Genomics in general practice


Pharmacogenomics: Summary
☰ Table of contents


Practice point


While there are international guidelines about potential uses of pharmacogenomic testing, there is limited evidence from randomised controlled trials of the clinical utility and cost effectiveness of using pharmacogenomics to tailor prescribing, especially in primary care.1,2


What do I need to know?


The term ‘pharmacogenomics’ describes how common gene variants influence drug metabolism and response.
There are common variants in cytochrome P450 enzymes (CYPs) and drug receptors that influence the rate of metabolism of many commonly prescribed drugs. Individuals are typically classified as ‘poor’, ‘intermediate’, ‘extensive’ or ‘ultrarapid’ metabolisers depending on their CYP variants. Poor and ultrarapid metabolisers may require different dosages, or be more susceptible to adverse drug effects.3

Table 1 provides examples of drugs that may be affected by common gene variants.

 

Table 1. Examples of drugs affected by common gene variants1

Gene name

Examples of drugs affected

Clinical consequence

CYP2D6 (5–10% of Caucasians are poor metabolisers; 1–2%
of Caucasians are ultrarapid metabolisers)

Codeine

Poor metabolisers have no response to codeine; ultra- metabolisers are at an increased risk of side effects

Selective serotonin reuptake inhibitors (SSRIs; eg paroxetine, fluvoxamine)

Ultrarapid metabolisers have no response to SSRI; poor metabolisers may need 50% lower dose

CYP2C19 (poor metabolisers 2–15%)

Clopidogrel

Poor metabolisers may require alternative anti-platelet

VKORC1 and CYP2C9

Warfarin

CYP2C9 and VKORC1 genotypes may be useful in determining the optimal initial dose of warfarin

SLCO1B1

Simvastatin (not other statins)

Low-function genotype associated with increased of myopathy; consider alternative statin or lower dose

 


Genetic testing


Pharmacogenomic testing may be considered for patients with:

  • significant side effects from drugs for where pharmacogenomic variation in response is known (refer to Table 1)
  • poor therapeutic response to specific medications
  • potential suitability for using doses outside the usual range.

Pharmacogenomic testing is not subsidised under the Medicare Benefits Schedule (MBS), but can be ordered by general practitioners through a number of commercial providers.

 

More information


Refer to ‘Pharmacogenomics: More information’ for a detailed discussion of this topic.

 
 

Resources for patients


Centre for Genetics Education, Fact sheet 21: Pharmacogenetics/pharmacogenomics
National Library of Medicine, What is pharmacogenetics?


Genomics in general practice




 

 

  1. Clinical Pharmacogenetics Implementation Consortium. CPIC guidelines. Stanford, CA: CPIC, 2014.[Accessed 8 January 2018].
  2. Relling MV, Evans WE. Pharmacogenomics in the clinic. Nature 2015;526(7573):343.
  3. Abbasi J. Getting pharmacogenomics into the clinic. JAMA 2016;316(15):1533–35.

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