Quinone Methide Signal Amplification: Covalent Reporter Labeling of Cancer Epitopes using Alkaline Phosphatase Substrates

Diagnostic assays with the sensitivity required to improve cancer therapeutics depend on the development of new signal amplification technologies. Herein, we report the development and application of a novel amplification system which utilizes latent quinone methides (QMs) activated by alkaline phosphatase (AP) for signal amplification in solid-phase immunohistochemical (IHC) assays. Phosphate-protected QM precursor substrates were prepared and conjugated to either biotin or a fluorophore through an amine-functionalized linker group. Upon reaction with AP, the phosphate group is cleaved, followed by elimination of the leaving group and formation of the highly reactive and short-lived QM. The QMs either react with tissue nucleophiles in close proximity to their site of generation, or are quenched by nucleophiles in the reaction media. The reporter molecules that covalently bind to the tissue were then detected visually by fluorescence microscopy in the case of fluorophore reporters, or brightfield microscopy using diaminobenzidine (DAB) in the case of biotin reporters. With multiple reporters deposited per enzyme, significant signal amplification was observed utilizing QM precursor substrates containing either benzyl difluoro or benzyl monofluoro leaving group functionalities. However, the benzyl monofluoro leaving group gave superior results with respect to both signal intensity and discretion, the latter of which was found to be imperative for use in diagnostic IHC assays.