Peptide Receptor Radionuclide Therapy (PRRT) with <sup>177</sup>Lu-DOTA-peptides requires <sup>177</sup>Lu with high specific activity (SA) and values >740 GBq <sup>177</sup>Lu per mg Lu to maximise the atom% of<sup>177</sup>Lu over total Lu. Vendors provide SA values which are based on activity and mass of the target, whereas due to "burn-up" of target, these SA values are not accurate. For a radiochemist the SA of <sup>177</sup>Lu is of interest prior to radiolabeling. An alternative method to determine SA was developed by HPLC, which includes a metal titration of a known amount of DOTA-peptide with a known amount of activity (<sup>177</sup>Lu), and a unknown amount of metal (<sup>177+na</sup>tLu). Based on an HPLC separation of radiometal-DOTA-peptide and DOTA-peptide, and the concordant ratio of these components the metal content (<sup>177+na</sup>tLu) can be calculated, and eventually the SA of <sup>177</sup>Lu can be accurately determined. These experimentally determined SA values exceeded the estimated values provided by vendors by 27±16%, (range 6-73 %). The deviation of SA values for samples from the same Lu batch was <2% (n≤10). In conclusion: the SA of <sup>177</sup>Lu is apparently often higher as stated by vendors in comparison to the experimentally determined actual values. For this reason, the SA of <sup>177</sup>Lu-DOTA-TATE and other Lu-DOTA-peptides could be increased accordingly.

177Lu-DOTA-TATE, DOTA-peptides, HPLC, Lutetium-<sup>177</sup>, Specific activity, Titration
Current Radiopharmaceuticals
Erasmus MC: University Medical Center Rotterdam

Breeman, W.A.P, de Zanger, R, Chan, H.S, & de Blois, E. (2015). Alternative method to determine specific activity of 177Lu by HPLC. Current Radiopharmaceuticals, 8(2), 119–122. Retrieved from