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Pharmacology and Therapeutics

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Glossary of Pharmacology 

Pharmacogenetics

Definition: 
Pharmacogenetics is defined as the influence of variation in single gene on a person’s response to drugs.  The word is derived from the Latinized version of the Greek pharmakon (drug) and gene (create).  Pharmacogenetics is distinguished from pharmacogenomics in that pharmacogenetics is concerned the effects of single genes on drug response whereas pharmacogenomics is concerned with the effects of multiple genes (i.e. the genome) on drug response.
Relevance: 
While Sir Archibald Garrod suggested in 1931 that is was likely that genetics played a role in determining drug response, the term “pharmacogenetics” was first termed by Vogel in 1959 and then popularized by Kalow based on observations of the toxicity of succinylcholine in patients with pseudocholinesterase deficiency and of variations in isoniazid clearance due to genetically determined deficiency of N-acetyltransferase.  A number of genetically determined variations have subsequently been described which produce clinically impactful changes in drug disposition and response that impact on drug efficacy and safety.  Thus knowledge of the importance of genetic factors in determining drug response is a critical skill for pharmacologists and for health care workers.   

Examples of How Pharmacogenetics Influences Drug Therapy: 

Drug 

Genomic Association  

Clinical Consequences 

Citralopram 

CYP2C19 

Patients who are poor metabolizers have higher systemic concentrations and are at risk of QT prolongation 

Codeine 

CYP2D6  

Patients who are ultrarapid metabolizers metabolize codeine to morphine more efficiently and may have respiratory depression and death 

Irinotecan 

UGT1A1 

 

Patients who are poor metabolizers can have higher systemic concentrations and increased risk of drug toxicity such as neutropenia and severe diarrhoea 

6-mercaptopurine 

TPMT 

Patients who are homozygous for the poor metabolizer phenotype require dose adjustment to avoid serious bone marrow toxicity  

 

Teaching tips:  

Considerations around how pharmacogenetics used to teach about: 

  • How a genetically determined variation in drug clearance can impact on drug disposition and response 

  • Understanding how genetic and other factors including concurrent therapy impact on drug efficacy and response 

  • Developing a management plan which includes consideration of the genetic make-up of the patient

Suggested articles on this topic:
    • Chang KL, Weitzel K, Schmidt S.  Pharmacogenetics: Using genetic information to guide drug therapy.   Am Fam Physician.2015 ; 92 :588-594. 

      This manuscript provides examples of genetically determined variations in drug clearance and effect that impact on clinical care. 

    Linked terms:  Clinical Pharmacology, Pharmacogenomics, Haplotype, Allele   

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