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Introduction

Secondary loss of response to Infliximab(IFX) occurs in approximately half of patients with Inflammatory Bowel Disease(IBD), due to drug levels falling below the therapeutic range. This study evaluated IFX levels and related clinical and laboratory parameters and their impact on clinical outcomes.

Aims & Methods

Pre-infusion drug levels, associated laboratory data, and clinical parameters were evaluated in IBD patients receiving maintenance doses of IFX(5–15 mg/kg every 4–8 weeks) between Nov2021-Apr2025. Drug levels were measured using a bedside kit capable of quantitatively detecting values between 0.4–20µg/mL.

Results

A total of 1,059 single pre-infusion measurements from 455 patients and consecutive dual measurements at weeks 4 and 8 from 64 patients were evaluated. In the single and double measurement groups, 75.4% and 84.4% of patients had Crohn’s disease(CD), respectively. The most commonly used doses were 5 mg/8 weeks and 10 mg/4 weeks(Table 1) Drug levels were below 5 µg/mL in 33% of the measurements. Antibody positivity was present in 47.5% of 120 available measurements ranging from 0.4-20 µg/mL, showing no significant correlation with drug levels or immunomodulatory(IM) use (p=0.62 and p=0.61) However, for all positive antibody measurements drug levels were ≤ 3.5 µg/mL. CRP differed significantly across drug level groups, whereas albumin did not(Table 1). A weak negative correlation was found between IFX levels and CRP(r = -0.13, p = 0.001), with no correlation for albumin(p=0.54). In UC patients, the correlation between CRP and IFX levels was slightly stronger(r= -0.280, p=0.004).No significant associations were found between IM use and IFX levels >5 µg/mL(p=0.55), >10 µg/mL(p= 0.93), or >20µg/mL(p = 0.87). Abdominal pain was more common in patients with IFX levels <5 µg/mL (p=0.032) and <10 µg/mL (p=0.044). Smoking status was not associated with achieving IFX levels >5 µg/mL( p=0.57), >10 µg/mL (p=0.89), and >20 µg/mL (p=0.633). Among CD patients, those with IFX levels >5 µg/mL had significantly lower CDAI scores (p = 0.039), and lower daily stool frequency (p=0.002).Regression analysis showed that higher BMI (p=0.045), lower CRP(p < 0.001), higher IFX dose(p < 0.001), and CD diagnosis(p =0.014) to be independently associated with achieving IFX levels >5 µg/mL. These factors also predicted levels >10 µg/mL and >20 µg/mL, except for BMI at the >20 µg/mL threshold. In the dual measurement group, the median CRP levels at weeks 4 and 8 were 2.1(5.1) and 2.5(6.3), respectively (p = 0.108). Among patients with IFX >20 µg/mL at week 4, 75% maintained levels >5 µg/mL at week 8, whereas this rate was 64% for those with levels >10 µg/mL. Only 59.6% of those with >5 µg/mL at week 4 retained >5 µg/mL at week 8(all p <0.001). Week 4 IFX levels was 7.3 times higher than at week 8 for cases within measurable range.

Parameter	>5 µg/ml	>10 µg/ml	>20 µg/ml	Patient Demographics
* >5 vs <5; >10 vs <10 ; >20 vs <20 # >5 and <5; >10 and <10 ; >20 and <20 respectively
Disease(All / CD / UC %)	67 / 69.8 / 56.5	51 / 53 / 43	25 / 25.4 / 21.8	Sex M / F %	56 / 44
p (CD vs UC)	<0.001	0.012	0.29	Age at Diagnosis	29 (19)
CRP # median(IQR)	2.6 (5.2) ; 4.9 (11.1)	2.5 (5.2); 4.1 (9.3)	2.1 (3.9); 3.6 (7.2)	Age at Measurement	38 (21)
p*(CRP)	<0.001	<0.001	<0.001	Duration of IFX Treatment	20 (25)
Dose Groups (%)	5mg/8w: 46 5mg/4w: 84 10mg/8w: 64 10mg/4w: 81 15mg/4w: 70	5mg/8w:28 5mg/4w:75 10mg/8w:38.7 10mg/4w: 71 15mg/4w: 56	5mg/8w: 4 5mg/4w: 43 10mg/8w:15.3 10mg/4w: 42 15mg/4w: 34
p (Dose)	<0.001	<0.001	<0.001
Week 4 (%) / Week 8 (%)	89 / 53.1	83 / 31.3	62.5 / 9.4

Conclusion

One-third of patients demonstrated sub-therapeutic drug levels, which were associated with elevated CRP levels and worse clinical outcomes. Notably, the 5mg/4 weeks group exhibited higher drug concentrations compared to the 10mg/8 weeks group, suggesting that increased infusion frequency may contribute to achieving higher levels. Moreover, maintaining sufficient drug levels at week 8 requires substantially higher levels at week 4. Measurement of anti-TNF antibodies may not be fruitful if drug level is > 3.5µg/mL. These findings support the drug monitoring and personalized dosing to reduce the risk of loss of response and improve outcomes.

THERAPEUTIC DRUG LEVEL MONITORING OF ANTI-TNF(INFLIXIMAB) IN IBD; ASSOCIATION WITH BIOCHEMICAL AND CLINICAL PARAMETERS

Mert Berke Gur 1, oguz bakkaloglu 1, Tugce Eskazan 1, Atilla Akpinar 1, Ali Ibrahim Hatemi 1, Yusuf Ziya Erzin 1, Aykut Ferhat Çelik 1

1 Istanbul University-Cerrahpasa, Cerrahpasa Medical Faculty, Istanbul, Türkiye

Conference

UEG Week Berlin 2025

Topics

IBD

Submission format

Abstract

Session

IBD (Posters)

Citation

United European Gastroenterology Journal 2025; 13 (Supplement 8)

Published

2025

	Group A INDUCTION	B Group MAINTENANCE
Pacients / Determinations	25 / 25	33 /42
Sex ( M / F )	12 (48%) / 13 (52%)	17 (51%) / 16 (48%)
Age (median)	12 (RIC 2-18)	12 (RIC 1-18)
Phenotype ( CD / UC )	22 (88%) / 3 (12%)	25 (81%) / 8 (18%)
VEOIBD	14 (56%)	17 (51%)
BMI body mass index (median)	17.1 (RIC 13.4-26.3)	16.4 (RIC 12.4-26.1)
Biosimilar	14 (56%)	11 (24%)
Optimized dose	12 (48%)	26 (57%)
Immunomodulators	13 (52%)	28 (62%)

	CD patients		UC patients
	Switch vs. Swap to vedolizumab N=1,987	Switch vs. Swap to ustekinumab N=5,317	Switch vs. Swap to vedolizumab N=4,324	Switch vs. Swap to ustekinumab N=815
	aHR [95%CI]	aHR [95%CI]	aHR [95%CI]	aHR [95%CI]
Short-term IBD treatment discontinuation (1 year after switching/swapping)	1.23 [1.11-1.36] *	0.76 [0.70-0.81] *	0.81 [0.75-0.87] *	0.58 [0.49-0.69] *
Long-term IBD treatment discontinuation (5 years after switching/swapping)	1.14 [1.06-1.23] *	0.72 [0.68-0.76] *	0.83 [0.78-0.87] *	0.62 [0.54-0.71] *
IBD-related hospitalizations	1.06 [0.94-1.19]	0.77 [0.71-0.83] *	1.11 [1.02-1.20] *	0.84 [0.68-1.03]
IBD-related surgeries	0.94 [0.66-1.32]	1.14 [0.93-1.39]	1.29 [0.89-1.87]	1.08 [0.42-2.79]
Hospitalizations for serious infection	1.20 [0.87-1.67]	0.74 [0.59-0.93] *	0.96 [0.74-1.23]	0.58 [0.30-1.14]
*p-value <0.05; aHR: adjusted hazard ratios; CD: Crohn’s disease; CI: confidence interval; UC: ulcerative colitis Covariates used to estimate propensity score: age, gender, previous infections, clostridium difficile infections, cardiovascular diseases, end stage kidney disease/kidney transplant, chronic liver diseases, chronic pulmonary diseases, venous thromboembolism, diabetes mellitus, nature of 1L anti-TNF, time to first line of anti-TNF from diagnosis date, disease duration, hospitalisation at induction of new treatment. Additional covariate for CD patients only: history of major IBD-related surgery (colectomy or intestinal resection).

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