Optimization of the small-scale synthesis of DOTA-Tyr3 -octreotide

The clinical potential of 111In and 90Y labelled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid conjugated Tyr3-octreotide (DOTA-TOC) have been reported in a number of publications, and Phase II clinical trials of 90Y-DOTA-TOC are currently in progress. However, to date, only a summary of the large-scale preparation of these radiopharmaceuticals has been published. This publication aims to describe our experience of the small-scale synthesis of DOTA-TOC in the hope that this will assist others in the preparation of this and other similar radioconjugates. DOTA in the form of the tri-t-butyl ester was coupled to the Lys5 (BOC) protected Tyr3-octreotide in N,N-dimethylformamide or N-methyl-2-pyrolidinone, in a three-step reaction involving conjugation, using O-(7-azabenzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) and diisopropylethyamine (DIPEA) as coupling reagents, deprotection with trifluoroacetic acid and HPLC purification of the conjugates. The product was obtained in final yields of 60+/-5%. The purified product was characterized by mass spectroscopy, showing a molecular weight of 1421.55+/-0.08. In somatostatin receptor binding assays, the unlabelled DOTA-TOC showed an effective displacement of 99mTc labelled HYNIC-TOC (where HYNIC is hydrazinonicotinamide) (IC50=0.31+/-0.07 nm), confirming the retention of receptor-binding affinity. The conjugate could be efficiently labelled with 111In by addition of 111InCl3 and ammonium acetate buffer pH5 and heating (95 degrees C, 20 min).