Drug Metabolism Flashcards
Routes of drug administration
Depends on drug and target area (tissue):
- Oral (most favourable for pharm. companies, convenient and can be self administered)
- Sublingual (via tissues underneath tongue) e.g. glyceryl trinitrate “angina spray”
- Rectal e.g. diazepam (useful when oral/injection isn’t an option)
- Other epithelial surfaces: skin (analgesics, antibiotics, acne cream), cornea, vagina, nasal mucosa
- Injection e.g. insulin
- Inhalation e.g. salbutamol (oral inhalation) to treat asthma
Topical administration definition
On to the skin
Systemic administration definition and subtypes
Into the body
2 types:
- Enteral (oral, rectal, sublingual): GI-tract route e.g. tablets or capsules
- Parenteral (injection, IV, inhalation, cutaneous, application to other epithelial surfaces): Non-GI tract route e.g. inhalers or injections
Oral administration
- Most common form of drug administration
- Tablets/capsules easy for patients to take
- Rate of absorbance can be altered by manipulation of the formation e.g. “enteric-coated” tablets allow slow release of drug
- Require GI absorption
- Sites of absorption are: Stomach, small intestine (most important) and large intestine (colon)
Rectal administration
- Useful for drugs required to produce local effect e.g. ulcerative colitis
- Useful for patients who:
- Are unable to take medication orally either post-operatively or due to vomiting
- Can’t be administered via IV e.g. diazepam used due to status epilepticus (seizures)
Sublingual administration
- Absorption directly from oral cavity
- Useful when rapid response required e.g. angina attack
- Good for drugs which are unstable at a gastric pH or rapidly metabolised by liver
Administration by injection
- Types inc. subcutaneous, intramuscular (e.g. upper arm/buttock), intradermal, intravenous, intra-arterial, intrathecal (delivers drug directly to spinal cord), intraperitoneal (delivers drug to peritoneal cavity or area containing abdominal organs)
- Drug usually absorbed faster via injection than orally
- Absorption rate depends greatly on diffusion through local tissue and removal by local blood flow (faster blood flow, faster absorption)
“Bolus” injection
- An IV injection that has fastest and most certain route
- Rapidly produces high conc. in the right heart, pulmonary vessel and systemic circulation
- Good for administering morphine or adrenaline
- Peak conc. dependent on rate of injection
IV infusion and uses
- Steady IV infusion avoids high peak systemic concentrations and uncertainty over absorption e.g. antibiotics
- Provides most complete drug availability with minimal delay
- Commonly used for chemotherapies, antibiotics and pain relief medications
Adminstration by inhalation
- Drugs for local lung effects e.g. bronchodilators (salbutamol)
- Achieve high local drug concentrations
- Minimise systemic effects
- Administration of volatile and gaseous anaesthetics e.g. nitrous oxide
- Large surface area and blood flow of lungs allow rapid changes of systemic conc.
Cutaneous administration
- Used when local effect on skin required e.g. topical steroid creams
- Significant absorption can occur leading to systemic effects
- Exploited therapeutically e.g. rub-on gels containing non-steroidal anti-inflammatoires (“Voltarol”) and transdermal patches
Application to other epithelial surfaces
Nasal sprays
- Allergies (e.g. Hay Fever)
Eye drops
- Used for localised eye treatment
- No side effects associated with systemic exposure
Small intestine
- Main site for absorption of most drugs
- Large surface area due to microvilli (approx, 200 m^2 compared to 1 m^2 in stomach)
- High blood flow: approx. 1 litre/min (in the stomach only 0.15 litre/min)
- Bile helps solubilize some drugs
Mechanisms of absorption
2 types: Transcellular (goes through cells) and paracellular (goes between cells)
Transcellular:
- Passive diffusion: Non-polar chemicals pass passively through cell membrane down conc. gradient
- Facilitated diffusion: Polar chemicals pass via channel protein down conc. gradient
- Active transport: Polar chemicals pass against conc. gradient, requires ATP
Paracellular:
- Drug passes through gaps between cells
- When cells damaged, gaps bigger and absorption increased
Factors that can influence GI absorption (5)
Typically, 75% of a drug given orally is absorbed in 1-3 hours. Factors that can alter this are:
- Physicochemical (water solubility, lipophilicity, ionisation)
- Formulation (from best to worst: solution > emulsion > suspension > capsule > tablet)
- Biological (gut content, gut pH, gut motility, blood flow, bile flow, malabsorption states, gut flora/microorganisms in gut)
- Interaction with food (e.g. chelation of tetracycline with calcium/milk)
- Drug-drug interaction (DDIs with: anticholinergics affect stomach emptying time, laxatives affect motility, cardiovascular drugs decrease blood circulation, antacids and ion exchange resins affect adsorption e.g. cholestyramine charcoal adsorption for treatment of poisoning)
Factors affecting passage of drugs through cell membranes
- Water solubility
- Lipid solubility
- Degree of ionisation
- Molecular weight
- Active transport
- Free drug (unbound) concentration gradient
Partition coefficient
Determines conc. of drug in organic solvent.
Partition coefficient (P) = [conc. in organic solvent]/[conc. in aqueous phase]
Log P indicates lipophicility of drug. If log P > 0, drug rapidly absorbed via transcellular route. If log P < 0, drug slowly absorbed via paracellular route
Ionisation of weak organic acids and bases
- Many drugs either weak acids or bases, existing in both ionised and unionised forms
- Ratio of the 2 forms varies with pH
- Membranes are impermeable to the ionised form of drug
- Henderson-Hasselbach equation defines dissociation constant (pKa) for weak acids and bases. pKa is the pH of an acid/base when it is 50% dissociated.
- Weak acids: pH = pKa + log([A-]/[HA])
- Weak bases: pH = pKa + log([B]/[BH+])
pH partition theory
- Ionisation affects steady state distribution of drug molecules between 2 aqueous compartments if pH difference exists
- Weak acids accumulate in high pH compartments, weak bases in relatively low pH
pH partition hypothesis (Acids)
E.g. Aspirin (pKa 3.5)
- Ionisation greatest at alkaline pH (Urine with pH 8)
pH partition hypothesis (Bases)
E.g. Pethidine (pKa 8.6)
- Ionisation greatest at acid pH (Gastric juice with pH 3)
- Always given by injection rather than oral dose as it would not be able to be absorbed through GI tract in ionised form in stomach
Binding of drugs to proteins
- Drug can bind to protein to form complex which stays in plasma (isn’t absorbed into target tissue/organ)
- Only unbound drug pharmacologically active
- In therapeutic drug concentrations, most of drug in “bound” form due to high quantity of proteins in plasma (only small amount of drug gets into target tissue)
- 3 main proteins that bind to drugs in plasma:
- Albumin (bind to acidic drugs)
- Beta-globulin (bind to some basic drugs)
- Acid glycoprotein (bind to some basic drugs)
- Amount of binding depends on drug e.g. diazepam 99% bound, phenylbutazone 60% bound, theophylline 15% bound
- Binding results in: reduced excretion, reduced pharmacological effect, potetnial displacement of other drugs already bound
- Drug-protein binding affinity detemrine preferred administration route e.g. for high affinity drug:
- Treatment by infusion (low continuous dose, most drug bound to plasma, no effect/poor patient response)
- Treatment by IV injection (high single dose, drug can diffuse out of plasma before protein binding, good patient response)
Interaction with body fat
- Many drugs designed to be lipophilic (prefer lipid environment to dissolve) to help diffusion across membranes
- Also means they’re soluble in other fat sources e.g. adipose tissue
- Drug accumulation (potentially to toxic levels)
- Can be ‘trapped’ in adipose tissue for years
- Amount of adipose tissue varies from person to person (3-4% to >45%)
- Important for setting dosing regime (mg per kilogram)
What is drug metabolism?
- Enzyme-catalysed conversion of drug into chemically-distinct product (metabolite)
- Most drugs lipophilic/non-polar to get into target tissue
- Must be made more hydrophilic/polar
- Allows easier excretion (urine/faeces)
- Liver is main site of drug metabolism (lungs, skin, kidneys also have metabolic capacity)
- Natural reactions within body to remove chemicals
Drug metabolism phases
- Split into two phases: I and II
- Phase I reaction introduces functional group to drug molecule, prepares molecule for phase II reaction (non-polar to polar)
- Phase II reaction normally adds large or charged molecule to drug, increases water solubility and excretion of drug
Chemical processes of phase I and II
Phase I:
- Oxidation
- Reduction
- Hydrolysis
Functionalisation:
- Addition of reactive group
- Unmasking of reacting group
- Consequences: small decrease in lipophicility, slight increase in excretion, alter pharmacological effect
Phase II:
- Glucuronidation
- Sulphation
- Acetylation
- Amino acid conjugation
- Glutathione conjugation
Conjugation:
- Addition of large group (often charged)
- Consequences: large decrease in lipophicility, increase in excretion, usually decreases pharmacological effect (not always the case)
Examples of phase I reactions
- Cytochrome P450 oxidation
- Non-P450 oxidation
- Reduction
- Hydrolysis
Examples of Cytochrome P450-mediated oxidation reactions
Aliphatic hydroxylation
Aromatic hydroxylation
Epoxidation
N-Dealkylation
O-Dealkylation
S-Dealkylation
Oxidative deamination
N-Oxidation
S-Oxidation
Alcohol oxidation
Dehydrogenation
Dehalogenation
Cytochrome P450
- Over 1200 individual P450 enzymes described in animals, plants, yeast and bacteria (very few organisms have no P450 e.g. E.coli)
- P450 is the terminal oxidase component of electron transfer system in the smooth endoplasmic reticulum (SER)
- Absolute requirement for molecular oxygen, co-factor NADPH, and cytochrome P450 reductase
Cytochrome P450 enzymes (CYP enzymes)
- Quantitatively most important enzymes involved in drug metabolism
- Superfamily of enzymes with overlapping substrate specificities (also involved in steroid biosynthesis as well as vitamin A/D metabolism)
- Have characteristic absorption max. at 450 nm in presence of carbon monoxide (CO)
- Apoprotein varies, can be classified according to amino acid sequence
- All P450s contain prosthetic group ferriprotoporphyrin IX
CYP enzyme cycle
- Substrate binds e.g. aromatic hydrocarbon benzene
- Oxygen associates with Fe atom of prosthetic group
- An oxygen atom forms water with 2 H+
- 2nd oxygen atom reacts with substrate to form oxidised product
- Substrate leaves active site
Human hepatic P450 enzymes
- 57 genes encoding P450 enzyme isoforms identified in humans
- Divided into 18 families
- Many have role in synthesis of endogenous (internal origin) compounds inc. steroids/cholesterol and vitamin D
- Number of key P450 isoforms are responsible for majority of hepatic drug metabolism
Aliphatic hydroxylation
- R-CH3 –> RCH2OH (addition of hydroxyl group)
- E.g. Oxidation of tolbutamide (hypoglycaemic agent used to treat diabetes)
Aromatic hydroxylation
- Benzene (C6H6) –> Phenol (C6H5OH)
- E.g. Oxidation of salicylic acid (treatment for psoriasis; also analgesic) into gentisic acid
Epoxidation
- Benzene (C6H6) –> Arene oxide (C6H6O)
- E.g. Oxidation of carbamazepine (anticonvulsant used to treat epilepsy) to carbamazepine epoxide
O-dealkylation
- ROCH3 –> ROH + HCHO (aldehyde)
- E.g. Oxidation of phenacetin (analgesic related to paracetamol previously used to treat pain) to paracetamol and acetaldehyde
N-dealkylation
- R2NCH3 (tertiary amine) –> R2NH (secondary amine) + HCHO
- E.g. Oxidation of diazepam (treatment for anxiety disorders and alcohol withdrawal symptoms) to nordiazepam and formaldehyde
Deamination
- RCH(CH3)NH2 –> RC(=O)CH3 (ketone) + NH3
- E.g. Oxidation of amphetamine (indirectly acting sympathomimetic) to inactive metabolite and ammonia
N-oxidation
- R2NH –> R2NOH (hydroxylamine, toxic)
- E.g. Oxidation of clozapine (antipsychotic used in treatment of schizophrenia) to clozapine N-oxide
S-oxidation
- R2S –> R2SO (sulpoxide) –> R2SO2 (sulphone)
- Oxygens held by dative bonds
Alcohol oxidation
- CH3CH2OH –> CH3C(=O)H (acetaldehyde) + H2O
Dehydrogenation
- Paracetamol –> N-acetyl-p-benzoquinoneimine + H2O
Dehalogenation (quite rare)
- F3C-CH(Cl)Br (halothane) –> F3C-C(=O)OH
- Halothane is a gaseous anaesthetic
Examples of non-P450 mediated oxidation reactions and their enzymes
Alcohol oxidation (Alcohol dehydrogenase)
N-oxidation (Flavin monooxygenase, FMO)
S-oxidation (same as above)
Oxidative deamination (Monoamine oxidase, MAO)
Aldehyde oxidation (Aldehyde oxidase)
Flavin monooxygenase (FMO)
- Found alongside P450s in liver microsomes (SER)
- Mediates N- and S-oxidation reactions
- Needs NADPH as co-factor
- Many metabolites generated by FMO also arise from P450 reaction (often difficult to distinguish between actions of the two)
- E.g. nicotine –> nicotine-1-oxide
Monoamine oxidase (MAO)
2 types:
- MAO-A (found in liver, pulmonary vascular endothelial, GI tract and placenta)
- MAO-B (found in blood platelets)
- Both found in neurons and astroglia (type of neural cell)
- Vital role in inactivation of endogenous neurotransmitters
- Inhibitors of MAOs inc. antidepressants and caffeine
- E.g. (For MAO-A) sumatripan (used to treat migraines) –> indole-acetic acid derivative (active)
Aldehyde oxidase (AO)
- Located in cytosolic compartment of tissues in many organisms
- Catalyses oxidation of aldehydes into carb. acids
- Catalyses hydroxylation of some heterocycles
- Catalyses oxidation of both CYP450 and MAO intermediate products
- E.g. benzaldehyde –> benzoic acid + hydrogen peroxide
Xanthine oxidation
- Catalyses oxidation of hypoxanthine to xanthine to uric acid
- Uric acid –> gout
- In rare examples, known to catalyse xenobiotic (substance foreign to body) metabolism reactions
- E.g. 6-mercatopurine (antitumour drug) –> 6-thioxanthine –> 6-thiouric acid (inactive)
Reduction
- “Opposite” of oxidation
– i.e. the removal of oxygen or addition of hydrogen - As with oxidation it creates polar functional groups which are more readily conjugated and eliminated.
- Much less common than oxidation reactions but still important.
- Can be catalysed by Cytochrome P450s
- Other reductases are implicated
Cytochrome P450 reductase (POR)
Phase II reactions
- Usually catalysed by a transferase enzyme that
transfers a polar group from a donor or
conjugating agent to the phase I metabolite. - Phase II metabolites are generally more polar
and can be excreted more easily. - Exceptions are Acetylation and Methylation, in
which polar groups such as OH or NH2 groups
are masked.
Examples of toxic phase I metabolites
- Epoxides
- Hydroxylamines
- Quinoneimines
- Free radicals
