B-273 Therapeutic Drug Monitoring (TDM) Assays to Quantify Risankizumab Drug and Anti-Risankizumab Antibodies in Individuals Treated with IL23 Inhibitor Risankizumab for Chronic Autoimmune Diseases

Background Risankizumab (RSK) (Skyrizi®), a humanized monoclonal antibody against the p19 subunit of interleukin 23 (IL23), is used to treat Crohn’s disease (CD), ulcerative colitis (UC), plaque psoriasis (PsO), and psoriatic arthritis (PsA). By blocking IL23 binding to its receptor, RSK inhibits IL23-dependent cell signaling and release of proinflammatory cytokines. Therapeutic drug monitoring of biologic drugs and their antibodies can expedite clinical decision-making, improve efficacy, and may lead to reduced cost. Here, analytic performance characteristics of a new lab developed electrochemiluminescence immunoassays for RSK and anti-RSK antibody concentrations are presented. Methods We developed and validated electrochemiluminescence immunoassays for RSK and its anti-drug antibody (ADA) in accordance with FDA guidance for therapeutic protein immunogenicity testing and bioanalytical method validation. The drug assay utilizes antibodies against RSK and is designed to detect free (ADA-unbound) pharmacodynamically active RSK. The ADA assay measures total binding antibodies (including IgM and IgG) against RSK. Analytical performance including accuracy, precision, lower limit of quantitation (LLOQ), upper limit of quantitation (ULOQ), dilutional linearity, specificity, and drug tolerance (for the ADA assay) was assessed. Results The RSK dug assay has an analytical measurement range (AMR) of 0.75 – 60 µg/mL with an 80-fold maximum dilution (maximum quantifiable value of 4,800 µg/mL). Lower limit of quantitation (LLOQ) is 0.75 ug/mL where steady state RSK drug trough concentrations are reported to be around 4 - 9 µg/mL in UC and CD depending upon the dose. Within the AMR, bias for the assay is = ±7.5% and imprecision is = 9.8% CV. There is no cross-reactivity with other biologics used to treat similar diseases or with other anti-IL23 inhibitor drugs. The ADA AMR is 25 – 5,000 ng/mL with a maximum allowable dilution of 100-fold (maximum reportable concentration of 500,000 ng/mL). Assay LLOQ is 25 ng/mL. Assay bias is = 14.0% and imprecision is = 14.5% CV for samples within the AMR. Drug tolerance was confirmed by sample recovery of 80 – 120 % for samples with drug levels as high as 120 µg/mL and no cross-reactivity was detected for ADAs to other biologics. All positive ADA samples undergo a confirmatory test step where signal suppression upon the addition of high concentrations of RSK confirms the RSK-specificity of the anti-drug antibodies. Conclusion This study demonstrates the robust analytic performance of two new TDM assays to quantify RSK drug and anti-RSK antibodies. Accurate quantitation of RSK drug enables dose optimization at post-induction and maintenance time points, while this drug-specific, drug tolerant, and high resolution anti-RSK assay allows for accurate immunogenic assessment at signs of lack or loss of response. Together, these TDM assays aim to optimize the use and longevity of RSK through precise drug and immunogenicity monitoring.