What are PCSK9 inhibitors?

PCSK9 inhibitors are a new type of cholesterol-lowering drug. Those inhibitors are monoclonal antibodies that target and inactivate a liver protein, known as proprotein convertase subtilisin kexin 9. Knocking out this protein, the amount of harmful LDL cholesterol circulating in the bloodstream reduces dramatically. This LDL reduction translates into fewer strokes, heart attacks and other problems related to cholesterol-clogged arteries, once arteries becomes healthier.

Premature cardiovascular disorders and associated mortality are significatively related to heterozygous familial hypercholesterolaemia (HeFH), which is one of the most frequent genetic disease. The role of PCSK9 in such and similar health issues is regulation of hepatic LDL receptor action; blocking PCSK9 binding to the LDLR with a monoclonal antibody has been accepted as an eficient LDL lowering.

PCSK9 was discribed in 2003 by Nabil Seidah, in the Clinical Research Institute of Montreal in Canada. The new protein convertase, the gene for which was located on the short arm of chromosome 1. Meantime, families presenting HeFH had been followed by Catherine Boileau at a famous Parisian hospital , where a mutation on chromosome 1 were identified. Both laboratories worked together and identified the PCSK9 to the condition. Afterwards, diverse experiments from different laboratories demonstrated that high levels of PCSK9 are responsible for LDL receptors stopped function.

Since the late 1980s, statins have been the first-line drugs for lowering cholesterol serum levels but unfortunately, statins doesn’t trigger the expected response in some people and also there are groups of people that can’t take those drugs because of side effects like liver damage, muscle pain or even development of diabetes.

Although PCSK9 inhibitors are an experimental drug, the results of clinical trials are positive, lowering cholesterol levels in patients under the treatment with PCSK9 inhibitors and suggesting clinic benefits as lowering incidence of cardiovascular events.

Cholesterol metabolism and PCSK9 inhibitors

Why manage the levels of cholesterol?

Atherosclerotic plaques in formation.

Elevated levels of cholesterol are associated with the formation of atherosclerotic plaques which occur inside the arteries. Plaque is made of cholesterol, fatty molecules, cellular products, calcium and fibrin and it may occlude or break off. The result of atherosclerosis is: coronary heart diseases, agina, carotid artery disease, peripheral artery disease (PAD) and chronic kidney disease.  

Did you know that? 

Ischaemic heart disease is first leading cause of death worldwide and stroke is the second.

Cholesterol - A basic review

Cholesterols are fundamental molecules for animals. Belonging to sterols class, these organic molecules take role on the composition of cell membranes, maintaining fluidity and integrity. Raised serum levels are highly associated with increased risk of stroke and heart disease. The World Health Organization points that one third of ischaemic heart disease can be attributed to high cholesterol (HC). The concern about treatment and prevention of HC and its consequences is global, once it is present in both developing and developed word as a risk factor for other health complications.

The average amount of deaths in the world due to cardiovascular diseases.

Functions:

Precursor to bile acids, steroid hormones in mammals, vitamin D;

Component of cell membranes, maintaining membranes fluidity and permeability: cholesterol molecules interact with fatty acyl chains of phospholipids to increase packing density, produce condensing effect, reduce permeability and maintain membrane fluidity;

Covalent link to proteins.

Gateway

Intestinal Absorption of Cholesterol

Intestinal absorption of cholesterol is a key regulatory in human sterol metabolism because it determines what percentage of cholesterol produced by liver and what percentage of dietary cholesterol is released into the blood. In normal metabolism, approximately 55% of the intestinal absorption enters the blood through the enterocyte per day. Also, there is a mechanism to remove the excess of cholesterol from the enterocyte. This process are made by ATP-binding cassette (ABC) protein family, ABC1, ABCG5 and ABCG8. These proteins transport excessive cholesterol from enterocyte back into the gut. Then, cholesterol is eliminated in the faeces as unreabsorbed sterols and bile acids.

Cholesterol Synthesis

Biosynthetic pathway

The first stage of cholesterol synthesis is the production of mevalonate, which is the rate-limiting step in cholesterol formation. In this pathway, two molecules of acetyl-CoA condense forming acetoacetyl-CoA which then condenses with a third molecule of acetyl-Coa forming β-hydroxy-β-methylglutaryl-CoA (HMG-CoA). Then, HMG-CoA is reduced to mevalonate catalyzed by HMG-CoA reductase. The regulation of the activity of HMG-CoA reductase is controlled by:

Transcriptional Control: the rate of synthesis of HMG-CoA reductase mRNA is controlled by sterol-regulatory element-binding proteins (SREPBs);

Proteolytic Degradation of HMG-CoA reductase: high levels of cholesterol and bile salts can lead to enzyme proteolysis;

Regulation by covalent modification: the activity of the reductase is also regulated by phosphorylation and dephosphorylation of the HMG-CoA reductase.

The second stage is the conversion of mevalonate to two activated isoprenes. Three phosphate groups are transferred from ATP molecules to mevalonate, forming isopentenylpyrophosphate and dimethylallyl pyrophosphate. The next stage is the reactions between isoprenes forming squalene containing 30 carbons. In the last stage, the enzyme squalene monooxygenase adds one atom of oxygen to the squalene molecule forming an epoxide and NAPH reduces the other oxygen atom to H2O. Then, it allows the conversion in a cyclic structure leading to formation of lanosterol and finally, cholesterol.

How does the cholesterol flows inside the bloodstrem? How is it internalized by cells?

Receptor-mediated endocytosis of LDL:

Two different types of lipoprotein receptors have been identified. The first one present receptors that bind lipoproteins presenting exogenous cholesterol absorbed from the intestine (chylomicron remnant receptors); the other one is responsible for uptake of cholesterol carrying lipoproteins derived from the liver and other nonintestival sources (LDL receptors) (Goldstein and Brown, 1987). Since these receptors are regulated by different genes, they promote different models of metabolic regulation.

Focusing on the LDL receptors:

The main ligand on LDL receptors is ApoB-100 on the LDL molecule; nonetheless this receptor can also be activated by VLDL, IDL and HDL due to existence of multiple copies of ApoE in such lipoproteins (Daniels, 2009). The LDLR is composed by three protein modules: a domaisn with seven contiguous cysteine-rich repeats, a400-amino acid sequence and a 58-residue sequence rich in serine and threonine.

Endocytosis

After the endocytosis, a receptor-ligand complex (called lysosome) is formed, in which all of the components of LDL are absolutely degraded by acid hydrolases. The LDLR peptide is recovered back to the membrane; meantime, the larger portion of lipids (cholesteryl esters) is hydrolysed by lysosomal acid lipase (LIPA) into free cholesterol. This cholesterol may be integrated into the cell membranes or play an important role in efflux to cellular adaptors, conversion back into cholesterol esters, metabolism into bile acids or synthesis of steroids, according to different cell types.

When there is cholesterol overloading, the efflux process, by macrophage foam cells, become extremely important for homeostasis maintenance. One of the possible cholesterol efflux pathways is related to the gene ABCA1. It occurs when the ApoA1 (Apolipoprotein A-1) binds to ABCA1, avoiding its deterioration and expanding the level of ABCA1 transporter in the plasma membrane. The ABCA1 transporter and ApoA-1 interaction promote the movement of free cholesterol across the cell membrane, resulting in the formation of discoidal HDL molecules. The efflux process is known by its importance in preventing atherosclerosis in humans.

. Summary of  ABCA1 activity in the plasmamembrane of cells and formation of nascent HDL particles.

Basis of Transport of Cholesterol by The Blood Lipoprotein

Cholesterol is hydrophobic and must be transported through the bloodstream packaged as lipoproteins.  The major carriers of lipids are chylomicrons, VLDL (metabolism leads to IDL and LDL) and HDL. The apoproteins add to the hydrophilicity  and structural stability of the particle, activate enzymes for lipoprotein metabolism and act as ligands on the surface of lipoprotein that target specific receptors on peripheral tissues.

• Chylomicrons: are synthesized from dietary lipids within the epithelial cells of small intestine and secreted in to the lymphatic vessels draining the gut. The apoprotein apoCII activates lipoprotein lipase (LPL), which hydrolyze the chilomicrons releasing the free fatty acids derived from core triacylglycerides into target cells.

Finally, the excess of LDL are available for nonspecific uptake by macrophages (scavenger cells) present near the endothelial cells of arteries.

Important: This exposure of vascular endothelial to high levels of LDL is related to induce the inflammatory process and initiate the formation of atherosclerosis.

• High-Density Lipoprotein (HDL): HDL particles can be created by various mechanisms. The first is the synthesis by the liver and intestine whose shell, like that of other lipoproteins, lipids and apoproteins. The second is the budding of apoproteins from chylomicrons and VLDL and the third method for HDL generation is free apoAI, which adquires cholesterol and phospholipids from other lipoproteins and cell membranes. The benefit of HDL particles is the ability to remove cholesterol from cholesterol-laden cells and to return the cholesterol to the liver, making a reverse transport. Consequently, the likelihood that foam cells (macrophages that engulf LDL-cholesterol) will form within vessels wall is reduced. Reverse cholesterol transport is the directional movementVery-Low-Density Lipoprotein (VLDL): the excess of carbohydrate are converted to triacylglycerols which along with free and esterified cholesterol, phospholipids, and the major apoB-100 are packaged forming VLDL. These particles are secreted from the liver into bloodstream and accept apoproteins from high-density lipoprotein (HDL) particles. The apoproteins activate lipoprotein lipase which facilitates the hydrolysis of the triacylglicerol causing the release of fatty acids and glycerol. These fatty acids are oxidized as fuel by muscle cells, used in resynthesis or milk production in lactating breast. The VLDL remnants are take up from the blood by liver cells mediated by apoproteins.

• Intermediate-Density Lipoprotein (IDL): approximately half of the VLDL remnants are not taken by the liver and form IDL. Then, the hepatic triglyceride lipase removes the additional triacylglycerols from IDL forming LDL.

• Low-Density Lipoprotein (LDL): approximately 60% of the LDL is transported back to the liver to be endocytosed. The remaining 40% of LDL is carried to tissues such as adrenocortical and gonadal cells that also contain apoprotein receptors and they use the cholesterol for the synthesis of steroid hormones, membranes and vitamin D.  of cholesterol from the cell to the lipoprotein particle.  The HDL clearance is through its uptake by scavenger receptor SR-B1 or specific receptors on hepatocytes.

What the future holds for this drug?

Further studies are needed to strengthen evidence for some suggested clinical benefits, as stroke and cardiovascular disease. Those outcomes are already been assessed in current studies but full cardiovascular impact is unknown. Side effects were also reported, including nasopharyngitis, injection site reactions (PCSK9 inhibitors are given by subcutaneous injection one or two times per month), allergy, flu and itching, even so the drug is considered well-tolerated.

It is an expensive drug (cost could run $7.000 - $12.000 per year) but seems to be a good new for millions, but the personal lifestyle is still an important adjunct fundamentally linked with the maintenance of good cholesterol levels.

Quit smoking!

Eat low fat/salt diet! 

Exercise!

If cholesterol still remains high, drugs can be used to help!