Methotrexate has been shown to be effective in the induction and maintenance of remission of CD in several controlled trials. Unfortunately, these studies did not address fistulizing disease. To date, only two retrospective case series have been published which examined the use of methotrexate for CD [88, 89]. The first, by Vandeputte et al., analyzed 20 patients, 8 of whom had fistulae, refractory to azathioprine and requiring continuous corticosteroid treatment [88]. The authors reported improvement in 70% of patients overall with parental methotrexate within 12 weeks but did not specify the outcome of the patients with fistulae. The other series, by Mahadevan et al., included 37 courses of intramuscular and/or oral methotrexate given to 33 patients, 16 of whom had fistulae and were intolerant or refractory to 6MP [89]. Complete fistula closure was achieved in 25%, with another 31% showing improvement. Similar to other medications, fistulae often recurred when the dose of intramuscular methotrexate was decreased or when the route of administration was changed to oral. Thus, methotrexate may represent a reasonable alternative to patients who fail or cannot tolerate azathioprine or 6MP, and long-term maintenance therapy is likely necessary; however, prospective randomized placebo-controlled trials are still needed to evaluate formally the efficacy of methotrexate for fistulizing CD. The initial dose of methotrexate suggested is 25 mg intramuscularly every week. Concurrent administration of folate is advocated to lessen nausea. Adverse events are common and include hepatic fibrosis, bone marrow suppression, pneumonitis and pulmonary fibrosis, nausea, and teratogenicity [90, 91].

There are no controlled trials documenting the efficacy of cyclosporine for the treatment of fistulizing CD. Ten case series, with a total of 64 patients, assessing cyclosporine in Crohn’s fistula have been published [92–101]. The largest series, by Present and Lichtiger, looked at 16 patients with various types of CD fistulae (perianal, rectovaginal, and enterocutaneous) treated with intravenous cyclosporine at a dose of 4 mg/kg/day and observed improvement in 88% with complete fistula closure in 44% [96]. The mean time to response was rather short at just over 7 days. The authors noted that 36% of patients relapsed when converted to oral cyclosporine. Taken collectively, the 10 case series showed an initial response rate of fistulizing CD to intravenous cyclosporine of 83% at doses of 2.5–5 mg/kg/day (mostly 4 mg/kg/day). The overall rate of fistula recurrence after discontinuing oral cyclosporine was 62%, however, and thus, most authorities will use cyclosporine as a bridge to other maintenance therapies, such as azathioprine or 6MP [9, 26, 102]. The recommended initiation intravenous dose of cyclosporine is 4 mg/kg/day for 1 week, followed by oral formulation, typically 6–8 mg/kg/day, all dosed by levels. Adverse events are common and include paresthesias, hirsutism, hypertension, tremor, renal insufficiency, headache, opportunistic infections, gingival hyperplasia, seizures, and hepatotoxicity [90, 103].

Several uncontrolled case series, with a total of 16 patients with CD fistulae, have suggested that tacrolimus may have efficacy in the management of fistulizing disease [104–107]. The only controlled trial of tacrolimus for fistulizing CD is a randomized, double-blind, placebo-controlled study of 48 patients with perianal or enterocutaneous fistulae by Sandborn et al. [108]. In this study, patients received oral tacrolimus at 0.2 mg/kg/day or placebo for 10 weeks. The primary endpoint, fistula improvement (defined as closure of >50% of draining fistulae and maintenance of closure for at least 4 weeks), occurred in 43% of patients receiving tacrolimus, compared to 8% of patients on placebo (p = 0.004). There was no difference in the secondary endpoint, fistula remission (defined as closure of all fistulae and maintenance of that closure for at least 4 weeks), between the two groups (10% of tacrolimus-treated patients vs. 8% of placebo-treated patients). Of note, 38% of patients treated with tacrolimus developed increases in serum creatinine to >1.5 mg/dL, necessitating dose reduction. Gonzalez-Lama et al. conducted a small, uncontrolled, prospective, open-label study of long-term oral tacrolimus at a dose of 0.1 mg/kg/day in 10 patients with Crohn’s fistulae refractory to all conventional therapy, including infliximab [109]. Patients in the study had perianal, enterocutaneous, and rectovaginal fistulae. The authors found that after 6–24 months of follow-up, 50% of patients achieved complete response and an additional 40% showed improvement. Importantly, no relapses and no cases of nephrotoxicity occurred throughout the follow-up period. In addition to nephrotoxicity, other adverse events associated with tacrolimus include headache, insomnia, paresthesias, tremor, and leg cramps [108].

Given that inflammation in CD is associated with high levels of tissue tumor necrosis factor-α (TNF-α) expression, therapies directed against this cytokine have become a recent focus of interest. Infliximab, a chimeric (75% human, 25% murine) IgG1 monoclonal antibody directed against TNF-α, is the prototype anti-TNF-α agent, and has now become the cornerstone in medical therapy of fistulizing CD. Several uncontrolled studies have shown efficacy of infliximab in this regard [110–112]. Infliximab has also been shown to be efficacious in the treatment of CD in two multicenter randomized, double-blind, placebo-controlled trials [113, 114], and thus has the most robust data of any medication for the treatment of CD fistulae. The first, by Present et al., randomized 94 patients with draining abdominal (10% of patients) or perianal (90% of patients) fistulae to placebo, infliximab at a dose of 5 mg/kg, or infliximab at 10 mg/kg, administered intravenously at weeks 0, 2, and 6 [113]. The primary endpoint was a reduction in the number of draining fistulae by >50%, which was maintained for at least 4 weeks, and a secondary endpoint was closure of all fistulae. The authors found that the primary endpoint was achieved in 68% of patients who received infliximab at 5 mg/kg and 56% of patients who received infliximab at 10 mg/kg, compared to 26% of patients who received placebo (p = 0.002 and p = 0.02, respectively). Closure of all fistulae was achieved in 55% of patients who received infliximab at 5 mg/kg and 38% of patients who received infliximab at 10 mg/kg, compared to only 13% of patients who received placebo (p = 0.001 and p = 0.04, respectively). The median time to response was 14 days for infliximab-treated patients versus 42 days for patients assigned to placebo, and the majority of infliximab-treated patients achieved fistula closure prior to the third infusion. Eleven percent of infliximab-treated patients developed a perianal abscess, possibly resulting from premature closure of the cutaneous end before the closure of the rest of the fistula tract. However, the overall rates of infection did not differ between the infliximab and placebo groups.

In the study by Present et al., the median duration of response was 3 months, suggesting that, similar to the treatment of CD fistula with other medications, maintenance therapy may be required. In addition, since the treatment of luminal CD with infliximab often necessitates maintenance therapy, it should not come as a surprise that maintenance infliximab may be of benefit in the management of fistulizing CD. The other multicenter, randomized, double-blind, placebo-controlled trial of infliximab for CD fistula, the ACCENT II trial (A Crohn’s Disease Clinical Trial Evaluating Infliximab in a New Long-Term Treatment Regimen in Patients with Fistulizing Crohn’s Disease) reported by Sands et al., followed 282 patients with draining perianal, abdominal, and rectovaginal fistulae [114]. All patients were induced with infliximab at 5 mg/kg at weeks 0, 2, and 6, and response, defined as a reduction in the number of draining fistulae by >50% for at least 4 weeks, was achieved in 195 patients (69%), similar to the induction response rate reported by Present et al. At week 14, these 195 responders were then randomly assigned to receive infusions of either infliximab 5 mg/kg or placebo every 8 weeks until week 54. The primary endpoint was time to loss of response. The authors observed a median time to loss of response of 40 weeks in infliximab-maintained patients versus 14 weeks in placebo-assigned patients (p = 0.001). Overall, 42% of patients in the infliximab group had a loss of response, compared to 62% in the placebo group. At week 54, 46% of patients treated with infliximab still had a response, versus 23% of patients treated with placebo (p = 0.001). In addition, at week 54, 36% of patients in the infliximab group had a complete absence of draining fistulae, compared to 19% in the placebo group (p = 0.009). Sands et al. performed a post-hoc analysis of the ACCENT II data looking at the efficacy of infliximab induction and maintenance in the subset of women with rectovaginal fistulae [115]. Twenty-five of the original 138 women had a total of 27 draining rectovaginal fistulae at baseline. At week 14, 64% of these 25 women had responded and were then randomized to receive infliximab or placebo maintenance therapy. The authors reported a median time to loss of response of 46 weeks for the infliximab group versus 33 weeks in the placebo group.

The social impact of infliximab in patients with active fistulizing CD has also been investigated in two recent studies. Cadahia et al. were interested in the effect of infliximab induction treatment on health-related quality of life, and thus, they conducted a prospective observational study of 25 patients who received three-dose induction infliximab therapy for single or multiple draining abdominal or perianal fistulae [116]. The authors found that health-related quality of life, as measured by the SF-36, demonstrated significant improvement in the physical domain after 4 and 10 weeks. In addition, a significant increase in IBDQ score was seen after 4 weeks. More recently, Lichtenstein et al. evaluated the impact of infliximab maintenance therapy on the number of hospitalizations, surgeries, and procedures in patients with fistulizing CD [117]. Using data from the ACCENT II trial, they revealed that compared to patients who received placebo, patients who received maintenance infliximab had significantly fewer number of mean hospitalization days (0.5 vs. 2.5 days), hospitalizations (0.11 vs. 0.31), total surgeries and procedures (65 vs. 126), inpatient surgeries and procedures (7 vs. 41), and major surgeries (2 vs. 11).

Mechanistically, infliximab’s effects on mucosal cytokine profiles may predict which patients with fistulizing CD will relapse. Agnholt et al. recently collected tissue samples for cytokine analysis from 26 patients with CD fistulae [118]. They observed that fistula healing was associated with decreased production of TNF-α, interferon-γ, and interleukin-10, while relapse was associated with increased production of interferon-γ.

Despite all of its reported success, the use of infliximab may not obviate the need for surgical management of CD fistulae in many cases. Poritz et al. retrospectively examined surgical rates in patients treated with infliximab for fistulizing CD at a single institution [119]. Among the 26 patients with various types of fistulae, 46% experienced a partial response to infliximab, and an additional 23% had fistula closure. However, 54% of patients overall still required surgery after infliximab therapy, and another 23% continued to open fistulous drainage but refused surgery. Of note, none of the patients with either enterocutaneous or peristomal fistulae were healed with infliximab treatment.

The effectiveness of infliximab in combination with other medical therapies for fistulizing CD has also been investigated in several studies [120–122]. West et al. conducted a double-blind, placebo-controlled trial of ciprofloxacin overlapping with infliximab in patients with perianal CD fistulae [120]. In this study, 24 patients were randomized to receive either ciprofloxacin at 1000 mg/day or placebo for 12 weeks in addition to infliximab at 5 mg/kg at weeks 6, 8, and 12. Patients were followed for 18 weeks, and the primary endpoint was reduction in the number of draining fistulae by > 50%. The authors reported that 73% of the ciprofloxacin-treated patients responded, compared to 39% in the placebo group. One caveat is that the response rate to infliximab alone was much less than in other infliximab studies, in which at least 60% of patients responded. Infliximab has also been evaluated in combination with immunomodulator therapy. Ochsenkühn et al. performed an uncontrolled pilot study of long-term azathioprine (at 2–2.5 mg/kg/day) or 6MP (at 1 mg/kg/day) in combination with induction infliximab in 16 patients [121]. They found that 75% of patients achieved complete fistula closure, which persisted for more than 6 months (median time of 10 months). As seen previously, the median time to fistula closure was 14 days. A similar uncontrolled pilot study by Schröder et al. followed 12 consecutive patients with CD fistulae intolerant or resistant to azathioprine [122]. Patients were treated with induction infliximab and long-term methotrexate at 20 mg/week (intravenously for 6 weeks, followed by oral thereafter). The authors observed that 33% of patients experienced complete fistula closure for at least 6 months (median 13 months), and 25% had a partial response. While providing a suggestion of efficacy of combination therapy for the treatment of fistulizing CD, controlled trials have yet to be performed.

The combination of infliximab with surgical intervention (i.e., seton placement) in the treatment of CD perianal fistulae has been assessed in several studies [6, 7, 123–125]. Three single-center retrospective case series, from Calgary, Leeds, and Oxford, each of which included 21 patients, have documented favorable rates of fistula healing with seton placement followed by induction and maintenance infliximab, with complete and partial healing rates of 67% and 19%, 47% and 53%, and 21% and 42%, respectively [7, 123, 124]. Two studies were able to compare the outcomes of patients treated with infliximab and seton placement to those treated with infliximab and/or seton placement alone [6, 125]. The first, by Regueiro and Mardini, retrospectively analyzed 32 consecutive patients with perianal CD fistulae, all of whom had received at least three induction doses of infliximab and some of whom had additionally undergone an EUA with seton placement prior to infliximab treatment [6]. Response was defined as complete closure and cessation of drainage from the fistula. They found that compared to patients treated with infliximab alone (n = 23), patients who had a preinfusional EUA with seton placement (n = 9) had a significantly higher rate of initial response (100% vs. 83%, p = 0.014), lower rate of recurrence (44% vs. 79%, p = 0.001), and longer time to recurrence (13.5 months vs. 3.6 months, p = 0.0001). The second, by Scaudione et al., prospectively subdivided 35 consecutive patients with complex perianal fistulae into three groups: infliximab with seton placement (n = 14), infliximab alone (n = 11), and seton placement alone (n = 10) [125]. The authors reported that patients in the combination group had a nonsignificantly higher rate of complete response, defined as closure of all draining fistulae and cessation of drainage for 3 months, of 79% vs. 64% and 70%, respectively, and a significantly longer time to recurrence of 10.1 months vs. 2.6 and 3.6 months, respectively (p < 0.02).

The combination of infliximab with immunomodulators and seton placement has also been investigated more recently. A prospective open-label study of 34 patients from three hospitals in France, by Roumeguere et al., had patients undergo seton placement 3 months prior to the start of medical therapy, followed by initiation of methotrexate 25 mg/week, followed by induction infliximab, after which patients were maintained on methotrexate alone [126]. At 14 weeks, 74% of patients had a complete response and another 11% had a partial response. Of patients with initial response, 90% had maintained at least a partial response after 56 weeks. A prospective study of 41 patients from St. Mark’s Hospital in London, by Tozer et al., assessed long-term fistula response and remission rates after treatment with infliximab (or adalimumab in nine patients who lost response to infliximab) combined with thiopurines, in which 73% of patients had seton placement which was removed after 2–6 weeks [127]. They reported rates of fistula response and remission at 2 years of 35% and 29%, respectively, and at 3 years of 37% and 21%, respectively. A large retrospective study from two referral centers in France, by Bouguen et al., assessed long-term rates of initial and sustained fistula closure in 156 patients treated with infliximab and immunomodulators (in 58%) and seton placement (in 62%) [128]. They observed rates of initial fistula closure of 59%, 73%, and 88% at 3, 5, and 10 years, respectively, and rates of sustained fistula closure of 22%, 43%, and 57% at 3, 5, and 10 years, respectively. Interestingly, the use of infliximab for more than 118 weeks and the use of combination therapy were associated with significantly higher rates of initial fistula closure.

Adverse events with infliximab treatment are common and include infusion reactions, delayed-type hypersensitivity reactions, formation of human antichimeric antibodies, formation of antinuclear and anti-double-stranded DNA antibodies, and drug-induced lupus-like reactions [129–131]. In addition, infectious complications seem to be increased, but serious infections, such as pneumonia, sepsis, tuberculosis, and opportunistic infections, including listeriosis, aspergillosis, histoplasmosis, coccidiomycosis, and Pneumocystis carinii pneumonia, occur only rarely [132–136]. Finally, there have been isolated case reports of hepatic necrosis and non-Hodgkin’s lymphoma in patients treated with infliximab, although it has not been determined whether these events were the direct consequence of infliximab therapy.

Like infliximab, the other commonly used anti-TNF-α medications for CD treatment, adalimumab and certolizumab pegol, have shown efficacy in the treatment of fistulizing disease. However, although data in patients with fistulae for both adalimumab and certolizumab pegol were obtained from randomized placebo-controlled studies, fistula healing was not a primary endpoint in these studies. Adalimumab, a fully human IgG1 monoclonal antibody, was found to be efficacious for maintenance of remission in the Crohn’s Trial of the Fully Human Antibody Adalimumab for Remission Maintenance (CHARM), in which 854 patients received open-label induction treatment with subcutaneous adalimumab 80 mg at week 0 and 40 mg at week 2, followed by randomized maintenance treatment with adalimumab 40 mg every week or every other week or placebo up to week 56, with a co-primary endpoint of clinical remission at weeks 26 and 56 [137]. In this study, 117 patients had draining fistulae, and 113 of these had perianal fistulae. A subgroup analysis of these patients, in which complete fistula healing was defined as the absence of draining fistulae at the last two consecutive post-baseline evaluations, reported complete fistula healing rates of 30% for the combined adalimumab groups versus 13% for the placebo group at week 26 (p < 0.05), and 33% for the combined adalimumab groups versus 13% for the placebo group at week 56 (p < 0.05) [138]. The authors also observed that these rates of fistula healing were largely maintained for up to 2 years of follow-up in a long-term extension study of CHARM called ADHERE [139].

Certolizumab pegol, a pegylated humanized Fab fragment of an anti-TNF-α monoclonal antibody, was shown to have efficacy in the maintenance of remission in active Crohn’s disease in the Pegylated Antibody Fragment Evaluation in Crohn’s Disease: Safety and Efficacy 2 (PRECISE 2) randomized placebo-controlled trial, in which 668 patients received open-label induction treatment with subcutaneous certolizumab pegol 400 mg at weeks 0, 2, and 4, followed by randomized maintenance treatment with certolizumab pegol 400 mg or placebo every 4 weeks through week 24 and followed to week 26 [140]. In this study, 58 patients had draining fistulae, and 55 of these had perianal fistulae. A subgroup analysis of these patients, in which complete and partial fistula closure were defined as closure of 100% and at least 50%, respectively, of all draining fistulae at two consecutive post-baseline evaluations at least 3 weeks apart, reported complete fistula healing rates of 36% for the certolizumab pegol group versus 17% for the placebo group (p = 0.038), and partial fistula healing rates of 54% for the certolizumab pegol group versus 43% for the placebo group (p = NS) at week 26 [141].

Adverse events associated with adalimumab and certolizumab pegol are similar to those seen with infliximab.

Other anti-TNF-α medications, including CDP571 and thalidomide, have also been preliminarily investigated for the treatment of fistulizing CD; of note, golimumab has not been studied for the treatment of CD fistulae. CDP571, a humanized (95% human, 5% murine) IgG4 monoclonal antibody, has been assessed for efficacy in the treatment of CD fistulae in two multicenter, randomized, double-blind, placebo-controlled trials [142, 143]. The first study, by Feagan et al., published only in abstract form, treated 71 patients with steroid-dependent CD with intravenous CDP571 at 20 mg/kg or placebo at week 0, followed by a second infusion of CDP571 at 10 mg/kg or placebo at week 8 [142]. At week 16, among the subgroup of patients with draining perianal fistulae, fistula closure was achieved in 25% of patients who received CDP571, compared to none in the placebo group. The other study, by Sandborn et al., followed 169 patients for 24 weeks, during which patients received an initial infusion of CDP571 at either 10 or 20 mg/kg or placebo, followed by CDP571 at 10 mg/kg or placebo every 8–12 weeks [143]. This study included 37 patients with open perianal or enterocutaneous fistulae and reported that 50% of patients treated with CDP571 achieved fistula closure versus 15% of patients who received placebo. Adverse events due to CDP571 include infusion reactions, formation of anti-idiotype antibodies, development of new antinuclear or anti-double-stranded DNA antibodies, insomnia, pruritus, and rash [142, 143].

Thalidomide has also been preliminarily evaluated in the treatment of fistulizing CD in two open-label pilot studies [144, 145]. The first study, by Ehrenpreis et al., enrolled 22 patients with refractory CD to receive oral thalidomide at 200 or 300 mg/day for 12 weeks [144]. At week 4, of the 13 patients with fistulae, 9 patients (69%) responded, 3 patients (23%) achieved remission, and 2 patients (15%) had closure of all fistulae. Nine patients with fistulizing disease completed the 12 weeks of treatment. Of these nine patients, all (69%) were responders, six patients (46%) achieved remission, and five patients (38%) had complete closure of all fistulae. The other pilot study, by Vasiliauskas et al., treated 12 patients with steroid-dependent CD with 50 or 100 mg/day of thalidomide for 12 weeks [145]. Of the six patients with active perianal fistulae at the time of entry into the study, five (83%) had improvement in symptoms after 4 weeks. Four of these six patients with fistulizing disease completed 12 weeks of treatment. Fistula closure was achieved in one patient (17%) at week 12, with improvement in another two patients (33%). Adverse events are common with thalidomide therapy and include severe somnolence, peripheral neuropathy, teratogenicity, peripheral edema, constipation, seborrheic dermatitis, hypertension, muscle spasm, and diffuse rash [144, 145].

Anti-integrin therapy has been used more recently in the treatment of CD as a way to target reduction of lymphocyte trafficking to the gut. The α4β7 integrin, a cell-surface glycoprotein expressed on lymphocytes, helps to regulate lymphocyte migration into inflamed intestinal tissue via interaction with mucosal addressin-cell adhesion molecule 1 (MAdCAM-1) on intestinal blood vessels [146]. Natalizumab, which is not gut-specific as it binds both α4β7 and α4β1 integrins (the latter which are located in the central nervous system), was shown to be effective for the treatment of CD in a large randomized controlled trial, but patients with draining fistulae were exluded [147]. However, the use of natalizumab is associated with an increased risk of progressive multifocal leukoencephalopathy [148], and thus vedolizumab was developed as a purely gut-selective blocker of α4β7. Vedolizumab was shown to be efficacious for the treatment of moderate-to-severe active Crohn’s disease in the GEMINI 2 double-blind randomized placebo-controlled trial [149]. Although fistula treatment was not the primary endpoint in this trial, 57 patients had actively draining fistulae at baseline. Treatment with vedolizumab 300 mg every 8 weeks was associated with a significantly higher rate of fistula closure than treatment with placebo after 52 weeks (41% vs. 18%, p = 0.03).

A variety of other therapies for fistulizing CD have been suggested to be of possible benefit in uncontrolled case series or anecdotally. These include elemental diets, bowel rest with total parental nutrition, mycophenolate mofetil, granulocyte-colony stimulating factor, hyperbaric oxygen, and coagulation factor XIII [150–170]. However, controlled trials are required before any of these modalities can be recommended for routine use. Other novel therapies are also currently under investigation (refer to www.​clinicaltrials.​gov).