ACS APPL MATER INTER

Among bio-inspired protein materials, secretory proteinmicroparticlesare of clinical interest as self-contained, slow protein deliveryplatforms that mimic secretory granules of the human endocrine system,in which the protein is both the drug and the scaffold. Upon subcutaneousinjection, their progressive disintegration results in the sustainedrelease of the building block polypeptides, which reach the bloodstreamfor systemic distribution and subsequent biological effects. Suchentities are easily fabricated in vitro by Zn-assistedcross-molecular coordination of histidine residues. Using cationicZn for the assembly of selected pure protein species and in the absenceof any heterologous holding material, these granules are expectedto be nontoxic and therefore adequate for different clinical uses.However, such presumed biosafety has not been so far confirmed andthe potential protein dosage threshold not probed yet. By selectingthe receptor binding domain (RBD) from the severe acute respiratorysyndrome coronavirus 2 (SARS-CoV-2) spike protein as a model proteinand using a mouse lab model, we have explored the toxicity of RBD-madesecretory granules at increasing doses up to & SIM;100 mg/kg ofanimal weight. By monitoring body weight and biochemical blood markersand through the histological scrutiny of main tissues and organs,we have not observed systemic toxicity. Otherwise, the bioavailabilityof the material was demonstrated by the induction of specific antibodyresponses. The presented data confirm the intrinsic biosafety of artificialsecretory granules made by recombinant proteins and prompt their furtherclinical development as self-contained and dynamic protein reservoirs.