Increasing molar activity by HPLC purification improves 68Ga-DOTA-NAPamide tumor accumulation in a B16/F1 melanoma xenograft model

Abstract

Purpose Melanocortin receptor 1 (MC1R) is overexpressed in melanoma and may be a molecular target for imaging and peptide receptor radionuclide therapy. 68 Gallium ( 68 Ga) labeling of DOTA-conjugated peptides is an established procedure in the clinic for use in positron emission tomography (PET) imaging. Aim of this study was to compare a standard labeling protocol against the 68 Ga-DOTA peptide purified from the excess of unlabeled peptide. Procedures The MC1R ligand DOTA-NAPamide was labeled with 68 Ga using a standard clinical protocol. Radioactive peptide was separated from the excess of unlabeled DOTA-NAPamide by HPLC. Immediately after the incubation of peptide and 68 Ga (95°C, 15 min), the reaction was loaded on a C18 column and separated by a water/acetonitrile gradient, allowing fractionation in less than 20 minutes. Radiolabeled products were compared in biodistribution studies and PET imaging using nude mice bearing MC1R-expressing B16/F1 xenograft tumors. Results In biodistribution studies, non-purified 68 Ga-DOTA-NAPamide did not show significant uptake in the tumor at 1 h post injection (0.78% IA/g). By the additional HPLC step, the molar activity was raised around 10,000-fold by completely removing unlabeled peptide. Application of this rapid purification strategy led to a more than 8-fold increase in tumor uptake (7.0% IA/g). The addition of various amounts of unlabeled DOTA-NAPamide to the purified product led to a blocking effect and decreased specific tumor uptake, similar to the result seen with non-purified radiopeptide. PET imaging was performed using the same tracer preparations. Purified 68 Ga-DOTA-NAPamide, in comparison, showed superior tumor uptake. Conclusions We demonstrated that chromatographic separation of radiolabeled from excess unlabeled peptide is technically feasible and beneficial, even for short-lived isotopes such as 68 Ga. Unlabeled peptide molecules compete with receptor binding sites in the target tissue. Purification of the radiopeptide therefore improved tumor uptake.

Impact of the mouse model and molar amount of injected ligand on the tissue distribution profile of PSMA radioligands

“…This situation makes the comparability between new and existing radiopharmaceuticals often challenging or even impossible. While the tumor mouse model is an important determinant for the resultant data, it is in particular the molar amount of injected ligand that was previously shown to have an impact on the biodistribution of different peptide-based radiopharmaceuticals [1][2][3][4]. Indeed, depending on the choice of these parameters, the overall picture of the radioligand's properties may vary and lead to controversial conclusions.…”

Clinically Applicable Cyclotron-Produced Gallium-68 Gives High-Yield Radiolabeling of DOTA-Based Tracers

“…Increased in vitro cell uptake with AMA was also reported for the same radiotracer by Sanchez-Crespo et al [28]. In a study by von Hacht et al, the low AMA of the DOTA-based 68 Ga-labeling was resolved by preparative HPLC purification, thereby improving the detection of small metastases [29]. The level of AMA and its impact in diagnostics is an interesting and important aspect, which is made available also for DOTA-based 68 Ga-labeled radiopharmaceuticals by the results from this present study.…”