Preclinical Validation of [²¹¹At]mABG, Synthesized via GMP-Friendly Electrophilic Astatodeboronylation Chemistry

Introduction: Neuroblastoma is a highly malignant pediatric tumor with a high membrane expression of the norepinephrinetransporter (NET). Iodine-131 labeled meta-iodobenzylguanidine (mIBG) as a substrate for the norepinephrine transporter can be used as treatment of neuroblastoma. However, [¹³¹I]MIBG has a limited therapeutic effect and requires 4-6 day isolation to shield parents and families from unwanted irradiation. Recently, the alpha emitter based derivative meta-astatobenzylguanidine ([²¹¹At]mABG) has been suggested as a more potent antitumor agent and patient-friendly alternative. In this work, we selected and validated the preferred chemical method for [²¹¹At]mABG synthesis towards GMP production for clinical implementation to treat pediatric neuroblastoma patients. Methods: Astatine-211 was obtained by dry distillation using the Atley C100 module. Two different methodologies for the synthesis of [²¹¹At]mABG by electrophilic aromatic substitution on either a trimethylsilyl or boronic acid precursor were compared. Time-dependent uptake was tested in transporter positive (MaxNB293, SK-N-SH) and low/negative (AMC691T, Kelly) neuroblastoma cells. Norepinephrine transporter specificity was tested by co-incubation with the transporter substrate norepinephrine or inhibitor desipramine. Cytotoxicity of [²¹¹At]mABG was determined by measuring cell viability in both NET positive and negative cells. Finally, a large [²¹¹At]mABG dose was synthesized and stability was monitored for up to 24 h. Results: Radiochemical conversion into [²¹¹At]mABG using a trimethylsilyl and boronic acid precursor reached 78 ± 8 % and 95 ± 1 %, respectively (based on radioTLC analysis, (n = 3 for each condition). When [²¹¹At]mABG was synthesized from the boronic acid precursor, the final formulated product was obtained in 79 ± 8% yield with a radiochemical purity >95%. Time dependent and NET specific [²¹¹At]mABG uptake in neuroblastoma cells was observed with 268 ± 46 and 309 ± 58 % of added activity/mg protein after 4 h incubation for the NET positive cells MaxNB293 and SK-N-SH, respectively. Uptake in NET-negative cells AMC691T and Kelly only showed 31 ± 6 and 36 ± 9 % of added activity/mg protein, respectively. Cell viability was NET expression dependent, with an EC₅₀ value of 0.036 kBq for SK-N-SH and 16 kBq for Kelly cells, as positive and low expressing cell lines. Ultimately, starting with 345 MBq of astatine-211, 236 MBq of [²¹¹At]mABG was afforded (68 % non-decay-corrected, n = 1). Radiochemical purity of the formulated product remained >95% over 24 h. Conclusion: We have developed a mild synthesis procedure for reliable and high-yielding production of [²¹¹At]mABG using GMP compatible reagents. Efforts towards fully automated and GMP compliant synthesis are underway.