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How do Drugs molecules move through the body?
►Bulk flow: Via the vascular, lymphatic and spinal fluids
►Diffusion: Areas of high concentration to areas of low concentration
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What are 3 ways drugs transfer across biological membranes?
- -Passive diffusion
- -Carrier transport
- -Pinocytosis- plasma membrane buds off and creates vesicle
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describe passive diffusion
- Drugs transported down their concentration gradient
- ►Law of mass action
- ►Non-saturable
- Lipid-soluble drugs pass directly across plasma membrane.
- Some ionic drugs can move through protein channels in the membrane
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ATP-binding cassette (ABC) transporters:
- ►"Primary active transporters”
- ►Use energy from ATP hydrolysis
- ►Can pump molecules against their concentration gradient
- ►Saturable
- ►Exhibit varying substrate selectivity
- ►ex: P-glycoprotein
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Solute linked carriers:
- ►Some use energy of passively diffusing solutes to pump drugs against their concentration gradients
- ►“Secondary active transport”
- ►e.g. organic cation/anions transporters ►Some facilitate diffusion of drugs down concentration gradient (no energy required)►“facilitated transport”
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Drug effectiveness in part dependent on:
- ►affinity for target receptor (agonist or antagonist)
- ►And possibly efficacy at target receptor (agonist)
- ►drug’s ability to get to target receptor from site of administration
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To be absorbed and distributed to target tissue, drug must first:
- ►be released from its formula
- ►be water soluble (Either in the free form, or through binding to water-soluble plasma proteins)
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Drug formulation: give examples of solids, semi-solids, polymers, and liquids?
- Solids
- ►Tablets
- ►Capsules
- ►Powders
- ►Suppositories
- Semi-solids
- ►Ointments
- ►Creams
- ►Pastes
- Liquids
- ►Suspensions
- ►Solutions
- Polymers
- ►Transdermal patches
- ►Drug-eluting stents
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what is drug chemistry and what does it depend on?
Determines solubility in biological fluids and ability to penetrate biological membranes.
Depends on:
Size and shape
- Ionization state and charge
- ►Many drugs are weak acids or weak bases
- ►Exist in both ionized and unionized states depending on pH
- Lipid-water partition coefficient
- ►The higher the coefficient the more easily the drug will pass across biological membranes
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The relative concentrations of the ionized and unionized forms of a weak acid are given by what equilibrium?
- HA <-> H+ + A-
- HA= nonionized drug
- H+= proton
- A-= ionized drug
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For a weak acid in a basic solution, which way does the equilibrium moves in favor of?
ionization (rightwards)
HA <-> H+ + A-
- HA= nonionized drug
- H+= proton
- A-= ionized drug
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For a weak acid in an acidic solution, which way does the equilibrium move in favor of?
the non-ionized form (leftwards)
HA <-> H+ + A-
- HA= nonionized drug
- H+= proton
- A-= ionized drug
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What is a drug pKa?
the pH at which 50% of a weak acid drug is ionized
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What is the Henderson-Hasselbalch equation?
the concentration ratio of the ionized and nonionized forms of a weak acid drug
Log [HA]/[A-] = pKa- pH
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The relative concentrations of the ionized and unionized forms of a weak base at equilibrium are given by:
- BH+ <-> B + H+
- BH+ = ionized drug
- B = nonionized drug
- H+ = proton
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For a weak base in an acidic solution, which way does the equilibrium moves in favor of?
of ionization (leftwards)
BH+ <-> B + H+
- BH+ = ionized drug
- B = nonionized drug
- H+ = proton
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For a weak base in a basic solution, which way does the equilibrium move in favor of?
the non-ionized form (rightwards)
BH+ <-> B + H+
- BH+ = ionized drug
- B = nonionized drug
- H+ = proton
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In a solution of given pH, the concentration ratio of ionized and nonionized forms of a weak base drug is given by:
Log [BH+]/ [B]= pKa- pH
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what is pH trapping?
- since pH is not the same in all body compartment, the level of drug ionization depends on where the drug is.
- pKa of morphine is 7.9 (weak base) so it gets trapped in the stomach (pH=2).
- highly acidic pH causes ionization of weak base.
- [BH+)/[B]= pKa- pH= 5.9
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How can acid-base chemistry affect elimination of a drug?
forced diuresis- changing urine pH in order to speed elimination of drugs
Na2CO3 will increase urine pH
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what is drug absorption?
refers to drug transport from site of administration to the bloodstream
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what is bioavailability (F)?
amount of drug in the bloodstream/ amount of drug administered.
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what is first-pass (pre-systemic) metabolism?
orally admin drugs absorbed from intestine into_______?
Pass thru _____ prior to entering systemic circulation.
First-pass metabolism substantial for which drugs?
Fraction of administered drug metabolized between administration site and systemic circulation.
hepatic portal system
liver
aspirin, morphine, & ethanol
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Bioavailabilty is affected by:
- -diseased states
- -drug interactions
- -genetic factors
- -route of administration
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routes of administration:
- oral or rectal (gut)
- percutaneous (skin)
- intravenous (plasma)
- intramuscular (muscle)
- intrathecal (CSF)
- inhalation (lung)
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2 categories of the route of admin?
- Enteral - involves GI absorption
- -oral
- -rectal (suppository)
- -sublingual
- Parenteral- no GI absorption
- -intravenous
- -intramuscular
- -intrathecal
- -subcutaneous
- -topical
- -inhalation
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Drug distribution between compartments influenced by:
- ►Permeability between compartments
- ►Binding to compartments
- ►pH partition
- ►Fat:water partition
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Where is the initial drug distribution?
- ►Cardiac output and regional blood flow
- ►Drugs reach brain, lungs, kidney, liver first (high blood flow)
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drug redistribution:
- Drug accumulates in organs and tissue with binding affinity for drug
- ►Includes the specific high affinity target receptors…
- ►But also non-specific low affinity sites
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Volume of distribution:
►Apparent volume (l) in which drug is dissolved to produce known plasma concentration
Vd= dose/ [drug] plasma
12 mg/ (6 mg/5L)= 10 L
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Vd is influenced by:
- Plasma protein binding
- ►Serum albumin, glycoproteins, steroid binding proteins
- ►Can markedly affect free [Drug]Plasma
- Tissue binding
- ►Drugs can be sequestered tissue protein and fat
- ►Drug stored in tissue can be released back into circulation as plasma concentration falls
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2 processes involved in drug elimination:
- Biotransformation
- ►Anabolic or catabolic alteration of the drug’s structure
- Excretion
- ►Removal of drug or drug metabolite from the blood plasma
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Are drugs lipophilic or hydrophilic? why?
- Many drugs are lipophilic
- ►Because this facilitates absorption across biological membranes
- ►… but excretion apparatus favors elimination of hydrophilic molecules
- ►Because these are readily soluble in urine
Therefore… Lipophilic drugs have to be made hydrophilic
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2 processes for drug elimination:
- Phase I – catabolic (break down)
- ►Activate “prodrugs” – e.g. codeine
- ►Generate reactive species for Phase 2 reaction
- Phase II – anabolic (putting together)
- ►“Conjugation” of water soluble chemical group to assist elimination
some drugs can be excreted after phase 1 metabolism
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what are the phase I rxns?
- -oxidation (cytochrome P450, 80% of oxidations)
- CYP3A4- 30% of hepatic CYP, metabolizes most drugs
- non-CYP pathways: monoamine oxidase, OH- dehydrogenase
- -reductions (cytochrome P450 reductase)
- -hydrolysis
- ex. esterases; carboxylesterases, acetylcholinesterase
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what are the phase II rxns?
- Conjugation reactions
- ►Of parent drug…
- ►…or Phase I metabolite
ex: acetaminophen
- Addition of polar chemical group
- e.g. Glucuronide, Glutathione, Acetate, Sulfate
increase water solubility increase elimination
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Excretion pathways include:
- ►Kidneys
- ►Bile
- ►Sweat
- ►Expiration
- ►Breast milk
drugs have to be water soluble to get out
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Kidneys filter approx. how many liters/day of plasma?
180
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3 processes for renal excretion:
- ►Glomerular filtration
- ►Tubular secretion
- ►Tubular reabsorption
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Glomerular filtration involves what type of diffusion?
Glomerular filtration rate and renal blood flow determine:
Eliminates:
- -passive diffusion
- - rate of drug clearance
- - freely dissolved small molecules
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Tubular secretion involves:
its ideal for:
►Involves active transport (ABCs, SLCs etc)►Ideal for removing protein-bound drug that escapes filtration
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Tubular reabsorption:
►Ionized drugs concentrated in distal tubule ►e.g. digoxin
- ►Depending on urine pH, some drug may become de-ionized…
- ►… and reabsorbed
- ►e.g. aspirin
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what is the challenge of clinical pharmacokinetics?
Challenge: Producing a [Drug]Plasma that is therapeutic, not toxic
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what is the therapeutic window?
range of drug dosage within therapeutic range but outside of toxic range
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(kinetics of elimination)
Drugs elimination dictated by what two possible time-dependent patterns?
- First order kinetics
- ►Most of the time
- ►exponential decay. ex: if 50% drug eliminated per hr, then in 1 hr:
- 100 mg/ml-> 50 mg/ml
- 500 mg/ml-> 250 mg/ml
- Higher [], greater change in 1 hr
- Zero order kinetics
- ►When an elimination step is saturated
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what is Kel?
elimination rate constant
Kel= slope= change conc/ change time
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T 1/2=
T 1/2= Ln2/Kel
How long it takes a [] to drop by ½. How slow the drug is being eliminated
- ►Longer the T1/2, slower the elimination
- ►4-5 half-times required to eliminate 95% of drug
- ►Important for designing repeated administration regimens
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first order drug elimination: plasma clearance-
- Rate at which a given volume of plasma is cleared of all drug
- e.g. ml/min
- clearance: Vd x Kel
- Allows calculation of rate of drug administration required to maintain therapeutic plasma concentration
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continuous administration: IV administration
dosage rate=
clearance x steady state [drug] plasma
- ►Drug can be introduced at constant rate to offset elimination
- ►Rate of administration must exceed T1/2 of elimination for [Drug]Plasma to be elevated
- ►Takes ~4-5 half-lives to reach steady state plasma concentration
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continuous administration: non-IV administration
dosage rate=
clearance x steady state [drug] plasma / bioavailability
- ►Drug can be introduced at constant rate to offset elimination
- ►Rate of administration must exceed T1/2 of elimination for [Drug]Plasma to be elevated
- ►Takes ~4-5 half-lives to reach steady state plasma concentration
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Repeated clearance
Avg. [Drug] plasma = dose x bioavailability/ dosage interval x clearance
- More commonly used ►e.g. oral tablets
- ►Consecutive administration of fixed doses produces plateau in [Drug]Plasma
- ►Both dose interval and dose can be modified to alter plasma concentration
For tablet drugs: Need to measure the time in btwn the tablets, don’t have as much control over the [],so you need to figure how often to administer
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Loading dose:
= desired [drug] plasma x V d
When you want to get to a plasma [] quickly. General anesthesia: propofol (IV adminster), and then after use isoflurane (inhaled substance.
- ►Necessary when rapid increase in [Drug]Plasma to therapeutic levels is required
- ►Usually IV to reduce time and circumvent bioavailability limitations
- ►Subsequent smaller “maintenance” doses administered to maintain desired [Drug]Plasma
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