April 1998
`
`Autoantl oies an Common Vira I esses
`
`‘A E Posterior Tibial Neuropathy From Ruptured Baker's Cyst
`
`Methotrexate in Rheumatoid Arthritis:A11 Update With
`Focus on Mechanisms Involved in Toxicity
`
`The Case of the Golden Veklnture
`
`_
`
`iii’ Progressive Scnsorineural Hearing Impairment in Systemic
`Vascu itides
`
`High Incidence of Malignancies in Patients With
`7 _'
`‘o ;__.
`
`__: Table 0fContents
`
`1'1'i
`
`APR 1 7 1993
`J5/120
`CENT;g
`Boo
`. M53792
`
`W. B. Saunders Compan
`A l">l'» i.\1Un of H:llmlll'I Hrzlve A”
`<
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`Seminars
`
`in
`
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`
`
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`s
`
`4 Arthritis and Rheumatism
`
`_ RoY D. ALTMAN, MD, NORMAN L. GOTFLIEB, MD,
`DA'v"1D S. HOWELL, MI), E ‘itors
`'
`
`
`
`.n,.:anh.r|.»Mann...
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`
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`-
`M-clthotrexate in Rheumatoid Arthritis:
`With Focus on Mechanisms Involved in Toxicity
`
`Annelies E. van Ede, Roland F.J.M. Laan, Henk J. Blom,
`Rooney A. De Ab:-ea, and Leo B.A. van da Putts
`
`Objectives: To provide an update of the current knowledge of the mechanismof
`action of low-dose methotrexate (MTX) in the treatment of patients with rheuma-
`toid arthritis (RA), with an emphasis on the mechanisms involved intoxicity. We also
`considered strategies currently used to prevent or decrease toxicityof
`Methods: We reviewed the Iiterarlure dealing with the subiecls of N|'l'X treatment of
`RA, the mechanisms of actionof low-dos MTX regarding efficacy and toxicity, and
`strategies used to prevent or decrease MTX toxicity.
`-
`Results: M‘l'X is a fast working and effective second-line antirheumatic agent ($LAl.
`Its use is limited mainly because of side effecls. The mechanisms of action regaiding
`efficacy and toxicity are probably determined by different metabolic pathways.
`Recent data indicate that the anliinflammatory effect of MTX is mediated by
`adenosine. However, MTX side effects n only partly be explained by folate
`antagonism and may also depend on its action on other related metabolic path-
`The latter include the homocysteine-methioninepolyamino pathway and
`purine metabolism. Variants in these metabollc routes (ie, the C677T mutatlon In the
`methylene-tetrahydrofolate reductase [MTHFR] gene), may predispose to the devel-
`opment of side effects. Currently .the mom promising strategy to decrease or
`preventtoxicity of MTX is concomitant prescription of folic acid or folinic acid. Other
`strategies are currenfly under investigation.
`‘
`.
`Conciusions: MTX benefits a niaiority of FIA patients. Approximately 30% of
`patients, however, abandon treatment because of drug-related side effects. Folic '
`acid or folinic acid likely reduces MTX toxicity. More data, however, are needed to
`evaluate a potentialdetrimental efiect on the antirheumatic efficacy of MTX.
`. Semin Arthritis Rheum 27:277-292. Copyright © 1998 by W.B. Saunders Com-’
`Fv¢‘u"'r'
`-
`
`INDEX WORDS: Methotrexate; rheumatoid arthritis; mechanism of action;
`toxicity.
`
`ETHOTREXATE (MTX, amethopterin) is a
`folic acid antagonist that was introduced into
`the treatment of childhood leukaemia in 1947. In
`1951 Gubner et al reported a favorable effect in
`patients
`psoriasis, pscxiatic mfliritis, and rheu-
`matoid arthritis (RA) (l). Low-dose MTX has been
`commonly used in the treatment of psoriasis since
`the 19605 and was approved for this purpose by the
`Food and Drug Administration in 1971. However,
`its use in the treatment. of RA only begun,du._rLng the
`19805 and it was not approved by the Food and
`Drug Administration for this indication until 1988
`(2-5).
`
`FmmtheDeparbrimtafRhzwnatologyandIheLabarm‘oryof-
`Pediatrics andNeurology University q'Nijrnegen. The Nerherlaruis
`Address reprint request:
`to Aruuliex E. van Ede. MD,
`Department ofRheumatology, University ofNijmcgen, PO Box
`9101, 6500 HB Niimegen, The Netherlands.
`Amelie: E. van Ede, MD: Deparrrnen: of Rheumatology,
`University ¢'zfNxjrn¢gen; Roland ELM. Laan. MD. PhD: Depart-
`ment ofRheurnaralogy, University afNijrnegen; Honk J. Blom.
`PhD: Laboratory of Pediatrics and Neurology, University cf
`Nijrnegen; Ronney A. De Abreu, P)i.D: Laboratory ofPed:'atricr
`and Neurology, University ofNiimegen; Leo BA. van de Purte.
`Ilm. PhD: Prbfelrsoii Hood of Depamnent. Department q‘
`Rheurnatology, University ofMjmegen.
`Copyright ¢ 1998 by WB. Saunders Company
`0049-0172/98/2705-0003.38.00/0
`
`Seminars in Arthritis and Rheumatism, Vol 27, No 5 (Aprill, 1998: pp 277-292
`
`277
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`
`278
`
`VAN EOE ET AL
`
`Currently, MTX is prescribed by rheumatolo-
`glss sfl over the world. MTX has proved to be a.
`very eflective, fast-working, second-line antirheu-
`matic agent (SLA) with the best eflicacy-toxicity
`ratio (6-8'). Nevertheless,
`the main reason for
`discontinuation of MTX is not inefficacy but toxic-
`ity (9-12). Because of its clear-cut and long-lasting
`eflicacy, much efion is currently being made to
`develop strategies to decrease or prevent its toxic-
`ity. ln approximately 30% of RA patients, toxicity
`leads to discontinuation of MTX therapy. There-
`fore, diminishing of toxicity may result in better
`treatment of this group of RA patients (9-22).
`MTX acts as a folate antagonist, but this does not
`directly explain the full spectrum of side effects.
`However, MTX exerts its action also on other
`related metabolic pathways. Recently, a gene muta-
`tion was discovered in one of these metabolic
`routes that may predispose to the development of
`side effects (23-25). Other promising research
`focuses on the homocysteine-methionine-poly
`amine pathway and on purine metabolism.
`To better. understand the" possible mechanisms
`operative in both eflicacy and toxicity of MTX in
`the treatment of RA patients, we describe folate
`metabolism and folate dependent metabolic path-
`uL.__ al.';.a....;..;. I_...-. «L... .....-. _..I.. .. 4.. ..i........ ..._.-I
`ways in which MTX is involved. Subsequently, we
`LI.lCl.l menus; IJUW LLICBC Judy lcldlfl LIJ BLLlbil\4y E1111
`
`side effects. Finally, we dismiss strategies to pre-
`vent or decrease toxicity of MTX.
`
`FOLATE METABOLISM
`
`Figure 1 gives an outline of folate metabolism
`and folate dependent pathways. Because humans
`are not capable of forming folic acid from their
`basic constituents, dietary intake is essential (26).
`Dihydrofolate-reductapse (DI-IFR) catalyzes the con-
`version of dihydrofolate (DI-IF’) into tetrahydrofo—
`late (TI-IF), which serves as a central component of
`folate dependent pathways (27,28).'TI-[F is also
`formed from 5-methyl-THF during the conversion
`of homocysteine (the methyl group acceptor) into
`methionine, and in the steps involved in purine
`synthesis. The biologically active folates are deriva-
`tives of tetrahydrofolates and participate in one 4
`carbon transfer reactions. In the cells, these folates
`are converted into polyglutarnated forms and thereby
`retained intracellu.lar1y. Serum folate consists mainly
`of 5-methy1—THF, which is the predominant circu-
`lating form of folate. In rapidly dividing tissues,
`
`5-methyl-THF and 10-formyl-THF are equally pre-
`domant. 5-Methyl-T!-;F is more sttsmpfible to
`both intra- and extracellular changes in folate status
`(29).
`.
`_
`Folates are involved in several important meta-
`‘ bolic routes: 5,10-rnethenyl-'I'HF/10-forrnyl-TI-IIF,
`5.l0—rnet.hylene-'l"HLF. and 5-methyl-TI-IF deliver
`one-carbon units for the synthesis of purines,
`pyrimidines and methionine respectively.
`
`'
`
`Pun'ne-Metabolism"
`Purines arerrecessary for the synthesis of the
`nucleic acids adenosine monophosphate (AMP)
`and guanosine monophosphate (GMP), and finally »
`deoxyribonucleic acid (DNA) and ribonucleic acid
`(RNA). The purinermetabolism is composed of a
`de novo synthesis and a salvage route (Fig 2).
`Complete purine de novo synthesis (PDNS). exists
`only in proliferating cells such as of bone marrow
`and liver and is partly performed in lymphocytes
`and mononuclear cells (30). The central enzyme is
`5-phosphoribosyl-1-pyrophosphate (PRPP) synthe-
`tase. In PDNS the 5,10-methenyl-THF dependent
`enzyme glycinamide ribosyl-5-phosphate (GAR)
`formyluansferase and the 10-formylr'I'}IF depen-
`dent enzyme amino-irnidazolcarboxarnide ribosy1-
`5-phosphate (AICAR) formylnansferase are in-
`volved. Salvage probably takes p1aee.in all body-
`cells, but measurements have been done"main1y in
`lymphocytes and erythrocytes. In the purine sal-
`vage route,
`the enzymes hypoxanthine-guanine
`phosphoribosyl
`transferase (HGPRT), purine
`nucleoside phosphorylase (PNP), adenosine deami-
`nase (ADA), and purine-5’nucleotidase (5’NT) are
`involved.
`AICAR and its metabolites inhibit adenosine
`-4‘ -.l-_.-.-.'_-
`....I.:..I. L..- ;._a.':_A-..._...a_______-_
`lcinase (AK) and ADA with the consequent increase
`U1 duDJAUhJ.|lE, Wl.I.l\ol.I LIA) d.I.II.l.I.I.I.Lld.I.I.I.ll.lAI.\Jl’ PAUIEL‘
`
`ties (31-33). _
`
`Pyfimidine-Metabolism
`The 5,10-methylene-TI{F dependent enzyme thy-
`midyiate syntherase Ci‘S), catalyzing the conver-
`sion of deoxyuridylate (dUMP) to deoxythyrni-
`dylate (dTMP), forms a rate limiting step in DNA
`synthesis (26).
`-
`z1.......-.......:..... x,(...r..-.....‘..,. D,.1..,._..r..,. n...z....,...
`Luuu./r._yoLc Le-in uuvrbutc-A uI._yuuun.c A uuuvuy
`
`l-lomocysteine and folate status are inversely
`relawd. Hornocysteine is probably an even better
`
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`
`METHOTREXATE IN RA
`
`279
`
`I DNA I‘
`
`folic acid
`
`HOMOCYSTEINBMETHIONINE
`
`
`
`\ 5,10-c
`
`2-THF —n—> 5-CH3-THF
`mthfr
`
`5,10-CH-THI=<—— 5-CHO—THF
`
`dTMP
`
`dUMP
`
`GAR
`
`->—>': 4+):i—:':?:<(—71‘L11 3'=.''
`
`
`
` Polyamines
`
`4
` et ———> SAM
`
`|
`
`L
`
`‘
`
`ms
`
`'
`
`F‘
`
`-
`
`R-CH3
`
`mz—u—§—x<1
`
`5
`
`R
`I
`N
`E
`
`+
`
`lfolinic acid)
`
`_
`
`THF
`
`10_CH0_THF
`
`,
`
`l
`
`FGAR
`W
`
`V
`
`AICA
`
`FAICAR
`_ V
`V
`PURINES
`V
`. :V
`_ .
`| DNNRNA |
`
`‘ i-icy
`
`L
`
`.
`
`_
`_
`»
`cystathlonme
`_[
`v
`cysteine
`
`»
`
`.
`
`'
`
`_ V
`SAH
`
`gadenosine
`
`-
`
`"
`
`_
`
`I
`
`.
`
`'
`
`.
`
`'
`
`Fig 1. Simplified metabolic scheme illustrating folate metabolism and" its relation to purine,
`pyrimidine, and homocysteine-methionine metabolism. Known inhibition of enzymes by methotrex-
`ate is indicated by ——|—Abbvevlatlons: DNA, deoxyribonucleic acid; dTMP, deoxythymidylate; ts,
`t.':','.'::idi!e'.e syntheteee; e."..'!‘.‘.P, decx‘,-urid-y-late: GAR, glyclnarnida .-lbasyl-5-p_,hoiphate;, FGAR, torm-
`glycinamide rihosylJ5-phosphate; AIOAR, amino-imidazolcarboxamide ribosyl-5-phosphate; FAIOAR,
`form-amino-imidazolcarboxamide ribosyl-,5-phosphate; RNA, ribonucleic acid; DHF, dihydrofolate;
`dhfr, dihydrololate-reductase; THF, tetrahydrofolate; 5,10-CH2-THF, 5,10-methylene tetrahydrofolate;
`5,10-CH-THF, 5,10-methenyltetrahydrofolate; 10-CHO-THF, 10-formyl tetrahydrofolate; mthfr, methy-
`
`lene-tehahydrofolete re¢_!_I_Ic!
`:5-G!-lg-T!-!F, 5-methyl tetrehydrcfolete; 5-G!-l0-'!'!-!F Efer.-ny! *.et.-eh‘,-d.-c-
`folate lfollnlc acid); Met, methionine; SAM,S-adenosyl-L-methionine; ms, methionine synthetaee; Hey,
`homocysteine; SAH, S-adenosyl-L-homocysteine; R. methyl-acceptor; Fl-CH3.
`
`arameter for measuring the effective capacity of
`folate metabolism than folate blood levels (34-38).
`Homocysteine is not derivedfrom the diet but
`from transmethylation of methionine. It is reused
`by rcmethylation to methionine and tra.nssulfura-
`tion to cysteine, regulated by methylene—tetrahydm-
`folate reducmse (MTHIFR) and cystathionine-&
`synthase (CS) respectively (39). 5-Methyl-THF is
`necessary as a methyl donor for the conversion of
`homocysteine to methionine, which in nnn can be
`converted into S—adenosyl-L-methionine (SAM)
`by adenosine Iriphosphate (ATP). SAM is the
`most important methyl group donor in the cell;
`during these methyltransferase reactions, SAM is
`
`converted into S-adenosyl-I.-homocysteine (SAH).
`SAH is degraded by SAH—hydrolase into homo-
`cysteine and adenosine. I-Iomocysteine is an in-
`hibitor of methyltransferases. Therefore, the ratio
`between SAM and SAH is an important determi-
`nant of intracellular transmethylation capacity
`(40,41).
`SAM is not only converted into SAH, but also
`into dccarboxy-SAM, which is the substrate for the
`synthesis of polyarnines. In polyamine synthesis,
`the decarboxylation of SAM by SAM-decarboxyl-
`'ase is arate-li.mitin'g step (41,42). The polyamines
`putrescine, spermidine, and spermine are essential
`for cell functions including proliferation, differen-
`
`/ p
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`

`
`I;
`
`.
`
`lil|BOSE-5-phosphate
`’
`+PRPP
`ivPRA
`
`vlrGAR
`V
`
`FGAR
`vi
`SAICAR
`
`AICAR
`'
`‘i’
`FAICAR
`
`280
`
`I
`
`‘
`
`‘
`
`"70
`
`O<OZ
`
`mc:><I->m'
`
`l
`
`l
`
`1
`
`-
`
`VAN EDE ET AL
`
`MTX METABOLISM
`
`L
`
`MTX (amedlopicrin, 4-amino 4-dcoxy-N10-)
`methylptemyl-glutamic acid) has a structural for-
`mula that is rather similar to DHF (Fig 3). MIX
`consists of one more methyl-group and has a
`NH,-group instead of an OH-group. MTX in low
`Arsnna nix
`mu.
`n1vva
`:\r-1:111: in
`an
`/~.-.vnn1=oaIu nkau-.-‘Ln.-3
`uuawe snyu u.|a.u_y so uuuv
`vvuayivwiy uuovn uma,
`although there is a significant variability (47). The
`initial (distribution) half-life‘ is 1.5 to 3.5 hours; the
`terminal (elimination) half life is 8 to 15 hours.
`MTX and its metabolites are eliminated mainly '
`through the 1r_id_n.ey (2.9); MLX enters the cell by
`active transport. It is a prodrug that is convened by
`folylpolyglutamylsynthetase to its polyglutamate
`form, through addition of 1 to-4 glutamate groups’
`(48). The polyglutatnate form is probably present
`in all cells. and measurements have been performed
`in erythrocytes (49), liver (50), fibroblasts (51), and
`myeloid precursors in bone marrow (52). MTX-
`. polyglutamates are retained within the cell, replace
`Dl-IF in its binding to DHFR, and have a stronger
`afiinity for folate dependent enzymes like TS, GAR
`formyltransferasc, and AICAR forrnyltransferase
`(48,53,54) than MTX—trit>noglutamate" and DHF.
`M'l‘X levels in serum remain detectable for only a
`short time and are not a good indicator for the
`efficacy of MTX. However, the process of polyglu-
`tamation. is crucial for the cellular retention of
`MTX and its effectiveness (55).
`‘
`
`-THE MECHANISM OF ACTION OF MTX
`AS A FOLATE ANTAGONIST
`Figure 1 shows the modes of action of MTX
`acting as a folate antagonist Chabner et a1 postu-
`lated two theories in explaining the mechanism of
`action of MTX: (A) the depletion theory, based on
`blockade of DHFR resulting in depletion ofintracel-
`lular reduced folates; and G5)
`the competition
`theory, based_ on the direct inhibition of distal steps
`in the synthesis of nucleotides by MTX and by _
`accumulated DHF (48).
`.:-._ -c .._.1___-.1 -I-r
`_ ..--
`..c ._.......:_-
`..n
`MTX inhibits DHFR, thereby causing a deple-
`uuu U1 Luuuucu 111.1‘ 3,
`lJGdI.I.|§U UL uuyplug U1
`
`DI-IF-polyglutamates (27,2§,55-58). Moreover, di-
`' rect enzymatic inhibition by DI-ll-' and/or MTX
`polyglutamates effects 5,10-methylene-THF reduc-
`tase, TS (59,60), GAR formyltransferase (61 ,62),
`and AICAR for-.1‘-.§.".traz;si'aasc, lee&g to inhibi-
`tion of purine and pyrimidine metabolism, subse-
`quent
`inhibition of DNA- and RNA-synthesis.
`Reduced availability of 5-methyl-Tl-IF will reduce
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`GUANOSINE
`Tpnn
`it / \
`GUANINE
`
`.
`
`d
`INO$lNE<p-—- ADENOSINE
`Trmp
`l
`HYFOXANTHINE
`
`Flg 2.- Pathways In purlne-metabollsm, which
`is composed of a dc no-.-e synthesis l-.:p..%r part
`of the figure) and a salvage route (lower part of
`the figure). Abbreviations: PRPP, 5-phosphoribo-
`_syl-1-pyrophosphata; PRA, phosphoribosyl-
`amine; GAR, glyeinamide ribosvl-5-phosphate;
`
`FEAR, form-glyeina
`busy!-5-phosphate;
`
`SAICAR, suocinyl ammo-Iml azolcarboxamide
`ribosyl-5'-phosphate; AICAB. amino-imidazolcan
`boxamide ribosyl—5-phosphate; FAICAR, form-
`amino-imidazolcarboxamide
`ribosyl-5-phos-
`nhate: XMP, xanthine monophosphate; GMP,
`guanosine monophosphate; IMP, inosine mono-
`phosphate; s-AMP, suceinyl-adenosine mono-
`phosphate; AMP, adenosine monophosphato;
`AK, adenosine kinase; HGPRT, hypoxanthine-
`guanine phosphoribosyl trans-ferase; 5'N1', pu-
`rine—5’nuclootidase; ADA, adenosine deami-
`nase; PNP, purine nucleoslde phosphorylase.
`
`‘t-‘mfinn - nu-nu‘-in
`...........,
`,,..,......
`
`an/l Gm.-nun.-_n.nA:-nprl
`umrlm-eh
`c,.....wo..., «Ann .............v ...~\......~u
`
`cellular reactions (43-46). A decrease in poly-
`amines results in antiproliferative and immuno-
`modulating efiects.
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`METHOTREXATE IN RA
`
`281
`
`"*“./"N/"\
`1
`'
`A c:*=
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`
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`
`methotrexate
`
`“fill:
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`CH
`
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`z~ N
`c—N— cl-l—COzH
`\=/
`
`dihydrofolate
`
`‘ N ~
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`7
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`v
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`N
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`folic acid
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`Fig
`rurautfire 4-,: rruéii'rcirex-
`ate, dihydrofolate, folic
`acid, andfolinic acid.
`
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`
`folinic acid
`
`hem-ccystcixie remethylat-Lon, causing an increase
`of homocysteine and adenosine (31,32). Also,
`MTX may directly inhibit methionine-synthetase
`(48), and even methionine transport (63), leading to
`an additional decrease of SAM, which next to
`reduced tmnsmethylatinn may also 1'er_h_Ir:.e.
`I11;-.
`synthesis of polyamines (41,43).
`’
`
`V EFFICACY AND SIDE EFFECTé OF MTX IN RA
`Ejficacy
`of MTX uno uuvn
`1.... 1.-....
`In 511: 135; Annoria’ (la: emmmy
`extensively studied. Nowadays MTX has proved its
`efficacy in RA on both a short-term and long-term
`basis (3,9.12,l5-18,‘64-‘7_2). The longest clinical
`studies vary from 7 to 10 years (15,69,70).
`The effect of
`on ra.dio!ogi¢:J.=_l joint damage
`is somewhat uncertain (19). Some authors found
`delay or prevention of structural damage within one
`year (73.74). Other studies show no difference in
`
`effect on radiological progression when conipared
`with other active antirheumatic treatment (71,75-
`79). In a metaanalysis MTX showed a slower rate
`of radiological progression compared with azathio-
`Jr“ as IQf\\
`prine, but not compared with all other slow-acting
`\.u use \uv;.
`
`Several studies compared the efficacy and toxic-
`ity of MTX with other antirheumatic drugs (13).
`MIX proved to be more effective and less toxic
`than auranofin and azathioprine (78,81). Felson et
`91 performed two .'ne‘:.°..'=.na.'.yses in which MTX had
`the best eflicacy/toxicity ratios. MTX was as effec-
`tive, but less toxic than sulfasalazine (6,7). MTX
`works fast and has the best probability of continu-
`ing therapy of all SLAS (7,8,82-85).
`,
`was
`prescribsd. in a dose of 5_ mg’
`week, but the weekly dose has gradually increased
`over the last 10 years, showing a dose~response
`relationship (86,87). In many studies the mean dose
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`282
`
`VAN eoerr AL
`
`=
`
`is 10 to 15 mg/week andthe maximum dose is 25 to
`30
`(2(}22,79,88,89).
`-
`-
`Despite the fact that low-dose MTX has become
`standard therapy for RA, its mechanisms of action
`with respect to efficacy as well as side effects are
`still obscure (l4.27,90-109).
`_
`An.'z'prolzf£rative ejfecrs.
`in cancer neatment,
`high-dose MTX is used for its antiproliferative
`properties. However, antiproliferafive efl"ects prob-
`ably do not explain its mechanism of action in
`low—dose treatment of RA. Hirata describes direct
`inhibition of endothelial cell proliferation by low-
`dose MTX in RA (110), but several authors de-
`'1 scribe conflicting results about inhibition of lympho-
`cyte proliferation (3,44,45,92,94-96,105,111).
`Immunomodulatingv eflects.
`Immune mecha-
`nisms play a key role in the pathogenesis of RA
`(90). Several studies show suppression as well as
`enhancement of activities of both numbers of B-
`and T-cells
`and their
`function by MTX
`(75,94,95,l07). Cronstein postulates that MTX
`modulates the production ofcytokines (tumor necro-
`sis factor .[TNF]oL, Interleukin [IL]-1, IL-6, H.-8)
`by its cytotoxic efiect on cells that generate these
`cytoldnes_(l07).
`The immunomodulating effect of MTX may be
`related to its potential to inhibit the synthesis of
`polyarnines (105). Wolos et al showed am inhibi-
`tion of polyamincs prevented the development of
`collagen-induced arthritis in an animal model (112).
`- Polyamines may play a role in the pathogenesis of
`RA, because increased levels of polyamines have
`been found in lymphocytes, synoviai fluid, and
`urine of patients with RA (113-115). Fururnitsu et
`al showed correlation of the levels of urinary
`polyamines with radiological damage (114).
`_
`Irnmunomodulating efifects of MTX‘ may also be
`the result of interference with purine meiabolisrn.
`The association of disturbances of purine metabo-
`lism with immune disorders shows that purine
`metabolism can effect the immune response (116).
`There are several inborn errors of purine metabo-
`that lead to irnrniine disorders. Parrirlc r.ucico-
`side phosphorylase (PNP)-deficiency leads to B-
`and T—cell disorders and is associated with autoim-
`mune disorders like systemic lupus erythematosus, '
`idiopathic thrombocytopenic purpura, and autoim-
`'-'u.z"-....
`lieir-.clytic arlérnie (117-122). Ade-.~.osine~
`deaminase (ADA) deficiency is well recognized as’
`a cause to severe combined immunodeficiency
`disease (SCID)
`(123). Purine-5’nucleotidase
`
`‘
`
`.-
`
`(5’NT)-deficiency is associated with primary hypo-
`gannnnaglobufiernia (124,125). Disturbances of
`purine metabolism might be implicated in the
`pathogenesis of RA (126,127). Although Stolk et al
`(128) found no influence of early RA or disease
`activity on purine enzymes, several other studies of
`patients with RA have found lower purine en-
`zymes, especially 5’NT and ADA (126,129). Ker-
`stens et al found a positive correlation between low
`activities of purine enzymes and response to treat-
`ment as well as,.toxicity in azathioprine-treated
`.patients (130-132).
`Antiinflammatory eflectr. The rapid improve-
`ment and exacerbation after respectively introduc- _
`tion and discontinuation of MTX suggests the
`possibility of an antiinflammatory effect (230).
`Cronstein postulates two possible biochemical
`mechanisms of action of MTX: (A) inhibition of
`homocysteine remethylation and (B) adenosine
`release.(l07). Adenosine seems to play a central
`role in antiinflarnmanory response.
`(‘It occupies
`specific receptors on fibroblasts and endothelial
`cells, thereby diminishing the adiiererice and de-
`structive capacity of neutrophils (3,l0§; 106). Aden-
`osine inhibits the function of natural killer cells,
`monocytes/macrophages, and T-lymphocytes (33),
`the toxicity of 02-metabolites (133,134) and the
`
`s of l"NFa and comp "merit (135,135). In
`vitm studies show that low—_dose MTX leads to
`adenosine release in fibroblasts and endothelial
`cells (137). Cronstein et al were the first to prove
`that MTX exerts an adehosine-induced anu'inflam-
`matory efiect
`in vivo in a mouse model.
`' suggests new possibilities for drug development-
`agents inhibiting inflammation by increasing aden-
`osine-release (cg, inhibitors of AICAR-formy1trans~
`ferase, ADA, AK, or adenosine uptake) (105-107).
`
`Side Efiecrr
`Side effects are quite common during treatment
`with MTX‘ (Table 1) (10.14,19,82,l01,l38,139).'
`The severity varies, but most side effects are mild.
`reversible, and can be treated conservatively. Side
`effects like nausea, changes in nansaminases, and
`stomatitis are often encbuntered and are dose
`dependent; others like pneumonitis and.hepat0C61-
`lular changes are not (140). However, in approxi-
`mately 30% of parientswith RA, toxicity 188118 I0
`discontinuation of MTX therapy within one year
`(9-22). Only a few determinants for toxicity are
`known, such as increasing age and poor renal
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`MEFHOTREXATE IN RA
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`283
`
`Table 1: Side Effects of Low-Dose MTX
`Frequent
`Less Frequent
`Rare
`Elevation of
`Central nervous Nephrotoxicity
`transa'mi-
`system (head- Dermatitis
`: nases
`ache,vertigo) Photosensitivity
`Gastrointestinal Pneumonitis
`Gynecornastia
`(nausea,
`Leucopenia and Oiigospermia
`anorexia, diar-
`thrombopenia Progression of
`Hair loss
`rhea)
`subcutaneous
`' Stomatitis infections
`
`‘ nodules
`
`function (11,16, 14]). Another important problem is
`that although the risk of side effects may be slightly
`higher in the first six months, the risk for all sorts of
`adverse efiects is permanent, implying a need for
`long-term monitoring (9.12,I5;69;79,90).
`Although the precise mechanism of toxicity is
`still obscure, some side effects have been directly
`related to MTX involvement
`the previously
`mentioned metabolic pathways. Other side efiects
`like pnetimonitis and progressive subcutaneous
`nodules probably have a more complex origin
`(90,l42,143); ,
`\
`Folate me'taboli.rm. At least part of the side
`effects of MTX seems to be directly related to its
`folate antagonism and its cytostatic effects, espe-
`cially in tissues with a high cell turnover‘(bone
`marrow and gastrointestinal tract), which have a
`high requirement for purines, thymidine, and me-
`thionine (56,57, 144-146). Supplementation of folic
`acid or folinic acid may diminish toxicity (as is
`mentioned later) (147,148).
`Kremer et al were the first to show accumulation
`of MIX-polyglutarnates in liver biopsies from
`patients with RA, accompanied by folate defi-
`ciency. Although no correlation could be shown
`between those parameters and u-ansaminase eleva-
`tions, one could speculate whether MTX-po1ygluta—
`' mates cause hepatotoxicity because of folate defi-
`ciency (149).
`r\"I|Q .. ‘aw.
`- "a... 1....
`.....:.....'5... at .....:_.. -..
`Purine metabolism.‘ Kerstens et al found a
`Cuuulufluu b»i.'»-vuun LUW auuviuca U1 yuuuc cu-
`zymes and response to treatment as well as toxicity ‘
`in azathioprine-treated patients (130-132). In a
`similar way part ofthetoxicity of MIX may be
`caused by its inhibition ofpurine metabolism.
`Adenosine.
`Inhibition ofADA leads to eccur.-mu.-
`lation of‘ metabolites like deoxyadenosine and
`deoxyadenosine trlphosphate (dATP) through alter-
`native routes. It is possible that high concentrations
`
`of adenosine, deoxyadenosine, and methylated aden-
`osine-me%lites are directly tome. Dwxyarleno-
`sine causes chromosomal fractures and inactivates
`SAH-hydrolase, which is needed for adequate
`transmethylation. dATP inhibits iibonucleotide re-
`ductase, which is necessary for DNA-synthesis
`(150-152.); Baggott et 21 suggest a parallel htween
`SClI)_, caused by ADA-deficiency, and MTX-
`treatment, which also induces inhibition of ADA
`(123,153).
`_
`Homocysteine-methianine-polyamine pathway.
`MTX leads to hyperhomocysteinemia, a we_:]_l_—
`known risk-factor for vascular disease (154,155).
`Roubenofiet al found that RA patients have higher
`fasting homocysteine levels (156).
`In a study
`among RA patients treated‘ with MTX, Haagsma et
`al