Conjugation of the closo-borane mereaptoundecahydrododecaborate (BSH) to a tumour selective peptide

The non-toxic disodium mercaptoundecahydrododecaborate (BSH) is one of the most promising agents known for boron-neutron-capture therapy (BNCT). In order to enhance the intratumoral concentration of the boron cluster by receptor mediated endocytosis, a synthetic method for the conjugation of borane clusters to peptides was developed. A Michael addition was performed to link BSH to the thiol reactive maleimido modified Tyr(3)-octreotate. Tyr(3)-octreotate, synthesized by Fmoc-solid phase peptide synthesis, is a selective ligand for somatostatin receptors. The modified boron cluster was obtained in 68 % yield after purification. Characterisation of the BSH conjugate was performed with HPLC, MALDI-TOF and electrospray mass spectrometry. The BSH-Tyr(3)-octreotate conjugate is considered to target receptorpositive tumours which might further improve the potential of BNCT.