(R)-NODAGA-PSMA: A Versatile Precursor for Radiometal Labeling and Nuclear Imaging of PSMA-Positive Tumors

Purpose The present study aims at developing and evaluating an urea-based prostate specific membrane antigen (PSMA) inhibitor suitable for labeling with In-111 for SPECT and intraoperative applications as well as Ga-68 and Cu-64 for PET imaging. Methods The PSMA-based inhibitor-lysine-urea-glutamate-coupled to the spacer Phe-Phe-D-Lys (suberoyl) and functionalized with the enantiomerically pure prochelator (R)-1-(1-carboxy-3-carbotertbutoxypropyl)-4,7-carbotartbutoxymethyl)-1,4 ,7-triazacyclononane ((R)-NODAGA(tBu)(3)), to obtain (R)-NODAGA-Phe-Phe-D-Lys(suberoyl)-Lys-urea-Glu (CC34). CC34 was labeled with In-111, Ga-68 and Cu-64. The radioconjugates were further evaluated in vitro and in vivo in LNCaP xenografts by biodistribution and PET studies. Biodistribution studies were also performed with Ga-68-HBED-CC-PSMA (HBED-CC: N,N'-bis[2-hydroxy5-( carboxyethyl)benzyl]ethylenediamine-N,N'-diacetic acid) and In-111-PSMA-617 for comparison. Results Ga-68-CC34, Cu-64-CC34, and In-111-CC34 were prepared in radiochemical purity >95%. Ga-68/nat-CC34, Cu-64/nat-CC34, In-111/nat-CC34, Ga-68/nat-HBED-CC-PSMA, and In-111/nat-PSMA- 617 exhibited high affinity for the LNCaP cells, with K-d values of 19.3 +/- 2.5 nM, 27.5 +/- 2.7 nM, 5.5 +/- 0.9 nM, 2.9 +/- 0.6 nM and 5.4 +/- 0.8 nM, respectively. They revealed comparable internalization profiles with approximately 75% of the total cell associated activity internalized after 3 h of incubation. Ga-68-CC34 showed very high stability after its administration in mice. Tumor uptake of Ga-68-CC34 (14.5 +/- 2.9% IA/g) in LNCaP xenografts at 1 h p.i. was comparable to Ga-68-HBED-CC-PSMA (15.8 +/- 1.4% IA/g) (P = 0.67). The tumor-to-normal tissue ratios at 1 and 2 h p.i of Ga-68-CC34 were also comparable to Ga-68-HBED-CC-PSMA (P>0.05). Tumor uptake of In-111-CC34 (28.5 +/- 2.6%IA/g) at 1 h p.i. was lower than In-111-PSMA-617 (52.1 +/- 6.5% IA/g) (P = 0.02). The acquisition of PET-images with Cu-64-CC34 at later time points showed wash-out from the kidneys, while tumor uptake still remained relatively high. This resulted in an increased tumor-to-kidney ratio over time. Conclusions Ga-68-CC34 is comparable to Ga-68-HBED-CC-PSMA in terms of tumor uptake and tumor to normal tissue ratios. Cu-64-CC34 could enable high contrast imaging of PSMA positive tissues characterized by elevated expression of PSMA or when delayed imaging is required. Cu-64-CC34 is currently being prepared for clinical translation.