The structural characteristics of a mucin glycopeptide motif derived from the N-terminal fragment STTAV of the cell surface glycoprotein CD43 have been investigated by NMR. In this study, a series of molecules prepared by total synthesis were examined, consisting of the peptide itself, three glycopeptides having clustered sites of alpha-O-glycosylation on the serine and threonine side chains with the Tn, TF, and STF carbohydrate antigens, respectively, and one with the beta-O-linked TF antigen. Additionally, a glycopeptide having the sequence SSSAVAV, triglycosylated with the Le(y) epitope, was investigated. NMR data for the tri-STF-STTAV glycopeptide were used to solve the structure of this construct through restrained molecular dynamics calculations. The calculations revealed a defined conformation for the glycopeptide core rooted in the interaction of the peptide and the first N-acetylgalactosamine residue. The similarity of the NMR data for each of the alpha-O-linked glycopeptides demonstrates that this structure persists for each construct and that the mode of attachment of the first sugar and the peptide is paramount in establishing the organization of the core. The core provides a common framework on which a variety of glycans may be displayed. Remarkably, while there is a profound organizational effect on the peptide backbone with the alpha-linked glycans, attachment via a beta-linkage has little apparent consequence.

The syntheses of two tumor-associated carbohydrate antigens, Tn and TF, have been achieved using glycal assembly and cassette methodologies. These synthetic antigens were subsequently clustered (c) and immunoconjugated to a carrier protein (KLH or BSA) or a synthetic lipopeptide (pam) for immunological study. Three Tn conjugates were used to vaccinate groups of mice, and all preparations proved to be immunogenic. The Tn(c) covalently linked to KLH (27−KLH) plus the adjuvant QS-21 was the optimal vaccine, inducing high median IgM and IgG titers against Tn(c) by ELISA. These antibodies were strongly reactive with the Tn(c) positive human colon cancer cell line LS-C but not the Tn(c) negative colon cancer cell line LS-B by FACS. The antibodies' reactivities with natural antigens were inhibited with synthetic Tn(c) but not with structurally unrelated compounds. On the basis of these results, vaccines containing 27−KLH and 30−pam plus QS-21 are being tested in patients with prostate cancer.

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The feasibility of using carbohydrate-based vaccines for the immunotherapy of cancer is being actively explored at the present time. Although a number of clinical trials have already been conducted with glycoconjugate vaccines, the optimal design and composition of the vaccines has yet to be determined. Among the candidate antigens being examined is Lewis y (Le y ), a blood grouprelated antigen that is overexpressed on the majority of human carcinomas. Using Le y as a model for specificity, we have examined the role of epitope clustering, carrier structure, and adjuvant on the immunogenicity of Le y conjugates in mice. A glycolipopeptide containing a cluster of three contiguous Le y -serine epitopes and the Pam3Cys immunostimulating moiety was found to be superior to a similar construct containing only one Le y -serine epitope in eliciting antitumor cell antibodies. Because only IgM antibodies were produced by this vaccine, the effect on immunogenicity of coupling the glycopeptide to keyhole limpet hemocyanin was examined; although both IgM and IgG antibodies were formed, the antibodies reacted only with the immunizing structure. Reexamination of the clustered Le y -serine Pam3Cys conjugate with the adjuvant QS-21 resulted in the identification of both IgG and IgM antibodies reacting with tumor cells, thus demonstrating the feasibility of an entirely synthetic carbohydrate-based anticancer vaccine in an animal model.

The total syntheses of complex sialylated cell-surface antigens have been accomplished. The target systems include 2,3-STF, STn, 2,6-STF, and glycophorin antigens. In addition, an α-O-linked serine glycoside of an entire Lewis blood group (Y) antigen has been assembled.

The epothilones are naturally occurring cytotoxic molecules that possess the remarkable ability to arrest cell division through the stabilization of microtubule assemblies. Our in vivo studies with 12,13-desoxyepothilone B (dEpoB), have established that the desoxy compound is well tolerated and virtually curative against a variety of sensitive and resistant xenograft tumors in animal models. In light of these discoveries, we sought a chemical synthesis of dEpoB that would be able to support a serious and substantial discovery research program directed toward the clinical development of this molecule. The overall strategy for this endeavor assumed the ability to synthesize dEpoB from three constructs which include an achiral β,δ-diketo ester construct A, an (S)-2-methylpentenal moiety B, and the thiazoyl-containing vinyl iodide moiety C. We envisioned that a diastereoselective aldol condensation between an achiral C5−C6 (Z)-metalloenolate derived from construct A and an (S)-2-methylalkanal fragment, B, would generate the desired C6−C7 bond. Second, a B-alkyl Suzuki coupling between the vinyl iodide construct C and an alkyl borane would form the C11−C12 bond. Finally, a late-stage reduction of the C3 ketone to the requisite C3 alcohol with high asymmetric induction would permit us to introduce the β,δ-diketo ester fragment A, into the synthesis as a readily accessible achiral building block. The governing concepts for our new synthesis are described herein.

Cell-surface mucin glycoproteins are altered with the onset of oncogenesis. Knowledge of mucin structure could be used in vaccine strategies that target tumorassociated mucin motifs. Thus far, however, mucins have resisted detailed molecular analysis. Reported herein is the solution conformation of a highly complex segment of the mucin CD43. The elongated secondary structure of the isolated mucin strand approaches the stability of motifs found in folded proteins. The features required for the mucin motif to emerge are also described. Immunocharacterization of related constructs strongly suggests that the observed epitopes represent distinguishing features of tumor cell-surface architecture.The exterior of most cells is dominated by glycolipids, proteoglycans, and glycoproteins, including mucin-like proteins. These display polyvalent ␣-O-linked carbohydrates on proximal serine and threonine residues (1-4). Altered expression of cell-surface mucin character is often characteristic of malignant cells (1,3,5). Accordingly, tumor-associated mucins are good targets for a vaccination strategy (3, 5-9). We have surmounted the synthetic challenges of constructing polypeptides bearing clustered glycodomains (7,8), and one such construct is currently in human clinical trials. Given this access, we probed the effects of clustered glycosylation patterns on peptide conformation and recognition (9-18). We have conducted extensive NMR and restrained molecular dynamics calculations (19, 20) on fully synthetic clustered carbohydrate tumor antigens (1-4) corresponding to a fragment of CD43, a glycoprotein aberrantly expressed on the surface of acute myelogenous leukemia cells (21-24). Our findings demonstrate how clustered glycosylation induces the peptide backbone into an unprecedented rigid scaffold corresponding to a polypeptide secondary conformation, which is consistent with elongated mucin glycoprotein structure and function (1,(25)(26)(27)(28)(29). Remarkably, the glycosylation-induced structure approaches the stability of motifs found in globular proteins.The CD43, or leukosialin, protein presented an ideal candidate from which to select a substructure for synthesis. In consequence of its possible role in inducing immunologic response, the system is relatively well characterized (22-24). The sequence STTAV is a glycosylation locus found in the amino terminus of the protein. We have accessed through chemical synthesis the clustered 2,6-STF trisaccharide 1 (7), which is present on CD43 when expressed on acute myelogenous leukemia cells (24). In addition, we have synthesized the TF disaccharide 2 and the monosaccharide Tn antigen 3. Through our synthetic methods, we also gained access to the ␤-linked stereoisomer of the TF antigen 4 (Fig.