Comparison of the dosimetry and cell survival effect of 177Lu and 161Tb somatostatin analog radiopharmaceuticals in cancer cell clusters and micrometastases

Abstract

Background: ¹⁷⁷Lu-based radiopharmaceuticals (RPs) are the most used for targeted radionuclide therapy (TRT) due to their good response rates. However, the worldwide availability of ¹⁷⁷Lu is limited. ¹⁶¹Tb represents a potential alternative for TRT, as it emits photons for SPECT imaging, β^--particles for therapy, and also releases a significant yield of internal conversion (IE) and Auger electrons (AE). This research aimed to evaluate cell dosimetry with the MIRDcell code considering a realistic localization of three ¹⁶¹Tb- and ¹⁷⁷Lu-somatostatin (SST) analogs in different subcellular regions as reported in the literature, various cell cluster sizes (25-1000 µm of radius) and percentage of labeled cells. Experimental values of the α- and β-survival coefficients determined by external beam photon irradiation were used to estimate the survival fraction (SF) of AR42J pancreatic cell clusters and micrometastases.

Results: The different localization of RPs labeled with the same radionuclide within the cells, resulted in only slight variations in the dose absorbed by the nuclei (AD_N) of the labeled cells with no differences observed in either the unlabeled cells or the SF. AD_N of labeled cells (MDLC) produced by ¹⁶¹Tb-RPs were from 2.8-3.7 times higher than those delivered by ¹⁷⁷Lu-RPs in cell clusters with a radius lower than 0.1 mm and 10% of labeled cells, due to the higher amount of energy emitted by ¹⁶¹Tb-disintegration in form of IE and AE. However, the ¹⁶¹Tb-RPs/¹⁷⁷Lu-RPs MDLC ratio decreased below 1.6 in larger cell clusters (0.5-1 mm) with > 40% labeled cells, due to the significantly higher ¹⁷⁷Lu-RPs cross-irradiation contribution. Using a fixed number of disintegrations, SFs of ¹⁶¹Tb-RPs in clusters with > 40% labeled cells were lower than those of ¹⁷⁷Lu-RPs, but when the same amount of emitted energy was used no significant differences in SF were observed between ¹⁷⁷Lu- and ¹⁶¹Tb-RPs, except for the smallest cluster sizes.

Conclusions: Despite the emissions of IE and AE from ¹⁶¹Tb-RPs, their localization within different subcellular regions exerted a negligible influence on the AD_N. The same cell damage produced by ¹⁷⁷Lu-RPs could be achieved using smaller quantities of ¹⁶¹Tb-RPs, thus making ¹⁶¹Tb a suitable alternative for TRT.

Keywords: 161Tb; 177Lu; Cell dosimetry; Radiopharmaceuticals; Somatostatin analogs; Targeted radionuclide therapy; Theranostics.

Fig. 1

A 3D culture of AR42J cells embedded in alginate beads. Representative microscopic images of the 3D culture AR42J cells: B phase contrast image, C cytoplasmic distribution of green AM (green) in alive cells, D nuclear imaging, staining with propidium iodide (red), and E merged image of C and D

Fig. 2

Survival curve obtained from A clonogenic assay and B EdU proliferation assay at 24 h after irradiation. Each point represents the mean value of the six experiments, and the bars indicate the standard errors obtained. The red line represents the best fit to the experimental data points using the linear-quadratic model

Fig. 3

¹⁶¹ Tb-DOTATOC/¹⁷⁷Lu-DOTATOC ratios of the mean doses to nuclei of labeled cells (MDLC) calculated for the three different cluster sizes (radius of 100, 500, and 1000 µm) and 10, 40, or 70% of labeled cells. For each cluster size and percentage of labeling, four points are reported in the plot, corresponding to the activity levels presented in Table 5. A two-way ANOVA analysis with a significance level of 0.05 (three columns and twelve rows, with a total of 36 observations) revealed significant differences in the MDLC ¹⁶¹Tb-DOTATOC/¹⁷⁷Lu-DOTATOC ratios among cluster sizes (54.67% of total variation; P < 0.0001) and percentages of labeled cells (54.67% of total variation; P < 0.001). Tests of Tukey's multiple comparisons showed significant differences between the following combinations of labeled cells: 10% vs. 40% (P < 0.001), 10% vs. 70% (P < 0.0001), and 70% vs. 40% (P < 0.0001). Significant differences were also observed when comparing 100 µm vs. clusters of 500 µm (P < 0.0001) and clusters of 100 µm vs. clusters of 1000 µm (P < 0.0001). However, a comparison of clusters of 500 µm vs. clusters of 1000 µm revealed no significant difference (P = 0.9977)

Fig. 4

The relative contribution of cross-irradiation to the MDLC for different percentages of labeled cells (10%, 40s%, and 70%) using a mean number of disintegrations per cell of 18,900 for both ¹⁷⁷Lu-RPs and ¹⁶¹Tb-RPs versus the radius of the spherical cell cluster

Fig. 5

Mean dose (AD_N) to all cells (MDC), unlabeled cells (MDUC), and labeled cells (MDLC) calculated using a mean number of disintegrations per cell of 18,900 and 10%, 40%, or 70% of cells labeled with ¹⁷⁷Lu-RPs or ¹⁶¹ Tb-RPs versus the radius of the cell cluster

Fig. 6

Mean dose to all cells (MDC), labeled cells (MDLC), and unlabeled cells (MDUC) versus the radial position within the cluster with a radius of 1000 µm and 40% of cells labeled with A) ¹⁷⁷Lu-DOTATOC and B) ¹⁶¹Tb-DOTATOC. The self- and cross-contributions to MDLC were plotted separately

Fig. 7

Mean dose to all cells (MDC), to 40% labeled cells (MDLC), and to unlabeled cells (MDUC) versus the radius of cell cluster, considering a mean number of disintegrations per cell of 18,900, for all A ¹⁷⁷Lu-RPs and B ¹⁶¹Tb-RPs

Fig. 8

MDUC versus the size of the cluster radius considering A) the same mean number of disintegrations per cell for all RPs and B) a mean number of disintegrations per cell of 18,900 and 13,734 for ¹⁷⁷Lu-RPs and ¹⁶¹Tb-RPs, respectively

Fig. 9

Survival fraction versus the mean number of disintegrations per cell after administration of ¹⁷⁷Lu-NLS or ¹⁶¹Tb-NLS considering 10, 40, or 70% of cells labeled and a cluster size of 100 µm (A and C) and 1000 µm (B and D)

Fig. 10

Survival fraction versus the mean number of disintegrations per cell after administration of ¹⁶¹Tb-LM3 in cell clusters with sizes ranging between 100 µm and 1000 µm considering (A) 40% and (B) 70% of labeled cells

Fig. 11

Survival fraction versus the mean number of disintegrations per cell for 100, 500, and 1000 µm cluster sizes considering the 70% of the cells labeled with A) ¹⁷⁷Lu-RPs or B) ¹⁶¹Tb-RPs

Fig. 12

Comparison of survival fractions versus the mean number of disintegrations per cell following ¹⁷⁷Lu- or ¹⁶¹Tb-LM3 treatment of 100, 500, and 1000 µm cell clusters considering 70% of cells labeled A using the same mean number of disintegrations per cell for the two radionuclides, while in B scaling the mean number of disintegrations of ¹⁶¹Tb by a factor 1.37