M. Imad Damaj, Ph.D. Associate Professor Pharmacology and ...
Transcript of M. Imad Damaj, Ph.D. Associate Professor Pharmacology and ...
PHARMACOGENETICS & DRUG IDIOSYNCRASY
M. Imad Damaj, Ph.D.Associate Professor
Pharmacology and ToxicologySmith 656A, 828-1676, [email protected]
Toxicity No EffectOops!Oops!
Too Much Too Little↓ Dose ↑ Dose
No effect↑ Dose
Toxicity↓ Dose
Inter-individual Differences in Drug Efficacy
Group Incomplete/absent efficacy
AT2-antag 10-25%SSRI 10-25%ACE -I 10-30%Beta blockers 15-25%Tricycl. AD 20-50%HMGCoAR-I 30-70%Beta-2-agonists 40-70%
N C NH
NH2
N C NH
NH2
CYP2D6
OH
DEBRISOQUINE4-HYDROXYDEBRISOQUINE
May 1975: Five interns at St. Mary’s Hospital in London participated in a study of the effects of debrisoquine (40 mg), an antihypertensive agent.
Robert Smith only: dizziness and severe orthostatic hypotension
PharmacogeneticsSame symptomsSame findingsSame disease (?)
Same Drug….
Different Effects
?
Genetic Differences
Possible Reasons: Non-Compliance… Drug-drug interactions…Chance…
Or….
PEOPLE ARE DIFFERENT!Exposure Exposure Exposure
The Gene for…
PHARMACOGENETICSPHARMACOGENETICS
The study of genetically controlled variations in drug
response
Much individuality in drug response is inherited (polymorphism)
Human genome contains 30,000 to 40,000 genes
100,000 different proteins: possible drug targets
99.9% Identical
0.1% = almost 3 million single nucleotide polymorphism in which a nucleotide is exchanged
for another at a given position
To be important SNPs must affect either function or amount of a protein
Ethnic Differences Correlate well with
Genetic Background
GENETIC GENETIC POLYMORPHISMSPOLYMORPHISMS
PharmacokineticPharmacokinetic PharmacodynamicPharmacodynamic••Metabolizing EnzymesMetabolizing Enzymes••TransportersTransporters••Plasma protein bindingPlasma protein binding
••Target ProteinsTarget Proteins•• ReceptorsReceptors••Ion channelsIon channels
Idiosyncratic ResponseIdiosyncratic Response
Variation in Target ProteinsVariations in target proteins or their pathways
can influence the outcome of pharmacotherapy
AmountAmount StructureStructure FunctionFunction
Variation in β2-Adrenergic Receptorβ2-Receptor mediates the relaxation of smooth
muscle in small airways
β2-Receptor agonists are used for asthma
Variation in β2-Adrenergic Receptorβ2-Receptor is a G-protein receptor: highly polymorphic
Amino Acid 16: 60% Hispanic/Caucasian SNP for glycine G16
The other 40% arginine R16
Variation in β2-Adrenergic Receptor
Individuals with G16 variant acutely
downregulate β2- receptor in response to
agonists
Albuterol is less effective in G16
individuals
First step toward developing an effective
pharmacological strategy for such patients
Variation in Enzymes of Drug Metabolism
Poor MetabolizerPoor Metabolizer UltraUltra--MetabolizerMetabolizer
Multiple genesMultiple genes
Normal ResponseNormal ResponseToxicityToxicityDiminishedDiminishedResponseResponse
Deleted geneDeleted gene
GCCCACCTC
GCCCGCCTC
Patient A
Patient B
Wild type
Mutation
Wild type
Con
cent
ratio
n
Mutation
Con
cent
ratio
n
Time
Time
CYP450
CYP450
Same dose but different plasma concentrations
PGt and Drug Metabolism
PGt: Possible Impact on PK and Dose-Response
Efficacy: reduction in anxiety and symptoms of depression
Safety: tachycardia, arrhythmias and drowsiness
Example: Nortriptyline
Individualizing Therapy is Important for Drugs witha Narrow Therapeutic Range
e.g. penicillinSafe to prescribe dose effective for
>90% of population
e.g. anticancer drugDifficult to prescribe effective
non-toxic dose
narrowtherapeutic range
Dose
100
efficacy
toxicity100
efficacy
toxicity
wide therapeuticrange
Dose
Some drugs have such a wide therapeutic range that individualizing the dose is not important
Atypical Plasma Cholinesterase
O C CH2CH2
O
(H3C)3NH2CH2C CO
O CH2CH2N(CH3)3+ +
SUCCINYLCHOLINE
•a rapid acting, rapid recovery muscle relaxant - 1951•usual paralysis lasted 2 to 6 min in patients•occasional pt exhibited paralysis lasting hrs•cause identified as an “atypical” plasma cholinesterase
Hydrolysis by pseudocholinesterasecholine succinylmonocholine
•Atypical plasma cholinesterase has 1/100 the affinity for succinylcholine as normal enzyme•occurs in 1:2500 individuals
020406080
100
-6 -4log molar dibucaine conc.
% In
hibi
tion
Typical Atypical
Adapted from: Pharmac Ther 47:35-60, 1990.
normal enzyme inhibited > 70%abnormal inhibited < 30%
Atypical Plasma Cholinesterase
CH2
CH2
CH
O NO2
O NO2
O NO2
Nitroglycerin(Glyceryl Trinitrate);NITRO-BIDNITROSTAT
CH2 CH CH CH CH CH2
O NO2
O NO2
O
O
CH2 CH CH CH CH CH2
O NO2
O H
O
O
Isosorbide Dinitrate;ISORDIL,SORBITRATE
Isosorbide -5-Mononitrate;IMDUR, ISMO
Antianginal Drugs (Nitrates)
Antianginal Drugs & Methemoglobin Reductase
Methemoglobin is hemoglobin that has been oxidized from the ferrous (Fe++) to the ferric (Fe+++) state, thus unable to bind oxygen.
The NADH- methemoglobin reductase enzyme reduces methemoglobin to hemoglobin.
Methemoglobinemia results from either inadequate enzyme activity or too much methemoglobin production
Antianginal Drugs & Methemoglobin Reductase
Deficiency of methemoglobin reductase is inherited as an autosomal recessive trait and occurs with increased frequency in Inuit and Alaskan Native Americans.
Heterozygotes have approximately 50% enzyme activity but without cyanosis, although they are predisposed to the development of toxic methemoglobinemia when exposed to nitrates and other stress.
A family illustrating the inheritance of a deficiency in methemoglobin reductase.
N-ACETYLTRANSFERASE POLYMORPHISM
Distribution of plasma isoniazid concentration in 483 subjectsafter and oral dose. Reproduced from Evans DAP. Br Med J 2:485, 1960.
XENOBIOTICS SUBJECT TO POLYMORPHIC ACETYLATION IN MAN
Hydrazinesisoniazid
hydralazinephenylzine
acetylhydrazinehydrazine
Arylaminesdapsone
procainamidesulfamethazinesulfapyridine
aminoglutethimide
CarcinogenicArylaminesbenzidine
β-naphthylamine4-aminobiphenyl
Drugs metabolized to aminessulfasalazine nitrazepamclonazepam caffeine
ETHNIC DIFFERENCES IN THE DISTRIBUTION OF ACETYLATOR PHENOTYPE
Population % Slow % Hetero Fast % Homo Fast
South Indians 59 35.6 5.4Caucasians 58.6 35.9 5.5Blacks 54.6 38.6 6.8Eskimos 10.5 43.8 45.7Japanese 12 45.3 42.7Chinese 22 49.8 28.2
From: Kalo W. Clin Pharmacokinet 7:373-4000, 1982.
Many CYP450 Enzymes Are Polymorphic: Example CYP 2C19 & CYP 2D6
Family: CYP 2Subfamily: CYP 2D6Gene: CYP 2D6*3
• Responsible for metabolism of 40% of all Rx drugs
OXIDATION POLYMORPHISM
Family: CYP 2Subfamily: CYP 2C19Gene: CYP 2C19*3
CYP2C19 PolymorphismFirst detected from unusual response to anti-epileptic drug mephenytoin (dysphoria/sedation)3-6% of Whites and African Americans, but up to 25% of
Chinese/Japanese/Koreans are PMsThe common true null mutations leading to PM status result from splicing defects (*2) or the loss of start (*4) and stop (*3) codons
c.G636A Exon 4
Pro Trp Ile GlnCYP2C19*1 ….CCC TGG ATC CAG gta…
Pro StopCYP2C19*3 ….CCC TGA ATC CAG gta…
(Truncation of protein at aa 211 - loss of heme/substrate binding domains)
c.G681A Exon 5
Ile CysCYP2C19*1 ….cttag ATA TGC…GGGAA
GluCYP2C19*2 ….cttag atatgc………agGAA
(40 bp deletion from mRNA and premature stop 20 aa downstream in new exon-5)
Phenytoin
A non-linear kinetics after moderate doses
Phenytoin Metabolism
Aromatic hydroxylation
OHO
R R R
phenytoin
Deficient para-hydroxylation will lead to increase phenytoin levelsIncreased toxicity: nystagmus, ataxia and motor impairment
CYP-2C19
Frequency of CYP2C19 Poor Metabolizers
AfricansAfrican-AmericansCaucasiansChinese JapaneseKoreansAmerindians
4.11.42.8
13.6 20.313.7
3.83.32.1
13.8 17.016.85.7
Phenotype Genotype
CYP2C19 SubstratesS-mephenytoinhexobarbitalR-mephobarbitalphenytoindiazepamcitalopram
omperazolelansoprazolepantoprazole
R-warfarin (8-OH)propranolol (in part)imipramineclomipramineamitryptyllineproguanilteniposidenilutamideindomethacinmoclobemide
Cure rates for H. pylori infections may depend upon CYP2C19 genotypes
0102030405060708090
100
genotype
% c
ured
wild-type
heterozygote
homozygousvariant
62 patients with duodenal or gastric ulcertreated with omeprazole 20 mg and amoxicillin20% of Asian and 4% of whites are homo. variantN = 28 25 9
Ann Intern Med 1998;129:1027-30
CYP 2D6 POLYMORPHISMThe polymorphismus of CYP 2D6 (debrisoquine 4-hydroxylase) has been studied in great detail, as metabolic differences have first been described for debrisoquine and sparteine (antipsychotics)
localized on chromosome 22Of the 75 allels, 26 exprime CYP2D6 proteinessee http://www.imm.ki.se/CYPalleles/cyp2d6.htm
CYP 2D6 Polymorphism (II)
Lit: J. van der Weide et al. Ann. Clin. Biochem 36 (1999) 722
CYP 2D6 Polymorphism (III) variability of debrisoquine-4-hydroxylation
N
NH
NH2 N
NH
NH2
HOH
CYP2D6
homocygote extensive metabolizers
= metabolic rate
= number of individuals (european population)
heterozygote extensive metabolizers
homocygote poor metabolizers
Lit: T. Winkler Deutsche Apothekerzeitung 140 (2000) 38
Frequency of CYP 2D6 Poor & Rapid Metabolizers
CaucasiansAsiansEthiopiansSpaniards
5-10%1-2%
rare
133
Poor Rapid
CYP2 D6 Substrates
AntidepressantsHaloperidolβ-Blockers
DextrometorphanCodeine
AmphetamineFlecanide
Phenformin
IDIOSYNCRATIC EFFECTS
Not caused by variations in target proteins or metabolizing enzymes
Chance interaction between the drug and some unusual aspect of the physiology
Hard to predict
Types of Idiosyncratic ResponsesTypes of Idiosyncratic Responses
Drug toxicity due to deficient
metabolism
Increased sensitivity to drug effect
Novel drug effect
Decreased responsiveness to drug
Abnormal distribution of material
Increased Sensitivity to Drug Effect
Nitrites and other drugs causing methemoglobinemia (due to oxidizing effects) -- basis is abnormal hemoglobins (M and H)
Aminoglycoside antibiotic-induced deafness* -- basis unknown; apparent transmission by females
Chloramphenicol-induced bone marrow depression* -- basis unknown
Drugs and Chemicals Unequivocally Demonstrated to Precipitate Hemolytic Anemia
in Subjects with G6PD Deficiency
Acetanilide Nitrofurantoin PrimaquineMethylene Blue Sulfacetamide NalidixicAcidNaphthalene Sulfanilamide
Sulfapyridine Sulfamethoxazole
Glucose-6-Phosphate Dehydrogenase Activity
Effects >100 million worldwide
INCIDENCE OF G6PD DEFICIENCY IN DIFFERENT ETHNIC POPULATIONS
Ethnic Group Incidence(%)Ashkenazic Jews 0.4Sephardic Jews
Kurds 53Iraq 24Persia 15Cochin 10Yemen 5North Africa <4
Iranians 8Greeks 0.7-3
INCIDENCE OF G6PD DEFICIENCY IN DIFFERENT ETHNIC POPULATIONS
Ethnic Group Incidence(%)Asiatics
Chinese 2Filipinos 13Indians-Parsees 16Javanese 13Micronesians <1
African-Americans 10
PrimaquineHaemolysis is pronounced in individuals who are glucose 6-phosphate dehydrogenase deficient (~ 10% of black American males). Primaquine itself is not toxic to erythrocytes.It has been proposed that there is extensive metabolism to unstable catechols and quinones.Primaquine metabolites can place the erythrocyte under oxidative stress.If not rectified, oxidative stress results in oxidation of haemoglobin and critical protein thiols, with Heinz body formation and lysis.
Acute Intermittent Porphyria
Porphyrias are associated with overproduction of porphyrins: acute abdominal pain, psychosis, “purple pee”.Acute intermittent porphyria the exacerbation is induced by barbiturates, sulfonamides, and griseofulvin
Porphyrin
succinyl-CoA+
glycine ALAsynthase
Deaminase(PBG)
Porphobilinogen
Malignant Hyperthermia & HalothaneMalignant Hyperthermia- 1/20,000 with succinylcholineClassic-- rapid rise in body temperature, muscle rigidity, tachycardia, rhabdomyolysis, acidosis, hyperkalemiaLife threatening-I.v. dantroleneGenetic defect in the muscleMutations in the Ryanodine receptor (calcium release channel)Halothane induces potentiation of Ca activity in susceptible patients
Distinguishing Toxic, Idiosyncratic and Allergic Responses to Drugs
Anti-H; SADAntagonistsAntagonistsEffect antagonized
Ag-AbReceptorReceptorMechanism
√Prior exposure
ManyFewAllWhat drugs
√√Dose-related
AllergicIdiosyncraticToxic
Allergic skin reaction
Penicillin rash
PHARMACOGENETICS & CLINICAL PRACTICE
Genetically Based Optimization of Drug Dosing
Responders
Non-responders
Toxic responders
Genetically Based Optimization of Drug Dosing
Non-responders
Toxic responders
Current Applications of Pharmacogenetics
EfficacyHER2Breast CancerHerceptin
SafetyTSColorectal Cancer5-FU
SafetyNot publishedMelanomaMelacine
Safety & EfficacyTPMTALL6-MP
PGx Application
GeneDiseaseDrug
ALL, 6MP and polymorphic TPMT
Children TPMT Toxicity
Children TPMT Poor response
6MP
6MP
Pharmacogenetics: Use in drug delivery
The Wall Street Journal, Friday, April 16, 1999
Pharmacotherapy of Tomorrow?
informed physician diagnosisSavings: time, money & illness
drug d
drug a
drug c
drug b
diagnostic
todayempirical prescription
“mass market”
individual physician experienceCost: time, money & well-being
drug b
drug c
futurerational prescription
“individualized”
define&
treatdrug d
drug a
PharmacogeneticsPharmacogenetics: to deliver : to deliver ‘right ‘right medicine, right dose, to right patient’medicine, right dose, to right patient’
Physician Dx;clinical info