This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in **drug metabolism** and **synthesis of cholesterol, steroids and other lipids**. The specific substrate of this CYP2C18 protein has not yet been determined, but it is part of this metabolic enzyme family.

This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in **drug metabolism** and synthesis of **cholesterol, steroids and other lipids**. This protein localizes to the endoplasmic reticulum and is one of the most important enzymes involved in the metabolism of xenobiotics; it metabolizes many **therapeutic drugs**.

Accumulating evidence indicates that CYP2C9 ranks among the most important drug-metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs.

Many CYP2C9 substrates are weak acids, but CYP2C9 also has the capacity to metabolize neutral, highly lipophilic xenobiotics, suggesting substrate specificity may be broader than is currently believed.

The cytochromes P450 (CYP) are a superfamily of hemoproteins which **metabolize foreign chemicals as well as a number of endogenous compounds such as steroids**. The human CYP2C subfamily appears to **principally metabolize a number of clinically used drugs**. Four members of this subfamily have been identified in humans: **CYP2C8, CYP2C9, CYP2C18, and CYP2C19**, indicating that both CYP2C18 and CYP2C9 belong to a drug‑metabolizing subfamily that also acts on endogenous compounds.

CYP2C9 is one of the major cytochrome P450 enzymes involved in **drug metabolism in humans**. There is increasing evidence that **CYP2C9 is important in the oxidative metabolism of acetic acid, propionic acid, fenemate and oxicam NSAIDs**. The enzyme also contributes to the metabolism of other clinically important drugs such as **warfarin and phenytoin**, illustrating its central role in hepatic **xenobiotic metabolism**.

Human **CYP2C9 is one of the most abundant drug metabolizing cytochrome P450 (CYP) enzymes**, being expressed in the liver at protein level similar to CYP3A4. CYP2C9, one of the most abundant hepatic cytochrome P450 enzymes, is involved in **metabolism of 15–20% of clinically important drugs** (warfarin, sulfonylureas, phenytoin, non-steroid anti-inflammatory drugs). Inhibition of CYP2C9 activity or changes in its expression can therefore have major consequences for **drug response and safety** in patients, underscoring its physiological importance in **hepatic metabolism**.

This review notes that cytochrome P450 (CYP) enzymes in extrahepatic tissues often play a dominant role in **target tissue metabolic activation of xenobiotic compounds** and may determine drug efficacy and influence the tissue burden of foreign chemicals or bioavailability of therapeutic agents. It lists many CYPs expressed in respiratory and gastrointestinal organs, including **CYP2C8, CYP2C9, CYP2C18, and CYP2C19**, indicating that CYP2C18 and CYP2C9 are part of tissue-expressed CYPs contributing to local metabolism and responses to chemicals in human physiology.

This gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in **drug metabolism and synthesis of cholesterol, steroids and other lipids**. This protein localizes to the endoplasmic reticulum but its specific substrate has not yet been determined. ... A cytochrome P450 monooxygenase **involved in retinoid metabolism. Hydroxylates all trans‑retinoic acid (atRA) to 4‑hydroxyretinoate and may modulate atRA signaling and clearance**.

The cytochrome P450 family mediates the metabolism of xenobiotics and endogenous compounds. CYP2C9 is one isoform that mediates the oxidation of several important drugs, and the study examines substrate interactions and sites of metabolism for CYP2C9.

NCBI’s pharmacogenetics review describes **CYP2C9** as a member of the CYP2C subfamily that **plays a major role in the oxidative metabolism of numerous clinically important drugs**. It explains that genetic polymorphisms in CYP2C9 significantly affect drug clearance, leading to variability in drug response and risk of adverse effects, which is a key pharmacological and physiological consideration in human therapy.

This study aimed to characterize the intestinal and hepatic expression and function of CYP2C enzymes in the same set of subjects. **The rank order for CYP2C mRNA expression in the intestine was CYP2C9 = CYP2C18 > CYP2C19 > CYP2C8, whereas that in the liver was CYP2C9 > CYP2C8 > CYP2C18 > CYP2C19.** The rank order for expression of CYP2C protein in the intestine was CYP2C9 > CYP2C19 > CYP2C8 (content below limit of quantification) > CYP2C18 (not detected) and that in the liver was CYP2C9 > CYP2C8 > CYP2C19 > CYP2C18 (not detected). ... Collectively, these results show that **CYP2C8, CYP2C9 and CYP2C19 are expressed as functional enzymes in the human small intestine**, and further suggest that CYP2C genes are independently regulated in human intestine and liver.

CYP2C8, CYP2C9, and CYP2C19 are expressed in the adult liver at significant levels; however, the expression of CYP2C enzymes in **extrahepatic tissues such as the brain** is less well characterized. ... **CYP2C9 and CYP2C19 were both expressed in the microsomal fractions of the cortex, hippocampus, amygdala, basal ganglia, and cerebellum of the human brain.** ... Moreover, **CYP2C enzymes in animals have been proposed to contribute to the regulation of cerebral blood flow via generation of epoxyeicosatrienoic acid metabolites from arachidonic acid.** Human CYP2C family enzymes may play a similar role in regulating cerebral blood flow; **CYP2C8 and CYP2C9 were shown to have arachidonic acid epoxygenase activity** and CYP2C9 has been suggested to regulate blood flow in skeletal muscle during exercise.

Members of the CYP2C subfamily, including **CYP2C18**, contribute to the oxidative metabolism of clinically used drugs and other xenobiotics. In this study, **CYP2C18 mRNA and protein were detected in human liver and extrahepatic tissues**, although at lower levels than CYP2C9 and CYP2C19, indicating a potential role in **drug metabolism**. Recombinant CYP2C18 showed catalytic activity toward selected substrates, confirming that the enzyme participates in **xenobiotic biotransformation** in humans.

Cytochrome P450 (CYP) enzymes in **extrahepatic tissues often play a dominant role in target tissue metabolic activation of xenobiotic compounds.** They may also determine drug efficacy and influence the tissue burden of foreign chemicals or bioavailability of therapeutic agents. This review focuses on xenobiotic-metabolizing CYPs of the human respiratory and gastrointestinal tracts. **Many CYPs are expressed in one or more of these organs, including CYP2C8, CYP2C9, CYP2C18, CYP2C19**, among others. The preferential expression of certain CYPs in the respiratory tract and the regional differences in CYP expression profile in different parts of the gastrointestinal tract are described, indicating roles in local metabolism and tissue-selective responses.

CYP2C enzymes, including **CYP2C9 and CYP2C18**, not only metabolize drugs but also act as **epoxygenases of polyunsaturated fatty acids**, generating epoxyeicosatrienoic acids and related lipid mediators. These metabolites have important **physiological functions**, such as regulation of **vascular tone, inflammation, and cell proliferation**. Thus, CYP2C9 and related CYP2C enzymes contribute to both **xenobiotic metabolism** and the modulation of **endogenous signaling pathways** relevant to human health and disease.

PharmGKB notes that **CYP2C9 is one of the most important drug‑metabolizing enzymes in humans**, responsible for the metabolism of about **15% of all clinically used drugs**, including warfarin and many NSAIDs. It further highlights that variation in CYP2C9 activity alters **drug pharmacokinetics and clinical outcomes**, underscoring its significant role in human physiology related to drug response.

CYP2C18 and CYP2C19 are two closely related members of the CYP2C subfamily. In this study, **CYP2C18 mRNA was detected in several extrahepatic tissues, including the small intestine, colon, and skin, whereas CYP2C18 protein was detectable only at low levels or not at all in liver.** The authors note that **CYP2C18 exhibits catalytic activity toward certain substrates in vitro**, but its contribution to systemic drug metabolism appears minor due to low hepatic expression. By contrast, **CYP2C19 showed significant activity in the activation of tamoxifen**, illustrating that different CYP2C enzymes have distinct metabolic roles despite overlapping tissue expression.

CYP2C9 is an **enzyme that is responsible for breaking down (metabolizing) several of the drugs that are commonly used today**. Some medications, such as **celecoxib, warfarin, and phenytoin require CYP2C9** in order for the medication to be metabolized to forms that are not active and are more easily eliminated from the body. Differences in the CYP2C9 gene lead to production of an enzyme with normal to no function, and patients are classified into **normal, intermediate, and poor metabolizers**, categories that clinicians use to guide drug therapy, illustrating the enzyme’s role in **physiological handling of drugs**.

Cytochrome P450 2C9 (CYP2C9) is known primarily for its role in drug metabolism, but this study examined its activity toward endogenous fatty acids. The authors report that **CYP2C9 metabolizes arachidonic acid to epoxyeicosatrienoic acids (EETs)**, demonstrating arachidonic acid epoxygenase activity. These **EETs are biologically active eicosanoids involved in the regulation of vascular tone and other physiological functions**, suggesting that CYP2C9 contributes to endogenous signaling pathways in addition to xenobiotic metabolism.

Losartan is an angiotensin II receptor antagonist that undergoes extensive first-pass metabolism. In this work, the authors conclude that **CYP2C9 in human small intestine contributes to the first-pass metabolism of losartan**, alongside CYP3A4. They state that **intestinal CYP2C9 activity, although lower than hepatic activity, is sufficient to influence the systemic availability of losartan**, illustrating a role for CYP2C9 in extrahepatic drug metabolism that can affect pharmacokinetics.

This study investigated the role of cytochrome P450 enzymes in serotonin metabolism. The authors found that **CYP2C9 catalyzes the 5‑hydroxylation of serotonin in human liver microsomes**, indicating that **CYP2C9 participates in the metabolism of the endogenous neurotransmitter serotonin**. This supports the concept that CYP2C9 has functions beyond drug metabolism, contributing to the clearance and regulation of endogenous bioactive amines.

The **CYP2C9 gene** provides instructions for making an enzyme that is a member of the cytochrome P450 family. These enzymes are involved in the **processing of many drugs**. The CYP2C9 enzyme is especially important for breaking down a blood-thinning drug called **warfarin** and several **nonsteroidal anti-inflammatory drugs (NSAIDs)**. Changes in the CYP2C9 gene can affect how well the enzyme works, which can alter a person’s **response to medications** and risk of side effects.

CYP2C18 and CYP2C19 are human CYP2C enzymes whose brain expression is difficult to study directly in humans. In this transgenic mouse model, **human CYP2C18 and CYP2C19 mRNA were expressed in the brain**, and the study shows that **CYP2C18 can catalyze the metabolism of retinoids and certain xenobiotics in neural tissue**. The authors suggest that **CYP2C18 may influence local concentrations of signaling molecules such as retinoic acid in the brain**, implying potential roles in neurophysiological processes, although the precise in vivo functions remain to be fully defined.

Cytochrome P450 2C18 is a protein that in humans is encoded by the **CYP2C18** gene. This gene encodes a member of the cytochrome P450 superfamily of enzymes; the cytochrome P450 proteins are monooxygenases which **catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids**. CYP2C18 also possesses **epoxygenase activity** and can attack various long‑chain polyunsaturated fatty acids to form epoxide products that **act as signaling agents**, suggesting roles in physiological signaling pathways beyond xenobiotic metabolism.

Cytochromes P450 are a group of heme-thiolate **monooxygenases**. In liver microsomes, this CYP2C18 enzyme is involved in an **NADPH-dependent electron transport pathway**. It **oxidizes a variety of structurally unrelated compounds, including steroids, fatty acids, and xenobiotics**, which places CYP2C18 directly in pathways of endogenous metabolism (steroids, fatty acids) as well as xenobiotic/drug metabolism.

General function: **A cytochrome P450 monooxygenase involved in retinoid metabolism**. It **hydroxylates all-trans-retinoic acid (atRA) to 4-hydroxyretinoate**, and may be involved in regulating levels of this active retinoid. Retinoic acid is a signaling molecule that affects gene expression, cell differentiation, and other physiological processes, so CYP2C18’s role in retinoid metabolism links it to broader physiological functions.

Cytochrome P450 2C9 (CYP2C9) is an enzyme that in humans is encoded by the **CYP2C9** gene. CYP2C9 is one of the major **drug-metabolizing enzymes** in the liver and is responsible for the oxidative metabolism of many clinically important drugs, including **warfarin, phenytoin, tolbutamide, and numerous nonsteroidal anti-inflammatory drugs (NSAIDs)**. As a typical cytochrome P450, it also participates in the metabolism of **endogenous substrates such as arachidonic acid**, linking it to both xenobiotic and endogenous metabolic pathways.

DrugBank describes human **CYP2C9** as a cytochrome P450 monooxygenase that **catalyzes the oxidation of a broad range of xenobiotics and endogenous compounds**. It notes that CYP2C9 is involved in the **metabolism of many therapeutic drugs** (including S‑warfarin, phenytoin and some oral hypoglycemics) and also **metabolizes fatty acids such as arachidonic acid**, suggesting roles in both drug clearance and endogenous lipid signaling pathways.

Cytochrome P450 2C9 is a specific enzyme belonging to the cytochrome P450 family, known for its role in **metabolizing a wide range of drugs**. CYP enzymes that are responsible for xenobiotic metabolism are most densely distributed in the **liver and intestines**, but are also found in the **placenta, lung, lymphocytes, macrophages, kidney, and even the brain**, indicating potential contributions to **physiological processes** in these tissues. CYP2D6, CYP2C9 and CYP2C19 are among the most highly polymorphic CYP enzymes, and genetic variants can result in altered rates of substrate metabolism.

An overview of CYP2C18 in ScienceDirect notes that CYP2C18 belongs to the human CYP2C subfamily and has been detected in several tissues, though its **exact role in drug metabolism is less well characterized** than that of CYP2C9 or CYP2C19. It is discussed in the context of the CYP2C subfamily’s contribution to **xenobiotic metabolism** and possible involvement in the metabolism of **endogenous substrates**, indicating a potential though not fully defined contribution of CYP2C18 to human metabolic processes.

Across pharmacology and biochemistry literature, CYP2C enzymes (including CYP2C9 and CYP2C18) are consistently described as **microsomal monooxygenases** that participate in phase I metabolism. They contribute to the **oxidation of xenobiotics (drugs and environmental chemicals)** and also transform **endogenous molecules such as fatty acids, steroids, and retinoids**, thereby linking them to both detoxification and normal physiological signaling and homeostasis in humans.