Evaluation of PSMA-Targeted TREN-CAM Conjugates for Targeted Imaging of Cancer with 68Ga(III) and 45Ti(IV)

Chelation approaches that are compatible with a multitude of isotopes are an important area of development. Here, we introduce the design, synthesis, and evaluation of 2,3-dihydroxyterephthalate/catechol chelator conjugates compatible with the positron emission tomography (PET) isotopes 68Ga3+ and 45Ti4+, targeting the prostate-specific membrane antigen (PSMA). The conjugates are made in a multistep organic synthesis incorporating 2,3-dihydroxyterephthalate, linked to the amino hexanoic acid-extended, urea-dipeptides EuE or KuE (substrates of the PSMA active site). The radiochemical complexes, [45Ti][Ti(TREN-CAM-hex-EuE)]2-, [45Ti][Ti(TREN-CAM-hex-KuE)]2-, and [68Ga][Ga(TREN-CAM-hex-KuE)]3- form readily at room temperature within 15 min with a molar activity of 24-29 mCi/μmol. The corresponding chelates are stable in phosphate-buffered saline (PBS) solution prior to injection. Subsequent in vivo studies in a bilateral tumor xenograft mouse model were conducted, including 90- and 270-min PET, followed by biodistribution and metabolite analysis at 2 or 5 h postinjection. These studies demonstrated selective uptake of the radiochemical complexes in the PSMA-expressing tumor (17.25 ± 4.15, 13.84 ± 3.85, 15.64 ± 6.37% ID/g for [45Ti][Ti(TREN-CAM-hex-EuE)]2-, [45Ti][Ti(TREN-CAM-hex-KuE)]2- and [68Ga][Ga(TREN-CAM-hex-KuE)]3- respectively), with pharmacokinetics dominated by renal clearance. Delayed clearance of the [45Ti][Ti(TREN-CAM-hex-KuE)]2- complex is observed when compared with that of [68Ga][Ga(TREN-CAM-hex-KuE)]3- as indicated by elevated activity retention in the blood, which we attribute to the charge difference and partial complex dissociation. Urine metabolite analysis shows that [68Ga][Ga(TREN-CAM-hex-KuE)]3- is excreted >98% intact, while [45Ti][Ti(TREN-CAM-hex-KuE)]2- exhibited signs of dechelation. Conclusively, our data support further investigation of bifunctional TREN-CAM derivatives as a synthetically accessible bifunctional chelator class for 68Ga3+ and 45Ti4+ isotopes.