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Efficacy and safety of PCSK9 monoclonal antibodies: an evidence-based review and update

Abstract Objective Treatment of dyslipidemia lowers cardiovascular (CV) risk. Although statin use is a cornerstone therapy, many patients are not achieving their risk-specific low-density lipoprotein cholesterol (LDL-C) goals. The proprotein convertase subtilisin/kexin type 9 (PCSK9) monoclonal antibodies have been extensively studied as lipid-lowering therapy (LLT). Herein, we present an updated evidence-based review of the efficacy and safety of PCSK9 monoclonal antibodies in the treatment of familial and non-familial hypercholesterolemia. Methods PubMed database was searched to review Phase III studies on PCSK9 monoclonal antibodies. Then, the US National Institutes of Health Registry and the WHO International Clinical Trial Registry Platform were searched to identify and present the ongoing research. Results PCSK9 monoclonal antibodies were investigated for the treatment of dyslipidemia, as a single therapeutic agent or as an add-on therapy to the traditional LLT. They proved effective and safe in the treatment of familial and non-familial hypercholesterolemia, and in the prevention of adverse CV events. Conclusions The use of PCSK9 monoclonal antibodies in the treatment of dyslipidemia is currently recommended to achieve risk-specific LDL-C goal to reduce adverse CV events. Future results of the ongoing research might expand their clinical generalizability to broader patient populations.


Introduction
It has been well established that lipid-lowering therapy (LLT) targeting low-density lipoprotein cholesterol (LDL-C) lowers cardiovascular (CV) risk. This is largely based on significant benefits observed with statin therapy in the prevention of primary and secondary atherosclerotic cardiovascular disease (ASCVD) 1,2 . Although statins remain the cornerstone of LLT, yet patients on statin therapy may still experience CV events 2 , statin intolerance, or inability to achieve target LDL-C levels despite maximally-tolerated doses. Thus, alternative LLT to further decrease LDL-C levels and improve CV outcomes has been investigated 1 . Ezetimibe in combination with statin therapy has shown incremental lowering of LDL-C levels and improvement in CV outcomes. However, such improvements are modest and the overall outcome data on ezetimibe monotherapy are scarce 1,3 . Monoclonal antibodies are novel lipid-lowering agents that reduce LDL-C levels by inhibiting proprotein convertase subtilisin/kexin type 9 (PCSK9). The currently available antibodies, alirocumab and evolocumab, are fully human immunoglobulin-G (IgG) subtypes 4 and are approved by the Food and Drug Administration (FDA) for heterozygous familial hypercholesterolemia (HeFH) and for the prevention of CV events in patients with established cardiovascular disease (CVD).
Evolocumab is also indicated for homozygous familial hypercholesterolemia (HoFH) 5 . International guidelines have been updated to include PCSK9 antibodies in their recommendations [6][7][8] . The development of a third PCSK9 antibody, bococizumab, in advanced phase III clinical trials was abandoned in 2016. After the completion of six bococizumab studies, an unexpected attenuation of effect on LDL-C over time was observed, in addition to high rates of injection-site reaction due to high immunogenicity 9 . However, the development of new investigational monoclonal antibodies to inhibit PCSK9 is well under way with promising initial results [10][11][12][13] . This review will discuss the efficacy and safety profile of PCSK9 monoclonal antibodies or inhibitors in the treatment of familial and non-familial hypercholesterolemia. For the purpose of this review, PCSK9 monoclonal antibodies or inhibitors will be used interchangeably.

Materials and methods
Search strategy for studies on PCSK9 monoclonal antibodies following search thread: ("alirocumab" [Supplementary Concept] OR "evolocumab" [Supplementary Concept] OR "bococizumab" [Supplementary Concept]). The search was limited to "Humans" and "Clinical Trial". The references' lists of the identified trials were manually searched to identify further potentially useful articles. Several meta-analyses, prespecified, post hoc and pooled analyses of PSCK9 inhibitors trials have been included to emphasize their findings. The literature search was updated on March 1, 2020. Figure 1 presents the approximate publication period of time of Phase III clinical trials discussed in the review.

Search strategy for registered clinical trials (ReCTs)
Literature search on the ongoing active research on PCSK9 monoclonal antibodies was conducted on 15 March 2020. The search strategy included the individual agents separately: alirocumab and evolocumab. The United States (US) National Institutes of Health Registry (http://clinicaltrials.gov/) was searched for Phase III trials and recruitment status of "Active, not recruiting", "Recruiting" and "Not yet recruiting" and the World Health Organization (WHO) International Clinical Trial Registry Platform (ICTRP) (http://www.who.int/ ictrp/network/en/) for Phase III trials.

Mechanism of action of PCSK9 monoclonal antibodies
The PCSK9 is a low-abundance circulating plasma protein that is synthetized and secreted by hepatocytes 14 . PCSK9 regulates LDL receptor (LDLR) on hepatocytes surfaces, which is the primary receptor that is responsible for clearing the circulating LDL particles by endocytosis. PCSK9 binds to LDLR, stabilizes the LDL/LDLR complex and prevents LDLR from being released from the endocytosed vesicle causing lysosomal destruction of the LDLR and thus preventing its recycling 5,15 . Subsequently, there is reduction in the LDLR density on the hepatocytes surfaces which affects the ability to eliminate the circulating LDL particles leading to an elevated plasma cholesterol level 14 . Preclinical trials have shown that PCSK9 has direct proinflammatory action on the vessel walls which could be explained by its effect on LDLR-related protein 1 (LRP1) which regulates plaque macrophages 5 . PCSK9 circulating in the plasma has also been postulated to flow through the arterial plaque adding up to the plaque PCSK9 concentration which controls LDLR expression in the atheroma and reduces levels of LRP1 5 . Increased LDLR expression through the inhibition of PCSK9 will result in profound uptake of lipoprotein cholesterol into the atheroma theoretically while the reduction of LRP1 levels results in induction of inflammation 4,5 . Monoclonal antibodies such as alirocumab and evolocumab, self-administered subcutaneously every two or four weeks, bind to PCSK9 preventing its binding to LDLR. When PCSK9 is absent or its binding to LDLR is inhibited, LDL is degraded while the LDLR can be recycled then accumulate on the hepatocyte's membrane leading to an accelerated LDL clearance and reduced levels in blood 5 . Accordingly, PCSK9 inhibition have two counteracting actions with a net effect of reduction of LDL-C levels and atheroma volume 4 . An injection of monoclonal antibodies can completely inactivate all the circulating PCSK9 within hours 5,14 , and all the newly secreted PCSK9 after several days 5 . Marked reduction in LDL-C levels starts one day after an injection 14 . Inhibition of PCSK9 can also be achieved by  targeting PCSK9 synthesis in hepatocytes. Inclisiran, a longacting small interfering RNA (siRNA), that inhibits translation of PCSK9 mRNA leading to a reduced PCSK9 synthesis. Inclisiran is a valid alternative to PCSK9 inhibitors with an advantage of twice-a-year injection to produce an LDL-C reduction by 50% or more 16 . Bempedoic acid is another promising agent that could be a potential alternative to statins. It is an oral once-daily molecule that acts as an inhibitor to adenosine triphosphate (ATP) citrate lyase. The latter is an enzyme that upstream from 3-hydroxy-3-methylglutaryl-coenzyme A. Thus, reducing cholesterol synthesis 16,17 .

Lipid outcomes
For the purpose of Figure 2, when the monoclonal antibody was tested in several arms i.e. in different strengths and/or in combination with other LLT, the LDL-C values were averaged using the values of different arms of the Phase III trials presented in the review.

24-Week trials
In ODYSSEY EAST 58

Longer-term trials (!48 weeks)
Evolocumab 420 mg Q4W combined with diet-alone or with atorvastatin 10 mg or 80 mg (with or without ezetimibe), showed an average of 57% (p < .001) reduction in LDL-C at Week 12 maintained through Week 52 when studied in hyperlipidemic patients with CV risk in the DESCARTES 44 trial (n ¼ 900). Around 82.3% of patients on evolocumab achieved LDL-C level < 1.8 mmol/L (70 mg/dL) versus 6.4% on placebo.
In the diet-alone group, the least-square mean reduction in LDL-C was 55.7%, in the atorvastatin 10 mg group was 61.6%, in the atorvastatin 80 mg group was 56.8% and in the atorvastatin 80 mg plus ezetimibe 10 mg group was 48.5% (p < .001, all comparisons). The ODYSSEY COMBO I 43 trial (n ¼ 316) in hypercholesterolemic patients at high CV risk, showed estimate mean difference in baseline LDL-C of À45.9% (p < .0001) with alirocumab 75Q2W at 24 weeks. Furthermore, 75% of patients achieved LDL-C < 1.8 mmol/L (70 mg/dL) in alirocumab group compared to 9% in the placebo group. The reductions were sustained till Week 52. In ODYSSEY CHOICE I 46 trial (n ¼ 803) in hypercholesterolemic patients at moderate-to-very-high CV risk, alirocumab 300 mg Q4W without statins showed mean LDL-C change from baseline by À52.7% compared to þ0.3% with placebo (p < .0001) at Week 24. While those on statins showed a mean change of À58.8% compared to À0.1% with placebo (p < .0001). The results were maintained through Week 48 in both groups. When compared to ezetimibe in hypercholesterolemic patients at high CV risk in the ODYSSEY COMBO II 50 trial (n ¼ 720), alirocumab 75Q2W reduced LDL-C by a mean of 50.6% as compared to a mean of 20.7% with a difference of À29.8% (p < .0001) at Week 24. The reductions were sustained till Week 52 and the percentage of patients who achieved LDL-C < 1.8 mmol/L (70 mg/dL) was 77% on alirocumab versus 45.6% on ezetimibe (p < .0001).

Familial hypercholesterolemia
Alirocumab 150Q2W when added to statin therapy in ODYSSEY HIGH FH 47 trial (n ¼ 107) in patients with HeFH and LDL-C ! 4.14 mmol/L (160 mg/dL), resulted in a percent change in LDL-C from baseline of À45.7% compared to À6.6% by placebo with least-square mean difference between the groups of À39.1% (p < .0001). Forty one percent of the patients on alirocumab achieved LDL-C risk-specific goals versus 5.7% in the placebo group at Week 24.

Mixed populations
In Alirocumab expanded-use program 42 , an open-label single arm trial, in patients with HeFH and/or CHD and baseline LDL-C of !4.14 mmol/L (160 mg/dL) on LLT, a reduction of 55% in LDL-C level was observed with alirocumab 150Q2W arm by Week 24. The single-arm ODYSSEY APPRISE 66 study (n ¼ 955) is the first study in a real life setting prior commercial availability of alirocumab in subjects (63% with HeFH; 68% with a history of CVD) inadequately controlled on their maximally-tolerated LLT. In the interim data of the first 843 patients, LDL-C level reduction from baseline was 56% at Week 12. The ODYSSEY JAPAN 45 trial (n ¼ 216) was a randomized double-blind trial in Japanese patients with HeFH or non-FH and at high CV risk who were on stable statins. Alirocumab (75/ 71 ) who had fasting triglyceride (TG) levels of !1.7 mmol/L (150 mg/dL), a mean percentage change in LDL-C of À67% versus placebo and À42% versus ezetimibe was observed (p < .001, all comparisons), which was similar to the observed LDL-C reductions in patients without elevated TG levels.

Statin intolerance
It is estimated that about 10 of patients 73,74 may be unable to tolerate any dose of statins (i.e. complete intolerance) or the dose that effectively reduces LDL-C (i.e. partial intolerance). More than half of the patients who experience statinrelated adverse effects, discontinue their statin therapy. Thus, this precludes patients from achieving the target LDL-C level 4 , and exposing high CV risk patients to increased risk of mortality or adverse CV events 5 . Statin adverse muscle symptoms are considered the most predominant cause for statin discontinuation 4,5 . Therapy with PCSK9 inhibitors has been effective and well tolerated in statin-intolerant patients in the short-term (12-24 weeks) 71,74-77 or longer-term (up to two years) 78 pivotal trials. Statin-intolerance in such trials was defined as the inability to tolerate two or more statins at the lowest available dosage 71,76,77 . Evolocumab therapy in three randomized double-blind trials, the 12-week Phase II (GAUSS; n ¼ 236) 74 , the Japanese 12-week Phase III (GAUSS-4; n ¼ 61) 75 and the 24-week Phase III (GAUSS-2; n ¼ 307) 71 , significantly reduced LDL-C levels and was associated with short-term tolerability. The efficacy and safety persisted over 24 weeks then up to two years, as shown in the two-stage GAUSS-3 76 (n ¼ 491) trial and the subset analysis 78 of the OSLER open-label extension studies, respectively. In GAUSS-2 71 and GAUSS-3 76 trials, evolocumab reduced LDL-C by 53 to 56 and 52.8% (p < .001, both comparisons), respectively. Adverse muscle events occurred in 12 and 20.7% of patients on evolocumab as compared to 23 and 28.8% of patients on ezetimibe, respectively. In the GUASS-4 75 trial, the percent change in mean LDL-C was À40.1% as compared to ezetimibe (adjusted p < .0001) with diarrhea (9.5%) and nasopharyngitis (12.5%) being the most common adverse events. Similarly, alirocumab in the ODYSSEY ALTERNATIVE 77 trial (n ¼ 314) was welltolerated and produced significant reductions in LDL-C levels by 45% (p < .0001) at Week 24 in patients with statin intolerance due to muscle symptoms. Skeletal muscles adverse events were less frequent with alirocumab as compared to ezetimibe (hazard ratio (HR), 0.71, 95% CI 0.47 À 1.06, p ¼ .096).

Cardiovascular outcomes
The use PCSK9 inhibitors has been associated with a benefit in CV outcomes. One interpretation of the potential mechanism is their ability to lower the atherogenic lipids levels (namely LDL-C, non-HDL-C, lipoprotein(a) (Lp(a)) 79 . A number of studies has shown that PCSK9 inhibitors had significant reduction in atherogenic lipid fractions i.e. LDL-C, non-HDL-C, apolipoprotein-B (apo-B), and Lp(a) levels. In a post hoc analysis of a Phase II trial, alirocumab reduced total LDL particle 134 R. KADDOURA ET AL.
(LDL-P) and very-low-density lipoprotein (VLDL), and increased HDL particles by 63.3, 36.4, and 11.2% (p < .01, all), respectively 80 . Evolocumab in the BERSON 56 87 of the ODYSSEY OUTCOMES trial, a reduction in Lp(a) was an independent factor of lowering the risk of major adverse CV events (MACE), and that a 1 mg/dL reduction in Lp(a) was associated with an HR of 0.994 (95% CI, 0.990 À 0.999, p ¼ .0081). In addition, there has been a positive correlation between plasma levels of PCSK9 and the carotid intima media wall thickness 88 , the platelet count in patients with coronary artery disease (CAD) 89 , and the necrotic core tissue fraction within the coronary plaques 90 , suggesting that the PCSK9 protein may have a key modulating effect in atherosclerosis 91 . The GLAGOV 92 trial (n ¼ 968) has confirmed that evolocumab in addition to statin therapy led to significant reduction in atheroma volume and induced plaque regression in higher percentage of patients. Alirocumab was evaluated in the Japanese Phase IV ODYSSEY J-IVUS 93 study (n ¼ 206) which enrolled hypercholesterolemic patients recently hospitalized with ACS. Using intravascular ultrasound imaging analysis, there was a numerically more percent reduction in normalized total atheroma volume which did not reach the statistical significance. It remains to be established whether PCSK9 inhibition has important clinical effects on arterial stiffness, endothelial function, and inflammatory responses. Circulating PCSK9 was significantly associated with arterial stiffness 94 and drove a pro-inflammatory response on macrophage that stimulates cytokines 95 . To date, two patient cases have reported with impressive results on the use of PCSK9 inhibitors through improvement in carotid-femoral pulse wave velocity 96 . Two-month use of alirocumab has improved endothelial function as assessed by brachial artery vasoreactivity test 97 .
Prior to the major clinical outcome trials, FOURIER 98 and ODYSSEY OUTCOMES 99 , findings from the published evidence gave an important signal that the reduction of LDL-C using PCSK9 inhibitors lowered the adverse CV events. In the OSLER 100 study (n ¼ 4465), evolocumab as compared to the control group reduced LDL-C by 61% (p < .001) and adverse CV events rate by 53% (HR, 0.47, 95% CI 0.28 À 0.78; p ¼ .003) after one year of treatment. Alirocumab in the ODYSSEY LONG TERM 63 trial (n ¼ 2341) over a period of 78 weeks, significantly reduced LDL-C by 62% (p < .001) and in a post hoc analysis of it, there was an evidence of a reduction in the rate of MACE (HR, 0.52, 95% CI 0.31 À 0.90; nominal p ¼ .02). A post hoc analysis 101 from ten ODYSSEY trials (n ¼ 4974), showed that the incidence of MACE was reduced by 24% for every 39 mg/dL reduction in LDL-C (adjusted HR, 0.76, 95% CI 0.63 À 0.91; p ¼ .0025) and by 29% for every additional 50% reduction in LDL-C from baseline (HR, 0.71, 95% CI 0.57 À 0.89; p ¼ .003). A meta-analysis 102 of 24 randomized control trials (RCTs) (n ¼ 10,159) showed that the use of PCSK9 inhibitors was associated with reductions in LDL-C by 47.49% (p < .001), all-cause mortality (odds ratio (OR), 0.45, 95% CI 95% 0.23 À 0.86; p ¼ .015; heterogeneity p ¼ .63; I 2 ¼ 0%), MI (OR, 0.49, 95% CI, 0.26 À 0.93; p ¼ .03; heterogeneity p ¼ .45; I 2 ¼ 0%), and CV mortality (OR, 0.50, 95% CI, 0.23 À 1.10; p ¼ .084; heterogeneity p ¼ .78; I 2 ¼ 0%) compared with no PCSK9 inhibitors treatment. A study 103 predicted the individual lifetime benefit of PCSK9 inhibition, expressed in terms of gain in life expectancy free of (recurrent) stroke or MI, in statin-treated patients with stable CAD based on competing two risk models developed in data from the Treating to New Targets (TNT) trial. The individual lifetime benefit varied substantially, ranging from < 6 months free of stroke or MI in 61% of patients to !12 months in 10% of patients. The expected benefit was the highest in younger patients (aged 40 À 60 years) with risk factors, especially if LDL-C level is >1.8 mmol/L (70 mg/dL).
Three Phase III trials, FOURIER 98 , ODYSSEY OUTCOMES 99 , and SPIRE 104 , investigated PCSK9 inhibitors in patients with a history of clinically evident CVD. The FOURIER trial (n ¼ 27,564) randomized patients with stable ASCVD and fasting LDL-C level ! 1.8 mmol/L (70 mg/dL) or non-HDL !2.6 mmol/L (100 mg/dL) who were on statin therapy. At Week 48, evolocumab 140 mg Q2W or 420 mg QM, reduced LDL-C by 59% compared to placebo. The primary outcome of a composite of CV death, MI, stroke, hospitalization for unstable angina (UA), or coronary revascularization was significantly lower (HR, 0.85; 95% CI, 0.79 À 0.92; p < .001) at a median duration of follow-up of 2.2 years 98 ( Table 1). The reduction of the composite of CV events was greater in patients with high-risk features i.e. more recent MI (HR, 0.80, 95% CI, 0.71 À 0.9), multiple prior MI (HR, 0.82; 95% CI, 0.72 À 0.93), and residual multivessel CAD (HR, 0.79; 95% CI, 0.69 À 0.91) with absolute risk reductions that exceeded 3% (3.4, 3.7, and 3.6%, respectively) versus around 1% in the low-risk groups (0.8, 1.3, and 1.2%) 105 . A Pre-specified secondary analysis 106 of the FOURIER trial evaluated the effect of evolocumab on total CV events. The total CV events were reduced by 18% (incidence rate ratio (RR), 0.82; 95% CI, 0.75-0.90; p < .001) which was driven by decreases in MI, stroke, and coronary revascularization. In another secondary ad hoc analysis 107 of the FOURIER trial, the reduction of adverse CV events was equally effective in patients with stable ASCVD regardless of the baseline LDL-C whether it was 1.8 mmol/L (70 mg/dL) or the background statins whether it was of a maximal or submaximal potency. Thirteen percent of patients in the FOURIER trial had PAD; evolocumab significantly lowered the primary end point in PAD patients (HR, 0.79; 95% CI 0.66 À 0.94; p ¼ .0098) and the risk of major adverse limb events in all patients (HR, 0.58; 95% CI, 0.38 À 0.88; p ¼ .0093). The reduction in LDL-C levels lowered the risk of major adverse limb events 108 . Evolocumab induced a relative reduction in venous thromboembolism (VTE) events by 29% (HR, 0.71; 95% CI, 0.50-1.00; p ¼ .05) in the first year and by 46% (HR, 0.54; 95% CI, 0.33-0.88; p ¼ .014) thereafter, in a pre-specified analysis of the FOURIER trial 109 . Moreover, evolocumab in the FOURIER trial reduced adverse CV events across the strata of baseline high-sensitivity C-reactive protein (hs-CRP) ( < 1, 1 À 3, and >3 mg/dL) with more absolute risk reductions in the patients with higher-baseline hs-CRP 110 . A meta-analysis 111 found that PCSK9 inhibitors do not impact the circulating hs-CRP levels in contrast to statins 112 or bempedoic acid 17,113 . A recent post hoc analysis of the FOURIER trial found that after the first 12 months of evolocumab use, there was a 52% relative reduction (HR, 0.48; 95% CI, 0.25-0.93) in aortic stenosis (AS) events rates in evolocumab group, which was attributed to the reduction in Lp(a) levels 114 . A previous study have suggested that PCSK9 inhibitors may potentially have a role in reducing the risk of AS based on the finding that PCSK9 R46L loss-of-function mutation was associated with lower Lp(a) and LDL-C level as well as reduced risk of AS 115 . The ODYSSEY OUTCOMES 99 trial (n ¼ 18,924) randomized patients on statin therapy with inadequate LDL-C control who had a recent ACS event rather than the more stable ones enrolled in the FOURIER trial. Alirocumab 75 mg or 150 mg Q2W over a median follow up of 2.8 years, significantly reduced the four-point composite of CV death, nonfatal MI, fatal or nonfatal ischemic stroke, or UA requiring hospitalization (HR, 0.85, 95% CI, 0.78 À 0.93; p < 0.001) ( Table 1). The absolute benefit was greater among patients who had a baseline LDL-C of ! 2.6 mmol/L (100 mg/ dL) than among patients with lower baseline LDL-C level. In a pre-specified analysis 116 , alirocumab prevented total (first and recurrent) non-fatal CV events (HR, 0.87, 95% CI, 0.82 À 0.93) and all-cause mortality (HR, 0.83, 95% CI, 0.71 À 0.97) in the presence of a strong association between nonfatal and fatal event risk. In another pre-specified analysis 117 , alirocumab has also reduced the total (first and subsequent) hospitalizations (HR, 0.96 [95% CI, 0.92-1.00]; p ¼ 0.04) and slightly increased days alive and out of hospital (RR, 1.003 [95% CI, 1.000-1.007]; p ¼ 0.05) due to a decrease in days dead (RR, 0.847 [95% CI, 0.728-0.986]; p ¼ 0.03), defined as the time from a patient's death to the study end date. A third pre-specified analysis 118 of the ODYSSEY OUTCOMES trial investigated the effect of alirocumab in ACS patients with a history of coronary artery bypass grafting. Alirocumab was associated with large absolute reductions in MACE and death. In a prespecified analysis of other vascular events in the ODYSSEY OUTCOMES trial, alirocumab lowered PAD events (HR, 0.69; 95% CI, 0.54-0.89; p ¼ .004) significantly, but missed the statistical significance in reducing VTE events (HR, 0.67; 95% CI, 0.44-1.01; p ¼ .06) 119 . When combined data from the FOURIER and ODYSSEY OUTCOMES trials (n ¼ 46,488) were analyzed, it resulted in a 31% reduction in VTE events [HR, 0.69; 95% CI, 0.53-0.90; p ¼ .007] 109 . Evidence also suggests the association with Lp(a) levels reduction 109,119 . The two parallel, SPIRE-1 and SPIRE-2, CV outcome trials (n ¼ 27,438) 104 were conducted in 2013 to assess the efficacy and safety of bococizumab, at a dose of 150 mg Q2W, in patients at high CV risk. However, the trials were terminated early by the sponsor after a median follow up of 10 months. The sponsor opted to stop the development of bococizumab due to the high rates of the anti-drug antibodies (ADAs), as detected in the data from other studies in the SPIRE program that consisted of a total of eight studies (i.e. six lipid-lowering and two CV outcome trials). In a combined analysis of SPIRE-1 and SPIRE-2 trials, bococizumab showed no benefit in term of MACE 9,104 ( Table 1).

Safety of PCSK9 monoclonal antibodies
In general, the safety profile of both evolocumab and alirocumab is excellent. Nasopharyngitis and mild injection-site reactions are considered the most common adverse reactions 5 .

Adverse effects
In two meta-analyses 1,79 of RCTs, treatment with PCSK9 inhibitors was not associated with the adverse effects commonly described with statin therapy such as myalgia and elevations in serum aminotransferases or creatine kinase, with overall serious adverse events that were comparable to the control group. The aforementioned meta-analyses did not report the injection site reactions which are the most frequent adverse events with the PCSK9 inhibitors use 4 . The allergic local injection-site reactions (e.g. itching, redness, swelling) rates were 3.8% in alirocumab group versus 2.1% in placebo (p < .001) 99 and 5.9 versus 4.2% over a period of 78 weeks 63 . Injection-site reactions are usually mild and selflimited 99 . The reactions rates with evolocumab were 2.1% versus 1.6% in the placebo group 98 , and there was no increase in hypersensitivity with longer treatment 120 . Bococizumab caused higher injection-site reactions as compared to placebo (10.4 versus 1.3%, p < .001) 104 and 12.7 per 100 person-years in six trials evaluating bococizumab. However, the rates did not increase with longer time 121 .

Anti-drug antibodies
Therapeutic molecules may have an immunogenicity potential that may generate ADAs. The ADAs can be generated in response to fully human (e.g. alirocumab and evolocumab) or humanized (e.g. murine-derived bococizumab) monoclonal antibodies. High-titer ADAs have substantially developed in response to treatment with bococizumab (i.e. high degree of immunogenicity), while treatment with either alirocumab or evolocumab has not significantly been associated with ADAs generation 104,121 . This immunologic difference explains the higher injection-site reactions rate reported with bococizumab 104 . ADAs can be either neutralizing or binding 122 . Binding ADAs bind to therapeutic proteins without affecting the molecules' function 120 , while neutralizing ADAs can directly impair their function 122 . In the ODYSSEY OUTCOMES 99 trial, neutralizing ADAs were observed in 0.5% of patients on alirocumab versus 0.1% of those on placebo. Roth et al. 123 reported data from 10 trials and found that ADAs against alirocumab were observed in 5.1% of patients as compared to 1.0% in the control group with persistent ADAs found in 1.4% versus 0.2% after 12 weeks or more, respectively. Neutralizing ADAs were reported in only 1.3% of the patients in alirocumab group. In the FOURIER 98 trial, evolocumab generated binding ADAs in 0.3% of the patients and no neutralizing ADAs have been detected. These findings were consistent over a period of four years as observed in the OSLER-1 120 study. In four RCTs that reported data regarding ADAs for evolocumab and alirocumab, the ADAs were transient and their titers decreased over time 79 . Almost half of the patients who received bococizumab in the SPIRE studies had high-titer ADAs with neutralizing antibodies developed in 29% of the patients 104 . Comparisons of ADAs incidence among the different PCSK9 inhibitors may be misleading because immunogenicity potential is dependent on various factors 123 . Cross-reactivity between PCSK9 inhibitors is unlikely to occur due to the specificity of the binding domains 121 . ADAs directed against PCSK9 inhibitors are one of the theoretical mechanisms for the PCSK9 inhibitors hyporesponsiveness (i.e. < 15% reduction in LDL-C level) due to the physiological impairment of monoclonal antibodies once absorbed into the circulation 14 . The occurrence of ADAs with PCSK9 inhibitors has attracted widespread attention.
Although autoantibodies were detected in some patients on alirocumab or evolocumab, reductions in LDL-C levels induced by either monoclonal antibody were not attenuated 79 . ADAs did not appear to significantly attenuate the lipid-lowering effect of alirocumab 99,123 or evolocumab 98 , unlike that of bococizumab 121 . The high immunogenicity rates and the wide variation in LDL-C response among patients on bococizumab led to the discontinuation of further clinical development of bococizumab 9,121 .

Neurocognitive events
Low LDL-C levels in the clinical trials of PCSK9 inhibitors have raised a concern about the association of their use with cognitive impairment 124 . Neurocognitive adverse events were reported as delirium, attention disorders, amnesia, dementia, disturbances in thinking and perception, or mental impairment disorders 4,5 . Neurocognitive adverse events reported in the clinical trials of alirocumab and evolocumab were not significantly different from those reported in the control groups 63,98,99 . Furthermore, similar findings were found in two meta-analyses of RCTs 1,79 and in the EBBINGHAUS 124 study (n ¼ 1204), a sub-study of the FOURIER trial, that evaluated the cognitive deficits. The findings of the latter study were supported by a Mendelian randomization study which found no causal relationship between inhibition of PCSK9 function and neurodegenerative diseases 125 .

Diabetes
It has been hypothesized that the significant LDL-C-lowering effect of the PCSK9 inhibitors may negatively affect the glycemic status, namely the new-onset diabetes, as suggested with statin therapy to be associated with an increase in diabetes incidence 104,126 . It has been postulated that the genetic variations in the targets of statins and PCSK9 inhibitors i.e. 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and PCSK9 respectively, have been associated with desirable protective effect against atherosclerosis but on the other hand with adverse effects in term of diabetes 104 . The currently available evidence did not suggest significant increase in new-onset diabetes or worsening of preexisting diabetes on shorter or longer terms with either alirocumab 39,63,99,127 or evolocumab 56,57,98,120,[128][129][130] . In the SPIRE 104 studies, although bococizumab use was not associated with significant increase in new-diabetes, the early termination of the trials precluded the ability to assess adverse effect on glycemic control. The results of three recent meta-analyses 1,79,126 are in line with the findings of the previous studies. However, one 126 of the aforementioned meta-analyses has elaborated that the imbalance in the background LLT of the control arms may have masked the effect of PCSK9 inhibitors on diabetes.

Fat-soluble vitamins and steroid hormones
Fat-soluble vitamins and steroid hormones levels were measured as part of prespecified safety analyses in trials with PCSK9 inhibitors such as ODYSSEY LONG TERM 63 and DESCARTES 131 . In the alirocumab group, although the levels of vitamin E or vitamin K were below the lower limit of the normal range, there were no clinically important changes. Moreover, there was no clinically meaningful effect with regards the changes in the levels of cortisol or other fat-soluble vitamins 63 . Similarly, vitamin E levels were lower in evolocumab-treated subjects but the results were not statistically significant after the adjustment of LDL-C level, and there were no adverse effects reported for steroid or gonadal hormones 131 .

Cost-effectiveness of PCSK9 monoclonal antibodies
Evaluation of cost effectiveness of PCSK9 inhibitors has been of interest to several trials. In 2015, they were not proven to be cost effective as their use increased annual prescription spending by $125 billion over ezetimibe in patients with HeFH or ASCVD 132 . Another study 133 recommended price reduction ($14,000-15,000) by 70% as they possessed incremental cost of about $350,000 per quality adjusted life years (QALYs) when compared to statins. A treatment cost analysis 134 of evolocumab versus ezetimibe to prevent one MACE in T2DM using data from the FOURIER and IMPROVE-IT trials had similar conclusion. In all analyses performed, ezetimibe was consistently a cost-saving strategy compared with evolocumab except for the case where evolocumab price is significantly reduced and the branded ezetimibe is used. In a review 135 of 10 health economic evaluations from United States of America (USA) and Europe, cost effectiveness of PCSK9 inhibitors remained uncertain and price reduction was recommended. Following the CV outcome trials, a study showed that relevant costs saved per CV event annually remained very small in Apulia, Italy 136 . In response to 2018 price reductions, National Lipid Association issued a statement to highlight in which subgroups of patients PCSK9 monoclonal antibodies are currently considered a cost-effective reasonable option ( < USD 100,000/QALY). The subgroups included (a) extremely high-risk patients with ASCVD of >40% 10-year ASCVD risk and LDL-C ! 1.8 mmol/L (70 mg/ dL), (b) very-high-risk patients with ASCVD of 30-39% 10-y ASCVD risk and LDL-C ! 2.6 mmol/L (100 mg/dL) and (c) highrisk patients with 20-29% 10-year ASCVD risk and LDL-C ! 3.36 mmol/L (130 mg/dL). All patients should receive a maximally tolerated statin therapy and ezetimibe prior starting PCSK9 monoclonal antibodies therapy 137 .

Registered clinical trials
The documented significant benefit of the PCSK9 monoclonal antibodies in decreasing lipid parameters and adverse CV events encouraged further investigations on their efficacy and safety. Research is ongoing in patients with familial and non-familial hypercholesterolemia. New research is targeting special patient populations such as pediatrics and human immunodeficiency virus (HIV) subjects. Relevant ongoing Phase III trials are listed in Table 2.

Conclusions
The currently-available PCSK9 monoclonal antibodies, alirocumab and evolocumab, are safe and effective when used as an add-on or as monotherapy. They are effective in lowering LDL-C levels by more than 50% in pooled populations. The outcome studies have confirmed that the significant LDL-Clowering capacity was translated into a reduction in CV events without causing excessive adverse effects. The benefit in CV outcomes was attributed to the ability of PCSK9 inhibitors to lower atherogenic lipid fractions such as LDL-C, non-HDL-C, apo-B, and Lp(a) levels and atheroma volume. The ability to lower Lp(a) levels could potentially reduce PAD, VTE, and AS events as well.

Declaration of funding
Nothing to declare.

Declaration of financial/other relationships
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.