Imaging small human prostate cancer xenografts  after pretargeting with bispecific bombesin-antibody  complexes and targeting with high specific radioactivity  labeled polymer-drug conjugates.
OBJECTIVE
Pretargeting with bispecific monoclonal antibodies (bsMAb) for tumor imaging was developed to enhance target to background activity ratios. Visualization of tumors was achieved by the delivery of mono- and divalent radiolabeled haptens. To improve the ability to image tumors with bsMAb, we have combined the pretargeting approach with targeting of high specific activity radiotracer labeled negatively charged polymers. The tumor antigen-specific antibody was replaced with bombesin (Bom), a ligand that binds specifically to the growth receptors that are overexpressed by many tumors including prostate cancer. Bomanti- diethylenetriaminepentaacetic acid (DTPA) bispecific antibody complexes were used to demonstrate pretargeting and imaging of very small human prostate cancer xenografts targeted with high specific activity ¹¹¹In- or ⁹⁹mTc-labeled negatively charged polymers.


METHODS
Bispecific antibody complexes consisting of intact anti-DTPA antibody or Fab′ linked to Bom via thioether bonds (Bom-bsCx or Bom-bsFCx, respectively) were used to pretarget PC-3 human prostate cancer xenografts in SCID mice. Negative control mice were pretargeted with Bom or anti-DTPA Ab. 111In-Labeled DTPA-succinyl polylysine (DSPL) was injected intravenously at 24 h (7.03 ± 1.74 or 6.88 ± 1.89 MBq ¹¹¹In-DSPL) after Bom-bsCx or 50 ± 5.34 MBq of ⁹⁹mTc-DSPL after Bom-bsFCx pretargeting, respectively. Planar or single photon emission computed tomography (SPECT)/CT gamma images were obtained for up to 3 h and only planar images at 24 h. After imaging, all mice were killed and biodistribution of 111In or 99mTc activities were determined by scintillation counting.


RESULTS
Both planar and SPECT/CT imaging enabled detection of PC-3 prostate cancer lesions less than 1-2 mm in diameter in 1-3 h post 111In-DSPL injection. No lesions were visualized in Bom or anti-DTPA Ab pretargeted controls. 111In-DSPL activity in Bom-bsCx pretargeted tumors (1.21 ± 0.36 %ID/g) was 5.4 times that in tumors pretargeted with Bom or anti-DTPA alone (0.22 ± 0.08, p = 0.001). PC-3 xenografts pretargeted with Bom-bsFCx and targeted with ⁹⁹mTc-DSPL were visualizable by 1-3 h. Exquisite tumor uptake at 24 h (6.54 ± 1.58 %ID/g) was about 15 times greater than that of Bom pretargeted controls (0.44 ± 0.17, p = 0.002).


CONCLUSIONS
Pretargeting prostate cancer with Bom-bsCx or Bom-bsFCx enabled fast delivery of high specific radioactivity ¹¹¹In- or ⁹⁹mTc-labeled polymer-drug conjugates resulting in visualization of lesions smaller than 1- 2 mm in diameter within 3 h.
Department of Pharmaceutical Sciences, Northeastern University, School of Pharmacy, Mugar Bldg, Rm 205, 360 Huntington Avenue, Boston, MA 02115, USA.
Vishwesh
V
Patil
Keyur
K
Gada
Rajiv
R
Panwar
Alexandra
A
Varvarigou
Stan
S
Majewski
Andrew
A
Weisenberger
Craig
C
Ferris
Yared
Y
Tekabe
Ban-An
BA
Khaw
D000818:Animals; D018033:Antibodies, Bispecific / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D000911:Antibodies, Monoclonal / Q000276:immunology; D000970:Antineoplastic Agents / Q000737:chemistry; D001839:Bombesin / Q000276:immunology*; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000276:immunology; D007205:Indium Radioisotopes; D007553:Isotope Labeling; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D057054:Molecular Imaging / Q000379:methods*; D015609:Organotechnetium Compounds; D004369:Pentetic Acid / Q000276:immunology; D011107:Polylysine / Q000737:chemistry*; D011471:Prostatic Neoplasms / Q000473:pathology*; D013440:Sulfides / Q000737:chemistry; D047368:Tumor Burden
D018033:Antibodies, Bispecific; D000911:Antibodies, Monoclonal; D000970:Antineoplastic Agents; D007140:Immunoglobulin Fab Fragments; D007205:Indium Radioisotopes; D015609:Organotechnetium Compounds; D013440:Sulfides; D011107:Polylysine; D004369:Pentetic Acid; D001839:Bombesin