A new 68 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 68 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-NHCH 2 CH 3 ) and ATBBN ( d -Phe-Gln-Trp-Ala-Val-Gly-His-Leu-NHCH 2 CH 3 ), were conjugated with 1,4,7-triazacyclononanetriacetic acid (NOTA) and labeled with 68 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 68 Ga-NOTA-MATBBN were also evaluated in PC-3 prostate tumor model and compared with those of 68 Ga-NOTA-ATBBN. NOTA-conjugated BBN analogs were labeled with 68 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 68 Ga-NOTA-MATBBN and 68 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. 68 Ga-NOTA-MATBBN was more hydrophilic than 68 Ga-NOTA-ATBBN, as indicated by their log P values (−2.73 ± 0.02 vs. −1.20 ± 0.03). The IC 50 values of NOTA-ATBBN and NOTA-MATBBN were similar (102.7 ± 1.18 and 124.6 ± 1.21 nM). The accumulation of 68 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 68 Ga-NOTA-MATBBN and 68 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 68 Ga-NOTA-ATBBN in the liver and intestines. In contrast, the abdominal area does not have much retention of 68 Ga-NOTA-MATBBN. Biodistribution data were in accordance with the PET results, showing that 68 Ga-NOTA-MATBBN had more favorable pharmacokinetics and higher tumor to background ratios than those of 68 Ga-NOTA-ATBBN. At 1 h postinjection, the tumor to liver and intestine of 68 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. 68 Ga-NOTA-MATBBN exhibited GRPR-targeting properties both in vitro and in vivo. The favorable characterizations of 68 Ga-NOTA-MATBBN such as convenient synthesis, specific GRPR targeting, high tumor uptake, and satisfactory pharmacokinetics warrant its further investigation for clinical cancer imaging.