Direct Reversible Conjugation of an Adenosine Receptor Antagonist to an Axonal Transport Protein Carrier

This paper describes gold-free central nervous system (CNS) drug conjugation with a neural tracing protein, which represents a significant advance toward clinical relevance of the underlying blood-brain barrier (BBB)-bypassing drug delivery nanotechnology. Retrograde neural tracing proteins, such as wheat germ agglutinin (WGA), have been widely used for histochemical staining to map neuronal connections between peripheral nerve terminals and CNS neurons. Here, we demonstrate that WGA on its own can simultaneously function as a nanocarrier, transporter, and targeting agent by its direct chemical conjugation to a CNS drug, dipropylcyclopentyl xanthine (DPCPX), using minimal synthetic steps and reagents without requiring an additional nanoparticle linker. DPCPX, an A₁ receptor antagonist, has potential for treating spinal cord injury (SCI)-associated breathing dysfunction. Chemical characterization of the protein-drug nanoconjugate revealed selective conjugation predominantly at arginine residues, with some modifications at lysine residues, providing insights into the stability of protein-aldehyde linkages. We then evaluated the nanoconjugate's in vitro properties, including drug release kinetics, cytotoxicity, and cellular internalization in differentiated NSC-34 cells. Our findings indicate a slow drug release profile, ensuring that the drug remains conjugated to the WGA carrier during retrograde transport and reaches the relevant neural sites in the spinal cord and brainstem, where it can induce a compensatory recovery mechanism via adenosine receptor antagonism.