DTPA-(PABn)-Leu5]-des-acyl ghrelin(1-5) as a new carrier of radionuclides and potential precursor of radiopharmaceuticals.

Abstract

Ghrelin is a peptide consisting of 28 aminoacids and an octadecyl side chain (acyl group) binding to the growth hormone secretagogue receptor type 1a (GHS-R1a). Its des-acylated form, des-acyl ghrelin (DAG) binds to the corticotropin releasing factor receptor type 2a (CRF2a) located on endocrine cancer cells such as the prostate carcinoma cell line DU 145. The aim of this study is to develop a new DAG-based carrier of radionuclides with potential application in therapy. Trunctated C-terminal five aminoacids chain of the DAG peptide (H2N-Gly-Ser-Ser-Phe-Leu-COOH) was linked to DTPA to obtain [DTPA-(PABn)-Leu5]-DAG(1-5). For therapeutic application the lutetium-177 (177Lu) radionuclide was coordinated to the peptide. To determine biological and chemical properties of newly synthesized radiopharmaceutical, two iodine-131 (131I)-labelled compounds were used: [131I]-Tyr4-DAG(1-5) and full length [131I]-DAG(1-28) together with their nonradioactive forms: DAG(1-28) and DAG(1-5). Identical HPLC elution profiles of [177Lu-DTPA-(PABn)-Leu5]-DAG(1-5) before and after incubation with human serum proved its stability. The lipophilicity profile of [177Lu-DTPA-(PABn)-Leu5]-DAG(1-5) was log DO/W=-2.68±0.05, pH 7.4. Receptor affinity of the nonradioactive conjugate [Lu-DTPA-(PABn)-Leu5]-DAG(1-5) was IC50 (21.06 nmol/l), as shown against the [131I]-DAG(1-28) used as a competitor. The 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyltetrazolium bromide assay indicated the significant cytotoxicity of the newly synthesized compounds, similar to that of [131I]-Tyr4-DAG(1-5). The results obtained suggest the potency of the [DTPA-(PABn)-Leu5]-DAG(1-5) as a new carrier of radionuclides in radiopharmacy.