Normal thiopurine methyltransferase levels do not eliminate 6-mercaptopurine or azathioprine toxicity in children with inflammatory bowel disease

6-mercaptopurine (6-MP) and azathioprine (AZA) are used to treat inflammatory bowel disease (IBD). Side effects include infection, leukopenia, hepatitis, and pancreatitis. The level of thiopurine methyltransferase (TPMT), which metabolizes 6-MP to 6-methylmercaptopurine, may reflect the risk of side effects. We sought to evaluate the relationship between the side effects of these medications and the TPMT level of pediatric patients with IBD. The medical records of our patients who were diagnosed with IBD and who received 6-MP or AZA were reviewed for measured TPMT levels. All red blood cell (RBC) TPMT levels were determined at the Mayo Medical Laboratories, Rochester, MN. The occurrence of leukopenia, elevated aminotransferases, and pancreatitis was evaluated. Twenty-two patients, mean age 13.7 years, received 6-MP or AZA and had TPMT levels measured. The TPMT levels ranged 10.7-27.5 U/mL RBC with a mean of 17.2 +/- 3.2 U/mL RBC. Two children had levels below the accepted norm of 13.8 U/mL RBC. One of these patients (50%) developed both elevation of aminotransferases and leukopenia. Of all, 20 children had normal levels, 3 (15.0%) exhibited side effects: hepatitis (n = 2) and leukopenia (n = 1). We conclude that side effects of 6-MP or AZA occur despite normal TPMT levels.     

Effective maintenance of inflammatory bowel disease remission by azathioprine does not require concurrent 5-aminosalicylate therapy.

OBJECTIVES: To assess the effect of 5-aminosalicylate treatment in conjunction with azathioprine on remission maintenance in inflammatory bowel disease patients. METHOD: This retrospective study was based on a total of 186 inflammatory bowel disease patients (104 with Crohn's disease; 82 with ulcerative colitis), who were stable on azathioprine for a minimum of 6 months. The median duration of follow-up was 4.3 years (range 0.6-15.5 years). Relapse rates per year of follow-up were compared in an azathioprine + 5-aminosalicylate group (n = 103) and an azathioprine alone group (n = 83); survival curves for cumulative remission rates were compared by log-rank test. Discontinuation of azathioprine in both groups was also recorded, as was the incidence of malignancy. RESULTS: In ulcerative colitis patients (n = 82), mean relapse rates for the azathioprine + 5-aminosalicylate group were 0.21/year compared with 0.19/year for the azathioprine alone group (P = not significant). In Crohn's disease patients (n = 104), mean relapse rates for the azathioprine + 5-aminosalicylate group were 0.27/year compared with 0.3/year for the azathioprine alone group (P = not significant). The cumulative remission percentage (determined from time to first relapse) was used in Kaplan-Meier survival analysis and showed no difference between the azathioprine + 5-aminosalicylate group and the azathioprine alone group by log-rank analysis, for ulcerative colitis and Crohn's disease as well as for all inflammatory bowel disease patients. Concurrent use of 5-aminosalicylates was no more frequent in patients who discontinued azathioprine due to adverse events. The only malignancy recorded was Waldenstr?m's macroglobulinaemia after 7 years of azathioprine therapy. CONCLUSION: Concurrent use of 5-aminosalicylate drugs did not reduce the relapse rates of inflammatory bowel disease patients who were established on azathioprine therapy. The use of 5-aminosalicylate drugs did not lead to any increase in discontinuation of azathioprine due to adverse events.     

Relationship between 6-mercaptopurine dose and 6-thioguanine nucleotide levels in patients with inflammatory bowel disease

BACKGROUND: 6-Thioguanine nucleotides (6-TGN) are active metabolites of azathioprine (AZA) and 6-mercaptopurine (6MP). Higher remission rates have been observed in patients with higher 6-TGN levels. However, many physicians prescribe AZA/6MP using milligrams per kilogram (mg/kg) dosing regimens without measuring 6-TGN levels. The aim of this study was to examine the association between 6MP dose and 6-TGN levels. METHODS: We conducted a cross-sectional study of patients treated for inflammatory bowel disease (IBD) with AZA or 6MP, whose 6-TGN levels were measured. Patients with low or intermediate thiopurine methyl transferase (TPMT) activity were excluded. AZA dose was converted to 6MP equivalents. The relationship between dose and 6-TGN levels was assessed with the Spearman correlation coefficient. We used logistic regression to assess the relationship between dose and 6-TGN levels of >230 pmol/8 x 10(8). RESULTS: In this study, 155 patients met our inclusion criteria (median dose, 1.01 +/- 0.40; range, 0.61-1.41 mg/kg). There was a weak correlation between 6-TGN levels and the absolute dose (rho = 0.18, P = 0.04) and the dose in mg/kg (rho = 0.19, P = 0.03). The correlation between mg/kg dosage and 6-TGN levels was slightly stronger in those using concomitant 5-aminosalicylate (5-ASA) medications (rho = 0.24, P = 0.02). Compared with <1.0 mg/kg per day, doses of > or =1.5 mg/kg per day were strongly associated with 6-TGN levels of >230 pmol/8 x 10(8) RBC (adjusted odds ratio [OR] = 3.7, 95% confidence interval [CI] = 1.1-11.8). However, only 37% of patients receiving > or =1.0 mg/kg per day had 6-TGN levels of >230 pmol/8 x 10(8) RBC. CONCLUSIONS: 6MP dose is weakly associated with 6-TGN levels. The use of standard mg/kg dosing regimens will result in low 6-TGN levels in most patients.     

Treatment of inflammatory bowel disease with azathioprine and 6-mercaptopurine

6-Mercaptopurine and azathioprine have become important therapeutic options for patients with inflammatory bowel disease (IBD). Although accumulating data in the literature have supported the use of these immunomodulators in the management of IBD, marked variation exists in the pattern of clinical practice regarding azathioprine or 6-mercaptopurine therapy in patients with IBD. This article provides a critical review of the data on the clinical efficacy and toxicities of 6-mercaptopurine and azathioprine in the management of IBD. Emerging literature on the potential application of pharmacogenetic testing and metabolite monitoring are also discussed.     

Utilisation of erythrocyte 6-thioguanine metabolite levels to optimise azathioprine therapy in patients with inflammatory bowel disease

BACKGROUND AND AIM: The immunosuppressive properties of 6-mercaptopurine and its parent compound azathioprine are mediated by their intracellular metabolism into active 6-thioguanine (6-TG) metabolites. Measurement of erythrocyte 6-TG metabolite levels has been proposed as a useful clinical tool for assessing treatment efficacy in patients with inflammatory bowel disease (IBD). AIM: The purpose of the study was to establish a therapeutic index of treatment efficacy based on measurement of erythrocyte 6-TG metabolite levels, and apply it clinically to guide therapy. METHODS: Heparinised blood was obtained from 82 adult patients with IBD on long term (more than three months) antimetabolite therapy (63 Crohn's disease; 19 ulcerative colitis). Erythrocyte 6-TG metabolite levels were measured using reverse phase high performance chromatography, and correlated with treatment efficacy. In 22 patients with refractory Crohn's disease despite long term azathioprine therapy, their dosage was increased by 25 mg/day at eight week intervals as needed. Serial erythrocyte 6-TG metabolite levels were measured at each clinic visit and correlated with treatment efficacy. RESULTS: Clinical remission, as defined by a low disease index score in patients weaned off or on a low alternate day dose (<20 mg on alternate days) of corticosteroid, was achieved in 68% of patients on long term antimetabolite therapy. Treatment efficacy correlated with erythrocyte 6-TG levels greater than 250 pmol/8x10(8) red blood cells in patients with colonic and fistulising Crohn's disease (p<0.01) but not in patients with ileocolonic disease. Eighteen of 22 patients with incompletely responsive Crohn's disease achieved disease remission by optimising their dose of azathioprine therapy. Median (range) erythrocyte 6-TG metabolite levels increased from 194 (67-688) to 303 (67-737) pmol/8x10(8) red blood cells (p<0.05). Clinical response associated well with a reduction in corticosteroid requirements. Mean (SEM) white blood cell count decreased from 8.6 (0.9) to 6.9 (0.6) x10(3)/microl with adjustment in azathioprine dosage. No patient incurred azathioprine induced leucopenia. CONCLUSION: Measurement of erythrocyte 6-TG metabolite levels is helpful in determining the adequacy of azathioprine dosage and can be used to optimise the dose of antimetabolite therapy to achieve an improved clinical response without inducing leucopenia. Patients who are clinically refractory to azathioprine therapy despite achieving high erythrocyte 6-TG levels (>250) should be considered for adjunct or alternative forms of immunosuppressive therapy or surgery.     

Optimizing 6-mercaptopurine and azathioprine therapy in the management of inflammatory bowel disease

The thiopurine drugs,6-mercaptopurine(6-MP) and azathioprine,are efficacious in the arsenal of inflammatory bowel disease(IBD) therapy.Previous reports indicate that 6-thioguanine nucleotide(6-TGN) levels correlate with therapeutic efficacy,whereas high 6-methylmercaptopurine(6-MMP) levels are associated with hepatotoxicity and myelotoxicity.Due to their complex metabolism,there is wide individual variation in patient response therein,both in achieving therapeutic drug levels as well as in developing advers...     

Epstein-Barr virus-positive lymphoma in patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine.

BACKGROUND & AIMS: The use of azathioprine and 6-mercaptopurine for inflammatory bowel disease increased in the early 1990s. We sought to determine the effect of this change in therapy on the risk of lymphoma in patients with inflammatory bowel disease. METHODS: All patients with inflammatory bowel disease at a single tertiary care medical center who developed lymphoma between 1985-2000 were identified and the pathologic features of the lymphoma including presence of Epstein- Barr virus were determined. The patients were divided into two 8-year periods (1985-1992, 1993-2000) corresponding with the introduction of azathioprine and 6-mercaptopurine in 1993. RESULTS: Eighteen patients with lymphoma were identified, 6 between 1985-1992 and 12 between 1993-2000. Six of 18 lymphomas occurred in patients treated with azathioprine or 6-mercaptopurine, all between 1993-2000. Seven patients developed Epstein-Barr virus-positive lymphoma (1 from 1985-1992, 6 from 1993-2000). Five of 7 Epstein-Barr virus-positive lymphomas occurred in patients treated with azathioprine or 6-mercaptopurine compared with 1 of 11 Epstein-Barr virus-negative lymphomas (P = 0.01). Approximately 1200 patients with inflammatory bowel disease were treated with these agents between 1993-2000. CONCLUSIONS: Treatment of inflammatory bowel disease with azathioprine or 6-mercaptopurine appears to be associated with a small increased risk of Epstein-Barr virus-positive lymphoma.     

Erythrocyte mean corpuscular volume as a surrogate marker for 6-thioguanine nucleotide concentration monitoring in patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine

Mean corpuscular volume may correlate with erythrocyte 6-thioguanine nucleotide concentrations in patients treated with azathioprine and 6-mercaptourine. We conducted a study of 166 patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine to determine the relationship between mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentrations, disease activity as measured by the Inflammatory Bowel Disease Questionnaire (active disease <170, remission >170), and leukopenia. Blood was submitted for mean corpuscular volume, whole blood 6-thioguanine nucleotide concentration, and leukocyte count. The mean +/- SD mean corpuscular volume during treatment was 94.7 +/- 6.6 fL and the mean +/- SD change in mean corpuscular volume was 7.5 +/- 6.3 fL. There were significant correlations between mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentration (r(s) = 0.33, p < 0.001) and between change from baseline in mean corpuscular volume and erythrocyte 6-thioguanine nucleotide concentration (r(s) = 0.26, p = 0.001). There was no correlation between Inflammatory Bowel Disease Questionnaire scores and mean corpuscular volume values (r(s) = 0.01, p = 0.94). The mean corpuscular volume values in 55 patients with active disease and 111 patients in remission were similar (95.1 vs. 94.5 fL, p = 0.57). There was a weak negative correlation between the mean corpuscular volume and the leukocyte count, (r(s) = -0.18, p = 0.022). In patients with inflammatory bowel disease treated with azathioprine or 6-mercaptopurine, mean corpuscular volume and change from baseline in mean corpuscular volume correlated with erythrocyte 6-thioguanine nucleotide concentrations and negatively with leukocyte counts, but did not correlate with disease activity as measured by the Inflammatory Bowel Disease Questionnaire. Measurement of mean corpuscular volume is a simple and inexpensive alternative to measurement of 6-thioguanine nucleotide concentrations in patients treated with azathioprine or 6-mercaptopurine.     

Low-dose 6-mercaptopurine in inflammatory bowel disease is associated with minimal hematologic toxicity

A feared complication of therapy with 6-mercaptopurine (6-MP) is myelosuppression. To evaluate whether rigorous blood count monitoring is necessary, we prospectively followed the hematologic profiles of 57 patients with inflammatory bowel disease who were treated with low-dose 6-MP. Most patients (97%) were treated initially with a single dose of 50 mg/day and 79% never used more than 50 mg/day. Blood counts were obtained at weekly intervals over the first month, every two weeks for the second month, and monthly thereafter in the first year. Sixteen (28%) developed mild leukopenia (white blood count <4.5×10³/mm³). No patient had a white blood cell count <2.8×10³/mm³ and no patient developed leukopenia prior to three months of treatment. In only five patients did the leukopenia prompt a change in 6-mercaptopurine dose. Very mild thrombocytopenia (platelet count of <145×10³/mm³) developed in three (5%) and macrocytosis (mean cell volume >101 fl) was seen in nine (16%). In conclusion, leukopenia was not uncommon in patients treated with low-dose 6-MP, but was not clinically significant. Leukopenia occurred no earlier than three months and as late as 42 months into therapy. Thrombocytopenia was uncommon, mild, and was not associated with apparent bleeding. Macrocytosis may occur in the absence of vitamin B₁₂ and folate deficiencies. Patients can be spared from weekly blood count monitoring when using low-dose 6-mercaptopurine treatment.     

6-mercaptopurine in patients with inflammatory bowel disease and previous digestive intolerance of azathioprine

OBJECTIVE: Azathioprine and 6-mercaptopurine are useful therapies in inflammatory bowel diseases. Despite their efficacy, their use is limited owing to treatment intolerance or toxicity in 10-15% of patients. It has been suggested that both drugs could be interchangeable. MATERIAL AND METHODS: All patients treated with 6-mercaptopurine because of previous digestive intolerance of azathioprine in four Spanish hospitals were reviewed. Tolerance of 6-mercaptopurine therapy was assessed. RESULTS: Fifteen patients (11 Crohn's disease, 4 ulcerative colitis) were included. Immunosuppressant therapy was prescribed for steroid-dependent disease in 13 cases, and for perianal disease in 2. Main symptoms of digestive intolerance were epigastric pain, nausea and vomiting, which developed within the first weeks of treatment. Acute pancreatitis was ruled out in all the cases. Five patients commenced 6-mercaptopurine immediately after azathioprine discontinuation and 7 patients within the first month. Eleven patients (73.3%) tolerated 6-mercaptopurine and reached the therapeutic goals; only two patients had to discontinue 6-mercaptopurine because of adverse effects. CONCLUSIONS: Treatment with 6-mercaptopurine is a safe alternative in patients with inflammatory bowel diseases and previous digestive intolerance of azathioprine.     

Efficacy of 6-mercaptopurine treatment after azathioprine hypersensitivity in inflammatory bowel disease

AIM：To investigate the efficacy of 6-mercaptopurine （6-MP） in cases of azathioprine （AZA） hypersensitivity in patients with inflammatory bowel disease. METHODS： Twenty nine previously confirmed Crohn’s disease （CD） （n = 14） and ulcerative colitis （UC） （n = 15） patients with a known previous （AZA） hypersensitivity reaction were studied prospectively. The 6-MP doses were gradually increased from 0.5 up to 1.0-1.5 mg/kg per day. Clinical activity indicies （CDAI/CAI）, laboratory variables and daily doses of oral 5-ASA, corticosteroids, and 6-MP were assessed before and in the first, sixth and twelfth months of treatment. RESULTS： In 9 patients, 6-MP was withdrawn in the first 2 wk due to an early hypersensitivity reaction. Medication was ineffective within 6 mo in 6 CD patients, and myelotoxic reaction was observed in two. Data were evaluated at the end of the sixth month in 12 （8 UC, 4 CD） patients, and after the first year in 9 （6 UC, 3 CD） patients. CDAI decreased transiently at the end of the sixth month, but no significant changes were observed in the CDAI or the CAI values at the end of the year. Leukocyte counts （P = 0.01）, CRP （P = 0.02）, and serum iron （P = 0.05） values indicated decreased inflammatory reactions, especially in the UC patients at the end of the year, making the possibility to taper oral steroid doses. CONCLUSION： About one-third of the previously AZA- intolerant patients showed adverse effects on taking 6MP. In our series, 20 patients tolerated 6MP, but it was ineffective in 8 CD cases, and valuable mainly in ulcerative colitis patients.     

Long term efficacy and safety of allopurinol and azathioprine or 6-mercaptopurine in patients with inflammatory bowel disease

Background and aimsWe previously reported that IBD patients who are non-responders to thiopurines with preferential shunting of metabolites to hepatotoxic 6-methylmercaptopurine ribonucleotides compared to 6-thioguanine nucleotides can reverse the ratio of 6-MMP/6-TGN and respond to thiopurines with the addition of allopurinol. The objective of this study is to report long term efficacy and safety, along with results for an additional 11 patients.MethodsRetrospective chart review of patients at the University of Chicago IBD Center treated with allopurinol in addition to thiopurines.ResultsTwenty five patients with Crohn's disease or ulcerative colitis were enrolled. Within the first month of therapy 6-TGN metabolite levels increased from a mean of 186.5 ± 17.4 (SE) to 352.8 ± 37.8 pmol/8 × 10⁸ (p = 0.0001). Over the same period 6-MMP levels decreased from a mean of 11,966 ± 1697 to 2004 ± 536 pmol/8 × 10⁸ (p < 0.0001). The mean daily dosage of prednisone decreased from 19.8 ± 3.8 mg to 5.3 ± 2.7 mg (p = 0.03). Thirteen patients have a minimum of one year follow-up. Nine of these thirteen patients have continued on therapy for at least 2 years. All thirteen of these patients continue to be in clinical remission at the last follow-up visit. No patients have had evidence of sustained thrombocytopenia or abnormal liver enzymes.ConclusionsIn AZA/6-MP non-responders with increased 6-MMP/6-TGN ratios, addition of allopurinol continues to demonstrate safety and efficacy for long-term maintenance and steroid-sparing in IBD.     

Neutropenia is not required for clinical remission during azathioprine therapy in inflammatory bowel disease

Inflammatory bowel disease is an idiopathic chronic inflammatory process of the gastrointestinal tract. The aetiology remains unknown but probably involves a combination of genetic susceptibility, environmental triggers and abnormal immune regulation. Immunomodulators are effective in treating inflammatory bowel disease. Azathioprine and 6-mercaptopurine (6MP) are the most frequently used immunomodulator agents. These agents are probably underused by many clinicians because of concerns about myelosuppression, pancreatitis, allergic reactions and hepatotoxicity, which can occur in a fraction of patients taking these drugs. Therefore, clinicians have sought ways to optimize therapeutic response and limit toxic side effects. Neutropenia, although uncommon, can occur in patients taking azathioprine or 6MP. The question of neutropenia effecting clinical response has been raised as a possible indicator of therapeutic response. In the study from Campbell and Ghosh [7] in this issue of the European Journal of Gastroenterology and Hepatology, no difference in relapse rates was noted between neutropenic and non-neutropenic patients. In fact, severe life-threatening neutropenia was seen in four patients.     

6-mercaptopurine or methotrexate added to prednisone induces and maintains remission in steroid-dependent inflammatory bowel disease

BACKGROUND: As treatment of steroid-dependent patients with inflammatory bowel disease (IBD) is controversial, we analysed the efficacy and tolerance of 6-mercaptopurine (6-MP) and methotrexate (MTX) added to prednisone in increasing and maintaining the disease remission rate. METHODS: Seventy-two steroid-dependent IBD patients, 34 with ulcerative colitis (UC) and 38 with Crohn's disease (CD), receiving treatment with prednisone were randomly assigned in a 2:2:1 ratio to additionally receive, orally, over a period of 30 weeks 1.5 mg/kg/day of 6-MP (group A) or 15 mg/week of MTX (group B), or 3 g/day of 5-aminosalicylic acid (5-ASA) (group C). All patients who achieved remission were included in a maintaining remission study for 76 weeks. Remission was defined after stopping prednisone as a CD activity index of <150 and normal serum orosomucoid concentration for CD patients and a Mayo Clinic score <7 for UC patients. RESULTS: With regard to achieved remission, a significantly higher (P< 0.05) rate existed for UC patients in group A (78.6%) than in group C (25%), with no statistical differences in group B (58.3%) versus C. For CD patients, the rates were significantly higher (P< 0.001 and 0.01, respectively) in groups A (93.7%) and B (80%) versus C (14%). With regard to maintaining remission, UC patients in group A (63.6%) presented significantly higher rates (P < 0.0015 and P < 0.001, respectively) versus 14.3% in group B and none in group C. For CD patients, statistical differences (P < 0.001) existed when comparing rates in groups A (53.3%) and B (66.6%) versus none in group C. Noticeable side effects appeared in 13.3% of patients from group A and 11.5% from group B. CONCLUSIONS: These results suggest that 6-MP or MTX added to prednisone could be effective in steroid sparing, as well as in achieving and maintaining remission in steroid-dependent IBD patients. MTX was less effective in maintaining remission in UC patients.     

Increased risk of lymphoma among inflammatory bowel disease patients treated with azathioprine and 6-mercaptopurine

BACKGROUND: Inflammatory bowel disease (IBD) is commonly treated with immunomodulators such as azathioprine and 6-mercaptopurine (6-MP). Studies examining lymphoma risk in IBD patients treated with these medications have been underpowered and have yielded conflicting conclusions. AIMS: The purpose of this meta-analysis was to provide a more precise estimate of the relative risk of lymphoma among IBD patients treated with azathioprine or 6-MP. METHODS: Studies were included if they were English language, full article, cohort studies specifically designed to evaluate cancer as an adverse outcome of treatment with azathioprine or 6-MP. Pooled standardised incidence ratios were calculated to estimate the relative risk of lymphoma associated with therapy. Heterogeneity was assessed using Poisson regression. Sensitivity analyses examined the influence of individual studies on risk estimate and heterogeneity statistics. RESULTS: Six studies were identified that met our inclusion criteria. When the data were combined across all studies, the pooled relative risk was 4.18 (95% confidence interval 2.07-7.51; 11 observed cases, 2.63 expected). Sensitivity analysis showed that exclusion of any one study had a relatively small effect on the pooled relative risk estimate (range 3.49-5.21) but excluding either the study with the highest or lowest estimated relative risk eliminated the statistically significant heterogeneity. CONCLUSIONS: Our data suggest an approximate fourfold increased risk of lymphoma in IBD patients treated with azathioprine/6-MP. The increased risk of lymphoma could be a result of the medications, the severity of the underlying disease, or a combination of the two.     

Utility of measuring 6-methylmercaptopurine and 6-thioguanine nucleotide levels in managing inflammatory bowel disease patients treated with 6-mercaptopurine in a clinical practice setting

BACKGROUND: Measuring levels of 6-mercaptopurine (6-MP) metabolites (6-thioguanine nucleotides [6-TGNs] and 6-methylmercaptopurine [6-MMP]) has been proposed as a method to adjust 6-MP dose to optimize therapeutic response while minimizing toxicity in patients with inflammatory bowel disease. A 6-TGN level of >230 pmol/8 x 108 red blood cells (RBCs) has been reported to be associated with a higher efficacy rate, and a level of >450 pmol/8 x 108 RBCs has been reported to be associated with myelotoxicity. A 6-MMP level of >5,700 pmol/8 x 108 RBCs has been reported to be associated with an increased frequency of abnormal results of liver function tests (LFTs). GOALS: To report our experience with 6-MMP and 6-TGN levels in a clinical practice setting. STUDY: Using outpatient clinic medical records, we identified 53 measurements. Indications for measurement, 6-MP dose, and subsequent adjustments were documented. RESULTS: Indications for measurements included the following: persistent symptoms, 31 cases (58.5%); abnormal LFT results, 7 (13.2%); steroid dependency, 6 (11.3%); anemia, 4 (7.5%); and leukopenia, 2 (3.8%). Of the 31 cases with persistent symptoms, 12 had "therapeutic" 6-TGN levels and other interventions were undertaken. 6-TGN levels were "subtherapeutic" in 19. The 6-MP dose was increased, and remission was achieved in 10 cases after a mean period of 3.6 weeks. Among the cases with abnormal LFT results, 6-MMP levels were high in five and low in two. Among the steroid dependency cases, 6-TGN levels were "subtherapeutic" in five. The dose was increased and steroids were weaned in three cases. The 6-TGN level was high in one of the leukopenia cases and the 6-MP dose was decreased. 6-TGN levels were not above the "target range" in any of the anemia cases. CONCLUSION: Measuring levels of 6-MP metabolites may have a role in customizing 6-MP dosing. This role is not completely clear and needs to be explored in larger well-controlled studies.