Outstanding Challenges in IBD Treatment

Inflammatory bowel disease affects over 6.8 million people worldwide (GBD 2017, Lancet Gastroenterol 2020) and incurs substantial costs in surgery, hospitalisation, and lost productivity. Biological therapies have transformed IBD management since the approval of infliximab in 1998, yet a majority of patients do not achieve or sustain deep remission — defined as combined clinical, biochemical, and endoscopic remission — with any currently available agent.

Two interrelated failures drive this treatment gap. First, clinicians lack reliable, non-invasive tools to objectively measure mucosal inflammation in real time, making treat-to-target strategies difficult to implement. Second, even when disease activity is quantified, the available therapies fail a large proportion of patients either from the outset (primary non-response) or over time (secondary loss of response). Understanding these limitations is prerequisite to designing the next generation of IBD therapeutics and monitoring strategies.

The ideal IBD biomarker would be non-invasive, cheap, rapid, and capable of accurately reflecting mucosal inflammatory activity across all disease phenotypes and locations. No current biomarker meets these criteria. The three most widely studied — C-reactive protein (CRP), fecal calprotectin (FC), and fecal lactoferrin (FL) — each carry significant limitations that restrict their clinical utility.

CRP is the most accessible serum inflammatory marker and correlates reasonably well with disease activity in Crohn's disease. However, its performance in ulcerative colitis is markedly inferior: the sensitivity for active UC is approximately 50%, meaning half of patients with endoscopically active disease have normal CRP values. Vermeire et al. (2006) demonstrated that CRP is the best-performing conventional serum marker but emphasised that it is neither disease-specific nor adequate for monitoring mucosal healing.

A key practical limitation is that CRP elevation is triggered by many non-IBD processes — bacterial infection, malignancy, cardiovascular disease, and obesity — all of which are relevant comorbidities in IBD patients. Furthermore, up to 28% of children with Crohn's disease and 42% of paediatric UC patients exhibit normal CRP despite endoscopically active disease, making it unreliable in this population (Shine et al., 1985; Tibble & Bjarnason, 2001).

Fecal calprotectin (a neutrophil-derived S100A8/9 heterodimer released during intestinal inflammation) has emerged as the gold-standard non-invasive marker for mucosal inflammation and now anchors most treat-to-target protocols. A landmark meta-analysis by Lin et al. (2018) covering 19 studies reported a pooled sensitivity of 81% and specificity of 74% for detecting endoscopic activity in Crohn's disease (AUC = 0.85), and superior performance in UC (sensitivity 87%, specificity 77%, AUC = 0.91). A subsequent meta-analysis focused on mucosal healing (Chen et al., 2021) confirmed AUC 0.88 in UC at a cut-off of 60–75 µg/g.

However, the performance of FC is critically dependent on disease location — a major limitation given that isolated small bowel (ileal) Crohn's disease accounts for approximately one-third of CD cases (Ungaro et al. 2021). A systematic review by Tham et al. (2019) demonstrated that sensitivity for ileal CD falls to just 36% at standard cut-offs — rendering FC near-useless as a screening tool for the most common CD phenotype without colonic involvement.

Additional confounders include proton pump inhibitor use, NSAID consumption, colorectal cancer, and coeliac disease, all of which can elevate FC independently of IBD activity. Cut-off values lack standardisation across assays (ranging from 50 to 250 µg/g in published guidelines), and pre-analytical variability from stool homogenisation and storage temperature further undermines reproducibility.

Fecal lactoferrin (FL), a glycoprotein released by activated neutrophils, offers broadly comparable performance to FC in CD: pooled sensitivity 75%, specificity 80%, AUC 0.81 (Luu et al. 2023). Its clinical uptake has been limited by cost and assay availability relative to the widely deployed ELISA-based FC tests. Fecal S100A12 shows promise in paediatric IBD (pooled sensitivity 95%, specificity 97%, AUC 0.99; Islek et al. 2023), though adult data remain limited.

Serum biomarker panels combining CRP, albumin, platelet count, and white cell count (e.g. the Harvey-Bradshaw Index input variables) improve accuracy over any single marker but still cannot replace endoscopy for determining mucosal healing — the key target in modern treat-to-target protocols. Blood transcriptomic classifiers (Lee et al. 2019) and plasma proteomics-based scores are in early validation; none have yet been approved for routine clinical use.

Modern IBD therapy encompasses five mechanistic classes: anti-TNF agents (infliximab, adalimumab, certolizumab, golimumab), anti-integrin agents (vedolizumab), anti-IL-12/23 and anti-IL-23 agents (ustekinumab, risankizumab, mirikizumab, guselkumab), JAK inhibitors (tofacitinib, upadacitinib, filgotinib), and S1P receptor modulators (ozanimod, etrasimod). Despite this mechanistic breadth, a consistent pattern emerges across trials: induction remission rates are modest, maintenance remission decays over time, and net remission across all enrolled patients is substantially lower than headline trial data suggest.

Critically, patients who fail one biologic are systematically less likely to respond to subsequent agents — regardless of mechanism — due to shared pathways of secondary resistance, the development of anti-drug antibodies, and progressive fibrotic remodelling in the gut wall (Qiu et al. 2020).

A critical reanalysis by Fumery et al. (2022) re-examined pivotal CD trial data and found that net remission rates — accounting for all patients who enrolled, including those who withdrew, experienced adverse events, or failed induction — were substantially lower than the per-protocol figures reported in publications. For the best-performing agents, net remission across all treated patients rarely exceeded 40% at one year. The authors concluded that clinicians should communicate realistic expectations to patients, with the majority not achieving sustained remission from any single agent.

Response rates are further eroded in biologic-experienced populations. A meta-analysis by Qiu et al. (2020) demonstrated that primary non-response to one anti-TNF agent is strongly associated with inferior response to second-line biologics irrespective of mechanism, suggesting that shared downstream immunological failure modes — not simply target escape — drive sequential therapeutic attrition.

In UC, the 2024 AGA evidence synthesis (Singh et al. 2024) conducted a network meta-analysis across all approved advanced therapies in biologic-exposed patients. Upadacitinib ranked highest for induction of remission (P-score 0.93), followed by tofacitinib and ustekinumab. However, even the best-performing agent achieved remission in only ~55–70% of biologic-naïve patients in pivotal trials, with rates substantially lower (~30–50%) in biologic-experienced populations — the patients most urgently in need of new treatments.

A fundamental reason for variable biomarker performance and inconsistent treatment response is that neither "Crohn's disease" nor "ulcerative colitis" is a single, homogeneous entity. Both represent broad clinical syndromes with heterogeneous underlying immunobiology, genetic architecture, microbiome composition, and disease behaviour. Ungaro et al. (2021) and colleagues argued in Gastroenterology that the current Montreal classification — which stratifies CD by location (L1/L2/L3), behaviour (B1/B2/B3), and upper GI modifier — is fundamentally insufficient to guide treatment decisions or predict outcomes.

Molecular subtyping efforts have identified at least two transcriptomic endotypes of UC (inflammatory vs. fibrostenotic) and multiple immunological CD subtypes (Th1-dominant, Th17-dominant, innate immune-predominant) that respond differently to available therapies. Patients with OSM/OSMR-high gene expression in mucosal biopsies show markedly inferior response to anti-TNF therapy (West et al. 2017, Nat Med), yet no OSM pathway inhibitor is approved and OSMR transcriptomics is not part of any routine clinical workup.

The absence of validated predictive biomarkers for treatment selection means that current practice remains empirical sequence therapy: patients cycle through agents by class until one works or all have failed, with each failure increasing the risk of surgical intervention and reducing the probability of subsequent drug response.

Several promising strategies are in development or early clinical adoption. For biomarker improvement, point-of-care FC testing now enables near-real-time results in outpatient settings, reducing the lag between sample collection and treatment adjustment. Serum MMP-3 and REG1B (regenerating islet-derived protein 1B) are being evaluated as markers of small bowel inflammation that are less dependent on colonic disease contribution. Plasma cell-free DNA methylation profiling and faecal microbiome diversity indices show promising signals in small cohorts.

For treatment optimisation, proactive therapeutic drug monitoring guided by treat-to-target algorithms has been shown to substantially improve anti-TNF durability (Zhao et al. 2021). Combination biological therapy — exemplified by the guselkumab + golimumab DUET programme — targets complementary pathways simultaneously and may overcome the ceiling effect imposed by single-target blockade. Early-phase data suggest substantially higher rates of endoscopic remission than either agent alone.

Ultimately, resolving IBD's treatment challenges likely requires moving from reactive, symptom-driven management to a model of precision IBD care: molecular endotyping at diagnosis, biomarker-informed monitoring using disease-location-appropriate tests, proactive TDM, and timely escalation before irreversible structural damage accrues.

IBD is predominantly diagnosed between the ages of 15 and 35 — precisely the years during which education is completed, careers are established, and long-term earning trajectories are set. This timing means the disease does not merely disrupt employment: it can permanently reshape lifetime occupational achievement. A 2024 population-based Norwegian study by Assa et al. (J Crohn's Colitis 2025) found that IBD onset before young adulthood was independently associated with reduced labour market participation and lower income compared with age-matched healthy controls, with the strongest effects in patients with high disease severity and comorbid mental health conditions.

A landmark 2024 systematic review and meta-analysis by Zeitz et al. (J Crohn's Colitis), synthesising data from 154 studies and over 170,000 IBD patients, quantified the scale of professional impairment. Pooled estimates showed that only 65.6% of IBD patients were employed — and of those, nearly one in three had previously lost a job due to their disease. Work-related absence was substantial: patients with active disease missed an average of 23.9 working days per year and lost 4.1 hours per week through absenteeism alone. Even while present at work, a further 3.9 hours per week were lost to presenteeism (reduced on-the-job effectiveness). The pooled overall productivity loss across all IBD patients was 39.4%.

Long-term outcomes are similarly stark. The Norwegian IBSEN cohort, followed for 10 years after IBD diagnosis, found an overall work disability rate of 18.8% — with a relative risk of 1.8 in UC and 2.0 in CD compared to age- and sex-matched general population controls (Bernklev et al. 2010). Women with Crohn's disease were most severely affected, with a disability pension rate of 24.6%. Disease activity is the primary driver: IBD patients with active disease have a 20% lower odds of being employed (OR 0.8) and over twice the odds of receiving a disability pension (OR 2.1) compared with those in remission — underscoring the direct link between inadequate treatment and occupational harm.

The professional burden is further compounded by indirect effects on career trajectory. IBD patients are disproportionately likely to choose lower-demand occupations, reduce working hours, decline promotion opportunities, and forgo further training — a set of accommodations that reduce lifetime earnings even when formal disability status is not reached. A scoping review by Boal Carvalho et al. (2022) identified workplace disclosure, physical demands, unpredictable symptom flares, and bathroom access as the primary barriers facing employed IBD patients. Yet formal workplace accommodations remain rare: fewer than half of IBD patients who needed accommodations had received them, often due to reluctance to disclose a stigmatised gastrointestinal condition to employers.

Critically, a Swedish nationwide registry study by Eriksson et al. (2020) found that societal costs — encompassing healthcare utilisation, sick leave, and disability pensions — were consistently 2 to 3 times higher in IBD patients than in matched general population controls, both in the incident year of diagnosis and over subsequent years. Effective treatment that achieves and maintains deep remission is therefore not merely a clinical goal but an economic one: sustained remission is the strongest predictor of preserved employment and reduced disability pension uptake.

IBD imposes a three-layered financial burden on patients: direct out-of-pocket healthcare costs, indirect costs from lost productivity and disability, and the long-term wealth penalty from reduced lifetime earnings. Across all three layers, IBD patients fare substantially worse than the general population — and the gap has widened since the advent of expensive biologic therapies in the 2000s.

The most comprehensive US cost analysis, the Crohn's & Colitis Foundation's IBD Cost Initiative (Buie et al. 2019), estimated total direct IBD-related health system expenditure of $14.6–31.6 billion annually in the United States alone — a figure that, accounting for more recent biologic pricing and expanded prevalence, is now estimated to approach $50 billion per year. On a per-patient basis, average annual direct costs range from approximately $9,000–12,000 across the IBD population (Buie et al. 2019), rising to $30,000–60,000 or more in patients receiving biologic or small-molecule therapies.

A landmark analysis of lifetime costs by Park et al. (2020, Clin Gastroenterol Hepatol) found that patients diagnosed with Crohn's disease in their 20s face lifetime direct healthcare costs exceeding $400,000 — substantially more than age-matched controls — with hospitalisation and biologic drugs accounting for the majority of expenditure. Earlier age at diagnosis correlated directly with higher lifetime cost, reinforcing the unique economic vulnerability created by IBD's early onset.

Out-of-pocket costs for patients vary enormously by insurance status. Under US Medicare Part D (2022), annual retail prices for biologic maintenance therapies ranged from approximately $60,000 for adalimumab to over $150,000 for ustekinumab, corresponding to annual out-of-pocket costs of $5,400–$10,200 for patients without supplemental coverage (Medscape 2022). These figures — representing a substantial fraction of median household income — may result in cost-related non-adherence, drug discontinuation, and preventable disease flares. For commercially insured patients with manufacturer copay assistance, costs can be near zero, but this benefit does not extend to government insurance programmes, disproportionately affecting lower-income and elderly IBD patients.

The indirect cost burden is equally significant and frequently underestimated. A US productivity analysis by Ballester et al. (2022) found that IBD patients with active disease incurred an additional $2,168 per patient per year in work-loss-related indirect costs compared with matched non-IBD controls — costs largely absorbed by patients themselves and their employers rather than healthcare systems. When aggregated across the 3.1 million US IBD patients (Dahlhamer et al. 2016), the total indirect cost burden exceeds $4–6 billion per year in lost economic output (Ballester et al. 2022).

The financial burden also falls unevenly. Patients with more severe, refractory, or surgically managed disease — often those in whom available therapies have failed — bear the highest costs at every level: more hospitalisations, more procedures, higher drug expenditure, and greater productivity loss. This creates a reinforcing cycle: suboptimal treatment efficacy drives higher costs, while cost-related barriers to care impede access to the treatments most likely to induce remission.