Transcript Phenotype

Dosing Regimen
Individualization
Pharmacogenomics: Use of
genetic information to guide DR
Variability in Drug Response
Environmental





diet
other drugs
diseases
pollutant exposure
smoking
Genetic





receptors
transporters
drug metabolism
enzymes
susceptibility to
disease
susceptibility to toxic
effects of drug
Genetic-Induced Variability
Pharmacogenetics: Study of unusual drug response
that is inherited.
Early Example: prolonged muscle relaxation after
suxamethonium and an inherited deficiency of plasma
cholinesterase. W. Kalow. Lancet 211:576, 1956.
Muscle paralysis after suxamethonium is terminated by
elimination of the drug by cholinesterase-mediated
hydrolysis.
Other examples
Protein = enzyme
Phenotype
Drug
Modified Response
Plasma
pseudocholinesterase
slow
hydrolysis
succinylcholine
prolonged apnea
N-acetyltransferase
slow, rapid
acetylators
isoniazid
slow: toxic neuritis
procainamide disease susceptibility
dapsone
slow: bladder cancer
Aldehyde
dehydrogenase
slow, rapid
metabolizers
alcohol
slow: facial flushing
rapid:  liver cirrhosis
CYP2C19
slow, rapid
hydroxylators
proguanil
slow: increased
toxicity; ineffectiveness
Dihydropyrimidine
dehydrogenase
slow
inactivation
5-fluorouracil
enhanced toxicity
W. Sadee. Br. Med. J. 319:1, 13 Nov. 1999.
Other examples
Protein = receptor
Phenotype
Drug
Modified Response
2-adrenoceptor
enhanced
downregulation
salbutamol
reduced effectiveness
in asthma
5-H2TA serotonergic
receptor
various
polymorphisms
clozapine
variable efficacy
HER2
overexpression
in breast and
other cancers
Herceptin
 efficacy
W. Sadee. Br. Med. J. 319:1, 13 Nov. 1999.
Other examples
Protein = transporter
Phenotype
Drug
Modified Response
Multiple drug resistance
transporter
overexpression
in cancer
vinblastin
doxorubicin
paclitaxel
drug resistance
W. Sadee. Br. Med. J. 319:1, 13 Nov. 1999.
Genotype
22 pairs of identical chromosomes
2 sex chromosomes
Genotype is the collection of genes that an
individual has.
For the 22 pairs, each individual has 2 similar
genes; one paternal and one maternal. These
are alleles.
An allele is dominant if it expresses itself and
recessive if it does not.
Phenotype
Homozygous: an individual with a pair of identical
alleles, either dominant or recessive.
Heterozygous: an individual with one dominant and one
recessive allele.
Phenotype: outward characteristic expression of the
gene pair.
•homozygous & dominant = one phenotype
•heterozygous
= same as above
•homozygous & recessive = another phenotype.
Genetic Polymorphism
Phenotype (e.g., CL value, therapeutic window) is
variable as a result of inheritance of particular genes.
single gene = monogenic
multiple genes = polygenic
 polymodal distribution
 unimodal distribution
3
0.9
0.8
2.5
0.7
0.6
Frequency
Frequency
2
1.5
1
0.5
0.4
0.3
0.2
0.5
0.1
0
0
5
10
15
Value
20
25
0
0
5
10
15
Value
20
25
Monogenic
Polygenic
Detection
Abnormal drug
response.
Polymodal distribution.
Twin studies
Example
Isoniazid
Nortriptyline
Female, dark; Male, light
Figs. 14-3 & 14-4, Rowland and Tozer, p. 225.
Race &
Ethnicity
Propranolol, 80 mg
p.o. @ steady state
Until the modern era, lack of mobility led
to genetic differences among racial and
ethnic groups. With regard to genetic
polymorphisms in genes that control drug
effect, this produced different frequencies
of polymorphisms.
Asians
Caucasians
10
10
59.8 ± 42.7
27.4 ± 12.0
t1/2 [h]
4.0 ± 0.9
5.1 ± 3.8
CLm PG [mL/min]
337 ± 202
196 ± 89
72
CLm HOP [mL/min]
515 ± 304
197 ± 187
161
CLm NLA [mL/min]
158 ± 47
131 ± 22
fup [%]
16.7 ± 5.1
11.5 ± 1.6
n
CLo [mL/min/kg]
difference, %
118
45
Race & Ethnicity
Propranolol, 80 mg
p.o. @ steady state
w/ 14C i.v. dose.
African
Americans
Caucasians
13
12
CLiv [mL/min]
947 ± 271
771 ± 142
23
CLo [mL/min]
3255 ± 1723
2125 ± 510
53
F [%]
34 ± 10
37 ± 7
t1/2 [h]
4.2 ± 0.8
4.1 ± 0.5
QH [mL/min]
1449 ± 327
1241 ± 277
17
fub [%]
16.9 ± 3.0
14.6 ± 3.4
16
Vss [L]
329 ± 98
273 ± 32
21
1960 ± 553
1960 ± 491
n
Vss,u [L]
difference, %
Race & Ethnicity: Slow Acetylators
Population
Black
White
Chinese
Eskimo
Japanese
Sudan
Nigeria
E. Africa
U.S.
Britain
Germany
U.S.
Taiwan
Britain
Hong Kong
Mainland
Canada
Alaska
Japan
U.S.
N
Frequency [%]
102
109
204
242
472
524
481
127
59
184
108
328
157
1990
209
65
49
55
42-51
55-62
57
52-58
22
22
22
13
5
21
7-12
10
Race & Ethnicity: Slow debrisoquine-type
hydroxylation (CYP2D6)
Population
N
Frequency [%]
Britain
Germany
U.S.
Sweden
Hungary
Spain
Canada
China
154
80
123
116
258
360
156
1188
100
377
13
269
0.7
5
8
3
9
5
7
5.4-8.8
10
6.6
31
0.7
Panama
51
0
102
1
300
0-0.5
Ghana
Black
White
Chinese
Native
American
Nigeria
Arab
Japanese
Japan
Pharmacogenomics
“… pharmacologic effects … are determined by the
interplay of several genes encoding proteins involved in
multiple pathways of drug metabolism, disposition, and
effects.” Evans and Relling. Science 286:487-491,1999
Pharmacgenomics refers to the entire spectrum of
genes that determine drug behavior and sensitivity. It
uses DNA and protein sequencing technology to identify
genetic polymorphisms, especially single-nucleotide
polymorphisms (SNPs). Technology to identify SNPs in
individual patients will become widely used.
SNP Detection
Nanogen, Inc. Ad in Drug
Discovery World, Winter, 2001.
SNP
Peakman and Arlington, Drug Discovery World, Winter 2000/01, pp. 35-40.
SNP Incidence
Human Genome: 3 billion nucleotides
Error rate: about 0.1%; i.e., one in every 1000
nucleotides shows variability from one person to the
next. About 750,000 SNPs have been found and 3
million are thought to exist.
SNPs account for interperson variability in height and eye
color, and in susceptibility to disease and response to
therapy.
SNP Consequence
When the wrong nucleotide is in the sequence of
nucleotides that make up the code for a protein, then
either the wrong amino acid is inserted (substitution) in
the protein, or no amino acid is inserted (deletion).
Often, the protein functions normally. Sometimes the
protein is functional but impaired, and sometimes it
lacks function.
CYP 2D6 Polymorphisms
Allele
Nucleotide; Protein changes
wild type
Activity
29
L1
1726 G  C;
2938 C  T; 296 Arg  Cys
A
2637 A
B
Size
[kb]
4268 G  C; 486 Ser  Thr
29
29
44
9 + 16
1934A (+ 6 other mutations)
D
Deletion
E
3023 A  C; His  Pro
T1795
1795 T; 152 Try  Gly
29
Normal
11.5
29
153 Stop
29
Absent
CYP 2D6 Polymorphisms con’t
Allele
Nucleotide; Protein changes
Size
[kb]
C
2705-5 AGA; 281 Lys
J
188 C  T; 34 Pro  Ser
1749 G  C
4268 G  C; 486 Ser  Thr
W
188 C  T; 34 Pro  Ser
4268 G  C; 486 Ser  Thr
29/44
Ch1
188 C  T; 34 Pro  Ser
1127 C  T
1749 G  C;
4268 G  C; 486 Ser  Thr
29/44
(L)12
Amplification of L
175
(L)2
Duplication of L
42
Activity
29
29/44
Decreased
Increased
CYP2D6 activity may be as low as zero and as high as 5
times the population average.
Nortriptyline: CYP2D6 substrate
Daily dosage [mg/day]
poor metabolizer
normal
ultra-rapid metabolizer
10
50-100
500
Prodrug substrate for CYP2D6
Poor metabolizers show a reduced response:
Prodrug
Active Metabolite
Poor Metabolizers show:
encainide
O-desmethyl encainide
low antiarrythmic activity
codeine
O-demethylation
 morphine
low analgesic action
CYP2D6 Blocking Interactions
Ajmalicine
Fluoxetine
Quinidine
Chinidin
Lobelin
Trifluperidol
Chorynanthine
Propidin
Yohimbine
Pronounced in rapid metabolizers
Not apparent in slow metabolizers
Polymorphism in Drug
Metabolism Enzymes
Evans and Relling. Science 286:487, 1999.
Polymorphism in Drug
Transporters - Pgp
MDR1 is the gene that codes P-glycoprotein.
15 mutations in MDR1
C3435T polymorphism correlates w/ Pgp expression in
the intestine.
C/C = high expression of intestinal Pgp
C/C is prevalent in West Africans (83%) and African
Americans (61%), and less so in caucasians (26%)
and Japanese (34%).
High intestinal Pgp causes reduced bioavailability of
certain Pgp substrates: HIV drugs nelfinavir, ritonavir,
and saquinavir; also cyclosporine.
AAPS Newsmagazine, January 2002.
Why one drug does’t fit all
Evans and Relling. Science 286:487, 1999.
Effectiveness of Drugs across
the Population
“Blockbuster” Model: drugs are produced to serve the
entire population.
Problem: Genetic variability means that many drugs are
effective in 60% of the population at best.
•Beta blockers do not work for 15-35% of
population.
•Tricyclic depressants do not work for 20-50%
•Interferons do not work for 30-70%
Peakman and Arlington, Drug Discovery World, Winter 2000/01, pp. 35-40.
Pharmacogenomic Model
Individualized Drug: Only used to treat those patients
whose genotypes showed that they would respond.
Each drug used in a subpopulation, in which it would
be efficacious for all.
In future,
•the pertinent parts of the genome of each patient
are known.
•from a list of several drugs available to treat the
patient’s illness, the drug that best matches the
patient’s genotypes is selected.
DNA Array
Evans and Relling. Science 286:487, 1999.