alpha-Melanocyte-stimulating hormone (
alpha-MSH) is known to bind to the
melanocortin receptor 1 (MC1R) which is overexpressed on melanotic and
amelanotic melanoma cells.
alpha-MSH analogues are potential candidates for specific targeting of
melanoma metastases. Several linear and cyclic radiolabeled
MSH peptides have been designed and tested in the past, showing both high affinity for the MC1R in vitro and good incorporation in
tumor xenografts in vivo. However, considerable kidney reabsorption of the radiopeptides could not be avoided. With the aim to increase the
tumor-to-kidney ratio, we synthesized six glycosylated derivatives of
NAPamide, an
alpha-MSH octapeptide analogue with high
tumor selectivity and coupled them to the
chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The
peptides were evaluated in vitro for MC1R binding and bioactivity and, after labeling with (111)In, for in vitro cellular uptake and in vivo tissue distribution in mice carrying B16F1
melanoma tumors. The
glycopeptides showed excellent binding affinities in the low nanomolar to subnanomolar range using both murine and human
melanoma cell lines. However, five
glycopeptides displayed lower selectivity in vivo than the parent
DOTA-
NAPamide, because of either a lower
tumor uptake or a higher kidney uptake. In particular C-terminal extension of the
amide group by a galactosyl moiety increased the kidney retention dramatically. By contrast, an N-terminally positioned
galactose residue in
DOTA-Gal-
NAPamide improved the
tumor-to-kidney ratio (4-48 h AUC of 1.34) by
a factor of about 1.2 as compared to the parent
DOTA-
NAPamide (4-48 h AUC of 1.11), thus serving as new lead compound for MC1R-targeting molecules.