Molecular amplifiers: synthesis and functionalization of a poly(aminopropyl)dextran bearing a uniquely reactive...

The synthesis and characterization of the versatile dextran-based molecular amplifier 6 is described. Dextran (Mr = 40,200) was selectively monofunctionalized in high yield at its reducing terminus via reductive amination with 2-(4-nitrophenyl)ethylamine to give 1. The nitro group in 1 serves as a masked amino group which is eventually converted into a reactive isothiocyanato group used for monovalent attachment of the completed assembly to a target molecule. Cyanoethylation of 1 gave the terminally nitrophenylated poly(cyanoethyl)dextran 5 which was selectively reduced to the corresponding poly(aminopropyl) derivative 6 with BH3.THF, a reagent which preserved the end nitro group. Conjugation of amplifier 6 with the isothiocyanate-derivatized Gd(III) chelate 7 gave conjugate 9 containing about 22 mol of chelate/mol of amplifier. The T1 relaxivity per Gd(III) ion of 9 in H2O was 15.0 mM-1 s-1, about 3-fold higher than that of free Gd(III)DTPA in H2O. The nitro group of 9 was then selectively reduced to the corresponding amine 10, which was converted into isothiocyanate 11. The reactivity of the single isothiocyanate group in 11 was demonstrated by coupling to 5-aminoeosin, giving conjugate 12. Amplifier 6 was also conjugated with the acid-labile N-cis-aconityl derivative 8 of the potent anticancer agent daunomycin. The nitro group of the resulting conjugate 13 was then reduced and the resulting amine 14 was converted into mono isothiocyanate 15. Compound 15 reacted with a water-insoluble amine-containing solid support to give 16. Free daunomycin was released from 16 by exposure to citrate-phosphate buffer at pH 4.0.