Synthesis and initial tumor affinity testing of iodine-123 labelled EGFR-affine agents as potential imaging probes for hormone-refractory prostate cancer.
The epidermal growth factor receptor (EGFR) is over-expressed in a variety of human cancers, including in hormone-refractory prostate carcinomas, in which the EGFR has been associated with advanced disease stage, resistance to standard treatment and poor prognosis. Therefore, the EGFR is considered to be a promising molecular target for molecular imaging and therapy for hormone-refractory prostate cancer. This work describes the synthesis and initial tumor affinity testing of the EGFR antagonist (123)I-mAb425 and the EGF receptor tyrosine kinase (EGFR-TK) inhibitor (123)I-PD153035 as potential imaging probes for studying EGFR-expressing prostate cancer using single photon emission tomography.


METHODS
(123)I-mAb425 and (123)I-PD153035 were prepared, starting from the IgG2a antibody and EGFR antagonist mAb425, that binds to the external domain of the EGF receptors, and from the EGFR-TK inhibitor PD153035, targeting the intra-endothelial tyrosine kinase domain of the EGFR, respectively. The potential of (123)I-mAb425 and (123)I-PD153035 to target EGFR-positive prostate carcinoma was tested on androgen-insensitive PC-3 and DU-145 prostate carcinoma cell lines, and on the androgen-sensitive LNCaP prostate cancer cell line for comparison. In vivo, the capability of (123)I-mAb425 and (123)I-PD153035 to target hormone-refractory prostate cancer was assessed in RNU rats or nu/nu mice bearing human PC3 prostate cancer xenografts.


RESULTS
(123)I-mAb425 was obtained in >90% radiochemical yield using the IODO-GEN method. (123)I-PD153035 was synthesized by a non-isotopic [(123)I]iodo-debromination of PD153035 in 50-60% radiochemical yield in a total synthesis time including HPLC separation of 70 min. In vitro (123)I-mAb425 and (123)I-PD153035 accumulated highly in human PC-3 and DU-145 prostate cancer cells. Radioactivity incorporation into PC-3 and DU-145 tumor cells following 15-min incubation at 37 degrees C varied from 25% to 48% of the total loaded activity per 10(6) tumor cells (560-1230 cpm/1000 cells). In comparison, the uptake of the EGFR-affine probes into LNCaP prostate carcinoma cells was significantly low (105 +/- 25 cpm/1000 cells). Inhibition experiments revealed that (123)I-mAb425 is taken up into tumor cells via the same pathway as the naturally occurring epidermal growth factor (EGF), while (123)I-PD153035 accumulation in prostate cancer cells occurs presumably via the same pathway as the selective EGFR-Tyrosine kinase antagonist AG1418. In vivo, the human prostate cancer xenografts in mouse war accurately visualized after i.v. administration of (123)I-mAb425 by a gamma camera.


CONCLUSIONS
These data suggest that (123)I-mAb425 and (123)I-PD153035 are promising candidates as imaging probes for EGFR-positive prostate cancer and warrant further in vivo validations to ascertain their potential as imaging agents for clinical used.
Department of Nuclear Medicine, Saarland University Medical Center, Homburg, Germany.
Thierry
T
Fozing
Claudia
C
Scheuer
Samuel
S
Samnick
D000818:Animals; D000911:Antibodies, Monoclonal / Q000032:analysis* / Q000737:chemistry / Q000276:immunology; D001692:Biological Transport; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D066246:ErbB Receptors / Q000037:antagonists & inhibitors* / Q000276:immunology / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic; D006728:Hormones / Q000494:pharmacology*; D006801:Humans; D007457:Iodine Radioisotopes; D008297:Male; D051379:Mice; D057054:Molecular Imaging / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000235:genetics / Q000378:metabolism*; D047428:Protein Kinase Inhibitors / Q000378:metabolism / Q000494:pharmacology; D011799:Quinazolines / Q000378:metabolism / Q000494:pharmacology; D011877:Radionuclide Imaging; D051381:Rats; D014054:Tomography; D023041:Xenograft Model Antitumor Assays
D000911:Antibodies, Monoclonal; D006728:Hormones; D007457:Iodine Radioisotopes; D047428:Protein Kinase Inhibitors; D011799:Quinazolines; D066246:ErbB Receptors; C088860:4-((3-bromophenyl)amino)-6,7-dimethoxyquinazoline