Prostate
carcinoma is the most common non-cutaneous
cancer in developed countries and represents the second leading cause of death. Early stage
androgen dependent prostate
carcinoma responds well to conventional
therapies, but relatively few treatment options exist for patients with
hormone-refractory
prostate cancer. One of the most suitable targets for antibody-mediated approaches is prostate specific membrane
antigen (PSMA) which is a well known tumour associated
antigen. PSMA is a type II integral cell-surface
membrane protein that is not secreted, and its expression density and enzymatic activity are increased progressively in
prostate cancer compared to normal prostate epithelium, thereby making PSMA an ideal target for
monoclonal antibody imaging and
therapy. To obtain a small
protein that can better penetrate tissue, we have engineered a
single-chain variable fragment (scFv) starting from the variable heavy and light domains of the murine anti-PSMA
monoclonal antibody D2B. scFvD2B was analysed in vitro for activity, stability, internalisation ability and in vivo for targeting specificity. Maintenance of function and immunoreactivity as well as extremely high radiolabelling efficiency and radiochemical purity were demonstrated by in vitro assays and under different experimental conditions. Despite its monovalent binding, scFvD2B retained a good strength of binding and was able to internalise around 40% of bound
antigen. In vivo we showed its ability to specifically target only PSMA expressing
prostate cancer xenografts. Due to these advantageous properties, scFvD2B has the potential to become a good
theranostic reagent for early detection and
therapy of
prostate cancers.