Development and basic performance verification of a rapid homogeneous bioassay for agonistic antibodies against the thyroid-stimulating hormone receptor

Assessing a Novel Thyroid-Stimulating Antibody Bioassay as a Predictor of Radioactive Iodine Therapy Efficacy in Graves' Disease

OBJECTIVE

No studies have reported the use of the Biosensor TSAb assay (TSAb (Bio)) for predicting the therapeutic efficacy of radioactive iodine therapy (RAIT) for Graves' disease (GD). Therefore, we evaluated the usefulness of this novel thyroid-stimulating antibody (TSAb) bioassay kit, which uses a cyclic adenosine monophosphate (cAMP) biosensor, in predicting the therapeutic efficacy of RAIT in 20 patients with GD.

METHODS

We defined the RAIT outcome using two criteria: (i) thyroid weight reduction to <8 g and (ii) thyroid weight reduction to >80% at 12 months post-RAIT. For criteria (i) and (ii), regression analysis was performed for thyroid-stimulating hormone (TSH) receptor autoantibody (TRAb), TSAb (Bio), TSAb (Bio)/TRAb, I-131 absorbed dose/TSAb (Bio), and I-131 absorbed dose/(TSAb (Bio)/TRAb). After evaluating the significance of each factor, receiver operating characteristic (ROC) analysis was also performed.

RESULTS

Among the five aforementioned anti-TSH receptor antibody parameters, only I-131 absorbed dose/(TSAb (Bio)/TRAb) showed statistical significance in the multivariate analysis (p = 0.004 and p = 0.021 for each criterion). Although I-131 absorbed dose/(TSAb (Bio)/TRAb) correlated significantly with the thyroid weight (r = -0.54, p = 0.015), it did not correlate with thyroid weight reduction rates at 12 months post-RAIT. ROC analysis also demonstrated a good predictive value (area under the curve of approximately 0.8 with a cut-off value of 3.9) for I-131 absorbed dose/(TSAb (Bio)/TRAb) in predicting thyroid weight at 12 months post-RAIT.

CONCLUSIONS

TSAb (Bio) is a useful predictor of RAIT efficacy when evaluated as I-131 absorbed dose/(TSAb (Bio)/TRAb). This index was defined by dividing a factor that positively affects RAIT outcomes (I-131 absorbed dose) by a factor negatively affecting it (i.e., (TSAb (Bio)/TRAb)). The therapeutic efficacy of RAIT can be expected with an I-131 absorbed dose/(TSAb (Bio)/TRAb) value of ≥3.9. Adjusting the I-131 dosage based on this index may lead to more effective treatment outcomes.

A cAMP-biosensor-based assay for measuring plasma arginine-vasopressin levels

Arginine-vasopressin (AVP), a cyclic peptide hormone composed of nine amino acids, regulates water reabsorption by increasing intracellular cyclic adenosine monophosphate (cAMP) concentrations via the vasopressin V2 receptor (V2R). Plasma AVP is a valuable biomarker for the diagnosis of central diabetes insipidus (CDI) and is commonly measured using radioimmunoassay (RIA). However, RIA has several drawbacks, including a long hands-on time, complex procedures, and handling of radioisotopes with special equipment and facilities. In this study, we developed a bioassay to measure plasma AVP levels using HEK293 cells expressing an engineered V2R and a cAMP biosensor. To achieve high sensitivity, we screened V2R orthologs from 11 various mammalian species and found that the platypus V2R (pV2R) responded to AVP with approximately six-fold higher sensitivity than that observed by the human V2R. Furthermore, to reduce cross-reactivity with desmopressin (DDAVP), a V2R agonist used for CDI treatment, we introduced a previously described point mutation into pV2R, yielding an approximately 20-fold reduction of responsiveness to DDAVP while maintaining responsiveness to AVP. Finally, a comparison of plasma samples from 12 healthy individuals demonstrated a strong correlation (Pearson's correlation value: 0.90) between our bioassay and RIA. Overall, our assay offers a more rapid and convenient method for quantifying plasma AVP concentrations than existing techniques.