Pharmacokinetics and Autonomic Drugs Flashcards
what is bioavailability?
- bioavailability (F) = % of administered drug reaching systemic circulation unchanged
when is bioavailability usually 100%?
F=100% for an IV dose
why is bioavailability usually less than 100% for an oral drug?
due to incomplete absorption and first pass metabolism
what is volume of distribution (Vd)?
- amount of drug in the body / [drug] in the plasma
compartment and drug types when low volume of distribution
- compartment: blood
- drug types: large/charged molecules; plasma protein bound
compartment and drug types when medium volume of distribution
- compartment: ECF
- drug type: small hydrophilic molecules
compartment and drug types when high volume of distribution
- compartment: all tissues including fat
- drug type: small lipophilic mcs, especially if bound to tissue protein
what is the equation for half life for a first order elimination?
t1/2=0.693 x Vd / CL
what is clearance (CL)?
- volume of plasma cleared of drug per unit time
- may be impaired with defects in cardiac, hepatic, or renal function
- CL=(rate of elimination of drug / plasma drug concentration) = Vd x Ke
- Ke= elimination constant
what is loading dose?
loading dose = Cp x Vd / F
Cp = target plasma concentration at steady state
what is maintenance dose?
maintenance dose = Cp x CL x t / F
Cp = target plasma concentration at steady state
t = dosage interval (time b/w doses), if not administered continuously
maintenance dose and loading dose in renal or liver disease
- maintenance dose decreases
- loading dose is usually unchanged
what does the time to steady state depend on?
- half life
- independent of dose and dosing frequency
what is the difference between zero order elimination and first order elimination?
- zero order–rate of elimination is constant regardless of Cp
- constant amount of drug eliminated per unit time
- Cp dec linearly with time
- first order–rate of elimination is directly proportional to drug concentration
- constant fraction of drug elminated per unit time
- Cp dec exponentially with time
- applies to most drugs
which drugs are zero order?
-
PEA–a pea is round shaped like the “0” in zero order
- Phenytoin
- Ethanol
- Aspirin
what are weak acid drugs?
where do they get trapped?
how do you treat overdose?
- phenobarbital, methotrexate, aspirin
- trapped in basic environments
- treat overdose with bicarbonate
what are weak base drugs?
where do they get trapped?
how do you treat overdose?
- amphetamines
- trapped in acidic environments
- treat overdose with ammonium chloride
what are phase 1 types of drug metabolism?
what enzyme do they require?
what do they yield?
what occurs in geriatric patients?
- reduction, oxidation, hydrolysis
- require cytochrome P 450
- yield slightly polar, water soluble metabolites
- geratric patients lose phase I first
what are phase 2 type of drug metabolism?
what do they yield?
- conjugation–Methylation, Glucuronidation, Acetylation, Sulfation
- geriatric patients have More GAS (phase 2)
- yields very polar, inactive metabolites (renally excreted)
what occurs in patients who are slow acetylators?
- have increased side effects from certain drugs b/c of decreased rate of metabolism
effect of competitive antagonist on receptor binding
- shifts curve right (decreased potency)
- no change in efficacy
- can be overcome by increasing the concentration of agonist substrate
effect of noncompetitive antagonist on receptor binding
- shift curve down (decrease efficacy)
- cannot be overcome by inc agonist substrate concentration
effect of partial agonist (alone) on receptor binding
- acts at same site as full agonist, but with lower maximal effect (dec efficacy)
- potency is an independent variable
what is the therapeutic index?
- measurement of drug safety
-
Therapeutic Index = TD50/ED50 = median toxic dose / median effective dose
- TITE
- safer drugs have higher TI values
- drugs with lower TI values frequently require monitoring
- therapeutic window–dosage range that can safely and effectively treat dz
alpha 1 receptor:
type of receptor
G protein class
major functions
- sympathetic
- q
- functions:
- inc vascular smooth muscle contraction
- inc pupillary dilator muscle contraction (mydriasis)
- inc intestinal and bladder sphincter muscle contraction
alpha 2 receptor:
type of receptor
G protein class
major functions
- sympathetic
- i
- functions:
- dec sympathetic (adrenergic) outflow
- decrease insulin release
- dec lipolysis
- inc platelet aggregation
- dec aqueous humor product
beta 1 receptor
type of receptor
G protein class
major functions
- sympathetic
- s
- functions:
- inc heart rate
- inc contractility
- inc renin release
- inc lipolysis
beta 2 receptor:
type of receptor
G protein class
major functions
- sympathetic
- s
- functions:
- vasodilation
- bronchodilation
- inc lipolysis
- inc insulin release
- dec uterine tone (tocolysis)
- ciliary muscle relaxation
- inc aqeuous humor production
beta 3 receptor:
type of receptor
G protein class
major functions
- sympathetic
- s
- functions:
- inc lipoysis
- inc thermogenesis in skeletal muscle
M1 receptor
type of receptor
G protein class
major functions
- parasympathetic
- q
- functions:
- CNS
- enteric nervous system
M2 receptor
type of receptor
G protein class
major functions
- parasympathetic
- i
- functions:
- dec heart rate
- dec contractility of atria
M3 receptor
type of receptor
G protein class
major functions
- parasympathetic
- q
- functions:
- inc exocrine gland secretions
- lacrimal, sweat, salivary, gastric acid
- inc gut peristalsis
- inc bladder contraction
- bronchoconstriction
- inc pupillary sphincter muscle contraction (miosis)
- ciliary muscle contraction (accommodation)
- inc exocrine gland secretions
D1 receptor
type of receptor
G protein class
major functions
- dopamine
- s
- functions:
- relaxes vascular smooth muscle
D2 receptor:
type of receptor
G protein class
major functions
- dopamine
- i
- functions:
- modulate transmitter release, especially in brain
H1 receptor:
type of receptor
G protein class
major functions
- histamine
- q
- functions:
- inc nasal and bronchial mucus production
- inc vascular permeability
- contraction of bronchioles
- pruritus
- pain
H2 receptor:
type of receptor
G protein class
major functions
- histamine
- s
- functions:
- increase gastric secretion
V1 receptor:
type of receptor
G protein class
major functions
- vasopressin
- q
- functions:
- inc vascular smooth muscle contraction
V2 receptor:
type of receptor
G protein class
major functions
- vasopressin
- s
- functions:
- inc water permeability and reabsorption in collecting tubules of kidney
- “V<strong>2</strong> is found in 2 kidneys”
- inc water permeability and reabsorption in collecting tubules of kidney
how to remember G protein linked 2nd messengers
“After qisses (kisses), you get a qiq (kick) out of siq (sick) sqs (super kinky sex).
tyramine–mechanism
- normally degraded by monoamine oxidase (MAO)
- levels inc in patients taking MAO inhibitors who ingest tyramine rich foods (ie. cheese, wine)
- excess tyramine enters presynaptic vesicles and displaces other neurotransmitters (ie. NE) –> inc active presynaptic neurotransmitters –> inc diffusion of neurotransmitters into synaptic cleft –> inc sympathetic stimulation
what does tyramine classically cause?
hypertensive crisis
name the 4 direct cholinomemetic agonists
- bethanechol
- carbachol
- methacholine
- pilocarpine
bethanechol–mechanism
- activates bowel and bladder smooth muscle
- resistant to AChE
- “Bethany call (bethanchol) me to activate your bowels and bladder.”
bethanechol–use
- postoperative ileus
- neurogenic ileus
- urinary retention
carbachol–mechanism
- carbon copy of acetylcholine
carbachol–use
- constricts pupil and relieves intraocular pressure in open angle glaucoma
methacholine–mechanism
- stimulates muscarinic receptors in airway when inhaled
methacholine–use
- challenge test for diagnosis of asthma
pilocarpine–mechanism
- contracts ciliary muscle of eye–open angle glaucoma
- pupillary sphincter–closed angle glucoma
- resistant to AChE
- “You cry, drool, and sweat on your pilow”
pilocarpine–use
- potent stimulator of sweat, tears, saliva
- open angle and closed angle glaucoma
- xerostomia–Sjorgren syndrome
name the 7 indirect cholinomimetic agents (anticholinesterases)
- donepezil
- galantamine
- rivastigmine
- edrophonium
- neostigmine
- physostigmine
- pyridostigmine
donepezil, galantamine, rivastigmine–mechanism
- inc ACh
donepezil, galantamine, rivastigmine–use
- Alzheimer disease
edrophonium–mechanism
- inc ACh
edrophonium–use
- historically, diagnosis of myasthenia gravis–extremely short acting
- myasthenia now diagnosed by anti AChR Ab (anti acetylcholine receptor antibody) test
neostigmine–mechansm
- inc ACh
- “Neo CNS = No CNS penetration (quaternary amine)
neostigmine–use
- postoperative and neurogenic ileus and urinary retention
- myasthenia gravis
- reversal of neuromuscular junction blockade–postoperative
physostigmine–mechanism
- inc ACh
- “Physostigmine ‘phyxes’ atropine overdose”
physostigmine–use
- anticholinergic toxicity
- crosses blood brain barrier –> CNS (tertiary amine)
pyridostigmine–mechanism
- inc ACh
- inc muscle strength
pyridostigmine–use
- myasthenia gravis (long acting)
- “Pyridostigmine gets rid of myasthenia gravis”
- does not penetrate CNS–quaternary amine
what is important to watch for when administering any cholinomimetic agents?
- exacerbation of COPD
- asthma
- peptic ulcers
what causes cholinesterase inhibitor poisoning?
- often due to organophosphates, such as parathion, that irreversibly inhibit AChE
in which population is cholinesterase inhibitor poisoning usually seen and why?
- farmers b/c organophosphates are often components of insecticides