**8. Drug delivery with the help of empty virus capsid**

The viral nanoparticles (VNPs) consist of protein core which ranges in complexity from small capsid-protein homomers to larger protein-based heteromers capable of internalizing oligo‐ nucleotides and being enveloped by lipids. Chemical modification process and genetic mutation provide the viral coat proteins with receptor binding domain that helps in cell specific targeting of VNPs [69]. Even fusion of terminal / internal proteins on the surface or inside the VNPs can be utilized for introduction of heterologous peptides, and in some cases entire proteins. VNPs can be genetically engineered by inserting amino acids for bioconjugation, peptide based affinity tags and peptides as targeting ligands for stimulation of immune response. [70].

High sequence variability due to the influence of the immune system in viral life-cycles is often seen on the surface loops of viral capsid proteins. This variability makes the loops highly susceptible to insertion of foreign sequences. VP1, the major coat protein of viruses of Polyo‐ maviridae family, when expressed in insect cells, yeast and *Escherichia coli* self-assembles as protein cages and shows natural affinity for a cell surface glycoprotein with a terminal a 2,3 linked N-acetylneuraminic acid and attaches to a4h1-integrin receptors [71]. Virus like particle (VLPs) constructed from the virus are used to deliver therapeutic genes to human fetal glial cells. Another technique Cell-docking involves attachment of antibodies to the surface of brain natriuretic peptide (BNPs). Coupling reaction between murine polyoma-virus and antitumor antibody B3 yielded polyoma VLPs with 30 to 40 antibody fragments bound to the surface, allowing the modified VLPs to bind to the breast carcinoma cells with high efficiencies [72].
