Preparation and Preclinical Evaluation of [11C]3Br-pHPG to Determine its Feasibility as a Bromine-76/77 Theranostic Agent to Target Norepinephrine Transporter in Neuroendocrine Tumors

Introduction: The development of ⁷⁶Br/⁷⁷Br as a theranostic pair compatible with small molecule radiopharmaceuticals offers potential to label scaffolds when metal chelation is not feasible. To achieve this goal, the identification of a suitable small molecule with potential for targeting and treating cancer is a necessary step. In this work we prepare the bromine containing analogue of 3F-pHPG, which has recently been demonstrated to image pheochromocytoma and paraganglioma.[1] Carbon-11 radiolabeling at the guanidine with [¹¹C]cyanogen bromide (CNBr) allowed us to quickly access the feasibility of pursuing 3Br-pHPG for evaluation as a ⁷⁶Br/⁷⁷Br theranostic pair.

Methods: The standard for 3Br-pHPG was prepared in 3 steps from tyramine. Treatment with N-bromosuccinimide yielded 3-bromo-tyramine, which was reacted with 1,3-Bis(tert-butoxycarbonyl)-2-methyl-2-thiopseudourea to produce the biscarbamate of the desired guanidine. Upon treatment with 4 M HCl in dioxane the 3Br-pHPG was precipitated as the HCl salt. To radiolabel with carbon-11, 3Br-tyramine-HCl salt (1 mg) was dissolved in 0.1 M sodium borate pH 8.0 (250 uL) and placed in the reactor of a GE FX_M module. [¹¹C]CNBr was prepared as previously described and sparged into reactor and held at room temperature for 2 min.[2] The intermediate was then treated with 35% NH₄Cl in NH₄OH solution (250 uL) and heated at 130 ^°C for 8 min under pressure (~300 kPa). The reaction was quenched with 100 mM NH₄OAc pH 4.5 (500 uL) and glacial acetic acid (200 uL). The resulting solution was purified by reverse phase HPLC (Luna 10u C18(2); 100x10 mm) at a flow rate of 4 mL/min using an injectable eluant (10% EtOH, 40 mM NH4OAc, pH 5.0). The product was collected (r.t. around 9 min) and passed through 0.22 u sterile filter into a sterile vial prior to QC analysis and release for use. Small animal PET imaging was performed in female Sprague Dawley rats with heart in frame to evaluate uptake and cardiac retention as a measure of norepinephrine transporter uptake with and without desipramine blocking (1.5 mg/kg i.v. 10 min prior to injection).

Results: [¹¹C]3Br-pHPG was prepared in 2% non-decay corrected yield (24±?? mCi; n=3) 35 min from end of beam. Prepared standard can be utilized for future studies with bromine-76, and -77. Rat imaging demonstrated cardiac uptake and desipramine blocking showed decreased uptake at late time points consistent with blocking of uptake by desipramine.

Conclusions: Radiolabeling with [¹¹C]CNBr provided a fast means to evaluate 3Br-pHPG uptake via norepinephrine transporter. Studies demonstrated uptake of the radiotracer and indicate its ability to image pheochromocytoma, paraganglioma and related neuroendocrine tumors.

Acknowledgements: We thank the SNMMI Mars Shot Fund for supporting this research.

[1] Wong KK, Else T, Viglianti BL, Brooks AF, Frey KA, Raffel DM, Eur. J. Nucl. Med. Mol. Imaging, 2022, 49(6), 2098–2099. [2]Zhao AY, Brooks AF, Raffel DM, Stauff J, Arteaga J, Scott PJH, Shao X, ACS Med. Chem. Lett. 2020, 11(11), 2325–2330.

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