Mass spectrometric analysis of protein species of biologics

The global market for therapeutic proteins has been growing fast in the past decade. In comparison to conventional small-molecule drugs therapeutic proteins have a very complex chemical composition. These proteins are typically comprised of dozens to hundreds of amino acids which often are post-translational modified. Recombinant therapeutic proteins are produced by living organisms resulting in a heterogeneous mixture of recombinant protein species (isoforms, proteoforms). The pharmaceutical function and biological and toxicological properties of biologics and follow-on biosimilars are critically related to their exact chemical composition. Therefore, analytical tools capable of identifying and quantifying therapeutic protein species and their accompanying species, often differing in a few chemical moieties only, are necessary to guarantee efficacy and safety of therapeutic proteins. In this review we focus on the application of mass spectrometry for analyzing the exact chemical composition of therapeutic protein species, highlighting especially top-down mass spectrometry.