Cytochrome P450 (CYP450) enzymes cause and cure many disorders of the central nervous system, including stroke. The brain has its own swarm of CYP450 genes that are controlled in a different way from the rest of the body. They make a number of key signalling molecules such as neurosteroids and eicosanoids, and breakdown all sorts of substrates, from vitamins to cholesterol to drugs. They are involved in neurotrophic maintenance, neuroprotection, blood-flow and heat regulation, neurotransmitters, and brain function and growth.

Ace Therapeutics offers comprehensive services to analyze the impact of cytochrome P450 metabolism on stroke drugs, including CYP450 inhibition, CYP450 reaction phenotyping, and CYP450 sensing.

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Cytochrome P450 Enzymes in the Brain

CYP450 is a superfamily of enzymes that play an important role in metabolizing a large number of compounds. CYP enzymes were originally described in hepatocytes, and the localization of CYPs at the cellular and subcellular levels was subsequently demonstrated in the brain. Some 15 CYP450 isotypes belonging to three families were involved in drug metabolism. CYPs in the brain breakdown exogenous chemicals (xenobiotics, foods, and pollutants) as well as endogenous chemicals (steroids, cholesterol, bile acids) causing neurological diseases, such as stroke.

Fig.1. The expression of human cytochrome P450 enzymes involved in the metabolism of xenobiotics in the selected brain regions shown in coronal, sagittal, and horizontal brain sections. (Kuban, et al., 2021)

Expression and Activity of CYPs in the Brain

CYPs were found in the brains of rats, mice, dogs, monkeys, and human. Overall, the distribution of drug-metabolizing CYPs is uneven, with different expression in different brain regions. The human brain's expression of CYP2B6 is 2.5 times more variable. Within specialized regions of the brain, CYP can only be expressed in certain neuron or glial cell populations. In the frontal cortex, for example, CYP2B6 is most abundant in astrocytes on the walls of blood vessels in layer I; CYP2D6 is abundant in pyramidal neurons in layers III–V and white matter. In the cerebellum, CYP2B6 and CYP2D6 are found in non-smoker's molecular and granular layer neurons, but not in Purkinje cells. But in smokers, these enzymes are concentrated in Purkinje cells. Such region- and cell-specific expression also hints at function (e.g., CYP2B6 at the blood-brain barrier is perhaps squelching drug and poison access to the brain).

Although overall CYP levels in the brain are low (0.5–2% of liver levels), their localized expression in specific brain regions and cell types can significantly impact metabolism within certain microenvironments. In some neurons, CYP levels may match or exceed those in liver hepatocytes.

Fig.2. Brain CYP expression is cell-specific, region-specific and inducible. (Ferguson, et al., 2011)

Factors Influencing CYP450 Function in the Brain

Drug interactions and CYP450 enzymes

The role of cyP450 enzymes in drug interactions is particularly important in polytherapy, where co-administration of drugs affecting CYP expression or activity can lead to unexpected pharmacokinetic or pharmacodynamic outcomes.

Hemodynamic and rheological factors and CYP450 enzymes

Cerebral blood flow plays a critical role in modulating brain CYP expression and activity, and directly or indirectly affects brain CYP expression and activity.

Genetic polymorphisms of CYPs

Hepatic P450 mutations dramatically alter systemic drug metabolism and distribution. However, the presence of P450 mutations or polymorphic variants in the brain may also alter brain drug metabolism.

Substance abuse and CYP450 enzymes

Alcohol consumption and smoking have been linked to increased levels of CYP2B6, CYP2E1, and CYP2D6 in specific brain regions, reflecting an adaptive response to chronic ethanol or nicotine exposure. These elevated CYP levels can alter how addicted individuals respond to drugs and endogenous compounds.

Dietary factors and CYP450 enzymes

Atherosclerosis and ischemia-reperfusion injury are caused by hyperlipidemia, CYP2E1 expression leads to tissue damage, inflammation and neuronal death, leading to increased chances of neuronal degeneration in ischemic patients.

Environmental factors and CYP450 enzymes

The brain CYPs participate in the metabolic breakdown of environmental chemicals and their pharmacological and toxicological effects. CYPs, for example, disintegrate pesticides like chlorpyrifos, which CYP2B makes neurotoxic through overstimulation of cholinergic neurons. Metabolism of cholesterol following uranium depletion requires CYP46A1.

Fig.3. Factors regulating CYP450 enzymes in the brain. (Ghosh, et al., 2016)

CYP Regulation in the Brain

CYP expression in the brain is regulated by nuclear receptors (NRs), including AHR, PXR, CAR, GR, FXR, RXR, and PPARs. While NRs in the liver and intestine modulate P450 enzymes in response to xenobiotics or endogenous compounds, their regulatory mechanisms in the brain differ and are less understood. Studies show that AhR activation by environmental pollutants like TCDD increases CYP1A1 and CYP1B1 expression in human brain cell lines, suggesting NR involvement in CYP regulation in the CNS.

NR distribution varies across brain regions. For instance, CAR is found in the human caudate nucleus, PXR in the thalamus, pons, and medulla, and AhR is highly expressed in the rat olfactory cortex but minimally in the amygdala. These differences highlight the need for further research on NR localization, their cellular specificity, and their roles in regulating CYPs. Additionally, the potential involvement of the PXR/CAR system in neurological diseases and CNS drug resistance remains to be explored.

Elevance of Brain CYP450 to Stroke Drug Discovery

Brain CYP450 enzymes play a critical role in stroke drug discovery, particularly in understanding brain pharmacokinetics and drug metabolism. Insights into brain CYP450 functions can aid in developing prodrugs activated within the brain and designing novel therapeutics. Knowledge of CYP450 and transporters at the neurovascular interface can enhance targeted drug delivery to the brain while minimizing drug interactions. Studies have shown that stabilizing epoxide eicosatrienoic acid (EET) levels by inhibiting or deleting soluble epoxide hydrolases (sEH) provides neuroprotection against ischemic stroke and subarachnoid hemorrhage (SAH) in stroke studies. Despite progress in understanding brain CYP450 and their distribution, further research is needed to address interindividual variability in stroke drug responses, as well as risk factors for neurotoxicity and stroke.

1. Kuban, W., & Daniel, W. A. (2021). Cytochrome P450 expression and regulation in the brain. Drug Metabolism Reviews, 53(1), 1-29.
2. Ferguson, C. S., & Tyndale, R. F. (2011). Cytochrome P450 enzymes in the brain: emerging evidence of biological significance. Trends in pharmacological sciences, 32(12), 708-714.
3. Ghosh, C., et al. (2016). Pathophysiological implications of neurovascular P450 in brain disorders. Drug discovery today, 21(10), 1609-1619.