Designed Ankyrin Repeat Proteins for detecting prostate-specific antigen expressionin vivo

Abstract

AbstractLate-stage prostate cancer often acquires resistance to conventional chemotherapies and transforms into a hormone-refractory, drug-resistant, and non-curative disease. Developing non-invasive tools to detect the biochemical changes that correlate with drug efficacy and reveal the onset of drug resistance would have important ramifications in managing the treatment regimen for individual patients. Here, we report the selection of new Designed Ankyrin Repeat Proteins (DARPins) that show high affinity toward prostate-specific antigen (PSA), a biomarker used in clinical monitoring of prostate cancer. Ribosome display andin vitroscreening tools were used to select PSA-binding DARPins based on their binding affinity, selectivity, and chemical constitution. Surface plasmon resonance measurements demonstrated that the four lead candidates bind to PSA with nanomolar affinity. DARPins were site-specifically functionalised at a uniqueC-terminal cysteine with the hexadentateaza-nonamacrocyclic chelate (NODAGA) for subsequent radiolabelling with the positron-emitting radionuclide68Ga. [68Ga]GaNODAGA-DARPins showed high stability toward transchelation and were stable in human serum for >2 h. Radioactive binding assays using streptavidin-loaded magnetic beads confirmed that the functionalisation and radiolabelling did not compromise the specificity of [68Ga]GaNODAGA-DARPins toward PSA. Biodistribution experiments in athymic nude mice bearing subcutaneous prostate cancer xenografts derived from the LNCaP cell line revealed that three of the four [68Ga]GaNODAGA-DARPins displayed specific tumour-bindingin vivo. For DARPin-6, tumour-uptake in the normal group reached 4.16 ± 0.58 %ID g-1(n= 3; 2 h post-administration) and was reduced by ∼50% in the blocking group (2.47 ± 0.42 %ID g-1;n= 3;P-value = 0.018). Collectively, the experimental results support the future development of new PSA-specific imaging agents for potential use in monitoring the efficacy of androgen receptor (AR)-targeted therapies.