Development of Novel Glycopeptide-based Cancer Immunotherapies
by David McDonald and Richard Payne
Over 120,000 Australians are diagnosed with cancer each year, and more than 43,000 died of cancer in 2011 alone. Our laboratory is among a number around the world working towards finding better treatments for cancer sufferers, and one of nearly 500 supported by Cure Cancer Australia, an initiative that began in 1967 to provide research support to Australian laboratories searching for cures for cancer.
Traditionally, most non-surgical interventions for the treatment of cancer have involved the use of cytotoxic agents, attempting to target cancer cells by their highly proliferative nature. Over the past few decades, however, new strategies have emerged including immunotherapeutic approaches. The concept of harnessing the immune system of patients to target and eradicate tumour cells via vaccination represents an extremely attractive strategy for the treatment of cancer. Vaccines offer several advantages over traditional treatments, including low toxicity, exquisite selectivity, the potential for long term therapeutic effects through immunological memory and the potential to overcome drug resistance. One of the main difficulties with this approach is that, because cancer cells are nearly identical to healthy cells, the vast majority of proteins expressed by cancer cells are also present in healthy tissue. In order to create a useful vaccine we must therefore utilise proteins that are either unique or highly over-expressed in tumours.
During cancer progression, the expression of glycosyltransferase enzymes that are responsible for the transfer of carbohydrates ("glycans") onto proteins becomes dysregulated, such that certain glycosyltransferases are highly over-expressed while others are under-expressed. As a result of this change, proteins on the surface of cancer cells tend to display unusual glycosylation patterns. This includes highly truncated carbohydrate chains which, when linked to serine or threonine amino acids in proteins, are known as Tumour-Associated Carbohydrate Antigens (TACAs) (see Figure 1 for examples). Since these glycans are rarely displayed on cells of healthy tissue, they have attracted significant attention for use as cancer-specific antigens in vaccine candidates. Our laboratory is particularly interested in a protein called mucin 1 (MUC1), a transmembrane protein normally expressed in epithelial tissue. MUC1 possesses a polymeric extracellular variable number tandem repeat (VNTR) domain of 20 amino acids (GVTSAPDTRPAPGSTAPPAH, amino acids in one letter code) that can be repeated up to 120 times on the cell surface. Each VNTR domain possesses five potential glycosylation sites (underlined) that can be occupied by a TACA. MUC1 is highly over-expressed in many forms of cancer, including carcinomas of the breast, colon, pancreas, prostate, ovary, rectum and stomach. The MUC1 proteins in these cancers are also decorated with TACAs in the VNTR domains. The aberrant glycosylation and over-expression of MUC1 is strongly correlated with tumour metastasis and poor prognosis. For this reason, glycopeptide fragments of the VNTR sequence bearing TACAs have emerged as promising targets for the development of synthetic cancer vaccines. Indeed MUC1 was ranked 2nd on a list of the top priority cancer antigens in 20112.
Our vaccine design strategy involves the incorporation of synthetic MUC1 VNTR glycopeptides (containing a variety of TACAs) which are conjugated to a foreign peptide called a T cell helper epitope (included to stimulate immune cells called T cells) and a lipopeptide (called an immunoadjuvant) which stimulates pattern recognition receptors on immune cells, which then activate the immune system to recognise and attack the cancer-associated MUC1 antigen. Our approach has been to produce each fragment of the vaccines by Fmoc-strategy solid-phase peptide synthesis, using synthetic glycosylamino acid building blocks to incorporate the TACAs into the MUC1 VNTR sequence. These fragments can then be joined together to afford pure self-adjuvanting, tri-component vaccine candidates, where each of the components necessary to stimulate an anti-tumour immunological response are covalently linked (Figure 2). Using this approach, we have generated two different libraries of vaccine candidates to date2. These candidates have been subsequently evaluated in preliminary immunological studies. They were shown to stimulate the production of high-affinity, class-switched IgG serum antibodies capable of selectively binding MUC1 on the surface of breast cancer cells decorated with TACAs, and not on healthy mammary cells that display a different glycosylation pattern.
We have very recently developed novel and efficient synthetic routes to the glycosylamino acid building blocks required to incorporate TACAs that contain sialic acid (e.g. sialyl TN and 2,6-sialyl T in Fig. 1) into vaccine constructs3. With this synthetic technology in hand, we are now equipped to expand our range of self-adjuvanting vaccines into a library of novel vaccine candidates. It is envisaged that the preparation of a larger vaccine library, possessing a range of adjuvants and antigens, will facilitate the immunological characterisation of novel immunotherapies. Crucially, the synthetic methods used will enable the preparation of homogeneous macromolecular vaccines, which is important for the development of therapeutics, including better methods of manufacture and acceleration of clinical approval. We are ideally positioned and committed to the development of immunotherapies which are capable of preventing or reversing tumours in model systems. Our upcoming studies will lay the foundations for translation of these vaccine candidates into pre-clinical studies in the future, which is the ultimate goal of our research program that is gratefully supported by a Priority Driven Young Investigator Grant, supported by RAMS and Cure Cancer Australia.
*David McDonald is a PhD student working in the Payne Research Group.
- Cheever, M.A; Allison, JP; Ferris, AS; Finn, OJ; Hastings, BM; Hecht, TT; Mellman, I; Prindiville, S. A; Viner, JL; Weiner, LMand Matrisian, LM. The prioritization of cancer antigens: A national cancer institute pilot project for the acceleration of translational research. Clin. Cancer Res. 15, 5323-5337, 2009.
- Wilkinson, BL; Day, S; Malins, LR; Apostolopoulos, V and Payne, RJ. Self-adjuvanting multicomponent cancer vaccine candidates combining per-glycosylated MUC1 glycopeptides and the toll-like receptor 2 agonist Pam3CysSer. Angew. Chem. Int. Ed. 50, 1635-1639, 2011.
- Wilkinson, BL; Day, S; Chapman, R; Perrier, S; Apostolopoulos, V and Payne, R. J. Synthesis and immunological evaluation of self-assembling and self-adjuvanting tricomponent glycopeptide cancer-vaccine Candidates. Chem. Eur. J. 18 (51), 16540-16548, 2012.
- Corcilius, L and Payne, RJ. Stereoselective synthesis of sialylated tumor-associated glycosylamino acids. Org. Lett., 15 (22), 5794–5797, 2013.
|A/Prof Richard Payne|
Richard J. Payne graduated with a PhD from the University of Cambridge in 2006. After a period as a Lindemann Fellow at The Scripps Research Institute (La Jolla) he began his independent career (in January 2008) as a Lecturer in Organic Chemistry and Chemical Biology at The University of Sydney. Associate Professor Payne's research focusses on utilising the power of synthetic organic chemistry to interrogate biological systems and address problems of medical significance. As a result of his research endeavours he has been the recipient of several prestigious awards including the RACI Biota Medal in Medicinal Chemistry (2008), a NSW Young Tall Poppy Science Award (2010), the Rennie Memorial Medal (2012), the Athel Beckwith Lectureship (2013), the Tregear Award for Peptide Science (2013), an ARC Future Fellowship (2013) and the Le Févre Memorial Prize.
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