A third generation PSMA-targeted agent [²¹¹At]YF2: Synthesis and in vivo evaluation

Targeted α-particle therapy agents have shown promising responses in patients who have developed resistance to β^--particle emitting radionuclides, albeit off-target toxicity remains a concern. Astatine-211 emits only one α-particle per decay and may alleviate the toxicity from α-emitting daughter radionuclides. Previously, we developed the low-molecular-weight PSMA-targeted agent [²¹¹At]L3-Lu that showed suitable therapeutic efficacy and was well tolerated in mice. Although [²¹¹At]L3-Lu had good characteristics, we now have evaluated a closely related analogue, [²¹¹At]YF2, to determine the better molecule for clinical translation.

The tin precursors and unlabeled iodo standards for [²¹¹At]YF2 and [²¹¹At]L3-Lu each were synthesized and a new one-step labeling method was developed to produce [²¹¹At]YF2 and [²¹¹At]L3-Lu from the respective tin precursor. RCY and RCP were determined using RP-HPLC. Cell uptake, internalization and in vitro cell-killing (MTT) assays were performed on PSMA⁺ PC-3 PIP cells in parallel experiments to compare [²¹¹At]YF2 and [²¹¹At]L3-Lu directly. A paired-label biodistribution study was performed in athymic mice with subcutaneous PSMA-positive PC-3 PIP xenografts as a head-to-head comparison of [¹³¹I]YF2 and [¹²⁵I]L3-Lu. The tissue distribution of [²¹¹At]YF2 and [²¹¹At]L3-Lu were determined individually in the same animal model.

The syntheses of tin precursors and unlabeled iodo standards were accomplished in reasonable yields. A streamlined and scalable radiolabeling method (1 h total synthesis time) was developed for the radiosynthesis of both [²¹¹At]YF2 and [²¹¹At]L3-Lu with 86 ± 7 % (n = 10) and 87 ± 5 % (n = 7) RCY, respectively, and > 95 % RCP for both. The maximum activity of [²¹¹At]YF2 produced to date was 666 MBq. An alternative method that did not involve HPLC purification was developed that provided similar RCY and RCP. Significantly higher cell uptake, internalization and cytotoxicity was seen for [²¹¹At]YF2 compared with [²¹¹At]L3-Lu. Significantly higher uptake and longer retention in tumor was seen for [¹³¹I]YF2 than for co-administered [¹²⁵I]L3-Lu, while considerably higher renal uptake was seen for [¹³¹I]YF2. The biodistribution of [²¹¹At]YF2 was consistent with that of [¹³¹I]YF2.

[²¹¹At]YF2 exhibited higher cellular uptake, internalization and cytotoxicity than [²¹¹At]L3-Lu on PSMA-positive PC3 PIP cells. Likewise, higher uptake and longer retention in tumor was seen for [²¹¹At]YF2. Experiments to evaluate the dosimetry and therapeutic efficacy of [²¹¹At]YF2 are under way.