Conjugation of the closo‐Borane Mercaptoundecahydrododecaborate (BSH) to a Tumour Selective Peptide

Abstract 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 Tyr3‐octreotate. Tyr3‐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‐Tyr3‐octreotate conjugate is considered to target receptor‐positive tumours which might further improve the potential of BNCT.