Fatty acid amide hydrolase
# Fatty acid amide hydrolase

Fatty acid amide hydrolase or FAAH (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) in 1993. In humans, it is encoded by the gene FAAH. FAAH also regulate the contents of N-acylethanolamine in Dictyostelium discoideum, as they modulate their levels in vivo through the use of a semispecific FAAH inhibitor.

## Function

FAAH is an integral membrane hydrolase with a single N-terminal transmembrane domain. In vitro, FAAH has esterase and amidase activity. In vivo, FAAH is the principal catabolic enzyme for a class of bioactive lipids called the fatty acid amides (FAAs). Members of the FAAs include:

* Anandamide (N-arachidonoylethanolamine), an endocannabinoid
* 2-arachidonoylglycerol (2-AG), an endocannabinoid.
* Other N-acylethanolamines, such as N-oleoylethanolamine and N-palmitoylethanolamine
* The sleep-inducing lipid oleamide
* The N-acyltaurines, which are agonists of the transient receptor potential (TRP) family of calcium channels.

FAAH knockout mice display highly elevated (>15-fold) levels of N-acylethanolamines and N-acyltaurines in various tissues. Because of their significantly elevated anandamide levels, FAAH KOs have an analgesic phenotype, showing reduced pain sensation in the hot plate test, the formalin test, and the tail flick test. Finally, because of their impaired ability to degrade anandamide, FAAH KOs also display supersensitivity to exogenous anandamide, a cannabinoid receptor (CB) agonist.

Due to the ability of FAAH to regulate nociception, it is currently viewed as an attractive drug target for the treatment of pain.

A Scottish woman with a previously unreported genetic mutation (dubbed FAAH-OUT) in her FAAH gene with resultant elevated anandamide levels was reported in 2019 to be immune to anxiety, unable to experience fear, and insensitive to pain. The frequent burns and cuts she suffered due to her hypoalgesia healed quicker than average.

A 2017 study found a strong correlation between national percentage of very happy people (as measured by the World Values Survey) and the presence of the A allele in the FAAH gene variant rs324420 in citizens' genetic make-up.

A mutation in FAAH was initially provisionally linked to drug abuse and dependence but this was not borne out in subsequent studies.

Studies in cells and animals and genetic studies in humans have shown that inhibiting FAAH may be a useful strategy to treat anxiety disorders, as inhibition produce analgesic, anxiolytic, neuroprotective, and anti-inflammatory effects by elevated N-acylethanolamines (NAE's) and their activation of cannabinoid receptors.

## Inhibitors and inactivators

Activation of the cannabinoid receptor CB1 or CB2 in different tissues, including skin, inhibit FAAH, and thereby increases endocannabinoid levels.

Based on the hydrolytic mechanism of fatty acid amide hydrolase, a large number of irreversible and reversible inhibitors of this enzyme have been developed.

Some of the more significant compounds are listed below;

* AM374, palmitylsulfonyl fluoride, one of the first FAAH inhibitors developed for in vitro use, but too reactive for research in vivo
* ARN2508, derivative of flurbiprofen, dual FAAH / COX inhibitor
* BIA 10-2474 (Bial-Portela & Ca. SA, Portugal) has been linked to severe adverse events affecting 5 patients in a drug trial in Rennes, France, and at least one death, in January 2016. Many other pharmaceutical companies have previously taken other FAAH inhibitors into clinical trials without reporting such adverse events.
* BMS-469908
* CAY-10402
* JNJ-245
* JNJ-1661010
* JNJ-28833155
* JNJ-40413269
* JNJ-42119779
* JNJ-42165279 in clinical trials against social anxiety and depression, trials suspended as a precautionary measure following serious adverse event with BIA 10-2474
* LY-2183240
* Cannabidiol
* MK-3168
* MK-4409
* MM-433593
* OL-92
* OL-135
* PF-622
* PF-750
* PF-3845
* PF-04457845 "exquisitely selective" for FAAH over other serine hydrolases, but failed in clinical trials against osteoarthritis
* PF-04862853
* RN-450
* SA-47
* SA-73
* SSR-411298 well tolerated in clinical trials but insufficient efficacy against depression, subsequently trialled against cancer pain as an adjunctive treatment.
* ST-4068, reversible inhibitor of FAAH
* TK-25
* URB524
* URB597 (KDS-4103, Kadmus Pharmaceuticals), is an irreversible inactivator with a carbamate-based mechanism, and appears in one report as a somewhat selective, though it also inactivates other serine hydrolases (e.g., carboxylesterases) in peripheral tissues.
* URB694
* URB937
* VER-156084 (Vernalis)
* V-158866 (Vernalis) in clinical trials for neuropathic pain following spinal injury, and spasticity associated with multiple sclerosis. Structure not revealed though Vernalis holds several patents in the area.

### Inhibition and binding

Structural and conformational properties that contribute to enzyme inhibition and substrate binding imply an extended bound conformation, and a role for the presence, position, and stereochemistry of a delta cis double bond.

## Enhancements of FAAH and its activity

Insulin medication and intraoperative doses of insulin increase the production and activity of fatty acid amide hydrolase, which, beside of other N-acylethanolamines (NAE's), degrade the endocannabinoid NAE 20:4 (AEA: $C_{22}H_{37}NO_2;$ 20:4, ω-6), also suggest that insulin may play a key role in the obesity-linked dysregulation of the adipose ECS at the gene level, as the European Medicines Agency, EMA, in 2023 is investigating several reports from European countries about suicidal thoughts and thoughts of self-harm in patients who have been treated with Novo Nordisk's popular medicines for obesity and diabetes, revealed by the danish newspaper Belingske, an outcome also seen in the CB1 receptor blocker rimonabant, an anorectic antiobesity drug that was first approved in Europe in 2006, but several years later was withdrawn worldwide due to serious psychiatric side effects, and happening at the same time as BT reveal that EMA also has raised a safety alert for Wegovy, and the companies diabetes medication Ozempic, based on a study that suggests that the active substance in the two preparations, can increase the risk of thyroid cancer in patients with type 2 diabetes.

The phytocannabinoid $Δ^9-tetrahydrocannabinol,$ that can mimic endocannabinoid activation at CB1 and CB2 receptors in different tissues, thereby inhibit FAAH, is found to induce apoptosis in different cancer cells by increased de novo synthesized ceramide levels in a CB2 dependent manner, and moreover, cannabinoid administration selectively increased apoptosis in pancreatic tumor cells, but not in normal tissue.

And by its cannabinoid receptor activation, the influence of cannabis on insulin observed in an human study, is in line with the established role of the endocannabinoid system (ECS) in regulating glucose metabolism and, at large, energy balance, an homeostatic function that is carried out via interactions between ECS and multiple central and peripheral pathways (e.g., brain, pancreas, liver). And through autocrine, paracrine, and endocrine mechanisms, endocannabinoids modulate pancreatic β-cells function, proliferation, and survival, as well as insulin production, secretion, and resistance, through CB1 and CB2 receptors also widely expressed in islets of Langerhans, but the direction and magnitude of the relationship between cannabinoids and insulin is not linear, and may depend of duration and frequency of exposure, as chronic cannabis use (daily consumption) reduces the risk of obesity, insulin resistance, and diabetes mellitus (DM), where acute cannabis administration in cannabis users also blunt the insulin spike secondary to a brownie intake, and the found activation of cannabinoid receptors expressed by pancreatic β-cells, can stimulate insulin secretion and, therefore, may be beneficial in treating impaired glucose tolerance and diabetes mellitus, as the lower odds of DM among marijuana users is found significant, as well as former and, to a greater extent, chronic (daily) cannabis use, is consistently associated with smaller waist circumference, lower BMI, and lower risks of overweight, obesity, and central obesity in patients with chronic hepatitis C virus (HCV) infection, where obesity is found with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis.

## Assays

The enzyme is typically assayed making use of a radiolabelled anandamide substrate, which generates free labelled ethanolamine, although alternative LC-MS methods have also been described.

## Structures

The first crystal structure of FAAH was published in 2002 (PDB code 1MT5). Structures of FAAH with drug-like ligands were first reported in 2008, and include non-covalent inhibitor complexes and covalent adducts.

