Endocannabinoid reuptake inhibitors (eCBRIs), also called cannabinoid reuptake inhibitors (CBRIs), are drugs which limit the reabsorption of endocannabinoid neurotransmitters by the releasing neuron. [1] [2]
The method of transport of endocannabinoids through the cell membrane and cytoplasm to their respective degradation enzymes has been rigorously debated for nearly two decades, and a putative endocannabinoid membrane transporter was proposed. [3] However, as lipophilic molecules endocannabinoids readily pass through the cell lipid bilayer without assistance and would more likely need a chaperone through the cytoplasm to the endoplasmic reticulum where the enzyme FAAH is located. More recently fatty acid-binding proteins (FABPs) and heat shock proteins (Hsp70s) have been described and verified as such chaperones, and their inhibitors have been synthesized. [1] [4] The inhibition of endocannabinoid reuptake raises the amount of those neurotransmitters available in the synaptic cleft and therefore increases neurotransmission. Following the increase of neurotransmission in the endocannabinoid system is the stimulation of its functions which, in humans, include: suppression of pain perception (analgesia), increased appetite, mood elevation and inhibition of short-term memory. [5] [6]
Anandamide (ANA), also known as N-arachidonoylethanolamine (AEA), an N-acylethanolamine (NAE), is a fatty acid neurotransmitter. Anandamide was the first endocannabinoid to be discovered: it participates in the body's endocannabinoid system by binding to cannabinoid receptors, the same receptors that the psychoactive compound THC in cannabis acts on. Anandamide is found in nearly all tissues in a wide range of animals. Anandamide has also been found in plants, including small amounts in chocolate. The name 'anandamide' is taken from the Sanskrit word ananda, which means "joy, bliss, delight", plus amide.
Cannabinoids are several structural classes of compounds found in the cannabis plant primarily and most animal organisms or as synthetic compounds. The most notable cannabinoid is the phytocannabinoid tetrahydrocannabinol (THC) (delta-9-THC), the primary psychoactive compound in cannabis. Cannabidiol (CBD) is also a major constituent of temperate cannabis plants and a minor constituent in tropical varieties. At least 113 distinct phytocannabinoids have been isolated from cannabis, although only four have been demonstrated to have a biogenetic origin. It was reported in 2020 that phytocannabinoids can be found in other plants such as rhododendron, licorice and liverwort, and earlier in Echinacea.
URB597 (KDS-4103) is a relatively selective and irreversible inhibitor of the enzyme fatty acid amide hydrolase (FAAH). FAAH is the primary degradatory enzyme for the endocannabinoid anandamide and, as such, inhibition of FAAH leads to an accumulation of anandamide in the CNS and periphery where it activates cannabinoid receptors. URB597 has been found to elevate anandamide levels and have activity against neuropathic pain in a mouse model.
The endocannabinoid system (ECS) is a biological system composed of endocannabinoids, which are endogenous lipid-based retrograde neurotransmitters that bind to cannabinoid receptors, and cannabinoid receptor proteins that are expressed throughout the vertebrate central nervous system and peripheral nervous system. The endocannabinoid system remains under preliminary research, but may be involved in regulating physiological and cognitive processes, including fertility, pregnancy, pre- and postnatal development, various activity of immune system, appetite, pain-sensation, mood, and memory, and in mediating the pharmacological effects of cannabis. The ECS plays an important role in multiple aspects of neural functions, including the control of movement and motor coordination, learning and memory, emotion and motivation, addictive-like behavior and pain modulation, among others.
Fatty-acid amide hydrolase 1 or FAAH-1(EC 3.5.1.99, oleamide hydrolase, anandamide amidohydrolase) is a member of the serine hydrolase family of enzymes. It was first shown to break down anandamide (AEA), an N-acylethanolamine (NAE) in 1993. In humans, it is encoded by the gene FAAH. FAAH also regulate the contents of NAE's in Dictyostelium discoideum, as they modulate their NAE levels in vivo through the use of a semispecific FAAH inhibitor.
Methoxy arachidonyl fluorophosphonate, commonly referred as MAFP, is an irreversible active site-directed enzyme inhibitor that inhibits nearly all serine hydrolases and serine proteases. It inhibits phospholipase A2 and fatty acid amide hydrolase with special potency, displaying IC50 values in the low-nanomolar range. In addition, it binds to the CB1 receptor in rat brain membrane preparations (IC50 = 20 nM), but does not appear to agonize or antagonize the receptor, though some related derivatives do show cannabinoid-like properties.
The fatty-acid-binding proteins (FABPs) are a family of transport proteins for fatty acids and other lipophilic substances such as eicosanoids and retinoids. These proteins are thought to facilitate the transfer of fatty acids between extra- and intracellular membranes. Some family members are also believed to transport lipophilic molecules from outer cell membrane to certain intracellular receptors such as PPAR. The FABPs are intracellular carriers that “solubilize” the endocannabinoid anandamide (AEA), transporting AEA to the breakdown by FAAH, and compounds that bind to FABPs block AEA breakdown, raising its level. The cannabinoids are also discovered to bind human FABPs that function as intracellular carriers, as THC and CBD inhibit the cellular uptake and catabolism of AEA by targeting FABPs. Competition for FABPs may in part or wholly explain the increased circulating levels of endocannabinoids reported after consumption of cannabinoids. Levels of fatty-acid-binding protein have been shown to decline with ageing in the mouse brain, possibly contributing to age-associated decline in synaptic activity.
Cannabinoid receptor 1 (CB1), is a G protein-coupled cannabinoid receptor that in humans is encoded by the CNR1 gene. The human CB1 receptor is expressed in the peripheral nervous system and central nervous system. It is activated by endocannabinoids, a group of retrograde neurotransmitters that include anandamide and 2-arachidonoylglycerol (2-AG); plant phytocannabinoids, such as docosatetraenoylethanolamide found in wild daga, the compound THC which is an active constituent of the psychoactive drug cannabis; and synthetic analogs of THC. CB1 is antagonized by the phytocannabinoid tetrahydrocannabivarin (THCV).
The cannabinoid receptor 2(CB2), is a G protein-coupled receptor from the cannabinoid receptor family that in humans is encoded by the CNR2 gene. It is closely related to the cannabinoid receptor 1 (CB1), which is largely responsible for the efficacy of endocannabinoid-mediated presynaptic-inhibition, the psychoactive properties of tetrahydrocannabinol (THC), the active agent in cannabis, and other phytocannabinoids. The principal endogenous ligand for the CB2 receptor is 2-Arachidonoylglycerol (2-AG).
A reuptake inhibitor (RI) is a type of drug known as a reuptake modulator that inhibits the plasmalemmal transporter-mediated reuptake of a neurotransmitter from the synapse into the pre-synaptic neuron. This leads to an increase in extracellular concentrations of the neurotransmitter and an increase in neurotransmission. Various drugs exert their psychological and physiological effects through reuptake inhibition, including many antidepressants and psychostimulants.
An N-acylethanolamine (NAE) is a type of fatty acid amide where one of several types of acyl groups is linked to the nitrogen atom of ethanolamine, and highly metabolic formed by intake of essential fatty acids through diet by 20:4, n-6 and 22:6, n-3 fatty acids, and when the body is physically and psychologically active,. The endocannabinoid signaling system (ECS) is the major pathway by which NAEs exerts its physiological effects in animal cells with similarities in plants, and the metabolism of NAEs is an integral part of the ECS, a very ancient signaling system, being clearly present from the divergence of the protostomian/deuterostomian, and even further back in time, to the very beginning of bacteria, the oldest organisms on Earth known to express phosphatidylethanolamine, the precursor to endocannabinoids, in their cytoplasmic membranes. Fatty acid metabolites with affinity for CB receptors are produced by cyanobacteria, which diverged from eukaryotes at least 2000 million years ago (MYA), by brown algae which diverged about 1500 MYA, by sponges, which diverged from eumetazoans about 930 MYA, and a lineages that predate the evolution of CB receptors, as CB1 – CB2 duplication event may have occurred prior to the lophotrochozoan-deuterostome divergence 590 MYA. Fatty acid amide hydrolase (FAAH) evolved relatively recently, either after the evolution of fish 400 MYA, or after the appearance of mammals 300 MYA, but after the appearance of vertebrates. Linking FAAH, vanilloid receptors (VR1) and anandamide implies a coupling among the remaining ‘‘older’’ parts of the endocannabinoid system, monoglyceride lipase (MGL), CB receptors, that evolved prior to the metazoan-bilaterian divergence, but were secondarily lost in the Ecdysozoa, and 2-Arachidonoylglycerol (2-AG).
RVD-Hpα (pepcan-12) is an endogenous neuropeptide found in human and mammalian brain, which was originally proposed to act as a selective agonist for the CB1 cannabinoid receptor. It is a 12-amino acid polypeptide having the amino acid sequence Arg-Val-Asp-Pro-Val-Asn-Phe-Lys-Leu-Leu-Ser-His and is an N-terminal extended form of hemopressin, a 9-AA polypeptide derived from the α1 subunit of hemoglobin which has previously been shown to act as a CB1 inverse agonist. All three polypeptides have been isolated from various mammalian species, with RVD-Hpα being one of the more abundant neuropeptides expressed in mouse brain, and these neuropeptides represent a new avenue for cannabinoid research distinct from the previously known endogenous lipid-derived cannabinoid agonists such as anandamide. Recently it was shown that RVD-Hpα (also called Pepcan-12) is a potent negative allosteric modulator at CB1 receptors, together with other newly described N-terminally extended peptides (pepcans).
LY-2183240 is a drug which acts both as a potent inhibitor of the reuptake of the endocannabinoid anandamide and as an inhibitor of fatty acid amide hydrolase (FAAH), the primary enzyme responsible for degrading anandamide. This leads to markedly elevated anandamide levels in the brain, and LY-2183240 has been shown to produce both analgesic and anxiolytic effects in animal models. While LY-2183240 is a potent inhibitor of FAAH, it has relatively poor selectivity and also inhibits several other enzyme side targets. Consequently, it was never developed for clinical use, though it remains widely used in research, and has also been sold as a designer drug.
N-Arachidonylglycine (NAGly) is a carboxylic metabolite of the endocannabinoid anandamide (AEA). Since it was first synthesized in 1996, NAGly has been a primary focus of the relatively contemporary field of lipidomics due to its wide range of signaling targets in the brain, the immune system and throughout various other bodily systems. In combination with 2‐arachidonoyl glycerol (2‐AG), NAGly has enabled the identification of a family of lipids often referred to as endocannabinoids. Recently, NAGly has been found to bind to G-protein coupled receptor 18 (GPR18), the putative abnormal cannabidiol receptor. NaGly is an endogenous inhibitor of fatty acid amide hydrolase (FAAH) and thereby increases the ethanolamide endocannabinoids AEA, oleoylethanolamide (OEA) and palmitoylethanolamide (PEA) levels. NaGly is found throughout the body and research on its explicit functions is ongoing.
4-O-Methylhonokiol is a neolignan, a type of phenolic compound. It is found in the bark of Magnolia grandiflora and in M. virginiana flowers.
IDFP is an organophosphorus compound related to the nerve agent sarin.
The endocannabinoid transporters (eCBTs) are transport proteins for the endocannabinoids. Most neurotransmitters are water-soluble and require transmembrane proteins to transport them across the cell membrane. The endocannabinoids on the other hand, are non-charged lipids that readily cross lipid membranes. However, since the endocannabinoids are water immiscible, protein transporters have been described that act as carriers to solubilize and transport the endocannabinoids through the aqueous cytoplasm. These include the heat shock proteins (Hsp70s) and fatty acid-binding proteins for anandamide (FABPs). FABPs such as FABP1, FABP3, FABP5, and FABP7 have been shown to bind endocannabinoids. FABP inhibitors attenuate the breakdown of anandamide by the enzyme fatty acid amide hydrolase (FAAH) in cell culture. One of these inhibitors (SB-FI-26), isolated from a virtual library of a million compounds, belongs to a class of compounds that act as an anti-nociceptive agent with mild anti-inflammatory activity in mice. These truxillic acids and their derivatives have been known to have anti-inflammatory and anti-nociceptive effects in mice and are active components of a Chinese herbal medicine used to treat rheumatism and pain in human. The blockade of anandamide transport may, at least in part, be the mechanism through which these compounds exert their anti-nociceptive effects.
PF-3845 is a selective inhibitor of fatty acid amide hydrolase. It results in increased levels of anandamide and results in cannabinoid receptor-based effects. It has anti-inflammatory action in mice colitis models. Antidiarrheal and antinociceptive effects were also seen in mouse models of pain.
An endocannabinoid enhancer (eCBE) is a type of cannabinoidergic drug that enhances the activity of the endocannabinoid system by increasing extracellular concentrations of endocannabinoids. Examples of different types of eCBEs include fatty acid amide hydrolase (FAAH) inhibitors, monoacylglycerol lipase (MAGL) inhibitors, and endocannabinoid transporter (eCBT) inhibitors. An example of an actual eCBE is AM404, the active metabolite of the analgesic paracetamol and a dual FAAH inhibitor and eCBRI.
Guineesine is a compound isolated from long pepper and black pepper.