Therapeutic biologics (TBs), which represent diverse entities of biologics, including antibodies, antibody-drug conjugates (ADCs), therapeutic replacement enzymes, peptides, small interfering RNAs, and antisense oligonucleotide (ASO) drugs have revolutionized the treatments of a wide range of diseases. TBs are receiving major attention in drug development pipelines. Generally, TBs perform highly specific and complex functions with low off-target toxicity, the features that could be challenging for small molecule drugs. Since the introduction of insulin as the first TB drug, the progression of novel class entities from relatively small and native proteins to the next generation TBs with more and more complex structures, such as conjugated peptides, fusion proteins, and ADCs, has introduced many new challenges related to the characterization of catabolism, biotransformation, distribution, and elimination. In addition, the advancement in protein engineering further diversifies the group of available TB molecules by introducing bispecific antibodies, multifunctional antibody-based biologic therapeutics, and non-antibody-based nanobodies (e.g., caplacizumab, which contains only a single-domain fragment of an antibody).The distribution and elimination of TBs are determined by target-mediated and/or non-target-mediated processes, as well as physiochemical properties, such as molecule sizes and surface charges. Renal filtration can be dominant for TBs with small molecular weights, e.g., <50 kDa, and large net positive charges. Antibodies with a molecular weight of greater than 150 kDa are limited to the intravascular space, with little distribution to the interstitial space of extravascular organs and tissues. The elimination of TBs is commonly facilitated by nonspecific proteolytic degradation in lysosomes. Cellular uptake of antibodies can be mediated by either target-mediated internalization or non-target-mediated pinocytosis, followed by intracellular lysosomal degradation to small peptides and amino acids. The rate of target-mediated drug disposition (TMDD) is related to the expression of the target antigen, the affinity of antigen bound, the rate of internalization, and the extent of lysosomal catabolism. TMDD is more efficient than pinocytosis and can be a major elimination process for therapeutical proteins (TPs). In addition, many antibodies and their derivatives can interact with immunoglobulin-specific receptors, such as the neonatal Fc receptor (FcRn) and Fcc receptor. These receptors can prevent IgG antibodies from proteolytic degradation and reduce the systemic clearance (CL) of the TPs (Ryman and Meibohm, 2017).The size, charge, and modification of TBs need to be taken into consideration in the prediction of elimination pathways. Since the appropriate tools to characterize absorption, distribution, metabolism, and excretion (ADME) of TBs are still lacking, characterizing the exposure, metabolic or catabolic biotransformation, TMDD, and elimination of TBs in the preclinical phase is more difficult as compar...
