Drug biotransformation (katzungs, Ch 4, trans 5) Flashcards
A few transformations occur in the intestinal lumen or intestinal wall. In general, all of these reactions can be assigned to one of two major categories called phase I and phase II reactions. These reactions usually convert the parent drug to a more polar metabolite by introducing or unmasking a functional group (−OH, −NH 2, −SH).
Phase I
- *Often these metabolites are inactive, although in some instances activity is only modified or even enhanced
- *many phase I products are not eliminated rapidly and undergo a subsequent reaction in which an endogenous substrate such as glucuronic acid, sulfuric acid, acetic acid, or an amino acid combines with the newly incorporated functional group to form a highly polar conjugate. Such conjugation or synthetic reactions are the hallmarks of phase II metabolism
Many drug-metabolizing enzymes are located in the lipophilic endoplasmic reticulum membranes of liver and other tissues
Rough endoplasmic reticulum - protein synthesis
Smooth endoplasmic reticulum - rich in enzymes responsible for oxidative drug metabolism
REMEMBER Smooth endoplasmic reticulum contain class of enzymes known as the mixed function oxidases (MFOs) or monooxygenases
The activity of these enzymes requires both a reducing agent (nicotinamide adenine dinucleotide phosphate [NADPH]) and molecular oxygen
** 1 molecule is consumed (reduced) per substrate molecule; one oxygen atom appearing in the product and the other in the form of water
What is the rate-limiting step in hepatic drug oxidations
P450 heme reduction
Microsomal drug oxidations require P450, P450 reductase, NADPH, and molecular oxygen.
STEPS:
- oxidized (Fe 3+ ) P450 combines with a drug substrate to form a binary complex
- NADPH donates an electron to the flavoprotein P450 reductase, which in turn reduces the oxidized P450-drug complex
- A second electron is introduced from NADPH via the same P450 reductase, which serves to reduce molecular oxygen and to form an “activated oxygen”-P450-substrate complex
- This complex in turn transfers activated oxygen to the drug substrate to form the oxidized product
PHASE II reactions
Parent drugs or their phase I metabolites that contain suitable chemical groups often undergo coupling or conjugation reactions with an endogenous substance to yield drug conjugates
In general, conjugates are polar molecules that are readily excreted and often inactive.
PHASE II reactions Glucuronidation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Glucuronidation
Endogenous Reactant: UDP glucuronic acid
Transferase (Location): UDP glucuronosyltransferase (microsomes)
Types of Substrates: Phenols, alcohols, carboxylic acids, hydroxylamines, sulfonamides
Examples: Nitrophenol, morphine, acetaminophen, diazepam, N-hydroxydapsone, sulfathiazole, meprobamate, digitoxin, digoxin
PHASE II reactions Acetylation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Acetylation
Endogenous Reactant: Acetyl-CoA
Transferase (Location): N–Acetyltransferase
(cytosol)
Types of Substrates: Amines
Examples: Sulfonamides, isoniazid, clonazepam, dapsone, mescaline
PHASE II reactions Glutathione conjugation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions Glutathione conjugation Endogenous Reactant: Glutathione (GSH) Transferase (Location): GSH-S-transferase (cytosol, microsomes) Types of Substrates: Epoxides, arene oxides, nitro groups, hydroxylamines Examples: Acetaminophen, ethacrynic acid, bromobenzene
PHASE II reactions Glycine conjugation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Glycine conjugation
Endogenous Reactant: Glycine
Transferase (Location): Acyl-CoA glycinetransferase (mitochondria)
Types of Substrates: Acyl-CoA derivatives of carboxylic acids
Examples: Salicylic acid, benzoic acid, nicotinic acid, cinnamic acid,
cholic acid, deoxycholic acid
PHASE II reactions Sulfation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Sulfation
Endogenous Reactant: Phosphoadenosyl phosphosulfate
Transferase (Location): Sulfotransferase (cytosol)
Types of Substrates: Phenols, alcohols, aromatic amines
Examples: Estrone, aniline, phenol, 3-hydroxycoumarin, acetaminophen, methyldopa
PHASE II reactions Methylation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Methylation
Endogenous Reactant: S-Adenosylmethionine
Transferase (Location): Transmethylases (cytosol)
Types of Substrates: Catecholamines, phenols, amines
Examples: Dopamine, epinephrine,
pyridine, histamine, thiouracil
PHASE II reactions Water conjugation Endogenous Reactant: Transferase (Location): Types of Substrates: Examples:
PHASE II reactions
Water conjugation
Endogenous Reactant: Water
Transferase (Location): Epoxide hydrolase (microsomes)
Types of Substrates: Arene oxides, cis-disubstituted
and monosubstituted oxiranes
Examples: Benzopyrene 7,8-epoxide, styrene 1,2-oxide, carbamazepine epoxide
REMEMBER
Biotransformation
- is the chemical transformation of the drug in the body into a metabolite
- the primary site of occurrence is the LIVER, however pre-systemic metabolism occurs in the GIT (presence of CYP450)
- it main purpose is to render the drug to be more water soluble allowing it to be excreted faster
- it does not always render the drug into its inactive form, rather it converts the drug to its active or inactive form
a. For better DISTRIBUTION, drugs must be unionized, lipid soluble, and non-polar
b. For better EXCRETION, drugs must be ionized, water soluble, and polar
c. Unionized form of WEAK ACID: PROTONATED form; WEAK BASE: NON-PRTONATED form
Metabolite – the byproduct of metabolism
Active
- capable of pharmacologic activity
- still reabosorbable
- less polar, less ionized, lipid soluble
Inactive
- more polar, more ionized, water soluble
DRUG METABOLIZING SYSTEMS
- Enzymes
- Non-enzymatic processes
- Rearrangement of UNSTABLE metabolites into STABLE complexes
ENZYMES
1. Substrate Specific Enzymes - can degrade the drug analogues of normal body substances
Ex: Acetylcholinesterase, Monoamine oxidase (MAO) and Catecholamine-o- methyl-transferase (COMT) degrade norepinephrine and other catecholamine
2. Broad Substrate Specific Enzymes - Degrade a broad spectrum of substances, does not degrade specific analogues of normal physiologic constituent
Ex: Broad Plasma Pseudocholinesterase degrades non-specific esters like succinylcholine (muscle relaxant) and procaine (ester-type anesthetic)
Non-enzymatic processes
Ex: hydrolysis (isoniazid)
LOCATION OF DRUG METABOLIZING ENZYMES
- LIVER – main site
- LUNGS, GIT (intestinal mucosa), SKIN, PLACENTA, KIDNEY – other tissues with enzymes
- SUBCELLULAR – endoplasmic reticulum, mitochondria, cytosol, lysosomes, nuclear envelope, plasma membrane
REMEMBER
- drug can undergo phase I then it is converted into a metabolite which inactive from there it could readily be excreted OR
- if the drug is active it has to pass through phase II making it polar, rendering it inactive, then excreted
HOWEVER, this is not always the case!
• a drug can undergo phase II first before phase I
• Isoniazid - anti TB drug; possesses a functional group that is actually acetylated in phase II and is hydrolyzed into nicotinic acid or your isonicotinic acid in phase I
PHASE 1 (NONSYNTHETIC)
- drugs are oxidized or reduced to a more polar form by introducing or unmasking a functional group (e.g., -OH, -COOH, -SH, -O-, or NH2)
- these moieties do little to increase the water solubility of the drug but usually lead to drug inactivation
- OXIDATIVE REACTIONS
- HYDROLYTIC REACTIONS
- REDUCTIVE REACTIONS
Phase I
Oxidative reaction
- utilizes microsomal cytochrome P450 monooxygenase and cytoplasmic enzymes
- most predominant (50% of drug interactions)
- occurs in ER and microsome
Paracetamol : N-oxidation
Ibuprofen: Hydroxylation
Important CYP450 Enzymes
CYP3A4: accounts for over 50% of drug metabolism in liver
CYP2D6: metabolizes opioid analgesics like codeine
CYP2C9: metabolizes warfarin and phenytoin
Phase I Hydrolytic reactions - involves ester and amide drug - compound broken in parts - intracellular (ER, cytoplasm) or extracellular (circulating in plasma)
Example of hydrolytic enzymes:
Procaine (ester): pseudocholinesterase;
Lidocaine (amide): liver microsomal cP450
Phase I Reductive reactions - involves azo, nitro, and carbonyl functional groups of drugs - hepatic nitroreductase - enzymes in cell cytoplasm
Dantrolene – nitro reduction in malignant hyperthermia
PHASE 2 (SYNTHETIC)
- drugs are conjugated with acetate, glucuronate, sulfate, or glycine
- most conjugated drug metabolites are inactive
CONJUGATION
- parent drug or metabolite coupled to endogenous substances
- acetyl CoA, glucuronic acid
- in ER and cytoplasm
- Clonazepam (acetylation); Isoniazid (acetylation)
- Glucuronidation is the most common conjugation reaction
EFFECTS OF DRUG METABOLISM
Increased polarity (increased renal elimination)
- highly polar drugs are poorly absorbed, poorly transported across membranes, readily excreted
- the higher the lipid solubility and lower polarity, the higher the half-life of the drug
clinical correlation
Question: If the drug is highly lipid soluble and highly localized, usually the estimated half life is 100 days but you cannot give the drug every 100 days because you need to maintain a steady state plasma concentration. What will you do?
Answer: You should increase the dose (double the dose). For example, you are administering a 500mg drug, give a loading dose, you double the dosage, make it 1000mg. After which, you give maintenance doses
