[⁸⁹Zr]ZrCl₄ for direct radiolabeling of DOTA-based precursors

Zirconium-89 (⁸⁹Zr) is a positron emitter with several advantages over other shorter-lived positron emission tomography (PET) compatible radiometals such as gallium-68 or copper-64. These include practically unlimited availability, extremely low cost, greatly facilitated distribution logistics, positron energy fit for medical PET imaging, and sufficiently long physical half-life to enable PET imaging at later time points for patient-specific dosimetry estimations. Despite these apparent benefits, the reception of ⁸⁹Zr in the nuclear medicine community has been tepid. The driving factor for the absence of broader adaptation is mostly routed in its final formulation - [⁸⁹Zr]zirconium oxalate. While serving as a suitable precursor solution for the gold standard chelator deferoxamine (DFO), [⁸⁹Zr]Zr-oxalate is inaccessible for the most commonly used chelators, such as the macrocyclic DOTA, due to its pre-chelated state. Consequently, pioneering work has been conducted by multiple research groups to create oxalate-free forms of [⁸⁹Zr]Zr⁴⁺, either via chemical conversion of oxalate into other counterion forms or via direct radiochemical isolation of [⁸⁹Zr]ZrCl₄, showing that [⁸⁹Zr]Zr-DOTA complexes are possible and stable. However, this success was accompanied by challenges, including complex and labor-intensive radiochemical processing and radiolabeling procedures as well as the relatively minuscule conversion rates. Here, we report on the direct production of [⁸⁹Zr]ZrCl₄ avoiding oxalate and metal contaminants to enable efficient radiolabeling of DOTA constructs.

We based our direct production of [⁸⁹Zr]ZrCl₄ on previously reported methods and further optimized its quality by including an additional iron-removing step using the TK400 Resin. Here, we avoided using oxalic acid and effectively minimized the content of trace metal contaminants. Our two-step purification procedure was automated, and we confirmed excellent radionuclide purity, minimal trace metals content, great reactivity over time, and high specific molar activity. In addition, DOTA-based PSMA-617 and DOTAGA-based PSMA-I&T were radiolabeled to demonstrate the feasibility of direct radiolabeling and to estimate the maximum apparent specific activities. Lastly, the biodistribution of [⁸⁹Zr]Zr-PSMA-617 was assessed in mice bearing PC3-PIP xenografts, and the results were compared to the previously published data.

A total of 18 batches, ranging from 6.9 to 20 GBq (186 to 541 mCi), were produced. The specific molar activity for [⁸⁹Zr]ZrCl₄ exceeded 0.96 GBq (26 mCi) per nanomole of zirconium. The radionuclidic purity was >99 %, and the trace metals content was in the <1 ppm range. The [⁸⁹Zr]ZrCl₄ remained in its reactive chemical form for at least five days when stored in cyclic olefin polymer (COP) vials. Batches of 11.1 GBq (300 mCi) of [⁸⁹Zr]Zr-PSMA-617 and 14.4 GBq (390 mCi) of [⁸⁹Zr]Zr-PSMA-I&T, corresponding to specific activities of 11.1 MBq/μg (0.3 mCi/μg), and 14.4 MBq/μg (0.39 mCi/μg), respectively, were produced. [⁸⁹Zr]Zr-PSMA-617 animal PET imaging results were in agreement with the previously published data.

In this work, we report on a suitable application of TK400 Resin to remove iron during [⁸⁹Zr]ZrCl₄ radiochemical isolation. The breakthrough allows for direct radiolabeling of DOTA-based constructs with [⁸⁹Zr]ZrCl₄, leading to high apparent molar activities and excellent conversion rates.