Drug Metabolism Flashcards
Importance of Drug Metabolism:
- Termination of drug action
- Drug activation
- Drug-drug interactions
Termination of drug action:
- For many drugs, the metabolites are more easily excreted from the body
- Metabolites are most often more polar and hydrophilic
- For many drugs, metabolism inactivates the drug
Active drug ⇒ inactive metabolite
Drug activation:
- Some drugs (prodrugs) are converted to their active form by metabolic enzymes
Prodrug (inactive) ⇒ active drug
Drug-drug interactions:
- Many result from drug metabolism issues
What are some drugs that were withdrawn because of drug-drug interactions due to drug metabolism?
- terfenadine
- mibefradil
- astemizole
- cisapride
Phase I metabolism:
-
Oxidation, reduction, dealkylation, or hydrolysis reactions
- often introduce or reveal a functional group
Phase II metabolism:
Conjugation of the drug or drug metabolite to an endogenous substrate molecule
How are drugs localized in the body?
- Organ distribution
- Subcellular distribution
Organ DIstribution:
- Liver has the highest content overall of drug metabolizing enzymes
- Other organs with significant content
- gastrointestinal tract
- kidneys
- lungs
- All tissues have some drug metabolizing enzymes
- Distribution varies significantly for specific enzymes
Subcellular distribution:
- Phase I enzymes: usually smooth ER (microsomal fraction)
- Phase II enzymes: most are cytosolic
What is the first-pass effect?
- Applies to orally administered drugs
- Following absorption from the GI tract, the portal venous system transports them to the liver
- Significant metabolism can occur prior to reaching the general circulation
- Drug metabolizing enzymes in the liver and/or intestine contribute to the first-pass effect
- Can greatly lower the oral bioavailability of a given drug
_______________ are hemeproteins that are major catalysts of _________ biotransformation reactions.
- Cytochrome P450s
- Phase I
Drugs with a _____________ need higher oral doses to match the effects or efficacy seen with IV administration
large first pass effect
What are two reasons for a low F (bioavailability)?
- Poor absorption
- Large first-pass effect
Name some drugs with a large first-pass effect:
Drug (F)
- morphine (0.17 – 0.33)
- meperidine (0.52)
- aspirin (0.68)
- propranolol (0.26)
- labetalol (0.18 – 0.25)
- metoprolol (0.38 – 0.50)
- diltiazem (0.44)
- verapamil (0.22)
- nortriptyline (0.51)
Example: First-pass effect and oral dosing of morphine
If:
- first-pass effect results in an oral F=0.33
- an IV dose of 10 mg effectively relieves pain
Then:
- an oral dose of 30 mg would be needed for same degree of pain relief
- if the oral F=0.17, then an oral dose of 60 mg would be needed
- Essentially, a patient will need 3-6x as much morphine than the IV dose
P450 Catalytic Cycle:
- Required components:
- cytochrome P450
- P450 reductase (flavoprotein)
- NADPH
- O2
- drug (substrate)
- Monooxygenase-type reactions:
- S + O2 + 2e– + 2H+ ⇒ SO + H2O
- where S = substrate (drug)
Subcellular localization:
- anchored to the cytoplasmic face of the smooth ER
-
one isoform of P450 reductase, but many cytochrome P450 isoforms
- approx. 10–20 P450 molecules per P450 reductase
Human P450s:
- 18 families
- (CYP1, CYP2, CYP3)
- 57 human P450 genes
- (15 families largely involved)
- Significant inter-individual variation
- drug metabolism
How are drug metabolized by human P450s:
- Broad substrate specificity
- each isoform can have several to hundreds of drug substrates
- Cytochrome P450 isoform contribution to human drug metabolism:
% of drugs handled by major isoforms:
- CYP3A: 50%
- CYP2D6: 25%
- CYP2C9:15%
- ≤5% each: CYP1A2, CYP2E1, CYP2A6, CYP2C19
- ***DOES NOT equate to the amount of substrates each P450 can handle***
Other Phase I Enzymes
Flavin-containing monooxygenase (FMO):
- Flavoprotein localized in smooth ER
- Catalyze monooxygenation reactions, primarily of soft nucleophiles:
- N, S, P & Se moeities
- Cannot handle C (job of the P450s)
- S + O2 + 2e– + 2H+ —> SO + H2O
- Primarily N-oxidation and S-oxidation reactions
- Hundreds of potential substrates:
- some are P450 substrates, others are unique to FMOs
- products are more polar and less toxic/active
Required components for FMOs:
- FMO
- NADPH
- O2
- drug (substrate)
FMO isoforms:
5 isoforms (FMO1-FMO5):
- FMO3
- Liver, brain, kidney
- 2–3% of hepatic protein (no other protein is this abundant)
- FMO1
- Kidney, intestine
- FMO2
- Lung (26% African-
Americans) - Non-functional in
Caucasians
- Lung (26% African-
- FMO4
- Kidney, brain
- FMO5
- Liver, kidney
Examples of drug substrates
for FMO:
- nicotine
- cimetidine
- ranitidine
- spironolactone
- imipramine
- clozapine
- olanzapine
Dehydrogenases & Hydrolases:
Dehydrogenases
- Alcohol dehydrogenase (ADH)
- Aldehyde dehydrogenase (ALDH)
Hydrolases
- Epoxide hydrolase (EPHX)
- Carboxyl esterases
- acetylcholine, procaine, cocaine, aspirin
- Amidases
- lidocaine, peptide drugs
Phase II Drug Metabolizing Enzymes:
Conjugative Reactions
- conjugation of an endogenous chemical group to the drug
- other examples:
- methylation
- glutathione conjugation
- glucoside conjugation
- amino acid conjugation
What are the possible orders of phase I and phase II enzymes? What does this cooperativity depend on?
- depends on drug, its functional groups, and available enzymes:
- drug → phase I → excretion
- drug → phase I → phase II → excretion
- drug → phase II → excretion
- drug → phase II → phase I → excretion
Comparison of major classes of Phase II reactions:
Glucuronidation
-
High energy intermediate:
- UDP-glucuronic acid
-
Functional groups needed on drug:
- -OH, -COOH, -NH2, -NR2, -SH
-
Responsible Enzyme(s):
- UDPglucuronosyl
transferases (UGT)
- UDPglucuronosyl
-
Enzyme localization:
- endoplasmic reticulum (lumenal face)
Comparison of major classes of Phase II reactions:
Acetylation
-
High energy intermediate:
- acetyl-CoA
-
Functional groups needed on drug:
- -OH, -NH2
-
Responsible Enzyme(s):
- N-acetyltransferases (NAT)
-
Enzyme localization:
- cytosolic
Comparison of major classes of Phase II reactions:
Sulfation
-
High energy intermediate:
- Adenosine-3’- phosphate- 5’-phosphosulfate (PAPS)
-
Functional groups needed on drug:
- -OH, -NH2
-
Responsible Enzyme(s):
- sulfotransferases (SULT)
-
Enzyme localization:
- cytosolic
Comparison of major classes of Phase II reactions:
Glutathione conjugation
-
High energy intermediate:
- drug itself: arene oxides, epoxides, etc.
-
Functional groups needed on drug:
- aryl halide, arene oxide, epoxide, carbonium ion
-
Responsible Enzyme(s):
- glutathione S-transferases (GST)
-
Enzyme localization:
- cytosolic, some endoplasmic reticulum
What is the net effect of most phase II reactions?
- Metabolites are usually:
- more polar (easier to excrete)
- inactive or less toxic
Exceptions: acetyltransferases and methyltransferases
% of drugs handled by phase II enzymes:
UGTs > STs > NATs, GSTs > TPMT
What are some other properties of phase II enzymes?
- Some are inducible
- Most have many substrates
- Vmax limited by conventional enzyme kinetics and conjugant supply
- drug + conjugant → drug-conjugant complex
Conjugation capacity & Abundance raw materials for cojugation:
Glucoronidation
- Conjugation Capacity Abundance of Raw
- High
- Materials for Conjugation
- High
Conjugation capacity & Abundance raw materials for cojugation:
Acetylation
- Conjugation Capacity Abundance of Raw
- Variable
- Materials for Conjugation
- Variable
Conjugation capacity & Abundance raw materials for cojugation:
Sulfation
- Conjugation Capacity Abundance of Raw
- Low
- Materials for Conjugation
- Low
Conjugation capacity & Abundance raw materials for cojugation:
Glutathione conjugation
- Conjugation Capacity Abundance of Raw
- Low*
- Materials for Conjugation
- Low*
Why is the conjugation capacity and abundance of GSH low in humans if the initial amount of GSH high?
It is not rapidly replenished
What is enzyme induction?
Exposure to some drugs and environmental chemicals can markedly upregulate enzyme amount and/or activity
- usually transcriptional increases:
- sometimes translational or protein stabilization
- more enzyme = faster metabolism
-
most cytochrome P450s (several isoforms) are inducible
- CYP2D6 not as inducible as others
-
some phase II enzymes (e.g. UGTs, GSTs) are inducible
- induced by environmental chemicals and some drugs
- changes the overall proportion of drug-metabolizing enzymes
Which P450 does ethanol induce?
CYP2E1
Which P450 does tobacco smoke induce?
CYP1A
What are the consequences of induction?
Can increase or decrease drug effects
-
Broad substrate specificity of many of these enzymes: a single inducer will simultaneously upregulate the ability to metabolize several drugs
- will decrease effectiveness of drugs whose metabolites are inactive
- will increase the effects/toxicity for drugs that are activated by the induced enzyme
- Increase metabolism dramatically
- Inducers are not quantitatively equal
How long does induction take?
If transcriptional,
- Maximal effects are seen in 1 – 2 days
Is it possible for an inducer to be a substrate?
Inducers may or may not be substrates
_________________ may inhibit the metabolism of several drugs.
Drug or environmental chemical
Potential types of inhibition:
-
Competitive
- competitive substrates are a major cause of drug-drug interactions
- quinidine (CYP2D6)
- furafylline (CYP1A2)
- Many others
-
Bind CYP heme – disrupts catalytic activity (non-competitive)
- cimetidine
- ketoconazole, itraconazole
- erythromycin
- others
-
Suicide inhibitors (irreverisble—non-competitive)
- ethinyl estradiol
- levonorgestrol
- secobarbital
How long does inhibition take?
Immediate
Extent of inhibition:
- Highly variable:
- Depends on enzyme and inhibitor
- Small effects ⇒ very large effects
- Several inhibitors of CYP2D6 can reduce activity to near zero:
- amiodarone, bupropion, chloroquine, quinidine
- diphenhydramine, fluoxetine, haloperidol, paroxetine
- propoxyphene, terbinafine
How does grapefruit juice interact with drugs?
Grapefruit juice increases drug absorption
- Furanocoumarin is the responsible ingredient that inhibits intestinal CYP3A
- Increasing the net amount of drug that reaches the general circulation
How are FMOs affected by induction & inhibition?
- Not significantly induced by clinically used drugs
-
Not significantly inhibited by clinically used drugs
- less susceptible to competitive substrate inhibition than are P450s
- Less potential for metabolic drug-drug interactions
What are some other factors that influence drug
metabolism activity?
-
Age
- Old and young
- efficacy and/or toxicity
- Genetics
-
Disease States
- Hepatic diseases
- Some cancers
- Infections
- cytokines decrease expression of some P450s
- Gender
Explain how gender is significant in regards to drug metabolism?
- Clear differences in sex hormone metabolism
- More recent findings:
- Women require half the dose of zolpidem (Ambien®)
- While CYP3A expression is overall similar between men and women, the clearance in women is actually slower
- 6–7% of drugs have >40% pharmacokinetic differences between men and women
What is the most common cause of acute hepatic failure?
Acetaminophen overdose
What factors contribute to acetaminophen hepatotoxicity?
Interplay of:
- multiple phase II reactions
- phase I reaction
- CYP2E1 induction
