[161Tb]Tb-DOTA-LM3

Preclinical data have shown that ¹⁶¹Tb-labeled peptides targeting the somatostatin receptor are therapeutically more effective for peptide receptor radionuclide therapy than are their ¹⁷⁷Lu-labeled counterparts. To further substantiate this enhanced therapeutic effect, we performed cellular dosimetry to quantify the absorbed dose to the cell nucleus and compared dose-response curves to evaluate differences in relative biological effectiveness in vitro. Methods: CA20948 cell survival was assessed after treatment with [¹⁶¹Tb]Tb- and [¹⁷⁷Lu]Lu-DOTATATE (agonist) and with [¹⁶¹Tb]Tb- and [¹⁷⁷Lu]Lu-DOTA-LM3 (antagonist) via a clonogenic assay. Cell binding, internalization, and dissociation assays were performed up to 7 d to acquire time-integrated activity coefficients. Separate S values for each type of particle emission (Auger/internal conversion [IC] electrons and β^- particles) were computed via Monte Carlo simulations, while considering spheric cells. Once the absorbed dose to the cell nucleus was calculated, survival curves were fitted to the appropriate linear or linear-quadratic model and corresponding relative biological effectiveness was evaluated. Results: Although the radiopeptide uptake was independent of the radionuclide, [¹⁶¹Tb]Tb-DOTATATE and [¹⁶¹Tb]Tb-DOTA-LM3 delivered a 3.6 and 3.8 times higher dose to the nucleus, respectively, than their ¹⁷⁷Lu-labeled counterparts on saturated receptor binding. This increased nucleus-absorbed dose was mainly due to the additional emission of IC and not Auger electrons by ¹⁶¹Tb. When activity concentrations were considered, both [¹⁶¹Tb]Tb-DOTATATE and [¹⁶¹Tb]Tb-DOTA-LM3 showed a lower survival fraction than did labeling with ¹⁷⁷Lu. When the absorbed dose to the nucleus was considered, no significant difference could be observed between the dose-response curves for [¹⁶¹Tb]Tb- and [¹⁷⁷Lu]Lu-DOTATATE. [¹⁶¹Tb]Tb-DOTA-LM3 showed a linear-quadratic dose response, whereas [¹⁶¹Tb]Tb-DOTATATE showed only a linear dose response within the observed dose range, suggesting additional cell membrane damage by Auger electrons. Conclusion: The IC, rather than Auger, electrons emitted by ¹⁶¹Tb resulted in a higher absorbed dose to the cell nucleus and lower clonogenic survival for [¹⁶¹Tb]Tb-DOTATATE and [¹⁶¹Tb]Tb-DOTA-LM3 than for the ¹⁷⁷Lu-labeled analogs. In contrast, [¹⁶¹Tb]Tb-DOTATATE showed no higher dose response than [¹⁷⁷Lu]Lu-DOTATATE, whereas for [¹⁶¹Tb]Tb-DOTA-LM3 an additional quadratic response was observed. Because of this quadratic response, potentially caused by cell membrane damage, [¹⁶¹Tb]Tb-DOTA-LM3 is a more effective radiopeptide than [¹⁶¹Tb]Tb-DOTATATE for labeling with ¹⁶¹Tb.

Accurate cellular dosimetry is essential to investigate fundamental mechanisms of targeted radionuclide therapy. The aim of this study was to assess how morphological cellular geometry models influence cellular dosimetry estimates, in comparison to simplified spherical models that do not properly represent an adherent cell geometry.

Virtual cell models of the CA20948 cell line were generated by confocal microscopy of SSTR2 and DAPI staining and served as input to derive morphological S-values for ¹⁷⁷Lu and ¹⁶¹Tb. Absorbed dose-response relationships were established for [¹⁷⁷Lu]Lu-DOTA-TATE, [¹⁶¹Tb]Tb-DOTA-TATE, [¹⁷⁷Lu]Lu-DOTA-LM3 and [¹⁶¹Tb]Tb-DOTA-LM3 using S-values from both morphological and spherical cell geometries.

Thirty-four cell geometries were modeled and a spherical model with equivalent volume was generated with a radius for the cell and nucleus of 8.6(7) µm and 5.5(6) µm, respectively. Compared to spherical cell models, morphological cell models significantly changed the S-value with an increase of 13% (¹⁷⁷Lu) and 22% (¹⁶¹Tb) with the cell membrane as source region and a decrease of 11% (¹⁷⁷Lu) and 12% (¹⁶¹Tb) with the cytoplasm as source region. Absorbed dose-response relationships based on morphological cell geometries showed a linear dose-response model for [¹⁷⁷Lu]Lu-DOTA-TATE and [¹⁶¹Tb]Tb-DOTA-TATE with α = 0.22[0.18,0.26] Gy-1, and a linear-quadratic dose-response model for [¹⁷⁷Lu]Lu-DOTA-LM3 and [¹⁶¹Tb]Tb-DOTA-LM3 with α = 0.000[0.000,0.022] Gy-1 and β = 0.064[0.055,0.072] Gy-2. The assumption of a spherical cell model did not significantly affect the dose-response models, while underestimating the cell dimensions did induce a rescaling of the dose-response models.

These findings validate the use of simplified spherical models for CA20948 cells but highlight the importance of a correct estimation of the cell dimensions.