1. Immunological tolerance to tumor-associated carbohydrate antigens (TACAs) is the central problem for cancer vaccine design. In order to circumvent this problem, research described in this thesis has exploited a new strategy for selective immunotargeting of cancer based on glycoengineering of cancer cells, using melanoma as a model. The basic concept is to inoculate the immune system of cancer patients with a synthetic vaccine consisting of a structurally modified sialooligosaccharide cancer antigen, and after an immune response specific to the artificial antigen is established, tumors are engineered to force their expression of the modified antigen. The activated immune system will thereby eliminate the antigen-tagged tumors.
The main aims of this research are to identify the precursors that can be used to engineer tumor antigens and the proper antigens that can induce efficient immune responses. For this purpose, several N-acyl derivatives of D-mannosamine were synthesized and their ability to modify cell surface sialic acid and sialo-TACAs, such as GM3, was investigated through their enzymatic reactions with NeuNAc aldolase and metabolic engineering of tumor cells. In addition, several N-modified GM3 antigens and their protein conjugates were prepared and their immunological properties were studied by ELISA.
It was revealed that N-phenylacetyl-D-mannosamine (ManNPAc) is an excellent precursor for metabolic modification of tumor cells, and chemically modified GM3 antigens are much more immunogenic than natural GM3. N-Phenylacetyl GM3 (GM3NPAc) is the most immunogenic, and its antisera is the least cross reactive with the natural GM3. The results also showed that the bioengineered cancer cells treated with ManNPAc can be selectively and efficiently targeted by anti-GM3NPAc sera from mice immunized with the corresponding glycoconjugate vaccine.
In view of these results, it is anticipated that the new strategy holds great promise as an effective immunotherapy of cancer with GM3 as the primary target. In the future, this project will study the immunodiagnosis, as well as passive and active immunotherapy for melanoma and other cancers based on this strategy.
2. The highly potent anticancer steroidal saponin OSW-1 was synthesized from commercially available diosgenin, L-arabinose and D-xylose in a total of 37 steps with the longest linear sequence of 17 steps and in 7.6% overall yield. Alternative synthetic routes to OSW-1 aglycone were also studied.