Key Pharmacokinetic Concepts – Single
Dose and Steady State Drug
Administration
Pankaj B. Desai. Ph.D.
Professor of Pharmacokinetics and
Biopharmaceutics
Director, Drug Development Graduate Program
Morning Agenda: Wake Up and Smell the
Coffee (Cytochrome P450 1A2 Substrate)
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CYP1A2
Substrate
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Overview of ADME principles
Important PK Parameters
First Pass Metabolism
Compartmental & NonCompartmental Analyses
Single Dose Kinetics
Multiple Dose Kinetics
Drug-Drug Interactions
Inter-Subject Variability
ADME
ADME ISSUES IN ANTIssues
I-CANCER DRUG DEVELOPMENT
ADME
Clinical Pharmacology
• First in Human -Pharmacokinetically Guided Dose Escalation/
Drug Tolerance Study
• Pharmacokinetics-Pharmacodynamics
• Drug Metabolism
• Mass Balance with Radiolabeled Compounds
• Bioequivalence:Generic compounds
• Single and multiple doses
• Conventional versus controlled release formulations
• Bioavailability of metabolites
• Drug-Drug/Drug Dietary Product Interactions
• Special Populations
Drug Input & Different Routes of Administration
1.
2.
I.V. and I.A. injections:
•
Bolus dosing
•
Zero-Order Input (Infusions)
Extravascular Administration
• First Order (mostly passive diffusion)
• Zero Order (active transport and controlled release systems)
Factors Affecting Drug Distribution
• Phyisco-chemical properties of the drug
• Small vs. Large mol.wt. Compounds
• Hydrophilic vs. Lipophilic compounds
• pH of the milieu and pKa of the drug
• Perfusion rate (blood flow/min/g tissue)
• Protein binding
• Anatomical restrictions
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CNS- protected by the blood brain barrier
Transport across placenta
Salivary Drug Excretion (S/P ratios)
Excretion of the drug in milk (M/P ratios)
Apparent Volume of Distribution
• Mathematical term to correlate amount & concentration
• Merely a tool to understand the EXTENT of drug
distribution- not a real physiological volume
• Compare to the volume of body waters
• Best calculated from I.V. Dosing as
I.V. Dose/Cpo
Drug
Sulfisoxazole
Phenytoin
Phenobarbital
Diazepam
Digoxin
L/Kg
0.16
0.63
0.55
2.4
7
L/70 kg
11.2
44.1
38.5
168
490
Apparent Volume of Distribution
Plasma Water-3.5 L,
~4.5 % body wt (w/w)
100 mg
Conc = 2 mg/ml
Vd = 50 ml
Beaker without
Charcoal
100 mg
Conc = 0.2 mg/ml
Vd = 500 ml
Beaker with
Charcoal
T
B
W
E
C
W
Total extracellular
water - 15 L, 20 %
body wt (w/w)
Total Intracellular water
–20 L, 30 % body wt
(w/w)
Total body Water 40 L,
~55 % body wt (w/w)
Major Drug Elimination Pathways (Coordinated
defense mechanism)
Biotransformation
HEPATIC
Excretion
Extra-Hepatic
Renal
Phase I
Phase II
Biliary
Glomerular Filtration
• Kidney receives 1.1 L of blood (20 – 25%) of
cardiac output
• 10 % is filtered at the glomerulus
• Compounds with Mol.wt < 20,000 filtered
• GFR = 120 ml/min
• CLR of Inulin - a measure of GFR
• Filtered freely into the tubule
• Not influenced by protein binding and neither secreted nor
reabsorbed
• Rate of filtration = Fu. Cp.GFR
• Not a very effective drug extraction process
(maximal ~ 0.11 or 10 %)
Active Secretion
•
•
•
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Detected when the overall rate of
urinary drug excretion exceeds the rate
of filtration
Secretory processes (proteins) located
predominantly within the proximal
tubules
Mechanisms exist for secreting acids
(anions) and bases (cations) from plasma
into the tubular lumen
• Energy-dependent
• Saturable processes
• Subject to competitive
inhibition
Effect of Protein-Binding
• Depends upon secretion
efficiency and contact time at
the secretory sites
• Restrictive (dependent on the
Fub) vs. Non-Restrictive
(perfusion-rate limited)
Reabsorption
• Must occur when CLR < fu.GFR
• Reabsorption occurs all long the nephron, associated with
reabsorption of water; majority however occurring from
the proximal tubules
• Predominantly a passive diffusion process
• Driven by concentration-gradient across the tubular
lumen
• Active secretion occurs for many endogenous
compounds such as vitamins, electrolytes, glucose
and amino acids
• Urine-Plasma Ratio (U/P) based on HendersonHasselbalch equation
• Influence of pKa and pH of urine
Major Tissues Involved in
Drug Metabolism
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•
•
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Liver
Small intestines
Kidney
Lung
Other portals of entry into the body and
protected organs.
-e.g. nasal mucosa
Representation of drug metabolism and
excretion by the hepatocyte
Biliary Excretion is Transporter Mediated
Phase I and Phase II Drug
Metabolizing Enzymes
Phase I enzymes: Predominantly cytochrome P450 (CYP)
Drug Metabolism by CYPs
Theophylline,
caffeine,
Olanzapine
CYP2C8
Paclitaxel
Rosiglitazon
e
CYP2C9
cerivastatin(15%)
Includes:
warfarin
phenytoin
tolbutamide
Losartan
CYP2A6 (Coumarin)
CYP2E1
(Chlorzoxazone)
CYP2B6
CYP1A2
bupropion,
5%
tamoxifen,
efavirenz
CYP3A (50%)
Includes:
lovastatin
cyclosporin
nifedipine
midazolam
CYP2D6
ethinylestradiol
(25%)
Ritonavir
Includes: Tricyclic antidepressants,
Midazolam
SSRI's, haliperidol, propanolol, atomoxetinetestosterone
Detxromethorphan,
Phase II Reactions
• Also known as Synthetic (conjugation)
reactions
• Major reaction: Transfer of the conjugating
moiety to the drug
• Enzymes involved are “transferase”
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•
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Glucuronosyl transferase
Sulfotransferases
N-acetyltransferase
Methyltransferase
Glycine transferase
Glutathione-S-transferase
Drug Biotransformation Reactions
• Active Drug to Inactive Metabolite
• Amphetamine
• Phenobarbital
• Taxol
Phenylacetone
Hydroxyphenobarbital
6-hydroxytaxol
• Active Drug to Active Metabolite
• Codeine
• Procainamide
• tamoxifen
Morphine
N-acetylprocainamide
4-hydroxytamoxifen
Drug Biotransformation Reactions
• Inactive Drug to Active Metabolite
• Hetacillin
• Sulfasalazine
• Cyclophosphamide
Ampicillin
Sulfapyridine + 5 ASA
Nitrogen mustard
• Active Drug to Reactive Intermediates
• Acetaminophen
Reactive metabolites
(hepatic necrosis)
• Benzo(a)pyrene
Reactive metabolite
(carcinogenic)
Nomenclature
• Basis: Amino acid sequence
• Families: Less than 40 % a.a. sequence
assigned to different gene families
(gene families 1, 2, 3, 4 etc.)
• Subfamilies: 40 – 55 % identical sequence
(2A, 2B, 2C, 3A etc.)
CYP3A4
Family
Subfamily
Isoform
CYP Nomenclature (Contd.)
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Cytochrome P450 Nomenclature, e.g. for CYP2D6
CYP = cytochrome P450
2 = genetic family
D = genetic sub-family
6 = specific gene
NOTE that this nomenclature is genetically based: it
has NO functional implication
Examples of CYP mediated Oxidative
Examples of reactions catalyzed by cytochrome P450:
Metabolism
Hydroxylation of aliphatic carbon
Examples of CYP mediated Oxidative
Heteroatom dealkylation
Metabolism
Examples of reactions catalyzed by cytochrome P450:
Clearance Concepts
Compartmental Modeling
One-Compartment Open Model
I.V. bolus
DB1 Cp1
Vd
k10
K10 = overall
Elimination
Rate Constant
I.V. Bolus
k
10  t
Cp  C p   e

D
Cp  
Vd
Two-compartment Open model
λ
t
λZt
Cp  C1
 Cz
 1
Central or Plasma
I.V. bolus
Cp1 VC
Dp
k12
k21
Tissue
Dt
Ct
Vt
1- hybrid rate constant (distribution)
z- hybrid rate constant (terminal)
Two-compartment Open Model
Elimination
only
Blood flow to human tissues
Tissue
Percent Body
Weight
Percent Cardiac
Output
Adrenals
0.02
1
550
Kidney
0.4
24
450
Liver
2.0
25
Hepatic
Portal
Blood Flow
(ml/100 g
tissue/min)
5
20
20
75
Brain
2.0
15
55
Skin
7.0
5
5
Muscle
(basal)
40.0
15
3
Connective
Tissue
7.0
1
1
Fat
15.0
2
1
Extravascular dose
e.v. dose
Dp
Cp
Vd
ka
Site of
absorption
k10
16
Cp'
Conc(ug/ml)
12
8
Cp
4
Cp'-Cp
0
0
5
10
Time(hrs)
Cp=
F.Dose.Ka
V(ka-k)
(e-k.t- e-ka.t)
15
NCA
Used to estimate
• AUC
• Bioavailability
• Clearance
• Volume of Distribution
• Average Steady State Concentration
AUC
Trapezoidal Rule
AUC= ½(t3-t2)(C2+C3)
AUC
Example
AUC(0-)
AUC(ug.hr/ml)
10
0
0.25
0.69
2.40
7.41
18.11
17.46
14.51
11.41
8.71
6.46
19.38
106.80
8
Conc(ug/ml)
Conc
(ug/ml)
0
2.025
3.53
6.07
8.75
9.36
8.1
6.41
5
3.71
2.75
6
4
2
0
0
2
4
6
8
10
12
14
16
Tim e(hr)
Cp(last)=
2.75/0.1419
Conc Time Profile (Oral Dose)
y = 20.245e-0.1419x
10
R2 = 0.9981
Conc (ug/ml)
Time
(hr)
0
0.25
0.5
1
2
4
6
8
10
12
14
1
0
2
4
6
8
Time (hr)
10
12
14
16
Bioavailability
• Absolute
Bioavailability
• Relative
Bioavailability
F=
[AUC]e.v/[DOSE]e.v
[AUC]i.v/[DOSE]i.v
F=
[AUC]e.v/[DOSE]e.v
[AUC]std/[DOSE]std
Bioequivalence
• Two products are considered to be bioequivalent
if the concentration time profiles are so similar
that they are likely to produce clinically relevant
differences in either efficacy or toxicity.
• Common measures used to assess differences are
Tmax, Cmax and AUC.
Other Parameters
• CL = Di.v/AUC
• AUMC = ½(t2-t1)(C1t1 +C2t2)
• MRT (Mean Residence Time)
= AUMC/AUC
or MRT = 1/K or CL/V
• Vss = CL. MRT
Multiple Dosing –Overall Aims
• Key Concepts
• Principle of Superposition
• Drug Accumulation and Steady State
• Persistence Factor and Accumulation Factor
• Peak, Trough and Steady State Average Levels
• Applications
• Determination of drug concentrations and amounts following
multiple i.v. and e.v. doses (Ka > > K10)
» max, min and during a dosing interval
• Determination of dosing regimens
– Doses (Maintenance and Loading) and Dosing Interval
» Cpmax consideration
» Cpmin consideration
» Cpmax and Cpmin consideration
• Practical Considerations in Decision Making
Drug Accumulation Depends on
Frequency of Administration
Multiple I.V. Dosing
The AUC within a dosing interval at steady state
is equal to the total AUC of a single dose.
Peak, Trough and Css Average
Accumulation Index - Cssmax/Cmax1
AUC at Steady State = AUC0
∞
Impact of Half-life and dosing interval Half-Li
on
Goals of the Dosing Regimen
Dosing Regimen: Loading and
Maintenance Doses
Constant Rate Regimens
Sources
Sourcesof
of Variability
Variability
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•
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Genetic factors
• Genetic differences within
population
• Racial differences among
different populations
Environmental factors and drug
interactions
• Enzyme induction
• Enzyme inhibition
Physiologic considerations
• Age
• Gender
• Diet/nutrition
• Pathophysiology
Drug dosage regimen
• Route of drug administration
Dose dependent (nonlinear)
pharmacokinetics
Examples of CYP3A Inducers
Therapeutic Class
Anti-epileptic
Drugs
Anti-Infective
Agents
Anti-Cancer
Drugs
Carbamazepine
Phenobarbital
Phenytoin
Topiramate
Felbamate
Rifampicin
Paclitaxel
Rifabutin
Docetaxel
Rifapentine
Cyclophosphamide
Clotrimazole
Ifophosphamide
Sulfadimidine
Tamoxifen
Suflinpyrazone
4-hydroxyEfavirenz
tamoxifen
Amprenavir
SU5416
Nelfinavir
Ritonavir
Capravirine
Miscellaneous
Lovastatin
Troglitazone
Omeprazole
Prednisolone
Probencid
Phenylbutazone
Diazepam
fexofenadine
Hyperforin
Induction of CYP1A2 (Ethoxyresorufin O-deethylase)
by SU5416 in Primary Human Hepatocytes
Stopeck et.al. Clin. Cancer Research, 2002
Salzberg et.al, Investigational New Drugs 24: 299–304, 2006)
Example of Auto-Induction – SU5416
Oral
Treatment
AUC Day 8
AUC Day 15
AUC Day
21/22
Induction of
clearance
Once weekly
(n=3)
156 ± 117
131 ± 140
141 ± 90
10%
Twice weekly
(n=3)
329 ± 187
117 ± 92
198 ± 321
40%
Daily dosing
(n=3)
412 ± 111
21 ± 36
9 ± 16
98%
Stopeck et.al. Clin. Cancer Research, 2002
Salzberg et.al, Investigational New Drugs 24: 299–304, 2006)
Effect of Tamoxifen (TAM) Mediated
CYP3A4 Induction
Letrozole Alone
Letrozole + Tamoxifen
( 6 weeks & > 4 months)
Dowsett, M. et al. Clin Cancer Res 1999;5:2338-2343
62
PXR
Pharmacogenomics. 2008 November; 9(11): 1695–1709.
Midazolam
Plasma
Conc.
Profile
Effect
of CYP3A/PXR
Genotypes
on CYP3A
Induction
3
ID: 1
7
ID: 3
3
ID: 4
ID: 5
7
30
10
Midazolam Conc. (ng/ml)
30
ID: 6
ID: 7
ID: 8
ID: 9
ID: 10
ID: 11
ID: 12
ID: 14
10
30
10
30
ID: 15
Time(hrs)
10
3
7
Day 0
Day 1
Day 42
64
Inhibition of Drug Metabolizing Enzymes
Inhibitor present
Inhibitor absent
CYP3A
CYP3A
Active drug
Active drug
Inactive drug
Inhibitor
Inactive drug
Saquinavir
+
Ritonavir
AIDS. 1997 Mar
15;11(4):F29-33
Saquinavir
Plasma Rosuvastatin concentration-time profile in the
absence and presence of Darunavir/Ritonavir
Before DRV/RTV
After DRV/RTV
Desai Lab with the UC President
•
Graduate Students
- Rucha Sane
– Niresh Hariparsad
– Fang Li
– Ganesh Mugundu
•Former Student/Post-Doc
Srikanth Nallani, Ph.D., FDA
• Collaborators
– Arthur Buckley, Ph.D., College of
Pharmacy
– Julie Nelson, Ph.D., Department of
Molecular Genetics, Biochemistry and
Microbiology
- Elizabeth Shaughnessy, MD
- Judith Feinberg, MD Brian Goodwin,
Ph.D., GlaxoSmithKline
– Stephen Storm, Ph.D. University of
Pittsburgh
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• Funding Sources
- Aventis Pharmaceutical, Eli Lily & Co, Bristol Myers
- Womens Health (UC), American Cancer Society
- NIH, Susan G. Komen Breast Cancer Foundation
Squibb