The development of an indirect competitive immunomagnetic-proximity ligation assay for small-molecule detection

The development of an indirect competitive immunomagnetic-proximity ligation assay (ICIPLA), which is a novel method for detecting small molecules, is described in this report. Free small molecules in samples can be detected using a proximity ligation assay (PLA); the detection is based on the proximity effect caused by a high concentration of small molecule–BSA conjugates bound to streptavidin magnetic beads. As an indirect format competitive immunoassay, the ICIPLA method has the advantage in that the quantity of monoclonal antibody (mAb) used for small-molecule detection is 8-fold lower than that required for the competitive immunomagnetic-proximity ligation assay (CIPLA) described in our previous work. Small molecules can be detected using a single monoclonal antibody, and the PLA method can be used to amplify high-performance signals. In this work, the small molecular compound ractopamine (RAC) was selected as a target for ICIPLA. The limit of detection (LOD) was 0.01 ng ml−1, and the method exhibited a broad dynamic range of up to six orders of magnitude. We also employed the ICIPLA method to detect RAC in serum, urine, and muscle extracts; the results indicated that the LOD and dynamic range were not altered. The cross-reactivity studies showed that the cross-reactivity values for all RAC analogs were below 0.01%. These results suggest that ICIPLA is a sensitive, specific and practical method for small-molecule detection. This is the first report of the improved PLA technology for small-molecule detection by indirect competitive formats in the biological samples.