Background: Anti-tumour necrosis factor (TNF) drugs have revolutionised Crohn's disease management but treatment failure is common. We sought clinical and pharmacokinetic factors that predict and mitigate primary non-response at week 14, non-remission at week 54, and adverse events leading to drug withdrawal. Methods: We conducted a prospective observational UK-wide study in 1610 biologic naive patients with active luminal Crohn's disease (955 infliximab [753 Remicade; 202 biosimilar CT-P13] and 655 adalimumab). Findings: Primary non-response (PNR) occurred in 23·8% (95% CI 21·4%-26·2%), non-remission in 63·1% (95% CI 60·3%-65·8%), and adverse events curtailed treatment in 7·8% (95% CI 6·6%-9·2%) patients. The only independent risk factor for PNR was low drug level. The optimal week 14 drug levels associated with remission at weeks 14 and 54 were 7 mg/L and 12 mg/L for infliximab and adalimumab, respectively. Continuing standard dosing regimens after PNR was rarely helpful: only 12·4% (14/113; 95% CI 6·9%-19·9%) entered remission by week 54. Obesity, smoking, low albumin levels, higher baseline markers of disease activity, and development of anti-drug antibodies were associated with low drug levels, which mediated week 54 non-remission. Immunogenicity rates at week 54 were 62·8% (95% CI 59·0-66·3%) for infliximab and 28·5% (95% CI 24·0-32·7%) for adalimumab. For both drugs, sub-optimal week 14 drug levels were the main determinant of immunogenicity; whilst subsequent anti-drug antibody formation was the principle factor accounting for low drug levels. Combination thiopurine or methotrexate therapy mitigated this risk with similar effect sizes for both anti-TNF drugs and for infliximab led to lower week 54 non-remission rates. Interpretation: Anti-TNF treatment failure is common and associated with low drug levels, partly mediated by immunogenicity Clinical trials are required to investigate whether personalised induction regimens and treatment-to-target dose intensification improve outcomes. Funding: Charities: CORE, CCUK, C3. Unrestricted grants: Abbvie, Merck Sharp & Dohme, NAPP, Pfizer and Celltrion. Declaration of Interest: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and declare: N.A.K has consulted for Falk and received honoraria from Falk, Allergan, Pharmacosmos and Takeda for unrelated topics and is a deputy editor of Alimentary Pharmacology & Therapeutics Journal; G.A.H reports non-financial support from AbbVie, outside the submitted work; and that he is now an employee of AbbVie and owns stock in the company; M.R. is an employee of AbbVie and owns stock in the company; D.G. reports honoraria from AbbVie; G.J.W has consulted for AbbVie and received honoraria from Falk and AbbVie for unrelated topics and a fellowship from NIHR; A.H has served as consultant, advisory board member or speaker for AbbVie, Atlantic, Bristol-Myers Squibb, Celltrion, Falk, Ferring, Janssen, MSD, Napp Pharmaceuticals, Pfizer, Pharmacosmos, Shire and Takeda and also serves on the Global Steering Committee for Genentech outside the submitted work;. S.S reports personal fees from AbbVie, Merck, Takeda, grants from Takeda, Tillotts, AbbVie, Merck, outside the submitted work; P.M.I reports personal fees from Abbvie, Janssen, Pfizer, Sandoz, VH squared, Samsung Bioepis, and Ferring , grants from MSD, grants and personal fees from Takeda, non-financial support from Falk, outside the submitted work; D.McG reports grants from NIH and Helmsley Charitable Trust, during the conduct of the study; grants and personal fees from Janssen, Precision IBD Inc, Second Genome, Qu Biologics, Pfizer, Gilead and Takeda, outside the submitted work; C.W.L reports grants and personal fees from Abbvie and Gilead personal fees from GSK, Janssen, Takeda, Amgen, Pfizer, Samsung Bioepis, Celgene, grants outside the submitted work; J.R.F.C reports personal fees from Abbvie, grants and personal fees from Takeda, Hospira and Biogen, personal fees from Janssen, MSD, Celltrion, NAPP, and Sandoz, outside the submitted work; J.L has served as consultant, advisory board member or speaker for Shire (International/UK) AbbVie, Atlantic, BMS, Celltrion, Falk, Ferring (International/UK), Janssen, MSD, Napp Pharmaceuticals, Pfizer, GSK, Allergan, Cornerstone US, and Takeda and has received travel support from AbbVie, Warner Chilcot, Takeda and Pfizer outside the submitted work; R.K.R reports honoraria from Abbvie, Ferring , Therakos and Celltrion, grants from Nestec, outside the submitted work; J.R.G received honoraria from Falk, Abbvie and Shield therapeutics for unrelated topics. T.A has received unrestricted research grants, advisory board fees, speaker honorariums and support to attend international meetings from AbbVie, Merck, Janssen, Takeda, Ferring, Tillotts, Ferring, Pfizer, NAPP, Celltrion, Hospira for unrelated topics, no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work. H.G, B.H, C.B, M.H.P, T.M, A.T, R.N, 473 S.B, N.C, N.M.H, P.H, S.L, M.P, J.C.M have no conflicts of interest to declare. Ethical Approval: The South West Research Ethics committee approved the study (REC Reference: 12/SW/0323) in January 2013.
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