By David N. Leff
The first handful of prostate cancer patients entered a Phase I pilot clinical trial last month at Memorial Sloan-Kettering Cancer Center in New York, to test a "second-generation" tumor vaccine. Eventually, several dozen subjects will be immunized with a series of shots containing six separate tumor antigen targets, designed to elicit antibody firepower against their malignant tumor cells.
The novel vaccine was synthesized under the direction of synthetic organic chemist Samuel Danishefsky, who directs Sloan-Kettering's Laboratories of Bio-Organic Chemistry. He reported a first-generation human trial in the current Proceedings of the National Academy of Science (PNAS), dated March 13, 2001. It bears the title: "Immunization of metastatic breast cancer patients with a fully synthetic globo H conjugate: A Phase I trial." Danishefsky is its senior author.
"This PNAS paper," he told BioWorld Today, "deals with our first-generation concept. We synthesized a single vaccine target, globo H, which is a breast tumor-associated antigen. We connected it to a carrier protein, keyhole limpet hemocyanin (KLH), to augment the antigen's immunogenicity. It verified in mice that it was indeed antigenic. The antibodies bound to the antigen on the cell surface of that tumor and didn't bind to tumors that don't have that globo H antigen.
One distinguishing feature of many types of cancer cells is an abnormal abundance of certain carbohydrate molecules on the cell surface.
"Globo H, commonly found on breast cancer cells," Danishefsky observed, "is a potential target for vaccine therapy. It is a constellation of sugars - carbohydrate entities, which occurs in the glycoproteins of breast, prostate and other epithelial cancers. The expressed levels of these antigens are magnified in the context of such tumors. So one would like the immune system, when it sees such a high concentration, to attack the tumor. But you have to have an educated immune system to do that - which is what our vaccine is about.
"Of course, before vaccinating those breast cancer patients," he added, "we asked whether the process is harmful, because it might have had an autoimmune response."
Pilot Breast Cancer Cohort Looking Good
Between September 1997 and August 1999, the 27 breast cancer patients received five vaccinations each, which they tolerated well, and successfully passed their Phase I safety trials. As an added bonus beyond the Phase I endpoints, vaccine-stimulated antibodies bound significantly to 16 of the 27 patients. "The numbers are statistically insignificant," Danishefsky commented, "but the possible incidental clinical benefit may be deduced from the fact that we're going on to Phase II."
Meanwhile, an article in the Journal of the American Chemical Society (JACS), dated March 7, 2001, describes what Danishefsky, its senior author, terms "really futuristically but potentially very significant data. It tells us where to go from the clinical trial results to the next generation, and the generation after that."
The JACS paper is titled, "Pursuit of optimal carbohydrate-based anticancer vaccines: Preparation of a multiantigenic, unimolecular glycopeptide containing the Tn, MBR1 and Lewisy antigens."
"In our non-paper," Danishefsky pointed out, "the ongoing multiantigenic prostate cancer trial, very serious regulatory issues have been raised concerning such a vaccine, which would have six or seven independent vaccines in it. Convincing everyone that they were necessary, and making and conjugating all of them, we have a huge issue arising.
"The JACS paper shows," he continued, "the way in which all the antigens can be put into a single molecule containing all of the biological information. In this one there were three antigens that are built into a single molecule, and conjugated to the KLH carrier protein. They are found in almost all prostate cancer cells, and anticipate our third-generation vaccine. The three simply tell us that at some point we can aspire to put them all in the same molecule."
Back-to-back with Danishefsky's paper in the March 13 PNAS is a companion article, which looks toward that future. Its title: "Toward optimized carbohydrate-based anticancer vaccines. Epitope clustering, carrier structure and adjuvant all influence antibody responses to Lewisy conjugates in mice." Its senior author is immunologist Kenneth Lloyd, who directs Sloan-Kettering's Tumor Antigen Laboratory. He did the analysis; Danishefsky's group the synthesis.
Here, the operative word is "clustering."
"Somehow, in the real world of cell-surface architecture," Danishefsky noted, "the tumor-associated antigens are often presented in clustered form. They are not expressed simply as carbohydrates. Carbohydrates in turn are attached to proteins in the context of glycoproteins.
"Now, it turns out for reasons that are not at all clear," he went on, "in these aberrant cancerous glycoproteins, the antigens are presented clustered. In other words, you might have a row of tumor antigens on the side chains of the relevant amino acid - serine for example. Imagine a row of five serine residues along the protein, each one carrying the same tumor antigen. That's called clustering. Nobody knows why it happens."
Simulating Tumor Call's Antigenic Surface
"Looking toward ever-increasing realism in the vaccines of the future," Danishefsky recounted, "we synthesized the clustered form, not of globo H, but of another tumor-associated antigen called Lewisy. This candidate antigen is a carbohydrate specificity belonging to the A, B, H Lewis blood group family. It is overexpressed on the majority of carcinomas, including ovary, pancreas, prostate, breast, colon and non-small-cell lung cancers. We made a construct that presents this Lewisy on three consecutive amino acids, trying to get it to look more and more like the eventual tumor cell surface.
"And the second-generation results, though only in mice, were actually quite encouraging in that they showed two important things: one, that the clustered presentation is particularly antigenic, and two, that we could economize and considerably simplify the presentation mode of the vaccine construct because there didn't have to be that KLH carrier protein when we used the clustered motif.
"So that's a very important finding, potentially," Danishefsky summed up, "that the clustering motif, which was set up to simulate the cell surface, has been successful in mice."
"The development of a totally synthetic vaccine," his paper concludes, "in contrast to carbohydrate-protein conjugate vaccines, would greatly facilitate the production of the vaccine for large-scale clinical trials and could simplify regulatory approval."