Evaluation of a simplified radiolabeling method for a PARP inhibitor in an animal model of breast cancer

Background: Several poly (adenosine diphosphate–ribose) polymerase (PARP) inhibitors were recently approved by the US Food and Drug Administration for use in cancer treatment. To facilitate the discovery of novel PARP-targeting ligands, a radioiodinated ligand, I-125-KX1, was developed and validated for its specificity to PARP-1; however, its preparation procedure is time-consuming. The present study employed a solid-phase extraction (SPE) method in the radiolabeling procedure of I-123/125-KX1 and evaluated its binding specificity by using receptor binding assays, autoradiography, and in vivo single photon emission computed tomography (SPECT) imaging technique. Results: Through the incorporation of the SPE purification method as the final step in the radioiodination procedure, the resultant product I-123/125-KX1 exhibited high radiochemical purity (> 99%) and an acceptable radiochemical yield (58.6% for I-123-KX1, 73.3% for I-125-KX1). The binding characteristics of this radiotracer were validated through saturation binding assays conducted on MDA-MB-231 and MCF-7 cells. The K_d values obtained for the tracer (~ 1.0 nM) was consistent with values reported in the literature, and the B_max values of these two cell lines (2017 ± 178 fmol/mg on MDA-MB-231 vs. 1393 ± 105 fmol/mg on MCF-7) were in line with the results from Western blot analyses. To demonstrate the in vivo imaging ability of I-123-KX1 prepared in this study, an MDA-MB-231 tumor animal model was used and the tracer displayed a suitable uptake on the tumor tissues (6.9 ± 0.8%ID/mL). The binding specificity of the SPE-purified I-125-KX1 was further verified using in vitro autoradiography in conjunction with various PARP inhibitors. Additionally, an anti-PARP-1 immunohistochemistry experiment was conducted, which revealed that the autoradiograms of the radiotracer displayed a similar pattern. Conclusions: This suggests that the I-123/125-KX1 prepared using the SPE method showed some comparable properties to those from the traditional method, indicating its potential suitability for future radioligand preparation in PARP studies. However, further characterization studies may be needed to confirm its efficacy.