Antibodies and their derivatives radiolabelled with positron and gamma emitting radiometals enable sensitive and quantitative molecular Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) imaging of antibody distribution in vivo.
Chelators that are covalently attached to antibodies allow radiolabelling with metallic PET and SPECT radioisotopes. Conventional strategies for chelator-protein conjugation generate heterogeneous mixtures of bioconjugates that can exhibit reduced affinity for their receptor targets, and undesirable biodistribution and pharmacokinetics.
Recent advances in bioconjugation technology enable site-specific modification to generate distinct constructs with superior properties. Herein we survey existing site-specific chelator-protein conjugation methods.
These include chelator attachment to cysteines/disulfide bonds or the glycan region of the antibody, enzyme-mediated chelator conjugation, and incorporation of sequences of amino acids that chelate the radiometal. Such technology will allow better use of PET and SPECT imaging in the development of antibody-based therapies.
Monoclonal antibodies (mAbs) have demonstrated exquisite sensitivity and selectivity for their target cell surface receptors in vivo. As well as being important in clinical therapies mAbs can be used as in vivo vectors, to deliver an additional therapeutic payload (e.g. small-molecule cytotoxic compounds or radiotherapeutic isotopes or, in combination with an imaging probe (e.g. a gamma or positron-emitting radionuclide, or an optically active molecule), to visualize the in vivo distribution of target cell surface receptors.
Advances in conjugation chemistry and protein engineering have enabled development of homogenous radiolabelled antibodies. New site-specific antibody modification strategies that have not yet been applied to PET/SPECT imaging with radiolabelled antibodies could similarly be adapted to generate well-defined chelator-antibody constructs.
Several preclinical studies have highlighted that site-specifically modified antibodies have improved in vivo behavior (higher affinity for target receptors, lower off-target accumulation/persistence, better conjugate stability) relative to those modified using conventional, less specific methods.
Site-specifically radiolabelled antibody-based radiopharmaceuticals will deliver new clinically-useful contrast agents for molecular PET/SPECT imaging, by (i) providing clinicians with better molecular imaging tools to predict whether a patient will respond to a particular treatment or intervention, and (ii) providing scientists with reliable tools to quantitatively map the in vivo behaviour of new antibody-based therapies and/or newly-discovered receptors that are drug targets.
Such technology will also be critically important in providing antibody conjugates with precisely defined structures and stoichiometries that are acceptable to regulatory authorities.