LONDON ¿ Sometime during the next millennium, someone who visits the doctor is likely to be able to undergo a battery of rapid tests ¿ one set to show his or her genetic susceptibility to a range of diseases, another to indicate which of many infections pose a particular threat, and yet another to tell the physician which drugs the patient has difficulty metabolizing, thus risking severe side effects if they are prescribed.
A study in the Lancet last month reports one of the first steps towards the third of these scenarios: the identification of a human genetic polymorphism which affects the metabolism of a commonly prescribed drug, warfarin. Ann Daly, senior lecturer in pharmacology at the University of Newcastle Upon Tyne Medical School, and senior author on the paper, told BioWorld International: ¿Many of the advances in the area of human gene polymorphisms are not really being applied at the moment to the clinical situation, but this report provides a good example of bringing molecular biology to bear on an applied medical science.¿
Rapid on-the-spot tests that would allow physicians to diagnose a certain genotype in the clinic and adjust drug treatment accordingly are not yet available, of course. While it is too early to say with certainty that such tests would be clinically useful, Daly said preliminary indications are ¿encouraging.¿
The Lancet paper is titled: ¿Association of polymorphisms in the cytochrome P450 CYP2C9 with warfarin dose requirement and risk of bleeding complications.¿ The study it reports, by Guruprasad Aithal and colleagues in the department of medicine at the University of Newcastle upon Tyne, together with Daly, found that people with certain genetic variants of an enzyme belonging to the family of cytochrome P450s were more than six times as likely to need a lower dose of the anticoagulant drug warfarin, because they have difficulty metabolizing it.
Warfarin is a commonly prescribed drug which is used to treat various cardiovascular conditions such as myocardial infarction (heart attack). It is an anticoagulant, reducing the blood¿s tendency to clot. Warfarin is given as a mixture of two slightly chemically different forms, R-warfarin and S-warfarin. It works by inhibiting an enzyme ¿ vitamin K reductase ¿ which is involved in the synthesis of vitamin K. S-warfarin is more efficient at this than R-warfarin.
Polymorphisms May Modulate Enzyme
Unusually, the optimum dose of warfarin must be individually determined for each patient. The effective daily dose ranges from 0.5 milligrams to 60 milligrams. If the dose is too high, the patient will be at risk of serious hemorrhage, which may be life-threatening; if too low, clots may form.
Patients being prescribed warfarin for the first time are therefore usually kept in the hospital until the correct dose for them can be determined.
One of Daly¿s main research interests has been the genetics of the cytochrome P450s, which metabolize many commonly prescribed drugs. Cytochrome P450 CYP2C9 metabolizes S-warfarin, as well as some non-steroidal anti- inflammatory drugs, such as ibuprofen, and a drug called phenytoin, which is used to treat epilepsy.
Recently, two polymorphisms were discovered in the gene encoding cytochrome P450 CYP2C9, which seemed to modulate the enzyme¿s activity. The wild type allele is known as CYP2C9*1, but two variants have also been identified: CYP2C9*2, where cysteine substitutes for arginine at amino acid 144; and CYP2C9*3, where leucine substitutes for isoleucine at amino acid 359. The second of these two variants has less than 5 percent of the activity of the wild type enzyme with S-warfarin, while the first has about 12 percent.
Daly said: ¿As we had a polymerase chain reaction test for these polymorphisms, this made it relatively easy to screen patients for their genotype. Because warfarin is important in clinical practice, and because the dose has to be individualized, we thought it would be interesting to look at the relationship between polymorphisms of cytochrome P450 CYP2C9 and warfarin dose requirement.¿
The researchers therefore examined the frequency of the two variant alleles in three groups of patients. The first comprised patients already on warfarin, whose requirement for warfarin was low ¿ a dose of 1.5 milligrams or less. The second was patients already on warfarin, but on a wide range of doses. The third comprised controls recruited through local general practices.
Genotype Predictor Of Warfarin Dose?
They found that 81 percent of the 36 patients in the low-dose warfarin group had one or more of the variant alleles, compared with 40 percent of the 100 patients in the control group. Daly said: ¿We saw quite a strong increase in the number of people heterozygous for one of these variant alleles in the low dose group. We thought originally that perhaps it would be only those who were homozygous for the variants who would be particularly affected, but there is a strong effect in heterozygotes too.¿
In their paper, the researchers write: ¿An individual requiring a low warfarin dose is six times more likely to be positive for one or more of the variant alleles associated with impaired S-warfarin metabolism (CYP2C9*2 and CYP2C*3) compared with the general population. CYP2C9 genotyping appears to have the potential to identify a subgroup of individuals who are poor metabolizers of warfarin, hence they require a very low dose of drug.¿ Their findings indicate, they add, that many of these individuals have an increased risk of bleeding complications when on warfarin, when compared with clinic controls.
¿We believe,¿ she concluded, ¿that genotype for this particular polymorphism is going to be a predictor of warfarin dose requirement, although other factors will also contribute, such as diet and the patient¿s age. There may also be other genetic factors which we don¿t know about at present, but this polymorphism does seem to be a significant factor and it might be useful to use it in initial screening prior to starting a patient on warfarin for the first time.¿
She hopes next to embark on a larger prospective study which would examine the genotypes of patients being prescribed warfarin for the first time. The progress of these patients would be followed over several years. However, the group has not yet obtained funding to carry out this study. n