Rachmadi NR.
Ethnic variability in methotrexate.
Ethnic variability in methotrexate pharmacogenetics among rheumatoid arthritis populations: a narrative review Nabila Risti Rachmadi1.
Lathifa Nabila2 Zullies Ikawati3*.
Ayu Paramaiswari4 Master of Clinical Pharmacy.
Faculty Pharmacy.
Universitas Gadjah Mada.
Yogyakarta.
Indonesia, 2Doctoral in Pharmaceutical Science.
Faculty Pharmacy.
Universitas Gadjah Mada.
Yogyakarta.
Indonesia, 3Department of Pharmacy.
Faculty Pharmacy.
Universitas Gadjah Mada.
Yogyakarta.
Indonesia, 4Faculty of Medicine.
Public Health, and Nursing.
Universitas Gadjah Mada.
Yogyakarta.
Indonesia https://doi.
org/10.
22146/ijpther.
ABSTRACT
Submitted: 04-03-2026 Accepted : 25-05-2026 Ethnic variability.
Polymorphism.
Methotrexate.
Pharmacogenetics.
Rheumatoid arthritis Methotrexate is the recommended first-line conventional disease-modifying antirheumatic drug for rheumatoid arthritis.
interindividual variability in treatment response and toxicity remains a major clinical challenge.
Pharmacogenomics provides a biological basis for this variability, yet reported associations between genetic polymorphisms and methotrexate outcomes are often inconsistent across This narrative review synthesizes current evidence on ethnic variability in methotrexate pharmacogenetic associations among patients with rheumatoid A structured literature search was conducted in PubMed/MEDLINE.
Scopus, and Google Scholar up to January 2026, including human studies evaluating genetic polymorphisms in methotrexate-related pathways and reporting outcomes related to treatment effectiveness or toxicity.
The available evidence is dominated by observational studies in populations of European and East Asian ancestry, with limited data from Southeast Asian cohorts.
Polymorphisms in genes involved in methotrexate transport, intracellular activation, folate-dependent metabolism, and drug efflux demonstrate substantial interethnic differences in allele frequencies and reported clinical associations.
Variants in folate metabolism genes are more consistently associated with methotrexate-related toxicity than with treatment effectiveness, whereas transporter and downstream pathway variants show population-dependent and heterogeneous associations with effectiveness.
These inconsistencies are further influenced by methodological heterogeneity, including differences in outcome definitions, methotrexate dosing, folate supplementation, sample size, and adjustment for concomitant therapies.
Overall, ethnic variability appears to be a key determinant of methotrexate pharmacogenetic associations in rheumatoid arthritis.
Populationspecific studies using standardized disease activity outcomes are required, particularly in underrepresented Southeast Asian populations especially Indonesia, to support equitable implementation of precision medicine strategies.
ABSTRACT
Metotreksat merupakan obat lini pertama yang direkomendasikan untuk artritis reumatoid, namun variasi respons klinis dan kejadian toksisitas antar pasien masih menjadi tantangan utama dalam praktik klinik.
Farmakogenetik memberikan dasar biologis terhadap variasi tersebut, tetapi hubungan antara polimorfisme genetik dan luaran terapi metotreksat sering dilaporkan tidak konsisten antar populasi.
Tinjauan naratif ini bertujuan merangkum bukti terkini mengenai variasi etnis pada asosiasi farmakogenetik metotreksat pada pasien artritis reumatoid.
Pencarian literatur terstruktur dilakukan pada PubMed/MEDLINE.
Scopus, dan Google Scholar hingga Januari 2026, mencakup studi pada manusia yang mengevaluasi polimorfisme genetik pada jalur farmakologi metotreksat serta melaporkan luaran efektivitas atau toksisitas terapi.
Bukti yang tersedia didominasi oleh studi observasional pada populasi leluhur Eropa dan Asia Timur, sementara data dari populasi Asia Tenggara masih terbatas.
Polimorfisme pada gen yang berperan dalam transport metotreksat, aktivasi intrasel, metabolisme bergantung folat, dan pengeluaran obat menunjukkan perbedaan frekuensi alel dan asosiasi klinis antar etnis.
Varian pada gen metabolisme folat lebih konsisten berhubungan dengan toksisitas dibandingkan efektivitas terapi, sedangkan varian transporter dan jalur hilir menunjukkan asosiasi yang bergantung pada populasi dan heterogen.
Variasi metodologis, termasuk perbedaan definisi luaran, dosis metotreksat, suplementasi folat, ukuran sampel, dan pengendalian terapi penyerta, turut memperkuat ketidakkonsistenan temuan.
Secara keseluruhan, variasi etnis diduga menjadi determinan penting dalam farmakogenetik metotreksat pada artritis reumatoid, sehingga diperlukan penelitian spesifik populasi dengan luaran aktivitas penyakit yang terstandarisasi, khususnya pada populasi Asia Tenggara terkhususnya Indonesia.
*corresponding author: zullies_ikawati@ugm.
Indonesian Journal of Pharmacology and Therapy is licensed under CC BY-NC 4.
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INTRODUCTION
Precision emerged as a central paradigm in the management of autoimmune playing an essential role in explaining interindividual variability in drug Pharmacogenomics affecting therapeutic efficacy and (RA), where treatment response is influenced by complex inflammatory and immunological pathways, genetic contribute to therapeutic variability and clinical outcomes.
Rheumatoid
(RA)
disease characterised by persistent synovial inflammation, progressive joint destruction, and substantial functional impairment.
4,5 The global prevalence of RA has continued to rise, with an estimated 17.
6 million affected individuals worldwide in 2020, and projections indicating further increases over the coming decades.
6 Disease activity is commonly evaluated using a validated composite instrument such as the Disease Activity Score-28 (DAS-.
, which integrates clinical and laboratory parameters including swollen joints, tender joints, acute phase response, and overall health to provide a continuous assessment of rheumatoid arthritis disease activity to guide therapeutic decision-making.
Methotrexate (MTX) is the firstline conventional synthetic diseasemodifying .
sDMARD) for RA due to its established efficacy, flexible dosing, and favourable cost-effectiveness profile.
5 Despite its widespread use, considerable variability exists in treatment response and 8 Methotrexate is particularly sensitive to genetic variability because on active cellular transport and intracellular polyglutamation processes.
MTX requires transporter-mediated uptake and intracellular conversion into methotrexate polyglutamates to exert its anti-inflammatory effects.
Consequently, polymorphisms in genes involved in methotrexate transport, polyglutamation, and drug efflux pathways may substantially influence intracellular drug exposure, treatment response, and toxicity risk.
A key factor contributing to these discrepancies is ethnic variability in genetic architecture.
Ethnicity, often used as a surrogate for shared genetic ancestry, may influence the distribution variants, reflecting population-specific allele frequency distributions, haplotype structures, and linkage disequilibrium 10,11 Polymorphisms methotrexate-related genes, including Solute Carrier Family 19 Member 1 (SLC19A.
Methylenetetrahydrofolate Reductase (MTHFR).
ATP-Binding Cassette Subfamily G Member 2 (ABCG.
, and 5-Aminoimidazole-4-Carboxamide Ribonucleotide Transformylase (ATIC) exhibit marked differences in allele frequencies across Caucasian.
East Asian.
South Asian, and other populations.
These genes play important roles in the pharmacokinetic and pharmacodynamic pathways of methotrexate.
Ethnic variability in these polymorphisms may therefore influence transporter activity, intracellular methotrexate exposure, treatment efficacy, and toxicity profiles among rheumatoid arthritis patients.
Recent pharmacogenetic studies and reviews further support the clinical relevance of these genes in personalized methotrexate therapy and precision medicine approaches in rheumatoid arthritis management.
Failure to account for ethnic variability and population structure may lead to inconsistent pharmacogenetic findings across studies.
Differences in allele frequencies and genetic Rachmadi NR.
Ethnic variability in methotrexate.
backgrounds between populations can influence the effects of methotrexaterelated genetic variants on treatment response and toxicity.
In addition, methodological differences, such as variations in outcome definitions, methotrexate dosing regimens, folate supplementation, sample sizes, and contribute to conflicting results.
These multifactor may limit the consistency, interpretation, and applicability of existing methotrexate pharmacogenetic 10,15 Several pharmacogenetic markers have been increasingly investigated in rheumatoid arthritis to better and toxicity.
Variants involved in methotrexate transport, polyglutamation, and drug efflux
SLC19A1,
MTHFR.
FPGS.
GGH, and ATP-Binding Cassette (ABC) transporters, have been associated with interindividual variability in treatment response and adverse drug reactions.
Recent advances in pharmacogenomics have further emphasized the potential role of population-specific genetic profiling in supporting personalized methotrexate therapy and improving therapeutic rheumatoid arthritis.
Along with these considerations, a comprehensive narrative synthesis of methotrexate pharmacogenetics that explicitly addresses ethnic variability is warranted.
This review aims to population-level association studies, and methodological sources of heterogeneity to provide a coherent framework for understanding interethnic differences in methotrexate response among rheumatoid arthritis By highlighting gaps in current evidences and methodological limitations, this review seeks to inform future research directions and support the advancement of precision medicine in rheumatoid arthritis management.
METHODS
This study was conducted as a narrative review employing a pathwaybased synthesis approach, aimed to provide an interpretative synthesis of published evidence focusing on ethnic variability in methotrexate with rheumatoid arthritis.
A structured literature search was conducted to identify relevant studies examining ethnic variability in methotrexate with rheumatoid arthritis.
Electronic databases including PubMed/MEDLINE.
Scopus, and Google Scholar were searched for eligible studies published between 2000 and January 2026.
Search terms were combined using Boolean operators AND and OR, including AumethotrexateAy AND Aurheumatoid arthritisAy AND (AupharmacogeneticsAy OR Augenetic polymorphismA.
AND (Auethnic differencesAy OR ethnicity OR ancestr.
AND (AuSLC19A1Ay OR AuRFC1Ay OR AuMTHFRAy OR AuFPGSAy OR AuATICAy OR AuABC transportersA.
Additional populationrelated terms such as AuAsian ancestryAy were applied to capture region-specific Additional population-related terms such as AuAsian ancestryAy were also incorporated to capture region-specific Detailed search strategies for each database are presented in TABLE 1.
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TABLE 1.
Literature Search Strategies Across Electronic Databases Database Search Strategy PubMed/MEDLINE (AumethotrexateAy AND Aurheumatoid arthritisA.
AND (AupharmacogeneticsAy OR Augenetic polymorphismA.
AND (Auethnic differencesAy OR ethnicity OR ancestr.
Scopus TITLE-ABS-KEY (AumethotrexateAy AND Aurheumatoid arthritisAy AND pharmacogenetic.
Google Scholar Aumethotrexate pharmacogenetics rheumatoid arthritis ethnicity SLC19A1 MTHFRAy Inclusion criteria were human studies involving RA patients receiving MTX therapy, evaluation of MTX-related genetic polymorphisms, and reporting of clinical outcomes related to effectiveness or toxicity.
Animal studies, in vitro studies, and studies not related to MTX Reference lists of relevant articles were also screened to identify additional Identified synthesized using a pathway-oriented framework, grouping findings according to methotrexate transport, intracellular metabolism, folate-dependent pathways, and drug efflux mechanisms.
Within each pathway, evidence was further compared across ethnic populations to highlight in allele frequency distribution, and sources of methodological heterogeneity.
RESULTS
The literature search identified investigations evaluating methotrexaterelated genetic polymorphisms and arthritis populations.
After screening included studies mainly investigated polymorphisms involved in methotrexate transport, intracellular polyglutamation, folate-dependent metabolism, and drug efflux pathways.
The reviewed evidence was predominantly derived from Caucasian and East Asian populations, whereas comparatively limited data were available from Southeast Asian and Middle Eastern cohorts.
Key genes frequently evaluated across studies included SLC19A1 (RFC.
MTHFR.
Folylpolyglutamate Synthase (FPGS).
Gamma-Glutamyl Hydrolase (GGH), 5-Aminoimidazole-4-Carboxamide Ribonucleotide Transformylase (ATIC), and ATP-Binding Cassette (ABC) transporters such as ATP-Binding Cassette Subfamily B Member 1 (ABCB.
ATP-Binding Cassette Subfamily C Member 2 (ABCC.
, and ATPBinding Cassette Subfamily G Member 2 (ABCG.
Across studies, genetic variants were primarily evaluated within key methotrexate (MTX) pharmacological pathways, including cellular transport, intracellular polyglutamation, folatedependent metabolism, and drug efflux.
However, substantial heterogeneity sample size.
MTX dosing regimens.
Consequently, the comparability and generalisability of pharmacogenetic findings between Caucasian.
Asian, and Middle Eastern populations remain These studies have primarily examined polymorphisms involved in methotrexate transport, intracellular metabolism, folate-dependent pathways, and drug efflux, using diverse clinical endpoints related to treatment response and toxicity.
15,21
Methotrexate Transport Pathway Genetic Rachmadi NR.
Ethnic variability in methotrexate.
methotrexate cellular transport have been among the most extensively determinants of treatment response.
In particular.
SLC19A1/RFC1 (Solute Carrier Family 19 Member 1 / Reduced Folate Carrier .
, which encodes the reduced folate carrier responsible for methotrexate uptake into cells, has been repeatedly examined across ethnic 22,23 In Caucasian populations, the rs1051266 (G80A) variant has been associated with a protective effect against methotrexate discontinuation due to adverse events in a univariate analysis involving 333 individuals of Caucasian ancestry.
24 However, other studies have reported inconsistent associations between this variant and methotrexate treatment response.
25,26
In Egyptian cohorts, the variant was associated with increased susceptibility to methotrexate treatment response.
Evidence from Asian ancestry has been more heterogeneous, with several studies reporting positive correlation27-29 while others remained conflicted between efficacy and toxicity profiles22 findings of its polymorphism.
In contrast, studies conducted in Indonesian populations remain limited.
Notably, rs1051266 (SLC19A.
was excluded in one large replication study due to a low genotype call rate, suggesting technical limitations in genotyping rather than absence of the variant in European populations.
This methodological issue may partly explain the inconsistent associations reported for this single nucleotide polymorphism (SNP) across studies.
Collectively, findings across populations indicate an ethnicity-dependent pattern of association for SLC19A1.
Intracellular Methotrexate Metabolism Genes involved in intracellular methotrexate metabolism have been less frequently evaluated and demonstrate greater inconsistency across studies.
FPGS (Folylpolyglutamate Synthas.
, intracellular drug retention, has shown variable associations with treatment response and toxicity in FPGS variants in Caucasian-Spain study is associated with treatment response and persistence, with concomitant associations with toxicity.
Evidence from Asian cohorts is limited, research conducted with Japanese patients indicate that FPGS profile in the patient are different significantly from Caucasian population also a major determinant of intracellular conversion of MTX to MTXPG3-5.
FPGS
catalyzes the sequential addition of glutamate residues to methotrexate, methotrexate polyglutamates (MTXPG.
MTXPG3Ae5, retained intracellularly and contribute to prolonged pharmacological activity.
Variations in FPGS may therefore accumulation, treatment persistence, therapeutic response, and toxicity Studies involving South Indian Tamil rheumatoid arthritis patients have reported associations between FPGS variants and methotrexate-related adverse events.
31 FPGS variants affect treatment persistence hence correlated with 330 South Indian Tamil patients with RA were associated with MTX adverse events32 while in other research it is associated with MTX therapy response8 hence the clarity of diverse outcome in ethnicity is remain clear.
Similarly, genetic variation in GGH (Gamma-Glutamyl Hydrolas.
, enzyme responsible for methotrexate deglutamation, has been associated with inconsistent clinical outcomes.
Studies in Caucasian-Spain populations have reported heterogeneous findings,8 while in the UK-based ancestor GGH are found to be associated .
rend pO 0.
with MTX 33 Asian based studies find that GGH does not affect the effectiveness of methotrexate therapy.
15,32,33
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Folate-Dependent Pathways Drug Efflux Transporters Polymorphisms in genes regulating folate metabolism have been primarily examined in relation to methotrexaterelated toxicity rather than treatment Variants MTHFR (Methylenetetrahydrofolate Reductas.
C677T
A1298C,
polymorphisms and were observed in Chinese ancestry patients, these variants did not demonstrate reliable predictive value for clinical methotrexate response While rs17421511, and rs1476413 are having Positive response to MTX treatment (GG) in active flaring while rs4846051 MTHFR Increased the risk of MTX toxicity in 100 European descendant RA patients.
While in Tunisian descendant study, the involvement of the MTHFR C677T polymorphism is correlated to MTX In contrast, polymorphisms in ATIC .
-Aminoimidazole-4-Carboxamide Ribonucleotide Transformylas.
a key enzyme involved in purine synthesis adenosine-mediated anti-inflammatory effects, have been evaluated in a smaller number of Folate-dependent pathways were distinguished from core folate metabolism and included purine and pyrimidine synthesis pathways requiring folate-derived cofactors.
Latest study in 647 Malaysian ethnicity population evaluate that polymorphism in ATIC C347G .
and ATIC T675C .
, have a significant effect on the efficacy of MTX in RA patients.
Caucasian ancestry based research also finds the efficacy of the research.
33,37
While in Chinese population patients this polymoprhism remains unassociated with the outcomes of MTX therapy.
Evidence from various ethnic varieties remains conflicted, therefore with its limited previous studies.
Further studies with Indonesian ancestry studies are Genetic variation in ABC (ATPBinding Cassett.
transporters, including ABCC2 (ATP-Binding Cassette Subfamily C Member .
and ABCG2 (ATP-Binding Cassette Subfamily G Member .
, which mediate methotrexate efflux.
Variants in these genes were most frequently associated with differences in methotrexate toxicity and, to a lesser extent, drug disposition, while associations with treatment efficacy were less consistently reported across Pharmacogenetic analyses have demonstrated that genetic variation ABCB1 (ATP-Binding Cassette Subfamily B Member .
, particularly rs1045642, is associated with variability in clinical response to methotrexate (MTX) among patients with rheumatoid Ethnic-specific differences have been consistently reported.
East Asian populations, including Han Chinese,20 and Japanese cohorts,28 ABCB1 polymorphisms have been predominantly associated with increased MTX-related toxicity, notably MTXinduced elevations in hepatic enzyme In contrast, studies conducted in Turkish populations have shown that the heterozygous ABCB1 C/T genotype is associated with non-response to MTX Furthermore, evidence from multi-country meta-analyses suggests that ABCB1 variants may contribute to higher rates of MTX drug resistance, underscoring substantial heterogeneity in pharmacogenetic effects across In populations of European ancestry.
ABCB1 polymorphisms have also been linked to MTX toxicity, with studies in adult RA cohorts followed for approximately three months reporting associations between transporter gene variants and adverse treatment outcomes.
MTX efflux is mediated primarily by members of the ATP-binding cassette (ABC) transporter family, including ABCB1.
ABCC1Ae4, and Rachmadi NR.
Ethnic variability in methotrexate.
ABCG2, highlighting the central role of drug efflux mechanisms in determining intracellular MTX exposure.
Beyond ABCB1, polymorphisms in ABCC2, particularly rs3740066, been significantly associated with European Alliance of Associations for Rheumatology (EULAR) good or moderate response, while rs3740066 and rs717620 were also associated with achieving low disease activity (LDA) based on DAS28ESR.
In addition, variants in ABCB1 .
s1128503, rs4148.
and ABCC3 .
s2277624, rs4148.
were significantly associated with changes in DAS28-ESR, indicating improved clinical efficacy.
Consistent with these findings.
ABCC2 polymorphisms have been proposed as potential predictors of clinical response to MTX in Chinese RA patients.
Studies on pharmacogenetic risk factors for MTX toxicity and efficacy have produced conflicting data, potentially due to limited sample size, ancestry heterogeneity, and lack of accounting for comorbidities and other confounding Summary Ethnic-Specific Pharmacogenetic Patterns Overall, highlights substantial ethnic variability associations involving drug transport.
Although the underlying the biological plausibility of these associations, ethnic variability may substantially influence their clinical significance and consistency across While data from Caucasian and Asian populations provide a foundation for understanding pharmacogenetic variability, evidence from Southeast Asian populations remains limited and A comparative synthesis of pharmacogenetic findings across interethnic groups is presented in TABLE 2.
While numerous studies have investigated the association between methotrexate response or toxicity in rheumatoid arthritis, the reported findings remain highly heterogeneous and, in some cases, contradictory.
Several have demonstrated particularly in outcome definition, population structure, and methotrexate inconsistent findings across ethnic groups15,25,33,43 Moreover, adjustment for important clinical confounders, such as corticosteroid use and concomitant conventional synthetic DMARDs, as an additional risk of biased To contextualise the heterogeneity of reported associations, methodological limitations across key domains are summarised in TABLE 3.
Addressing these limitations through population-specific study designs, and robust analytical frameworks will be essential for advancing the clinical utility of pharmacogenetic markers in rheumatoid arthritis.
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TABLE 2.
Ethnic variability in methotrexate pharmacogenetic associations and clinical Role in MTX Pharmacological Pathway Caucasian Ancestry Asian Ancestry Middle Eastern Direction of Association Cellular uptake of MTX Positive response15,21 and, protection effects against MTXrelated toxicity25 Potential efficacy,27,28 Chinese studies report associations with efficacy and Favorable Ethnicitydependent.
F o l a t e Associations with MTX toxicity and Chinese reported associations with disease activity MTX L i m i t e d higher toxicity MTX36 elevated hepatic V a r i a b l e .
e t h n i c i t y dependent, limited predictive Variable.
for toxicity than Intracellular
MTX
Associated MTX t r e a t m e n t European (Spanis.
cohorts Japanese and South Indian Tamil studies demonstrate effects on MTX polyglutamate Malaysian heterogeneous15,28,32 Very limited or no populationspecific Populations p e c i f i c .
associated with both efficacy and MTX
Heterogeneous MTX efficacy or 40,41 Predominantly associations reported across multiple Asian No populationspecific data
Inconsistent ATIC
(C347G),
rs4673993 (T675C) F o l a t e dependent p a t h w a y .
de novo purine adenosinemediated antiinflammatory Several associations with MTX efficacy and treatment respons Malaysian .
showed significant associations with MTX efficacy.
Chinese cohorts reported no significant association15,20 Very limited Populations p e c i f i c .
ABC
transp o r t e r s ( A B C B 1
A B C C 2 ,
ABCG.
rs1045642 (C3435T), (G2677T/A), rs717620 (Ae24C>T), rs2273697 .
9G>A) Gene
SLC19A1
(RFC.
MTHFR
FPGS
GGH
Key Polymorphism.
rs1051266 (G80A) C677T.
A1298C.
r s 1 7 4 2 1 5 1 1 , rs1476413, rs4846051 rs10106, rs1544105.
A1994G and others MTX efflux Exploratory evidence that may have an efficacy relation of MTX42 More consistently reported associations with MTX Limited exploratory with no consistent Inconsistent / populationdependent Rachmadi NR.
Ethnic variability in methotrexate.
TABLE 3.
Methodological Sources of Heterogeneity in Methotrexate Pharmacogenetic Studies and Implications for Future Research Methodological Domain Outcome definition Population structure MTX exposure Sample size Confounding control Genetic modeling Identified Limitation Heterogeneous endpoints (EULAR response.
DAS28.
Underrepresentation of Southeast Asian Variability Small cohort sizes Limited adjustment for steroids and csDMARDs Inconsistent inheritance Impact on Evidence Interpretation Poor comparability across Implication for Future Studies Limited external validity Conduct population-specific pharmacogenetic studies Confounding Use standardized disease activity measures .
DAS.
Standardize MTX adjust analytically Low statistical power Risk of biased associations Prospective sample size estimation Multivariable regression analysis Divergent findings Pre-specified genetic models with sensitivity analyses DISCUSSION This narrative review highlights substantial ethnic variability in reported methotrexate (MTX) pharmacogenetic associations among patients with rheumatoid arthritis.
Although several genetic polymorphisms within the MTX pharmacological pathway demonstrate strong biological plausibility, their observed clinical relevance remains population-dependent, inconsistent associations reported across different ethnic groups.
8,32,38,44 These discrepancies likely reflect differences in allele frequencies, haplotype structures, population-specific genetic architectures.
In addition, methodological heterogeneity across studies, including variations in study design.
MTX dosing regimens, folate supplementation, outcome definitions, and analytical approaches, further limits the comparability and reproducibility of existing findings.
In this regard, genetic risk, as reflected by polygenic risk scores, represents the cumulative burden of multiple susceptibility variants that predispose individuals to immune dysregulation and chronic inflammation.
While genetic predisposition alone may be insufficient to initiate rheumatoid arthritis, its interaction with accelerated biological ageing appears to substantially amplify disease risk.
This interaction highlights a synergistic relationship between inherited vulnerability and age-related physiological decline, providing a more integrative framework for understanding disease susceptibility and therapeutic 45 Ethnic variability in MTX pharmacogenetic associations is likely influenced by differences in populationspecific genetic architecture, including allele frequency distributions, haplotype structures, and linkage disequilibrium Polymorphisms in key MTXrelated genes such as SLC19A1.
MTHFR.
ATIC, and members of the ATP-binding cassette (ABC) transporter family exhibit marked interethnic differences in allele 46,47 These variations may alter the functional impact of genetic variants on MTX pharmacokinetics and pharmacodynamics, thereby modifying IJPTher.
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8,44 Among these.
SLC19A1 (RFC.
has been the most consistently investigated, given its central role in MTX cellular uptake.
While several studies in Caucasian populations have reported positive associations between rs1051266 and treatment response, findings from Asian populations have been less consistent, and evidence from Southeast Asian cohorts remains limited.
These discrepancies may partly reflect ethnic differences in allele frequencies and linkage disequilibrium patterns, which influence the functional impact of genetic variants across populations.
The heterogeneity observed across studies reflects not only differences in genetic architecture between ethnic groups but also methodological variability in study design, treatment exposure, and outcome assessment.
Similarly, polymorphisms in MTHFR, a key regulator of folate metabolism, have shown more consistent associations with MTX-related toxicity than with treatment efficacy, particularly in East Asian populations.
This pattern suggests that folate pathway variants may exert a greater influence on adverse drug reactions than on therapeutic response.
However, inconsistent findings across ethnic groups underscore the complexity of MTX pharmacogenetics and caution against extrapolating results derived from one population to another without Genes involved in intracellular MTX metabolism, including FPGS and GGH, remain underexplored in Although these enzymes play a critical role in determining intracellular MTX polyglutamate levels, available evidence demonstrates inconsistent or null associations with clinical outcomes.
The lack of large, well-powered studies limits the ability to clarify their contribution to ethnic variability in MTX response.
These findings highlight a disconnect between strong mechanistic plausibility and limited clinical validation, a recurring challenge in pharmacogenomic research.
Polymorphisms in ATIC, which influence purine synthesis inhibition and adenosine-mediated anti-inflammatory effects, illustrate the importance of population-specific effects.
While some Caucasian studies have reported positive associations with MTX efficacy, findings in East Asian populations have been mixed, and data from Southeast Asia remain scarce.
This pattern suggests that ATIC-related associations may be context-dependent and influenced by population-specific genetic backgrounds.
Likewise, genetic variation in ABC transporters involved in MTX efflux has been examined primarily through exploratory analyses.
Although limited evidence suggests potential associations with MTX toxicity or drug disposition in Caucasian and East Asian populations, the absence of comprehensive studies in Southeast Asian cohorts precludes definitive conclusions.
These findings further emphasize the need for population-specific before clinical implementation can be Beyond represents a major contributor to inconsistent pharmacogenetic findings.
Differences in MTX dosing strategies, folate supplementation, concomitant therapies, and outcome definitions complicate cross-study comparisons and may obscure true genetic effects.
The use of heterogeneous clinical endpoints, such as EULAR response.
DAS28 scores, or treatment discontinuation, further limits comparability across ethnic groups.
Standardization of outcome measures and analytical approaches is therefore essential to advance the field.
19 The limited and fragmented evidence from Asian populations represents a critical gap in the current pharmacogenetic Given the ethnic diversity within Southeast Asia itself, reliance on data derived from Caucasian or East Asian populations may lead to inaccurate assumptions regarding MTX response and toxicity in this region.
Rachmadi NR.
Ethnic variability in methotrexate.
At present, available evidence does not support routine genotype-guided MTX therapy in Southeast Asian moreover Indonesia patients.
Instead, careful clinical monitoring remains the cornerstone of MTX management until robust, population-specific data become Future research on methotrexate arthritis should increasingly focus on genetic determinants that occupy central, biologically non-redundant positions within the methotrexate pharmacological pathway.
15,22 Among the wide range of candidate genes evaluated to date.
SLC19A1.
FPGS, and ATIC remain
particularly relevant due to their sequential and complementary roles in methotrexate transport, intracellular
activation, and downstream antiinflammatory SLC19A1
encodes the reduced folate carrier
responsible for cellular methotrexate uptake, thereby influencing intracellular
FPGS
polyglutamation, a critical process determining methotrexate retention and sustained inhibition of folate-dependent ATIC downstream in the folate pathway and contributes to AICAR accumulation and adenosine-mediated anti-inflammatory effects, a mechanism widely recognized as central to methotrexate efficacy in rheumatoid arthritis.
21,32,43,44 Given their mechanistic positioning, genetic variation in these loci is biologically well placed to contribute to interindividual differences in treatment response.
Despite this strong biological polymorphisms in SLC19A1.
FPGS, and ATIC and methotrexate efficacy have shown substantial variability across racial and ethnic populations.
Studies conducted in Caucasian.
European.
Middle Eastern, and Asian cohorts have reported inconsistent findings with respect to both the direction and magnitude of genetic effects, reflecting differences in genetic architecture, allele frequency distribution, linkage disequilibrium patterns, clinical outcome definitions, and study design.
18,30,48 From a global health perspective, addressing diverse populations is essential to ensure equitable development of precision Notably, the insufficient data from Southeast Asian populations limits extrapolation of findings and emphasizes the need for standardized outcome measures such as the Disease Activity Score-28 (DAS.
Populationspecific investigations are essential to clarify the clinical relevance of MTX pharmacogenetics in regional practice.
Indonesia represents a particularly pharmacogenetic research due to its pronounced population heterogeneity and regional substructure across the These allele frequency spectra and linkage disequilibrium structure, both of which may influence the detection.
This concern is clinically salient given that methotrexate remains the recommended first-line conventional disease-modifying antirheumatic drug in rheumatoid arthritis management, including in low- and middle-income settings, from Indonesian populations remains 5,49 Targeted pharmacogenetic studies in Indonesian cohorts focusing on SLC19A1.
FPGS, and ATIC variants would therefore contribute valuable evidence to clarify their clinical relevance, support context-appropriate optimization of methotrexate therapy, and strengthen the global pharmacogenetic knowledge base in rheumatoid arthritis.
Beyond gene selection, the choice of population is pivotal for interpreting Indonesia is a particularly informative priority setting because IJPTher.
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its population structure is highly heterogeneous, shaped by complex admixture and regional substructure across the archipelago.
such features can materially alter allele frequencies, linkage disequilibrium patterns, and observed effect sizesprecisely the conditions under which extrapolation from more genetically homogeneous cohorts becomes least reliable.
This methodological concern is clinically consequential because methotrexate remains the globally recommended first-line csDMARD yet pharmacogenetic evidence from genetically diverse and historically underrepresented populations remains 5,49 Therefore, well-designed Indonesian cohorts ideally multicenter and explicitly accounting for sub-ethnic structure would not only clarify the translational relevance of SLC19A1.
FPGS, and ATIC variants for methotrexate efficacy, but also contribute to reducing inequities in the evidence base that currently constrains equitable implementation of precision medicine worldwide.
18,19
CONCLUSION
Ethnic variability appears to play an important role in methotrexate pharmacogenetic associations among patients with rheumatoid arthritis and may partly explain the inconsistent findings reported across different Variations frequencies, population structure, and methotrexate-related pharmacological pathways can influence treatment response, toxicity, and overall clinical Current evidence suggests methotrexate transport, intracellular metabolism, folate-dependent pathways, and drug efflux mechanisms may exhibit population-specific effects, although the findings remain heterogeneous across studies due to differences in study design, outcome definitions, and analytical approaches.
In particular, pharmacogenetic data from Southeast Asian populations are still limited, highlighting the need for further population-specific Future studies using standardized clinical outcome measures and more diverse ethnic representation are needed to improve the consistency, reproducibility, and clinical applicability in supporting precision medicine approaches for rheumatoid arthritis ACKNOWLEDGEMENTS The authors acknowledge all researchers whose work contributed to this review.
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