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 for 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. Conclusions: 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.