Pharmacogenetics Flashcards
(35 cards)
Describe pharmacogenomics.
The genome wide approach to prediction of drug responsiveness.
What is the systemic drug concentration?
The systemic drug concentration is the concentration of drug that gives a therapeutic effect. Too high and it will be toxic, too low and it will be ineffective.
What might determine the systemic drug concentration?
The absorption of drug and its bioavailability,
The subsequent distribution of the drug in various body compartments,
The metabolism and excretion of the drug.
Genetic studies have contributed most to what determinant of systemic drug concentration?
From the point of view of pharmacogenetics, the area where genetic studies have contributed most is to those genes that encode enzymes which are responsible for the metabolism of the drugs and those genes that encode target cell receptors.
Why is avoiding adverse drug reactions important?
6.5% of hospital admission in the UK are due to adverse drug reactions.
It is estimated that the annual cost (in 2004) is approximately £500 million to the NHS.
The estimated death rate is more than 10,000 per year.
What is one example of where pharmacogenetics could be used for the avoidance of adverse drug reactions.
Detection of thiopurine methyl transferase deficiency which accounts for some cases of thiopurine drug toxicity.
What can cause some cases of thiopurine drug toxicity?
Thiopurine methyl transferase deficiency accounts for some cases of thiopurine drug toxicity.
Give three examples of thiopurine drugs.
1) . Azathioprine
2) . 6-mercaotopurinol
3) . Thioguanine
What are thiopurines used for therapeutically?
Thiopurine drugs are used as myelosuppressants in patients that have had organ transplants and patients with inflammatory bowel disease, rheumatoid arthritis and eczema that are unresponsive to other treatments.
Azothiprine is a prodrug. Describe the metabolism of Azothioprine into its active form.
Azothioprine is metabolised to 6-mercaotopurine. Thiopurine methyl transferase (TPMT) then methylates 6-mercaptopurine creating methyl-mercaptopurine.
If 6-mercaptopurine is not metabolised by TPMT into methylmercaptopurine then it is converted into Thioguanine nucleotides which can then go on to cause inhibition of DNA and RNA synthesis.
What happens when patients with TPMT deficiency are given thiopurine drugs?
In patients with TPMT deficiency the thiopurines cannot be metabolised efficiently and this is associated with severe bone marrow toxicity due to the accumulation of unmetablised drug.
What happens when patients with very high levels of TPMT are given Thiopurines?
There is a rapid metabolism of the thiopurines which is associated with a risk of ALL relapse and may be associated with hepatotoxicity.
What can cause TPMT deficiency?
There are a number of single nucleotide mutations that can cause amino acid substitutions within the enzyme TPMT.
This results in an increased degradation rate of the enzyme which accounts for the failure to efficiently metabolise the thiopurines.
About 1 in 300 individuals are homozygous for these single nucleotide mutations (or compound heterozygotes) and these individuals have absent TPMT activity.
Heterozygotes bearing one mutant allele are much more common (approximately 1 in 10 individuals) and they have intermediate TPMT activity.
What are the 4 most common TPMT variant alleles?
TPMT1,
TPMT2,
TPMT3A,
TPMT3C.
Have various polymorphisms at 238, 460, 719.
1 and 3A are most common in Caucasians and Africans.
1 and 3C are most common in Indians.
See the table in notes for more info.
How can the TPMT genotype guide Thiopurine treatment?
Homozygote or compound heterozygotes individuals have absent TPMT activity and have a high risk of marrow suppression. They should not be treated with thiopurine drugs.
Individuals that have heterozygosity for a mutant allele have intermediate TPMT activity. These individuals have increased risk of marrow suppression. These individuals may be treated with low dose thiopurines and achieve a therapeutic response.
How can TPMT activity be measured?
TPMT activity can be measured in whole blood within the red cells which is its main location. Nobody knows what the physiological function of TPMT is. Measuring the red cell activity has certain caveats. Firstly the activity decreases with red blood cell age. Therefore, anything that alters the red cell lifespan will likely affect the measured TPMT activity. Measured TPMT activity will also be affected by blood transfusions.
There is evidence that TPMT activity is increased in patients with chronic renal failure who are are a group of patients who are likely to be treated with immunosuppressants.
The measurement of TPMT activity does not predict all cases of marrow suppression.
Before prescribing thiopurine drugs should patients be genotyped, phenotyped or both?
Genotyping for SNPs is technically easy. It is unaffected by medication and illness but a problem is that it will not detect rare variants as current methods only target specific mutations. On the whole only common mutations will be affected. Certain variants are more prominent in some racial groups and there may be variants that we haven’t detected in some racial groups. Therefore it may be difficult to provide a completely comprehensive screen for all genetic variants.
Enzyme activity/phenotyping is technically tricky. It is subject to interference. However, this method will detect reduced TPMT activity due to any variant including rare ones.
The approach that has been adopted in centres that have taken up TPMT testing is to carry out enzyme activity measures as a first test and then follow up patients with low enzyme activities with genotyping tests.
TPMT genotyping/phenotyping can assist in deciding starting does of thiopurines.
Monitoring blood count and liver function remains mandatory whichever method is used.
Why might we want to use pharmacogenetics to assist in choosing an appropriate drug?
The use of an inappropriate drug delays effective treatment and exposes patients to unnecessary risk of adverse drug reactions. It also incurs unnecessary expense.
Give an example of one situation in which pharmacogenetics can be used to assist in the choice of appropriate drugs.
Herceptin and breast cancer.
How can pharmacogenetics be utilised in selecting breast cancer patients for which Herceptin will be an appropriate treatment?
The selection of the drug Herceptin for the treatment of breast cancer can be guided by genetic testing of the tumour tissue.
Her2 is a tyrosine kinase growth factor receptor that responds to EGF and other growth factors. Binding of the relevant growth factor to the Her2 receptor triggers gene activation and cell division and thus growth of the tumour.
Clinically, the importance of this is that there is Her2 gene amplification in 25-30% of breast tumours. Her2 is positively associated with more aggressive tumours. Her2 positive tumours are responsive to Herceptin.
In detecting patients for treatment with Herceptin, Her2 can be detected initially by IHC of tumour tissue and in selected cases the increased copy numbers of the Her2 gene can be established either by FISH or Her2 PCR.
There are ethical issues that arise from the utilisation of genetic tests to select an appropriate treatment. For example, is withholding a drug deemed to be ineffective on genetic testing acceptable? Will pharmacogenetics cause unacceptable disparity in prescribing to different ethnic groups? Will pharmaceutical developments be adversely affected by market segmentation as those with rarer genotypes are neglected in drug development because the market is smaller?
Why might you want to use pharmacogenetics for rapid optimisation of drug dose?
There is wider inter-individual variation in plasma levels and during drug responsiveness which are the result of complex interactions between genetic and environmental factors.
Currently we use a trial and error approach to drug dosing.
Describe an example of a situation in which pharmacogenetics can guide optimisation of drug dose.
Anticoagulantion with Warfarin.
Describe who warfarin acts and how optimisation of warfarin dose can be achieved with pharmacogenetic assistance.
Warfarin is prescribed to patients who are at risk of thromboembolism due to pulmonary embolus, DVT or atrial fibrillation.
Warfarin acts by inhibiting the enzyme Vitamin K reductase, which recycles oxidised vitamin K to its reduced form following use in carboxylation of prothrombin and coagulation factors VII, IX and X. Carboxylation is essential for the activation of these coagulation factors. Therefore if warfarin inhibits vitamin k reductase, vitamin k does not return to its reduced form and the coagulation factors cannot be carboxylated and so remain non-functional.
Warfarin is metabolised by cytochrome p450 enzyme CYP2C9. Optimising the warfarin dosage is extremely important because under treatment will fail to achieve reduction in risk of thromboembolism. Patients are often given a loading dose to achieve therapeutic levels rapidly. Overdose of warfarin increases the risk of haemorrhage.
Patients on warfarin are monitored using the International Normalised Ratio to assess the activity of the clotting factors. Typically the target INR is between 2 and 3.
The first gene that was of interest in the pharmacogenetics of warfarin was the CYP2C9 gene which encodes the cytochrome p450 enzyme. Cytochrome p450 enzyme metabolises not only warfarin but many other drugs glipizide and Ibuprofen.
Single base changes at aa144, 359 and 360 can result in amino acid substitutions which render the CYP2C9 enzyme less active.
The main allele of CYP2C9 is *1 on Caucasian and Asian populations, *2 is found in both populations also. *3 is only found in Caucasians. *4 and *5 is found in Japanese and African American populations only.
Possession of 1 or 2 copies of a variant CYP2C9 allele predicts low warfarin dosage. Other factors that predict low warfarin dosage include advanced age, low body weight, white race, low serum albumin, tobacco use, cardiac failure and liver disease, drug interactions.
What factors predict low warfarin dose?
Possession of 1 or 2 copies of a variant CYP2C9 allele predicts low warfarin dosage. Other factors that predict low warfarin dosage include advanced age, low body weight, white race, low serum albumin, tobacco use, cardiac failure and liver disease, drug interactions.
