In order to improve the efficacy of
doxorubicin (DOXO) in the treatment of
hepatocellular carcinomas, the
drug was conjugated with lactosaminated
human albumin (L-HSA), a hepatotropic
drug carrier. Conjugation was performed using the (6-maleimidocaproyl)hydrazone derivative of the
drug (
DOXO-EMCH). The
maleimide group of
DOXO-EMCH reacts with the aminoacidic residues of the carrier forming stable bonds, whereas the
hydrazone bond is rapidly hydrolysed in the acidic endosomal and lysosomal compartments of the cells allowing the intracellular release of DOXO. To identify the
amino acids of L-HSA involved in the bond with
DOXO-EMCH, in the present study we synthesized this compound with the 2,3
carbon atoms of the
maleimide moiety enriched in the (13)C
isotope and used this labelled
DOXO-EMCH to prepare two types of L-HSA conjugate. Type I was prepared in analogy to those studied in the anticancer experiments using
tris(2-carboxyethyl)phosphine (
TCEP) to reduce
l-cysteine disulfides and make the sulfhydryl groups available for the reaction with
DOXO-EMCH; type II was synthesized omitting
TCEP. By (13)C NMR spectroscopy we could demonstrate that in type I conjugate
cysteine was the only
amino acid residue that reacted with
DOXO-EMCH, whereas in type II conjugate
lysine was the only
amino acid in the reaction with
DOXO-EMCH. When hydrolysed in an acidic medium to cleave the
hydrazone bond, type I conjugate released only DOXO, whereas type II conjugate also released a derivative of the
drug.