A covalent linkage between daunorubicin and proteins that is stable in serum and reversible by lysosomal hydrolases, as required for a lysosomotropic drug-carrier conjugate: in vitro and in vivo studies.

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Abstract
Daunorubicin (DNR) has been conjugated to succinylated serum albumin by an amide bond joining the amino group of the drug and a carboxyl side chain of the protein either directly or with the intercalation of a peptide spacer arm varying at 1-4 amino acids. During in vitro incubation with lysosomal hydrolases, intact DNR could be released extensively only from conjugates prepared with a tri- or tetrapeptide spacer arm. These latter conjugates remained very stable in the presence of serum. When tested in vivo against the i.p. form of L1210 [mouse] leukemia, the conjugates in which DNR was linked to serum albumin directly or via 1 amino acid were completely inactive but the conjugate with a dipeptide spacer arm was not more active than free DNR. In parallel with the in vitro studies, the best therapeutic results were obtained with the conjugates formed with tri- and tetrapeptidic spacer arms; they were much more active than DNR, inducing a high percentage of long-term survivors. Use of a tri- or tetrapeptide spacer arm is essential to obtain DNR-protein conjugates that remain stable in serum and from which DNR can be released through the action of lysosomal hydrolases. The in vivo results suggest that these conjugates are endocytosed by L1210 cells and that DNR is released intracellularly after digestion by lysosomal enzymes. This conjugation method can be applied to other drugs possessing a free amino group and to various potential carriers such as antibodies, polypeptide hormones, and glycoproteins that have amino or carboxyl side chains.