Pharmacogenetic tests: the need for a level playing field

a level playing fieldMunir Pirmohamed and Dyfrig A. Hughes The delivery of more personalized medicine could be accelerated by addressing the substantial differences in the level of evidence required for the inclusion of pharmacogenetic tests in treatment guidelines, drug labelling and reimbursement schemes compared with that needed for non-genetic diagnostic tests.
There is an increasing drive, within both drug develop­ to those for patients with normal renal function. Indeed, ment and clinical practice, to stratify or personalize such a stipulation would be impractical and costly, and medicines to improve clinical outcomes for patients. would never be done during the drug development Pharmacogenetics is perhaps the most well­developed process, potentially disadvantaging vulnerable patient of the various technologies that can be used to personal­ populations.
ize medicines, but it has been criticized for not deliver­ Another major determinant of variability in drug ex ­ ing on this promise. There are several science­related po sure is genetic polymorphisms in drug­metabolizing reasons why many pharmacogenetic studies do not enzymes and drug transporters. For example, cyto­ progress beyond the stage of biomarker discovery, such chrome P450 2D6 (CYP2D6) is responsible for the as inadequate sample sizes, poor clinical phenotype or metabolism of ~25% of drugs, but ~7% of the Caucasian poor study design. However, we would also strongly population (termed poor metabolizers (PMs)) lack this argue that the slow progress in the implementation of enzyme, whereas another 2% of Northern Europeans, pharmacogenetic (and indeed other genetic) tests can 10% of Southern Europeans and up to 30% of some partly be explained by the fact that different criteria are African populations carry more than two copies of the applied when considering genetic testing compared gene, and are termed ultra­rapid metabolizers (URMs). with non­genetic diagnostic tests. Three specific areas Atomoxetine, a drug widely used for attention deficit hyperactivity disorder, is metabolized in the liver by First, drug response (efficacy or toxicity) is related CYP2D6. The states that the to the dose administered, but more importantly to dose should be reduced by 50% in patients with hepatic the systemic and cellular exposure to the drug and its impairment (Child­Pugh class B), as drug exposure goes metabolites. As most drugs and/or metabolites are elimi­ up by twofold. It is also known that drug exposure is nated via the kidney and/or the liver, there are numer­ increased by a similar amount in CYP2D6 PMs; how­ ous regulatory documents that provide guidance on ever, although the SmPC for atomoxetine mentions the performing pharmacokinetic and pharmacodynamic effect of CYP2D6 polymorphisms, it does not mandate assessments on patients with impaired liver or kidney testing for their presence. It has been stated that atomo­ function and using this data to develop dosing recom­ xetine is equally efficacious and well tolerated among mendations. For example, thpatients with the different CYP2D6 genotypes1, but Personalised Medicine, University of Liverpool, tates: “after an given the occurrence of numerous adverse effects in initial usual dose, the dosage of aztreonam should be children, including arrhythmias and hepatotoxicity, can halved in patients with estimated creatinine clearances we be sure? The role of such polymorphisms in predis­ between 10 and 30 mL/min/1.73 m2”. There are many posing to toxicity from increased drug and/or metabo­ other drugs that contain similar recommendations lite exposure is only now becoming clear, even with within their labelling. Of course, this is logical in order old drugs such as codeine; there is increasing concern to prevent variability in exposure and thus in the efficacy about the risk of respiratory depression in children who and/or toxicity of a drug. There is no regulatory require­ are CYP2D6 URMs because of increased conversion ment to undertake clinical trials to show that the dosing of codeine to morphine2. Notably, the new recommendations for patients with, for example, renal impairment are equivalent in terms of clinical outcomes has recommendations on NATURE REVIEWS | DRUG DISCOVERY
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2013 Macmillan Publishers Limited. All rights reserved dosing evaluation in patients with polymorphisms in treated with carbamazepine — do not enjoy these ben­ known metabolic pathways, an important pioneering efits. Subsidizing treatments to manage, but not tests development that will begin to ensure harmonization of to prevent, serious conditions appears inconsistent. dosing strategies as part of drug development.
Moreover, many treatments for rare diseases are very The second example relates to the use of diagnostic expensive and exceed conventional thresholds of cost­ tests. In general, the evidence required for implementa­ effectiveness. However, these are still available in most tion of non­genetic tests in clinical practice seems to be markets, implying that more lenient criteria are applied relatively haphazard and varies between genetic and non­ for reimbursement. Pharmacogenetic tests typical y cost genetic tests. For example, renal impairment is widely less than their value­based price (that is, the price that regarded as a risk factor for hypersensitivity to the gout results in a cost­effectiveness ratio of £30,000 per qual­ drug al opurinol, with drug package inserts recommend­ ity­adjusted life­year in the United Kingdom), and much ing the use of lower doses in patients with renal impair­ less than the premium prices of many orphan drugs. As ment. The origin of this association is from case reports with other diagnostics, however, their value is appropri­ and case series in the 1970s and 1980s, and it has become ated to the treatments they accompany. Whereas the widely accepted despite the fact that other studies have annual cost of abacavir is ~£6,100, for instance, the cost shown no differences in adverse events according to cre­ of HLA-B*57:01 testing is £50 (less than 1% of the cost atinine clearance, and there have been concerns about of abacavir). However, the value of testing is not lim­ under­dosing to control urate levels in clinical practice. ited to the reduction in the incidence of hypersensitivity By contrast, there is now extensive evidence showing reactions: it also increased HIV clinicians’ confidence that the presence of the HLA-B*58:01 allele predisposes in the drug and therefore led to more widespread use to serious cutaneous adverse drug reactions to allopu­ of abacavir. Different strategies for setting the price of rinol3, but this has not been implemented in guidelines pharmacogenetic tests may therefore be warranted, such or as yet in drug package inserts. Prospective studies are that their true value is reflected in their price.
quoted as being necessary to demonstrate evidence of Our arguments here are not meant to suggest that clinical utility. Indeed, this has been done for two other special consideration should be given to genetic tests, drugs that can cause severe hypersensitivity reactions: but more that the same level of evidence needs to be the anti­HIV drug abacavir4 (for which the presence of applied to genetic and non­genetic tests for adoption the HLA-B*57:01 al ele has been associated with hyper­ into guidelines and drug package inserts. Incentives for sensitivity) and the anti­epileptic drug carbamazepine5 the development of genetic tests, their pricing and reim­ (for which the presence of the HLA-B*15:02 allele has bursement arrangements must also be aligned with other been associated with a life­threatening hypersensitivity health technologies.
disorder known as Stevens–Johnson syndrome). Part of 1. Sauer, J. M., Ring, B. J. & Witcher, J. W. Clinical pharmacokinetics the reason for differences in evidential standards may of atomoxetine. Clin. Pharmacokinet. 44, 571–590 (2005).
be the familiarity and availability of non­genetic tests, 2. Ciszkowski, C., Madadi, P., Phillips, M. S., Lauwers, A. E. & Koren, G. Codeine, ultrarapid-metabolism genotype, and postoperative death. and the perceived low costs of these tests. However, it is N. Engl. J. Med. 361, 827–828 (2009).
important to note that genetic tests only need to be done 3. Zineh, I. et al. Allopurinol pharmacogenetics: assessment of potential clinical usefulness. Pharmacogenomics 12, 1741–1749
once, whereas protein­ or metabolite­based tests, such as renal function tests, need to be performed repeatedly. So, 4. Mallal, S. et al. HLA-B*5701 screening for hypersensitivity to abacavir. N. Engl. J. Med. 358, 568–579 (2008).
over a lifetime, the costs of the non­genetic tests may be 5. Chen, P. et al. Carbamazepine-induced toxic effects and equivalent and indeed may become greater as the costs HLA-B*1502 screening in Taiwan. N. Engl. J. Med. 364,
A final difference between genetic and other health Disclaimer
The views expressed in this article are those of the authors, and not of any technologies is apparent with respect to legislation on institutions that they represent.
orphan drugs in various regions. Such legislation pro­ Competing financial interests
vides incentives for manufacturers to develop treatments The authors declare no competing financial interests.
for life­threatening or chronically debilitating diseases that are rare (in the European Union, diseases that affect FURTHER INFORMATION
European Medicines Agency publishes guideline on use of
no more than 5 in 10,000 people). The incentives range pharmacogenetics in evaluating pharmacokinetics of medicines: from reductions in regulatory fees to 10 years of mar­ ket exclusivity once authorized (7 years in the United States). However, manufacturers of genetic tests that prevent equally severe and rare conditions — such as SmPC for aztreonam: http://www.medicines.org.uk/EMC/medicine/549/SPC/ testing for the presence of HLA-B*15:02 to avoid the Azactam+1g+or+2g+Powder+for+Solution+for+Injection+or+Infusion%2c+vial development of Stevens–Johnson syndrome in patients ALL LINKS ARE ACTIVE IN THE ONLINE PDF
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