The importance of radiochemical purity: Cellular binding and internalization of different radiometal chlorides in prostate cancer cells

Radiometals play an important role in nuclear medicine, both for imaging and therapy. Binding studies represent an important step in the development of new radiolabeled ligands, as valuable insights into the binding properties can be gained. However, this technique requires high radiochemical purity, otherwise results may lead to wrong assumptions or misinterpretations of affinities or uptake rates. Therefore, this in vitro study aimed at investigating the cell binding and internalization characteristics of different radiometal chlorides ([¹¹¹In]InCl₃, [⁶⁸Ga]GaCl₃ and [¹⁷⁷Lu]LuCl₃) commonly applied in nuclear medicine, as well as the clinically applied [¹⁷⁷Lu]Lu-PSMA-I&T in comparison, by using prostate cancer cells. PC-3 and LNCaP cells were incubated with 100 kBq of the respective radiometal chloride or [¹⁷⁷Lu]Lu-PSMA-I&T for 1 h. For [¹⁷⁷Lu]LuCl₃, nuclei isolations and colloid determinations in saline and cell medium were also performed. Results showed that [¹¹¹In]InCl₃ and [⁶⁸Ga]GaCl₃ bind and are internalized up to 3 % to PC-3 and LNCaP cells, whereas [¹⁷⁷Lu]LuCl₃ showed cell binding of up to 25 %, internalization up to 2.5 % and a nuclear uptake below 0.3 %. In comparison, [¹⁷⁷Lu]Lu-PSMA-I&T showed only 3 % total cell binding to LNCaP cells. Further analysis of [¹⁷⁷Lu]LuCl₃ stability in NaCl and cell medium showed only low amounts of colloids, which are not increasing over time, and negligible unspecific binding to the used cell culture plates. In conclusion, the results demonstrate the importance of high radiochemical purity, especially with regard to Lu-177 labeled compounds. Even if radiopharmaceuticals comply with common release-criteria, significant uptake can be derived from [¹⁷⁷Lu]LuCl₃ impurities and lead to wrong estimations of a compound's uptake behavior. Assuming an experimental result of 2 % membrane binding of the applied product, and 5 % residual [¹⁷⁷Lu]LuCl₃ in the final product (thereof 25 % membrane binding, as described above), would lead to 1.25 % membrane binding resulting from [¹⁷⁷Lu]LuCl₃ and only 0.75 % from the radiopharmaceutical.