*3.4. Radiosynthesis and Characterization of [165Er]PSMA-617*

Following successful isolation, the [165Er]PSMA-617 was synthesized with the labeling yields summarized in Table 5. Radioanalytical HPLC showed [165Er]PSMA-617 (UV

retention time (RT) = 6.45 min, radioactivity RT = 6.60), co-eluting with cold Er-PSMA-617 and Ho-PSMA-617 standards (230 nm RT = 6.45 min), shown in Supplementary Figure S7. Radiochemical purity of the [165Er]PSMA-617 was > 95%. Visible also in the 230 nm absorbance chromatograph of the [165Er]PSMA-617 radiopharmaceutical were multiple mass peaks with RT = 6.8–7.0 min, retention times corresponding to [natZn]PSMA-617 (230 nm RT = 6.90 min), [natFe]PSMA-617 (230 nm RT = 6.80 min), and [natCu]PSMA-617 (230 nm RT = 6.95 min) (see Supplementary Figure S8). The [165Er]PSMA-617 radiochemical purity remained > 95% after 12 h incubation in whole human serum, the longest time point assessed. The octanol-PBS distribution coefficient of [165Er]PSMA-617 was <sup>−</sup>3.3 <sup>±</sup> 0.3 (*n* = 5).


**Table 5.** [ <sup>165</sup>Er]PSMA-617 radiolabeling yields.

† Radioactivity and MA decay-corrected to end of bombardment.
