LSM3211 CA2 preparation

Plasma half-life of Histamine (T_1/2)

5 to 10 mins

Why is histamine broken down so fast?

Broken down via HNMT (major metabolic pathway) and DAO

What are the major histamine producing cells?

Mast cells in tissue and basophils in circulating

How is histamine released?

Granules in mast cells/basophils contain histamine, a preformed mediator that upon degranulation, is quickly released without additional synthetic processes

What are histamine receptors?

GPCR

H1 receptors

responsible for acute allergic reaction

G protein of H1 receptors

G alpha q

Effector of H1 receptors

Phospholipase Cbeta which generates DAG and IP

What does DAG do?

DAG activates PKC, which is responsible for activating PLA2 and producing arachidonic acids and eicosanoids

Chlorpheniramine

alkylamine, 1st gen antihistamine

Diphenhydramine

ethanolamine, 1st gen antihistamine

Cetirizine

piperazine, 2nd gen antihistamine

Fexofenadine

piperidine, 2nd gen antihistamine

What are H1-antihistamines?

inverse agonists that stabilize H1-receptors in its inactive form

What is an inverse agonist?

a drug that binds to the same receptor as an agonist, but induces a pharmacological response opposite to that of the agonist

Clinical uses of 2nd gen antihistamines

preferred choice for mild-moderate allergies

Clinical uses of 1st gen antihistamines

for non-allergic conditions such as motion sickness and vertigo, nausea and vomiting, insomnia and peri-operation (all usually diphenhydramine)

What is the main difference between 1st gen and 2nd gen antihistamines?

1st gen: sedating, low H1-receptor selectivity, longer onset of action and shorter duration

2nd gen: non-sedating, high H1-receptor specificity, shorter onset of action and longer duration

Contraindications for 1st gen antihistamines

patients with glaucoma or prostatic hyperplasia

should be avoided for pilots, drivers and any machinery operators that require psychomotor skills. Fexofenadine is recommended instead.

What is pharmacogenomics?

Study of how variation in multiple genes affect individuals’ response to drug

What is pharmacogenetics?

Study of how variation in single gene affect individuals’ response to drug

Type of Genetic Variations

Single Nucleotide Polymorphism

Deletion or Insertion

Variable Number Tandem Repeat

Copy Number Variation

What is SNP? (Single Nucleotide Polymorphism)

a single base change, leading to differencing behaviour in phenotype

What is deletion/insertion?

addition or loss of nucleotides

What is VNTR? (Variable Number Tandem Repeat)

Short sequence repeats

What is CNV? (Copy Number Variation)

Duplications or deletions of large DNA segments

What are Ultra-Rapid Metabolizers (UM)?

fast clearance, reduced efficacy of drug

higher dose may be needed

What are Extensive Metabolizers (EM)?

normal metabolism, standard dose

What are Intermediate Metabolizers (IM)?

slower metabolism, dose adjustment may be required

What are Poor Metabolizers (PM)?

slowest clearance, risk of toxicity

lower dose needed

What are the 4 metabolic phenotypes?

Ultra-rapid

Extensive

Intermediate

Poor

What is warfarin?

Vitamin K antagonist that acts as an oral anticoagulant, reducing the formation of blood clots

What is warfarin used for?

used to treat or prevent blood clots in veins or arteries, reducing the risk of stroke or heart attack

What are some challenges with warfarin?

Narrow therapeutic index, small changes in plasma levels can cause bleeding or clot formation

Requires individualized dosing based on international normalized ratio (INR)

MOA of warfarin

antagonizes the vitamin K-dependent clotting pathway, affecting both intrinsic and extrinsic pathways of the clotting cascade

Target of warfarin

Vitamin K epoxide reductase (VKOR)

R vs S enantiomers of Warfarin

S isomer is more potent than R

Challenges of Warfarin

Narrow therapeutic index, requires individualized dosing that is based on International Normalized Ratio (INR)

CYP2C9*1 gene (Wild-type)

Normal metabolism of warfarin

CYP2C9*2 and CYP2C9*3

Reduced function alleles, poor metabolizers of warfarin leading to higher bleeding risk

VKORC1 gene

produces VKOR enzyme which is warfarin’s target (Note: G - wild type, A - non-functional)

VKORC1 AA genotype

low VKOR enzyme expression, causing increased sensitivity to warfarin, lower dose required

VKORC1 AG genotyp

intermediate VKOR enzyme expression, moderate sensitivity to warfarin, requires standard dose

VKORC1 GG genotype

high VKOR enzyme expression, warfarin resistant, requiring higher dose

Ethnicity on Warfarin dose/response

Asians - lower doses, increased warfarin sensitivity

Caucasions - moderate dose, with some needing adjustments

African Americans - higher doses due to lower prevalence of CYP2C9 and VKORC1 variants

Effect of diet on warfarin

Consuming large amounts of vitamin K = higher dose of warfarin needed

Drug-drug interactions of warfarin

Fluconazole is a strong CYP2C9 inhibitor —> decrease warfarin metabolism

Medical conditions affecting warfarin dose/response

Liver disease - impair warfarin metabolism, increase bleeding risk

Heart failure - reduce warfarin clearance, requiring dose adjustments

MOA of Gefitinib

binds to the ATP cleft of the EGFR tyrosine kinase, blocking signaling in tumors

L858R Mutation (Exon 1)

Increases gefitinib sensitivity, improving progression free-survival (efficacy)

T790M Mutation (Exon 20)

Results in acquired resistance, decreasing gefitinib efficacy (occurs in 50% of resistance cases)

MOA of Irinotecan (prodrug)

inhibit topoisomerase 1, preventing relaxation of supercoiled DNA and blocking cell growth (used in cancer therapy)

Phase I metabolism of Irinotecan

Prodrug Irinotecan is converted in the liver by carboxylesterase into active SN-38

Phase II metabolism of Irinotecan

UDP-glucuronyltransferase (UGT) inactivates SN-38 to its SN-38G form

UDP-glucuronyltransferase (UGT)

transforms small lipophilic molecules, such as steroids, bilirubin, hormones and drugs into water-soluble excretable metabolites by conjugating with glucuronic acid

UGT1A1*28

promoter polymorphism, decreases activity and strongly associated with toxicity due to longer TATA box, usually in Caucasians

UGT1A1*6

associated with decreased UGT activity, usually in Asians

Sympathetic Division

in the thoracic and lumbar parts of spinal cord, for fight or flight response

Parasympathetic Division

in cranial and sacral parts of the spinal cord, for regulation of body functions during rest/digestion/waste eliminination

Preganglionic neurons

Cholinergic, ganglionic transmission occurs via niconitic ACh receptors

Postganglionic sympathetic neurons

mainly noradrenergic except for sweat glands

Postganglionic parasympathetic neurons

cholinergic, acting on muscarinic receptors

Parasympathetic activity

increases activity of GI and Urinary systems, decrease CVS

Synthesis of ACh

synthesized from choline and acetyl-CoA via choline transferase

Hemicholinium

inhibits choline uptake into the nerve terminal

Transport of ACh

transported into vesicles for storage by vesicular acetylcholine transporter (VAChT)

Vesamicol

inhibits VAChT, causing reduced acetylcholine loaded in synaptic vesicles and released

ACh release

presynaptic membrane depolarization —> opens Ca2+ channels, causing synaptic vesicle membranes to fuse with presynaptic membrane —> exocytosis of ACh

Botulinum toxin

prevent ACh release

Termination of Action of ACh

via acetylcholinesterase that converts ACh to choline + acetate

Acetylcholinesterase

2 distinct types: acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE)

Differs in distribution, substrate specificity and functions

AChE has high activity at synapse, usually in neurons. —> prevent overstimulation

Butyrylcholinesterase (BChE)

high activity in the liver, skin, brain, muscle and blood

Reversible AChE inhibitors

edrophonium, physostigmine, neostigmine

short lived action, used in treatments

Irreversible AChE

malathion and parathion

Pralidoxime

MOA: reactivate cholinesterase which has been inactivated by phosphorylation due to an organophosphate pesticide (malathion and parathion)

Phosphorylation of AChE

leads to irreversible inhibition, increasing cholinergic activity

Cholinergic Receptors

2 types: muscarinic and nicotinic

Acetylcholine

High affinity for cholinergic receptors

Cholinomimetics

consist of 2 subgroups:

1. Direct Acting —> at acetylcholine receptor namely (i.e muscarinic and nicotinic)

   1. Indirect Acting —> inhibition of cholinesterase and prolong ACh action

Anti-cholinergic drugs

consist of 3 subclasses:

1. Block muscarinic receptors

2. Block nicotinic receptors

3. Cholinesterase regenerators (not receptor blockers but are chemical antagonists of organophosphate acetylcholinesterase inhibitors)

Muscarinic receptors

G-protein coupled receptors, respond to muscarine and ACh

Signal Transduction Pathways used by Muscarinic Receptors

• Phosphatidylinositol pathway (M1 and M3)

• cAMP pathway (M2)

M1 receptors

mainly in brain, promotes cognitive activity and controls vomiting centre

also in stomach, influences acid production

M2 receptors

primarily in the heart, decrease cardiac activity

M3 receptors

important in smooth muscle contractions and increasing glandular secretions

Effects of Muscarinic Agonists

• Brain (M1)

• Cardiovascular effects (M2) - decrease in cardiac output

• Smooth muscle (M3) - increase in peristaltic activity of GI tract, contraction of bladder detrusor muscles (in urinary retention) and bronchial smooth muscles

• Sweating, lacrimation, salivation, bronchial secretion (mostly M3, M1 to a lesser extent)

  • Effects on the eye - adjust to changes in light intensity and intraocular pressure through smooth muscle contraction, tear production

Glaucoma

increased intraocular pressure, drainage of aqueous humour becomes impeded when pupil dilated

activation of constrictor muscle by muscarinic agonist lowers the intraocular pressure by improving drainage

Side Effects of Non-Selective Muscarinic agonist (BETHANECHOL)

• Blurred vision

• Increased lacrimation - tearing

• Increased salivation

• Bronchial constriction

• Hypotension

• Abdominal cramping

• increased gastric motility

• Diarrhea

• Urinary urgency

Non-selective Muscarinic Agonists

Pilocarpine, Bethanechol

Pilocarpine

used in acute glaucoma treatment, given alongside physostigmine which is a reversible cholinesterase inhibitor

Bethanechol

for urinary retention and gastrointestinal hypotonia, longer duration of action than ACh bc it is not broken down by cholinesterase

Non-selective Muscarinic Antagonist

Atropine

Atropine

used to dilate pupil/treatment of organophosphate poisoning, given with Pralidoxime to manage side effects

Relives SLUDGE/BBB syndrome (symptoms of excessive cholinergic activity)

Side Effects of Muscarinic Antagonists (ATROPINE)

• Heart rate - tachycardia (inhibits existing parasympathetic tone)

• Smooth muscle relaxation

• GI tract - inhibited at larger doses

• Inhibition of secretions - salivary, lacrimal, bronchial and sweat glands are inhibited, causing dry mouth/skin

• Eye - pupil dilated, unresponsive to light, may increase intraocular pressure

• CNS - mild restlessness, higher doses can cause agitation and disorientation. (CNS depression in toxic doses), chronic use may lead to cognitive decline

SLUDGE/BBB syndrome

S - salivation/excessive sweating

L - lacrimation

U - urination

D - diarrhea

GI - gi upset

E - emesis

BBB - bronchospasm, bronchorrhea, bradycardia

Nicotinic Receptors

Ligand gated ion channels, 2 major subtypes:

1. Muscle

2. Ganglion (for transmission at sympathetic and parasympathetic ganglia)

Drugs that target nicotinic receptors:

1. Ganglion stimulating drugs

2. Neuromuscular blocking drugs

Nicotine

ganglion stimulating drug

Neuromuscular-blocking drugs

Act by:
i) Presynaptically - inhibit ACh synthesis/release

ii) Postsynaptically - endplate of neuromuscular junction (NMJ)

Non-depolarizing competitive blockers

MOA: Block ACh receptors without depolarizing the motor end plate

Act as competitive agonists, compete with ACh at the receptor without stimulating it

By preventing depolarization of muscle cell membrane —> inhibit muscular contraction

At low doses, can overcome by administeration of cholinesterase inhibitors that increase conc of ACh (e.g neostigmine and edriphonium)