Fast and facile purification of ⁶⁸Ga-labeled compounds using solid-phase extraction

Objectives Chelate-conjugated compounds can be labeled with ⁶⁸Ga in a one-step procedure. During the labeling procedure formation of ⁶⁸Ga-colloid may occur. Upon i.v. injection ⁶⁸Ga colloid will accumulate in the liver and spleen. Here, we compared various methods to remove ⁶⁸Ga colloid from a ⁶⁸Ga-labeled tracer preparation.

Methods DOTA-Exendin-3 was labeled with ⁶⁸Ga and quality control was performed with ITLC-SG with two mobile phases to determine the content of unincorporated ⁶⁸Ga and ⁶⁸Ga colloid. The labeled compound was purified by (1) gel filtration (GF), (2) RP-HPLC on a C18 column or (3) solid-phase extraction (SPE) on a HLB cartridge (Waters). The in vivo characteristics were determined in BALB/c nude mice and PET images were acquired 1 h p.i. Unpurified ⁶⁸Ga-Exendin and ¹¹¹In-labeled Exendin-3 were used as a reference.

Results The colloid content of ¹¹¹In-Exendin-3 and unpurified, GF, RP-HPLC and SPE-purified ⁶⁸Ga-Exendin-3 was < 3%, 9%, 7% < 3% and < 3%, respectively. Unpurified Exendin-3 showed high liver and spleen uptake: 6.1 ± 1.0 and 4.5 ± 0.7 %ID, respectively. GF partly removed ⁶⁸Ga colloid, resulting in moderate liver and spleen uptake (3.0 ± 0.3 and 1.4 ± 0.3 %ID/g). SPE-purified Exendin-3 showed very low liver (0.8 ± 0.0 %ID/g) and spleen (0.5 ± 0.1 %ID/g) uptake and were similar to that of RP-HPLC purified Exendin-3 (0.6 ± 0.1 and 0.4 ± 0.1 %ID/g, respectively). SPE and RP-HPLC showed the same liver and spleen uptake as ¹¹¹In-Exendin-3: 0.7 ± 0.1 and 0.3 ± 0.1 %ID/g, respectively. The PET images clearly showed the enhanced liver uptake after injection of unpurifed ⁶⁸Ga-Exendin-3. Liver uptake of GF purified Exendin-3 was reduced, whereas it was absent in the PET images after injection of the HPLC and SPE-purified preparations (Figure 1).

Conclusions Solid-phase extraction using HLB is a fast and simple method to remove ⁶⁸Ga-colloid. The uptake in liver and spleen of the SPE-purified product was similar to ¹¹¹In-Exendin-3 or HPLC purified ⁶⁸Ga-Exendin-3, indicating complete removal of ⁶⁸Ga-colloid