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, Muscle paralysis after suxamethonium is terminated by elimination of the drug by cholinesterase-mediated hydrolysis.

Other examples Protein = enzymePhenotypeDrugModified Response Plasma pseudocholinesterase slow hydrolysis succinyl- choline prolonged apnea N-acetyltransferase slow, rapid acetylators isoniazid procainamide dapsone slow: toxic neuritis disease susceptibility 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-fluorouracilenhanced toxicity W. Sadee. Br. Med. J. 319:1, 13 Nov

Other examples Protein = receptorPhenotypeDrugModified Response  2 -adrenoceptor enhanced downregulation salbutamol reduced effectiveness in asthma 5-H2TA serotonergic receptor various polymorphisms clozapinevariable efficacy HER2 overexpression in breast and other cancers Herceptin  efficacy W. Sadee. Br. Med. J. 319:1, 13 Nov

Other examples Protein = transporterPhenotypeDrugModified Response Multiple drug resistance transporter overexpression in cancer vinblastin doxorubicin paclitaxel drug resistance W. Sadee. Br. Med. J. 319:1, 13 Nov

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  polymodal distribution multiple genes = polygenic  unimodal distribution

MonogenicPolygenic Detection Abnormal drug response. Polymodal distribution. Twin studies ExampleIsoniazidNortriptyline Figs & 14-4, Rowland and Tozer, p 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 steady state AsiansCaucasiansdifference, % n10 CL o [mL/min/kg]59.8 ± ± t 1/2 [h]4.0 ± ± 3.8 CL m PG [mL/min]337 ± ± 8972 CL m HOP [mL/min]515 ± ± CL m NLA [mL/min]158 ± ± 22 f up [%]16.7 ± ± 1.645

Race & Ethnicity Propranolol, 80 mg steady state w/ 14 C i.v. dose. African Americans Caucasiansdifference, % n1312 CL iv [mL/min]947 ± ± CL o [mL/min]3255 ± ± F [%]34 ± 1037 ± 7 t 1/2 [h]4.2 ± ± 0.5 Q H [mL/min]1449 ± ± f ub [%]16.9 ± ± V ss [L]329 ± ± 3221 V ss,u [L]1960 ± ± 491

Race & Ethnicity: Slow Acetylators PopulationNFrequency [%] Black Sudan Nigeria E. Africa U.S White Britain Germany U.S Chinese Taiwan Britain Hong Kong Mainland Eskimo Canada Alaska Japanese Japan U.S

Race & Ethnicity: Slow debrisoquine-type hydroxylation (CYP2D6) PopulationNFrequency [%] Black Ghana Nigeria White Britain Germany U.S. Sweden Hungary Spain Chinese Canada China Native American Panama510 Arab1021 JapaneseJapan

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: ,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

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 AlleleNucleotide; Protein changes Size [kb] Activity wild type 29 Normal L G  C; 2938 C  T; 296 Arg  Cys 4268 G  C; 486 Ser  Thr 29 A 2637  A 29 Absent B 1934A (+ 6 other mutations) D Deletion11.5 E 3023 A  C; His  Pro 29  T  T; 152 Try  Gly 153 Stop29

CYP 2D6 Polymorphisms con’t AlleleNucleotide; Protein changes Size [kb] Activity C  AGA; 281  Lys 29 Decreased J 188 C  T; 34 Pro  Ser 1749 G  C 4268 G  C; 486 Ser  Thr 29/44 W 188 C  T; 34 Pro  Ser4268 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 L175 Increased (L) 2 Duplication of L42 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 metabolizer10 normal ultra-rapid metabolizer500

Prodrug substrate for CYP2D6 Poor metabolizers show a reduced response: ProdrugActive MetabolitePoor Metabolizers show: encainideO-desmethyl encainidelow antiarrythmic activity codeine O-demethylation  morphine low analgesic action

CYP2D6 Blocking Interactions AjmalicineFluoxetineQuinidine ChinidinLobelinTrifluperidol ChorynanthinePropidinYohimbine 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

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.