Contrast agents
The invention relates to ultrasound contrast agents comprising vesicles comprising a protein capable of formation of gas-containing vesicles, wherein the vesicles contain gas which comprises sulphur hexafluoride or a low molecular weight fluorinated hydrocarbon. These contrast agents exhibit stability in vivo upon administration so as to permit ultrasound visualization while allowing rapid subsequent elimination from the system.
1 . Contrast agents for use in diagnostic ultrasound studies comprising microbubbles of gas or a gas precursor encapsulated in a protein shell characterised in that the said protein is crosslinked with crosslinking groupings containing biodegradable linkages.
2 . Contrast agents as claimed in claim 1 wherein the crosslinking groupings contain biodegradable linkages selected from amide, imide, imine, ester, anhydride, acetal, carbamate, carbonate, carbonate ester and disulphide groups.
3 . Contrast agents as claimed in claim 2 wherein the crosslinking groups contain biodegradable linkages of formula
—(Y) n —CO—O—C(R 1 R 2 )—O—CO—(Z) n —
(where Y and Z, which may be the same or different, are —O—, —S— or —NR 3 —; R 1 and R 2 , which may be the same or different, are hydrogen atoms or carbon-attached monovalent organic groups or together represent a carbon-attached divalent organic group; R 3 is a hydrogen atom or an organic group; and the symbols n, which may be the same or different, are zero or 1).
4 . Contrast agents as claimed in any of the preceding claims wherein the protein is albumin, gelatin or globulin.
5 . Contrast agents as claimed in claim 4 wherein the protein is human serum albumin.
6 . Contrast agents as claimed in any of the preceding claims further containing an inorganic particulate stabiliser.
7 . A process for the preparation of a contrast agent as claimed in claim 1 which comprises encapsulating a gas or gas precursor in a protein and crosslinking the protein with crosslinking groups containing biodegradable linkages before, during or after said encapsulation.
8 . A process as claimed in claim 7 wherein crosslinking is effected after encapsulation.
9 . A process as claimed in claim 7 or claim 8 wherein crosslinking is effected using a crosslinking agent of formula (I)
A 1 -X-A 2 (I)
(where X is a linking group containing one or more biodegradable linkages as defined in claim 2 or claim 3 and A 1 and A 2 , which may be the same or different, are functional groups reactive with proteins).
10 . A process as claimed in claim 9 in which A 1 and A 2 are both aldehyde groups.
11 . A process as claimed in any of claims 8 to 10 wherein encapsulation is effected by agitation or sonication of the protein in an aqueous medium to yield a protein foam which is dried and thereafter suspended in a solution of the crosslinking agent in a polar organic solvent
12 . A process as claimed in claim 11 in which the crosslinking agent is a compound of formula (I) as defined in claim 9 in which A 1 and A 2 are both O-linked sulphonated N-hydroxysuccinimidyl residues.