Evaluation of 134Ce/134La as a PET Imaging Theranostic Pair for 225Ac α-Radiotherapeutics

Abstract

²²⁵Ac-targeted α-radiotherapy is a promising approach to treating malignancies, including prostate cancer. However, α-emitting isotopes are difficult to image because of low administered activities and a low fraction of suitable γ-emissions. The in vivo generator ¹³⁴Ce/¹³⁴La has been proposed as a potential PET imaging surrogate for the therapeutic nuclides ²²⁵Ac and ²²⁷Th. In this report, we detail efficient radiolabeling methods using the ²²⁵Ac-chelators DOTA and MACROPA. These methods were applied to radiolabeling of prostate cancer imaging agents, including PSMA-617 and MACROPA-PEG₄-YS5, for evaluation of their in vivo pharmacokinetic characteristics and comparison to the corresponding ²²⁵Ac analogs. Methods: Radiolabeling was performed by mixing DOTA/MACROPA chelates with ¹³⁴Ce/¹³⁴La in NH₄OAc, pH 8.0, at room temperature, and radiochemical yields were monitored by radio-thin-layer chromatography. In vivo biodistributions of ¹³⁴Ce-DOTA/MACROPA.NH₂ complexes were assayed through dynamic small-animal PET/CT imaging and ex vivo biodistribution studies over 1 h in healthy C57BL/6 mice, compared with free ¹³⁴CeCl₃ In vivo, preclinical imaging of ¹³⁴Ce-PSMA-617 and ¹³⁴Ce-MACROPA-PEG₄-YS5 was performed on 22Rv1 tumor-bearing male nu/nu-mice. Ex vivo biodistribution was performed for ¹³⁴Ce/²²⁵Ac-MACROPA-PEG₄-YS5 conjugates. Results: ¹³⁴Ce-MACROPA.NH₂ demonstrated near-quantitative labeling with 1:1 ligand-to-metal ratios at room temperature, whereas a 10:1 ligand-to-metal ratio and elevated temperatures were required for DOTA. Rapid urinary excretion and low liver and bone uptake were seen for ¹³⁴Ce/²²⁵Ac-DOTA/MACROPA. NH₂ conjugates in comparison to free ¹³⁴CeCl₃ confirmed high in vivo stability. An interesting observation during the radiolabeling of tumor-targeting vectors PSMA-617 and MACROPA-PEG₄-YS5-that the daughter ¹³⁴La was expelled from the chelate after the decay of parent ¹³⁴Ce-was confirmed through radio-thin-layer chromatography and reverse-phase high-performance liquid chromatography. Both conjugates, ¹³⁴Ce-PSMA-617 and ¹³⁴Ce-MACROPA-PEG₄-YS5, displayed tumor uptake in 22Rv1 tumor-bearing mice. The ex vivo biodistribution of ¹³⁴Ce-MACROPA.NH₂, ¹³⁴Ce-DOTA and ¹³⁴Ce-MACROPA-PEG₄-YS5 corroborated well with the respective ²²⁵Ac-conjugates. Conclusion: These results demonstrate the PET imaging potential for ¹³⁴Ce/¹³⁴La-labeled small-molecule and antibody agents. The similar ²²⁵Ac and ¹³⁴Ce/¹³⁴La-chemical and pharmacokinetic characteristics suggest that the ¹³⁴Ce/¹³⁴La pair may act as a PET imaging surrogate for ²²⁵Ac-based radioligand therapies.

Keywords: 134Ce; 225Ac; PET imaging; PSMA-617; YS5 antibody; targeted α-radiotherapy.

Graphical abstract

FIGURE 1.

(Left) Radiolabeling of MACROPA.NH₂ and DOTA with ¹³⁴CeCl₃. (Right) Percentage radiolabeling at increasing L/M ratios for MACROPA.NH₂ and DOTA (n = 2) at 25 °C, as assayed by radio-TLC.

FIGURE 2.

Evaluation of PET imaging of ¹³⁴Ce and chelated complexes in wild-type mouse studies. (A) Coronal small-animal PET/CT images of free ¹³⁴CeCl₃, ¹³⁴Ce-MACROPA.NH₂, and ¹³⁴Ce-DOTA in wild-type mice. (B–D) Ex vivo biodistribution of ¹³⁴CeCl₃ (n = 2) and ²²⁵AcCl₃ (n = 3) (B), ¹³⁴Ce/²²⁵Ac-MACROPA.NH₂ (n = 3) (C), and ¹³⁴Ce/²²⁵Ac-DOTA (n = 3) (D). Error bars represent SD. ***P < 0.0008. ****P < 0.0001.

FIGURE 3.

Radiolabeling of prostate cancer–targeting agents PSMA-617 and MACROPA-PEG₄-YS5. (A) Radiolabeling of PSMA-617 (left) and radiolabeling yields at increasing molar ratios of PSMA-617 (right). (B) Radio-iTLC of ¹³⁴Ce-PSMA-617 (left), same reaction mixture diluted in saline scanned without waiting for 1-h decay (middle), and same radio-iTLC scanned after 1-h decay showing quantitative radiochemical yield (right). (C) Radiolabeling of MACROPA-PEG₄-YS5. (D) Radio-iTLC of ¹³⁴Ce-MACROPA-PEG₄-YS5 (left), same reaction mixture after PD10 column purification immediately scanned without waiting for 1-h decay (middle), and same radio-iTLC after 1-h decay (right). (E) ¹³⁴La dechelation due to recoil effect. PBS = phosphate-buffered saline.

FIGURE 4.

Small-animal PET imaging of ¹³⁴Ce-PSMA-617 in 22Rv1 xenograft at 1 h after injection.

FIGURE 5.

In vitro and in vivo analysis of radioimmunoconjugate ¹³⁴Ce-MACROPA-PEG₄-YS5. (A) Left: Magnetic bead–based radioligand assay for ¹³⁴Ce-MACROPA-PEG₄-YS5 (n = 3). Right: Saturation binding assay of ¹³⁴Ce-MACROPA-PEG₄-YS5 on 22Rv1 cells (dissociation constant, 3.7 nM) (n = 3). (B) Maximum-intensity-projection PET/CT and transverse small-animal PET/CT images obtained up to 7 d after ¹³⁴Ce-MACROPA-PEG₄-YS5 injection in mouse bearing 22Rv1 xenografts, demonstrating gradual increase in tumor uptake over time (n = 4). MIP = maximum-intensity projection.

FIGURE 6.

Ex vivo biodistribution analysis of ¹³⁴Ce/²²⁵Ac-MACROPA-PEG₄-YS5 in mouse bearing 22Rv1 xenografts at 7 d after injection. Higher tumor and liver uptake was obtained. Error bars represent SD (n = 5 at 7 d and 2 at 14 d for ¹³⁴Ce; n = 4 for ²²⁵Ac at 7 d).