Chemical glycosylation in the synthesis of glycoconjugate antitumour vaccines

Abstract

Therapeutic vaccines derived from carbohydrate antigen-adjuvant combinations are a promising approach for cancer immunotherapy. One of the critical limitations in this area is access to sufficient quantities of tumour-associated carbohydrate antigens and glycoconjugate adjuvants. At present, availability of the complex oligosaccharide constructs that are needed for the systematic design and evaluation of novel vaccine formulations relies on de novo chemical synthesis. The use of both state-of-the-art and emerging glycosylation technologies has led to significant advances in this field, allowing the clinical exploration of carbohydrate-based antigens in the treatment of cancer.

Figure 1. Glycosylation methods

a, b, Glycosylation of acetal-derived glycosyl donors. Activation of the anomeric leaving group (Lg, red) with an electrophilic promoter (El⁺, purple) is followed by nucleophilic attack of the acceptor (Nu–H, green) on the resulting electron-deficient anomeric carbon of the carbohydrate donor. c, d, Glycosylation with glycal donors. Activation of glycals with various electrophiles (El⁺) is followed by coupling with a glycosyl acceptor (Nu–H) at the anomeric carbon. These glycosylations result in functionalization of both the C1 and C2 positions of the donor. M, metal; R, various substituents; Tf, trifluoromethanesulphonyl; X, various leaving groups; Z, various functionalities.

Figure 2. Selected syntheses of tumour-associated Globo-H antigen hexasaccharide

a, The chemical structure of Globo-H. b, Synthesis through glycal assembly using oxidative C2-hydroxyglycosylation and oxidative C2-sulphonamidoglycosylation. c, Synthesis by an orthogonal two-directional glycosylation strategy using thioglycoside and glycosyl fluoride donors. d, Synthesis through a reactivity-based one-pot multiple glycosylation strategy. Bn, benzyl; Bz, benzoyl; Cbz, benzyloxycarbonyl; ClBn, 2-chlorobenzyl; coll, 2,4,6-collidine; Cp, cyclopentadienyl; Fuc, fucose; Lev, laevulinoyl (4-oxopentanoyl); LHMDS, lithium hexamethyldisilazide; NBz, 4-nitrobenzoyl; NIS, N-iodosuccinimide; PMP, 4-methoxyphenyl; TES, triethylsilyl; TIPS, triisopropylsilyl; Tol, toluyl (4-methylphenyl); Troc, trichloroethoxycarbonyl.

Figure 3. Selected syntheses of tumour-associated sialylated glycosphingolipid oligosaccharides

a, GM2, GD3 and GD2 have a similar trisaccharide core. b, Synthesis of GM2 tetrasaccharide through α-selective sialyl phosphite donor glycosylation. c, Synthesis of GD3 tetrasaccharide by thiosialoside glycosylation. Incorporation of the C5-trifluoroacetamide group in both donor and acceptor enhances efficiency of the coupling. Piv, pivaloyl (2,2,2-trimethylacetyl).

Figure 4. Mucin-related tumour-associated carbohydrate antigens

a, Structures of T_N, T, ST_N and 2,6-ST carbohydrate antigens. O-α-carbohydrate–amino-acid linkages can be synthesized by either (b) glycosylation of suitably protected Ser or Thr derivatives with various galactose-derived C2-azido donors, or (c) conjugate addition of amino-acid alkoxides into nitrogalactals. Boc, tert-butoxycarbonyl; Bu^t, tert-butyl; CAN, ceric ammonium nitrate; Y, various carbohydrate C1 groups; Z, various carbohydrate C2 groups.

Figure 5. Immunological adjuvant QS-21A

a, QS-21A is a heavily glycosylated saponin isolated from Quillaja saponaria Molina in both apiose (QS21A_api) and xylose (QS21A_xyl) forms. b, Synthesis of QS-21A_api. Glycosidic linkages were constructed by the use of Ph₂SO·Tf₂O-promoted dehydrative glycosylation and glycosyl trichloroacetimidate coupling in the construction of the trisaccharide–triterpene substructure. Ara, arabinose; PMB, 4-methoxybenzyl; Rha, rhamnose; TBS, tert-butyldimethylsilyl.