26566 Ustekinumab Quantitation with Antibodies, Serum (USTEK)

​​Stelara, Stelara antibodies​

​Evaluation of loss of response to therapy

Quantification of ustekinumab in human serum

Trough level quantitation for evaluation of patients treated with ustekinumab

Detection of antibodies to ustekinumab in human serum

​Patient Preparation: Collect immediately before the next dose of drug administration (trough level)

Centrifuge and aliquot serum into a plastic vial.​

Heat-inactivated specimen

​This assay measures free ustekinumab. This assay does not measure ustekinumab bound to anti-ustekinumab antibodies (immunocomplexes).

Presence of ustekinumab at concentrations greater than 1 mcg/mL may impair detection of antibodies to ustekinumab (ATU), as the ATU assay is not drug tolerant. This assay measures free ATU. This assay does not measure ATU bound to UTK (immunocomplexes).

Elevated rheumatoid factor (RF) may falsely increase results of ATU. During validation studies, negative ATU samples remained negative and positive ATU samples remained positive; however the quantitative result differed by more than 20% when compared to the non-RF spiked original samples. If patients are positive for RF, clinical correlation is recommended for ATU test interpretation.

​Ustekinumab (UTK) is a fully human IgG1 kappa monoclonal antibody (1) that binds with high affinity to the p40 subunit of human interleukin (IL)12 and IL23 and has been approved for the treatment of patients with moderate to severe Crohn disease (CD), moderate to severe ulcerative colitis (UC), psoriatic arthritis, and plaque psoriasis. The drug prevents IL12 and IL23 bioactivity by binding and neutralizing the shared p40 subunit, preventing interaction with the cell surface receptor protein IL12Rbeta1. Through this mechanism of action, UTK effectively neutralizes IL12 and IL23, proteins that are thought to be associated with gastrointestinal inflammation in CD and UC. In the setting of the inflammatory bowel diseases (IBD), CD and UC, the treatment regimen is started with a single weight-based loading dose of the t-mab administered intravenously (IV), and a maintenance regimen with standard (non-weight based) subcutaneous administration of ustekinumab 8 weeks after induction dose, and every 8 weeks thereafter. There is very little data supporting proactive therapeutic drug monitoring for ustekinumab.

The test is most useful in the evaluation of loss of response to therapy. A gradual decrease in efficacy over time following an initial response to biologics is common. In many cases, antibodies generated to the biologic are responsible for treatment failure, as they bind to the drug creating an immunocomplex and clearing the drug faster from circulation.

For IBD, measurements in nonresponders are indicated at post-induction (week 8) and concentrations of ustekinumab associated with favorable outcomes are greater than 3.5 mcg/mL. In addition, for measurements during maintenance stages of therapy, ustekinumab concentrations greater than or equal to 1 mcg/mL are associated with clinical response and clinical remission. At maintenance stages, ustekinumab concentrations greater than or equal to 4.5 mcg/mL are associated with mucosal healing.

In clinical trials, 6% to 12.4% of patients using ustekinumab for psoriasis or psoriatic arthritis developed antibodies-to-ustekinumab (ATU) over time. For IBD, between 2.9% and 4.6% of patients developed ATU when treated with ustekinumab for 1 year. Therefore, it is important to monitor trough concentrations of serum UTK to correlate drug levels with loss of response to therapy. ATU may increase drug clearance in treated patients or neutralize the drug effect, thereby potentially contributing to the loss of response. ATU could also cause adverse events such as serum sickness and hypersensitivity reactions.

Currently, ustekinumab quantitation is performed in conjunction with immunogenicity assessment for ATU.

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