When cyclosporine was introduced as rescue therapy for corticosteroid-refractory ulcerative colitis in the early 1990s, this marked a turning point in the management of these patients, for many of which colectomy had been the only remaining therapeutic option [1, 2]. Since then, the value of cyclosporine has been confirmed in numerous studies [3], and a recent randomized controlled trial carried out at 27 European inflammatory bowel disease centers found that its efficacy for inducing remission in severe ulcerative colitis refractory to intravenous steroids equaled that of infliximab [4]. However, the safety profile of cyclosporine, especially in the long run, appears rather unfavorable. Neurological side effects such as paresthesias, nephrotoxicity, hypertension, headache, and gingival hyperplasia have been reported in up to one third of patients [2, 3]. In addition, intravenous administration with therapeutic drug monitoring is generally required, although an oral formulation of cyclosporine exists.

As a consequence, an intense search for novel calcineurin inhibitors commenced which led to the isolation of a macrolide produced by Streptomyces tsukubaensis initially termed FK-506 in 1987 that was later renamed into tacrolimus (for Tsukuba macrolide immunosuppressant) [5]. Similar to cyclosporine, tacrolimus acts by inhibiting calcineurin, a phosphatase required for the translocation of the transcription factor NFAT (nuclear factor of activated T cells) into the nucleus, where it controls production of interleukin-2 in T lymphocytes. In addition, inhibition of other transcription factors such as NF-κB or Oct-1 has been demonstrated for these compounds, and consequently, both cyclosporine and tacrolimus act primarily by inhibiting T cell activation, although direct effects on B cell activation have been reported as well [6, 7].

While cyclosporine and tacrolimus exert similar effects, their mechanism of action differs with cyclosporine binding to cyclophilin and tacrolimus to a protein termed FKBP (for FK binding protein) that belongs to a group of cytosolic peptidylprolyl isomerases called immunophilins [7]. Due to this differential mode of action, the immunosuppressive potency of tacrolimus vastly exceeds that of cyclosporine [5, 6]. Moreover, both compounds exhibit important differences in terms of pharmacokinetics, and oral absorption of tacrolimus is more reliable compared to cyclosporine as it does not depend on bile flow or integrity of the intestinal mucosa. As a consequence, tacrolimus gradually replaced cyclosporine in many indications within transplantation medicine, which consequently piqued an interest in its potential usability in ulcerative colitis.

This development was paralleled by encouraging data obtained in animal models of IBD [8–11] and an anecdotal series with tacrolimus treatment of Crohn’s disease and ulcerative colitis as early as 1993 [6]. However, it was not until 1998 that a first detailed description of its use in adults was published in a form of a small open-label, uncontrolled pilot study. In this trial, 11 patients suffering from acute flares of ulcerative colitis or Crohn’s disease refractory to a standard therapy consisting of corticosteroids, azathioprine, and mesalamine received intravenous tacrolimus after about a week of unsuccessful intravenous steroid treatment [12]. Within 10 days, 9 out of the 11 patients displayed a favorable response, whereas the remaining 2 patients underwent colectomy. Subsequent case series involving 9–40 ulcerative colitis patients confirmed these data [13–15], and as a consequence, a first randomized multicenter trial on the use of tacrolimus in ulcerative colitis was published in 2006 [16]. This study compared two arms with low (5–10 ng/ml) and high (10–15 ng/ml) trough levels to a placebo group in a total of 60 hospitalized patients with moderately or severely active left-sided colitis or pancolitis and was carried out in 17 centers in Japan. Response as indicated by a drop in the disease activity score of more than 4 was observed in 68 % of patients in the high-dose group as compared to 10 % in the placebo group (p < 0.001) within 2 weeks of therapy. Similarly, more patients receiving the low dose displayed a clinical response (38 %), although statistical significance was not met in this group, most likely due to the small number of patients analyzed. Although this study has been criticized for its low number of patients and the potential inclusion of patients with less severe disease [17], it provided the first data on the short-term efficacy of tacrolimus in a randomized design.

Later on, the same group published results from a second double-blind placebo-controlled trial that involved 62 hospitalized patients with steroid-dependent or steroid-refractory disease and employed target tacrolimus trough levels of 10–15 ng/ml. Again, the primary endpoint was clinical response after 2 weeks of therapy as indicated by a drop in the disease activity index of at least 4 points which was met by 50 % in the tacrolimus group as opposed to 13.3 % in the placebo group (p = 0.003). Rates for mucosal healing (44 % vs. 13 %) and clinical remission (9 % vs. 0 %) were also higher in the tacrolimus group, although the latter difference was not statistically significant. None of the patients required colectomy during a 12-week open-label extension, which again raised objections on whether disease severity might have been lower than in other case series that reported colectomy rates between 10 and 50 % [18]. Although the incidence of adverse events in both prospective trials was reported to be not significantly different between the tacrolimus and placebo groups, experiences coming from the use of tacrolimus in other indications demonstrated that it is associated with infections, nephrotoxicity, changes in glucose metabolism, and neurological adverse events such as tremor or paresthesias [19, 20], suggesting that the apparent lack of relevant side effects in these trials might have been due to the short time of treatment and/or follow-up. Data from uncontrolled observational studies furthermore suggest that even when used for short times, tacrolimus therapy might be associated with side effects in up to 50 % of patients, although, in these series, the majority of adverse reactions were mild and very rarely required discontinuation of therapy [13–21].

In summary, the available evidence provides support for tacrolimus in the induction of remission when treatment with corticosteroids has failed. However, head-to-head comparison studies with infliximab and/or cyclosporine have not been conducted to date. Nonetheless, national and international professional societies have adopted tacrolimus into their current treatment guidelines for refractory ulcerative colitis [22, 23]. A target serum trough concentration of 10–15 ng/ml is supported by both Ogata trials, although several case series including more than 100 patients have demonstrated efficacy for trough levels below 10 ng/ml as well [13, 21, 24]. Thus, further appropriately designed trials will be required to determine the optimal dose of tacrolimus.

When administering the drug orally, target levels will be reached faster when therapy is started with higher doses (0.1–0.2 mg/kg daily divided into two doses), especially when patients are allowed to eat [3]. However, special caution has to be exercised in this scenario, and monitoring of drug serum levels is advised daily for the first days of therapy in order to avoid adverse effects due to overdosage. Alternatively, tacrolimus therapy can be initiated by continuous intravenous infusion in hospitalized patients in order to reach target levels faster. As a result of its narrow therapeutic window and its metabolization via the cytochrome C system (in particular CYP3A4), special caution is advised with respect to potential drug interactions, e.g., with macrolide antibiotics, certain antiepileptic and antifungal drugs, and antiretroviral medications [25].

Given the substantial risk of adverse reactions associated with systemic immunosuppression, strategies aimed at delivering pharmaceutical compounds selectively to sites of inflammation could pose an important improvement to the therapeutic options in ulcerative colitis. So far, both direct rectal application and use of carriers that facilitate drug release specifically to areas of inflamed mucosa have been tested. With respect to the first strategy, a report published in 2008 presented data from a total of eight patients suffering from either ulcerative proctitis, left-sided colitis, or extensive ulcerative colitis that had failed previous treatment with oral and rectal mesalamine, immunomodulators, and steroids [26]. After 8 weeks of rectal tacrolimus, remission was achieved in six out of these eight patients, and steroids could be reduced or discontinued in seven patients. Trough serum levels varied between undetectable and concentrations as high as 7 ng/ml, and no systemic adverse effects were reported. A second case series described a total of 17 patients treated with suppositories or enemas prepared from tacrolimus capsules. After 4 weeks of therapy, a clinical response was observed in 10 out of 12 (83 %) patients suffering from proctitis refractory to conventional therapy and 3 out of 5 (60 %) patients with left-sided colitis with a total of 5 patients showing mucosal healing by the end of therapy [27]. Again, no systemic side effects were observed with average whole blood trough levels of 2.5 and 0.7 ng/ml in patients receiving enemas or suppositories containing 2 mg tacrolimus, respectively, whereas peak tacrolimus concentrations measured in mucosal biopsies exceeded 100 ng/ml on an average. Taken together, these data suggest that topical application of tacrolimus might constitute a promising approach in particular for ulcerative proctitis refractory to standard therapy and further prospective, randomized studies in this challenging group of patients would be highly desirable.

It is conceivable, however, that the value of rectally administered tacrolimus will be limited when disease extends beyond the left flexure and strategies to modify pharmacokinetics in a way that allows selective release of the drug in areas of inflamed mucosa might represent an approach better suited for these cases. In this respect, it has been noted that mucus production is increased in inflamed mucosa, and evidence has been provided that this results in increased adhesion and selective accumulation of nanoparticle carriers in these areas [28]. As in addition, both paracellular permeability and local density of lymphocytes increase with active inflammation; these changes might provide a basis for the development of carriers able to selectively release immunomodulators to inflamed mucosa. Pilot studies with tacrolimus entrapped into nanoparticles yielded promising results in two animal models of colitis and demonstrated significantly increased concentrations of the drug in inflamed tissue as compared to healthy mucosa [29]. Modifications of this approach include coupling of nanoparticles with pH-sensitive microspheres to further increase specificity of drug delivery to areas of actively inflamed mucosa, and although up to now no clinical data are available for this approach, animal models provided first encouraging results supporting this concept [30, 31].

A key drawback of most studies on the use of tacrolimus in ulcerative colitis is the lack of data concerning its long-term safety and efficacy, and neither of the prospective randomized trials described above reported follow-up data beyond 12 weeks. Several case series tried to address this issue by investigating the long-term outcomes of patients in which remission was induced by tacrolimus and reported colectomy-free rates between 66 and 77.5 % within up to 39 months [13, 32, 33]. While these data indicate that colectomy can be avoided or at least delayed in a substantial percentage of patients who achieve remission upon treatment with tacrolimus, they do not allow for an assessment of its impact on maintaining remission as the majority of patients in these studies received maintenance therapy with thiopurines or biologics. To date, the only study investigating a potential role for tacrolimus in the maintenance of remission is a case series with 24 patients who were either thiopurine naive or intolerant (15 patients) or had previously failed maintenance therapy with thiopurines (9 patients). Treatment with tacrolimus for up to 3 years was compared to a retrospective control group of 34 patients receiving thiopurines as the standard therapy [34]. Among the subgroup naive or intolerant to thiopurines, remission (as defined by a Truelove-Witts severity index of 4 or less) was maintained after 1 and 3 years in 51 % and 19 %, respectively, as compared to 59 % and 36 % in patients receiving maintenance therapy with azathioprine or 6-MP. Although this difference did not reach statistical significance, again presumably due to the insufficient number of patients included, these observations seem to favor the use of thiopurines for maintenance of remission over tacrolimus in patients tolerating these compounds. Remission rates were even lower for patients who previously failed azathioprine therapy (25 % and 0 % after 1 and 3 years, respectively) with a significantly lower relapse-free survival compared to the control group receiving thiopurines for maintenance of remission. Adverse events requiring drug withdrawal occurred in 16.7 % of patients receiving tacrolimus compared to 14.7 % patients in the thiopurine group for infections (one patient receiving a combination of tacrolimus and azathioprine), rise in serum creatinine levels (tacrolimus), or leukopenia, pancreatitis, or nausea (thiopurine group). Other side effects observed with long-term tacrolimus therapy included tremor and impaired renal function (21 % and 17 %, respectively). Another small series demonstrated that tacrolimus therapy was effective for inducing clinical and endoscopic remission of steroid-refractory/steroid-dependent UC [35]. Endoscopic improvement was associated with favorable medium- and long-term prognosis. This study was retrospective and evaluated the medical records of 51 patients treated with tacrolimus for ulcerative colitis. Clinical remission and improvement were defined as a Lichtiger score of 4 or less and as a Lichtiger score of ≤10 and a reduction in the score of ≥3 compared with the baseline score, respectively. Endoscopic findings were evaluated based on the endoscopic activity index and Mayo endoscopic score. The endpoint, termed “clinical effectiveness” (as measured by a combination of clinical remission and improvement), was seen in 62.7 % of the patients at 3 months. Thirty-six patients underwent colonoscopy at 3 months, with 33.3 % (12 patients) and 27.8 % (10 patients) showing Mayo endoscopic scores of 0 and 1, respectively. On Kaplan-Meier analysis, the overall percentage of event-free survivors, who did not require colectomy nor switching to other induction therapy such as infliximab, was 73.0 % at 6 months, 49.9 % at 1 year, and 37.8 % at 2 years. Patients with a Mayo endoscopic score of 0–1 at 3 months showed significantly better medium- and long-term prognosis than those with a score of 2–3 (p < 0.01). Thus the finding of early mucosal healing was associated with a better long-term prognosis.

Therefore, although data are sparse, it appears that tacrolimus, while showing short-term efficacy, has only limited value for maintaining remission. Based on the current evidence, its place within the therapeutic armamentarium therefore resembles that of cyclosporine. It can be used to quickly induce remission in severe steroid-refractory ulcerative colitis and serve as a bridging agent until thiopurines started in parallel become effective. Caution and tight monitoring are needed with this strategy as patients receiving combined immunosuppression are particularly prone to infection [36, 37].

Sirolimus, another macrolide, was originally named rapamycin after its isolation from a soil sample derived from Easter Island (or Rapa Nui in the native language). It is produced by Streptomyces hygroscopicus and was initially characterized as a powerful antifungal compound [38, 39]. Further analyses, however, revealed its potent cytostatic and immunosuppressive activities, and as a result, sirolimus and its derivative everolimus are currently being used or evaluated for the treatment of a variety of pathological conditions including certain cancers [40], graft-versus-host disease [41], and polycystic kidney disease [42]. Although sirolimus resembles tacrolimus structurally and binds to the same intracellular target FKBP12, its mode of action does not involve inhibition of calcineurin signaling. Instead, the sirolimus-FKBP12 complex inhibits a serine/threonine kinase termed mTOR (for mammalian target of rapamycin) that is of pivotal importance for a variety of key developmental and cell biological functions [7, 43]. A fast growing body of evidence has revealed that this inhibition results in impaired function of dendritic cells and reduced T cell proliferation and associated mTOR signaling with the control of T cell antigen responsiveness [44]. In addition, mTOR has been demonstrated to have a key role in the regulation of autophagy [45] which in turn emerged as a pivotal component in the pathogenesis of inflammatory bowel diseases [46]. Thus, from a pathophysiological point of view, mTOR inhibition might hold some potential in the treatment of inflammatory bowel diseases. This is furthermore supported by results from animal studies in which sirolimus and P2281, a novel mTOR inhibitor, effectively improved histologic inflammation in the DSS model of colitis [47, 48]. Moreover, the rapamycin derivative everolimus significantly ameliorated colitis in the IL10^−/− model [49]. However, while case reports published in 2008 described significant improvement of two Crohn’s disease patients who previously failed established therapies upon treatment with sirolimus [50] and everolimus [51], a prospective randomized double-blind trial comparing everolimus to placebo and azathioprine for the treatment of moderately to severely active Crohn’s disease was prematurely terminated for lack of efficacy [52]. In this study, a total of 144 patients were enrolled when an interim analysis after 7 months suggested that everolimus was not superior to placebo for inducing a steroid-free remission. In addition, everolimus did not exert a positive effect on disease activity markers or quality of life, and 66 % of patients receiving the drug discontinued therapy, mostly for lack of efficacy. Given these results, it appears rather unlikely that future studies will further investigate mTOR inhibitors in the therapy of ulcerative colitis or Crohn’s disease.

Mycophenolate mofetil (MMF) is the oral ester prodrug of mycophenolic acid (MPA), a compound synthesized by Penicillium brevicompactum and related species [53]. MPA acts by reversibly inhibiting inosine-5′-monophosphate dehydrogenase (IMPDH), thereby preventing de novo guanosine synthesis. The resulting deprivation of deoxyguanosine triphosphate ultimately leads to reduced DNA synthesis [54]. Importantly, MPA preferentially inhibits the type II isoform of IMPDH that is almost exclusively expressed in activated T and B lymphocytes. As, in addition, lymphocytes critically depend on the de novo synthesis of guanosine triphosphate whereas salvage pathways exist in most other cells, MPA is relatively specific in its immunosuppressive mode of action and has been widely employed for the prevention of allograft rejection following solid organ transplantation [55].