Lesson 6 Flashcards
How do the doctors know when a patient should receive treatment for a dyslipidaemia? what are the drugs used for these diseases?
ormally the first screening to do is measuring the total cholesterol of our body. When this happens the physician will take into account the other risk factors, like diabetes, obesity, smoking, being male or females, in fact for example menopause represents a risk factor because the levels of estrogen that normally protect the heart for these diseases are very low. Then, depending on the risk factors and the levels of cholesterol, he will give the patient a treatment or not. There might also be problems with too low cholesterol, since it is a useful molecule to synthetise steroids and has many other important functions. That means that the cure should be personalised depending on the patient’s needs.
Even when the patient actually has high levels of cholesterol, a cure might not be necessary to keep it under control. The patient might just need to follow a series of guidelines: reduction of dietary saturated fat, weight reduction, increased activity, avoiding smoking and reducing stress. If this approach is unsuccessful or insufficient to normalize the lipid levels, drug therapy is generally recommended.
There are several classes of drugs for the treatment of dyslipidaemias:
- inhibitors of cholesterol synthesis
- bile acid sequestrants,
- cholesterol absorption inhibitors
- Fibrates, Niacin and others
- PCSK9 inhibitors
We do not know how fibrates and niacin exactly work, we only know that they do. It is probably a sum of many different mechanisms.
Also, when administering a drug it is important to take into account that cholesterol balance is regulated by both synthesis and absorption of cholesterol. The inhibition of cholesterol absorption may be compensated by an increase in synthesis. That said an optimal strategy to lower LDL cholesterol may be achieved by inhibiting both pathways (synthesis and absorption). Moreover it is better to accompany a drug therapy with a correct diet.
Speak about the two types of inhibitors of cholesterol synthesis
Drugs for lowering the cholesterol normally work in the liver, and since they lower the production of cholesterol they have a preventive effect towards cardiovascular diseases, meaning that they are a great secondary prevention drug. basically a secondary prevention drug is used after the occurrence of the symptoms of certain disorders, so by taking the drug we prevent a second event; while primary prevention is defined by taking a drug even though the patient never suffer from a specific disorder, with the goal of preventing the symptoms and the illness in the future.
There are many different enzymes involved in the production of cholesterol, the most important being HMG-CoA reductase. This enzyme is very important since it is the rate limiting enzyme in the synthesis of cholesterol. and it can be inhibited by statins, so that there is a modest reduction in the production of cholesterol, in fact statins are able to reduce the effects of cholesterol by 20%. Statins also have an indirect action, infact when the cholesterol concentration is lowered, a cellular signalling cascade in activated, culminating in the activation of sterol regulatory element binding protein 2 (SREBP2), a transcription factor that up-regulates expression of the gene encoding for the LDL receptor, increasing the clearance of LDL. That is why statins are very active drugs, capable of acting on general cholesterol but mainly LDL cholesterol. They represent the first line therapy to lower LDL.
We have around 7 different types of statins and the differences are bound to their efficacy from the most efficient to the less we have: rosuvastatin, atorvastatin and simvastatin.
Statins can be used on different dosages, depending on the state of the patient we might want to use the less potent drugs with a lower dosage, or a stronger drug with an higher dosage, or another combination of drug and dosage. Normally the first dosage will lower the concentration of cholesterol by around 40%, but from that point even by doubling the dosage we will not obtain double the effect, in fact there is only a 6% of extra effect every time the dosage is doubled.
Statins also have a pleiotropic effect, where pleiotropy includes all of a drug’s actions other than those for which the agent was specifically developed. Statins in particular are:
- able to increase by a small amount the concentration of HDL cholesterol
- reduce the concentration of triglyceride by up to 40%, this effect is due to the fact that in the liver less VLDL is produced because of the lower levels of cholesterols. **
- Statins can also decrease inflammation, this effect has been demonstrated by the measurement of C reactive protein in patients who take statins.
- Moreover, they are able to reverse endothelial dysfunctions, improving vasodilatation and preventing the blockage of the vases.
- Statins can also decrease thrombosis by decreasing the production of thrombin, and making the blood more fluid.
- They also improve the stability of the atherosclerotic plaques, infact if there is a rupture in the fibrotic layer breaking the plaque, a clot is going to form and the patient risks an ischemic attach, so by making this fiber thicker there is a lower ischemic attack risk.
Statins also have side effects: one scary side effect used to be myopathy, so pain in the muscles after taking statin, but nowadays this is easily solved by reducing and adjusting the dosage.
There are also inhibitors of VLDL secretion. They act by two different mechanisms, the first one is the inhibition of the lipid transfer by binding to MTP, which we know is responsible for the production of VLDL, so lowering it causes less VLDL. the most famous drug with this mechansim of action is called lomitapide, but this drug cannot be directly delivered to the liver because it might cause alterations in the fat absorption process and thus cause gastrointestinal problems. Inhibitors of VLDL secretions can also be *single-strand antisense oligonucleotide that binds to the apoB100 mRNA and thereby reduces ApoB protein levels this is the mechanism of action of *mipomersen. The net effect of each drug is always the reduction of VLDL secretion. These drugs are specific for people at a very high risk of cardiovascular problems, like people who are associated with familial hypercholesterolemia.
Speak about bile acid sequestrants, their effect and side effects
Another way to cure these diseases is related to the diet. As we know, we absorb cholesterol and fatty acid in the intestine thanks to the bile acid, so we would decrease the absorption by inhibiting the bile acids. Unfortunately bile acids are constantly recycled, because they are reused to digest fatty acids after being absorbed.
So, bile acid sequestrants are positively charged resins while bile acid are negative charged, in this way they bind to the resins and become very big, leading to be eliminated with the stool without getting reabsorbed. By blocking the recirculation of the bile acid the concentration of cholesterol also lowers.
The transformation of cholesterol in bile acids happens thanks to an enzyme called 7-alfa-hydrolase found ****in the liver. Since by using bile acid sequestrants the liver feels the reduction of of the concentration of these acids, it starts producing more of this enzyme by upregulating it, to get as much bile acid as possible for the digestion. But by having an up regulation of this enzyme the amount of cholesterol in the hepatocytes decreases since it is being used to form bile acids. And we know that when cholesterol decreases, SREBP2 up-regulates the expression of the gene encoding for the LDL receptor, increasing the clearance of LDL. This teaches us that when administering a drug it is very important to consider the various changes in the body that will intervene to keep the balance of these different processes.
Like for any drug, there are side effects: since bile acid are important for digestion and absorption, it means that there is going to be a reduction of absorption of different fats, and also vitamins like vitamin K, which is important for haemostasis control, in fact without vitamin k we risk internal bleeding.
Bile acid sequestrants are not very used, mostly they are given to very young people (25 years old) normally linked with familiar hypercholesterolemia, so that they can get a life long therapy and reduce the risk. Even tough taking a drug for a long time can cause problems, this one does not get absorbed in the blood flow, reducing the risks.
Speak about the two kinds of cholesterol absoption inhibitors
Another strategy is inhibiting cholesterol absorption. There are two types of compounds: one is a proper drug and the others are compounds naturally found in food. These inhibitors reduce the absorption of cholesterol in the intestine (exogenous pathway).
Cholesterol enters the intestine through a channel called NPCIL1, so the compounds of the first type block the transporter and thus the entrance of cholesterol into the enterocytes. This works on cholesterol coming from the diet but also for cholesterol forming the bile. By using these drugs there might also be a reduction of the LDL cholesterol, not only caused by the decreased absorption of cholesterol, but also because we have less VLDL production. Ezetimibe is a drug able to reduce the absorption of cholesterol by 50%, but does not have a strong effect on cholesterol present in the bloodstream. if we combine this drug with statins the effect is huge because it acts both on blood and diet.
the other class of compounds is the class of the sterols and stanols which are already present in food, plants, fruit and vegetables (this is also why a heaty diet is useful to fight dyslipidaemias). These compounds are similar to cholesterol but more hydrophobic, increasing the excretion of cholesterol with stool. Danacol, for instance, has sterols and stanols and helps us increasing the amount of these molecules which are usually not enough consumed through food, infact, in our diet we can consume around 200-400 mg of plant sterols but we need around 5 times more to have and actual effect on the absorption of cholesterol. These compounds when taken in the correct quantity can cause a reduction of LDL by around 15%, so this method is only used by people need a light drug.
the result of the reduced cholesterol absorption is a decrees in LDL cholesterol, infact a reduction in cholesterol absorption leads to a decrease in the cholesterol content of chylomicrons, which in turn leads to a decrease in the transport of cholesterol from the intestine to the liver, here normally cholesterol derived from chylomicron remnants contributes to the cholesterol that is packaged into VLDL particles but in this case we will have reduced cholesterol incorporation into VLDL and also decreased LDL-cholesterol concentrations in the plasma
Reduced hepatic cholesterol content also leads to up-regulation of the LDL receptor, as previously seen.
Speak about fibrates, niacin and omega3 for the cure of dyslipidaemias
Fibrates are able to bind to a receptor called PPARalpha which are intracellular receptors. When Fibrates bind to it, the receptors gets activated. These receptors do not dimerize with themselves like it usually happens, but with other receptors called RXR. When dimerized they move into the nucleus and stimulates many genes involved in the metabolism of lipids, in this way we obtain the decrease of triglycerides levels and the increase of HDL levels, but how? PPARalpha activation stimulates the genes that control the synthesis of ApoAI and II, increasing their production and thus, leading to an increase of HDL in the plasma. This enzyme also increases the production of lipoprotein lipase which is an enzyme that when expressed in the muscles absorb triglycerides and increases fatty acids oxidation, in this way Plasma triglycerides are lowered. PPARalpha also directly stimulates fatty acid oxidation in the hepatocytes, lowering the synthesis of tryglicerides. Also, they decrease the production of VLDL production, and modestly lower LDL levels. On top of that Fibrates have beneficial anti-inflammatory effects, decreasing the vulnerability of atherosclerotic plaques to rupture. It is important to remember that fibrates do not lower cholesterol itself but triglycerides and increase HDL.
The most famous Fibrate is fenofibrate along with gemofibrozil. These are the only drugs of these type used in the USA, while in Europe we have many others. All of them as a side effect have gastrointestinal stress.
Niacin is a water soluble vitamin also called vitamin B3, and it is fundamental for the production of NAD and NADP.
Even if the use of niacin as a drug dates back to 1955, we still do not know its precise mechanism of action, but we know that it is beneficial.
In the adipose tissue there is a specific receptor called niacin receptor that is able to decrease the activity of the hormone sensitive lipase and thus decrease plasma free fatty acids, causing a lower flux of fatty acids from the adipose tissue to the liver. In this way there are less fatty acids available for cholesterol. It is important to consider that lipids cannot freely travel in the bloodstream but need to bind to albumin. So trough the activation of niacin receptor we will have less LDL in the plasma. It has also be observed that niacin is able to decrease the degradation of ApoAI, which is fundamental for the production of HDL, this means that HDL is going to increase.
summarizing, we know that it is able to act on molecules like LDL, HDL, triglycerides, and that the effect is not very strong and is also very generic: it lowers LDL, rises HDL, lowers triglycerides, decreases APoB, reduces cardiovascular events and reduces plaque progression. If we need a more intense effect statins or other drugs can be added.
The major adverse effects of niacin are cutaneous flushing and pruritus, but these effects disappear in a couple of months. This drug is indicated for patients with an increase of both triglycerides and cholesterol. It is currently the most effective agent available for raising HDL and the drug of choice for patients with modestly elevated LDL and decreased HDL.
omega-3-fatty acids are another type of molecules which are found into our diet and can be increased thanks to the use of supplements, they are not useful for cholesterol but can lower triglycerides levels. It is said that they can reduce triglycerides in the blood by 50%, this is probably due to the fact that they are able to modulate some transcription factors: SREBP-1c and PPAR. Also, they are able to reduce the catabolism of fatty acids. The use of omega-3 fatty acids on clinical outcomes is uncertain, and we di bit exactly know their mechanism of action.
Speak about PCSK9 inhibitors
Another strategy is using PCSK9 inhibitors. PCSK9 is a protein involved in the degradation of LDL which binds to LDL-R preventing its action. It was discovered because some families suffering from hypercholesterolemia have an increased activity of this protein. There are also families with the opposite problem, in which people have a loss of function of this protein leading to a dramatic reduction of the incidence of cardiovascular problems (by 88%).
So, researchers started to look for a way to inhibit this protein, and now we have two monoclonal antibodies that target PCSK9 but unfortunately they have not been approved to be put on the market yet, on the other hand they have been approved for people that have familiar hypercholesterolemia. These antibodies are injected subcutaneously and act by decreasing LDL cholesterol by 50-72%. Like always there side effects: these antibodies can also influence other receptors, like receptors that are able to recognize viruses. For this reason, people that have been treated with PCSK9 will be more vulnerable to viral infections and need to be closely monitored.
What is hemostasis how does the formation of the blood clot happen?
As we have seen, when there is the formation of the plaque in the lumen of the vessel, the lumen becomes smaller. When the plaque is formed there is also the formation of a fibrose cap which is important since it prevents the break of said plaque. If the plaque brakes, our body will try to fix it by forming a blood clot which only blocks the flow of blood more. If this happens in the heart, we will have a part of the heart that does not get enough oxygen leading to an ischemic attack followed by necrosis of the tissue. The larger the damage, the more severe the problem is.
So, it is important to have a well regulated system of hemostasis, which is the mechanism that regulates bleeding and is able to keep the blood fluid or form clots and stop the bleeding. It’s easy to understand that this process must be very well regulated. When hemostatic processes are activated inappropriately thrombosis can occur.
Whenever we have a damaged endothelium we need the formation of a blood clot, which must be in the specific area of damage and not the nearing tissue. This happens in 4 steps
- In the area where the damage is registered there is an immediate vasoconstriction
- next we will have the primary hemostasis which is mainly mediated by platelets. Platelets are the first elements to arrive in the area and organize the clot, they are able to adhere here and start the formation of the clot, infact when there is an injury to the endothelium, its collagen gets exposed and attracts the platelets. this happens with different mechanisms, for example the formation of a bridge between platelets. These blood elements have many lipoproteins that are able to bind to other proteins. These lipoproteins are glycoproteins called GPIIb, GPIb and GPIIIa. Moreover there is another lipoprotein called glycoprotein GPVI which directly binds to collagen. So GPIIb and IIIa can bind to each other uniting the platelets while GPVI and GPIb are used to bind to collagen. Now, these adherent platelets undergo a process of activation during which the cells’ granule contents are released. The granules have many factors which are important for the formation of the clot. At this point platelets also release ADP which can bind to its receptors, found on the platelets themselves. This receptor is protein G bound and leads to less activation of phosphokinase A PKA which in turn leads to an increase of aggregation. Moreover, the activation of platelets can also be triggered by tissue factors present in the healthy endothelial cells found nearby, thrombin in particular is able to form more stable plaques by converting fibrinogen into a polymer that represents the matrix of the clot and is also capable of recalling other platelets. Thrombin also has a specific receptor called PAR, there are many types of PAR, PAR1, 2, 3, 4. On platelets we only find PAR1 and PAR4. PAR1 is the most important, it is also a G protein bound receptor, in particular it is bound to a Gq protein. It has a similar mechanism to TxA2 which we will see later, in the end it leads to platelets aggregation by an increase in calcium and phospholipase 2. This stimulates the formation of the bridge among platelets. Now the primary hemostatic plug, which is reversible, is formed. for this plug the fibrinogen-GPIIb–IIa interaction is critical.
- The third step is secondary hemostasis caused by other factors which make the plug more stable and permanent, in parallel we activate the factors that prevent the over-formation of thrombi so we also have the release of thrombolytic and anticoagulant factors. This step is also called platelet aggregation and consolidation. the factor Thromboxane A2 TxA2 plays an important role: it is activated by the activation of platelets and can in turn activate the receptors for TxA2. This receptor is Gq protein mediated and through the G protein it leads to activation of phospholipase C PLC which hydrolases the lipid PIP3 which in turn leads to the increase of cytosolic calcium and the activation of phospholipase A2. All these passages lead to the increase of the formation of the clot, infact Phospholipase A2 activates GPIIIa and GPIIb.
- The fourth step is the regulation of hemostasis obtained by natural anticoagulant and thrombolytic factors which limit the hemostatic process to the site of vascular injury.
