Evaluating Ga-68 Peptide Conjugates for Targeting VPAC Receptors: Stability and Pharmacokinetics

In recent years, considerable progress has been made in the use of gallium-68 labeled receptor-specific peptides for imaging oncologic diseases. The objective was to examine the stability and pharmacokinetics of [⁶⁸Ga]NODAGA and DOTA-peptide conjugate targeting VPAC [combined for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP)] receptors on tumor cells.

A VPAC receptor-specific peptide was chosen as a model peptide and conjugated to NODAGA and DOTA via solid-phase synthesis. The conjugates were characterized by HPLC and MALDI-TOF. Following Ga-68 chelation, the radiochemical purity of Ga-68 labeled peptide conjugate was determined by radio-HPLC. The stability was tested against transmetallation using 100 nM Fe³⁺/Zn²⁺/Ca²⁺ ionic solution and against transchelation using 200 μM DTPA solution. The ex vivo and in vivo stability of the Ga-68 labeled peptide conjugate was tested in mouse plasma and urine. Receptor specificity was determined ex vivo by cell binding assays using human breast cancer BT474 cells. Positron emission tomography (PET)/X-ray computed tomography (CT) imaging, tissue distribution, and blocking studies were performed in mice bearing BT474 xenografts.

The chemical and radiochemical purity was greater than 95 % and both conjugates were stable against transchelation and transmetallation. Ex vivo stability at 60 min showed that the NODAGA-peptide-bound Ga-68 reduced to 42.1 ± 3.7 % (in plasma) and 37.4 ± 2.9 % (in urine), whereas the DOTA-peptide-bound Ga-68 was reduced to 1.2 ± 0.3 % (in plasma) and 4.2 ± 0.4 % (in urine) at 60 min. Similarly, the in vivo stability for [⁶⁸Ga]NODAGA-peptide was decreased to 2.1 ± 0.2 % (in plasma) and 2.2 ± 0.4 % (in urine). For [⁶⁸Ga]DOTA-peptide, it was decreased to 1.4 ± 0.3 % (in plasma) and 1.2 ± 0.4 % (in urine) at 60 min. The specific BT474 cell binding was 53.9 ± 0.8 % for [⁶⁸Ga]NODAGA-peptide, 25.8 ± 1.4 % for [⁶⁸Ga]-DOTA-peptide, and 18.8 ± 2.5 % for [⁶⁸Ga]GaCl₃ at 60 min. Inveon microPET/CT imaging at 1 h post-injection showed significantly (p < 0.05) higher tumor to muscle (T/M) ratio for [⁶⁸Ga]NODAGA-peptide (3.4 ± 0.3) as compared to [⁶⁸Ga]DOTA-peptide (1.8 ± 0.6). For [⁶⁸Ga]GaCl₃ and blocked mice, their ratios were 1.5 ± 0.6 and 1.5 ± 0.3 respectively. The tissue distributions data were similar to the PET imaging data.

NODAGA is superior to DOTA in terms of radiolabeling kinetics. The method of radiolabeling was reproducible and yielded higher specific activity. Although both agents have relatively low in vivo stability, PET/CT imaging studies delineated BC tumors with [⁶⁸Ga]NODAGA-peptide, but not with [⁶⁸Ga]DOTA-peptide.