Where is methotrexate metabolized

The CT polymorphism was identified by Frosst et al. This polymorphism leads to an alanine to valine amino acid change at codon This has functional effects, leading to the formation of an enzyme with reduced activity. A second common polymorphism AC is a glutamic acid to alanine substitution at codon and was shown by Weisberg et al.

One might predict that patients with reduced MTHFR enzyme activity would be more susceptible to toxicities from MTX because of the effects on homocysteine metabolism, although it is more difficult to determine how such polymorphisms would influence drug efficacy. Van Ede et al. No difference in MTX efficacy was seen between the groups [ 25 ]. The predictive power of the genotype was independent of folate supplementation status.

Urano et al. In addition, there was a trend for improved efficacy with carriage of the C allele, with a greater change in the ESR and CRP although no significant differences in either tender or swollen joint counts. No association was observed between the AC polymorphism and either efficacy or toxicity [ 33 ]. Berkun et al. They found no association between CT and MTX toxicity but found that patients with the AA wild-type genotype were 5 times more likely to develop adverse events than those with the CC genotype OR 5.

Hughes et al. In contrast to other authors, they did not find an association between CT and adverse events on MTX. However, they found that carriage of the A variant allele was associated with at least one MTX adverse event OR In contrast, Wessels et al. In addition, carriers of the C allele developed more adverse events OR 2.

However, this study was based on clinical response at 6 months and to date no longer term outcome data is available.

The results for the AC SNP are inconsistent, with some studies suggesting an association between AA genotype and toxicity [ 34 , 35 ] and others with the CC genotype and toxicity [ 18 ]. To date, however, all of the studies have reported single cohorts where different definitions of drug efficacy and toxicity are used. In particular, different definitions for drug toxicity are used, with some authors reporting mild adverse events not requiring drug cessation meaning some adverse events may not be directly attributable to MTX and others reporting severe adverse events.

In addition, many of the studies reported are retrospective which may have led to errors in the estimation of MTX effectiveness, a problem that can be resolved by using clinical trial subjects.

Therefore, further studies using similar definitions for drug efficacy and toxicity are required to clarify these apparent inconsistencies. Whilst this has not been studied systematically in RA, in vitro studies in lymphocytic leukaemia suggest that low level gene amplification of DHFR or mutations in the enzyme may provide a mechanism of acquired resistance to MTX [ 10 ]. However, to date the effect of this polymorphism has not been studied in other populations or correlated with outcome of MTX treatment.

MTX has potent effects on adenosine metabolism and this provides a novel target for pharmacogenetic studies. Wessels et al. Examining the effects of adenosine pathway SNPs and toxicity, Wessels et al. In a small prospective analysis of 48 patients on MTX, Dervieux et al. Since the anti-inflammatory effects of adenosine are mediated by several receptors, we investigated the effect of polymorphisms within the adenosine receptor 2A ADORA2A and outcome of MTX therapy.

In subgroup analysis, this association appeared to be specific for GI adverse events [ 40 ]. Several recent studies have examined the effect of a number of genetic polymorphisms on MTX efficacy and toxicity. Patients with at least 1 homozygous variant were 3. When they expanded their study population to including patients on low-dose steroids and refined their scoring of the index, a lower pharmacogenetic score i.

Further prospective studies are required to determine the clinical utility of such a measure in predicting drug response. Serine hydroxyl methyltransferase SHMT is an enzyme that is important in the synthesis of 5,10 methylene tetrafolate.

Weisman et al. For each unit increase of the index there was a 1. However, since the adverse events were mild not requiring treatment cessation , again replication in a prospective cohort is required to determine the positive predictive value of such a measure in clinical practice.

In their prospective analysis of 48 patients, however, Dervieux et al. To date, a number of studies have reported potential genes that may be associated with toxicity or response to MTX. However, to date many of these associations have been reported in single cohorts and have either not been replicated or show inconsistent findings as exemplified by MTHFR.

There are a number of explanations for these apparent inconsistencies. First, the majority of studies have been from single cohorts with no replication sets and with relatively small numbers of patients, which both limits the study power and increases the likelihood of false positive associations. Given the small effect size quoted for many of the SNPs with reported ORs of around 2 , large sample sizes are required to prevent false negative results.

In addition, different populations have been studied using varying definitions of drug toxicity and efficacy, which may lead to selection bias. These different outcome criteria mean that it is difficult to compare results between studies. Other issues which need to be considered when interpreting genetic association studies include how the SNPs typed are selected since many studies report single or selected SNPs rather than whole gene coverage , genotyping quality, relevant data analysis with appropriate correction for the number of analyses performed together with the likely validity of interpretation.

These issues are reviewed in detail by Hattersley et al. However, given the strength of association observed for many of the individual SNPs, the predictive value of single SNPs may ultimately be limited. Given the complex cellular metabolism of MTX, it is more likely that a combination of genotypes may identify patients at greater risk of drug inefficacy or toxicity. In addition, the effect of genetic markers may be influenced by other environmental factors such as folate status, with potential gene—nutrient interactions.

Large-scale, prospective studies with more comprehensive gene coverage are necessary. In addition, toxicity and efficacy phenotypes need to be clearly defined with replication sets planned. Ultimately pharmacogenetics alone may not have sufficient predictive power, and therefore post-translational and biochemical changes around the MTX pathway such as polyglutamation and cytokine changes may need to be included to fully understand MTX response.

From a clinician's standpoint we are still a long way from being able to accurately predict an individual's likely outcome on MTX. Nevertheless, given its central importance in the treatment of RA and related diseases a major effort is justified to allow us to personalize the use of this therapy more effectively and safely. Google Scholar. Oxford University Press is a department of the University of Oxford.

It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Intracellular metabolism of MTX. Transporter pharmacogenetics. Cellular pharmacogenetics. Summary and future directions.

The pharmacogenetics of methotrexate. Hider , S. Oxford Academic. Revision received:. Cite Cite S. Select Format Select format. Permissions Icon Permissions. Abstract Methotrexate MTX is a cornerstone of therapy for rheumatoid arthritis. Rheumatoid arthritis , MTX , Pharmacogenetics. Open in new tab Download slide. T able 1. MTHFR polymorphism. Open in new tab. Google Scholar Crossref. Search ADS. Google Scholar PubMed. The human multidrug resistance protein MRP5 transports folates and can mediate cellular resistance against antifolates.

Differences in folylpolyglutamate synthetase and dihydrofolate reductase expression in human B-lineage versus T-lineage leukemic lymphoblasts: mechanisms for lineage differences in methotrexate polyglutamylation and cytotoxicity. Novel aspects of resistance to drugs targeted to dihydrofolate reductase and thymidylate synthase. Pharmacogenetic and metabolite measurements are associated with clinical status in patients with rheumatoid arthritis treated with methotrexate: results of a multicentred cross sectional observational study.

Pharmacogenomic and metabolic biomarkers in the folate pathway and their association with methotrexate effects during dosage escalation in rheumatoid arthritis. Polymorphism G80A in the reduced folate carrier gene and its relationship to methotrexate plasma levels and outcome of childhood acute lymphoblastic leukemia. Contribution of common polymorphisms in reduced folate carrier and gamma-glutamylhydrolase to methotrexate polyglutamate levels in patients with rheumatoid arthritis.

Polyglutamation of methotrexate with common polymorphisms in reduced folate carrier, aminoimidazole carboxamide ribonucleotide transformylase, and thymidylate synthase are associated with methotrexate effects in rheumatoid arthritis. There is a marked interindividual variability in the extent of absorption of oral methotrexate.

Conversely, the intraindividual variability is moderate even over a long time period. Intramuscular and subcutaneous injections of methotrexate result in comparable pharmacokinetics, suggesting that these routes of administration are interchangeable. Again, the unbound fraction exhibits a large interindividual variability. Methotrexate distributes to extravascular compartments, including synovial fluid, and to different tissues, especially kidney, liver and joint tissues.

Finally, the drug is transported into cells, mainly by a carrier-mediated active transport process. Methotrexate is partly oxidised by hepatic aldehyde oxidase to 7-hydroxymethotrexate. Both methotrexate and 7-hydroxy-methotrexate may be converted to polyglutamyl derivatives which are selectively retained in cells.

Methotrexate is mainly excreted by the kidney as intact drug regardless of the route of administration. The drug is filtered by the glomeruli, and then undergoes both secretion and reabsorption processes within the tubule. These processes are differentially saturable, resulting in possible nonlinear elimination pharmacokinetics. The usually reported mean values for the elimination half-life and the total body clearance of methotrexate are 5 to 8 hours and 4.

A positive correlation between methotrexate clearance and creatinine clearance has been found by some authors. Finally, the pharmacokinetics of low-dose methotrexate appears to be highly variable and largely unpredictable even in patients with normal renal and hepatic function. Furthermore, studies in patients with juvenile rheumatoid arthritis provide evidence of age-dependent pharmacokinetics of the drug.

These features must be considered when judging the individual clinical response to methotrexate therapy.