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Chapter 3
Pharmacokinetics
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2
Drug Administration
Drug Concentration in
Systemic Circulation
Drug in Tissues of
Distribution
Drug Metabolism or Excretion
Drug Concentration at Site of Action
Pharmacologic Effect
Clinical Response
Toxicity Efficacy
Ph
arm
aco
kine
tics dose-concentration
Ph
arm
aco
dyn
am
ics concentration-effect
Distribution Elimination
Absorption
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¨ Drug Transport¨ Process of Drug in vivo¨ Elimination Kinetics
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Chapter 3
Section 1Drug Transport
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Modes of Transport
1. Filtration
2. Simple diffusion
3. Carrier-mediated transport
1) Facilitated diffusion
2) Active transport
Passive transport
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¨ Aqueous diffusion, Aqueous channel;
1. Filtration
¨ Hydrosoluble , driven by concentration gradient
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1. Filtration
¨ Downhill movement.
Flux (molecules per unit time) =
Thickness
tcoefficientyPermeabiliAreaCC
21
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Simple diffusion
¨ Lipid diffusion
¨ The most common transport for drug
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2. Simple Diffusion, Passive Diffusion
¨ Ion trapping :– Nonionized form (uncharged) : low polarity,
hydrophobe, lipid soluble, permeation through membrane
– Ionized (charged) : high polarity, hydrophil, lipid unsoluble, unable permeation through membrane
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Ionization of weak acid and weak bases¨ A weak acid is best defined as a neutral molecule that
can reversibly dissociate into an anion and a proton.
C8H7O2COOH C8H7O2COO- +H+
Neutral aspirin
Aspirin anion proton
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Ionization of weak acid and weak bases¨ A drug that is a weak base can be defined as a neutral
molecule that can form cation by combining with a proton.
C12H11CIN3NH3 + C12H11CIN3NH2+H+
Pyrimethamine cation
Neutral pyrimethamine
Proton
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pH & pKa determine the degree of dissociation of drug
¨ pH and pKa are important in determining the fraction in the un-ionized form.
¨ pKa : the pH at which 50% of the molecules in solution are in the ionized form.
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13][
][10
][
][log
][
][log
][
]][[
HA
A
HA
ApKapH
HA
ApHpKa
HA
AHKa
AHHA
pKapH
Weak acid Weak base
][
][10
][
][log
][
][log
][
]][[
BH
B
BH
BpKapH
BH
BpHpKa
BH
BHKa
BHHB
pKapH
pH & pKa determine the degree of dissociation of drug
dissociation constant
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¨ Henderson-Hasselbalch equation:
pHpKedUnprotonat
otonateda
)(
)(log
Pr
¨ The lower the pH relative to the pKa, the greater will be the fraction of drug in the protonated form.
¨ The protonated form of a weak acid is the neutral, more lipid-soluble form.
¨ The unprotonated form of a weak base is the neutral form.
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Application of Henderson-Hasselbalch Equation
¨ Lipid-soluble form is reabsorbed by renal tubule¨ Weak acids are excreted faster in
urine;
weak bases are excreted faster in urine¨ Acidification: NH4Cl, Vc
¨ Alkalinization: NaHCO3, Acetazolamide( 乙酰唑胺)
alkaline acidic
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Quiz¨ We orally administer a weak acid drug(A) with a pKa of 3.4.
Gut pH is 1.4, and blood pH is 7.4. Assume the drug crosses membranes by simple passive diffusion. Which of the following observations would be true?
A. Only the ionized form of drug, will be absorbed from the gut.
B. The drug will be hydrolyzed by reaction with HCl and so cannot be absorbed
C. The drug will not be absorbed unless we raise gastric pH to equal pKa, as might be done with an antacid
D. The drug would be absorbed, and at equilibrium the plasma concentration of the A- would be 10000 times than the plasma concentration of nonionized moiety(HA)
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Carrier-mediated Active transport Facilitated transport
3. Carrier-Mediated Transport
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3. Carrier-Mediated TransportTrans-membrane protein
– Selectivity/specificity– Saturation– Competitive inhibition
¨ Active transport– Against gradient– Energy required
¨ Facilitated transport– Down gradient– Energy free
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Chapter 2
Section 2Process of Drug in vivo :
Absorption, Distribution, Metabolism, Excretion
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Ⅰ 、 Absorption
Process of drug leaving site of administration into systemic circulation.
¨ Inhalation¨ Intranasal¨ Intravenous Infusions, Intravenous Injections, Intravenous¨ Mucosal¨ Ophthalmic¨ Oral ¨ Buccal¨ Sublingual¨ Rectal¨ Topical
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Buccal/Sublingual
¨ absorbed though oral mucus membranes in mouth – buccal = cheek– sublingual (SL) = under tongue
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First pass elimination (first pass metabolism, first pass effect )
¨ pass through liver before reaching circulation
¨ undergo metabolism by liver
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Bioavailability
Bioavailability refers to the extent and rate at which the drug enters systemic circulation, thereby accessing the site of action.
If the drug is given by extravascular administration, less than 100% of a dose reach the systemic circulation.
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Bioavailability (F)
F = 100% A
D
F = AUC P.O
AUC I.V
p.o
i.V
AUC (area under the curve): The area under the plasma drug concentration-time curve , reflects the actual body exposure to drug after administration of a dose of the drug and is expressed in mg*h/L . It is directly proportional to the total amount of drug in the patient's blood.
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Routes of Administration, Bioavailability, and General Characteristics.
Route Bioavailability(%) Characteristics
Intravenous (IV) 100 (by definition) Most rapid onset
Intramuscular (IM)
75 to ≤ 100 Large volumes often feasible; may be painful
Subcutaneous (SC) 75 to ≤ 100 Smaller volumes than IM; may be painful
Oral (PO) 5 to < 100 Most convenient; first-pass effect may be significant
Rectal (PR) 30 to < 100 Less first-pass effect than oral
Inhalation 5 to < 100 Often very rapid onset
Transdermal 80 to ≤ 100 Usually very slow absorption; used for lack of first-pass effect; prolonged duration of action
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Ⅱ 、 Distribution
Drug goes to organs and tissue from circulation via its permeation
Dependent on its solubility, the rate of blood flow to the tissues, and the binding of drug molecules to plasma proteins
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1. Plasma protein binding
¨ Free drug Bound drug
][
][
][
][
thenprotein, plasma ofamount totalis If
][
]][[
drug bound is DP drug free is
DK
D
P
DP
P
KDP
PD
D
DPPD
DT
T
D
,
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¨ Unbound drug = active ¨ Reversible equilibrium¨ Saturable: albumin, most common protein to bind
drugs ¨ Nonspecific & competitive
Drug and Drug (especially, drug of high binding rate): phenylbutazone + warfarin
Drug and Endogenetic substance: billirubin
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+B drug :
98%
Free A drug: 1%A drug: 99%
plasma protein
Free A drug: 2%
Effect increase, even toxicity
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Distribution
Body’s barriers : Blood-brain barrier (BBB) : 1. Tight junction between endothelial cells
2. Astrocyte surrounding the endothelial cells
Placental-barrier Blood-eye-barrier
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血脑屏障 (Blood-brain barrier, BBB)
由毛细血管壁和 N 胶质细胞构成
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Ⅲ 、 Metabolism, Biotransformation Sites of biotransformation
– liver– Others: GIT, kidneys, brain, & plasma
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Results of metabolism :
1.transform into inactive substance;
2.inactive drug (pro-drug) → active metabolite; Codein morphine
3.active drug → other active substance; Phenylbutazone( 保泰松 ) and oxyphenbutazone (羟基保泰
松) Diazepam ( 安定 ) and oxazepam (去甲羟基安定) Carbamazepine ( 卡马西平 ) and 10, 11-epoxide carbamazepine
(环氧卡马西平)
4. transformed into toxicant. Isoniazid → acetyl isoniazid
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Steps of Metabolism
¨ Phase Ⅰ– oxidation, reduction and hydrolysis
¨ Phase Ⅱ– Conjugation with endogenous
compounds(glucuronic acid , glycine, sulfuric acid)
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matabolism
Phase I Phase II
Drug
Drug inactivaed
activityor
bound
Drug
Lipophilic Hydrophilic
Excretionbound
bound
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Metabolism Enzyme
¨ Specific enzymes– cholinesterase, monoamine oxidase (MAO), etc.
¨ Non-specific enzymes– hepatocyte microsomal enzymes (cytochrome P450
enzyme system, CYP 450).
– These isozymes involved in Phase I reactions.– If binds to carbon monoxide , spectrum with a maximum at
450 nm
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CYP1A1/2CYP1B1
CYP2A6
CYP2B6
CYP2E1
CYP3A4/5/7
CYP2C19
CYP2C9CYP2C8
Non-CYP enzymes
CYP 2D6
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Characteristic of hepatic drug enzyme
¨ low selectivity¨ great variability¨ enzyme activity is liable to be influenced by
outside factors.– enzyme inducer– enzyme inhibitor
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General inducers:
苯妥英 phenytoin 、奎尼丁 quinidine 、利福平rifampicin 、卡马西平 carbamazepine 、灰黄霉素griseofulvin 、巴比妥类 barbiturates (苯巴比妥为最) 、 甲丙氨酯 meprobamate ,格鲁米特 glutethimide 、保泰松 butazodine 、 chronic alcoholic intoxication 慢性酒精中毒
General inbibitors:
酮康唑 Ketoconazole 、西咪替丁 cimetidine 、异烟肼 isoniazide 、红霉素 erythrocin 、磺胺 sulfonamide 、氯霉素 chloramphenicol ,柚子汁 grapefruit juice , acute alcoholic intoxication 乙醇急性中毒者
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环孢素伊曲康唑
利福平
麦芽汁
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Ⅳ 、 Excretion
¨ Drug or metabolite → emunctory( 排泄器官 ) or secretory( 分泌器官 )→ outside of body
¨ Excetion organ: – kidney – bile duct – intestinal tract – salivary gland( 唾液腺 ) – galactophore( 乳腺 ) – sudoriferous gland( 汗腺 ) – lung
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Excretion
¨ Renal excretion: glomerular filtration, active tubule secretion, passive tubule reabsorption.
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2. 胃肠道:胆汁 - 粪便途径
Liver
Gut
Feces excretion
Portal vein
Bile duct
胆汁排泄(biliary excretion)
和肝肠循环(Enterohepatic
recycling)
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Chapter 2
Elimination Kinetics
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Kinetic process
¨ Drug elimination kinetics is the eliminating course of plasma or blood concentration of drug with its distribution, metabolism and excretion. It is expressed by mathematics equation:
This rate process is called N grade rate process. where K is rate constant, Minus of right-sideness denotes reduction of drug concentration.
)0( NKCdt
dc N
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First order elimination kinetics
n = 1 dC/dt = - kC
Zero order elimination kinetics n = 0 dC/dt = -k
dC/dt = - kCn
k : Rate constant for elimination
Log
Un
its o
f D
rug
Time
n=0n=1
n=0
n=1
Un
its o
f D
rug
Rate of elimination is proportional to C. t ½ is a constant
Rate of elimination is independent to C. t ½ is variable
T1/2: The time it takes for the [drug] in the body to be reduced by 50%
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Zero-order Kinetics First-order Kinetics
Contant Rate Process Rate is proportional to the drug concentration
dC/dt = -K0 dC/dt = - KC
t1/2 =0.5C0/K0 , depends on initial drug concentration
t1/2 =0.693/K , independent and is a constant value
Most drugs Ethanol, phenytoin, aspirin
Capacity-limited elimination, carrier based processes after saturation
Not linked with carriers or unsarurable if linked with carriers
Comparison
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Compartment Model房室模型
Section 4
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One compartment model一室模型
¨ Considers the body to be a single compartment. A drug is absorbed , immediately distributed, and subsequently eliminated by metabolism and excretion.
D
D
DKa
Ke
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Two compartment model二室模型¨ Central compartment : plasma, heart, lung,
kidney, endocrine system, et al ¨ Peripheral compartment : muscle, fat
tissue, bone, et al¨ Most drugs are transported with two
compartment model
D D DKa
D
K12
K21Ke 或K10
Central compartment
Peripheral compartmentDistribution
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Two compartment model
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Chapter 2
Section 5Important Parameters in
Pharmacokinetics
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1. Volume of distribution,Vd
Vd = D / C0
D for total dose of drugC for concentration in plasma―C0 equilibrium of distribution
Vd: theoretical volume that the total amount of administered drug would have to occupy (if it were uniformly distributed), to provide the same concentration as it currently is in blood plasma.Vd relates the amount of drug in the body to [C] of drug in blood or plasma
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Understand the drug distribution in the body :
• Vd≈5L indicates in the plasma ,• Vd≈30-40L in the total body fluid , • Vd > 40L in the tissue and organs , • Vd > 100L in the specific organ or big range
tissues , e.g thyroid , Skeleton, adipose tissue
Needed to calculate a loading dose :D=Vd×C
significances :
70kg person
42L Body fluid
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2. Clearance, CL
otherliverkidneysystem
otherother
liverliver
kidneykidney
RE
RE
RE
L/h)or ml/min ((RE)n eliminatio of Rate
CLCLCLCLC
CL
CCL
CCL
CCL
― Definition: volume of blood which is cleared off a drug per unit of time
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Calculations :1. Cl is constant in first-order kinetics
CL = KeVd
AKRE eCRECl /
CAVd /
CAKeCl /
dVKeCl
Reasoning:
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Calculations :2. CL = A / AUC ( A for total drug in body, = dose )
Reasoning:
AUC
A
AUC
CVVKCL
AUC
CK
K
CAUC
VKC
CVK
C
AK
C
RECL
CVAC
AV
AKRE
dde
ee
dedee
dd
e
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First-order Elimination Kinetics (linear kinetics)
as
Definition: The time it takes for the [drug] in the body to be reduced by 50%
Half-life, T1/2
dVKeCl
Cl
Vdt 693.02/1
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Constant repeated administration of drugs Steady-state concentration
Aim to let MTC>Css>MEC
Css-max < MTC
Css-min > MEC
RATE IN = RATE OUT
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Additive amount of eliminated drug T1/2 1 2 3 4 5
50 25 12.5 6.25 3.125
50 25 12.5 6.25
50 25 12.5
50 25
50sum 50 75 87.5 93.5 96.5
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时间 ( 半衰期 )
MTC
MEC
Dru
g C
once
ntra
tion
(ug/
ml)
Css is proportional to dose and dosing interval
时间 ( 半衰期 )
MTC
MEC
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63Css = AUCss/ τ
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Chapter 2
Section 6Dosage design and Optimization
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1. target concentration
Steady-state concentrion ( Css )
Css-max < MTC
Css-min > MEC
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Maintenance dose (MD) To maintain SS , the dosing rate must equal to
the rate of elimination. That is dosing rate = rate of elimination
TC : Target concentration
MD= CL×TC×Dosing interval
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Loading Dose ¨ Loading dose is dose required to achieve a specific
plasma drug concentration level immediately with a single administration.
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mm
tt
m
K
mL
K
mL
mK
LL
DD
e
D
e
DD
te
DD
DFeDFDF
e
e
e
2
21
111
1
2/12/1
2ln
2/1
时,当
即每隔一个 t1/2 给药一次时,负荷量为维持量的 2 倍。
首剂加倍
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The target concentration strategy
1. Choose the target concentration, TC.2. Predict volume of distribution (Vd) and clearance (CL) based
on standard population values with adjustments for factors such as weight and renal function.
3. Give a loading dose or maintenance dose calculated from TC, Vd, and CL.
4. Measure the patient's response and drug concentration.5. Revise Vd and/or CL based on the measured concentration.6. Repeat steps 3–5, adjusting the predicted dose to achieve TC.¨
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Important Pharmacokinetics Calculations
¨ Single-Dose Equations– Volume of distribution (Vd):
– Half-life (t1/2):
¨ Multiple Doses or Infusion Rate Equations– Loading dose (LD):– Maintenance dose (MD):
CL
Vt
C
DV
d
d
7.02/1
0
F
CCLMD
F
CVLD
CCLk
ss
pd
ss
0
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Brian is a 15 yr old patient who has been admitted to the hospital with a severe case of bacterial septicemia caused by a gram-negative organism that has been determined to be sensitive to Gentamycin. Gentamycin’s Vd=20 L. What i.v. loading dose would you give Brian to rapidly achieve a therapeutic plasma level of 5 mg/L?
A. 20 mg B. 25 mg C. 50 mg D. 100 mg E. 250 mgD
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¨ The following graph shows the elimination time course obtained after giving a 320 mg dose of a drug by both i.v. & oral routes. From the data shown, calculate the drugs elimination clearance. You may need to use a calculator.
A. 0.6 L/hrB. 1.75 L/hrC. 10 L/hr D. 32 L/hr E. 36 L/hr
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AIM get Cl,
1st to Calculate Vd,
Vd = D / C0
Vd = 320mg/32ug/ml=10L
2nd to get t1/2
t1/2=4h
Cl=0.693×10L/4h=1.75L/h
2/1
693.0t
VdCl
This formula can be used to calculate both Vd, t1/2 & Cl after giving a single drug dose
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Your pediatric patient is suffering from a bacterial infection & requires maintenance dosing with gentamicin. Gentamicin’s elimination clearance is 5.0 L/hr. What maintenance i.v. dose of gentamicin should you give every 8 hours to maintain an average steady-state plasma level of 5 ug/ml?
A. 25 mg B. 50 mg C. 100 mg D. 200 mg
Original question from USMLE
D
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The end