Preparation and evaluation of 99mTc-EDDA/HYNIC-[Lys3]-bombesin for imaging gastrin-releasing peptide receptor-positive tumours

Background Bombesin is a peptide that was initially isolated from frog skin and which belongs to a large group of neuropeptides with many biological functions. The human equivalent is gastrin-releasing peptide (GRP), whose receptors are over-expressed in a variety of malignant tumours. Aim To prepare a HYNIC-[Lys(3)]-bombesin analogue that could be easily labelled with Tc-99m from lyophilized kit formulations and to evaluate its potential as an imaging agent for GRP receptor-positive tumours. Methods HYNIC was conjugated to the E-amino group of Lys(3) residue at the N-terminal region of bombesin via succinimidyi-N-Boc-HYNIC at pH 9.0. Tc-99m labelling was performed by addition of sodium pertechnetate solution and 0.2 M phosphate buffer pH 7.0 to a lyophilized formulation. Stability studies were carried out by reversed phase HPLC and ITLC-SG analyses in serum and cysteine solutions. In-vitro internalization was tested using human prostate cancer PC-3 cells with blocked and non-blocked receptors. Biodistribution and tumour uptake were determined in PC-3 tumour-bearing nude mice. Results Tc-99m-EDDA/HYNIC-[Lys(3)]-bombesin was obtained with radiochernical purities > 93% and high specific activity (similar to 0.1 GBq.nmol(-1)). Results of in-vitro studies demonstrated a high stability in serum and cysteine solutions, specific cell receptor binding and rapid internalization. Biodistribution data showed a rapid blood clearance, with predominantly renal excretion and specific binding towards GRP receptor-positive tissues such as pancreas and PC-3 tumours. Conclusion Tc-99m-EDDA/HYNIC-[Lys(3)]-bombesin obtained from lyophilized kit formulations has promising characteristics for the diagnosis of malignant tumours that over-express the GRP receptor.