The Glycosylation The Somatostatin ~ (99m) Tc Marker And In Vivo And In Vitro Biological Evaluation

Somatostatin, first was discovered in 1973, exists in the central and peripheral nervous systems, and in peripheral tissues. It has widely physiological functions such as inhibition of hormone release and cell proliferation. The diverse actions of SMS peptides are mediated through interarion with a family of five specific SMS receptors expressed by a variety of normal and malignant tissues. Compared with normal tissues, somatostatin receptors (SSTRs) are over expressed in tumor tissues. Somatostatin has nanomolar affinity to all kinds of SSTRs. However, the short half-life of SMS in vivo (about 2-3 min ) prevented its application in the clinic. So lots of analogues were synthesized in order to overcome this disadvantage. The best known is the octreotide. In particular, the ¹¹¹In-DTPA-D-phe¹-octreotide is clinically established for diagnostic imaging of neuroendocrine tumours. Though these compounds have prolonged the biological half-life in a way, most of such analogues couldn't remain the high affinity to all of SSTRs. In the past 10 years, a lot of other strategies have been attempted for finding a compound that has high affinity to all kinds of SSTRs. Compared other compounds, glycosylated somatostatin has uniquely advantage. Conjugated to different molecular weight dextran, somatostatin could have different half-life and can be metabolized by different tissue. Furthermore most of dextran-protein conjugate retain the pharmacological activity. Some attempts have been doing in order to improve the half-life of somatostatin and retain the high affinity to all kinds of SSTRs at the same time.[Objective] As the suitable physicochemical characteristics of dextran, natural somatostatin, dextran-10 and the new bifunctional chelator MAG₂Lys were used to synthesize SMS-Dextran¹⁰-MAG₂Lys conjugate. The blood half-life of natural somatostatin was prolonged by changing the route of the metabolization. At the same time, the high affinity to all five SSTRs was not affected by the conjugation. The ^99mTc labelling, the purification and analysis method, the in vivo behavior and the scintigraphy of tumor-bearing nude mice were studied. The research aimed to discuss the possbility of ^99mTc-MAG₂Lys-Dextran¹⁰-SMS becoming an imaging agent for somatostatin receptor positive tumor and provide experimental basis for the study of diagnostic and therapeutic radiopharmaceutical.[Methods]â‘ The bifunctional chelator, MAG₂Lys, was synthesized by a series of condensation reaction.â‘¡Dextran was oxidized with sodium periodate to gain aldehyde groups, which was reacted with both somatostatin and MAG₂Lys. Consequently the Schiff base was reduced with sodium cyanoborohydride yielding stabilized compounds.â‘¢The in vitro somatostatin receptor competition binding study of SMS-Dextran¹⁰-MAG₂Lys was carded out by using rat brain cortex membranes and ¹²⁵I-Tyr³-octreotide as a radioligand.â‘£SMS-Dextran¹⁰-MAG₂Lys was radiolabeled with ^99mTc using sodium tartrate as a transehelator. The radiolabeling condition and the stability of the product were evaluated.⑤The biodistribution and half-life of radiolabeled SMS-Dextran¹⁰-MAG₂Lys were investigated in normal rats.â‘¥The tumor uptake and imaging property of radiolabeled SMS-Dextran¹⁰-MAG₂Lys were evaluated in nude mice bearing human pancreatic tumor.[Results]â‘ The bifunctional chelator, MAG₂Lys, was pure enough for next step from NMR.â‘¡The conjugate, SMS-Dextran¹⁰-MAG₂Lys, was analyzed by HPLC, and there was no polymers observed. The chemical purity of the compound was more than 99%.â‘¢The SMS-Dextran`(10)-MAG2Lys showed high somatostatin receptor binding affinity, i.e. in the same nM range as the reference ligand somatostatin (IC⁵⁰～1.0 nM).â‘£Under the optimum conditions, the labeling efficiency of ^99mTc-MAG₂Lys-Dextran¹⁰-SMS was approximately 40-50% and the radiochemical purity was more than 99% after purification. The in vitro stability of the radiolabeled conjugate was satisfied.⑤The half-life was 2.4 h post injection in normal rats. The excretion is mainly through the hepatobiliary and kidney system,â‘¥^99mTc-MAG₂Lys-Dextran¹⁰-SMS was localized in pancreatic tumor and showed visible tumor uptake at 4 h imaging. [Conclusion] ^99mTc-MAG₂Lys-Dextran¹⁰-SMS was synthesized, which maintained high affinity to all five SSTRs and suited ^99mTc labeling. The blood clearance of the conjugate conformed one-compartment model, and the half-life was 2.4 h. Dextran protected somastotatin from enzymolysis. The ^99mTc-MAG₂Lys-Dextran¹⁰-SMS was localized in pancreatic tumor and showed visible tumor uptake at 4 h imaging. The results indicate that ^99mTc-MAG₂Lys-Dextran¹⁰-SMS is a novel promising candidate imaging agent for somatostatin receptor positive tumor.