Engineering protein nanocages as carriers for biomedical applications

Bhaskar, Sathyamoorthy; Lim, Sierin

Environmental parameters such as temperature and pH have been used to control peptide self-assembly. 6 Additional functional ligands can be incorporated into naturally existing protein nanocages by genetic engineering and chemical modification techniques. Genetic engineering of the protein nanocages is achieved by recombinant DNA technologies. They are produced in small and large scales using both microbial and non-microbial systems as the cell factories. Bacterial cells, in particular, are popular hosts for producing a variety nanomaterials, including those for medical applications. 6 Chemical modifications can be used to prevent the interbatch variability of protein production observed in recombinant DNA technologies. 1 The Francis group has developed new methods for chemically modifying proteins that may be applicable to protein nanocages. 80 Application of protein nanocages as drug/gene carrier By leveraging on facile genetic or chemical modifications to promote molecular attachment and encapsulation, protein nanocages can be engineered to carry drugs, genes, imaging contrast agents or other active molecules. To date, small molecules such as doxorubicin, bleomycin, cisplatin, carboplatin, nucleic acids DNA/RNAs, peptides and proteins such as Cre recombinase, capase-8, interferon-γ and interleukin-2 have been delivered in vitro, whereas paclitaxel, doxorubicin, interleukin-2, CD40L, small interfering RNA, microRNA and CpG oligos have been applied in vivo as therapeutics using protein nanocages. 8 Covalent attachment of small molecules to the interior of protein nanocages prevents leakage, protects the drugs by preventing degradation and prevents nonspecific interaction. For instance, the low endosomal pH of the cell could be used as a trigger to release the encapsulated drug. The heat-shock protein cage interior and E2 protein are modified by genetically inserting cysteine residues at the interior surface for selective attachment of antitumor drugs with a pH-sensitive linker. 68, 81 The attachment of 6-maleimidocaprol, a hydrazone derivative of doxorubicin, to the inner surface of cysteine-modified heat-shock protein and E2 protein nanocages allows for the selective release of the drug under acidic conditions through the hydrolysis of the hydrazone linkage. Ferritin has been used to encapsulate the platinum-based anticancer drugs cisplatin and carboplatin. 82 Drugs encapsulated within a protein nanocage have been reported to retain their efficacy and exert cytotoxic effects on cancer cells. However, efficacy comparisons between the encapsulated drugs and free drugs and their effects on healthy cells are still under investigations.