Enteric pharmacokinetics of monomeric and multimeric camelid nanobody single-domain antibodies

Single-domain antibodies (sdAbs) derived from Camelidae heavy-chain-only antibodies (also called nanobodies or VHHs) have advantages over conventional antibodies in terms of their small size and stability to pH and temperature extremes, their ability to express well in microbial hosts, and to be functionally multimerized for enhanced properties. For these reasons, VHHs are showing promise as enteric disease therapeutics, yet little is known as to their pharmacokinetics (PK) within the digestive tract. To improve understanding of enteric VHH PK, we investigated the functional and structural stability of monomeric and multimeric camelid VHH-agents following in vitro incubation with intestinal extracts (chyme) from rabbits and pigs or fecal extracts from human sources, and in vivo in rabbits. The results showed that unstructured domains such as epitopic tags and flexible spacers composed of different amino acid sequences were rapidly degraded by enteric proteases while the functional core VHHs were much more stable to these treatments. Individual VHHs were widely variable in their functional stability to GI tract proteases. Some VHH-based agents which neutralize enteric Shiga toxin Stx2 displayed a functional stability to chyme incubations comparable to that of Stx2-neutralizing IgG and IgA mAbs, thus indicating that selected nanobodies can approach the functional stability of conventional immunoglobulins. Enteric PK data obtained from in vitro incubation studies were consistent with similar incubations performed in vivo in rabbit surgical gut loops. These findings have broad implications for enteric use of VHH-based agents, particularly VHH fusion proteins.