Primary Objective To determine the pattern of weight and body mass index (BMI) changes in biologic-treated patients with inflammatory bowel disease (IBD).

Secondary Objectives To evaluate the correlation between BMI and biologic drug dosing. To estimate the direct financial burden of biologic therapy according to BMI categories.

To assess whether higher BMI is associated with increased treatment requirements.

Inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), are chronic conditions associated with reduced quality of life and impaired productivity. Due to recurrent disease activity and progressive structural damage, IBD imposes a significant burden on both public and private healthcare systems.

The use of advanced therapies for moderate to severe IBD has provided substantial benefits in recent decades, including reductions in hospitalization and surgery rates. Anti-Tumor Necrosis Factor-alpha (anti-TNF-α) agents typically constitute the first line of advanced therapy for both CD and UC. However, between 10% and 40% of patients may not respond to treatment (primary non-responders), and approximately 12% may lose response over time, particularly with infliximab (IFX).

Several mechanisms may contribute to IFX treatment failure. Immunogenicity, where antibodies form against the monoclonal antibody, is a primary factor. Other factors affecting IFX clearance include overweight and obesity. Drug clearance is influenced by the volume of distribution, which depends on patient weight. Additionally, excess adipose tissue can alter the pharmacokinetics of anti-TNF agents and exert endocrine and immunological effects through the release of adipocytokines, potentially contributing to the pathogenesis of inflammatory conditions. Therefore, excessive body weight may partly explain IFX therapy failure in some cases.

Obesity appears to negatively affect the response to immunosuppressants and anti-TNF agents. However, studies examining the relationship between obesity, anti-TNF response, and autoimmune diseases have reported inconsistent results. For example, an analysis of three randomized controlled trials of IFX in dermatology and rheumatology found no statistically significant differences in response rates across BMI groups. In contrast, a prospective cohort study in patients with rheumatoid arthritis reported that obese patients treated with IFX had lower clinical response rates. A systematic review and meta-analysis found that obesity was associated with a 60% higher likelihood of losing response to anti-TNF agents compared to non-obese patients with rheumatoid arthritis, ankylosing spondylitis, and psoriasis. However, this association has not been confirmed in patients with IBD.

To date, few studies have examined the influence of obesity or overweight on endoscopic remission in IBD. In a recent cohort study of IFX-treated patients, Singh et al. (2018) found no significant reduction in mucosal healing in obese patients after adjusting for covariates (CI: 0.12-2.35; p=0.31). Other recent studies have demonstrated that body composition parameters, such as subcutaneous adiposity index, visceral adiposity index, and skeletal muscle mass index, are independent factors correlated with active Crohn's disease, with visceral adiposity negatively impacting disease activity.

Inclusion Criteria:

• Patients must have received infliximab (IFX) therapy for at least 14 weeks and completed the on-label induction regimen (5 mg/kg IV at weeks 0, 2, and 6), regardless of the dose at the time of blood collection (patients receiving optimized doses are eligible).
• Patients may be on IFX monotherapy or in combination with one of the following medications at the time of selection: aminosalicylates, antibiotics, prednisone, azathioprine, 6-mercaptopurine, or methotrexate.

Exclusion Criteria:

• Patients receiving IFX in combination with tacrolimus, cyclosporine, sirolimus, mycophenolate mofetil, another immunobiological agent, or tofacitinib.
• Patients with insufficient data in their electronic medical records.

• Patients underwent clinical, laboratory, endoscopic, and/or imaging evaluations as part of routine outpatient monitoring. Current medications and Serum IFX concentrations will offered for collection.
• Laboratory investigations (Complete blood picture, Liver function tests, C-reactive protein and ESR, Fecal calprotectin).
• The following variables were evaluated: age, sex, BMI [weight (kg)/height (m)²], smoking history, history of IBD-related surgeries, current and prior concomitant medications, disease duration, Montreal classification, and clinical activity indices for CD and UC. Clinical activity of CD was assessed using the Harvey-Bradshaw Index (HBI), with clinical remission defined as an HBI score ≤ 4 points.
• Clinical activity of UC was assessed using the partial Mayo score, with clinical remission defined as a score ≤ 2 points