Improved ⁶⁸ Ga-labeling method using ethanol addition: Application to the α-helical peptide DOTA-FAMP

We examined the ⁶⁸ Ga labeling of the α-helical peptide, DOTA-FAMP, and evaluated conformational changes during radiolabeling. ⁶⁸ Ga-DOTA-FAMP is a positron emission tomography probe candidate for atherosclerotic plaques. The labeling yield achieved by Zhernosekov's method (using acetone for ⁶⁸ Ga purification) was compared with that achieved by the original and 2 modified Mueller's methods (using NaCl solution). Modified method I involves desalting the ⁶⁸ Ga prior to labeling, and modified method II involves the inclusion of ethanol in the labeling solution. The labeling yield using Zhernosekov's method was 62% ± 5.4%. In comparison, Mueller's original method gave 8.9% ± 1.7%. Modified method I gave a slight improvement of 32% ± 2.1%. Modified method II further increased the yield to 66% ± 3.4%. Conformational changes were determined by circular dichroism spectroscopy, revealing that these differences could be attributed to conformational changes. Heat treatment affects peptide conformation, which leads to aggregation and decreases the labeling yield. Mueller's method is simpler, but harsh conditions preclude its application to biomolecules. To suppress aggregation, we included a desalting process and added ethanol in the labeling solution. These changes significantly improved the labeling yield. Before use for imaging, conformational changes of biomolecules during radiolabeling should be evaluated by circular dichroism spectroscopy to ensure the homogeneity of the labeled product.