Recent development of anticancer vaccines: Targeting tumor associated-carbohydrate antigens

Research conducted by the Department of Chemistry at Michigan State University

Traditional methods of treatment against cancer

Chemotherapy

Radiation

Surgery

These methods have been successful, but only to a certain extent and cancer– a very harmful disease, still remains difficult to cure and prevent. It is imperative that we find an alternative

approach to treating cancer.

Idea for vaccines against cancer and targeting TACAs

Anticancer vaccines and using immunotherapeutic methods (targeting antibodies/antigens) have seemed like an attractive alternative approach

Antibody: Protein used by the immune system to recognize/identify and neutralize pathogens like bacteria and viruses.

Antigen: Substance (foreign or anything else) causing the immune system to produce antibodies against it.

1) Much success has come with the use of vaccines against many infectious diseases

Why don’t we try that with cancer?

2) The power of the body’s immune system should be utilized to fight cancer

What are TACAs? Tumor associated-carbohydrate antigens that are present on the surface of

cancer cells, and can be divided into glycolipid and glycoprotein antigens

Glycolipid: Carbohydrates connect to waxy lipids and attach to the lipid bilayer on the cell surface through hydrophobic reactions.

Glycoprotein: Carbohydrates attach to proteins by attaching to a hydroxyl group.

o Expressed more highly on malignant cells and among most frequently found antigens on cancer cells

o Shared by many cancer-cell types (e.g. melanoma, sarcomas),variations of them are expressed on various epithelial cancers (e.g. breast, prostate)

Vaccines against TACAs could make it possible to use one medium for targeting different cancers

TACAs expressed on cancer stem cells

Have been observed to be largely expressed on cancer stem cells A study showed that Globo-H, a glycoprotein TACA, was

expressed in 8 of 40 (20%) breast cancer stem cells

These stem cells have characteristics of self-renewal and can regenerate/ differentiate into several cell types in specific cancers In this view, scientists have proposed that they should

be targeted to prevent the re-emergence of cancer

Challenges with developing anticancer vaccines targeting TACAs:

I. Isolating enough quantities of TACAs from natural resources because cell surface carbohydrates are heterogeneous (in different chemical phases)

o Possible for highly active trace contaminantso Synthesis is common method for getting enough

amounts of TACA in pure formo though, somewhat of a tedious and large-scale

preparation

II. The low immunogenicity of TACAs o TACAs are also strongly expressed in small amounts on

normal cells --- making them seem like self-antigens to the immune system

o Only weakly activate B cells* when administered alone--- need to share a link to an immunogenic carrier to fully activate it

*B cell: Type of white blood cell some producing antibodies to fight infections, and turning into memory cells to recognize and bond particular antigens.

Modification of TACAs: N-Glycolylneuraminic acid (Neu5Gc) GM3

One of the two common forms of sialic acid* in mammalian cells, in addition to Neu5AC GM3

Humans usually lack Neu5Gc acid, due to deletion of the Cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH*) gene--- unlike some other mammals (sheep, horse, goat)

Neu5c GM3 is actually largely expressed in human cancer cells (e.g. breast, melanoma) Acid’s role in tumor cell biology is still being researched, though,

a recent finding revealed that it lowered cluster of differentiation 4 ( CD4*) expression in mice and T lymphocytes*

Patients with anti- Neu5Gc antibody had increased 5-year survival rates

Neu5Ac GM3’s low immunogenicity makes Neu5Gc GM3’s tumor-specific character a cutting-edge target for the vaccine(s) development

• Sialic acid: Sugar involved in various cellular and regulation processes

• CMAH gene: Gene that encodes for enzyme responsible for synthesis of Neu5Gc

• CD4: Glycoprotein assisting in communication with antigen-presenting cell.

• T lymphocytes: White blood cells with a central role in adaptive immunity

Synthetic Modifications of TACAs

The glycosidic* bonds in TACAs can possibly be affected by acid-catalyzed hydrolysis and enzymatic degradation within living cells

To improve the antigens’ stability C-linked STn (glycoprotein antigen) and S-linked GM2/GM3 (ganglioside/hydroxy acid) were considered as substitutes of the corresponding native O-linked TACAs The Ye lab in Colorado prepared 40 STn analogs and

assessed the immune responses in mice after linking with Keyhole limpet hemocyanine (KLH) They found that the fluorine- containing STn compounds

had great increases in antibody production and cross-reacted well with STn-expressing tumor cells

*glycosidic: Covalent bond joining a carbohydrate molecule to another molecule that may not be another carb.

Immunogenic platforms for TACA delivery: Globo H- based vaccine

Globo-H:

- A globo-series glycolipid that’s a TACA and highly expressed on several tumors including those of the breast, lung, colon, ovary, pancreas, and prostate

- Expressed in the inner surface of normal epithelial cells, but the location inhibits its access by the immune system. This restricted distribution makes it seem like a strong target for anticancer vaccine development.

Globo H-based vaccine:

• In one clinical trial with five patients that had ongoing and recurring prostate cancer, a Globo H- KLH construct was immunized into them and the result was that they all generated strong igM* responses—two of them even showing high igG* concentrations

• Another clinical trial yielded similar antibody responses and demonstrated the safety and lack of toxicity of the vaccine; a third of them showed a decline in PSA concentration

*igM: Antibody produced in B cells and first to respond to initial exposure to an antigen

*igG: Main type of antibody in blood that can control infection of body tissues

Virus-Like Particle-Based TACA Delivery

Virus-like particles (VLPs) are capsids from viruses made of structural proteins that are self-assembled very orderly

Since B-cells respond well to highly organized antigens, researchers hypothesized that TACAs could be displayed on the surface of VLPs and then cross-link B-cell receptors* and lead to strong B-cell activation

In one study, Cowpea Mosaic Virus (CMPV) VLP was developed as a carrier to increase the immune responses to TACAs Preliminary results showed that there were high

concentrations of specific antibodies generated against TACAs with CMPV as a carrier

*B-cell receptors: Proteins on the outer surface of B cells

Conclusions

Over the past few decades, there’s been much progress with developing carbohydrate-based anticancer vaccines and using TACAs to do so

There have been many approaches and strategies for solving the challenges associated with targeting TACAs for anticancer vaccines and success that’s resulted from that

A lot of the studies have looked at the antibody concentrations generated and have contributed information on the immune responses

However, to move further in pursuing this, there must be more success established in the constructs of tumor protection and treatment

Sources and references

http://www.tandfonline.com/doi/full/10.1080/07328303.2012.659364#.VZYFHe1Viko

http://grantome.com/grant/NIH/R01-CA149451-01A1

http://medical-dictionary.thefreedictionary.com/immunogenicity

http://www.ncbi.nlm.nih.gov/pubmed/14533809

https://en.wikipedia.org/wiki/Sialic_acid#Sialic_acid_and_immunity

http://link.springer.com/article/10.1007/s000180050258#page-1

http://www.medicinenet.com/script/main/art.asp?articlekey=2413

https://www.collectiveip.com/grants/NIH:8403832