Summary

¹⁷⁷Lu (half-life 6.7d) is currently the most often applied radiometal for therapeutic purposes, as it has a particulate emission (β− or Auger electron) for effecting therapy and emits several accompanying γ-photons of 208 keV (11%) and 113 keV (6.4%), which are used for diagnostic evaluation and dosimetry.

¹⁶¹Tb is a more recently introduced radiolanthanide for therapeutic applications. ¹⁶¹Tb decays with a half-life of 6.89 days to stable ¹⁶¹Dy, while emitting β¯-particles (Eβ͞av = 154 keV) suitable for therapeutic purposes and γ-radiation (Eγ = 49 keV, I = 17.0%; Eγ = 75 keV, I = 10.2%) useful for SPECT imaging. ¹⁶¹Tb also emits a substantial number of low-energy conversion and Auger electrons, which makes this radionuclide exceptionally interesting for the treatment of disseminated cancers with multiple metastases ranging from a single cell (diameter: ~10μm) to micro cell clusters (diameter: < 1mm). Monte Carlo simulations assessed the dose delivered to 10μm spheres revealed a 3.5-fold increased value when using ¹⁶¹Tb as compared to ¹⁷⁷Lu. In larger tumors (diameter > 10mm), the emitted electron energy from ¹⁶¹Tb and ¹⁷⁷Lu respectively is almost entirely absorbed, resulting in a 1.3-fold higher absorbed electron energy fraction per decay for ¹⁶¹Tb compared to ¹⁷⁷Lu, making ¹⁶¹Tb more potent than ¹⁷⁷Lu.

The aim of the present study was to use dual-isotope SPECT imaging in order to demonstrate that ¹⁶¹Tb and ¹⁷⁷Lu are interchangeable without compromising the pharmacokinetic profile of the radiopharmaceutical.

After in vitro characterization, ¹⁶¹Tb- and ¹⁷⁷Lu-labeled somatostatin (SST) analogues DOTATOC (agonist) and DOTA-LM3 (antagonist) were injected to AR42J tumor-bearing nude mice. In vivo disptribution profiles were investigatd by dual-isotope SPECT/CT imaging. Results revealed identical pharmacokinetic profiles of the two peptides, irrespective of whether it was labeled with ¹⁶¹Tb or ¹⁷⁷Lu. Moreover, the visualization of the sub-organ distribution confirmed similar behavior of ¹⁶¹Tb- and ¹⁷⁷Lu-labeled SST analogues. These and previous findings suggest that any future (pre)clinical studies with ¹⁶¹Tb can be based on preclinical data obtained with its ¹⁷⁷Lu-labeled counterpart. This will allow the focusing of future investigations directly on the therapeutic efficacy of ¹⁶¹Tb, which is likely to be superior to the effect obtained with ¹⁷⁷Lu.

Results from nanoScan^® SPECT/CT

Five-week-old female CD1 nude mice were subcutaneously inoculated with AR42J tumor cells (5x106 cells in 100µl PBS). The scans were performed 10–14 days after tumor cell inoculation when the tumor size reached a volume of ~250mm3.

Mice were i.v. injected with a mixture of ¹⁶¹Tb-DOTATOC (~15MBq) and ¹⁷⁷Lu-DOTATOC (~15MBq) or a mixture of ¹⁶¹Tb-DOTA-LM3 and ¹⁷⁷Lu-DOTA-LM3 (~30MBq) at a ¹⁶¹Tb/¹⁷⁷Lu activity ratio of 1:1. For specificity test, blocking studies were performed under the same experimental conditions; however, in this case, an excess of unlabeled DOTATOC or DOTA-LM3 was added to the injection solution. SPECT/CT scans were acquired 2h, 4h, and 24h after injection of the radiopeptides using the dual-isotope SPECT acquisition protocol with a frame time of 60s resulting in a scan time of 45min.

For the SPECT/CT scan, simultaneous acquisition of counts stemming from ¹⁶¹Tb and ¹⁷⁷Lu, respectively, was performed by the selection of distinct energy windows for the two radionuclides. The two energy windows chosen for ¹⁶¹Tb were set at 47.7keV±10%, which enabled the detection of X-rays and γ-rays (46.0keV, 48.9keV and 52.0keV), and at 74.6keV±10%, enabling the detection of the γ-rays at 74.6keV. For ¹⁷⁷Lu, the windows were set at 112.9keV±10% and 208.4±10% to detect the γ-rays. Prior to animal studies, phantom scans were carried out in order to verify of the dual-isotope SPECT imaging protocol using Eppendorf vials filled with ¹⁶¹Tb or ¹⁷⁷Lu or both: analysis revealed no interference between ¹⁶¹Tb and ¹⁷⁷Lu in the acquired scans; each radionuclide was visualized independently of the other with high accuracy.

Analysis of the SPECT/CT images revealed:

• Equal in vivo distribution of simultaneously injected ¹⁶¹Tb-DOTATOC and ¹⁷⁷Lu-DOTATOC. The same observation was made for ¹⁶¹Tb-DOTA-LM3 and ¹⁷⁷Lu-DOTA-LM3. Images reconstructed using the energies of either radiolanthanide (red-to-yellow scale and green-to-yellow scale for ¹⁶¹Tb and ¹⁷⁷Lu, respectively), provided the distribution for each radiopeptide separately in the same mouse.

• Experiments performed by co-injection of excess unlabeled peptide resulted in SSTR blockade and, hence, accumulation of the radiopeptides in AR42J tumors was not observed. These additional studies proved that the uptake of the SST analogues in AR42J tumor xenografts was SSTR-specific.
• Quantification of the accumulated activity in AR42J tumors and kidneys confirmed equal distribution of the ¹⁶¹Tb- and ¹⁷⁷Lu-labeled counterparts. This was the ultimate proof that the chosen radiolanthanide did not have an impact on the tissue distribution profile of the radiopeptides.
• The SPECT/CT images showed activity accumulation in the AR42J xenografts, which was higher for the antagonist than for the agonist. In agreement with quantitative data from biodistribution studies, the activity was efficiently cleared through the kidneys over time and almost entirely excreted after 24h. Due to the favorable uptake of radiolabeled DOTA-LM3 in the tumor tissue, the tumor-to-kidney ratio was higher as compared to the ratio obtained after injection of radiolabeled DOTATOC.
• Dual-isotope SPECT image sections enabled, for the first time, visualization of the ¹⁶¹Tb- and ¹⁷⁷Lu-labeled peptide distribution at a sub-organ level in the same animal. Most important to note is that the pattern of activity distribution in tumors and kidneys was the same, irrespective of whether ¹⁶¹Tb or ¹⁷⁷Lu was used. The uptake in the tumor was quite homogenous, which can be ascribed to the well-vascularized AR42J xenograft. Accumulation of activity in the kidneys was more prominent in the cortex where the megalin-mediated reabsorption of radiopeptides occurs, and where various SSTR subtypes are known to be expressed

Full article on MDPI Pharmaceutics