Contrast agents

Abstract

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.