In this study, we investigate the usefulness of peptides as carriers for which the receptors are overexpressed on tumor cells. Covalent linking of the drug to the peptide could be used for chemotherapy and would lead to selective addressing of the tumor cells. As a model peptide, we used neuropeptide Y (NPY) because its receptors are produced in a number of neuroblastoma and the thereof derived cell lines. NPY is a 36-amino acid peptide of the pancreatic polypeptide family. It is expressed in the peripheral and central nervous systems and is one of the most abundant neuropeptides in the brain. Five distinct NPY receptors have been cloned, which have been named Y1, Y2, Y4, Y5, and y6 receptor subtypes. Upon binding to the G-protein coupled receptors, the ligand-receptor complex is internalized, which provides a convenient way to enter the cell by receptor-mediated endocytosis. Because structure-activity relationships (SARs) of NPY are well-known, position 15 of NPY was used for attaching maleimide anthracycline derivatives which would presumably not lead to a significant loss of binding activity for the Y1 receptor. Because the NPY-receptor complex is internalized and undergoes a pH shift from 7.4 to approximately 5.0 during endosomal trafficking, two different anthracycline derivatives that differ in the acid sensitivity of the bond between the drug and the spacer were selected (see Figure 1). Doxo-MBS and Dauno-MBS are characterized by a stable amide bond at the 3-amino position of the anthracycline; Dauno-HYD is a daunorubicin derivative incorporating an acid-sensitive hydrazone linker at the 13-keto position. The maleimide moiety was introduced into daunorubicin and doxorubicin in order to selectively label the sulfhydryl group of [C15]-NPY.

In this study, we investigate the usefulness of peptides as carriers for which the receptors are overexpressed on tumor cells. Covalent linking of the drug to the peptide could be used for chemotherapy and would lead to selective addressing of the tumor cells. As a model peptide, we used neuropeptide Y (NPY) because its receptors are produced in a number of neuroblastoma and the thereof derived cell lines. NPY is a 36-amino acid peptide of the pancreatic polypeptide family. It is expressed in the peripheral and central nervous systems and is one of the most abundant neuropeptides in the brain. Five distinct NPY receptors have been cloned, which have been named Y1, Y2, Y4, Y5, and y6 receptor subtypes. Upon binding to the G-protein coupled receptors, the ligand-receptor complex is internalized, which provides a convenient way to enter the cell by receptor-mediated endocytosis. Because structure-activity relationships (SARs) of NPY are well-known, position 15 of NPY was used for attaching maleimide anthracycline derivatives which would presumably not lead to a significant loss of binding activity for the Y1 receptor. Because the NPY-receptor complex is internalized and undergoes a pH shift from 7.4 to approximately 5.0 during endosomal trafficking, two different anthracycline derivatives that differ in the acid sensitivity of the bond between the drug and the spacer were selected (see Figure 1). Doxo-MBS and Dauno-MBS are characterized by a stable amide bond at the 3-amino position of the anthracycline; Dauno-HYD is a daunorubicin derivative incorporating an acid-sensitive hydrazone linker at the 13-keto position. The maleimide moiety was introduced into daunorubicin and doxorubicin in order to selectively label the sulfhydryl group of [C15]-NPY.