Synthesis and initial tumor affinity testing of iodine-123 labelled EGFR-affine agents as potential imaging probes for hormone-refractory prostate cancer

Abstract

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 123I-mAb425 and the EGF receptor tyrosine kinase (EGFR-TK) inhibitor 123I-PD153035 as potential imaging probes for studying EGFR-expressing prostate cancer using single photon emission tomography. Methods 123I-mAb425 and 123I-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 123I-mAb425 and 123I-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 123I-mAb425 and 123I-PD153035 to target hormone-refractory prostate cancer was assessed in RNU rats or nu/nu mice bearing human PC3 prostate cancer xenografts. Results 123I-mAb425 was obtained in >90% radiochemical yield using the IODO-GEN ® method. 123I-PD153035 was synthesized by a non-isotopic [ 123I]iodo-debromination of PD153035 in 50–60% radiochemical yield in a total synthesis time including HPLC separation of 70 min. In vitro 123I-mAb425 and 123I-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 °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 123I-mAb425 is taken up into tumor cells via the same pathway as the naturally occurring epidermal growth factor (EGF), while 123I-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 123I-mAb425 by a gamma camera. Conclusion These data suggest that 123I-mAb425 and 123I-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.