68Ga, 44Sc and 177Lu-labeled AAZTA5-PSMA-617: synthesis, radiolabeling, stability and cell binding compared to DOTA-PSMA-617 analogues

Abstract

Background The AAZTA chelator and in particular its bifunctional derivative AAZTA5 was recently investigated to demonstrate unique capabilities to complex diagnostic and therapeutic trivalent radiometals under mild conditions. This study presents a comparison of 68Ga, 44Sc and 177Lu-labeled AAZTA5-PSMA-617 with DOTA-PSMA-617 analogues. We evaluated the radiolabeling characteristics, in vitro stability of the radiolabeled compounds and evaluated their binding affinity and internalization behavior on LNCaP tumor cells in direct comparison to the radiolabeled DOTA-conjugated PSMA-617 analogs. Results AAZTA5 was synthesized in a five-step synthesis and coupled to the PSMA-617 backbone on solid phase. Radiochemical evaluation of AAZTA5-PSMA-617 with 68Ga, 44Sc and 177Lu achieved quantitative radiolabeling of > 99% after less than 5 min at room temperature. Stabilities against human serum, PBS buffer and EDTA and DTPA solutions were analyzed. While there was a small degradation of the 68Ga complex over 2 h in human serum, PBS and EDTA/DTPA, the 44Sc and 177Lu complexes were stable at 2 h and remained stable over 8 h and 1 day. For all three compounds, i.e. [natGa]Ga-AAZTA5-PSMA-617, [natSc]Sc-AAZTA5-PSMA-617 and [natLu]Lu-AAZTA5-PSMA-617, in vitro studies on PSMA-positive LNCaP cells were performed in direct comparison to radiolabeled DOTA-PSMA-617 yielding the corresponding inhibition constants (Ki). Ki values were in the range of 8–31 nM values which correspond with those of [natGa]Ga-DOTA-PSMA-617, [natSc]Sc-DOTA-PSMA-617 and [natLu]Lu-DOTA-PSMA-617, i.e. 5–7 nM, respectively. Internalization studies demonstrated cellular membrane to internalization ratios for the radiolabeled 68Ga, 44Sc and 177Lu-AAZTA5-PSMA-617 tracers (13–20%IA/106 cells) in the same range as the ones of the three radiolabeled DOTA-PSMA-617 tracers (17–20%IA/106 cells) in the same assay. Conclusions The AAZTA5-PSMA-617 structure proved fast and quantitative radiolabeling with all three radiometal complexes at room temperature, excellent stability with 44Sc, very high stability with 177Lu and medium stability with 68Ga in human serum, PBS and EDTA/DTPA solutions. All three AAZTA5-PSMA-617 tracers showed binding affinities and internalization ratios in LNCaP cells comparable with that of radiolabeled DOTA-PSMA-617 analogues. Therefore, the exchange of the chelator DOTA with AAZTA5 within the PSMA-617 binding motif has no negative influence on in vitro LNCaP cell binding characteristics. In combination with the faster and milder radiolabeling features, AAZTA5-PSMA-617 thus demonstrates promising potential for in vivo application for theranostics of prostate cancer.

Hybrid Chelator-Based PSMA Radiopharmaceuticals: Translational Approach

“…These findings correlate with the results published by Sinnes et al, who investigated the influence of the exchange of DOTA chelator in DOTA-PSMA-617 against AAZTA 5 . Both DOTA-PSMA-617 and AAZTA 5. -PSMA-617 displayed similar in vitro binding affinities and internalization ratios in LNCaP cells [40]. However, the reported binding affinities and internalization ratios of AATA 5 -PSMA-617 and DOTA-PSMA-617 were higher than those of the SA.KuE conjugates.…”

Prostat Kanserinde ²²⁵Ac-PSMA-617 ve ¹⁷⁷Lu-PSMA-617’nin Subsellüler Dozimetriye Dayalı Göreceli Etkinliği

“…Alternatively, extrapolation of 68 Ga-PSMA-617 uptake on positron emission tomography was previously used for dosimetry of 213 Bi-PSMA-617 ( 20 ). However, recent studies have shown that the degree of radiolabeled PSMA-617 uptake differs based on the radionuclide ( 23 , 24 ), which suggests that independent characterization of 225 Ac-PSMA-617 uptake will improve its dosimetry. At present, the only study that examined the tumor uptake of 225 Ac-PSMA-617 and 177 Lu-PSMA-617 is the pre-clinical study by Current et al ( 21 ), where ex vivo activity measurements were used for accurate uptake estimation.…”

Cold Kit Labeling: The Future of 68Ga Radiopharmaceuticals?

“…For instance, Varasteh et al investigated the influence of four different macrocyclic chelators on a bombesin analog targeting properties ( 97 ), while Renard et al evaluated seven different chelators coupled to a neurotensin receptor 1 antagonist ( 98 ). Other examples of such a strategy also include comparison of [ 68 Ga]Ga-THP-TATE with [ 68 Ga]Ga-DOTATATE and [ 68 Ga]Ga-AAZTA 5 -PSMA-617 with [ 68 Ga]Ga-DOTA-PSMA-617 ( 99 , 100 ).…”