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 examplesProtein = enzyme Phenotype Drug Modified Response
Plasma pseudocholinesterase
slow hydrolysis
succinyl-choline
prolonged apnea
N-acetyltransferaseslow, rapid acetylators
isoniazidprocainamidedapsone
slow: toxic neuritisdisease susceptibilityslow: bladder cancer
Aldehyde dehydrogenase
slow, rapid metabolizers
alcoholslow: facial flushingrapid: liver cirrhosis
CYP2C19slow, rapid hydroxylators
proguanilslow: increased toxicity; ineffectiveness
Dihydropyrimidine dehydrogenase
slow inactivation
5-fluorouracil
enhanced toxicity
W. Sadee. Br. Med. J. 319:1, 13 Nov. 1999.
Other examplesProtein = receptor
Phenotype Drug Modified Response
2-adrenoceptorenhanced downregulation
salbutamolreduced 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 DrugModified 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 PolymorphismPhenotype (e.g., CL value, therapeutic window) is variable as a result of inheritance of particular genes.single gene = monogenic
polymodal distribution
multiple genes = polygenic
unimodal distribution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 5 10 15 20 25
Value
Frequency
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25
Value
Frequency
Monogenic Polygenic
Detection
Abnormal drug response.Polymodal distribution.
Twin studies
Example Isoniazid Nortriptyline
Figs. 14-3 & 14-4, Rowland and Tozer, p. 225.
Female, dark; Male, light
Race & Ethnicity
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.
Propranolol, 80 mg p.o. @ steady state
Asians Caucasians difference, %
n 10 10
CLo [mL/min/kg] 59.8 ± 42.7 27.4 ± 12.0 118
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 45
Race & Ethnicity
Propranolol, 80 mg p.o. @ steady state w/ 14C i.v. dose.
African Americans
Caucasians difference, %
n 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
Vss,u [L] 1960 ± 553 1960 ± 491
Race & Ethnicity: Slow Acetylators
Population N Frequency [%]
Black
Sudan Nigeria E. AfricaU.S.
102109204242
654955
42-51
WhiteBritainGermanyU.S.
472524481
55-6257
52-58
Chinese
TaiwanBritainHong KongMainland
12759
184108
22222213
EskimoCanadaAlaska
328157
521
JapaneseJapanU.S.
1990209
7-1210
Race & Ethnicity: Slow debrisoquine-type hydroxylation (CYP2D6)
Population N Frequency [%]
Black
Ghana
Nigeria
15480
123116
0.7583
White
BritainGermanyU.S.SwedenHungarySpain
2583601561188100377
957
5.4-8.8106.6
ChineseCanadaChina
13269
310.7
Native American Panama 51 0
Arab 102 1
Japanese Japan 300 0-0.5
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 PolymorphismsAllele Nucleotide; Protein changes
Size [kb]
Activity
wild type 29
NormalL1
1726 G C;
2938 C T; 296 Arg Cys
4268 G C; 486 Ser Thr29
A 2637 A 29
Absent
B 1934A (+ 6 other mutations)
29
44
9 + 16
D Deletion 11.5
E 3023 A C; His Pro 29
T1795 1795 T; 152 Try Gly 153 Stop 29
CYP 2D6 Polymorphisms con’t
Allele Nucleotide; Protein changesSize [kb]
Activity
C 2705-5 AGA; 281 Lys 29
Decreased
J188 C T; 34 Pro Ser
1749 G C4268 G C; 486 Ser Thr 29/44
W 188 C T; 34 Pro Ser 4268 G C; 486 Ser Thr 29/44
Ch1188 C T; 34 Pro Ser
1127 C T
1749 G C;
4268 G C; 486 Ser Thr29/44
(L)12 Amplification of L 175Increased
(L)2 Duplication of L 42
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 10
normal 50-100
ultra-rapid metabolizer
500
Prodrug substrate for CYP2D6
Poor metabolizers show a reduced response:
Prodrug Active Metabolite Poor Metabolizers show:
encainideO-desmethyl encainide
low antiarrythmic activity
codeineO-demethylation morphine
low analgesic action
CYP2D6 Blocking InteractionsAjmalicine 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.