Systems for stabilizing and delivering active agents

A delivery system for an active agent, comprising:

• a ligand that is selective for transthyretin in the serum of a subject;
• and,
• a linker configured for operatively attaching the ligand covalently to an active agent, wherein the linker ranges from 14-20 angstroms in length;
• wherein the system has a structure comprising:
• where,
• n is 1 to 8;
• R1 and R3 are independently selected from a short chain alkyl having 1 to 4 carbon atoms;
• R2 is selected from the group consisting of a hydrogen, a short chain alkyl having 1 to 4 carbon atoms, and an aryl;
• Xa is C(R4)(R5), O, N—R5, or S;
• where R4 and R5 are independently selected from hydrogen, an alkyl having 1 to 4 carbon atoms, a substituted alkyl, an alkoxy, hydroxyl, an alkoxycarbonyl, an an amino;
• Ra is CHO, COOH, COOCH3, COOR6, CONR7R8, tetrazolyl, CONHOH, B(OH)2, CONHSO2Ar, CONHCH(R9)COOH, CF3, hydrogen, halogen, alkyl, substituted alkyl, acyl, substituted acyl, carboxyl, heterocyclic group, sulfonamide, sulfonyl fluoride, thioester, alkoxycarbonyl or substituted alkoxycarbonyl;
• Rb is the linker and is positioned ortho or meta to Ra, the linker including a CHO, COOH, COOCH3, COOR6, CONR7R8, tetrazolyl, CONHOH, B(OH)2, CONHSO2Ar, CONHCH(R9)COOH, CF3, hydrogen, halogen, alkyl, substituted alkyl, acyl, substituted acyl, carboxyl, heterocyclic group, sulfonamide, sulfonyl fluoride, ester, thioester, ether, thioether, triazolyl, alkoxycarbonyl or substituted alkoxycarbonyl;
• R6 is alkyl, haloalkyl, cycloalkyl, or heterocyclyl;
• R7 and R8 are each independently hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, or heteroaryl;
• and,
• R9 is the side chain of a naturally occurring α-amino carboxylic acid;
• or, a pharmaceutically acceptable salt, ester, enol ether, enol ester, amide, acetal, ketal, orthoester, hemiacetal, hemiketal, hydrate, solvate or prodrug thereof.