Abstract 1840: Isolation and evaluation of human nanobodies against hypoxanthine guanine phosphoribosyltransferase (HPRT) and their potential use for the detection and targeting of lung, breast and colon cancer cells

Since the clinical relevance of hypoxanthine guanine phosphoribosyltransferase (HPRT) as both a cancer biomarker and a potential tumor target is increasing, we isolated human nanobodies (Nbs) against HPRT and tested their potential to detect and target cytosolic and membrane associated HPRT in lung, breast and colon cancer cells. HPRT is a purine salvage pathway enzyme that catalyzes the conversion of hypoxanthine to inosine monophosphate and guanine to guanosine monophosphate. Until recently, HPRT was considered as a housekeeping gene and its clinical relevance was mainly limited to congenital central nervous disorders such as Lesch-Nyhan disease. However, recent gene expression analysis across all cancers in The Cancer Genome Atlas (TCGA) database and analysis of HPRT expression within cancer patients' tissues has revealed that the expression of HPRT is elevated in the majority of malignant tissues in comparison to normal tissues. Moreover, multiple studies have recently shown that HPRT may be associated with the cell membrane in lung, colon cancer and lymphoma cells, and not in normal cells. Thus, HPRT is a potential immunotherapeutic target for cancer. Using phage display technology we isolated 384 nanobody clones from the Christ human single domain antibody (dAb) library against human HPRT. Ten of the most sensitive Nbs were sequenced and evaluated to determine their sensitivity using dose response curves. Four parameter logistic regression analysis of the top 10 HPRT nanobodies showed R squared values ranging from 0.9388-0.9989 and a sigmoidal shape denoting a concentration-dependent antigen-ligand relationship. The anti-HPRT Nbs were able to detect minimum HPRT amounts ranging from 10-20 ng/ml. Validation of the anti-HPRT Nbs was conducted with a siRNA HPRT knockdown in A549 cells. A549 siRNA HPRT knockdown cell lysate showed at least a 5-fold reduction in the overall signal in phage ELISA compared to A549 wild type cell lysate (p