Sialic acid derivatives for protein derivatisation and conjugation
Derivatives are synthesized of starting materials, usually polysaccharides, having sialic acid at the reducing terminal end, in which the reducing terminal unit is transformed into an aldehyde group. Where the polysaccharide has a sialic acid unit at the non-reducing end it may be passivated, for instance by converting into hydroxyl-substituted moiety. The derivatives may be reacted with substrates, for instance containing amine or hydrazine groups, to form non-cross-linked polysialylated compounds. The substrates may, for instance, be therapeutically useful drugs peptides or proteins or drug delivery systems.
1. A process for producing an aldehyde derivative of a reducing terminal sialic acid of a starting material having a sialic acid at a non-reducing terminal, which process comprises:
a) a selective oxidation step to oxidise the non-reducing terminal sialic acid unit at the 7,8 vicinal diol group to form a 7-aldehyde;
b) a reduction step to reduce the 7-aldehyde group of the non-reducing terminal sialic acid to a corresponding alcohol, wherein the reduction step additionally reduces the reducing terminal sialic acid to effect ring opening of the reducing terminal sialic acid whereby a vicinal diol group is formed;
c) an oxidation step which oxidises the reducing terminal unit but does not oxidise the moiety produced in the oxidation and reduction reactions of the non-reducing terminal unit; and
d) a conjugation step to conjugate the reducing terminal sialic acid to a substrate.
2. A process according to claim 1 in which the starting material is a di-, oligo- or poly-saccharide.
3. A process according to claim 2 in which the polysaccharide comprises midchain sialic acid units.
4. A process according to claim 1 in which oxidation step a) is carried out in aqueous solution in the presence of periodate at a concentration in the range 1 μM to 1 M, a pH in the range 3 to 10, a temperature in the range 0 to 60° C. and a time in the range 1 min to 48 hours.
5. A process according to claim 1 in which the reduction step (b) is carried out in aqueous solution in the presence of borohydride at a concentration in the range 1 μM to 0.1 M, a pH in the range 6.5 to 10, a temperature in the range 0 to 60° C. and a period in the range 1 min to 48 h.
6. A compound of formula II
wherein Ac is acetyl;
m is 0 or more;
Gly 1 O is glycosyl; and
R 5 is an organic group conjugated to a reducing terminal sialic acid; in which R 5 is
and in which R 1 is a peptide, a polypeptide, a protein, a protein therapeutic, a drug, a drug delivery system, a liposome, a virus, a cell, or a synthetic polymer.
7. A pharmaceutical composition comprising a compound according to claim 6 and a pharmaceutical excipient.
8. A composition comprising a compound according to claim 6 and a diluent.
9. The process of claim 1 wherein the substrate is selected from the group consisting of a peptide, a protein, a drug, a drug delivery system, a liposome, a virus, a cell, and a synthetic polymer.
10. The process of claim 9 wherein the substrate is a polypeptide or protein.
11. The process of claim 1 wherein the starting material is polysialic acid.
12. The compound of claim 6 wherein R 1 is a polypeptide or protein.
13. The compound of claim 6 wherein R 1 is a peptide or protein therapeutic.