Induction of Drug Metabolism

nuclear receptors

"gene transcription agents" - receptors that mediate transcription activation (remember that a receptor is a macromolecule with which a hormone, drug, or other chemical interacts with to produce a certain effect)

% reduction of AUC can be modest to huge depending on the inducer/substrate pair. An example of a huge decrease in %AUC due to a DDI caused by induction of metabolism is

%AUC of Triazolam compared to the %AUC of Triazolam when taken with Rifampin - as a CYP3A4 inducer, Rifampin greatly increases the clearance of Triazolam thus dramatically decreasing the %AUC of Triazolam

What makes AhR unique compared to PXR and CAR?

AhR ligands not only include drug molecules (polycyclic aromatic hydrocarbons and benzimidazoles), but also include a molecule that we can get from food: flavonoids - flavonoids are a photosynthesizing cell commonly found in fruits, veggies, nuts, seeds, stems, flowers, wine, tea, honey, propolis, charred meat, and tobacco

PXR and CAR are also involved in the synthesis, metabolism, and uptake of lipids and involved in glucose homeostasis. Significance:

Lipid and glucose levels (factors related to physical condition and nutrition) can potentially interact with (and affect) drug metabolism via PXR and CAR

possible consequence of induction of metabolism when a prodrug is administered

↑ metabolism of a prodrug can ↑ concentration of the active drug ex.) codeine is administered as a prodrug and converted to its active form: morphine, if a normal dose of codeine is administered but there is an induction of metabolism that causes a faster rate of it being converted in morphine, toxic levels of morphine can result

Definiton and example: PK Tolerance

insufficient amounts of a a drug are reaching the site of action - ex.) clearance rate of the drug increases causing a a decrease in the level of drug circulating in the body

features that determine the function of a nuclear receptor (NR)

its two domains: 1. ligand-binding domain (LBD)- determines which drug is going to bind to it in the cytosol and activate it 2. DNA-binding domain (DBD) - determines which genes in the nucleus its going to activate

AhR uses a mechanism to induce metabolism that is similar to PXR with this one exception

- High levels of CYP2E1 are constantly being produced and being degraded in order to maintain a steady-state low level of the enzyme at all times - compound that induce metabolism via CYP2E1 bind to the proteasome that usually work to degrade it - normal levels of CYP2E1 are still being produced, but since less of it is being degraded, the level starts to increase - as levels of CYP2E1 increase, the metabolism reactions associated with it are induced

The constant production and degradation of CYP2E1 seems wasteful. Why do we think it was conserved in evolution?

- Induction of other CYPs have a slow turnover (a few days to a week) but the constant turnover of CYP2E1 allows it to be induced rapidly - rapid induction of this enzyme can be useful for an starving animal because starvation causes a shift from glucose oxidation to fatty acid and triglyceride oxidation - fatty acid/TG oxidation produces ketone body byproducts that have an intoxicating effect on the CNS that would impair the animal from being able to find food - ketone bodies are primarily metabolized by CYP2E1 and it would be imperative to to clear them as quickly as possible because the animal would not survive it it had to wait a few days for induction take effect

Consequence of promiscuity of Pregnane XR

- PXR is "promiscuous;" as the main inducer of the CYP3A4 enzymes, it has a LBD capable of being activated by a wide variety of drugs - its promiscuity increased the chance that two drugs taken together will both have their metabolisms induced by this NR

PXR and CAR regulate overlapping sets of genes that code for the synthesis of these groups of molecule

- Phase I enzymes - Phase II enzymes - Transorters

why aren't animal models effective for assessing the inductive effects of drugs in humans?

- a look at CYP34 inducers in other species (i.e. rabbit and rat) shows that there is great variation in induction of the same drug across different species - while PXR controls CYP34A production in most mammals and the DBD of PXR is conserved, - the LBD has much more variation - as a result, the same NR produces a variety of species-specific responses to the same inducers

clinical technique for resolving auto-induction

- dosage must be titrated for and determined emperically on a patient basis - titrate the dosage from a low initial dose - increase dosage until plasma concentration that produces the desired therapeutic effect can be maintained by the body (this is the body's saturation point of being able to produce CYP3A4 in the liver and intestine)

Constitutive Androstane Receptor (CAR) characteristics

- highly expressed in the liver and intestine - mostly induces CYP2B6 and CYP2C9 - does some induction induction of CYP3A4

characteristics of the kinetics of induction

- induction is slow to take effect (a few days to a week) - the inductive effect is slow to disappear once the patient has stopped taking the drug (several days to a week) - the timing of induction/de-induction will always depend on the specific inducer

CYP3A4 major characteristics

- major form in the liver (60%) and intestine (70%) - metabolizes many drugs (50-55% of drugs used clinically) - metabolizes most endogenous steroids - responsible for most first-pass metabolism - responsible for many Drug-Drug interactions

Which CYPs do Aryl Hyrdocarbon Receptors (ArH) induce?

- mostly transcriptionally induces CYP1A1 and CYP1A2 - some induction of CYP1B1 and others including some phase II metabolizing enzymes

Some dangers of herbal remedies

- often omitted by patients who don't consider them to be 'real drugs' - onset and disappearance of inductive effects can be a wide range (a few days to three weeks or more) - patient may abruptly terminate therapy due to a side effect or because they feel better - quality, purity, and content vary widely due to last of regulation; provided difficulty when predicting pharmacological effect (the exact specifics of the product are needed)

Pregnane X Receptor (PXR) significance

- one of the first NRs to be characterized - is highly active in the liver and intestine - has a mechanism of inception of metabolism that is slightly varied that the general mechanism - is the main inducer of CYP3A4 enzymes - has a highly promiscuous LBD that makes it capable of being activated by a wide variety of compounds

proteasome function and relevance to CYP2E1 induction

- recognizes and degrades unneeded, damaged, or faulty proteins (labeled by a ubiquitin chain) by cutting them into small peptides - bound by the molecules that induce metabolism via CYP2E1 to lower the normal rate of CYP2E1 degradation

how is auto-induction of metabolism mechanism resolved? give an example

- the body can only produce so much of CYP34 - this means that that there is eventually a level of saturation in which the dosage level cannot induce its own metabolism any longer - once this dosage level is found, by titrating from the lowest dose, it is maintained -ex.from lecture notes.) patient started taking initial dose of 200 mg of carbamazepine but slowly had to increase the dose to 1200 mg over a course of 4 weeks to maintain a plasma level that would give him the desired therapeutic effect

how do CYPs cause Drug-Drug Interactions

1. Different CYPs metabolize different subsets of drugs 2. Different CYPs are inhibited by different agents 3. Different CYPs are induced by different agents - these variations can work in concert to cause a drug-drug interaction

Basic mechanism of drug-drug interactions mediated by CYP34A induction by PXR

1. Drug A binds to PXR; gene synthesis is initiated; enzyme is produced 2. Metabolism of Drug A increases 3. Patient was also taking a Drug B that also binds to PXR and initiates CYP3A4 synthesis to induced metabolism 4. The increase of the production of CYP34A by Drug A causing an increase in the metabolism of Drug B 5. Drug B taken at its original dosage will now be cleared faster due to the increase in CYP3A4 synthesis, and less of Drub B will be available in the circulation * there will be a reduction in the plasma levels of Drug B because its being cleared more quickly

basis mechanism of auto-induction of metabolism

1. Drug A is metabolized by CYP34A 2. Drug A is also a PXR-ligand and it binds to PXR and activated the transcription of genes that synthesize more CYP3A4 3. More CYP3A4 is produced; Drug A is cleared faster over time because it is inducing its own metabolism

Therapeutic importance of Induction (of metabolism)

1. an inducer may cause "auto-induction" 2. induction is the main mechanism of PK tolerance 3. induction can cause failure of therapy 4. drugs often induce more than one CYP450 enzyme 5. inducer may induce CYPs that don't metabolize it

General mechanism steps of hepatic enzyme induction that increased protein synthesis

1. drug enters the cell and binds to a nuclear receptor 2. drug-receptor complex translocated to the nucleus 3. gene transcription is induced (↑ gene transcription) 4. protein synthesis is induced (↑ protein synthesis)

General procedure when a new drug is added to a patient's regimen

1. see if the new drug is a known inducer/inhibitor of any CYPs 2. if yes, see if the current drug(s) that the patient is taking is a substrate for any of that CYP 3. if yes, then there is a potential DDI to worry about and dosages may need adjustment / titration to desired effect

Although PXR and CAR regulate overlapping sets of genes, the sets of ligand that they bind to vary in this way

CAR binds to a set of ligands that are more size-restricted; PXR binds to a set of ligands that has a wider range of shape and size

which CYP is primarily involved in first pass metabolism and why

CYP3A4 - it's prevalent in the intestine; where more of first place metabolism takes place (in addition to the liver)

Two examples of drugs that are cleared more rapidly as a result of the addition of St. John's Wort

Hyperforin in St. John's Wort is and inducer of CYP3A4 via PXR - Indinavir (HIV-protease inhibitor and substrate of CYP3A4) % AUC decreased - Alprazolam (for panic and anxiety is a substrate of CYP3A4) % AUC decreased

Common CYP3A4 inhibitors (MAG)

M - macrolide anitbiotics (i.e. erythromycin) A - azole antifungals (i.e. ketoconazole) G - grapefruit juice

Common CYP3A4 inducers (PC GRP)

P - phenytoin C - cabamazepine G - glucocorticoids R - rifampin P - phenobarbital

Mechanism steps of induction of metabolism of PXR

PXR exists in the cytosol bound to other proteins in a complex 1. a drug that is a PXR Ligand enters the cell and binds to the complex 2. the other proteins fall off producing a PXR-drug complex 3. the complex translocates to the nucleus and binds to a partner receptor (RXR) 4. the newly formed dimer-drug complex binds to DNA and initiates gene transcription 5. protein synthesis is then initiated

Some CYPs have very few inducers while other have many. What structural property of PXR can be attributed to the long list of CYP3A4 inducers?

PXR has an extremely flexible binding site - the whole PXR structure can remodel to accommodate ligands of various shapes and sizes - since many drugs will induce PXR, and PXR is the main inducer of CYP3A4, many drugs are inducers of CYP3A4 - this is why CYP3A4 is inducible by more drugs than most other CYPs

What can be inferred from the very wide range of shapes and sizes of the ligands that PXR can bind?

Since CYP3A4 is controlled mainly by PXR, we can infer that CYP3A4 is induced by many different subsets of drugs and compounds

Definition of a drug

any chemical agent that interacts with specific target molecules or receptors in the body, thereby producing biological effect - does not matter whether it comes from a pharmacy, whether or not its FDA-approved, or if its an herbal remedy- its important that the pharmacist knows all of them to prevent a DDI

Definition: induction (of metabolism)

increased activity of biotransformation of enzymes - primarily due to increase in synthesis of proteins - in one case due to decrease in degradation of a protein (CYP2E1)

Possible consequence of induction of metabolism of an active drug into its inactive form

if a drug is administered in its active form and then metabolized into an inactive metabolite, there could be reduced concentration of the the active drug in the circulation and at the site of action (causing failure of therapy)

possible consequence of induction of metabolism of a toxic metabolite

if a toxic metabolite is produced (in any or increased quantities), there's increased risk of drug toxicity

possible consequence of induction of first phase of metabolism

if first pass metabolism is induced, the drug could be metabolized too much by the CYP into metabolites that are no longer active - causes a reduction in bioavailability of the drug

how does the CAR mechanism of induction differ from the PXR mechanism of induction?

main difference: CAR is "always ON" at some low level - PXR sits in the cytoplasm and waits for a drug to come bind to it for it to be active - at a low level, CAR is constantly moving into the nucleus and initiating gene transcription; drug binding to CAR just increased the amount of CAR that is always going into the nucleus anyway

PC BRASSS

mnemonic for memorizing common P450 enzyme inducers: P - phenytoin C - carbamazepine B - barbiturates R - rifampin A - alcohol (chronic) S - sulfonylureas S - smoking S - St. John's Wort

CYP enzyme genes in the nucleus are regulated by

nuclear receptor molecules in the cytoplasm

all CYPs catalyze these types of reactions

oxidation reactions

how do different drugs ultimately affect different CYPs?

since different CYPs under the control of different receptors, which receptor a drug bind to will determine which series of CYPs it will actually induce