Synthesis, biodistribution and renal handling of various chelate-somatostatin conjugates with metabolizable linking groups.

A series of DTPA-octreotide conjugates, with various linking groups, were synthesised to investigate the effect of different metabolizable linkers on the renal retention of radioactivity. All these newly synthesised octreotide conjugates retained the high binding affinity of octreotide for the somatostatin (SRIF) receptors either when unlabeled or radiolabeled with 111In. Some of the metabolizable linkers were rapidly degraded in vitro when incubated with a kidney homogenate. However, in vivo, all these conjugates displayed a significantly lower uptake in SRIF receptor-positive tissue compared to two conjugates with short, stable linkers. Additionally, the compounds with a potentially metabolizable linker had a higher whole-body retention of activity as opposed to the three metabolically stable compounds. Several of the linkers gave evidence of cleavage while in circulation in the blood, and it is probable that the lower tumour accumulation of most of the compounds tested was low due to the high enzymatic nature of the exocrine pancreatic tumour model used. In short, no increase in the tumour-to-kidney ratio was achieved with the analogues containing a metabolizable linker. The highest target-to-nontarget tissue ratios were obtained for the DTPA-octreotide conjugates that had short, metabolically stable linkers.