Polysaccharides Unique Targets for Immunotherapy

ORLANDO—Polysaccharides (a type of carbohydrate) are the most highly expressed antigens on the surface of cancer cells, and they can be uniquely effective targets for immunotherapy, said Philip Livingston, MD, of Memorial Sloan-Kettering Cancer Center and Cornell University Medical College.

He spoke at a symposium held in conjunction with the 38th Annual Meeting of the American Society of Clinical Oncology (ASCO) and supported by Anti-genics Inc. and the University of Connecticut School of Medicine.

The immune response to polysaccharide antigens is largely limited to the production of antibodies, Dr. Livingston said. In addition to cellular immune mechanisms, antibodies have been shown to be important mediators of autoimmunity, such as with graft rejections. "You can induce autoimmunity against almost any organ in the body experimentally," he said, "and some of this is antibody mediated."

Dr. Livingston explained that the efficacy of antibodies against polysaccharide targets on cancer cells has been demonstrated in animal models. In one study, mice were challenged with the EL4 lymphoma cell line, which naturally expresses the carbohydrate ganglioside GD2. Mice that received a monoclonal antibody directed against GD2 within 2 to 4 days after tumor challenge were protected from tumor progression. However, mice that received the antibody 7 to 10 days following tumor challenge showed little benefit.

He also pointed out that when lower numbers of tumor cells were used as a challenge, the window of opportunity for effective protection with the antibodies was longer. "The message is that for antibodies or vaccines that induce antibodies to be effective, you need to be thinking micrometastases and the adjuvant setting," he said.

There are several defined cancer cell surface antigens that can induce tumor cell regression through antibody mechanisms. The majority of these antigens, including the gangliosides GM2, GD2, and GD3, and fucosyl GM1, are carbohydrates, Dr. Livingston said. The GM2, GD2, and GD3 gangliosides are strongly expressed on the surface of more than 50% of tumors, including melanomas, neuroblastomas, and sarcomas.

On their own, he said, the tumor-related carbohydrate antigens are only weakly immunogenic. Immunogenicity is greatly increased by chemically linking the carbohydrate to a carrier protein and mixing that with an adjuvant.

Keyhole limpet hemocyanin (KLH) has been found to be an effective carrier protein and the saponin QS21 to be a potent adjuvant, he said. Vaccines so constructed are effective in inducing high titers of IgM antibodies and consistent IgG antibody titers.

Dr. Livingston described the results of vaccination with fucosyl-GM1-KLH with QS21 in 10 patients with small-cell lung cancer (ASCO abstract 1824). All of the patients developed substantial titers of both IgM and IgG antibodies with specific reactivity for fucosyl-GM1, and the majority of patients had antibodies capable of facilitating complement-dependent cell lysis. "These antibodies are just as effective as the optimal dose of a monoclonal antibody at inducing lysis of these tumor cells," he said.

Evidence of clinical benefit from carbohydrate vaccines has been noted in several studies. In a phase II trial conducted by Dr. Livingston and his colleagues enrolling 120 patients with melanoma, vaccination with GM2 and bacillus Cal-mette-Guérin (BCG) was compared with BCG alone.

"If we eliminated the patients who had antibody to GM2 at the initiation of the trial, there was a significant difference in survival between the two groups favoring GM2-BCG," he said. If all patients were included, the survival difference was no longer significant, "but there was a trend, and there was clear improvement in patients who had antibody against GM2 vs those who didn’t," he said.

However, a large randomized phase III trial comparing vaccination with GM2-KLH plus QS21 with high-dose interferon in patients with melanoma was stopped after 17 months of median follow-up because of clear benefits for interferon in both disease-free and overall survival. Dr. Livingston questioned whe-ther the survival differences would hold up long-term, and he indicated that the trial will be reevaluated in the near future.

Dr. G.D. MacLean and researchers at Biomira (Edmonton, Alberta) have been conducting trials for several years with synthetic sialyl-Tn (an epitope believed to have functional significance in metastasis) conjugated to KLH plus the Detox adjuvant (Theratope). They have reported that the majority of patients have an antibody response, and those with a strong response appear to have improved survival. Their phase III Theratope trial is ongoing "at multiple centers on both sides of the Atlantic," Dr. Livingston said.

According to Dr. Livingston, current research is moving toward polyvalent polysaccharide vaccines. Studies using multiple antigen vaccines are able to produce strong antibody responses across multiple melanoma cell lines. According to Dr. Livingston, "it isn’t until you pool your antibodies that you really can get a response in all the melanoma cell lines."

He indicated that polyvalent antigen vaccine trials are either planned or currently ongoing at his institution in patients with prostate, ovarian, breast, and small-cell lung cancer.