Abstract View

Objectives: Recent advances in production of theranostic radiobromine isotopes have renewed interest in their incorporation into radiopharmaceuticals, particularly for PET imaging (⁷⁶Br) and Auger electron therapy (⁷⁷Br). A key challenge in radiobromination is determining whether electrophilic (classical oxidative radiobromination) or nucleophilic (copper-mediated radiobromination) labeling strategies are more effective for a given scaffold. To address this question, we systematically investigated both approaches using a diverse set of aryl precursors, including boronated esters and corresponding stannanes.

Methods: Carrier-free [⁷⁷Br]Br was produced at the University of Wisconsin Cyclotron Facility via the ⁷⁷Se(p,n)⁷⁷Br reaction.¹ Electrophilic labeling conditions were optimized using [1,1′-biphenyl]-4-yltributylstannane, 4-(tributylstannyl)benzonitrile, and tributyl(4-methoxyphenyl)stannane by screening 16 oxidants, 5 additives, 5 solvents, and order of addition. Nucleophilic labeling conditions were optimized using [1,1′-biphenyl]-4-yltributylstannane by varying molar equivalents of Cu(OTf)₂ and pyridine, as well as the reagent addition order.² Substituent effects were evaluated using high-throughput experimentation under optimized nucleophilic conditions across 28 substrates, including stannanes, pinacol boronate esters (Bpin), boronic acids, and potassium trifluoroborates.³ All reactions were conducted in 0.2 mL polypropylene tubes and incubated for 30 min at room temperature with shaking at 1000 rpm using a thermomixer with a 96-well PCR smart block. Following incubation, reactions were analyzed using TLC and HPLC.

Results: The average activity per reaction [⁷⁷Br]Br was 14.7 ± 5.1 µCi in 35 µL. The optimized electrophilic condition was 1 µmol of substrate (15 µL) and 10 µmol of NCS (15 µL) in acetonitrile, with order of reaction was Br, aryl precursor, then oxidant. The optimized nucleophilic labeling was 1 µmol of substrate (15 µL) and 0.l µmol of Cu(OTf)₂(pyridine complex), (15 µL), where the order of addition was the Cu(OTf)₂(py)₄, followed by aryl precursor and then [⁷⁷Br]Br in ethanol (5 µL). 

For nucleophilic reactions, Bpin precursors generally outperform stannane precursor except the stannane phenol (RCC 96 ± 12) outperformed the Bpin phenol (82 ± 4). For electrophilic reactions, stannane precursors outperform Bpin except being methyl ester para (98 ± 2) and ortho substituted (28 ± 2) outperformed the stannane (para: 70 ± 2; ortho: 15 ± 1). For cases using boronic acids and potassium trifluoroborates, nucleophilic outperformed electrophilic. Commercial bioactive molecules showed stannane derivates had higher RCC using electrophilic reactions. Three phenylalanine substrates demonstrated stannane(electrophilic: 97 ± 2) and boronic acid (nucleophilic: 99 ± 1) outperformed the Bpin (electrophilic: 32 ± 4; nucleophilic: 0 ± 0). 

Conclusions: Ongoing work is expanding the substrate library, adding GeSn₃ and SiSn₃ precursors and increasing the number of bioactives. This work will be used to generate a machine learning model that also incorporate literature reported RCC.  This systematic approach aims to identify generalizable trends governing precursor selection and reaction conditions, thereby guiding the rational development of radiobrominated theranostic agents going forward.

Acknowledgments: Financial support from NIH (R01EB021155, T32EB035504), the Society of Nuclear Medicine (Mars Shot) and Rogel Cancer Center is gratefully acknowledged.

References:

1.     Ellison, P.A., et al. Nuc Med Bio. 2020

2.     Mixdorf et al. J. Org. Chem. 2023

3.     Webb et al., JACS, 2024

Figure 1. Radiolabeling results for electrophilic and nucleophilic radiobromination. Panel A shows optimization of radiolabeling conditions, including solvent, oxidant, and copper equivalent, displayed as a heatmap of average radiochemical conversion (RCC). Panel B presents high-throughput experimentation results both optimized radiolabeling conditions, illustrating substituent effects across 28 aryl substrates.