Synthesis of peptide conjugated chelator oligomers for endoradiotherapy and MRT imaging

The clinical impact of peptides that accumulate in tumours is determined by the number of particle emitting or paramagnetic isotopes attached. Therefore, attempts should be made to increase the cargo capacity of the peptide carriers. A general synthetic route to conjugates is described that allows insertion of multiple DOTA (1,4,7,10-tetraazacyclododecane-N′,N″,N^?,N?-tetraacetic acid) moieties at the N-terminal end of the cyclic peptide Tyr³-octreotate. The peptide moiety was assembled by Fmoc solid phase synthesis and oxidised to form the cyclic disulfide. Subsequently, the required number of DOTA-tris tert-butyl ester chelating units were attached to the side chains of lysines. The conjugates were purified and thoroughly studied by RP-HPLC, size exclusion HPLC and mass spectrometry. The labelling of the novel conjugates and of DOTA⁰-Tyr³-octreotate (DOTATATE) was exemplified for ⁹⁰Y and ¹¹¹In. The methodology described here allows the versatile introduction of multiple DOTA chelates into a peptide sequence, thus, introducing a new scope to the receptor affine peptides that can be synthesised using solid phase synthesis.