Preparation of [ 177 Lu]Lu-DOTA-Ahx-Lys40-Exendin-4 for radiotherapy of insulinoma: a detailed insight into the radiochemical intricacies.

INTRODUCTION: [177 Lu]Lu-DOTA-Ahx-Lys40-Exendin-4 ([177 Lu]Lu-DOTA-Exendin-4) is a potential agent for radiotherapy of insulinomas owing to its specificity towards GLP-1 (Glucagon like peptide-1) receptors over-expressed on such cancers. The objective of the present study is to optimize the various radiochemistry parameters for the consistent formulation of the agent with high radiolabeling yield using carrier added [177 Lu]LuCl3 and also to evaluate its biological behaviour in small animal model.

METHODS: In order to optimize the radiolabeling parameters, DOTA-Exendin-4 was radiolabeled with [177 Lu]LuCl3 in two different buffer systems (sodium acetate and HEPES) at three different temperatures (45, 65 and 95 °C) using three different ligand to metal ratios (3:1, 4:1 and 5:1). The radiolabeled peptide was characterized by both paper chromatography and HPLC. The effect of addition of three different radio-protectors on complexation yield was also studied. Bio-distribution studies were carried out in healthy Swiss mice to evaluate the pharmacokinetic behaviour of the radiolabeled peptide as well as to determine the in vivo specificity of the radiotracer towards GLP-1 receptors (blocking studies). Urine and kidney lysate of the animals were analyzed at various post-administration time-points in order to determine the in vivo stability of the radiolabeled peptide.

RESULTS: The [177 Lu]Lu-DOTA-Exendin-4 complex could be prepared consistently with >95% radiolabeling yield using the optimized reaction conditions. Bio-distribution studies revealed early accumulation of [177 Lu]Lu-DOTA-Exendin-4 in pancreas along with fast clearance via renal pathway. Significantly high accumulation of the radiotracer was observed in kidneys. Analyses of urine and kidney lysate of the animals revealed in vivo stability of [177 Lu]Lu-DOTA-Exendin-4. Blocking studies showed displacement of significant amount of radiotracer from GLP-1 receptor-positive organs such as, pancreas and lungs (p <0.05) in presence of unlabeled peptide, indicating the specificity of the radiolabeled preparation towards GLP-1 receptors.

CONCLUSIONS: Present study shows that [177 Lu]Lu-DOTA-Exendin-4 could be formulated for radiotherapeutic application with high radiochemical purity and adequate in vivo stability using [177 Lu]LuCl3 produced via direct neutron irradiation.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: Findings of the present study will be helpful in preparing the patient dose of [177 Lu]Lu-labeled Exendin for radiotherapy of insulinoma using carrier added [177 Lu]LuCl3 , produced in a medium flux reactor, without the requirement of post-labeling purification.