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.