A99mTc(CO)3-labeled benzylguanidine with persistent heart uptake

Synthesis and evaluation of 99mTc-analogues of [123/131I]mIBG prepared via [99mTc][Tc(CO)3(H2O)3]+ synthon for targeting norepinephrine transporter

INTRODUCTION

meta-[^123/131I]Iodobenzylguanidine (mIBG) is a clinical agent used for imaging neuroendocrine tumors, where uptake in tumor is via active transport mechanism through norepinephrine transporters (NET). Our group in past have evaluated a ^99mTc-analogue of the above tracer, based on ^99mTc-4 + 1 labeling approach, which exhibited significant affinity for NET but suffered from reduced specific uptake in comparison to reference standard no-carrier-added (n.c.a.) [¹²⁵I]mIBG. The present work attempts to synthesize two new ^99mTc-analogues of the radio-iodinated derivative following [^99mTc]Tc(CO)₃¹⁺ approach with an aim to improve the above specific uptake content.

METHODS

Two different precursors, xylylenediamine and 1,3-bis(chloromethyl)benzene, were synthetically modified to yield meta-functionalized benzylguanidine derivatives bearing iminodiacetate (IDA) and aminoethylglycine (AEG) tridentate chelating moieties, respectively. These ligands were labeled with technetium-99m via [^99mTc][Tc(CO)₃(H₂O)₃]⁺ synthon to form desired radioactive complexes 9 and 10. The radiolabeling yields of the complexes obtained were >90% as confirmed by radio-HPLC. The HPLC purified complexes were used for in vitro and in vivo evaluation to understand the true biological efficacy. Structural characterization of the radiolabeled complexes was carried after synthesizing and characterizing their Re-analogues.

RESULTS

Cell uptake studies with the radiolabeled complexes in SK-N-SH neuroblastoma cell lines revealed reduced uptake in the cells (<1% of incubated radioactivity/10⁶ cells) in comparison to n.c.a. [¹²⁵I]mIBG (~12%). However, limited specificity (~60%) was observed for the complexes as ascertained through desmethylimipramine (DMI) inhibition. Biodistribution studies in normal Wistar rats exhibited desired non-target clearance pharmacokinetics for the complexes but in vivo NET efficacy in myocardium for the neutral complex 10 could not be established.

CONCLUSIONS

Tridentate [^99mTc]Tc(CO)₃¹⁺ chelation approach severely affects biological behavior of the present small bioactive molecule under study to a significant extent in comparison to monodentate ligation in ^99mTc-4 + 1 strategy.

Synthesis and characterization of a novel 99mTc analogue of *I-mIBG showing affinity for nor-epinephrine transport positive tumors

¹²³I/¹³¹I labeled meta-iodobenzylguanidine (mIBG) is a radiopharmaceutical used for diagnosis of neuroendocrine tumors (NET) related to neural crest origin. Present work evaluates a newly synthesized ^99mTc analogue of this radioiodinated derivative.