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

In the last years Ga-68 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 Ge-68/Ga-68 generator system: the long-lived Ge-68 (t(1/2)=270.8 d) produces the short-lived Ga-68 (t(1/2)= 67.63 min) which decays to stable Zn-68. Ge-68 breakthrough compromises Ga-68 radionuclidic purity, while Zn-68 might affect the specific activity of the radiopharmaceutical. In this paper we investigated the weight of these impurities in Ga-68-DOTANOC synthesis. Zn-65 (t(1/2)=244.26 d; decay mode: EC 98.3%, beta(+) 1.7%) was used as a radiotracer of stable Zn-68; samples of the purification columns, wastes and product were recovered and measured with a calibrated HPGe gamma-ray spectrometry system. The results showed that Zn-68 competes with Ga-68 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 Ge-68 breakthrough providing a Ga-68-DOTANOC radionuclidic purity of (99.9999986 +/- 0.0000006)%, superior to 99.9% required by the Pharmacopoeia Monograph on Ga-68 Edotreotide injection. (C) 2013 Elsevier Ltd. All rights reserved.