# Dyslipidemia and Cardiovascular Risk Reduction

## Introduction

Dyslipidemia, particularly elevated low-density lipoprotein cholesterol (LDL-C), is a causal and modifiable risk factor for atherosclerotic cardiovascular disease (ASCVD). Statin therapy remains the cornerstone of treatment, but the therapeutic landscape has expanded significantly with the addition of ezetimibe, PCSK9 inhibitors, bempedoic acid, and inclisiran. A risk-based approach guides treatment intensity and target selection.

## Lipid Physiology and Atherogenesis

- **LDL particles** penetrate the endothelium, undergo oxidation, and trigger an inflammatory cascade leading to foam cell formation and plaque development
- **Mendelian randomization studies** confirm that lifelong exposure to lower LDL-C reduces ASCVD risk proportionally
- **"Lower is better"**: each 1 mmol/L (39 mg/dL) reduction in LDL-C reduces major vascular events by approximately 22% (CTT meta-analysis)
- **Non-HDL cholesterol** captures all atherogenic particles (LDL, VLDL, IDL, Lp(a)) and may be a superior treatment target
- **Apolipoprotein B (ApoB)**: one molecule per atherogenic particle; increasingly recognized as the best measure of atherogenic burden

## Cardiovascular Risk Assessment

- **10-year ASCVD risk**: Pooled Cohort Equations (PCE) estimate risk of first MI, stroke, or CV death
- **Risk enhancers**: family history of premature ASCVD, metabolic syndrome, CKD, chronic inflammatory conditions (RA, psoriasis, HIV), South Asian ancestry, elevated Lp(a), elevated hsCRP
- **Coronary artery calcium (CAC) score**: useful when risk-based decision is uncertain (borderline/intermediate risk); CAC = 0 generally supports deferring therapy

![ASCVD risk assessment and statin decision algorithm](images/ascvd-risk-algorithm.png)

## Statin Therapy

### Indications for Statin Initiation (2018 ACC/AHA)

- **Clinical ASCVD** (secondary prevention): high-intensity statin for all
- **LDL-C >= 190 mg/dL** (primary hypercholesterolemia): high-intensity statin without risk calculation
- **Diabetes, age 40-75**: at least moderate-intensity statin; high-intensity if multiple risk factors
- **10-year ASCVD risk >= 7.5%**: moderate-to-high intensity statin after clinician-patient risk discussion

### High-Intensity vs. Moderate-Intensity

| Intensity | LDL-C Reduction | Agents |
|----------|----------------|--------|
| High-intensity | ≥50% | Atorvastatin 40-80 mg, rosuvastatin 20-40 mg |
| Moderate-intensity | 30-49% | Atorvastatin 10-20 mg, rosuvastatin 5-10 mg, simvastatin 20-40 mg |
| Low-intensity | <30% | Simvastatin 10 mg, pravastatin 10-20 mg, lovastatin 20 mg |

### Statin Safety

- **Myalgias**: reported in 5-10% but nocebo effect is significant (SAMSON trial: two-thirds of statin-attributed symptoms also occurred with placebo)
- **Hepatotoxicity**: rare; routine LFT monitoring no longer recommended
- **New-onset diabetes**: modest risk (1 additional case per 255 patients treated for 4 years); cardiovascular benefit far outweighs risk
- **Drug interactions**: avoid simvastatin > 20 mg with amlodipine or diltiazem; rosuvastatin preferred with fewer CYP3A4 interactions

## Non-Statin Therapies

### Ezetimibe

- Blocks intestinal cholesterol absorption via **NPC1L1 transporter**
- Additional LDL-C reduction of 15-20% when added to statin
- **IMPROVE-IT**: ezetimibe + simvastatin reduced MACE vs. simvastatin alone in post-ACS patients

### PCSK9 Inhibitors

- **Evolocumab** (Repatha) and **alirocumab** (Praluent): monoclonal antibodies given subcutaneously every 2-4 weeks
- LDL-C reduction: 50-60% on top of maximally tolerated statin
- **FOURIER**: evolocumab reduced MACE by 15%; greatest benefit when LDL-C lowered below 20 mg/dL
- **ODYSSEY OUTCOMES**: alirocumab reduced MACE and all-cause mortality in post-ACS patients
- Indicated for ASCVD patients not at LDL-C goal despite maximally tolerated statin + ezetimibe

### Inclisiran

- **Small interfering RNA (siRNA)** targeting hepatic PCSK9 mRNA synthesis
- Administered subcutaneously twice yearly after initial loading doses
- LDL-C reduction approximately 50%; cardiovascular outcomes trial (ORION-4) ongoing
- Practical advantage: eliminates adherence burden of biweekly injections

### Bempedoic Acid

- **ACL inhibitor**: blocks cholesterol synthesis upstream of HMG-CoA reductase
- Prodrug activated in the liver (not in skeletal muscle, hence no myalgia)
- **CLEAR Outcomes**: reduced MACE by 13% in statin-intolerant patients
- LDL-C reduction: 18-25% as monotherapy; available in fixed-dose combination with ezetimibe

![Mechanisms of action of lipid-lowering therapies](images/lipid-lowering-mechanisms.png)

## Triglycerides and Residual Risk

- **Hypertriglyceridemia** (>= 500 mg/dL): risk of pancreatitis; treat with fibrates, omega-3 fatty acids, dietary modification
- **REDUCE-IT**: icosapent ethyl (EPA) 4 g/day reduced MACE by 25% in statin-treated patients with triglycerides 135-499 mg/dL
- **STRENGTH**: carboxylic acid omega-3 (EPA+DHA) showed no benefit, suggesting the EPA component is key
- **Lp(a)**: genetically determined, not meaningfully lowered by statins; emerging therapies (olpasiran, lepodisiran) show > 90% Lp(a) reduction

## LDL-C Treatment Thresholds

- **Very high-risk ASCVD**: LDL-C goal < 55 mg/dL (ESC/EAS) or >= 50% reduction (ACC/AHA); add ezetimibe, then PCSK9 inhibitor if not at goal
- **High-risk primary prevention** (LDL >= 190 or diabetes with risk factors): target >= 50% reduction
- **Moderate-risk primary prevention**: shared decision-making; consider risk enhancers and CAC score

![Stepwise approach to LDL-C lowering by risk category](images/ldl-treatment-steps.png)

## Key Clinical Pearls

- Statins remain the foundation of ASCVD risk reduction; address statin hesitancy with evidence from the SAMSON and StatinWISE trials showing high nocebo rates
- In secondary prevention, the goal is maximal LDL-C lowering; there is no lower threshold below which benefit ceases
- Bempedoic acid is an important option for truly statin-intolerant patients with proven MACE reduction
- Check Lp(a) at least once in every patient; it is an independent, causal risk factor not captured by standard lipid panels

## References

1. Grundy SM, Stone NJ, Bailey AL, et al. 2018 AHA/ACC Guideline on Management of Blood Cholesterol. *Circulation*. 2019;139(25):e1082-e1143.
2. Sabatine MS, Giugliano RP, Keech AC, et al. Evolocumab and Clinical Outcomes in Patients with Cardiovascular Disease (FOURIER). *N Engl J Med*. 2017;376(18):1713-1722.
3. Nissen SE, Lincoff AM, Brennan D, et al. Bempedoic Acid and Cardiovascular Outcomes in Statin-Intolerant Patients (CLEAR Outcomes). *N Engl J Med*. 2023;388(15):1353-1364.
4. Bhatt DL, Steg PG, Miller M, et al. Cardiovascular Risk Reduction with Icosapent Ethyl (REDUCE-IT). *N Engl J Med*. 2019;380(1):11-22.