Exploring the PET in vivo generator 134Ce as a theranostic match for 225Ac

Abstract

Purpose: The radionuclide pair cerium-134/lanthanum-134 (¹³⁴Ce/¹³⁴La) was recently proposed as a suitable diagnostic counterpart for the therapeutic alpha-emitter actinium-225 (²²⁵Ac). The unique properties of ¹³⁴Ce offer perspectives for developing innovative in vivo investigations not possible with ²²⁵Ac. In this work, ²²⁵Ac- and ¹³⁴Ce-labeled tracers were directly compared using internalizing and slow-internalizing cancer models to evaluate their in vivo comparability, progeny meandering, and potential as a matched theranostic pair for clinical translation. Despite being an excellent chemical match, ¹³⁴Ce/¹³⁴La has limitations to the setting of quantitative positron emission tomography imaging.

Methods: The precursor PSMA-617 and a macropa-based tetrazine-conjugate (mcp-PEG₈-Tz) were radiolabelled with ²²⁵Ac or ¹³⁴Ce and compared in vitro and in vivo using standard (radio)chemical methods. Employing biodistribution studies and positron emission tomography (PET) imaging in athymic nude mice, the radiolabelled PSMA-617 tracers were evaluated in a PC3/PIP (PC3 engineered to express a high level of prostate-specific membrane antigen) prostate cancer mouse model. The ²²⁵Ac and ¹³⁴Ce-labeled mcp-PEG₈-Tz were investigated in a BxPC-3 pancreatic tumour model harnessing the pretargeting strategy based on a trans-cyclooctene-modified 5B1 monoclonal antibody.

Results: In vitro and in vivo studies with both ²²⁵Ac and ¹³⁴Ce-labelled tracers led to comparable results, confirming the matching pharmacokinetics of this theranostic pair. However, PET imaging of the ¹³⁴Ce-labelled precursors indicated that quantification is highly dependent on tracer internalization due to the redistribution of ¹³⁴Ce's PET-compatible daughter ¹³⁴La. Consequently, radiotracers based on internalizing vectors like PSMA-617 are suited for this theranostic pair, while slow-internalizing ²²⁵Ac-labelled tracers are not quantitatively represented by ¹³⁴Ce PET imaging.

Conclusion: When employing slow-internalizing vectors, ¹³⁴Ce might not be an ideal match for ²²⁵Ac due to the underestimation of tumour uptake caused by the in vivo redistribution of ¹³⁴La. However, this same characteristic makes it possible to estimate the redistribution of ²²⁵Ac's progeny noninvasively. In future studies, this unique PET in vivo generator will further be harnessed to study tracer internalization, trafficking of receptors, and the progression of the tumour microenvironment.

Keywords: Actinium-225; Cerium-134; PET Imaging; Pretargeting; Progeny Release; Targeted Alpha Therapy.

Figure 1.

Decay scheme of ²²⁵Ac and ¹³⁴Ce and structures of the investigated radioligands. While ²²⁵Ac and ¹³⁴Ce show similar chemical behaviour and chelation chemistry, the PET-compatible progeny ¹³⁴La can serve as an imaging surrogate for the similar short-lived daughter ²²¹Fr.

Figure 2.

Biodistribution of [¹³⁴Ce]CeCl₃. Unbound ¹³⁴Ce accumulates predominately in the liver and spleen as demonstrated (left) via PET imaging (maximum intensity projections at 4 and 24 h p.i.) and (right) terminal biodistribution (3.7 MBq per mouse).

Figure 3.

Investigation of PSMA-617 in a subcutaneous PC3/PIP tumour mouse model. (A) The theranostic pair [²²⁵Ac]Ac-PSMA-617 and [¹³⁴Ce]Ce-PSMA-617. (B) Biodistribution data of ²²⁵Ac and ²¹³Bi at selected time points after administration of 37 kBq [²²⁵Ac]Ac-PSMA-617 in the tissue of interest. (C) Plot of the relative ²¹³Bi redistribution from tumour to kidneys in %ID/g. (D) Estimated relative biological effectiveness–weighted absorbed dose coefficients for [²²⁵Ac]Ac-PSMA-617 (in Gy-equivalent per MBq administered), disregarding and including the found redistribution of the progeny. (E) Direct comparison of [²²⁵Ac]Ac-PSMA-617 and [¹³⁴Ce]Ce-PSMA-617 in the tissue of interest.

Figure 4.

Evaluation of [¹³⁴Ce]Ce-PSMA-617 via PET imaging of ¹³⁴La. (A) PET imaging (maximum intensity projections) in a subcutaneous PC3/PIP tumour mouse model (3.7 MBq per mouse). Direct comparison of AM and PM imaging to visualize the redistribution of ¹³⁴La at different time points. One out of four mice is represented. (B) Plot of the relative ¹³⁴La redistribution from the tumours determined via comparative region-of-interest analysis of AM and PM images. (C) Maximum-intensity projection of a benchmark Jaszczak phantom (12.5 MBq [¹³⁴Ce]CeCl₃ in 20 mL).

Figure 5.

Illustration of the pretargeting approach, following the concept of reference [17]. The TCO (red star) modified mAb is administered, and after a certain time interval (here, 3 d), sufficient tumour uptake and clearance will be achieved. This is followed by injecting the radiolabelled tetrazine tracer — a small molecule that bio-orthogonally reacts with the TCO moiety. The tracer’s design allows for rapid renal clearance of unreacted excess. This figure was created with BioRender.

Figure 6.

Investigating mcp-PEG₈-Tz in a 5B1-TCO-pretargeted 5B1-TCO (100 μg, 0.7 nmol) subcutaneous BxPC-3 tumour mouse model. (A) Biodistribution data of [²²⁵Ac]Ac-mcp-PEG₈-Tz (2 nmol, 37 kBq) in the tissue of interest. (B) Direct comparison of ²²⁵Ac- and ¹³⁴Ce-labelled mcpPEG₈-Tz via terminal biodistribution. (C) The theranostic pair [²²⁵Ac]Ac-mcp-PEG₈-Tz and [¹³⁴Ce]Ce-mcp-PEG₈-Tz. (D) PET imaging (maximum intensity projections) in pretargeted mice receiving 3.7 MBq [¹³⁴Ce]Ce-mcp-PEG₈-Tz. Direct comparison of AM and PM imaging to visualize the redistribution of ¹³⁴La at the terminal time point. (E) Direct comparison of the tumour uptake of ²²⁵Ac (via PM biodistribution, in equilibrium) and ¹³⁴La (via AM ROI PET analysis) showing that PET imaging is underrepresenting the uptake, especially at early time points. The additional postmortem data point was determined via PM ROI PET analysis, highlighting the difference vs. the value-determined AM.

Figure 7.

Impact on the progeny redistribution of internalizing versus non-internalizing radiotracers. This figure was created with BioRender.