Having shown that [ 201 Tl(K222-PSMA)] + exhibits adequate stability under physiological conditions we investigated the ability of this conjugate to specifically bind PSMA in vitro. To this end, the PSMA-positive cell line, DU145-PSMA+, and the parental PSMA-negative DU145 cell line (~500 000 cells), were incubated with [ 201 Tl(K222-PSMA)] + for either 10 min or 1 h. Additionally, a control experiment was performed where both cell types were preincubated with the PSMA inhibitor, 2-PMPA (2-(phosphonomethyl) pentane-1,5-dioic acid) , to block PSMA receptors, prior to incubation with [ 201 Tl(K222-PSMA)] + for 1 h. After incubation, cells were washed and lysed, and cell lysates were collected and counted for radioactivity using a -counter (Supporting Information, Figure and Table ).