In vivo evaluation and small-animal PET/CT of a prostate cancer mouse model using ⁶⁴Cu bombesin analogs: Side-by-side comparison of the CB-TE2A and DOTA chelation systems

The BB2 receptor subtype, of the bombesin family of receptors, has been shown to be highly overexpressed in a variety of human tumors, including prostate cancer. Bombesin (BBN), a 14-amino acid peptide, has been shown to target the BB2 receptor with high affinity. ⁶⁴Cu (half-life = 12.7 h, β⁺: 18%, E_β+max = 653 keV; β^-: 37%, E_β-max = 578 keV) is a radioisotope that has clinical potential for application in both diagnostic imaging and radionuclide therapy. Recently, new chelation systems such as 1,4,8,11-tetraazabicyclo[6.6.2] hexadecane-4,11-diacetic acid (CB-TE2A) have been reported to significantly stabilize the ⁶⁴Cu radiometal in vivo. The increased stability of the ⁶⁴Cu-CB-TE2A chelate complex has been shown to significantly reduce nontarget retention compared with tetraazamacrocycles such as 1,4,7,10-tetraazacyclodoadecane-N,N′,N″,N‴-tetraacetic acid (DOTA). The aim of this study was to determine whether the CB-TE2A chelation system could significantly improve the in vivo stability of ⁶⁴Cu bombesin analogs. The study directly compares ⁶⁴Cu bombesin analogs using the CB-TE2A and DOTA chelation systems in a prostate cancer xenograft SCID (severely compromised immunodeficient) mouse model. Methods: The CB-TE2A-8-AOC-BBN(7-14)NH₂ and DOTA-8-AOC-BBN(7-14) NH₂ conjugates were synthesized and radiolabeled with ⁶⁴Cu. The receptor-binding affinity and internalization profile of each metallated conjugate was evaluated using PC-3 cells. Pharmacokinetic and small-animal PET/CT studies were performed using female SCID mice bearing PC-3 xenografts. Results: In vivo BB2 receptor targeting was confirmed by tumor uptake values of 6.95 ± 2.27 and 4.95 ± 0.91 %ID/g (percentage injected dose per gram) at the 15-min time point for the ⁶⁴Cu-CB-TE2A and ⁶⁴Cu-DOTA radioconjugates, respectively. At the 24-h time point, liver uptake was substantially reduced for the ⁶⁴Cu-CB-TE2A radioconjugate (0.21 ± 0.06 %ID/g) compared with the ⁶⁴Cu-DOTA radioconjugate (7.80 ± 1.51 %ID/g). The ⁶⁴Cu-CB-TE2A-8-AOC- BBN(7-14)NH₂ radioconjugate demonstrated significant clearance, 98.60 ± 0.28 %ID, from the mouse at 24 h after injection. In contrast, only 67.84 ± 5.43 %ID of the ⁶⁴Cu activity was excreted using the ⁶⁴Cu-DOTA-8-AOC-BBN(7-14)NH₂ radioconjugate because of nontarget retention. Conclusion: The pharmacokinetic and small-animal PET/CT studies demonstrate significantly improved nontarget tissue clearance for the ⁶⁴Cu-CB-TE2A8-AOC-BBN(7-14)NH₂. This is attributed to the improved in vivo stability of the ⁶⁴Cu-CB-TE2A chelate complex as compared with the ⁶⁴Cu-DOTA chelate complex.