Sortagging of liposomes with a murine CD11b-specific VHH increases in vitro and in vivo targeting specificity of myeloid cells

The therapeutic index of drugs can be increased via drug encapsulation in actively targeted, meaning ligand modified drug delivery systems. The manufacturing of such targeted drug delivery systems, in particular the conjugation between drug carrier and ligand, can be done by enzymatic conjugation methods, exploiting the site-specific, bioorthogonal nature of these reactions. The use of such enzymes like Sortase-A transpeptidase requires efficient purification methods, as residuals of the enzyme may be responsible for immunogenic potential and drug product instabilities. These instabilities may be based on the enzymatic reverse reaction, meaning here a cleavage between ligand and drug carrier. In the presented work, two differently PEGylated formulations were modified with variable fragments of camelid heavy chain-only antibodies (VHH) via Sortase-A, purified by different methodologies and tested for ligand cleavage upon storage. Strongly PEGylated liposomes (PEGhigh-LS) were found to retain higher amounts of Sortase-A than lowly PEGylated ones (PEGlow-LS) after dialysis purification. Surprisingly, this did not correlate with ligand stability during storage. PEGhigh-LS were less prone for degradation, compared to PEGlow-LS, which showed a ligand cleavage of 20% after an 8 weeks storage at 2-8 °C. Nonetheless, overall degradation could be minimized by an additional affinity bead purification procedure. Liposomes modified with a CD11b-specific VHH were tested for their in vitro and in vivo targeting ability towards CD11b+ cells. Specific targeting of CD11b was achieved in vitro and in vivo on various cell types. PEGylation decreased the targeting effect in vitro, however no differences between PEGhigh or PEGlow formulations were observed in vivo. The obtained results underline the need for a thorough characterization of novel conjugation strategies as well as an early in vivo characterization of such targeted drug delivery systems.