Selective receptor modulators
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A receptor modulator, or receptor ligand, is a general term for a substance, endogenous or exogenous, that binds to and regulates the activity of chemical receptors. They are ligands that can act on different parts of receptors and regulate activity in a positive, negative, or neutral direction with varying degrees of efficacy. Categories of these modulators include receptor agonists and receptor antagonists, as well as receptor partial agonists, inverse agonists, orthosteric modulators, and allosteric modulators, [1] Examples of receptor modulators in modern medicine include CFTR modulators, [2] selective androgen receptor modulators (SARMs), and muscarinic ACh receptor modulators.
Currently, receptor modulators are categorized in the Agonist, Partial Agonist, Selective Tissue Modulators, Antagonist, and Inverse Agonist categories in terms of the effect they cause. They are further divided into Orthosteric or Allosteric Modulators according to how they effect said result. Typically, a chemical acts in an agonist fashion whenever it instigates or else facilitates a particular reaction by binding to a particular receptor. In contract, a chemical acts as an antagonist whenever binding to a particular receptor blocks or inhibits a particular response. Between these endpoints exists a gradient defined by a number of variables. One example is Selective Tissue Modulators, which mean a given ligand can behave differently according to the tissue type it is in. As for orthosteric and allosteric modulation, this describes the manner in which the ligand binds to the receptor in question: if it binds directly to the prescribed binding site of a receptor, the ligand is orthosteric in this instance; if the ligand alters the receptor by interacting with it at any place other than a binding site, allosteric interaction occurred. Note that a drug's categorization does not dictate how another drug of the same family could be categorized or whether the same drug may also function in another category. An example is found in medications used to treat opioid addiction, with methadone, buprenorphine, naloxone, and naltrexone all in separate categories or in more than one simultaneously. In addition, depending on the cell type, the specific effect, whether agonist, antagonist, inverse agonist, etc., could have a unique specific effect. An example is seen in insulin, under "Receptor Agonists," as it interacts with multiple different cell types as an agonist, but incites multiple and different responses in both.
A receptor agonist is a chemical that binds to a receptor with the end result of directly inducing a conformational change in the bound receptor and activating a downstream effect. Some common examples are opium derivates, such as heroin and Toll-like receptor agonists. [3] Heroin functions in this manner, along with other opioids, when bound to μ-opioid receptors. [4] Opioids' manner of action are both concentration- and receptor-dependent, which provides a key difference between agonists and partial agonists. Another example is insulin, which activates cell receptors to instigate blood glucose uptake. [5]
Partial agonists are any chemical that can bind to a receptor without eliciting the maximum downstream response as compared to the response from a full agonist. A given partial agonist's affinity for a given receptor is also irrelevant to the consequent effect. An example is buprenorphine, a partial opioid receptor agonist used to treat opioid addictions by directly substituting for them without the same strength of effect.
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A receptor antagonist is any given ligand that binds to a receptor in some way without causing any immediate or downstream response, essentially neutralizing the receptor until something with a stronger affinity removes the antagonist or the antagonist itself unbinds. Generally, antagonists can act one of two ways: 1) they can either block the receptors directly, preventing the usual ligand from binding, such as in the case of atropine when it blocks specific acetylcholine receptors to provide important medical benefits. This is competitive antagonism, as they are competing for the same binding sites on the receptor. [6] The other is by binding to a receptor in a site other than the designated receptor site, inducing a conformational change to prevent the usual ligand(s) from binding and activating a downstream cascade. A commonly-seen and used receptor antagonist is naloxone, another opioid competitive antagonist typically used to treat opioid overdoses by blocking receptors outright. [7] Further elaboration can be found in "Orthosteric v. Allosteric Modulators."
Inverse agonists differ from regular agonists in that they effect receptors to which a regular agonist binds such that the bound receptors demonstrate reduced activity compared to when they are normally inactive. [8] In other words, inverse antagonists limit the efficacy of the bound receptor in some way. This is noted to be beneficial in instances wherein expression of receptors or up-regulated receptor sensitivity could be detrimental, thus making suppression of response the best recourse. A handful of examples of inverse agonist use in therapy include β-blockers, antihistamines, ACP-103 to treat Parkinson's disease, hemopressin, drugs to treat obesity, and more besides. [9]
In the fields of biochemistry and pharmacology an allosteric regulator is a substance that binds to a site on an enzyme or receptor distinct from the active site, resulting in a conformational change that alters the protein's activity, either enhancing or inhibiting its function. In contrast, substances that bind directly to an enzyme's active site or the binding site of the endogenous ligand of a receptor are called orthosteric regulators or modulators.
An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agonist, while an inverse agonist causes an action opposite to that of the agonist.
In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a receptor and produce physiological responses such as change in the electrical activity of a cell. For example, GABA, an inhibitory neurotransmitter, inhibits electrical activity of neurons by binding to GABAA receptors. There are three main ways the action of the receptor can be classified: relay of signal, amplification, or integration. Relaying sends the signal onward, amplification increases the effect of a single ligand, and integration allows the signal to be incorporated into another biochemical pathway.
A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor rather than activating it like an agonist. Antagonist drugs interfere in the natural operation of receptor proteins. They are sometimes called blockers; examples include alpha blockers, beta blockers, and calcium channel blockers. In pharmacology, antagonists have affinity but no efficacy for their cognate receptors, and binding will disrupt the interaction and inhibit the function of an agonist or inverse agonist at receptors. Antagonists mediate their effects by binding to the active site or to the allosteric site on a receptor, or they may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity. Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on the nature of antagonist–receptor binding. The majority of drug antagonists achieve their potency by competing with endogenous ligands or substrates at structurally defined binding sites on receptors.
Pharmacodynamics (PD) is the study of the biochemical and physiologic effects of drugs. The effects can include those manifested within animals, microorganisms, or combinations of organisms.
In pharmacology, an inverse agonist is a drug that binds to the same receptor as an agonist but induces a pharmacological response opposite to that of the agonist.
The GABAA receptor (GABAAR) is an ionotropic receptor and ligand-gated ion channel. Its endogenous ligand is γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system. Accurate regulation of GABAergic transmission through appropriate developmental processes, specificity to neural cell types, and responsiveness to activity is crucial for the proper functioning of nearly all aspects of the central nervous system (CNS). Upon opening, the GABAA receptor on the postsynaptic cell is selectively permeable to chloride ions and, to a lesser extent, bicarbonate ions.
In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from Latin ligare, which means 'to bind'. In protein-ligand binding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein. The binding typically results in a change of conformational isomerism (conformation) of the target protein. In DNA-ligand binding studies, the ligand can be a small molecule, ion, or protein which binds to the DNA double helix. The relationship between ligand and binding partner is a function of charge, hydrophobicity, and molecular structure.
An opioid antagonist, or opioid receptor antagonist, is a receptor antagonist that acts on one or more of the opioid receptors.
The μ-opioid receptors (MOR) are a class of opioid receptors with a high affinity for enkephalins and beta-endorphin, but a low affinity for dynorphins. They are also referred to as μ(mu)-opioid peptide (MOP) receptors. The prototypical μ-opioid receptor agonist is morphine, the primary psychoactive alkaloid in opium and for which the receptor was named, with mu being the first letter of Morpheus, the compound's namesake in the original Greek. It is an inhibitory G-protein coupled receptor that activates the Gi alpha subunit, inhibiting adenylate cyclase activity, lowering cAMP levels.
Diprenorphine, also known as diprenorfin, is a non-selective, high-affinity, weak partial agonist of the μ- (MOR), κ- (KOR), and δ-opioid receptor (DOR) which is used in veterinary medicine as an opioid antagonist. It is used to reverse the effects of super-potent opioid analgesics such as etorphine and carfentanil that are used for tranquilizing large animals. The drug is not approved for use in humans.
Ro15-4513(IUPAC: Ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo-1,4-benzodiazepine-3-carboxylate) is a weak partial inverse agonist of the benzodiazepine class of drugs, developed by Hoffmann–La Roche in the 1980s. It acts as an inverse agonist, and can therefore be an antidote to the acute impairment caused by alcohols, including ethanol, isopropanol, tert-butyl alcohol, tert-amyl alcohol, 3-methyl-3-pentanol, methylpentynol and ethchlorvynol.
An adrenergic antagonist is a drug that inhibits the function of adrenergic receptors. There are five adrenergic receptors, which are divided into two groups. The first group of receptors are the beta (β) adrenergic receptors. There are β1, β2, and β3 receptors. The second group contains the alpha (α) adrenoreceptors. There are only α1 and α2 receptors. Adrenergic receptors are located near the heart, kidneys, lungs, and gastrointestinal tract. There are also α-adreno receptors that are located on vascular smooth muscle.
Receptor theory is the application of receptor models to explain drug behavior. Pharmacological receptor models preceded accurate knowledge of receptors by many years. John Newport Langley and Paul Ehrlich introduced the concept that receptors can mediate drug action at the beginning of the 20th century. Alfred Joseph Clark was the first to quantify drug-induced biological responses. So far, nearly all of the quantitative theoretical modelling of receptor function has centred on ligand-gated ion channels and G protein-coupled receptors.
In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist while under other conditions, behaves as an antagonist.
In pharmacology and biochemistry, allosteric modulators are a group of substances that bind to a receptor to change that receptor's response to stimuli. Some of them, like benzodiazepines or alcohol, function as psychoactive drugs. The site that an allosteric modulator binds to is not the same one to which an endogenous agonist of the receptor would bind. Modulators and agonists can both be called receptor ligands.
Clinical neurochemistry is the field of neurological biochemistry which relates biochemical phenomena to clinical symptomatic manifestations in humans. While neurochemistry is mostly associated with the effects of neurotransmitters and similarly functioning chemicals on neurons themselves, clinical neurochemistry relates these phenomena to system-wide symptoms. Clinical neurochemistry is related to neurogenesis, neuromodulation, neuroplasticity, neuroendocrinology, and neuroimmunology in the context of associating neurological findings at both lower and higher level organismal functions.
Methocinnamox (MCAM) is an opioid receptor antagonist. It is a pseudo-irreversible non-competitive antagonist of the μ-opioid receptor and a competitive antagonist of the κ- and δ-opioid receptors. The drug has a very long duration of action of up to months with a single dose due to its pseudo-irreversibility. It is administered in animals by intravenous or subcutaneous injection.
Drug antagonism refers to a medicine stopping the action or effect of another substance, preventing a biological response. The stopping actions are carried out by four major mechanisms, namely chemical, pharmacokinetic, receptor and physiological antagonism. The four mechanisms are widely used in reducing overstimulated physiological actions. Drug antagonists can be used in a variety of medications, including anticholinergics, antihistamines, etc. The antagonistic effect can be quantified by pharmacodynamics. Some can even serve as antidotes for toxicities and overdose.
BMS‐986122 is a selective positive allosteric modulator (PAM) of the μ-opioid receptor (MOR).
