Polymeric dipicolylamine based mass tags for mass cytometry

Mass cytometry is an emerging powerful bioanalytical technique for high-dimensional single-cell analysis. In this technique, cells are stained with metal-isotope-tagged antibodies and are analyzed by an inductively coupled plasma time-of-flight mass spectrometer. While there are more than 100 stable isotopes available in the m/z 75 to 209 detection range of the instrument, only about 50 parameters can be measured per cell because current reagents are metal-chelating polymers with pendant aminocarboxylate chelators that only bind hard metal ions such as the rare earths and Bi(3+). Here we describe the synthesis and characterization of a new type of metal-chelating polymer with pendant dipicolylamine chelators suited to binding intermediate to soft metals such as rhenium and platinum. We introduce two different conjugation strategies, a thiol–maleimide reaction that works well for rhenium, and a DBCO-azide click reaction designed to avoid potential complications of Pt and other heavy metals interacting with thiol groups. We show that these polymers can serve as new elemental mass tags for mass cytometry. Antibody-polymer conjugates of CD20 and CD8a prepared by both coupling reactions were employed in conjunction with commercial metal-conjugated antibodies for multi-parameter single-cell immunoassays.