Two Novel Nanosized Radiolabeled Analogues of Somatostatin for Neuroendocrine Tumor Imaging

The somatostatin receptors (SR), which are overexpressed in a majority of neuroendocrine tumors, are targets for radiopeptide-based imaging using for example the Tc-99m-Tyr(3)-Octreotide peptide. Dendrimers are hyperbranched polymeric structures. The nanoscopic size and near-monodisperse nature properties give polyamidoamine (PAMAM) dendrimers an edge over linear polymers in the context of drug delivery. Gold nanoparticles (AuNPs) conjugated to peptides produces stable multimeric systems with target-specific molecular recognition. The aim of this research was to prepare two nanosized multimeric systems for neuroendocrine tumor imaging, Tc-99m-PAMAM-Tyr(3)-Octreotide and Tc-99m-AuNP-Tyr(3)-Octreotide, and to compare their in vitro uptake in SR-positive AR42J cancer cells as well as their biodistribution profile in athymic mice bearing AR42J tumors. [Tyr(3), Lys(Boc)(5)]-Octreotide was conjugated to the carboxylate groups of the PAMAM dendrimer (G3.5) with further Boc deprotection using TFA. Tc-99m labeling was carried out by a direct method. Tc-99m-Tyr(3)-Octreotide was conjugated to AuNPs (20 nm) by spontaneous reaction with the thiol group of cysteine. Radiochemical purity (RP) was determined by size-exclusion HPLC and ITLC-SG analyses. In vitro binding studies were carried out in AR42J cancer cells. Biodistribution studies were accomplished in athymic mice with AR42J-induced tumors with blocked and unblocked receptors. Elemental analysis demonstrated that 26 Tyr(3)-Octreotide molecules were successfully conjugated to one molecule of PAMAM. RP for both nanosized conjugates was > 94% and showed recognition for SR in AR42J cells. The tissue distribution of radioactivity 2 h after Tc-99m-PAMAM-Tyr(3)-Octreotide administration in mice showed specific tumor uptake (4.12 +/- 0.57% of injected dose/g) and high accumulation in the pancreas (15.08 +/- 3.11% of injected dose/g) which expresses SR. No significant difference in the tumor uptake was found between Tc-99m-PAMAM-Tyr(3)-Octreotide and Tc-99m-AuNPTyr(3)-Octreotide. However, the dendrimer-peptide conjugate showed a significant renal excretion. Both radiopharmaceuticals demonstrated properties suitable for use as target-specific agents for molecular imaging of tumors that overexpressed SR.