recurrent atherothrombotic event while on maximally tolerated statin therapy | IIb | B |
After Acute Coronary Syndrome
Lipid-lowering therapy should be initiated or continued with high-dose statin treatment as early as possible, irrespective of the initial LDL-C concentration. The LDL-C target is <1.4 mmol/L (<55 mg/dL), together with a reduction of at least 50% from baseline.
If the target is not reached after 4–6 weeks of maximally tolerated statin therapy, ezetimibe is recommended. If the goal remains unmet after a further period on statin therapy and ezetimibe, addition of a PCSK9 inhibitor is recommended. Lipid-lowering therapy should be intensified during the index hospitalisation in patients who were already receiving such treatment before admission.
In patients with recurrent atherothrombotic events within 2 years of the first acute coronary syndrome while receiving maximally tolerated statin-based treatment, an LDL-C target below 1.0 mmol/L (<40 mg/dL) may be considered.
Broader Secondary Prevention
Guideline-based cardiovascular risk reduction includes:
Smoking cessation.
A healthy, Mediterranean-style diet.
Restriction of alcohol intake.
Regular aerobic and resistance exercise.
Reduction of sedentary time.
Structured, multidisciplinary cardiac rehabilitation after acute coronary syndrome.
Control of blood pressure, with an ESC target below 130/80 mmHg.
Glycaemic control, with an HbA1c target below 7.0% as described in the source material.
The lipid strategy should be integrated with these measures rather than used in isolation.
Access and Treatment Selection
The source material indicates that evolocumab and alirocumab have received regulatory approval for patients with established clinical ASCVD or FH whose LDL-C remains above target despite maximally tolerated statin therapy, with or without ezetimibe. Inclisiran is approved as an adjunct to diet and maximally tolerated statin therapy in patients with heterozygous FH or ASCVD requiring additional LDL-C lowering.
Selection among agents is influenced by:
The degree of LDL-C reduction required.
The presence of ASCVD, acute coronary syndrome, or FH.
The highest tolerated statin dose.
Prior use of ezetimibe.
Statin intolerance.
The desired injection frequency.
Adherence considerations.
Monoclonal antibodies require administration every 2 weeks or monthly, depending on the agent and regimen, whereas inclisiran requires administration only twice yearly. The less frequent schedule of inclisiran may be advantageous where frequent self-injection is a barrier, although definitive cardiovascular-outcome evidence remains under evaluation.
Prognosis and Follow-Up
PCSK9 inhibition improves cardiovascular risk profiles primarily through substantial LDL-C lowering. Evolocumab and alirocumab reduce cardiovascular events in patients with established ASCVD and provide benefit in relevant diabetic subgroups without increasing new-onset diabetes. Long-term evolocumab data support persistence of benefit over extended follow-up, including reductions in cardiovascular death in the reported extension analysis.
Follow-up should assess:
Achievement of the LDL-C goal and the required percentage reduction from baseline.
Adherence to background statin and ezetimibe therapy.
Tolerability, particularly injection-site reactions and flu-like symptoms.
Ongoing cardiovascular risk-factor control.
The need for treatment intensification when LDL-C remains above target.
For inclisiran, the favourable LDL-C-lowering and tolerability data support its use in approved populations, but the source material notes that cardiovascular-outcome studies are still pending or ongoing. Accordingly, treatment decisions should distinguish the established event-reduction evidence for monoclonal antibodies from the currently evolving outcomes evidence for inclisiran.
