A new Ga-labeled BBN peptide with a hydrophilic linker for GRPR-targeted tumor imaging

Bombesin (BBN) is a peptide exhibiting high affinity for the gastrin-releasing peptide receptor (GRPR), which is overexpressed on several types of cancers. Various GRPR antagonists and agonists have been labeled with radiometals for positron emission tomography (PET) imaging of GRPR-positive tumors. However, unfavorable hepatobiliary excretion such as high intestinal activity may prohibit their clinical utility for imaging abdominal cancer. In this study, the modified BBN peptide with a new hydrophilic linker was labeled with Ga for PET imaging of GRPR-expressing PC-3 prostate cancer xenograft model. GRPR antagonists, MATBBN (Gly-Gly-Gly-Arg-Asp-Asn-d-Phe-Gln- Trp-Ala-Val-Gly-His-Leu-NHCHCH) and ATBBN (d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-NHCHCH), were conjugated with 1,4,7-triazacyclononanetriacetic acid (NOTA) and labeled with Ga. Partition coefficient and in vitro stability were also determined. GRPR binding affinity of both tracers was investigated by competitive radioligand binding assay. The in vivo receptor targeting potential and pharmacokinetic of Ga-NOTA-MATBBN were also evaluated in PC-3 prostate tumor model and compared with those of Ga-NOTA-ATBBN. NOTA-conjugated BBN analogs were labeled with Ga within 20 min with a decay-corrected yield ranging from 90 to 95 % and a radiochemical purity of more than 98 %. The specific activity of Ga-NOTA-MATBBN and Ga-NOTA-ATBBN was at least 16.5 and 11.9 GBq/μmol, respectively. The radiotracers were stable in phosphate-buffered saline and human serum. Ga-NOTA-MATBBN was more hydrophilic than Ga-NOTA- ATBBN, as indicated by their log P values (-2.73 ± 0.02 vs. -1.20 ± 0.03). The IC values of NOTA-ATBBN and NOTA-MATBBN were similar (102.7 ± 1.18 and 124.6 ± 1.21 nM). The accumulation of Ga-labeled GRPR antagonists in the subcutaneous PC-3 tumors could be visualized via small animal PET. The tumors were clearly visible, and the tumor uptakes of Ga-NOTA-MATBBN and Ga-NOTA-ATBBN were determined to be 4.19 ± 0.32, 4.00 ± 0.41, 2.93 ± 0.35 and 4.70 ± 0.40, 4.10 ± 0.30, 3.14 ± 0.30 %ID/g at 30, 60, and 120 min, respectively. There was considerable accumulation and retention of Ga-NOTA-ATBBN in the liver and intestines. In contrast, the abdominal area does not have much retention of Ga-NOTA-MATBBN. Biodistribution data were in accordance with the PET results, showing that Ga-NOTA-MATBBN had more favorable pharmacokinetics and higher tumor to background ratios than those of Ga-NOTA-ATBBN. At 1 h postinjection, the tumor to liver and intestine of Ga-NOTA-MATBBN were 8.05 ± 0.56 and 21.72 ± 3.47 and the corresponding values of unmodified counterpart were 0.85 ± 0.23 and 3.45 ± 0.43, respectively. GRPR binding specificity was demonstrated by reduced tumor uptake of radiolabeled tracers after coinjection of an excess of unlabeled BBN peptides. Ga-NOTA-MATBBN exhibited GRPR-targeting properties both in vitro and in vivo. The favorable characterizations of Ga-NOTA- MATBBN such as convenient synthesis, specific GRPR targeting, high tumor uptake, and satisfactory pharmacokinetics warrant its further investigation for clinical cancer imaging. © 2014 Springer-Verlag.