Use of 65Zn as a tracer for the assessment of purification in the 68Ga-DOTANOC synthesis

Abstract

In the last years (68)Ga has got into the focus of researchers and clinicians especially for radio-labeling of biomolecules; an important characteristic of this positron emitting isotope is its availability via the (68)Ge/(68)Ga generator system: the long-lived (68)Ge (t1/2=270.8 d) produces the short-lived (68)Ga (t1/2=67.63 min) which decays to stable (68)Zn. (68)Ge breakthrough compromises (68)Ga radionuclidic purity, while (68)Zn might affect the specific activity of the radiopharmaceutical. In this paper we investigated the weight of these impurities in (68)Ga-DOTANOC synthesis. (65)Zn (t1/2=244.26d; decay mode: EC 98.3%, β(+) 1.7%) was used as a radiotracer of stable (68)Zn; samples of the purification columns, wastes and product were recovered and measured with a calibrated HPGe gamma-ray spectrometry system. The results showed that (68)Zn competes with (68)Ga in labeling DOTANOC with a (95±2)% labeling yield; they also proved the effectiveness of the STRATA X-C cationic post-processing of the generator eluate in lowering the amount of this impurity to less than 1%. Moreover this approach, along with the purification of the final product through a STRATA X cartridge, effectively removes (68)Ge breakthrough providing a (68)Ga-DOTANOC radionuclidic purity of (99.9999986±0.0000006)%, superior to 99.9% required by the Pharmacopoeia Monograph on (68)Ga Edotreotide injection.

Keywords: (65)Zn; (68)Ga-DOTANOC; Radioactive tracer; Radionuclidic purity; Specific activity.