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.