Kocarek - Drug Metabolism Flashcards
Drug Metabolism
Chemical alteration of xenobiotics in the body. Converts lipophilic chemicals into hydrophilic chemicals that are readily excreted in urine or bile.
Xenobiotic
Any compound normally foreign to living systems
Issues of drug metabolism (4)
- Genetics
- metabolism-dependent toxicity
- Drug-drug interactions
- Pathophysiological conditions
When does acetaminophen become toxic?
When the dose overwhelms the glucuronidation and sulfation pathways and forms NAPQI via the cytochrome P450 path (mainly CYP2E1 and CYP3A4)
How can NAPQI be detoxified?
glutathione conjugation
What does MPTP do to the body?
Metabolism dependent.
It crosses the BBB and is metabolized by MAO to MPDP+ that then auto-oxidizes to MPP+. This concentrates in DA neurons where it impairs mitochondrial respiration.
Creates a Parkinson’s Disease-like condition
What does grapefruit juice inhibit?
Due to what chemical?
CYP3A4
Due to furanocoumarins
How is terfenadine cardiotoxic?
drug-drug interaction involving CYP3A4. CYP3A4 converts the prodrug terfenadine to fexofenadine in the intestine and liver. A CYP3A4 inhibitor, ketoconazole or erythromycin, causes terfenadine to accumulate to cardiotoxic levels
How can metabolism be affected by pathophysiological conditions
Hepatic metabolism may be compromised in liver disease
Cardiac disease can reduce blood flow to liver, reducing metabolism of flow-limited drugs
Four categories of biotransformation reactions
- Oxidation - P1
- Reduction - P1
- Hydrolysis - P1
- Conjugation - P2
Two major categories of biotransformation reactions
Phase I: oxidation, reduction or hydrolysis reactions
- Convert parent compound to more polar metabolite
- May make the compound more readily excreted
- May reveal a functional group that can serve as a site for additional metabolism
Phase II: conjugation of a small, endogenous substrate molecule with functional groups already present on drug or added/revealed by phase I metabolism
Important Point: either one of the above phases may occur first
How is procaine metabolized?
Hydrolysis of the ester linkage, which exposes a carboxylic acid group
Phase III metabolism
Describes the transporter proteins that pump xenobiotics or conjugates out of cells.
How is isoniazid metabolized? What product is hepatotoxic?
Undergoes phase II first: conjugated by acetylation (II) then the acetylated metabolite is then hydrolyzed (I) to produce isonicotinic acid and acetylhydrazine.
acetylhydrazine is hepatotoxic
What enzymes catalyze >50% of orally effective drugs in current use?
CYP2D6 and CYP3A4
What reactions are catalyzed by enzymes that participate in intermediary metabolism?
Cinnamic acid to benzene by mitochondrial fatty acid beta-oxidation enzymes
anti-HIV drug zidovudine to a triphosphate nucleoside by enzymes in the salvage pathway
Xenobiotic sensing receptors
Aryl hydrocarbon (Ah) receptor
Pregnane X receptor (PXR)
Constitutive androstane receptor (CAR)
Aryl hydrocarbon (Ah) receptor ligands:
prototype ligands are aromatic hydrocarbons and polychlorinated hydrocarbons
Regnane X receptor (PXR) ligands:
Ligands include a large number of drugs and other xenobiotics (ex rifampicin and hyperforin)
Constitutive androstane receptor (CAR) activated by:
Activiated by phenobarbital and many other drugs and xenobiotics
How is codine converted to morphine?
Phase I reaction (O-demethylation) catalyzed by CYP2D6)
Where is the highest level of CYPs found?
endoplasmic reticulum (microsomes) of liver, but are also present in virtually all cells
Cytochrome P450 is involved in what type of metabolism?
Phase I oxidation
of Human CYPs
57
18 families
708 gene families
Overall P450 rxn
Drug + NADPH + O2 –>Oxidized Drug + NADP+ + H2O
Incorporates an atom of oxygen into the substrate
CYP1
aromatic hydrocarbons
CYP2
phenobarbital, “phenobarbital-like” compounds
CYP3
Steroids, phenobarbital, “phenobarbital-like” compounds
CYP4
Peroxisome proliferators
How close are CYPs in the same family?
CYPs in the same family have at least 40% amino acid similarity
Cytochrome P450 catalytic cycle
o Drug adds first to cytochrome P450 (Fe3+)
o Addition of electron from NADPH (Fe2+)
o Addition of O2 (Fe2+−O)
o Addition of another electron from NADPH (Fe2+−O2(-))
o Addition of 2 protons (H+) and release of water (Fe3+−O)
o Transfer of O to drug and release of hydoxylated drug (d-OH) –> regeneration of P450 (Fe3+)
Componenets required for the P450 system
o NADPH
o NADPH-cytochrome P450 reductase (flavoprotein)
o Cytochrome P450
o Lipid environment
Types of oxidative reactions
Hydroxylation of an aliphatic or aromatic carbon
epoxidation of a double bond
heteroatom (O,S, and N) dealkylation
heteroatom (S,N, I) oxygenation and N-hydroxylation
oxidative group transfer
cleavage of esters
dehydrogenation
Example of Hydroxylation of an aliphatic carbon
tolbutamide to hydroxytolbutamide (via CYP2C9)
Example of Hydroxylation of an aromatic carbon
(S)-Mephenytoin to 4’-Hydroxy-(S)-Mephenytoin (via CYP2C19)
Example of an expoxidation
Carbamazepine (anticonvulsant) to carbamazepine-10,11-epoxide (stable)
and
Carbamazepine to carbamazepine-2,3-epoxide (unstable arene oxide)
Example of heteroatom oxygenation
S: omeprazole –>
N: NNK –> NNK N-oxide
Example of heteroatom dealkylation
Codeine to morphine via (CYP2D6): an example of O-dealkylation
Other oxidatative enzymes (10)
flavin monooxygenase alcohol dehydrogenase aldehyde dehydrogenase aldehyde oxidase xanthine oxidoreductase monoamine oxidases diamine oxidases polyamine oxidase semicarbazide-sense. amine oxidase peroxidase
Reduction: 3 types
Azo/nitro
Carbonyl
Quinone
Azo and nitro reduction
catalyzed by:
Catalyzed by intestinal microflora
sometimes by P450
Example of azo reaction
prontosil to 1,2,4-triaminobenzene and sulfanilamide (first sulfa drug)
Carbonyl reduction:
catalyzed by what?
Reduction of aldehydes to primary alcohols
and
ketones to secondary alcohols
catalyzed by NAD(P)H-dependent reductase
Two superfamilies of NAD(P)H-dependent reductase
- Aldo-ketoreductase
2. Short-chain dehydrogenases
Quinones can be reduced to hydroquinones by:
Quinones can be reduced to hydroquinones by NAD(P)H-quinone oxidoreductase-1 (NQO1,DT-diaphorase) and NQO2
What does hydrolysis cleave?
esters, amides, thioesters, phosphoric acid esters, acid anhydrides, and lactones
What enzymes hydrolyze xenobiotics?
Carboxylesterases (CE) and Epoxide hydrolases (main ones)
Cholinesterases (AChE, butyrylcholinesterase)
Paraoxonases (lactonases)
Alkaline phosphase
Peptidase
What do CE1 and CE2 hydrolyze?
xenobiotic esters and amides
ex: procaine by CE2
What are the 2 (of 5 total) epoxide hydrolases that function in xenobiotic metabolism?
Microsomal epoxide hydrolase (mEH)
Soluble epoxide hydrolase (sEH)
How do epoxide hydrolases work?
what are the products?
catalyze the trans-addition of water to alkene epoxides and arene oxides
Products are trans-1,2-dihydrodiols
What type of cell expresses mEH and sEH?
Generally, the same type of cell that expresses cytochromes P450
How can EH play a role in bioactivation?
polycyclic aromatic hydrocarbon “bay-regions” diolepoxides
Bay region epoxides are resistant to hydrolyation by EH, resulting from steric hindrance from the nearby dihydrodiol group
Conjugation reaction
xenobiotic (or endogenous substrate) is coupled with an endogenous molecule
How do the conjugated metabolites compare with substrate?
More polar
pharmcologiaclly inactive
more readily excreted
(Not always the case)
Major conjugation reactions
Glucuronidation* Sulfation (sulfonation)* Glutathione conjugation* Acetylation* (GGAS) Methylation Amino acid conjugation
Glucuronidation mechanism
transfer of glucuronic acid from the co-factor UDP-glucuronic acid to a nucleophilic heteroatom on a substrate
Glucuronidation substrates
aliphatic alcohols, phenols, carboxylic acids, amines, sulfhydryl groups
Glucuronidation is catalyzed by:
Uridine 5’-diphosphate (UDP)-glucuronosyltransferases (UGTs)
How many UGTs are encoded by the UGT1 locus? UGT2?
What are UGTs localized to?
UGT1: 9 (have different exon 1s)
UGT2: 10
localized to microsomes
How is morphine converted into an active metabolite?
UGT2B7 adds glucuronic acid to the 6-position of morphine
Sulfation (sulfonation) mechanism
transfer a sulfonate group from the co-factor 3-phosphoadenosine-5’-phosphosulfate (PAPS) to an available group on a substrate
Substrate of sulfation
aliphatic alcohols, phenols, amines, N-oxides, N-hydroxyls
Not carboxylic acids
Sulfation is catalyzed by:
How many are there?
SULTS
14 human SULTs
Example of sulfation rxn:
17 beta-estradiol to Estradiol-3-sulfate via SULT1E1
Glutathione conjugation mechanism
transfer of a glutathione (Glu-Cys-Gly) group to an electrophilic atom
Glutathione substrates
epoxides, arene oxides, nitro groups, hydroxylamines
Glutathione is catalyzed by:
How many are there?
Glutathione transferases (GSTs)
24human GSTs
Cytosolic, microsomal, mitochondrial enzymes
Acetylation
Mechanism:
Substrates:
Catalyzed by:
Phase II metabolism
Transfers acetyl group from co-factor acetyl coenzyme A to an amino group on a substrate
Substrates are amines
Catalyzed by N-acetyltransferases
NAT1 and NAT2 are located in cytosol
Methylation
Mechanism:
Substrates:
Catalyzed by:
Phase II metabolism
Transfers a methyl group from the cofactor S-adenosylmethionine (SAM)to an electron-rich heteroatom (O, N, or S) on a substrate; C-methylation is possible but rare
Substrates: phenols, catechols, aliphatic and aromatic amines, N-heterocyclic compounds, and sulfhydryl-containing compounds; certain metals can also be methylated (inorganic mercury, arsenic, selenium)
usually decreases the water solubility of xenobiotics (exception: N-methylation of pyridine-containing compounds, which produces quaternary ammonium ions, and S-methylation of thioethers which produces positively charged sulfonium ions)
Catalyzed by methyltransferases: COMT, POMT, PNMT, HNMT, NNMT, and others
