Ocrelizumab Concentration Is a Good Predictor of SARS-CoV-2 Vaccination Response in Patients with Multiple Sclerosis.

Ocrelizumab, an anti-CD20 monoclonal antibody, counteracts induction of humoral immune responses after severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) vaccinations in patients with multiple sclerosis (MS). We aimed to assess if serum ocrelizumab concentration measured at the time of vaccination could predict the humoral response after SARS-CoV-2 vaccination. In 52 patients with MS, we found ocrelizumab concentration at the time of vaccination to be a good predictor for SARS-CoV-2 IgG anti-RBD titers after vaccination (comparable to B-cell count). As the course of ocrelizumab concentration may be predicted using pharmacokinetic models, this may be a superior biomarker to guide optimal timing for vaccinations in B-cell depleted patients with MS. ANN NEUROL 2023;93:103-108.

Fingerprick blood samples to measure serum natalizumab concentrations.

BACKGROUND: Natalizumab via subcutaneous administration was recently approved for patients with multiple sclerosis. OBJECTIVE: In light of personalized extended dosing, in which treatment intervals are prolonged to a concentration cut-off, it would be preferable to measure natalizumab drug concentrations in capillary blood. METHODS: In this cross-sectional study in patients treated with intravenous (IV) natalizumab, capillary blood samples by fingerprick and venous blood samples were collected in 30 participants prior to IV administration of natalizumab. RESULTS: Natalizumab concentrations were similar with a mean bias of -0.36 µg/mL (95% CI: 1.3 to -2 µg/mL). CONCLUSIONS: This study shows that physicians can monitor natalizumab drug concentrations by a fingerprick, which could be used for personalized extended dosing.

Development of a SARS-CoV-2 Total Antibody Assay and the Dynamics of Antibody Response over Time in Hospitalized and Nonhospitalized Patients with COVID-19.

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 infections often cause only mild disease that may evoke relatively low Ab titers compared with patients admitted to hospitals. Generally, total Ab bridging assays combine good sensitivity with high specificity. Therefore, we developed sensitive total Ab bridging assays for detection of SARS-CoV-2 Abs to the receptor-binding domain (RBD) and nucleocapsid protein in addition to conventional isotype-specific assays. Ab kinetics was assessed in PCR-confirmed, hospitalized coronavirus disease 2019 (COVID-19) patients (n = 41) and three populations of patients with COVID-19 symptoms not requiring hospital admission: PCR-confirmed convalescent plasmapheresis donors (n = 182), PCR-confirmed hospital care workers (n = 47), and a group of longitudinally sampled symptomatic individuals highly suspect of COVID-19 (n = 14). In nonhospitalized patients, the Ab response to RBD is weaker but follows similar kinetics, as has been observed in hospitalized patients. Across populations, the RBD bridging assay identified most patients correctly as seropositive. In 11/14 of the COVID-19-suspect cases, seroconversion in the RBD bridging assay could be demonstrated before day 12; nucleocapsid protein Abs emerged less consistently. Furthermore, we demonstrated the feasibility of finger-prick sampling for Ab detection against SARS-CoV-2 using these assays. In conclusion, the developed bridging assays reliably detect SARS-CoV-2 Abs in hospitalized and nonhospitalized patients and are therefore well suited to conduct seroprevalence studies.

The effect of certolizumab drug concentration and anti-drug antibodies on TNF neutralisation.

OBJECTIVES: Tumour necrosis factor (TNF) inhibitors like certolizumab, elicit an immunogenic response leading to the formation of anti-drug antibodies (ADAs). We sought to mechanistically investigate the relationship between certolizumab concentrations, ADAs, and the effective TNF neutralising capacity in sera of rheumatoid arthritis (RA) patients. TNF neutralising capacity of certolizumab was compared to the neutralising capacity of adalimumab. METHODS: Serum samples were collected from 40 consecutive certolizumab-treated RA patients at baseline and 4, 16, 28 and 52 weeks after treatment initiation [Dutch Trial Register NTR (Nederlands Trial Register) Trial NL2824 no. 2965]. Certolizumab concentration and ADA titre were measured with a certolizumab bridging enzyme-linked immunosorbent assay (ELISA) and a drug-tolerant radioimmunoassay (RIA), respectively. TNF neutralisation by certolizumab and adalimumab, in presence or absence of ADAs, was analysed with the TNF-sensitive WEHI bioassay. RESULTS: Despite a high incidence of ADAs during one year of follow-up (65%; 26/40 patients), certolizumab levels of >10 µg/ml were measured in most patients. The capacity for TNF neutralisation highly correlated with certolizumab serum concentration, whereas no association with ADAs was observed. Similar results were obtained for adalimumab. The relative in vitro neutralising potency was higher for certolizumab compared to adalimumab. CONCLUSIONS: Anti-certolizumab antibodies were detected in a large proportion of patients, but in most cases where ADAs were detected, certolizumab was also present in high concentrations, directly correlating with in vitro neutralising capacity. These results indicate that measurement of certolizumab drug levels, rather than ADAs, have direct clinical significance.

High Mutation Frequency of the PIGA Gene in T Cells Results in Reconstitution of GPI Anchor-/CD52- T Cells That Can Give Early Immune Protection after Alemtuzumab-Based T Cell-Depleted Allogeneic Stem Cell Transplantation.

Alemtuzumab (ALM) is used for T cell depletion in the context of allogeneic hematopoietic stem cell transplantation (alloSCT) to prevent acute graft-versus-host disease and graft rejection. Following ALM-based T cell-depleted alloSCT, relatively rapid recovery of circulating T cells has been described, including T cells that lack membrane expression of the GPI-anchored ALM target Ag CD52. We show, in a cohort of 89 human recipients of an ALM-based T cell-depleted alloSCT graft, that early lymphocyte reconstitution always coincided with the presence of large populations of T cells lacking CD52 membrane expression. In contrast, loss of CD52 expression was not overt within B cells or NK cells. We show that loss of CD52 expression from the T cell membrane resulted from loss of GPI anchor expression caused by a highly polyclonal mutational landscape in the PIGA gene. This polyclonal mutational landscape in the PIGA gene was also found in CD52- T cells present at a low frequency in peripheral blood of healthy donors. Finally, we demonstrate that the GPI-/CD52- T cell populations that arise after ALM-based T cell-depleted alloSCT contain functional T cells directed against multiple viral targets that can play an important role in immune protection early after ALM-based T cell-depleted transplantation.

Loss of the GPI-anchor in B-lymphoblastic leukemia by epigenetic downregulation of PIGH expression.

Adult B-lymphoblastic leukemia (B-ALL) is a hematological malignancy characterized by genetic heterogeneity. Despite successful remission induction with classical chemotherapeutics and novel targeted agents, enduring remission is often hampered by disease relapse due to outgrowth of a pre-existing subclone resistant against the treatment. In this study, we show that small glycophosphatidylinositol (GPI)-anchor deficient CD52-negative B-cell populations are frequently present already at diagnosis in B-ALL patients, but not in patients suffering from other B-cell malignancies. We demonstrate that the GPI-anchor negative phenotype results from loss of mRNA expression of the PIGH gene, which is involved in the first step of GPI-anchor synthesis. Loss of PIGH mRNA expression within these B-ALL cells follows epigenetic silencing rather than gene mutation or deletion. The coinciding loss of CD52 membrane expression may contribute to the development of resistance to alemtuzumab (ALM) treatment in B-ALL patients resulting in the outgrowth of CD52-negative escape variants. Additional treatment with 5-aza-2'-deoxycytidine may restore expression of CD52 and revert ALM resistance.

Clinical Validation of a Capillary Blood Home-Based Self-Sampling Technique for Monitoring of Infliximab, Vedolizumab, and C-Reactive Protein Concentrations in Patients With Inflammatory Bowel Disease.

BACKGROUND: Therapeutic drug monitoring provides important guidance for treatment of patients with inflammatory bowel disease (IBD) and could help to early identify treatment failure. This study aimed to validate a finger prick-based capillary blood sampling technique to measure biological trough levels and C-reactive protein (CRP) and evaluate patient performance and -support. METHODS: In this prospective cohort study, patients with IBD receiving infliximab (IFX) or vedolizumab (VEDO) therapy performed finger prick-based capillary blood sampling at home. Additionally, blood was collected through routinely performed in-hospital venepuncture prior to biological infusion. IFX, VEDO, and CRP concentrations were measured by enzyme-linked immunosorbent assay. The concordance between methods was statistically evaluated and a survey was conducted to assess practicality and patient support. RESULTS: In total, 81 patients (46 IFX, 35 VEDO) were enrolled. Mean differences between both methods were 0.42 (95% confidence interval, -1.74 to 2.58) µg/mL for IFX and 0.72 (95% confidence interval, -5.50 to 6.94) µg/mL for VEDO. Passing-Bablok regressions demonstrated no evidence for systematic or proportional biases. Venous and capillary IFX (ρ = 0.96, P < .001) and VEDO (ρ = 0.97, P < .001) levels strongly correlated and showed high intermethod agreement (Cohen's kappa: IFX = 0.82; VEDO = 0.94). Similarly, venous and capillary CRP levels were strongly correlated (ρ = 0.99, P < .001). Most patients (>95%) were able to successfully perform the self-sampling at home without prior instructions. CONCLUSIONS: This study clinically validated a finger prick-based capillary blood self-sampling technique allowing concomitant home monitoring of biological levels and CRP for patients with IBD, who reported substantial support, tolerability, and practicality.

Subcutaneous administration of natalizumab can lead to lower drug concentrations compared to intravenous administration.

BACKGROUND: Several studies reported lower drug concentrations with subcutaneous natalizumab compared to intravenous natalizumab. With the emergence of extended interval dosing, gaining more insight into lower concentrations after subcutaneous administration is essential. METHODS: We compared serum trough concentrations between subcutaneous and intravenous administration within a matched cohort (n = 50). RESULTS: Subcutaneous administration (n = 25) was associated with lower concentrations compared to intravenous administration (n = 25) (log-B=-0.28, p = 0.01). In an exploratory group of 11 patients receiving extended interval dosing of subcutaneous natalizumab, the median trough concentration was even lower. CONCLUSION: Subcutaneous natalizumab can lead to lower drug concentrations, potentially limiting extended interval dosing.

Patient-perspective and feasibility of home finger-prick testing to complement and facilitate large-scale research in rheumatology.

BACKGROUND: During the COVID-19 pandemic, we developed a digital research platform to longitudinally investigate COVID-19-related outcomes in patients with rheumatic diseases and healthy controls. We used home finger-prick testing in order to collect serum samples remotely and increase the overall efficiency of the platform. The aim of the present study was to evaluate the success rate of the finger prick and patients' perspective towards the finger prick. METHODS: Serum samples were collected up to five times during follow-up, either via a venepuncture at the research institute or a finger prick from participants' home. Participants were asked to complete a digital evaluation questionnaire of the finger prick after their attempts. RESULTS: A total of 2135 patients and 899 controls performed at least one finger prick and were included in this study. The first finger prick was successfully done by 92% (95% CI: 90% to 93%) of patients, 94% (95% CI: 92% to 95%) of controls, 93% (95% CI: 92% to 94%) of all participants aged ≤70 years and 89% (95% CI: 86% to 92%) of all participants aged >70 years. Sex did not impact these success rates. Repeated failure occurred in 11/439 (0.8%) patients and 4/712 (0.6%) controls. Both patients and controls were less willing to perform a finger prick for individual healthcare compared with scientific research. CONCLUSION: The vast majority of participants, among which elderly and patients with rheumatic diseases, were able to successfully draw the required amount of blood for serological analyses. This shows that finger-prick testing is suitable for a high-throughput implementation to monitor patients remotely.

Belimumab concentration measurements using a homologous anti-idiotype immunoassay.

Monitoring belimumab concentrations in patients can be a valuable tool for assessing treatment response and for personalizing drug doses. Various assay formats may be used to measure concentrations of therapeutic monoclonal antibodies. A particularly useful format involves the use of anti-idiotype monoclonal antibodies, selected to be highly specific to the antibody of interest. Here, we describe the development of a specific, high-affinity anti-idiotype antibody to belimumab, and the application of this antibody in a homologous sandwich ELISA to measure belimumab concentrations.

Therapeutic Drug Monitoring of Vedolizumab in Inflammatory Bowel Disease Patients during Maintenance Treatment-TUMMY Study.

There are limited data on therapeutic drug monitoring (TDM) in inflammatory bowel disease (IBD) patients treated with vedolizumab (VDZ). Although an exposure-response relation has been demonstrated in the post-induction phase, this relationship is more uncertain in the maintenance phase of treatment. The aim of our study was to determine whether there is an association between VDZ trough concentration and clinical and biochemical remission in the maintenance phase. A prospective, observational multicenter study has been performed on patients with IBD on VDZ in the maintenance treatment (≥14 weeks). Patient demographics, biomarkers, and VDZ serum trough concentrations were collected. Clinical disease activity was scored by the Harvey Bradshaw Index (HBI) for Crohn's disease (CD) and the Simple Clinical Colitis Activity Index (SCCAI) for ulcerative colitis (UC). Clinical remission was determined as HBI < 5 and SCCAI < 3. Biochemical remission was defined as fecal calprotectin <250 mg/kg and serum CRP <5 mg/L. A total of 159 patients (59 CD, 100 UC) were included. In none of the patient groups, a statistically significant correlation between trough VDZ concentration and clinical remission was observed. Patients in biochemical remission had higher VDZ trough concentrations (p = 0.019). In this population, higher trough VDZ concentrations were associated with biochemical remission but not with clinical remission.

The clinical relevance of dupilumab serum concentration in patients with atopic dermatitis: a two-center prospective cohort study.

BACKGROUND: Dupilumab is prescribed in one dosage across adult atopic dermatitis patients. Differences in drug exposure may explain variation in treatment response. OBJECTIVE: Investigating the clinical relevance of dupilumab serum concentration in atopic dermatitis in real-world practice. METHODS: In two centers (Netherlands, UK), adults treated with dupilumab for atopic dermatitis were evaluated for effectiveness and safety pretreatment and at 2, 12, 24, and 48 weeks; trough serum samples were analyzed for dupilumab concentration at corresponding time points. RESULTS: In 149 patients, median dupilumab levels during follow-up ranged from 57.4 to 72.4 µg/mL. Levels showed high inter-patient and low intra-patient variability. No correlation was found between levels and ΔEASI. At 2 weeks, levels of ≥64.1 µg/mL predict EASI ≤7 at 24 weeks (specificity:100%, sensitivity:60%; p = .022). At 12 weeks, ≤32.7 µg/mL predicts EASI >7 at 24 weeks (sensitivity:95%, specificity:26%; p = .011). Inverse correlations were found between baseline EASI and levels at 2, 12, and 24 weeks (r = -0.25 to 0.36; p ≤ .023). Low levels were particularly observed in patients with adverse events, treatment interval deviation, and discontinuation. CONCLUSION: At the on-label dosage, the measured range of dupilumab levels does not seem to yield differences in treatment effectiveness. However, disease activity does seem to influence dupilumab levels - higher baseline disease activity results in lower levels at follow-up.

Optimization of a Quantitative Anti-Drug Antibodies against Infliximab Assay with the Liquid Chromatography-Tandem Mass Spectrometry: A Method Validation Study and Future Perspectives.

Monoclonal antibodies (mAbs), such as infliximab, are important treatment options for different diseases. Immunogenicity is a major risk, resulting in anti-drug antibodies (ADAs), being associated with adverse events and loss of response, influencing long-term outcomes. The development of ADAs against infliximab is primarily measured by immunoassays like radioimmunoassay (RIA). Although liquid chromatography-tandem mass spectrometry (LC-MS/MS) is increasingly utilized across different fields, this technique is currently not used for ADAs against infliximab measurements. Therefore, we developed the first LC-MS/MS method. Stable isotopically labeled infliximab antigen-binding fragments (SIL IFX F(ab')2) were used to bind and measure ADAs indirectly. Protein A magnetic beads were used to capture IgG, including ADAs, whereafter SIL IFX F(ab')2 was added for labeling. After washing, internal standard addition, elution, denaturation and digestion samples were measured by LC-MS/MS. Internal validation showed good linearity between 0.1 and 16 mg/L (R2 > 0.998). Sixty samples were used for cross-validation with RIA, and no significant difference between ADA concentrations was found. The methods had high correlation (R = 0.94, p < 0.001) and excellent agreement, intraclass correlation coefficient = 0.912 (95% confidence interval 0.858-0.947, p < 0.001). We present the first ADA against the infliximab LC-MS/MS method. The method is amendable for quantifying other ADAs, making it applicable as a template for future ADA methods.

Development of antidrug antibodies against adalimumab maps to variation within the HLA-DR peptide-binding groove.

Targeted biologic therapies can elicit an undesirable host immune response characterized by the development of antidrug antibodies (ADA), an important cause of treatment failure. The most widely used biologic across immune-mediated diseases is adalimumab, a tumor necrosis factor inhibitor. This study aimed to identify genetic variants that contribute to the development of ADA against adalimumab, thereby influencing treatment failure. In patients with psoriasis on their first course of adalimumab, in whom serum ADA had been evaluated 6-36 months after starting treatment, we observed a genome-wide association with ADA against adalimumab within the major histocompatibility complex (MHC). The association signal mapped to the presence of tryptophan at position 9 and lysine at position 71 of the HLA-DR peptide-binding groove, with both residues conferring protection against ADA. Underscoring their clinical relevance, these residues were also protective against treatment failure. Our findings highlight antigenic peptide presentation via MHC class II as a critical mechanism in the development of ADA against biologic therapies and downstream treatment response.

Case report: Pharmacokinetics of pembrolizumab in a patient with stage IV non-small cell lung cancer after a single 200 mg administration.

Background: Pembrolizumab is a well-tolerated biologic agent with a potentially stable and durable anti-tumor response. Unfortunately, discontinuation of therapy can occur as a consequence of immune-related adverse effects (irAEs). These irAEs appear independent of dose and exposure. However, such irAEs might also result from pembrolizumab's highly specific mechanism of action and current dosing regimens. However, the currently available pharmacokinetic (PK) and pharmacodynamic (PD) data to reassess dosing strategies are insufficient.To highlight the importance of additional PK/PD studies, we present a case describing the complexity of pembrolizumab's PK/PD after a single 200 mg pembrolizumab dose in a treatment-naive patient with non-small cell lung cancer (NSCLC). Case description: A 72-year-old man with stage IV NSCLC presented hepatotoxic symptoms 19 days after receiving the first 200 mg pembrolizumab dose. Hence, pembrolizumab therapy was paused, and prednisolone therapy was initiated, which successfully inhibited the toxic effect of pembrolizumab. However, repeated flare-ups due to prednisolone tapering suggest that the toxic effect of pembrolizumab outlasts the presence of pembrolizumab in the bloodstream. This further suggests that the T-cell-mediated immune response outlasts the programmed cell death protein 1 (PD-1) receptor occupancy by pembrolizumab, which challenges the need for the current fixed-interval strategies and their stop criteria.Furthermore, a validated ELISA quantified pembrolizumab levels in 15 samples within 123 days after administration. A shift in the pembrolizumab clearance rate was evident ensuing day 77 (0.6 µg/mL) after administration. Pembrolizumab levels up to day 77 (9.1-0.6 µg/mL) strongly exhibited a linear, first-order clearance (R2 = 0.991), whereas after day 77, an accelerated non-linear clearance was observed. This transition from a linear to non-linear clearance was most likely a result of full target receptor saturation to non-full target receptor saturation, in which the added effect of target-mediated drug disposition occurs. This suggests that pembrolizumab's targets were fully saturated at levels above 0.6 µg/mL, which is 43 to 61 times lower than the steady-state trough levels (Ctrough,ss) of the currently registered fixed-dosing regimens (3-5).

Antibody development and disease severity of COVID-19 in non-immunised patients with rheumatic immune-mediated inflammatory diseases: data from a prospective cohort study.

BACKGROUND: Research on the disease severity of COVID-19 in patients with rheumatic immune-mediated inflammatory diseases (IMIDs) has been inconclusive, and long-term prospective data on the development of SARS-CoV-2 antibodies in these patients are lacking. METHODS: Adult patients with rheumatic IMIDs from the Amsterdam Rheumatology and Immunology Center, Amsterdam were invited to participate. All patients were asked to recruit their own sex-matched and age-matched control subject. Clinical data were collected via online questionnaires (at baseline, and after 1-4 and 5-9 months of follow-up). Serum samples were collected twice and analysed for the presence of SARS-CoV-2-specific antibodies. Subsequently, IgG titres were quantified in samples with a positive test result. FINDINGS: In total, 3080 consecutive patients and 1102 controls with comparable age and sex distribution were included for analyses. Patients were more frequently hospitalised compared with controls when infected with SARS-CoV-2; 7% vs 0.7% (adjusted OR: 7.33, 95% CI: 0.96 to 55.77). Only treatment with B-cell targeting therapy was independently associated with an increased risk of COVID-19-related hospitalisation (adjusted OR: 14.62, 95% CI: 2.31 to 92.39). IgG antibody titres were higher in hospitalised compared with non-hospitalised patients, and slowly declined with time in similar patterns for patients in all treatment subgroups and controls. INTERPRETATION: We observed that patients with rheumatic IMIDs, especially those treated with B-cell targeting therapy, were more likely to be hospitalised when infected with SARS-CoV-2. Treatment with conventional synthetic disease-modifying antirheumatic drugs (DMARDs) and biological DMARDs other than B-cell targeting agents is unlikely to have negative effects on the development of long-lasting humoral immunity against SARS-CoV-2.

Immunogenicity of TNF-Inhibitors.

Tumor necrosis factor inhibitors (TNFi) have significantly improved treatment outcome of rheumatic diseases since their incorporation into treatment protocols two decades ago. Nevertheless, a substantial fraction of patients experiences either primary or secondary failure to TNFi due to ineffectiveness of the drug or adverse reactions. Secondary failure and adverse events can be related to the development of anti-drug antibodies (ADA). The earliest studies that reported ADA toward TNFi mainly used drug-sensitive assays. Retrospectively, we recognize this has led to an underestimation of the amount of ADA produced due to drug interference. Drug-tolerant ADA assays also detect ADA in the presence of drug, which has contributed to the currently reported higher incidence of ADA development. Comprehension and awareness of the assay format used for ADA detection is thus essential to interpret ADA measurements correctly. In addition, a concurrent drug level measurement is informative as it may provide insight in the extent of underestimation of ADA levels and improves understanding the clinical consequences of ADA formation. The clinical effects are dependent on the ratio between the amount of drug that is neutralized by ADA and the amount of unbound drug. Pharmacokinetic modeling might be useful in this context. The ADA response generally gives rise to high affinity IgG antibodies, but this response will differ between patients. Some patients will not reach the phase of affinity maturation while others generate an enduring high titer high affinity IgG response. This response can be transient in some patients, indicating a mechanism of tolerance induction or B-cell anergy. In this review several different aspects of the ADA response toward TNFi will be discussed. It will highlight the ADA assays, characteristics and regulation of the ADA response, impact of immunogenicity on the pharmacokinetics of TNFi, clinical implications of ADA formation, and possible mitigation strategies.

Biased anti-idiotype response in rabbits leads to high-affinity monoclonal antibodies to biologics.

Antibody formation to human(ized) therapeutic antibodies in humans is highly skewed toward anti-idiotype responses, probably because the idiotype is the only 'foreign' part of the antibody molecule. Here, we analyzed antibody responses to F(ab')2 fragments of a panel of 17 human(ized) therapeutic antibodies in rabbits. Homology between the rabbit germline and the human(ized) antibodies is moderate not only for the variable domains (both the complementarity-determining regions and the framework regions), but also for the constant domains (66% or less). Nevertheless, we observed a highly skewed anti-idiotype response in all cases, with up to >90% of the antibodies directed toward the idiotype. These results indicate that the idiotype may be inherently immunodominant. We used these biased responses to raise monoclonal rabbit anti-idiotype antibodies against secukinumab, ustekinumab, reslizumab, mepolizumab, palivizumab, and dupilumab and demonstrate the potential to develop sensitive pharmacokinetic assays with these antibodies.

Clinical Impact of Antibodies against Ustekinumab in Psoriasis: An Observational, Cross-Sectional, Multicenter Study.

Ustekinumab is an effective treatment for psoriasis, but response varies between patients. The formation of anti-drug antibodies (ADAs) may explain part of this variation by reducing the free ustekinumab level. Currently, published analyses of the clinical impact of ADAs are incomplete. In this observational cross-sectional multicenter study of 340 patients, we evaluated the impact of ADAs on ustekinumab level and clinical response as assessed by the PASI. Circulating ADA levels were measured using two assays: a drug-sensitive radioimmunoassay and a drug-tolerant ELISA. Circulating ustekinumab levels were measured using an ELISA. ADAs were detected in 3.8% (95% confidence interval [CI] = 3.2-4.2) and in 10.6% (95% CI = 7.9-13.9) of patients using the radioimmunoassay and drug-tolerant ELISA, respectively. At least 85% of the ADAs were neutralizing. Compared with patients negative for ADAs, ADA positivity in the radioimmunoassay and drug-tolerant ELISA were associated with lower median ustekinumab levels (-0.62 µg/ml [95% CI = -1.190 to -0.30] and -0.74 µg/ml [95% CI = -1.09 to -0.47], respectively) and higher absolute PASI (6.6 [95% CI = 3.0-9.9] and 1.9 [95% CI = 0.4-4.0], respectively). Absence of detectable ustekinumab regardless of ADA status correlated with poor clinical outcome (median sample PASI 10.1, 6.5 [95% CI = 3.9-8.8] compared with patients positive for ustekinumab). In conclusion, substantially reduced drug exposure resulting from ADAs formation is associated with impaired clinical response.