Development and validation of an assay for the characterization of SSTR2 expression on CTCs from liquid biopsies

SSTR2 is drawing renewed attention as a radiotherapeutic target in neuroendocrine tumors, and this AACR 2024 poster reports the development and analytical validation of an immunofluorescence assay that quantifies SSTR2 on circulating tumor cells recovered from a blood draw — screened across three antibody clones, characterized on spike-in cell lines, and applied to five patient samples.

Presented by RareCyte.

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• The clone chosen for the assay was the dimmest of the three, selected on signal-to-background rather than raw brightness. Three SSTR2 antibody clones were screened on spike-in cells: clone 402038 gave the lowest median positive signal of the three (MFI 1337.0, against 1923.3 for UMB1 and 1831.1 for 1C3) but the cleanest background (53.5, against 213.9 for UMB1), yielding a signal-to-background ratio of 25.0 where UMB1 reached 9.0 and 1C3 only 3.1.
• Performance was characterized on cell lines standing in as model CTCs, where the true answer is known. IMR-32 served as the SSTR2-positive control against HAP1 and BT-474 negatives, and the assay's reported metrics were 85% sensitivity and 100% specificity at a preliminary positivity threshold of 95 MFI; repeatability averaged a 14.4% CV across three replicate runs (25.7%, 13.4% and 4.1%), and inter-stainer run precision was 16.4%.
• Across five patient samples, SSTR2 positivity varied both between patients and between cells within a patient. CTC counts ran from 8 to 362 per sample; the four Merkel cell samples were 49%, 24%, 0% and 13% SSTR2-positive, while the single colorectal sample was 56% SSTR2-positive and 98% synaptophysin-positive — the authors describe this cohort as small and both positivity thresholds (95 MFI for SSTR2, 120 for synaptophysin) as preliminary.