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, beta+: 18%, E(beta+ max) = 653 keV; beta-: 37%, E(beta- 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.


CONCLUSIONS
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.
Garrison
Jered C
JC
Harry S. Truman Memorial VA Hospital, Columbia, Missouri 65201-5275, USA.
Rold
Tammy L
TL
Sieckman
Gary L
GL
Figueroa
Said Daibes
SD
Volkert
Wynn A
WA
Jurisson
Silvia S
SS
Hoffman
Timothy J
TJ
D000818:Animals; D001839:Bombesin / Q000031:analogs & derivatives*; D045744:Cell Line, Tumor; D002614:Chelating Agents; D003301:Copper Radioisotopes; D005260:Female; D006573:Heterocyclic Compounds, 1-Ring; D007553:Isotope Labeling; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D016513:Mice, SCID; D009942:Organometallic Compounds; D049268:Positron-Emission Tomography / Q000379:methods*; D011471:Prostatic Neoplasms / Q000175:diagnosis*; D019275:Radiopharmaceuticals; D014057:Tomography, X-Ray Computed / Q000379:methods*
C483895:(4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo(6.6.2)hexadecane)copper(II); D002614:Chelating Agents; D003301:Copper Radioisotopes; D006573:Heterocyclic Compounds, 1-Ring; D009942:Organometallic Compounds; D019275:Radiopharmaceuticals; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D001839:Bombesin