Novel targeted therapies are rapidly emerging for the treatment of cancer. With the advent of new immune targeting agents, understanding the changes in the tumor microenvironment (TME) is critical. Given the complexity and several cellular mechanisms and factors that play a role in the TME, novel imaging methods to assess and evaluate the dynamic changes in the TME during treatment are needed. Several techniques are being developed for imaging TME including optical, fluorescence and photoacoustic methods. Positron emission tomography (PET) imaging can be used to track the dynamics of different molecular targets in the TME in live animals and in humans. Several novel PET imaging probes including radiolabeled antibodies, antibody fragments, and small molecules have been developed with many more that are under development preclinically and in early human studies. This review is a brief overview of some of the PET agents that are either in the preclinical developmental phase or undergoing early clinical studies.

Site-specific protein labeling is important in biomedical research and biotechnology. While many methods allow site-specific protein modification, a straightforward approach for efficient N-terminal protein labeling is not available. We introduce a novel sortase-mediated swapping approach for a onestep site-specific N-terminal labeling with a near-quantitative yield. We show that this method allows rapid and efficient cleavage and simultaneous labeling of the N or C termini of fusion proteins. The method does not require any prior modification beyond the genetic incorporation of the sortase recognition motif. This new approach provides flexibility for protein engineering and site-specific protein modifications.