By Jessica Lu
Bacteriophages (phages) are viruses that specifically infect and replicate in bacterial cells. Although they are bacterial viruses, phages may be used in promising new methods to fight cancer because they are easy to genetically modify and are generally regarded as non-pathogenic (Abbaszadeh et al., 2021). These methods to fight cancer generally use a molecular biology technique called phage display. In phage display, phage genomes are modified so that desired proteins are expressed on the phage capsid (Fermin, Rampersad & Tennant, 2018). Phage display has great potential to be used in both cancer diagnosis and cancer treatment (Abbaszadeh et al., 2021).
Phages could be used to aid cancer diagnosis. Traditional cancer diagnosis using tumour biomarkers is inefficient and costly. To improve cancer identification, original tumour-targeting agents are needed (Abbaszadeh et al., 2021). Phages can act as tumour targeting agents if they are modified to display tumour-targeting peptides. Once targeted to the tumour, these phages can be used as molecular imaging probes (Abbaszadeh et al., 2021). For example, screening using a M13 phage library displaying random peptides found that the peptide sequence CAKATCPAC is specific towards human lung adenocarcinoma (Lee et al., 2016). By labelling the phages fluorescently, this allowed in vivo NIR fluorescent optical imaging in mice implanted with tumours. This kind of molecular imaging is advantageous because it allows rapid diagnosis and observation of cancer progression, making personalised care possible (Lee et al., 2016).
To treat cancer, phages can be used for immunotherapy (Abbaszadeh et al., 2021). Using phage display, antigens can be delivered to and recognised by immune cells. This can result in an intense, specific immune response against tumour cells (Hess & Jewell, 2019). One study used phages to stimulate invariant natural killer T cells (iNKT). iNKT cells function in both innate and adaptive immunity. They recognise lipid antigens and have shown anti-tumour potential. Alpha‐GalactosylCeramide (α‐GalCer) is an antigen recognised by iNKT cells that allows them to display anti-tumour effects (Zhang et al., 2019). One group conjugated the antigen α‐GalCer to filamentous fd phages. Therapeutic treatment using these phages in a mouse melanoma model led to delayed tumour growth and increased survival (Hess & Jewell, 2019).
Another strategy for cancer treatment is to use phages as delivery systems for chemotherapy drugs (Abbaszadeh et al., 2021). Phages such as the M13 phage can penetrate the gastrointestinal mucosal barrier, making oral delivery of a drug possible (Abbaszadeh et al., 2021). Displaying a class of peptides called cell-penetrating peptides (CPPs) on the surface of phages helps them internalise into cells through either receptor-mediated endocytosis or receptor-independent endocytosis. For example, screening phage libraries has identified two H1299 non-small cell lung cancer CPPs which facilitate internalisation via two different mechanisms of endocytosis (Ju & Sun, 2017). A proof-of-concept study conjugated the drug hygromycin to filamentous phages by a covalent amide bond. Anti ErbB2 and anti ERGR antibodies were used as targeting moieties. Treatment with the drug carrying phages led to growth inhibition of cancer cell lines in vitro with a potentiation factor of >1000 over the free drug (Bar, Yacoby & Benhar, 2008).
Phage therapy can also be combined with gene therapy, as tested on mice with glioblastoma (Przystal et al., 2019). Glioblastoma is the most common and deadliest primary brain tumour. It is commonly treated with temozolomide (TMZ), a chemotherapy drug that is well-tolerated but has limited efficacy. Grp78 is a protein produced in response to TMZ (Przystal et al., 2019). The phages were genetically modified to display the RGD4C ligand that binds to the αvβ3 integrin receptor, a receptor that is overexpressed on tumour cells. This allows phages to bind and specifically deliver a therapeutic AAV genome to the tumour cells. This AAV genome contained the gene encoding Herpes simplex virus type I thymidine kinase (HSVtk) under a Grp78 promoter. Thus, when tumour cells are treated with TMZ, Grp78 is produced, resulting in HSVtk expression. If cells are also treated with the nucleoside analogue prodrug ganciclovir (GCV), HSVtk phosphorylates GCV, transforming GCV into a nucleoside analogue triphosphate. GCV is subsequently incorporated into the tumour cell genome, inhibiting DNA polymerase and causing cell death by apoptosis (Przystal et al., 2019). When glioblastoma cells were implanted with mice, no mice survived beyond 38 days beyond implantation in the control group, whereas 80% survived past 62 days with this phage therapy treatment (Przystal et al., 2019).
Overall, the use of phages is very promising in the early detection and treatment of cancer, leading to better outcomes. Unlike many current treatments, phages act in a cell-specific manner, reducing the side effects of treatment. Phages may be a more harmless and lower cost alternative to current treatment methods (Abbaszadeh et al., 2021). To take full advantage of phage therapy for clinical use, more studies are needed in the future to better understand and optimise its applications.
Abbaszadeh, F., Leylabadlo, H. E., Alinezhad, F., Feizi, H., Mobed, A., Baghbanijavid, S. & Baghi, H. B. (2021) Bacteriophages: cancer diagnosis, treatment, and future prospects. Journal of Pharmaceutical Investigation. 51 (1), 23-34. Available from: doi: 10.1007/s40005-020-00503-x.
Bar, H., Yacoby, I. & Benhar, I. (2008) Killing cancer cells by targeted drug-carrying phage nanomedicines. BMC Biotechnology. 8 (1), 37. Available from: doi: 10.1186/1472-6750-8-37.
Fermin, G., Rampersad, S. & Tennant, P. (2018) Chapter 12 – Viruses as Tools of Biotechnology: Therapeutic Agents, Carriers of Therapeutic Agents and Genes, Nanomaterials, and More. In: Tennant, P., Fermin, G. & Foster, J. E. (eds.). Viruses. Academic Press. pp. 291-316.
Hess, K. L. & Jewell, C. M. (2019) Phage display as a tool for vaccine and immunotherapy development. Bioengineering & Translational Medicine. 5 (1), Available from: doi: 10.1002/btm2.10142.
Ju, Z. & Sun, W. (2017) Drug delivery vectors based on filamentous bacteriophages and phage-mimetic nanoparticles. Drug Delivery. 24 (1), 1898-1908. Available from: doi: 10.1080/10717544.2017.1410259.
Lee, K. J., Lee, J. H., Chung, H. K., Ju, E. J., Song, S. Y., Jeong, S. & Choi, E. K. (2016) Application of peptide displaying phage as a novel diagnostic probe for human lung adenocarcinoma. Amino Acids. 48 (4), 1079-1086. Available from: doi: 10.1007/s00726-015-2153-4.
Przystal, J. M., Waramit, S., Pranjol, M. Z. I., Yan, W., Chu, G., Chongchai, A., Samarth, G., Olaciregui, N. G., Tabatabai, G., Carcaboso, A. M., Aboagye, E. O., Suwan, K. & Hajitou, A. (2019) Efficacy of systemic temozolomide-activated phage-targeted gene therapy in human glioblastoma. EMBO Molecular Medicine. 11 (4), e8492. Available from: doi: 10.15252/emmm.201708492.
Zhang, Y., Springfield, R., Chen, S., Li, X., Feng, X., Moshirian, R., Yang, R. & Yuan, W. (2019) α-GalCer and iNKT Cell-Based Cancer Immunotherapy: Realizing the Therapeutic Potentials. Frontiers in Immunology. 10 Available from: doi: 10.3389/fimmu.2019.01126.