Radiosynthesis of 68Ga-labelled DOTA-biocytin (68Ga-r-BHD) and assessment of its pharmaceutical quality for clinical use

OBJECTIVES: Biocytin analogues labelled with indium-111, yttrium-90 and lutetium-177 have shown their effectiveness in the imaging of infections/inflammation in patients with osteomyelitis and function as efficient tools in pretargeted antibody-guided radioimmunotherapy. In this study, the labelling of a biocytin analogue coupled with DOTA (1,4,7,10- tetraazacyclododecane-1,4,7,10-tetraacetic acid), namely, r-BHD, with gallium-68 (Ga) was optimized, and the quality and stability of the preparations were assessed for clinical use. MATERIALS AND METHODS: Synthesis of Ga-r-BHD was carried out by heating a fraction of the Ge/Ga eluate in a reactor containing the biocytin analogue with the appropriate buffer. The influence of the precursor amount (from 2.5 to 140 nmol), the pH of the reaction (from 2 to 5.5) and the buffer species (1.5 mol/l sodium acetate, 1.5 mol/l sodium formate, 4.5 mol/l HEPES) on radiochemical yield and radiochemical purity was assessed. Studies on stability and binding to avidin (Av) were also conducted in different media. RESULTS: Under the best labelling condition (56 nmol of precursor, 3.8 pH, sodium formate buffer) synthesis of Ga-r-BHD resulted in a yield of 64±3% (not decay corrected). Radiochemical purity was around 95% because a Ga-coordinated sulfoxide form of the ligand was detected as a by-product of the reaction (Ga-r-SBHD). The by-product was identified and characterized by liquid chromatography-electrospray ionization tandem mass spectrometry. At the natural 1 : 4 Av/Ga-r-BHD molar ratio, affinity results were 62±2 and 80±2% in saline and human serum, respectively. Stability of Ga-r-BHD and of the radiotracer/Av complex remains almost constant over 180 min. Ga-r-BHD appears to be a good candidate for clinical applications.