The dominant approach to slowing atherosclerosis remains a mix of pharmaceuticals that can, separately, reduce blood pressure and LDL cholesterol (LDL-C) in the bloodstream. In the latter case, new therapies such as PCSK9 inhibitors and improved combinations of statins are capable of doing far more than just return raised LDL-C to normal levels. It is in fact possible to reduce blood cholesterol to something like a half or quarter of normal levels, and this produces incrementally greater benefits in reduction of atherosclerosis risk. But is it safe over the long term? And if it is, why did we evolve to have the observed normal levels of cholesterol in blood?
Atherosclerosis is the build up of fatty plaques that narrow and weaken blood vessels, ultimately leading to a fatal rupture of some form. Raised blood pressure accelerates this process through mechanisms that are incompletely explored – but it is obviously the case that, at later stages, more pressure and weaker blood vessels combines to increase the risk of fatal structural failure. Cholesterol is another input, arriving from the bloodstream. The final input is the activity of the immune system, and local inflammatory signaling, as the immune cells called macrophages attempts to clean up cholesterol from blood vessel tissues and return it to the liver to be disposed of.
Atherosclerotic plaques start and grow due to the presence of damaged, oxidized cholesterol more than overall cholesterol, but the more cholesterol in total, the more oxidized cholesterol is mixed in. That proportion increases with age, as rising levels of oxidative molecules throughout the body lead to ever more oxidative damage to molecules. Macrophages respond to the presence of cholesterol, arrive, become overwhelmed by oxidized cholesterol, and become inflammatory foam cells or die. In either case they produce signaling that leads to a further influx of macrophages, a feedback loop that only worsens with time. The bulk of atherosclerotic deposits is made up of the debris of dead cells and the cholesterol they failed to clear away, a significant fraction of it oxidized cholesterol.
Thus lower blood cholesterol is good in the sense that it will slow down this process by reducing one of the inputs. Unfortunately it doesn’t appear to significantly reverse atherosclerosis. Established atherosclerotic plaques remain, and the fatal end result is only put off to some degree, even for the very dramatic reductions in blood cholesterol discussed here. Better approaches are needed, such as ways to destroy oxidized cholesterol, or make macrophages resistant to oxidized cholesterol, or otherwise improve the process by which macrophages mine cholesterol from plaques and export it back to the liver. The past twenty years has seen a fair amount of innovation on the latter option, but sad to say that it has failed in human trials, even while producing as much as a 50% reversion of plaque in mice.
LDL-C is deposited in the arterial wall and promotes the inflammation process through the attraction of monocytes and macrophages at the site of cholesterol deposition, thus resulting in the development of atherosclerotic plaques and overt cardiovascular (CV) disease. An abundance of evidence has shown a linear relationship of LDL-C levels with the risk for CV events. Several lipid-lowering treatments such as statins, ezetimibe and the novel proprotein convertase subtilisin kexin 9 (PCSK9) inhibitors were found to offer significant benefits in the reduction in LDL-C and importantly in the amelioration of the overall CV risk of patients with hyperlipidemia with or without CV disease.
Towards this direction, the European Society of Cardiology and the European Society of Atherosclerosis recommend the reduction in LDL-C to lower than 70 mg/dl or a reduction of at least 50% if the baseline values are between 70 and 135 mg/dl in very high-risk patients, to lower than 100 mg/dl or a reduction of at least 50% from baseline values between 100 and 200 mg/dl in high-risk patients, and to less than 115 mg/dl in low to moderate risk patients. The 2017 American Association of Clinical Endocrinologists and American College of Endocrinology Guidelines for Management of Dyslipidemia and Prevention of Cardiovascular Disease suggest even lower LDL-C targets of <100 mg/dl, <70 mg/dl, and <55 mg/dl, in high, very high, or extreme risk diabetic patients.
The necessity for the reduction in LDL-C levels to provide significant CV beneficial effects has been shown and is recommended by all international guidelines. However, there are concerns for the optimal lower limit in which LDL-C can be reduced to achieve optimal CV benefit without causing potential adverse events. The purpose of this review is to present available data for the safety of reducing LDL-C to low or very low levels as it comes from studies of lipid-lowering drugs that achieved such levels.
In general, intensive lipid-lowering studies with statins showed that there is no increased risk of adverse events with reducing LDL-C to levels of approximately 40-50 mg/dl. The most important data for reducing LDL-C to such levels are provided from PCSK9 inhibitors studies where remarkable reductions in LDL-C levels were achieved and no increased rates of adverse events were noted with evolocumab. The slightly concerning findings with alirocumab in the ODYSSEY LONG TERM trial were not verified in the ODYSSEY OUTCOMES study. More importantly, the potential neurocognitive decline with low LDL-C was not observed in several post-hoc analyses and in the EBBINHAUS trial that was specifically designed to evaluate such events. However, it has to be noted that in most trials, the follow-up period and the exposure of the patients in low LDL-C was rather short and trials with longer study periods are needed to unveil potential harms.
Last, higher incidences of hemorrhagic stroke and cancer were not observed in these studies, even at very low LDL-C levels. In conclusion, reduction of LDL-C to less than 50 mg/dl seems safe and provides greater CV benefits compared with higher levels. Data for achieved LDL-C lower than 20-25 mg/dl is limited, although findings from the above mentioned studies are encouraging. However, further evaluation is needed for future studies and post-hoc analyses.