Azathioprine and 6-Mercaptopurine in Crohn Disease

The antimetabolite agents azathioprine and 6-mercaptopurine are purine analogs that interfere with nucleic acid metabolism and cell proliferation and have immunosuppressive properties. 6-Mercaptopurine, first synthesized in 1951, was initially used to treat leukemia. Azathioprine, its derivative, was synthesized in 1957. It has a longer serum half-life and perhaps fewer adverse effects and has been characterized as a slow-release form of 6-mercaptopurine. Azathioprine was initially used to prolong survival of renal allografts [1], and its use for Crohn disease was first reported in 1969 [2]. Many uncontrolled series have evaluated antimetabolite therapy for Crohn disease [2-12], and reports of the first randomized, double-blind, placebo-controlled trials appeared in 1971 [13, 14]. Although much experience has been gained with these drugs, their role as therapeutic agents remains controversial because they are slow to act and can cause serious adverse effects. Large randomized, controlled trials are the best way to solve important medical questions about therapy, but they are difficult, time-consuming, and expensive. Meta-analyses attempt to combine the results from previously published studies to enhance the statistical power of an analysis in lieu of recruiting the large number of patients that would otherwise be necessary. We did a meta-analysis of all the published, randomized, double-blind, placebo-controlled trials of azathioprine or 6-mercaptopurine therapy in patients with active or quiescent Crohn disease. Methods We did a literature search using the MEDLINE database (1966 to May 1994) with the Medical Subject Headings anti-metabolites, azathioprine, or mercaptopurine and Crohn disease or inflammatory bowel disease. Citations were limited to those published in English and French. We also did a manual search using references from these articles, review articles, and proceedings from major gastrointestinal meetings. Double-blind, placebo-controlled trials of oral azathioprine or 6-mercaptopurine therapy in patients older than 18 years who had active or quiescent Crohn disease were selected. Active disease was defined as the presence of serious or increasing symptoms from intestinal or extraintestinal Crohn disease. Quiescent disease, or disease in remission, was defined as the presence of mild or no symptoms before the patient entered the study, regardless of the use of prophylactic medication. Each study was independently reviewed by three authors, and methodologic criteria and the results of each study were recorded. We collated data according to the definition of success used in each study because different instruments were used to measure disease activity and no single definition of response was used in all the studies. Although some of the instruments have not been externally validated, they all appeared to have provided a reasonable measurement of clinically active and inactive disease. For studies of therapy in active disease, we recorded the number of patients entering clinical remission as defined by the study. For studies of maintenance therapy, we recorded the number of patients maintaining clinical remission as defined in the study. We also extracted data to measure the effect of dose and duration of azathioprine or 6-mercaptopurine therapy; the effect of concurrent therapy with sulfasalazine, 5-aminosalicylic acid, and steroids; the number of fistulae that improved or healed; and the number of patients who withdrew from the study because of toxicity. In crossover studies, we only incorporated data from the first portion of the study to avoid possible carryover effects of medication into the second part of the study and to make these studies more comparable to those that did not have a crossover design. For all studies, we enumerated adverse effects severe enough to require the withdrawal of a patient from a trial. All results were tabulated on an intention-to-treat basis. Two authors independently did a quality analysis of each study using criteria derived from those of Chalmers and colleagues [15]. The quality analysis was divided into three parts. Part one addressed the study protocol: patient selection, differences in drug and placebo regimens, randomization and blinding techniques, and a priori estimates of sample size. Part two addressed the analysis: details of the statistical significance of major end points, the presence of a posterior -error estimate for negative trials, use of the intention-to-treat rule, and analysis of adverse effects. Part three addressed the presentation of results: tests for equivalence of treatment and control groups, tabulation of remissions and relapses, and presentation of data on the timing of relapse and remission events. The complete quality analysis criteria may be obtained from the authors on request. Discrepancies between reviewers about retrieved data or quality assessment scores were settled by consensus. Statistical Analysis We separately analyzed active and quiescent disease therapy groups and used commercially available statistical software to calculate odds ratios and 95% CIs for individual studies [16, 17]. For the analysis of disease response and the effect of drug dose, duration of therapy, and cumulative dose, we calculated an estimate of the common odds ratio using a maximum likelihood estimate to produce a logistic regression analysis based on a binomial model [18]. For each model, the studies and treatment assignment were entered as indicator variables [19]. If the residual deviance was judged significant by a chi-square test after the model was fitted, then the model did not fully explain the difference between treatment and placebo groups after adjustment for study differences. This is an indication of the heterogeneity of the odds ratios, and it prompted a search for differences in the methods used by the studies, such as dose and duration of therapy, that could account for the heterogeneity. We then constructed a new model that incorporated these terms. A model was considered satisfactory if the residual deviance was not statistically significant (P > 0.05). In the primary analysis, the studies were weighted according to the total number of participants. We assessed the effect of quality scores by repeating the analysis using the quality scores to weight the studies according to the techniques of Detsky and colleagues [20]. We also repeated the analysis using the studies that scored at or above the median quality score, weighted by the number of study participants. This type of logistic regression analysis reproduces the traditional Mantel-Haenszel analysis [20-23] but also allows the modeling of more complex situations by including strata variables and other terms to measure the effect of variables such as drug dose, duration of therapy, and cumulative dose. We graphed the individual trial odds ratios and 95% CIs to confirm that a substantial overlap existed. We also did a sensitivity analysis by excluding each study in turn to ensure that no single study was solely responsible for a significant result. For the secondary analysis, we evaluated fistula response, steroid-sparing effect, and adverse effects. Data from the placebo and therapy groups were totaled, and pooled odds ratios and 95% CIs were calculated. Note that if an odds ratio exceeds 1.0, a beneficial effect of therapy is suggested; if the CI for an odds ratio includes unity, there is no statistical evidence for the superiority of treatment over placebo. Results Of the 222 studies we reviewed, 9 trials met the inclusion criteria [13, 14, 24-30]. Some of the randomized, double-blind, placebo-controlled trials were reported more than once, usually as an abstract at a symposium before formal publication. Results of one trial involving 11 patients [31] were published only in abstract form in 1974; this report contained insufficient information for analysis and was therefore excluded from further consideration. Results of another trial [30] were published in 1994 as an abstract, but the full-length manuscript is currently in preparation (Wright JP. Personal communication). The results of this trial as detailed in the abstract were included in the analysis, but we could not assign a quality score because the full-length report was not available. The nine selected trials had various designs (Tables 1 and 2): Four were exclusively concerned with therapy for active disease, two addressed maintenance of remission, and three had separate arms for patients with active and quiescent disease. Because the trials were done over 25 years on three continents, it is unlikely that many patients participated in more than one trial. We considered active disease and disease in remission separately so that patients participating in both these groups would not bias the results. There is a reasonable chance that the trials of O'Donoghue and colleagues [29] and Willoughby and associates [14], in which azathioprine was used for maintenance of remission, may have enrolled some of the same patients; both these trials were done at St. Bartholomew's Hospital in London, United Kingdom, and their results were reported in 1978 and 1971, respectively. The maximum potential number of shared patients is 10, which represents 3% of all patients included in the analysis of maintenance therapy. This small number and the fact that the odds ratios of both studies are similar to those in other studies suggests that no substantial bias from that source occurred. Table 1. Study Characteristics and Results: Therapy for Active Disease* Table 2. Study Characteristics and Results: Therapy for Quiescent Disease* Only Present and colleagues [25] used 6-mercaptopurine in their trial; all other investigators used azathioprine. Azathioprine is rapidly converted to 6-mercaptopurine in vivo; because the clinical indications of these two drugs are similar, combining these trials in the analysis is appropriate. The investigators