This document discusses pharmacogenetics and therapeutic drug monitoring. It describes how pharmacogenetics can help determine the best drug and dose for individuals by examining genetic factors that influence drug metabolism and response. Two key types of pharmacogenetic effects - analog and digital - are described. Several examples of pharmacogenetic testing for drugs like carbamazepine, allopurinol and cancer therapies are provided. The role of genes like HLA-B*1502, HLA-B*5801, CYP2C9 and VKORC1 in drug responses are outlined. Therapeutic drug monitoring is also discussed as a way to track drug levels in patients' blood to ensure efficacy and safety. Several classes of drugs that are commonly monitored

1. PHARMACOGENETICS AND
THERAPEUTIC DRUG MONITORING
PRACTICE
Larry Baum

2. Pharmacogenetics and
Therapeutic Drug Monitoring
Pharmacogenetics and therapeutic drug
monitoring can help select the best dose and drug
to prevent adverse events and achieve optimal
treatment.

3. Pharmacogenetics
 Many pharmacogenetic associations have been
studied.
 Only a handful are now examined in routine
clinical use.
 Obstacles to more widespread clinical
application of pharmacogenetics:
 Expensive
 Slow
 Evidence yet lacking on validity and usefulness

4. Two types of pharmacogenetic
effects
GG GT
 Analog
 Genetic variation affects level of drug
 Most changes in levels lie within
therapeutic window
 Thus most analog pharmacogenetic effects
not important
 Digital (or Binary)
 Genetic variation affects Safety vs. Toxicity
 Digital pharmacogenetic effects have more
clinical impact than analog effects
 Thus digital effects tend to get more
attention

5. Pharmacogenetics
 Digital
 Targeted cancer therapy
 Carbamazepine or allopurinol
 Analog
 Warfarin

6. Pharmacogenetics
 Targeted cancer therapy
 Pathologists examine tumor biopsies for
expression or mutations
 Expression of HER2 (ERBB2) is observed in
samples of breast tumor tissue to determine the
susceptibility to herceptin treatment.
 Presence of KRAS mutations in lung cancer
tumor tissue indicates that cetuximab would not
be effective.

7. Pharmacogenetics
 Carbamazepine

8. Pharmacogenetics
Highly effective
Serious
side effects
Ineffective
Epilepsy
Neuropathic
pain
Psychiatric illnesses
(e.g. mood disorders)
Carbamazepine

9. Drug Induced Severe Skin Allergies
SJS/TEN
Fauci AS et al, Harrison’s Principles of Internal Medicine, 17th
ed. www.accessmedicine.com

10. Fein and Hamann NEJM 2005;352:1696.
Stevens Johnson Syndrome (SJS)
<30% skin detachment

11. Hall JB, Schmidt GA, Wood LDH. Principles of Critical Care 3rd
Edition. www.accessmedicine.com
Toxic Epidermal Necrolysis (TEN)
>30% skin detachment
• Mortality up to 30%
• 1 in 1000 to 10000
• Occurs in first few
weeks of starting drug
• Unpredictable

12. Drug Induced Severe Skin Allergies
Stevens Johnson Syndrome/Toxic Epidermal Necrolysis
 Skin and mucosa (mouth, eyes) blistering, detachment, fever,
inflammation of lungs, liver
 Mortality up to 30%
 Induced by many drugs, incl. common antiepileptic drugs:
 Carbamazepine, phenobarbital, phenytoin, lamotrigine,
oxcarbazepine
 1 in 1000 to 10000 exposures
 Occurs in first few weeks of starting drug
 Difficult to predict who will develop the allergies
 Up to 10 times more common in Asians than Whites

13. Severe Skin Allergies and HLA-B*1502
 HLA
 Human leukocyte antigen
 Group of genes related to immunity
 Highly polymorphic
 HLA-B
 An HLA class I antigen
 Presents peptides inside cells
 Stimulates killer T cells
 Some variants associated with ankylosing spondylitis, AIDS, or malaria
 HLA-B*1502
 An allele of HLA-B
 Reported in several studies as associated with severe skin allergies and
carbamazepine

14. Severe Skin Allergies and HLA-
B*1502

15. Patient Sex Age at onset
(yrs)
Drug HLA-B*1502
1 F 28 Carbamazepine Positive
2 M 23 Carbamazepine Positive
3 F 53 Carbamazepine Positive
4 M 10 Carbamazepine Positive
5 F 53 Phenytoin Positive
6 F 41 Lamotrigine Positive
Man CBL, Kwan P, Baum L, Yu E, Lau KM, Cheng ASH, Ng MHL. Association between HLA–B*1502
allele and antiepileptic drugs-induced cutaneous reactions in Han Chinese. Epilepsia 2007;48:1015-8
Presence of HLA-B*1502 increased risk 72x
Severe Skin Allergies and HLA-B*1502

16. Rates of HLA-B*1502 in Different
Populations
China 10 – 20%
India 12 – 16%
Taiwan 11 – 15%
Thailand 14 – 16%
Europeans <2%

18. FDA News Dec 12, 2007
 “Patients with ancestry from areas in which HLA-
B*1502 is present should be screened for the HLA-
B*1502 allele before starting treatment with
carbamazepine. If they test positive, carbamazepine
should not be started unless the expected benefit
clearly outweighs the increased risk of serious skin
reactions.”
 “In HLA-B*1502 positive patients, doctors should
consider avoiding use of other antiepileptic drugs
associated with SJS/TEN when alternative therapies
are equally acceptable.”

19. Testing for HLA-B*1502
 Testing for HLA-B*1502 has greatly reduced the
incidence of SJS and TEN.
 Other epilepsy drugs
 Phenytoin, phenobarbital, zonisamide, and lamotrigine,
may also cause hypersensitivity
 Pharmacogenetic testing is not yet routinely performed
before prescribing them.

20. Pharmacogenetics
 Allopurinol

21. Severe Skin Allergies and HLA-
B*58:01
 HLA-B*5801
 An allele of HLA-B
 Increases the risk of severe cutaneous adverse drug reactions
during administration of allopurinol
 Genotyping
 Not yet routine before starting allopurinol
 No rapid, inexpensive test available

22. Warfarin pharmacogenetics
 Pharmacodynamics
From The Pharmacogenomics Knowledge Base: pharmgkb.org

23. Warfarin pharmacogenetics
 Pharmacodynamics
 VKORC1 is target of warfarin
o VKORC1 is subunit of vitamin K epoxide
reductase complex
o Reduces vitamin K
o S-warfarin is 3-5x more potent inhibitor than R
 GGCX
o Reduced vitamin K is needed for function
o Adds CO2 to Glu to form
γ-carboxyglutamic acid (Gla)
o Gla binds calcium
o Gla modification of clotting
factors activates them

24. Warfarin pharmacogenetics
AA AG or GA GG
~90%x90% ~90%x10%+
10%x90%
~10%x10%
~81% ~18% ~1%
 Pharmacodynamics
 Genetics
o VKORC1 -1639G>A
• A allele has lower expression
• Less VKORC1 means less warfarin needed
• A is 40-50% of alleles in Europeans
• A is ~90% of alleles in East Asians
• Where is this SNP more important: Europe or
Asia?
• Remember that each person has 2 alleles

25. Warfarin pharmacogenetics
 Pharmacokinetics

26. Warfarin pharmacogenetics
 Pharmacokinetics
 CYP2C9
o Major metabolism of S-warfarin
o Genetic variants
 Less CYP2C9 means less warfarin needed
 CYP2C9*2 (Arg144Cys)
• Cys has lower activity
• Cys ~10% in Europeans
• Cys ~0% in East Asians
 CYP2C9*3 (Ile359Leu)
• Leu has lower activity
• Leu ~10% in Europeans
• Leu ~3% in East Asians

27. Warfarin pharmacogenetics
 Genome-wide association study
 VKORC1, CYP2C9 polymorphisms had
effects
 No other genes had major effect
 Will genotyping these help clinically?
 Try randomized trial

28. Warfarin pharmacogenetics

29. Warfarin pharmacogenetics
 Genome-wide association study
 VKORC1 polymorphisms had biggest effect
 CYP2C9 polymorphisms had smaller effect
 No other genes had major effect
 Will genotyping these help clinically?
 Try randomized trial
 Proportion of all patients with wrong dose (out-
of-range INRs) wasn’t reduced significantly…
 but was among patients with extreme genotypes
 Reduced number and size of dosing changes
 Thus, genotyping may help somewhat
 Is it cost-effective?

30. Warfarin pharmacogenetics
 Cost-effective if:
 Short turn-around time (TAT)
 Low genotyping cost
 High risk patients
 Should we genotype patients?
 With current TAT and cost, it’s marginal
 Eventually (in a decade?), everyone may be sequenced at
birth
o TAT=0
o Incremental cost = 0
o Therefore, will be cost-effective
 Between now and then, reduced TAT and cost may make
warfarin pharmacogenetics clearly cost-effective

31. Warfarin pharmacogenetics

32. Warfarin pharmacogenetics

33. Warfarin pharmacogenetics

34. Warfarin pharmacogenetics

35. Warfarin pharmacogenetics
 Example: myself
 VKORC -1639: GA
 CYP2C9: *2/*3
 How did I get my genotype?
 http://23andMe.com
 ~US$300 for 1 million SNPs

36. Warfarin pharmacogenetics

37. Warfarin pharmacogenetics

38. Warfarin pharmacogenetics

39. Who offers tests?
 Genelex: http://www.genelex.com
 Gendia: http://www.gendia.eu/
 DNAvision: http://www.dnavision.com/pharmacogenetics.php
 Cincinnati Children’s Hospital:
http://www.cincinnatichildrens.org/service/g/genetic-pharmacology/defau
 Warfarin dosing: http://www.warfarindosing.org
 Many others

40. “Pharmacogenomics will undoubtedly become a very compelling part of
medical practice. The limiting factor right now is that oftentimes, if you are
ready to write a prescription, you do not want to wait a week to find out the
genotype before you decide whether you’ve got the right dose and the right
drug. But if everybody’s DNA sequence is already in their medical record
and it is simply a click of the mouse to found out all the information you
need, then there is going to be a much lower barrier to beginning to
incorporate that information into drug prescribing. If you have the evidence,
it will be hard, I think to say that this is not a good thing. And once you’ve
got the sequence, it’s not going to be terribly expensive. And it should
improve outcomes and reduce adverse events.” – Francis Collins
Pharmacogenomics

41. Cost of sequencing is falling
~US$1500
~RMB10,000

42.  Several classes of drugs
 Some older anti-epileptic drugs, including:
 Phenobarbital
 Phenytoin
 Valproate
 Immune suppressants, including:
 Cyclosporin A
 Everolimus
 Sirolimus
 Tacrolimus
 Methotrexate
 Mycophenolic acid
Therapeutic drug monitoring

43.  Several classes of drugs
 Other drugs, including:
 Digoxin
 Imatinib
 Salicylate
 Tricyclic antidepressants
 Theophylline
Therapeutic drug monitoring

44.  Some older anti-epileptic drugs, including:
 Phenobarbital
 Therapeutic range: 10-30 mg/L
 Method: Immunoassay
 Phenytoin
 Therapeutic range: 10-20 mg/L
 Method: Immunoassay or HPLC
 Valproate
 Serum trough therapeutic range: 50-100 mg/L
 Method: Immunoassay
Therapeutic drug monitoring

45.  Immune suppressants, including:
 Cyclosporin A
 C2 blood level (2 hours post-dose) of ~0.8-2 mg/L, then cut in
half later
 Method: LC-MS/MS, immunoassay (most popular)
 Everolimus
 Trough range of 3-8 µg/L
 Method: LC-MS/MS, immunoassay
 Sirolimus
 Trough range of ~5-15 µg/L
 Method: LC-MS/MS, immunoassay
 Tacrolimus
 Trough range of 5-15 µg/L
 Method: LC-MS/MS, immunoassay
Therapeutic drug monitoring

46.  Immune suppressants, including:
 Methotrexate
 C24 blood level (24 hours post-dose) <10 µM
 Method: HPLC, Immunoassay
 Mycophenolic acid
 Trough range of 1-3.5 mg/L
 Method: HPLC, immunoassay
Therapeutic drug monitoring

47.  Digoxin
 Digitalis has been used for over 2 centuries
 Used for atrial fibrillation and heart failure
 Narrow therapeutic index
 Check serum digoxin 2-4 weeks after starting digoxin if:
 Kidney function abnormal
 Used with drugs that alter digoxin level or effect
 Decrease clearance: verapamil, diltiazem, amiodarone, itraconazole
 Decrease gut bacteria metabolism: some antibiotics
 Affect serum potassium or kidney function
 Toxicity suspected
 Changes in above factors
Therapeutic drug monitoring

48. Rathore SS, et al. JAMA; 2003: 289: 871-878.
Therapeutic drug monitoring

49. Portrait of Dr. Gachet The Starry Night
Therapeutic drug monitoring

50.  Imatinib
 Ctrough > 1 mg/L
 Method: LC-MS
Therapeutic drug monitoring

51.  Salicylate
 Measure concentration in these situations:
 Suspect that too little or too much salicylate consumed
 Change in renal function, mental status, pH, or lung status
 Addition of drug that alters salicylate PK
 Toxic above 300 mg/L
Therapeutic drug monitoring

52.  Tricyclic antidepressants
 TDM recommended
 Reduce risk of intoxication
 Relation between concentration and effectiveness
 When dose changes
Drug Concentration range (ng/ml)
Amitriptyline 80-200
Desipramine 100-300
Dosulepin 45-100
Nortriptyline 70-170
Trimipramine 150-300
Therapeutic drug monitoring

53.  Theophylline
 For asthma or COPD
 Therapeutic range 10-20 mg/L
 Measure trough value at 1-2 half-lives and 5 half-lives
Therapeutic drug monitoring

54.  Labs
 Many drugs or toxic compounds are screened in the
chemical pathology departments of hospitals.
 Chinese medicine toxicology samples are often referred to
centralized poison treatment centers or toxicology
reference laboratories to identify the toxic ingredient.
Therapeutic drug monitoring

55.  “Be careful about reading health books. You may
die of a misprint.” – Mark Twain
Health education
 “I have never let my schooling interfere with my
education.” – Mark Twain

56. The End
Thank you