Site-specific bioconjugation and nuclear imaging.

Monoclonal antibodies and antibody fragments have proven to be highly effective vectors for the delivery of radionuclides to target tissues for positron emission tomography (PET) and single-photon emission computed tomography (SPECT). However, the stochastic methods that have traditionally been used to attach radioisotopes to these biomolecules inevitably produce poorly defined and heterogeneous probes and can impair the ability of the immunoglobulins to bind their molecular targets. In response to this challenge, an array of innovative site-specific and site-selective bioconjugation strategies have been developed, and these approaches have repeatedly been shown to yield better-defined and more homogeneous radioimmunoconjugates with superior in vivo performance than their randomly modified progenitors. In this Current Opinion in Chemical Biology review, we will examine recent advances in this field, including the development - and, in some cases, clinical translation - of nuclear imaging agents radiolabeled using strategies that target the heavy chain glycans, peptide tags, and unnatural amino acids.