The incidence of CD is higher in Western industrialized countries. The prevalence of CD in children younger than 20 years of age is 43 (95% CI, 40-45) per 100,000 people and for adults it is 201 (95% CI, 197-204) per 100,000 people.¹ However, a large-scale population-based study found that CD is becoming more common in Asia, especially in China (122/100,000) and Japan, and that the symptoms in patients with CD are often more severe in Asia than in the West.^2,3,4

Penner and Crohn⁵ first reported that perianal fistulas were a complication of CD in 1938. Sometimes perianal fistula is the first recognizable symptom of CD. Perianal fistulas in CD may arise from inflamed or infected anal glands and/or penetration of fissures or ulcers in the rectum or anal canal. Some studies have shown that CD patients with perianal fistulas often require a combined surgical and medical approach because of frequent recurrence and abscess formation.⁶

Infliximab (IFX) and adalimumab (ADA) have proven to be effective in the treatment of perianal fistulas in CD,^7,8 but it is not clear which is more effective. Therefore, the aim of this study was to determine the clinical efficacy of ADA compared with IFX in the treatment of perianal fistulas in CD, and to analyze the risk factors associated with recurrence or aggravation of the fistula tract.

TNF-α antibodies (IFX, ADA, and certolizumab) are effective for both induction and maintenance in moderate to severe CD in adults (including patients with a perianal fistula).⁹ IFX is a chimerical monoclonal antibody comprised of 75% human and 25% mouse sequences. However, the murine component is potentially antigenic for humans. Repeated infusion may cause the patient's immune system to produce antibodies against IFX that may reduce the efficacy of the drug, and may even cause anaphylactic shock.¹⁰ In order to lower the risk of negative immunogenic responses induced by chimeric antibodies, new studies attempted to replace the mouse-derived sequences to create fully human monoclonal antibodies. The result was the development of ADA, the first fully human antibody to be approved for clinical treatment. Some studies have found that ADA was more effective in short-term clinical response/remission, in long-term remission, and in obtaining complete fistula healing, compared with placebo. ADA has also proven to be effective in patients who no longer respond to or who develop intolerance to IFX.¹¹

Perianal fistula in CD is usually difficult to treat because of the high rate of relapse after cessation of treatment. Although the introduction of IFX has increased the efficacy, the results are still disappointing, especially for complex fistulas. A British study showed that only 29% and 42% of the patients achieved complete or partial remission, respectively, when treatment combined a surgical procedure with IFX.¹² Echarri et al.¹³ found that complete cessation of drainage occurred in 50% of the short-term cases, and that 87.5% were able to maintain this effect over time (follow-up period of 48 weeks). Sands et al.¹⁴ reported abscess development in patients with fistulizing CD was not dependent on cumulative IFX exposure. The efficacy of ADA for perianal fistula with CD is still not very clear because a dedicated controlled study has not yet been done. Castaño-Milla et al.⁸ reported retrospective data showing that ADA was effective for the treatment of perianal fistulas in CD patients naive to anti-TNF drugs. Our study examined the clinical efficacy of ADA versus IFX in the treatment of perianal fistulas in CD and the risk factors for fistula tract recurrence or aggravation were analyzed.

The retrospective nature of this study was a limiting factor. The treatment of the IFX group began 5 years earlier than that of the ADA group. Thus, the follow-up period was longer in the IFX group. We compared the efficacy of ADA and IFX and found that there were no differences in gender, disease duration, age of CD onset, intestinal location, fistula classification, seton placement, IOIBD score, CRP, and frequency of injection between the two groups.

The rate of nonrecurrence or aggravation of fistula was 62.5% in the ADA group and 83.9% in the IFX group, during a follow-up period of 24 months. We hypothesized that ADA was less effective than IFX for anal fistula in CD but found that there was no significant statistical difference between the two forms of treatment. However, we found that the seton was an independent risk factor for recurrence or aggravation of the fistula tract. Although this risk factor did not correlate with fistula classification, we hypothesized that a risk factor may be the fistula classification itself, because we only used a seton in patients with a complex fistula. Patients with a complex fistula were likely to experience recurrence or aggravation. The results were consistent with the findings of a study by Tozer et al.¹⁵ We also found that the risk factors may be gender specific because no female patients in our study experienced recurrence or aggravation. However, further research is needed to support these findings. The rates of loss of response were 12.9% and 25.0% in the IFX and ADA groups, respectively. When a biologic shows a loss of response and the patient experiences a recurrence of fistula, we reinsert a seton and use another biologic.

In conclusion, our results suggest that ADA may be less effective than IFX in the treatment of perianal fistula in CD. Fistula classification may also be an important risk factor, as complex fistulas are more likely to show relapse or aggravation.