Intro to Drug Metabolism & Phase I Metabolism Flashcards
Why are some drugs/xenobiotics metabolized?
It’s a defense mechanism
- To increase water solubility of hydrophobic molecules for excretion
- To protect against all types of “reactive” molecules
Where are drugs/xenobiotics metabolized?
In the liver
- gets absorbed in the gut and does the first-pass right after which has an impact on oral F (decreases the fraction that gets through)
- Every subsequent pass – CLH (metabolism of a portion of drug molecules in systemic circulation occurs on every pass back through the liver)
How are drugs/xenobiotics metabolized?
Phase I enzymes & Phase II enzymes
Two general sets of enzyme-catalyzed reactions
– Phase I enzymes: introduce or expose polar groups
– Phase II enzymes: “synthetic” – form conjugates
What affects the rates of drug/xenobiotic metabolism?
- Enzyme expression levels (when enzyme expression increases, the rate of clearance will increase because you have more enzyme that catalyzes the reaction)
- Inhibition (if you bind something to the enzyme and inhibit it, clearance decreases)
- Genetic polymorphisms/variability
The balanced molecular properties that drugs need:
- aqueous solubility high enough to dissolve in the
gut to drive absorption and to reach concentrations high enough to saturate receptor in tissues - hydrophobic enough to diffuse across membranes
and, in many cases, to enhance binding affinity to receptor
A drug is excreted unchanged via
kidney
What does it mean when a drug is metabolized?
chemical structures and properties are changed through enzyme-catalyzed reactions
These drugs may be excreted primarily
unchanged:
Polar and ionic drugs
Hydrophobic drugs are metabolized to become
polar, ionic and more water soluble
Orally delivered drugs are subject to both
prehepatic metabolism in the gut and more
extensive first-pass metabolism in the liver that
lowers bioavailability.
Two types of Enzyme-Catalyzed Reactions
- Primarily Xenobiotic
- Primarily Endobiotic
Primarily Xenobiotic
not expected to be in body
Primarily Endobiotic
synthesized in body
Phase I Class of Drug Metabolizing Enzymes
Introduce or Expose Polar Groups
Phase II Class of Drug Metabolizing Enzymes
Use “activated cofactors” to form drug-conjugates
Phase I Groups
Oxidation
Oxidation/Reduction
Hydrolysis/Hydration
Phase II Groups
Nucleophile in Drug
Electrophile in Drug
Oxidation
- Cytochrome P450 (CYP)
- Flavin Monooxygenase (FMO)
- Require NADPH + O2
- reveal a nucleophile: a Phase I Rxn
Oxidation/Reduction
- Alcohol Dehydrogenase (ADH)/Carbonyl Reductase (CBR)
- Aldehyde Dehydrogenase (ALDH)
- Require NAD+/NADH
Hydrolysis/Hydration
- Esterase (CES)
- Amidase
- Epoxide Hydrolase (EH)
- Reactions with H2O
Nucleophile in Drug
Glucuronosyl Transferase (UGT)
Sulfotransferase (SULT)
N-Acetyl Transferase (NAT)
Electrophile in Drug
Glutathione S-Transferase (GST)
CYPs
- aliphatic C-H -> C-OH
- N-, O-, S-dealkylation
- dehalogenation
- aromatic C-H -> C-OH
- epoxidation C=C
- N-H -> N-OH
CYPs & FMOs
N-, S-oxidation
ADH & ALDH
- R2CHOH –> R2C=O
- aldehyde oxidation RCH=O -> RCO2H
Esterase
Amidase
- ester hydrolysis
- amide hydrolysis
Epoxide hydrolase
epoxide hydration
- glucuronides – UGTs
- sulfates – SULTs
- peptide conjugates – N-acyl-amino acid transferases
Form conjugates to increase water solubility
- N-acetylation – NATs
- methylation – methyl transferases
Form conjugates to decrease nucleophilic reactivity
glutathione conjugates – GSTs
Form conjugates to eliminate reactivity and increase water solubility
Each drug may be metabolized via
one or more single or multi-step pathways
Which enzyme(s) are responsible for ”clearance” of a given drug depends on
the chemical structure of the drug
Common Path of Metabolism for Hydrophobic Drugs
nucleophile
- a chemical species or an atom within a molecule that has a tendency to donate a pair of electrons to another atom
- their electron-rich nature allows them to seek out and react with electron-deficient species
- Examples include negatively charged ions like hydroxide ions (OH-), chloride ions (Cl-), and cyanide ions (CN-)
Phase I reactions increase
water solubility to some extent
Phase II anionic conjugates dramatically increase
water solubility and lead to excretion
Predominant pathways for first metabolites depend on
key structural features
Phase I oxidations (CYP reactions) dominate in
the absence of polar nucleophiles
Phase II conjugations (UGT) dominate if
polar nucleophile present
Various outcomes from
these biotransformations
- active drug –> active metabolite
- active drug –> inactive metabolite
- active drug –> reactive “metabolite”
- inactive (prodrug) –> active metabolite (“drug”)
Pharmacologically inactive prodrugs become activated by
one of the drug metabolizing enzymes
The most common prodrug design utilizes
esters that are hydrolyzed to active acids or alcohols
Genetic polymorphisms/variability
presence of different versions of a gene or DNA sequence within a population, resulting in genetic diversity among individuals.
- may lower kcat –> lower rate
- may increase km –> lower rate
- may have opposite effect on km or kcat –> higher rate
- may have gene copies –> more total energy, higher rate
Rates are directly proportional to
Enzyme expression levels
Rates can be affected by
inhibitors of the drug-metabolizing enzymes
Rates in different individuals may vary because of
genetic
polymorphisms
Polar and ionic drugs may be excreted primarily
unchanged
Hydrophobic drugs are more extensively
metabolized
Phase I reactions primarily introduce or “expose” nucleophiles that are then subject to
Phase II conjugation reactions
Phase II reactions form conjugates of drug nucleophiles or electrophiles with groups that
- eliminate that reactivity
- increase the size of the conjugate
- introduce a low pKa anionic group that significantly increases water solubility
Enzymes catalyzing major initial transformations of parent drugs in the liver are key to
- clearance of drug
- potential drug-drug interactions
- variability in patient response
Orally delivered drugs are subject to both
- prehepatic metabolism in the gut
- more extensive first-pass metabolism in the liver
Biotransformations may convert
- active drugs into inactive metabolites
- active metabolites
- reactive metabolites that can lead to various types of toxicities
Absorption
Hydrophilic enough to dissolve in the gut
Distribution
Hydrophobic enough to diffuse across cell membranes
Metabolism
- increase water solubility of hydrophobic drugs to be excreted
- Protect against reactive molecules
- where: Liver (majority), gut, lungs
- how: first-pass metabolism, Phase I and Phase II reactions
Excretion
urine and/or feces
- Polar/ionic drugs mainly excreted unchanged by the kidney
- Metabolized drugs become more water soluble for entry into tubule lumen ⇨ urine
First-Pass Metabolism
Fraction of orally delivered drugs are subject to both pre-hepatic metabolism in the gut and more extensive first-pass metabolism in the liver which decreases bioavailability (F) before reaching systemic circulation
If you want to increase the bioavailability (F) of a drug substantially, what can you do?
Give the drug IV
What leads to the pk differences in IV vs oral administration?
IV avoids first-pass CYP enzymes in gut/liver, 100% enters systemic circulation
The fraction of oral drug that is absorbed and escapes both gut and liver first-pass metabolism ultimately becomes the fraction
that
is fully available for your body to use, aka F < 1.
Some drugs are administered as Prodrugs that become activated by
first-pass metabolism
Enzyme catalyzed reactions that introduce or expose
polar groups
Oxidation
- Require NADPH + O2
- CYPs and FMOs
Oxidation/Reduction
- Require NAD+/NADH
- ADH/CBR, ALDH
Hydrolysis
- Reactions with H2O
- Esterase, Amidase, Epoxide Hydrolase
Phase I Metabolism
- Increase water solubility to some extent
- Dominate in absence of polar nucleophiles
Phase II Metabolism
- Increase water solubility dramatically and lead to excretion (urine/feces)
- Dominate if polar nucleophile is present
In Phase II Metabolism, enzyme catalyzed reactions use
activated cofactors to form drug conjugates
Nucleophile in drug
- Form conjugates to increase water solubility (UGTs, SULTs, N-acyl-amino acid transferases)
- Form conjugates to decrease nucleophilic activity (NATs, methyl transferases)
Electrophile in drug
For conjugates to eliminate reactivity and
increase water solubility (GSTs)
Prodrugs and Possible Pathways
- active drug –> active metabolite
- active drug –> inactive metabolite
- active drug –> reactive metabolite
What are some factors that can affect the rate of a reaction and efficacy of the drug?
- enzyme levels
- enzyme inhibitors (DDIs)
- genetic polymorphisms
- enzyme saturation
Reactions that come from CYP enzymes
- Aliphatic hydroxylation
- N-H Hydroxylation
- Epoxidation
- Aromatic Hydroxylation
- N-Oxidation
- O-dealkylation
- N-dealkylation
- Dehalogenation
N-dealkylation
N-Oxidation
- O is added
Dehalogenation
N-H Hydroxylation
- you add an alcohol group
Aliphatic hydroxylation
hydroxyl group
O-dealkylation
methyl group is being removed
Aromatic
Epoxidation
What are the major CYP families that are
involved in ~90% of drug/xenobiotic metabolism?
- CYP 1A
- CYP 2C/D/E
- CYP 3A
Which subfamilies have the highest 3 proportions of drug metabolism?
- CYP 3A4/5
- 2D6
- 2C9
Amidases
Esterases
ADH- alcohol dehydrogenase
ALDH - aldehyde dehydrogenase
EH - epoxide hydrolase
Carbonyl reductase
ADH/CBR are
REVERSIBLE redox reactions of alcohols to
aldehydes/ketones (and vice versa)
Why do we need to remove aldehydes?
Aldehydes can react to amines on proteins and modify their function
Which drug do we use to treat alcohol abuse?
Disulfiram. By inhibiting ALDH we increase the buildup of aldehyde leading to bad SE.
Does 1st pass metabolism affect a drug’s clearance?
No! First pass metabolism affects the bioavailability (F) of the drug prior to it reaching systemic circulation. Since clearance is defined by the removal of parent drug from the body once it has been in systemic circulation, first pass metabolism would not be included
Phase I Reactions expose/introduce polar groups, revealing nucleophiles making the drug MORE
water soluble, and can lead to EASIER excretion.
If a drug is ANIONIC it will most likely need
TRANSPORTERS (and is usually highly water soluble!)
- Phase I rxns increase water solubility LESS than Phase II rxns
From a Phase I reactions, drugs can either
be completely eliminated or further metabolized by Phase II enzymes/rxns
