Matt Christ

For decades, HDL has been called the “good cholesterol.” Higher HDL levels are associated with lower cardiovascular risk. So why have nearly every major pharmaceutical intervention designed to raise HDL failed to prevent heart attacks? This is one of the most fascinating examples of the difference between correlation and causation in medicine. Let’s examine the clinical evidence. Torcetrapib — ILLUMINATE (2007) Torcetrapib dramatically increased HDL cholesterol. Unfortunately, the trial was terminated early after researchers observed increased cardiovascular events and mortality. The drug improved the biomarker while worsening clinical outcomes. Off-target effects, including increased blood pressure, complicated interpretation. Dalcetrapib and Evacetrapib Both successfully increased HDL. Neither demonstrated meaningful reductions in cardiovascular events in their major outcome trials. Both programs encountered trials stopped for futility. Niacin — AIM-HIGH and HPS2-THRIVE Niacin increases HDL and improves several traditional lipid measurements. Yet neither major trial demonstrated additional cardiovascular protection when added to contemporary lipid-lowering treatment. HPS2-THRIVE also identified increased serious adverse events. Anacetrapib — REVEAL Anacetrapib finally demonstrated a modest reduction in coronary events. However, researchers attributed much of the benefit to reductions in non-HDL cholesterol rather than increased HDL itself. Now consider the genetic evidence. A 2012 Lancet Mendelian randomization study examined genetic variants associated with naturally elevated HDL. One variant increased HDL without producing the expected reduction in myocardial infarction risk. That finding challenged the assumption that simply increasing HDL cholesterol should reduce cardiovascular events. So what does this actually tell us? HDL cholesterol is associated with cardiovascular health, but its concentration doesn’t necessarily reflect the functionality of HDL particles. Higher HDL frequently accompanies favorable metabolic characteristics, including lower triglycerides and improved insulin sensitivity. It may serve partly as a marker of metabolic health rather than a direct therapeutic target. That doesn’t mean HDL biology is irrelevant. HDL participates in reverse cholesterol transport, inflammation regulation, and other potentially protective processes. But increasing the number on a laboratory report doesn’t automatically improve those functions. My takeaway: This is why I find pharmacology and drug development so fascinating. A mechanism can make perfect biological sense. A medication can successfully manipulate its intended biomarker. And the clinical outcome can still be completely different from what researchers predicted. It’s also why ApoB and the burden of atherogenic lipoproteins deserve so much attention in cardiovascular risk assessment. Improving a biomarker isn’t necessarily the same as improving health outcomes. Sometimes the marker is associated with protection without being the mechanism responsible for it. Research: ILLUMINATE (2007), AIM-HIGH (2011), HPS2-THRIVE (2014), REVEAL (2017), Lancet Mendelian Randomization (2012). Not medical advice. #HDL #Cholesterol #ApoB #CardiovascularHealth #LipidResearch #Pharmacology #Longevity #MetabolicHealth