REVIEwS

Mycophenolate mofetil, azathioprine

and tacrolimus: mechanisms in
rheumatology
Jasper C. A. Broen1 and Jacob M. van Laar2 ✉
Abstract | The introduction of biologic DMARDs into rheumatology has resulted in a substantial
reduction of the burden of many rheumatic diseases. In the slipstream of the success achieved
with these biologic DMARDs, some conventional immunosuppressive drugs have also found use
in new indications. Notably , mycophenolate mofetil, azathioprine and tacrolimus have made their
way from solid organ transplantation drugs to become useful assets in rheumatology practice.
Mycophenolate mofetil and azathioprine inhibit the purine pathway and subsequently diminish
cell proliferation. Both drugs have a pivotal role in the treatment of various rheumatic diseases,
including lupus nephritis. Tacrolimus inhibits lymphocyte activation by inhibiting the calcineurin
pathway. Mycophenolate mofetil and tacrolimus are, among other indications, increasingly being
recognized as useful drugs in the treatment of interstitial lung disease in systemic rheumatic
diseases and skin fibrosis in systemic sclerosis. A broad array of trials with mycophenolate mofetil,
azathioprine and/or tacrolimus are ongoing within the field of rheumatology that might
provide further novel avenues for the use of these drugs. In this Review , we discuss the historical
perspective, pharmacodynamics, clinical indications and novel avenues for mycophenolate
mofetil, azathioprine and tacrolimus in rheumatology.

Over the past two decades, the outcomes of patients serendipitously discovered in patients with rheumatic
with rheumatic diseases have improved consider- disease being treated for a different indication. MMF,
ably with the introduction of biologic DMARDs. azathioprine and tacrolimus all originate in the field
Nevertheless, conventional DMARDs have stood the of transplantation medicine and have been developed
test of time as anchor drugs in various rheumatic dis- using ground-breaking research methods, performed
eases, notably in systemic lupus erythematosus (SLE) by Nobel-prize winning scientists1. Apart from their
and systemic vasculitis and increasingly in systemic common root in transplantation medicine, the drugs
sclerosis (SSc) and antisynthetase syndrome. The share similar mechanisms of action and affect common
mecha­nism of conventional DMARDs is often pleio- cellular pathways. In this Review, we discuss the his-
tropic and poorly understood. Investigations into the tory, common use and future of MMF, azathioprine and
mechanism of these drugs, however, might reveal path- tacrolimus within the field of rheumatology.
ways that are of interest for the development of new tar-
geted drug therapies or novel applications of these drugs Mycophenolate mofetil
Regional Rheumatology in additional rheumatic diseases. In this Review, we Historical perspective. Mycophenolic acid (MPA) was
1

Center, Máxima Medical

Center, Eindhoven and
focus on three drugs: mycophenolate mofetil (MMF), first described in 1913 (ref.2), following its isolation from
Veldhoven, Eindhoven, azathioprine and tacrolimus. Penicillium stoloniferum and decades after the discov-
the Netherlands. Unlike most of the contemporary biologic DMARDs, ery that penicillins have antibiotic properties. MPA was
2
Department of most conventional DMARDs were not initially devel- found not only to have antibiotic properties but also
Rheumatology and Clinical oped for the treatment of rheumatic conditions, but to have anti-viral and anti-inflammatory properties3–5.
Immunology, University were repurposed from other fields such as haematology, The anti-inflammatory effects of MPA were shown most
Medical Center Utrecht,
Utrecht, The Netherlands.
oncology or transplantation medicine. The introduc- clearly in trials investigating its use for the treatment of
✉e-mail: j.m.vanlaar@ tion of these DMARDs in rheumatic diseases was often patients with psoriasis6. In these trials, however, MPA
umcutrecht.nl a process of trial and error, driven by new pathogenetic use was associated with gastrointestinal adverse effects
https://doi.org/10.1038/ concepts of autoimmune disease; sometimes the bene- and viral infections; an association between MPA and
s41584-020-0374-8 ficial effects of these drugs in rheumatic disease were increased risk of malignancy was also suggested, but not

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Key points of MPA can increase and cause gastrointestinal adverse

effects12–14. The inter-individual variation of drug con-
• Mycophenolate mofetil (MMF), azathioprine and tacrolimus are three conventional centrations and the effect of impaired renal function
DMARDs that originate from the field of transplantation medicine, but have been on these concentrations have prompted researchers to
repurposed for the treatment of rheumatic diseases. investigate the benefits of using measurements of plasma
• MMF and azathioprine both interfere with the purine pathway to inhibit cell drug levels to guide dosing regimens in patients with
proliferation, whereas tacrolimus inhibits calcineurin activity and subsequent lupus nephritis. In two such studies, dosing using thera­
lymphocyte activation.
peutic targets of area under the concentration–time curve
• MMF and azathioprine are pivotal in the treatment of lupus nephritis. for 0–12 h (MPA-AUC0–12h) of 30–60 mg × h/l or MPA-
• MMF and tacrolimus are, among other indications, increasingly recognized as useful AUC0–12h of 60–90 mg × h/l, respectively, resulted in
drugs in the treatment of interstitial lung disease in systemic rheumatic diseases and excellent renal outcomes13,14; however, in the second
skin fibrosis in systemic sclerosis.
observational study13, adverse effects occurred in one
• Future avenues for these drugs include the optimization of dosing schemes and third of the patients13.
investigation of novel indications and effects when co-administered with other
DMARDs.
Molecular and cellular mode of action. MMF inhibits the
formation of guanine nucleotides15. Guanine is one of
proven beyond doubt6. Hence, at the end of the 1970s, the purine nucleobases necessary for the generation
use of MPA had mostly been discontinued7. of DNA and is thus required for cell replication. Cells are
The anti-inflammatory aspects of MPA, however, able to generate guanine nucleotides through two distinct
sparked an interest in the further optimization of the pathways: the de novo pathway and the salvage pathway.
drug. MMF, an ester of MPA, was developed and was MPA targets the de novo pathway by inhibiting the
found to have a better bioavailability and tolerance in the enzyme inosine monophosphate dehydrogenase, thereby
gastrointestinal tract than MPA5,7. In 1995, MMF was reducing the amount of guanine nucleotides formed15,16.
approved by the FDA for the prevention of transplant Unlike many other cell types (such as neurons,
graft failure7. Since 1995, MMF has been used in vari- hepatocytes and renal cells), lymphocytes rely solely on
ous clinical specialties, in particular for the treatment the de novo pathway to generate guanine nucleotides.
of rheumatic diseases (as discussed in the section on Therefore, lymphocytes are a relatively specific target of
current indications in rheumatology, below)5,7. MPA15,16 (Fig. 1). MPA mediates reversible inhibition
of B cell and T cell proliferation without causing mye-
Pharmacokinetics. MMF is a pro-drug of MPA. The lotoxicity15. MPA decreases the synthesis of guanine
molecular structure and pharmacokinetic parame- nucleotides, hinders DNA synthesis and consequently
ters of MMF are displayed in Table 1. When ingested, reduces lymphocyte proliferation, antibody production
MMF is almost completely absorbed in the small intes- and even microRNA expression15–17. The effect of MPA
tine, and the compound is then de-esterified by plasma on microRNA expression has been attributed to his-
esterases to form MPA8. Ninety-eight percent of MPA tone modifications at the promotor region of miR-142
is protein bound, and both the protein bound and free and miR-146a in CD4+ T cells17,18. Both microRNAs are
fraction are almost solely present in plasma8. Only small important in the regulation of immune responses17,18.
amounts of MPA enter cells. A large proportion of MPA MPA is thought to affect levels of histone modifications
is metabolized into phenolic glucuronide, which is, in by upregulating the global acetylation of histone proteins
Bioavailability turn, eliminated in urine8. The plasma concentrations H3 and H4 (ref.19). In CD4+ T cells from patients with
The proportion of a drug that of MPA can vary largely between individuals, some- SLE, this effect can result in a downregulation of CD40
reaches the circulation
unchanged, and so has an
times by up to 10-fold9. This variability might be par- ligand (CD40L)19. As CD40L-mediated costimulation
active effect, when introduced tially caused by variations in genes encoding uridine of antigen-presenting cells has a crucial role in various
into the body. diphosphate-glucuronosyltransferase enzymes, such as rheumatic diseases, the ability of MPA to reduce CD40L
UGT1A9 (ref.10). Proton pump inhibitors can impair the signalling is of interest and worth further investigation
Pro-drug
absorption of MMF, but not enteric coated mycopheno­ in SLE and other rheumatic diseases20.
A biologically inactive
compound that, after late sodium (the latter being the sodium salt of MPA), Although MMF has traditionally been viewed as a
administration, is metabolized probably because proton pump inhibitors increase the drug that targets lymphocyte proliferation, clinical and
into a pharmacologically pH of the gastrointestinal tract11. in vitro data suggest that MPA can also affect fibroblast
active drug. In addition to MMF, mycophenolate sodium is ano­ biology. Notably, increased activation of fibroblasts in
Tmax
ther pro-drug of MPA. The main difference between rheumatic diseases such as SSc and antisynthetase syn-
The time taken for a drug the two pro-drugs is that mycophenolate sodium has drome can lead to complications such as interstitial lung
to reach its maximum serum a slower absorption rate than MMF, which results in a disease (ILD)21. As with lymphocytes, MPA has antiprolif-
concentration in the body. longer Tmax (~3 h compared with 90 min for MMF) and erative effects on fibroblasts and probably mediates these
higher remaining plasma concentrations between doses8. effects by inhibiting the formation of guanine nucleo-
Area under the
concentration–time curve MPA concentrations are only modestly affected by sides22–25. Given the effects of this drug on fibroblasts,
The area under a plot of hepatic and mild renal disease. However, in patients with these cells probably rely not only on the salvage pathway
plasma concentration of a drug severe renal disease (chronic kidney disease stage 4, for purine nucleotide synthesis, but also on the de novo
versus time after dosage; this creatinine clearance below 30 ml/min), concentrations of purine nucleotide synthesis pathway, as only the latter is
time curve reflects the actual
body exposure to a drug after
MPA can double, which can necessitate adjustments targeted by MMF.
administration of a dose of of the drug dose9. In patients with impaired renal func- MPA reduces the proliferation of fibroblasts in vitro
the drug. tion, the concentration of the glucuronide metabolite in a dose-dependent fashion22,23; this effect occurs in rat

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Table 1 | Pharmacokinetics of mycophenolate mofetil, azathioprine and tacrolimus

Parameter mycophenolate mofetil7,8 azathioprine71,72 tacrolimus111–113
Molecular structure OH O O
– O
O O
N N+ OH OH
O N O
O O
O N S
H
N O
N
HO N
N O
N O
O O
O
O

Molecular formula C23H31NO7 C9H7N7O2S C44H69NO12

General Rapidly converted to mycophenolic acid Highly variable resorption Resorption (poor) throughout the
by plasma esterase; resorption throughout throughout the gastrointestinal gastrointestinal tract
gastrointestinal tract tract
Bioavailability 94% 47% (27–80%) 20–25%
Tmax 1.5 hour and a second peak between 1–2 hours 1–3 hours (capsules and granules)
6–12 hours owing to enterohepatic circulation or 6 hours (tablets)
Volume of distribution 0.71 l/kg 0.1–1.7 l/kg 0.68 l/kg
Protein-bound fraction 97% 20–30%  > 99%
Metabolism Conversion to inactive glucuronide Rapid conversion into CYP3A4-mediated metabolism in
metabolite in liver 6-mercaptopurine (a process that the liver and intestinal epithelium
is partially dependent on TPMT)
Elimination 93% in urine, the remaining with faeces Mainly with urine (as inactive 95% with faeces
thio-uricacid); a small fraction
eliminated with faeces
Elimination half-life 12–18 hours 60–120 minutes (after conversion 12–15 hours
to 6-mercaptopurine)
Common adverse effects Diarrhoea, nausea, vomiting, leukopenia, Nausea, vomiting, hepatic Hypertension, diarrhoea,
anaemia, increases in blood creatinine dysfunction, infection hyperglycaemia, anaemia,
concentration, cytomegalovirus viraemia, (viral, fungal or bacterial) headache, tremor, insomnia,
urinary tract infection, respiratory tract and anorexia pain and asthenia
infection and hepatitis
CYP3A4, cytochrome P450 3A4; Tmax, time taken to reach maximum concentration; TPMT, thiopurine S-methyltransferase

fibroblasts, human tenon fibroblasts and human der- convert to pro-fibrotic α-SMA+ myofibroblasts27. These
mal fibroblasts22,23. MPA treatment does not promote myofibroblasts are able to amplify the production of
apoptosis of fibroblasts in vitro, suggesting that this collagens and fibrogenic components27. Notably, in mice
drug does not have direct toxic effects on these cells. with bleomycin-induced fibrosis, treatment with MMF
Notably, addition of guanosine (a guanine nucleoside) lowers the serum concentrations of the pro-fibrotic
to cultures of MPA-treated fibroblasts recovers the pro- cytokine transforming growth factor-β (TGFβ) com-
liferation capacity of the cells22,23. Culture of fibroblasts pared with treatment with placebo26. The finding that
with the same concentrations of MPA as that found in MMF can reduce the transformation of resident fibro-
the plasma of patients can reduce their proliferation blasts into potentially pathogenic myofibroblasts is of
by ~50%24. In a study that investigated the effects of great interest in the treatment of progressive SSc and
various drugs (including methylprednisolone, cyclo- justifies treatment with MMF at an early disease stage
sporine, tacrolimus, azathioprine, MMF and everoli- to reduce fibrosis progression.
mus) on the proliferative capacity of fibroblasts from In vitro studies of human mesangial cells have also
transbronchial tissue of patients who underwent a lung provided useful insights. Mesangial cells are closely
transplantation24, tacrolimus and MMF were the most related to fibroblasts, but are only found in the renal
potent inhibitors of proliferation at clinically relevant mesangium and have the ability to remove aggregated
drug concentrations25. protein from the basement membrane of the kidney27.
In bleomycin-treated mice (a well-accepted model Similar to fibroblasts, mesangial cells can produce
of SSc), treatment with MMF has anti-fibrotic effects26. matrix proteins such as collagen I and fibronectin27.
In the skin of these mice, MMF treatment reduces leu- Emerging data implicate mesangial cells in the onset of
kocyte infiltration, hydroxyproline content and skin renal fibrosis in SLE28. MPA can also reduce the prolif-
thickness. Moreover, treatment with MMF decreases eration of human mesangial cells in vitro in response
the number of fibroblasts that express α-smooth muscle to stimulation with TGFβ, fetal calf serum or fibroblast
actin (α-SMA; a marker of myofibroblasts, the cells pri- growth factor29–31. Moreover, MMF can reduce the infil-
marily responsible for fibrosis) in the skin26. One hypoth- tration of myofibroblasts in renal interstitial tissue in the
esis is that during SSc progression, normal fibroblasts rat remnant kidney model23,29,30.

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PRPP MMF lupus nephritis (n = 4,222) provided no conclusion on

which drug was superior for induction therapy, but the
analysis suggested that MMF was the most effective
Plasma maintenance therapy44.
esterases In patients with anti-neutrophil cytoplasmic anti-
IMP body (ANCA)-associated vasculitis, low disease activity
and a low risk of organ damage, MMF is well-tolerated
IMPDH MPA for induction of remission45. As a maintenance therapy,
however, this drug is slightly less efficacious than azathi-
oprine45–47. Furthermore, MMF seems to be effective in
XMP
achieving a favourable clinical and biological response
in patients with myositis and has some beneficial
De novo GMP synthase eff­ects in the treatment of patients with lung diseases asso­
purine
synthesis ciated with myositis, SSc or RA37,48–50. However, whether
GMP MMF has beneficial effects on the extrapulmonary fea-
tures of RA is unknown. A clinical trial in 142 patients
with SSc-related lung disease showed that a 2-year treat-
ment course with MMF was safer and better tolerated
than a 1-year course with oral cyclophosphamide; how-
GTP and dGTP ever, MMF was no more effective than cyclophospha-
mide51. In post-hoc analyses of the Scleroderma Lung
Studies I and II, MMF was associated with improve-
ments in dyspnoea, lung function and skin thickening
DNA synthesis and cell proliferation compared with placebo51,52. MMF also has a modest
bene­ficial effect on skin thickening in patients with
Fig. 1 | Inhibition of nucleotide synthesis by mycophenolate mofetil. Mycophenolate early diffuse cutaneous SSc53,54. The clinical effects of
mofetil (MMF) is metabolized into mycophenolic acid (MPA) by carboxylesterases; MMF on skin thickening are paralleled by the attenuation
the latter inhibits the synthesis of guanine nucleotides through the de novo purine of inflammatory gene signatures in the skin32.
synthesis pathway. This pathway begins with the conversion of 5-ribose phosphate to
5-phosphoribosyl-1-pyrophosphate (PRPP). PRPP is subsequently converted to inosine Novel avenues for clinical use. The initial interest in
monophosphate (IMP), which is dehydrogenated to xanthine monophosphate (XMP) MMF as an immunosuppressive drug was mainly on the
by inosine monophosphate dehydrogenase (IMPDH) and subsequently dehydrogenated basis of its ability to reduce lymphocyte proliferation.
to guanosine monophosphate (GMP) by GMP synthase. GMP is converted to guanosine
Nowadays, most ongoing trials of MMF in the field of
triphosphate (GTP) and deoxyguanosine triphosphate (dGTP), which are needed for
DNA synthesis. MPA is a strong inhibitor of IMPDH and inhibition of IMPDH leads to low rheumatology are investigating its putative anti-fibrotic
availability of nucleotides (GMP, GTP and dGTP) and hence prevents DNA replication properties, including the effects on pulmonary fibro-
and subsequently cell proliferation. sis and skin fibrosis in SSc. Growing evidence suggests
that MMF can reduce the burden of pulmonary disease
and skin thickening in patients with SSc (as discussed
Taken together, the effect of MMF on fibroblasts in the previous section). Various trials are currently
seems at least in part to be dependent on the reduced underway to further substantiate these findings in SSc
availability of guanine nucleotides. This inhibition (Supplementary Table 1). These trials test the use of MMF
might lead to a reduction in numbers of myofibroblasts. alone or in combination with methotrexate, bortezomib
Moreover, treatment with MMF might disrupt an auto- and/or pirfenidone.
crine loop in which increased fibroblast proliferation As of January 2020, a total of 52 trials that involve
leads to the production of TGFβ and increased fibrosis32. treatment with MMF are ongoing or recruiting, accord-
ing to data from ClinicalTrials.gov55; most of these trials
Current indications in rheumatology. In the past dec- are in the fields of haematology, oncology and transplant­
ade, MMF has taken a central role in the treatment of ation medicine. In rheumatology, some of these trials
SLE, SSc, vasculitis and myositis33–37. For induction involve patients with SSc who are undergoing auto­
therapy in lupus nephritis, some reports suggest that logous haematopoietic stem cell transplantation (HSCT)
MMF is more efficacious and has a slightly better toler­ (Supplementary Table 1). After transplantation, various
ability than cyclophosphamide38. This finding, how- immune suppressive drugs (including MMF) are used
ever, was not replicated in a meta-analysis comprising to maintain remission. One study is underway to com-
618 patients with lupus nephritis, reporting that MMF pare treatment protocols and find the optimal dose to
was not superior to cyclophosphamide in terms of dis- use after HSCT in SSc56. As the mortality after HSCT in
ease remission and was only associated with a reduction SSc is relatively high, finding a balance between efficacy
in alopecia and amenorrhoea39. MMF is the preferred and safety of HSCT might be pivotal to reduce mortality.
treatment for maintenance therapy in lupus nephritis One Swedish trial is underway to investigate the bio-
after induction therapy with intravenous cyclophos- availability of MMF in patients with SSc with and without
phamide40–43. Notably, a meta-analysis of 53 randomized gastrointestinal complications57. This trial is of interest,
trials that assessed MMF, cyclophosphamide and cal- as many patients with SSc have gastrointestinal involve-
cineurin inhibitors for the treatment of proliferative ment that might affect the response of other disease

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manifestations (such as lung and skin manifestations) was investigated as a chemotherapeutic drug for the
to MMF. treatment of leukaemia and was further investigated for
MMF is being used as maintenance therapy in patients its anti-inflammatory effects by Robert Schwartz68. He
with lupus nephritis after induction therapy with cyclo- discovered in 1958 that treating rabbits with 6-mercapto­
phosphamide43. Several trials are currently underway to purine led to a decrease in the production of anti-
further optimize the balance between the efficacy and bodies against subsequently administered antigens68.
toxicity of MMF for use as maintenance therapy58–60. For Increased understanding of how organ transplant
instance, one study is investigating whether lymphocyte rejection progresses and the underlying immunologi-
subsets in the kidneys of patients newly diagnosed with cal mechanisms led to the hypothesis that 6-mercapto­
lupus nephritis can predict the efficacy of MMF as main- purine could reduce the risk of transplant rejection69.
tenance therapy8. Another trial is aimed at determining the In addition to 6-mercaptopurine, azathioprine was
ideal protocol and timing for the tapering of MMF treat- investiga­ted as a compound for preventing transplant
ment during maintenance therapy in lupus nephritis59. rejection69. Azathioprine proved to be better tolerated
Finally, a study is underway to investigate whether serum than 6-mer­captopurine and has since been used as
concentrations of MMF are a good parameter of whether a drug to prevent transplant rejection, in addition to
to taper or adjust drug doses during maintenance therapy60. prednisone, MMF and cyclosporin69. Given the success
Pegylated uricase is an effective therapy for the treat- of azathioprine in transplantation medicine, research-
ment of refractory gout;61 however, anti-polyethylene ers investigated this drug in other immune-mediated
glycol (PEG) antibodies can hinder the efficacy of this diseases and found success in the treatment of systemic
treatment62. A strategy that is currently being investi- inflammatory diseases, as described in the section on
gated to prevent the formation of these antibodies is current indications in rheumatology below70.
pretreatment with MMF63.
The aforementioned data on the mechanism of Pharmacokinetics. The main therapeutic effect of aza-
action of MMF might help in refining formulations thioprine relies on its metabolism to cytotoxic thiogua-
of MMF to better deliver high concentrations of MMF nine nucleotides, which inhibit de novo purine synthesis,
to the local site of fibroblast proliferation. For this rea- thereby decreasing leukocyte proliferation71. Hence,
son, investigators have developed a MMF ointment64. the metabolites of azathioprine need to reach the cell
Applying this ointment locally to active skin lesions in nucleus. The pharmacokinetic parameters and molecular
SSc might increase the local concentrations of the drug structure of azathioprine are displayed in Table 1.
and therefore increase the efficacy. Similarly, in patients The metabolism of azathioprine is complex71,72. After
with interstitial lung fibrosis, nebulization of MMF oral ingestion, azathioprine is absorbed from the whole
might lead to higher levels of the drug at the site of intestinal tract71,72. In the intestinal wall, liver and red
fibrosis and hence exert a larger effect than oral MMF65. blood cells, azathioprine is then converted to 6-mercap-
In conclusion, over the past century, MMF has moved topurine via a glutathione-mediated non-enzymatic pro-
from being used as an antibiotic to being used as an cess that involves the removal of an imidazole group67,68.
immunosuppressant drug in transplantation medicine Hypoxanthine-guanine phosphoribosyltransferase and
and rheumatology and is now increasingly being used other enzymes drive the formation of cytotoxic thi-
as an anti-fibrotic drug for the treatment of a wide array oguanine nucleotides71,72. By contrast, xanthine oxidase
of rheumatic diseases. and thiopurine S-methyltransferase (TPMT) metabo-
lize 6-mercaptopurine into inactive compounds. Hence,
Azathioprine TPMT and xanthine oxidase have an important role in
Historical perspective. The use of azathioprine stems keeping the balance between the tolerability and tox-
from research performed in 1957 by Nobel prize lau- icity of the drug71–73. Genetic variations in TPMT and
reates George Herbert Hitchings and Gertrude Elion1. co-administration of drugs that influence TPMT or xan-
Hitchings and Elion approached drug development in a thine oxidase activity, such as allopurinol, can affect the
novel way, namely by aiming to create drugs that specif- toxicity of azathioprine71,72. TPMT activity is more impor-
ically interfere in pathways contributing to cell homeo­ tant in determining the response to azathioprine than is
stasis. Nowadays, this approach is common practice. renal function74,75. Nevertheless, patients with stage 3 or
The two researchers looked at differences in nucleic higher chronic kidney disease are at an increased risk of
acid metabolism between normal cells, cancer cells toxicity caused by azathioprine compared with patients
and bacteria1. The aim of their research was to inter- with normal kidney function74,75.
fere with the purine pathway to reduce the production
of adenine and guanine nucleotides66. The idea behind Molecular and cellular mode of action. 6-mercapto­
this approach was that interference with nucleotide purine is a purine antagonist that inhibits leukocyte
production would consequently reduce DNA synthesis proliferation by interfering with nucleotide synthesis71,72
and inhibit cell growth67. By systematically investigating (Fig. 2). The therapeutic index of azathioprine is better than
Therapeutic index pathways, Hitchings and Elion were instrumental in the that of 6-mercaptopurine, and azathioprine has therefore
A ratio that denotes the margin development of various drugs that are still abundantly taken a more prominent role in the drug armamentarium
of safety of a drug, comparing used in clinical practice today, such as allopurinol, used to treat systemic autoimmune diseases71,72.
the dose of a drug that
produces the desired effect
acyclovir, trimethoprim and 6-mercaptopurine1,66,67. Cytotoxic thioguanine nucleotides reduce the forma-
and the dose that produces Azathioprine is a pro-drug of 6-mercaptopurine and tion of purine nucleotides through the de novo pathway
unwanted adverse effects. was synthesized in 1957 (ref.67). At first, 6-mercaptopurine by inhibiting amidotransferase enzymes and purine

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ribonucleotide interconversion71,72. This mechanism RAC1, which results in reduced production of B cell
is thought to explain the effect of azathioprine on leu- lymphoma-extra large (BCL-XL), a transmembrane
kocyte proliferation71,72. In addition, toxic thioguanine molecule with important anti-apoptotic functions76,77.
nucleotides are incorporated into DNA and RNA71,72, Interestingly, BCL-XL upregulation contributes to the
which is thought to mediate the cytotoxic effects of aza- survival of T cells following CD28 co-stimulation77,79.
thioprine. The imidazole derivative mercapto-imidazole These findings suggest that azathioprine and its metab-
is a metabolite of azathioprine and 6-mercaptopurine olites have an inhibitory effect on T cell survival and pro-
and potentially also has effects on lymphocyte function. liferation, after T cell co-stimulation, through inhibition
Although this metabolite has not been investigated of RAC1 and/or BCL-XL, and that these drugs make
in relation to the therapeutic effects of azathioprine, T cells more prone to apoptosis through this pathway80.
mercapto-imidazoles can reduce T cell proliferation In addition to T cells, 6-mercaptopurine can also inhibit
and nuclear factor of activated T cells (NFAT) signalling RAC1 in activated macrophages, which leads to a reduc-
following T cell receptor activation in mice76. tion in the expression of inducible nitric oxide synthase81.
In addition to the effects of azathioprine on purine The RAC1 pathway is also targeted by 6-mercapto­
nucleotide synthesis, azathioprine can directly promote purine in non-immune cells82. For instance, in endothe-
apoptosis and inhibit proliferation pathways. In vitro, lial cells stimulated with the pro-inflammatory cytokine
co-culture of stimulated primary human T cells with TNF, treatment with azathioprine reduces the expression
azathioprine or 6-mercaptopurine promotes apoptosis77. of leukocyte adhesion molecules VCAM1 and ICAM1
In these cells, 6-mercaptopurine directly interacts with and impairs NF-κB signalling83.
RAS-related C3 botulinum toxin substrate 1 (RAC1), Within the field of rheumatology, whether the
a GTP binding protein that has a critical role in T cell treatment outcome of azathioprine therapy correlates
migration and adhesion78. RAC1 deficiency in T cells with numbers of circulating leukocytes is unknown.
reduces interstitial migration of these cells within Azathioprine is often used after cyclophosphamide
and from lymph nodes78. 6-thioguanine triphosphate, induction therapy in ANCA-associated vasculitis,
a metabolite of 6-mercaptopurine, can also bind to and so leukocyte levels are already heavily affected by

Non-enzymatic cleavage Possible weak effect on

Azathioprine Imidazole derivatives lymphocyte function
and migration
Non-enzymatic
cleavage
Xanthine oxidase
6-Mercaptopurine 6-Thiouric acid

TPMT
HPRT 6-MMP

Possible reduction
6-Thioinosine monophosphate 6-Methylthioinosine in intracellular
5ʹ-monophosphate nucleotide levels
TPMT
IMPDH

6-Thioxanthosine monophosphate

GMP synthase

6-Thioguanosine monophosphate

DNA synthesis and cell proliferation

Fig. 2 | Immunomodulatory effects of azathioprine. Azathioprine is cleaved non-enzymatically into either

6-mercaptopurine or imidazole derivatives (such as mercapto-imidazole) by sulfhydryl-containing compounds; these
imidazole derivatives are thought to have weak immunomodulatory effects. 6-mercaptopurine is further metabolized by
xanthine oxidase and thiopurine S-methyltransferase (TPMT) into 6-thiouric acid and 6-methylmercaptopurine (6-MMP),
respectively , which are both non-toxic. In a step-wise manner, 6-mercaptopurine can also be metabolized to toxic
6-thioguanosine nucleotides, which inhibit effective DNA synthesis and cell proliferation. Firstly , 6-mercaptopurine is
converted to 6-thioinosine monophosphate by hypoxanthine-guanine phosphoribosyltransferase (HPRT); 6-thioinosine
monophosphate is metabolized to 6-thioxanthosine monophosphate by inosine-5′monophosphate-dehydrogenase
(IMPDH), which is subsequently converted to 6-thioguanosine monophosphate by guanosine monophosphate (GMP)
synthase. Inhibition of cell proliferation is the main immunosuppressive effect of azathioprine on lymphocytes.
6-thioinosine monophosphate can also be metabolized by TPMT to 6-methylthioinosine 5′-monophosphate, which is
thought to reduce the availability of nucleotides in cells.

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cyclophosphamide before treatment with azathioprine Two large retrospective studies are investigating,
begins84. One study in lupus nephritis showed that a among other drugs, the efficacy of azathioprine in the
higher number of circulating B cells, before treatment treatment of either therapy-resistant Takayasu disease or
initiation, is a predictor of responsiveness to azathio- sarcoidosis100,101. As discussed for MMF, azathioprine is
prine84. In several studies in Crohn’s disease and ulcer- also under evaluation for the treatment of patients with
ative colitis, the number of circulating lymphocytes SSc following HSCT, including the appropriate dosage
correlates with response to azathioprine85,86. Further and duration of therapy and the effect on disease activ-
studies are needed, however, to confirm the clinical ity56. Finally, efforts are underway to address if it is safe
utility of measuring circulating lymphocyte numbers as to taper azathioprine during maintenance therapy in
a predictor of responsiveness to azathioprine. patients with SLE or ANCA-associated vasculitis who
have end-stage renal disease102. As azathioprine treat-
Current indications in rheumatology. Azathioprine is ment is associated with adverse effects and an increased
currently used in the treatment of systemic autoimmune risk of skin and cervical cancer103,104, these studies could
disease, including overlap syndromes, but its role in the help to identify which patients might benefit from
treatment of RA is limited. Azathioprine is inferior to treatment with these drugs and for which patients these
and slower acting than methotrexate in RA87. Although drugs should be avoided or tapered103,105.
this drug is often prescribed for the treatment of SLE, Emerging data suggest that fine-tuning the patient
azathioprine is inferior to cyclophosphamide for induc- selection and dosing strategies with azathioprine ther-
tion of remission in patients with lupus nephritis88 or apy might lead to better therapeutic effects and reduced
systemic vasculitis89,90. However, azathioprine is effec- toxicity. This drug seems to mediate its main clinical
tive as maintenance therapy after induction therapy effects via the formation of 6-thioguanine nucleotides.
in both lupus nephritis and systemic vasculitis88–90. Over the past 15 years, researchers have performed
Although both azathioprine and MMF can be used as TPMT gene profiling in patients with inflammatory
maintenance therapy for lupus nephritis, azathioprine bowel diseases and other immune-mediated diseases to
is inferior to MMF in this indication and should be evaluate the ability of different variants to metabolize
used only when contraindications for MMF exist in azathioprine, with the aim of stratifying patients by risk
the patient (as discussed in more detail in the previ- of toxicity and/or by treatment response106. In 2014, var-
ous section on MMF). Azathioprine is widely used for iants in NUDT15 were identified in individuals of Asian
the treatment of other manifestations of SLE, such as descent that could be clinically relevant for the metab-
hepatitis and myositis38,88,91,92. Furthermore, azathio- olism of azathioprine106. NUDT15 encodes an enzyme
prine is used with or without glucocorticoids for the that hydrolyses 6-thioguanine nucleotides into inactive
treatment of patients with myositis, Behçet syndrome, metabolites. Consequently, patients who carry a variant
psoriatic arthritis or reactive arthritis93–97. Patients of NUDT15 that renders this protein less effective are at
with SSc and overlap syndromes, presenting with ILD an increased risk of adverse effects with azathioprine106.
or joint involvement, can also benefit from treatment In addition, phosphorylation of the metabolites of aza-
with azathioprine97 thioprine can affect the clinical efficacy of this drug106.
Further investigation of the genotypes and phospho-
Novel avenues for clinical use. As with MMF, azathi- rylation profiles that influence levels of 6-thioguanine
oprine has progressed from transplantation medicine nucleotides could help to optimize therapy with azathi-
into rheumatology practice. This drug is important in oprine106. Such an approach could enable personalized
the primary treatment or maintenance therapy of var- medicine and individual drug dosing, leading to less
ious systemic autoimmune diseases including antisyn- toxicity and better treatment results.
thetase syndrome-associated ILD. Twenty-nine trials
that include azathioprine are currently ongoing, as of Tacrolimus
January 2020, according to data from ClinicalTrials.gov55 Historical perspective. In 1982, following the success of
(Supplementary Table 1). Most of these studies are in cyclosporin, researchers in Japan tested fermentation
the field of gastroenterology, focusing on autoimmune products of Streptomyces spp.107 to determine whether
hepatitis, Crohn’s disease and ulcerative colitis. However, these products had immunosuppressant effects in a
nine studies are currently investigating azathioprine as a mixed lymphocyte reaction107,108. One of these products,
primary compound in rheumatology practice. The use dubbed FK506 and now called tacrolimus, was derived
of azathioprine for the treatment of antisynthetase syn- from Streptomyces tsukubaensis107,108 and was 100 times
drome is of particular interest98, as knowledge on this more potent in suppressing the response in a mixed lym-
disease and prospectively performed studies are scarce. phocyte culture than cyclosporin109. The first scientific
One ongoing study is comparing induction and main- report on FK506 was published in 1986 (ref.107) and the
tenance therapy with tacrolimus with induction ther- first clinical trials began in 1989 in Pittsburgh110. This
Mixed lymphocyte reaction apy with cyclophosphamide followed by maintenance trial was aimed at evaluating the efficacy of FK506 in
A test used to measure how treatment with azathioprine in patients with antisyn- rescuing liver allografts in patients who have undergone
T cells function in the presence thetase syndrome-associated ILD98. This condition is a liver transplant, but had chronic rejection or nephro-
of external stimuli; in this associated with a high morbidity and mortality99. Such a toxicity with cyclosporin. In this study, tacrolimus was
assay, populations of T cells
from different individuals are
study might help to define the best treatment strategy for well tolerated and had encouraging effects110. The use of
mixed to measure the reaction this condition and might give rise to an evidence-based tacrolimus subsequently disseminated into other fields
that occurs. treatment standard98. of transplantation medicine and is used in kidney, liver,

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APC of tacrolimus is the activity of P-glycoprotein, a

protein pump that pumps tacrolimus out of cells.
P-glycoprotein is present on cells in the liver and also
on barriers such as the blood–brain barrier, placenta
and intestinal epithelium. Hence, P-glycoprotein has a
direct effect on the bioavailability of tacrolimus111–113.
MHC In diseases that cause damage to the intestinal mucosa,
Antigen
such as infectious diarrhoea, P-glycoprotein activity
TCR
IL-2 is reduced, which leads to an increase in azathioprine
T cell bioavailabiality114. Hence, drugs that inhibit CYP3A4
or P-glycoprotein might increase tacrolimus concen-
trations111–113. The elimination half-life of tacrolimus
ranges from 12 to 15 h. Ninety-five percent of tacro­
↑ Intracellular Ca2+ limus metabolites are excreted through the biliary sys-
tem with faeces111–113. Although renal failure does not
have an effect on tacrolimus elimination, tacrolimus
can be nephrotoxic and should be avoided in patients
Ca2+
with renal failure111–113.
Calmodulin
Tacrolimus Molecular and cellular mode of action. The mode of action
Calcineurin
FKBP of tacrolimus has been best characterized in T cells.
Tacrolimus is a well-known calcineurin inhibitor, but
P
the actions of tacrolimus on T cells are thought to be
NFAT NFAT
pleiotropic and go beyond the inhibition of calcineu-
rin, affecting both mitogen-activated protein kinase
(MAPK) signalling and TGFβ signalling115,116.
Calcineurin is a calcium-dependent and calmodulin-
dependent phosphatase involved in T cell activation.
IL2 T cell receptor signalling results in upregulation of cal-
cineurin activity, which leads to activation of NFAT,
NFAT
nuclear translocation of NFAT and transcription of the
gene encoding IL-2 (ref.117). Hence, increased activity of
Nucleus
calcineurin leads to increased IL-2 expression and con-
Fig. 3 | Inhibition of T cells by tacrolimus. In T cells, T cell receptor (TCR) signalling sequently promotes the differentiation and survival of
increases levels of calcium in the cytoplasm, which leads to activation of the calcineurin– T cells117. After entering the cell, tacrolimus inhibits cal-
nuclear factor of activated T cells (NFAT) pathway. In this pathway , dephosphorylation of cineurin activity by binding to an immunophilin called
NFAT leads to activation and nuclear translocation of NFAT and transcription of IL2. FK506 binding protein (FKBP); notably, cyclosporin
Tacrolimus binds to FK506 binding protein (FKBP), and this tacrolimus–FKBP complex (a different calcineurin inhibitor) also binds to an immuno­
suppresses activation of the calcineurin–NFAT pathway , leading to a reduction in IL-2 philin (cyclophilin A)116. The immunophilins cyclophi-
production and inhibiting early activation of T cells. APC, antigen-presenting cell.
lin A and FKBP are naturally occurring calcineurin
inhibitors that bind to and reduce calcineurin activity in
intestine and heart transplantations110, as well as the field the absence of other calcineurin ligands118. The binding
of rheumatology. of cyclosporin or tacrolimus to cyclophilin A or FKBP,
respectively, increases the affinity of these immunophi-
Pharmacokinetics. The intestinal absorption of lins for calcineurin118. Hence, inhibition of calcineurin
tacro­limus is poor, with the majority being eliminated signalling through immunophilins increases with these
with faeces111. After absorption, approximately 99% of drugs and results in reduced NFAT signalling and IL-2
tacro­limus binds to erythrocytes. The remaining 1% production118 (Fig. 3). Calcineurin also promotes a sec-
of tacro­limus (unbound) enters the lymphatic sys- ond wave of IL-2 transcription via NF-κB. The inac-
tem and exerts its main therapeutic effects111,112. The tive form of NF-κB is bound to a protein called IκB119.
pharmaco­kinetic parameters and molecular structure Cal­cine­urin activity leads to degradation of IκB, enabl­ing
of tacrolimus are displayed in Table 1. NF-κB-mediated transcription of pro-inflammatory
The elimination and therefore the balance between genes120. Consequently, inhibition of calcineurin by
the efficacy and toxicity of a certain dose of tacrolimus either tacrolimus or cyclosporin increases the fraction
is mainly dependent on two cellular processes. The first of NF-κB bound to IκB and inhibits NF-κB-mediated
process is metabolism of tacrolimus via cytochrome transcription of pro-inflammatory genes120.
P450 3A4 (CYP3A4), an enzyme expressed in the In addition to calcineurin inhibition, tacrolimus can
liver and intestinal epithelium, into inactive metabo- reduce IL-2 transcription through the inhibition of the
lites111,112. Various polymorphisms in CYP3A4 can affect MAPK pathway115. Cyclosporine and tacrolimus (in
the clearance of tacrolimus and consequently the dose complex with their respective immunophilins) inhibit
of tacrolimus needed for effective treatment113. The the activity of components of this pathway: MAPK14
second process influencing the efficacy and toxicity (also known as p38α) by tacrolimus and MAPK8

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 Reviews

Box 1 | Future directions for research Current indications in rheumatology. Tacrolimus can
be a useful drug in the treatment of SLE and myositis-
Various research directions into mycophenolate mofetil (MMF), azathioprine and associated ILD. For example, topical ointments that
tacrolimus require further exploration. First, strategies for adjusting the dosage contain 1% tacrolimus are effective in the treatment
according to the pharmacokinetics and pharmacodynamics of an individual require of cutaneous lupus erythaematosus132. In patients with
further refinement. Various studies are currently focusing on the effect of tapering these
biopsy-confirmed active lupus nephritis, tacrolimus is
drugs on biological or clinical parameters. An important question to answer is when
to stop treatment. At what point can nothing more be gained or lost with treatment non-inferior to MMF for induction therapy when com-
(for instance, in chronic renal failure)? Another area of research related to dosing is the bined with prednisone and followed by azathioprine
effect of the dose and duration of treatment on the number and subsets of leukocytes. maintenance therapy133. To reduce the toxicity of tacroli-
This research should help to optimize dosing and reduce the toxicity of these drugs in mus, researchers have tested a dosing scheme of 4 mg/day
the circulation or organ of interest. A second direction is to test these drugs in novel (as opposed to the standard dose of 0.1–0.2 mg/kg/day)
indications, which could lead to better treatment of diseases and reductions in costs. in combination with low-dose MMF (1 g/day) for the
The patent rights of MMF, azathioprine and tacrolimus have expired, which makes them treatment of 368 patients with lupus nephritis. In this
relatively inexpensive to acquire. An exciting novel development in this regard is the randomized controlled trial, the combination of low-
possible addition of tacrolimus in the armamentarium to treat refractory rheumatoid dose MMF and tacrolimus was superior to intrave-
arthritis. Furthermore, a current head-to-head trial comparing MMF with tacrolimus for
nous cyclophosphamide pulses in achieving complete
the treatment of ILD in antisynthetase syndrome will carve out a stronger indication
of one of these two drugs in the treatment of antisynthetase syndrome98. A third renal response at 6 months. This outcome, however,
avenue is to test these drugs as co-medications in combination with other biologic or came at the expense of an increased risk of infection134.
conventional DMARDs. There is currently a surge in research into novel treatments of Data from smaller observational studies135 suggest that
ILD, including biologic and synthetic drugs. These drugs are often co-administered with so-called ‘multitarget’ therapy with low-dose MMF and
either MMF or tacrolimus. tacrolimus can be particularly effective in the treatment
of lupus nephritis unresponsive to standard regimens,
including MMF monotherapy135. Tacrolimus can also be
(also known as JNK1) by cyclosporine115. In T cells, considered in patients with ILD associated with polymyo­
co-stimulatory signals promote activation of MAPK14 sitis or dermatomyositis. In these patients, treatment
and MAPK8, which translocate to the nucleus to with tacrolimus in addition to standard of care therapy
exert multiple effects, including upregulation of IL-2 (prednisone, intravenous cyclophosphamide or cyclo-
expression121. Upstream inhibition of MAPK signal- sporin) is associated with improved survival compared
ling by cyclosporin and tacrolimus, therefore, leads to a with standard of care therapy alone136,137.
calcineurin-independent reduction in IL-2 signalling115.
Notably, some in vitro data suggest that tacrolimus Novel avenues for clinical use. One-hundred-and-nintey-
can promote TGFβ1 mRNA expression in T cells122. The two clinical trials of tacrolimus are ongoing, as of January
underlying mechanism of this phenomenon is unknown. 2020, the majority of which involve patients with solid
Cyclosporin can similarly promote TGFβ1 expression organ transplants or haematological malignancies55.
in peripheral blood cells of patients about to undergo Ten trials are of relevance for the field of rheumatol-
kidney transplantation123. Although these findings are ogy (Supplementary Table 1). For instance, one study is
preliminary, understanding this effect is important, as investigating tacrolimus droplets in the treatment of dry
TGFβ1 has an important role in fibrosis and has been eyes in patients with Sjögren syndrome. This study is of
associated with nephrotoxicity and pulmonary fibro- interest, as effective therapeutic options are lacking for
sis123, which are both complications of tacro­limus treat- sicca symptoms of Sjögren syndrome138. As discussed
ment124–127. A better understanding of the mechanism for azathioprine, tacrolimus is also under investigation
behind TGFβ upregulation by tacrolimus might lead to in a prospective clinical trial for the treatment of patients
strategies that can reduce the risk of these complications with antisynthetase syndrome and ILD98. In line with the
in patients. putative effect of tacrolimus on ILD in systemic auto­
Most research into tacrolimus has focused on T cells; immune diseases, investigators are also evaluating the
however, this drug can also affect other cell types. effect of tacrolimus on lung involvement in polymyositis
Tacrolimus can reduce cell degranulation and IFNγ pro- and dermatomyositis139. These studies are of interest and
duction by natural killer (NK) cells in vitro128. Similarly, it is hoped that the results will help with the management
cell degranulation and IFNγ production are reduced of pulmonary disease in these patients, which can some-
in NK cells derived ex vivo from patients treated with times be rapidly progressive. Other studies focusing on
calcineurin inhibitors compared with NK cells derived the safety and efficiency of tacrolimus in lupus nephritis
from healthy individuals128. Furthermore, tacrolimus are also underway140.
reduces the capability of dendritic cells to stimulate Currently, tacrolimus is seldomly used in the treatment
T cells and the production of IL-12 and CXC-chemokine of RA in most countries. Tacrolimus is, however, approved
ligand 10 (CXCL10) by these cells129. Dendritic cells for the treatment of RA in Japan, and an interesting ave-
incubated with tacrolimus have a tolerogenic pheno- nue of research is whether tacrolimus is indeed effective in
type, and infusion of dendritic cells pretreated with tacro­ the treatment of this disease. Although data from various
limus into mice with collagen-induced arthritis has clinical trials suggest that tacrolimus has beneficial effects
suppressive effects on CD4+ T cell proliferation130. Some in RA141,142, whether tacrolimus can reduce radiographic
data also suggest that tacrolimus has minor (although progression of the disease is unclear143. Various efforts are
clinically irrelevant) effects on the MAPK pathway underway for investigating tacrolimus as either an alter-
in monocytes131. native or an addition to methotrexate144,145. Moreover,

Nature Reviews | Rheumatology

Reviews

Box 2 | What can we expect from other transplantation drugs? the slow-release compound is much higher than that of
generic tacrolimus148.
Learning from history prepares us for the future. Tacrolimus, mycophenolate mofetil
and azathioprine all originated in transplantation medicine. Other drugs that are Conclusion
similar to those discussed in this Review are in use in transplantation medicine and
MMF, azathioprine and tacrolimus have a long history
could cross over to rheumatology practice in the future; for example, the mechanistic
and share a common root in solid organ transplantation,
target of rapamycin (mTOR) inhibitors sirolimus and everolimus.
In transplantation medicine, sirolimus and everolimus are used as maintenance but have found their way into rheumatology practice as
therapy after transplantation. Sirolimus seems to be a safe alternative for the effective drugs for the treatment of lupus nephritis, vas-
treatment of patients with lupus nephritis who do not tolerate standard treatments150. culitis and systemic autoimmune disease such as SSc and
A pilot study of the addition of everolimus to methotrexate therapy in the treatment antisynthetase syndrome.
of patients with rheumatoid arthritis yielded promising results, but needs further Tacrolimus is an inhibitor of the calcineurin pathway,
follow-up research151. Some data suggest that everolimus and sirolimus can have whereas azathioprine and MMF inhibit the de novo purine
both detrimental and beneficial effects in interstitial pulmonary disease152,153. Given synthesis pathway via different mechanisms. Further
the potential function of the mTOR pathway in various rheumatic diseases (including understanding of the purine synthesis pathway could
rheumatoid arthritis, Sjögren syndrome, systemic lupus erythematosus and gout), this
lead to better dosing schedules and drug delivery mech-
class of transplantation drugs might have a bigger role in rheumatology practice and
anisms. Notably, in addition to inhibiting lymphocyte
trials in the coming years150–155.
proliferation, MMF can also inhibit the proliferation of
fibroblasts, which could explain the beneficial actions
the efficacy of a combination of biologic DMARDs of this drug in SSc and ILD. Both MMF and tacrolimus
with either methotrexate or tacrolimus is being tested145. hold promise for use alone or as co-medication in the
If tacrolimus proves to be successful in these stud- treatment of ILD in rheumatic diseases. Furthermore,
ies, this drug might be a useful, low cost and relatively the inclu­sion of azathioprine or MMF in various pro-
safe alternative for patients with RA not responding to tocols for the treatment of patients with SSc following
other DMARDs. HSCT is under investigation. Additional areas of research
Finally, a slow-release compound of tacrolimus has that require further investigation are discussed in Box 1.
come on to the market146 and is being used as mainte- In addition to these three drugs, others are currently
nance therapy in kidney transplantation. The idea of this being used in transplantation medicine that might have
approach is that a once-daily formulation of tacrolimus potential in the treatment of rheumatic diseases (Box 2).
(rather than twice-daily dosing) might improve patient Taken together, MMF, azathioprine and tacrolimus
adherence. Various large studies in patients with kidney have come a long way as immunosuppressive drugs in
transplants showed that this formation was non-inferior various fields of clinical medicine and are likely to con-
to twice-daily tacrolimus146–148. This compound might tinue to have an important role in the future treatment
find its way into rheumatology practice, especially as of systemic autoimmune diseases.
poor adherence is a common feature in many rheumatic
diseases149. However, it should be noted that the cost of Published online xx xx xxxx

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