Significant efforts are made to characterize molecular liabilities and degradation of the drug substance (DS) and drug product (DP) during various product life-cycle stages. The in vivo fate of a therapeutic protein is usually only considered in terms of pharmacokinetics (PKs) and pharmacodynamics (PDs). However, the environment in the human body differs substantially from that of the matrix (formulation) of the DP and may impact on the stability of an injected therapeutic protein. Stabilizing excipients used in protein formulations are expected to undergo more rapid distribution and dissociation in vivo, compared to a protein as a highly charged macromolecule. Thus, in vivo stability may significantly differ from shelf-life stability. In vivo degradation of the therapeutic protein may alter efficacy and/or safety characteristics such as immunogenicity. Studying the stability of a therapeutic protein in the intended body compartment can de-risk drug development in early stages of development by improving the selection of better clinical lead molecules. This review assesses the considerations when aiming to evaluate the in vivo fate of a therapeutic protein by comparing the physiology of relevant human body compartments and assessing their potential implications on the stability of a therapeutic protein. Moreover, we discuss the limitations of current experimental approaches mimicking physiologic conditions, depending on the desired route of administration, such as intravenous (IV), subcutaneous (SC), intravitreal (IVT), or intrathecal (IT) administration(s). New models more closely mimicking the relevant physiologic environment and updated analytical methods are required to understand the in vivo fate of therapeutic proteins.
Keywords: developability; human body fluids; in vitro model; in vivo protein stability; therapeutic protein.