The terminal homologation by CH(2) insertion into the
peptides mentioned in the title is described. This involves replacement of the N-terminal
amino acid residue by a β(2) - and of the C-terminal
amino acid residue by a β(3) -homo-
amino acid moiety (β(2) hXaa and β(3) hXaa, resp.; Fig. 1). In this way, the structure of the
peptide chain from the N-terminal to the C-terminal stereogenic center is identical, and the modified
peptide is protected against cleavage by
exopeptidases (Figs. 2 and 3).
Neurotensin (NT; 1) and its C-terminal fragment
NT(8-13) are
ligands of the
G-protein-coupled receptors (GPCR) NT1, NT2, NT3, and NT analogs are promising tools to be used in
cancer diagnostics and
therapy. The affinities of homologated NT analogs, 2b-2e, for NT1 and NT2 receptors were determined by using cell homogenates and
tumor tissues (Table 1); in the latter experiments, the affinities for the NT1 receptor are more or less the same as those of NT (0.5-1.3 vs. 0.6 nM). At the same time, one of the homologated NT analogs, 2c, survives in human plasma for 7 days at 37° (Fig. 6). An NMR analysis of
NT(8-13) (Tables 2 and 4, and Fig. 8) reveals that this N-terminal NT fragment folds to a turn in CD(3)
OH. - In the case of the human
analgesic opiorphin (3a), a pentapeptide, and of the HIV-derived B27-KK10 (4a), a decapeptide, terminal homologation (→3b and 4b, resp.) led to
a 7- and 70-fold half-life increase in plasma (Fig. 9). With N-terminally homologated NPY, 5c, we were not able to determine serum stability; the
peptide consisting of 36
amino acid residues is subject to cleavage by endopetidases. Three of the homologated compounds, 2b, 2c, and 5c, were shown to be agonists (Fig. 7 and 11). A comparison of terminal homologation with other stability-increasing terminal modifications of
peptides is performed (Fig. 5), and possible applications of the
neurotensin analogs, described herein, are discussed.