The importance of high specific radioactivity in the performance of ⁶⁸Ga-labeled peptide

Abstract 

The use of ⁶⁸Ga-labeled peptides in diagnosis, dosimetry, therapy planning and follow-up of response to chemo- and radiotherapy requires accurate quantification of tracer binding characteristics in vivo, which may be influenced by the specific radioactivity (SRA) of the tracer.

Systematic study of the complexation reaction of DOTA-D-Phe¹-Tyr³-Octreotide (DOTATOC, where DOTA is the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) with ⁶⁷Ga, ⁶⁸Ga, ^69,71Ga and in the presence of competing metal cations [Al(III), Fe(III), In(III)] was performed using conventional and microwave heating techniques and assessed by mass spectrometry. Saturation binding of ⁶⁸Ga-DOTATOC to Rhesus monkey brain slices was performed using frozen section autoradiography.

High SRA was necessary in order to characterize the saturation binding of ⁶⁸Ga-DOTATOC to somatostatin receptors in Rhesus monkey brain sections. The complexation of Ga(III) with DOTATOC suggested more favorable formation compared to Fe(III) and In(III). The microwave heating mode might influence the selectivity of the complexation reaction, especially when comparing the behavior of Ga(III) and In(III). Al(III) was less critical with contamination and could be tolerated up to a concentration equal to that of the peptide bioconjugate. The SRA of ⁶⁷Ga-DOTATOC and ⁶⁷Ga-NODAGA-TATE (NODAGA-Tyr³-Octreotate, where NODAGA is 1,4,7-triazacyclononane-1-glutaric acid-4,7-diacetic acid) exceeded literature data by a factor of 7 and 5–15, respectively.

High SRA was critical for providing sufficient contrast and accurate quantification of PET images. Microwave heating mode apart from the acceleration of the labeling reaction also improved the selectivity of the complexation reaction towards gallium. Fe(III) was shown to be the most critical competitor deteriorating the ⁶⁸Ga-labeling efficiency.

Keywords: Specific radioactivity, PET, ⁶⁸Ga, Bioconjugate, DOTATOC