Cardiac fibrosis commonly refers to the excess deposition of extracellular matrix in the cardiac muscle, but the term may also refer to an abnormal thickening of the heart valves due to inappropriate proliferation of cardiac fibroblasts. [1] Fibrotic cardiac muscle is stiffer and less compliant and is seen in the progression to heart failure. The description below focuses on a specific mechanism of valvular pathology but there are other causes of valve pathology and fibrosis of the cardiac muscle.
Fibrocyte cells normally secrete collagen, and function to provide structural support for the heart. When over-activated this process causes thickening and fibrosis of the valve, with white tissue building up primarily on the tricuspid valve, but also occurring on the pulmonary valve. The thickening and loss of flexibility eventually may lead to valvular dysfunction and right-sided heart failure.
Following are types of myocardial fibrosis:
Certain diseases such as neuroendocrine tumor of the small intestine (also known by the obsolete term carcinoid), which sometimes release large amounts of 5-hydroxytryptamine, commonly known as 5-HT or serotonin into the blood, may produce a characteristic pattern of mostly right-sided cardiac fibrosis which can be identified with echocardiography. Cardiac fibrosis is a significant source of morbidity and mortality in patients with functional neuroendocrine tumors. This pathology has also been seen in certain East-African tribes who eat foods (Matoke —a green banana) containing excess amounts of serotonin.[ citation needed ]
Elevated prevalence of cardiac fibrosis and related valvopathies was found to be associated with use of a number of unrelated drugs following long-term statistical analysis once the drugs had been on the market for some time. The cause of this was unknown at the time, but eventually it was realised that all the implicated drugs acted as agonists at 5-HT2B receptors in the heart in addition to their intended sites of action elsewhere in the body. [5] [6]
The precise mechanisms involved remain elusive however, as while the cardiotoxicity shows some dose–response relationship, [7] it does not always develop, and consistent daily use over an extended period tends to be most strongly predictive of development of valvopathy. [8] [9] [10]
The drugs most classically associated with the condition are weight loss drugs such as fenfluramine and chlorphentermine, and antiparkinson drugs such as pergolide and cabergoline, which are prescribed for chronic use.[ citation needed ]
The heart valve changes seen with moderate and intermittent use can result in permanent damage and life-threatening heart problems if use of the causative drug is increased or continued, however longitudinal studies of former patients suggest that the damage will heal over time to some extent at least. [11] [12]
Some appetite suppressant drugs such as fenfluramine (which in combination with phentermine was marketed as Pondimin and commonly referred to as fen-phen), chlorphentermine, and aminorex (along with its analogue 4-Methylaminorex which has seen sporadic use as a recreational drug) induce a similar pattern of cardiac fibrosis (and pulmonary hypertension), apparently by overstimulating 5HT2B receptors on the cardiac fibroblast cells.[ citation needed ]
These drugs consequently tend to cause increased risk of heart valve damage and subsequent heart failure, which eventually led to them being withdrawn from the market.[ citation needed ]
Certain antimigraine drugs which are targeted at serotonin receptors as vasoconstrictive agents, have long been known to be associated with pulmonary hypertension and Raynaud's phenomenon (both vasoconstrictive effects), as well as retroperitoneal fibrosis (a fibrotic cell/fibrocyte proliferation effect, thought to be similar to cardiac valve fibrosis).[ citation needed ]
These drugs include ergotamine and methysergide and both drugs can also cause cardiac fibrosis. [13]
Certain antiparkinson drugs, although targeted at dopaminergic receptors, cross-react with serotoninergic 5-HT2B receptors as well, and have been reported to cause cardiac fibrosis. These drugs include pergolide and cabergoline.[ citation needed ]
Guanfacine may be a 5-HT2B agonist, based on the results of theoretical modeling and high-throughput screening. [14] [15]
Pergolide was an antiparkinson medications that was in decreasing use since reported in 2003 to be associated with cardiac fibrosis. [16] In March 2007, pergolide was withdrawn from the U.S. market due to serious valvular damage that was shown in two independent studies. [17] [18]
Like pergolide, cabergoline has been linked to cardiac damage. Among similar antiparkinsonian drugs, cabergoline exhibits the same type of serotonin receptor binding as pergolide. [19] Although lisuride, a related drug, also binds to the 5-HT2B receptor, it acts as an antagonist rather than as an agonist. [20]
In January 2007, cabergoline (Dostinex) was reported also to be associated with valvular proliferation heart damage. [21]
Several serotonergic recreational drugs, including the empathogens MDA and MDMA ("ecstasy"), [22] and some hallucinogens such as DOI [23] and Bromo-DragonFLY, [24] have all been shown to act as 5-HT2B agonists in vitro, but how significant this may be as a risk factor associated with their recreational use is unclear.
The piperazine derivative mCPP (a major metabolite of trazodone) is a 5-HT2B agonist in animal models, but actually behaves as a 5-HT2Bantagonist in humans. [25] [26] [27]
One study of human users of MDMA ("ecstasy") found that they did have heart valve changes suggestive of early cardiac fibrosis, which were not present in non-MDMA using controls, [28] suggesting that MDMA use certainly has the potential to cause this kind of heart damage.[ original research? ]
On the other hand, there is as yet no statistical evidence to establish or negate significant increases in rates of cardiac valvopathies in current or former MDMA users. Absent studies on point, it may be speculated that as with other 5-HT2B agonists, development of heart valve damage may be dependent on the frequency and duration of use and the total cumulative exposure over time. If that is the case, then the heaviest users are likely to face the greatest risk of heart damage.[ citation needed ]
The SSRI antidepressants raise blood serotonin levels [ citation needed ], and thus may be capable of the same risks, though it is thought that the risk is substantially lower with such drugs. The amino acid L-tryptophan also raises blood serotonin, and may present the same risk as well; though, again, the risk is considered to be low.[ citation needed ]
However, the tryptophan derivative 5-HTP (5-hydroxytryptophan), used in the treatment of depression, raises blood serotonin level considerably[ citation needed ]. It has yet to be reported to be associated with valve disease or other fibrosis, but for the previous theoretical reasons, it has been suggested as a possible danger.[ citation needed ]
When 5-HTP is used in medicine, it is generally administered along with carbidopa, [29] [30] which prevents the peripheral decarboxylation of 5-HTP to serotonin and so ensures that only brain serotonin levels are increased without producing peripheral side effects, however 5-HTP is also sold without carbidopa as a dietary supplement, and may have increased risks when taken by itself without carbidopa.[ citation needed ]
Cardiac fibrosis is common in non-human great apes in human care. The term idiopathic myocardial fibrosis was coined to emphasize this disease is likely different from the above described forms of cardiac fibrosis in humans. The etiology is not known, though vitamin D deficiency is a potential suspected cause at least in chimpanzees. [31]
The most obvious treatment for cardiac valve fibrosis or fibrosis in other locations, consists of stopping the stimulatory drug or production of serotonin. In the case of a functional neuroendocrine tumor, somatostatin analogs such as octreotide are used to reduce the production of serotonin by tumor cells, which often highly express inhibitory somatostatin receptors.[ citation needed ]
Surgical tricuspid valve replacement, sometimes combined with a pulmonary valve replacement, can be necessary in some patients. [32]
A compound found in red wine, resveratrol has been found to slow the development of cardiac fibrosis. [33] [34] [35] More sophisticated approaches of countering cardiac fibrosis like microRNA inhibition (miR-21, for example) are being tested in animal models.
The drug combination fenfluramine/phentermine, usually called fen-phen, was an anti-obesity treatment in the early 1990s that utilized two anorectics. Fenfluramine was marketed by American Home Products as Pondimin, but was shown to cause potentially fatal pulmonary hypertension and heart valve problems, which eventually led to its withdrawal in 1997 and legal damages of over $13 billion. Phentermine was not shown to have harmful effects.
Bromocriptine, originally marketed as Parlodel and subsequently under many brand names, is an ergoline derivative and dopamine agonist that is used in the treatment of pituitary tumors, Parkinson's disease, hyperprolactinaemia, neuroleptic malignant syndrome, and, as an adjunct, type 2 diabetes.
Pergolide, sold under the brand name Permax and Prascend (veterinary) among others, is an ergoline-based dopamine receptor agonist used in some countries for the treatment of Parkinson's disease. Parkinson's disease is associated with reduced dopamine activity in the substantia nigra of the brain. Pergolide acts on many of the same receptors as dopamine to increase receptor activity.
Fenfluramine, sold under the brand name Fintepla, is a serotonergic medication used for the treatment of seizures associated with Dravet syndrome and Lennox–Gastaut syndrome. It was formerly used as an appetite suppressant in the treatment of obesity, but was discontinued for this use due to cardiovascular toxicity before being repurposed for new indications. Fenfluramine was used for weight loss both alone under the brand name Pondimin and in combination with phentermine.
Phentermine (phenyl-tertiary-butylamine), with several brand names including Ionamin and Sentis, is a medication used together with diet and exercise to treat obesity. It is taken by mouth for up to a few weeks at a time, after which the benefits subside. It is also available as the combination phentermine/topiramate.
Cabergoline, sold under the brand name Dostinex among others, is a dopaminergic medication used in the treatment of high prolactin levels, prolactinomas, Parkinson's disease, and for other indications. It is taken by mouth.
Methysergide, sold under the brand names Deseril and Sansert, is a monoaminergic medication of the ergoline and lysergamide groups which is used in the prophylaxis and treatment of migraine and cluster headaches. It has been withdrawn from the market in the United States and Canada due to adverse effects. It is taken by mouth.
A dopamine agonist(DA) is a compound that activates dopamine receptors. There are two families of dopamine receptors, D2-like and D1-like, and they are all G protein-coupled receptors. D1- and D5-receptors belong to the D1-like family and the D2-like family includes D2, D3 and D4 receptors. Dopamine agonists are primarily used in the treatment of Parkinson's disease, and to a lesser extent, in hyperprolactinemia and restless legs syndrome. They are also used off-label in the treatment of clinical depression. The use of dopamine agonists is associated with impulse control disorders and dopamine agonist withdrawal syndrome (DAWS).
Lisuride, sold under the brand name Dopergin among others, is a monoaminergic medication of the ergoline class which is used in the treatment of Parkinson's disease, migraine, and high prolactin levels. It is taken by mouth.
A serotonin receptor agonist is an agonist of one or more serotonin receptors. They activate serotonin receptors in a manner similar to that of serotonin, a neurotransmitter and hormone and the endogenous ligand of the serotonin receptors.
Naphthylaminopropane (PAL-287) is an experimental drug under investigation as of 2007 for the treatment of alcohol and stimulant addiction.
Benfluorex, sold under the brand name Mediator, is an anorectic and hypolipidemic agent that is structurally related to fenfluramine. It may improve glycemic control and decrease insulin resistance in people with poorly controlled type-2 diabetes.
Norfenfluramine, or 3-trifluoromethylamphetamine, is a never-marketed drug of the amphetamine family that behaves as a serotonin and norepinephrine releasing agent and potent 5-HT2A, 5-HT2B, and 5-HT2C agonist. The action of norfenfluramine on 5-HT2B receptors on heart valves leads to a characteristic pattern of heart failure following proliferation of cardiac fibroblasts on the tricuspid valve, known as cardiac fibrosis. This side effect led to the withdrawal of fenfluramine as an anorectic agent worldwide, and to the withdrawal of benfluorex in Europe, as both fenfluramine and benfluorex form norfenfluramine as an active metabolite. It is a human TAAR1 agonist.
5-Hydroxytryptamine receptor 2B (5-HT2B) also known as serotonin receptor 2B is a protein that in humans is encoded by the HTR2B gene. 5-HT2B is a member of the 5-HT2 receptor family that binds the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT).
Terguride, sold under the brand name Teluron, is a serotonin receptor antagonist and dopamine receptor agonist of the ergoline family. It is approved for and used as a prolactin inhibitor in the treatment of hyperprolactinemia in Japan. Terguride is taken by mouth.
A serotonin releasing agent (SRA) is a type of drug that induces the release of serotonin into the neuronal synaptic cleft. A selective serotonin releasing agent (SSRA) is an SRA with less significant or no efficacy in producing neurotransmitter efflux at other types of monoamine neurons.
5-APB is an empathogenic psychoactive compound of the substituted benzofuran, substituted amphetamine and substituted phenethylamine classes. 5-APB and other compounds are sometimes informally called "Benzofury".
Levofenfluramine (INN), or (−)-3-trifluoromethyl-N-ethylamphetamine, also known as (−)-fenfluramine or (R)-fenfluramine, is a drug of the amphetamine family that, itself (i.e., in enantiopure form), was never marketed. It is the levorotatory enantiomer of fenfluramine, the racemic form of the compound, whereas the dextrorotatory enantiomer is dexfenfluramine. Both fenfluramine and dexfenfluramine are anorectic agents that have been used clinically in the treatment of obesity (and hence, levofenfluramine has been as well since it is a component of fenfluramine). However, they have since been discontinued due to reports of causing cardiovascular conditions such as valvular heart disease and pulmonary hypertension, adverse effects that are likely to be caused by excessive stimulation of 5-HT2B receptors expressed on heart valves.
5-HT2C receptor agonists are a class of drugs that activate 5-HT2C receptors. They have been investigated for the treatment of a number of conditions including obesity, psychiatric disorders, sexual dysfunction and urinary incontinence.
A prolactin modulator is a drug which affects the hypothalamic–pituitary–prolactin axis by modulating the secretion of the pituitary hormone prolactin from the anterior pituitary gland. Prolactin inhibitors suppress and prolactin releasers induce the secretion of prolactin, respectively.
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