Bone Uptake Studies in Rabbits Before and After High-Dose Treatment with ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP

MATERIALS AND METHODS

Scintigraphic Skeletal Uptake Measurement

Whole-body images were acquired simultaneously from anterior and posterior views at 3 min, 3 h, and 24 h after a bolus injection of the respective radiopharmaceutical (^99mTc-HDP, ¹⁸⁶Re-HEDP, or ¹⁵³Sm-EDTMP). The acquisition time was 1 min in the blood-pool phase and 10 min for the later images. For each rabbit, images were obtained under the same conditions using a double-head gamma camera (Bodyscan; Siemens, Erlangen, Germany) equipped with high-resolution collimators for low energy. The energy windows were 140 keV ± 15%, 137 keV ± 15%, and 103 keV ± 15% for ^99mTc, ¹⁸⁶Re, and ¹⁵³Sm, respectively.

For each image, the activities of the whole body, the urinary bladder, and the soft tissue of the flank were measured by region-of-interest (ROI) technique from both anterior and posterior views. Additionally, for uptake measurements of ¹⁸⁶Re-HEDP or ¹⁵³Sm-EDTMP, a rectangular ROI adjacent to the body was used to measure the bremsstrahlung of the β-particles of the radionuclide. An example of ROI positions is given in Figure 1. From these data, the geometric mean for each ROI was calculated after correction for different acquisition times; for radioactive decay; and, in cases of ¹⁸⁶Re-HEDP or ¹⁵³Sm-EDTMP, for bremsstrahlung. The initial whole-body activity of each rabbit was used as the reference value for further calculations of activity data, which are given as a percentage of initial whole-body activity. According to a simplified 3-compartment model, soft-tissue activity as compared with initial whole-body activity equals the activity of the flank as compared with the initial flank activity at 3 min after injection. Urinary excretion was determined by the difference of whole-body activities between the reference image at 3 min and the image of interest plus urinary bladder activity. From these data, bone uptake was calculated for 24 h after injection as 100% of initial whole-body activity minus both urinary excretion and soft-tissue retention as described in detail elsewhere (9,10).

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FIGURE 1.

ROI positions on posterior whole-body scintigram of rabbit 3 h after injection of 1,000 MBq of ¹⁸⁶Re-HEDP.

RESULTS

The mean bone uptake of ^99mTc-HDP at 24 h after injection before treatment with ¹⁵³Sm-EDTMP was 51.1% ± 5.5% of initial whole-body activity. Eight weeks after radionuclide therapy, the mean bone uptake value, 48.0% ± 6.1%, showed no significant difference (P > 0.05) from the pretreatment result (Table 1). Also, no significant differences between before- and after-therapy values could be proven for the remainder soft-tissue activities (16.4% ± 5.3% vs. 17.8% ± 3.5%) or for the urinary excretion rates at 24 h after injection (31.6% ± 3.0% vs. 34.2% ± 4.7%).

Similar results were obtained in rabbits treated with ¹⁸⁶Re-HEDP (Table 2). Both the mean bone uptake of ^99mTc-HDP (44.8% ± 6.7% vs. 40.4% ± 4.9%) and the urinary excretion of ^99mTc-HDP (37.0% ± 6.1% vs. 37.7% ± 4.9%) revealed no significant differences (P > 0.05 each) before and after treatment, although bone uptake values were lower in all animals after therapy. The remainder soft-tissue activity was significantly higher (P < 0.01) after radionuclide therapy (22.0% ± 0.4%) than before (17.7% ± 1.6%).

Between the 2 rabbit groups treated with either ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP, no significant differences (P > 0.05 each) could be proven for the mean bone uptake, soft-tissue retention, and urinary excretion of ^99mTc-HDP (Tables 1 and 2).

The mean bone uptake, soft-tissue retention, and urinary excretion of ¹⁵³Sm-EDTMP at 24 h after injection were 44.0% ± 6.5%, 9.8% ± 4.4%, and 46.3% ± 7.5%, respectively (Table 3). The corresponding results for ¹⁸⁶Re-HEDP were 31.2% ± 1.5%, 16.2% ± 2.7%, and 52.6% ± 3.4%. Thus, a significantly higher bone uptake was found for ¹⁵³Sm-EDTMP (P < 0.001), whereas soft-tissue retention was significantly lower for ¹⁵³Sm-EDTMP (P < 0.02). No differences were found between the urinary excretion rates for ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP (P > 0.05).

DISCUSSION

Because bone pain is relieved for only 2–3 mo in most patients undergoing radionuclide therapy with ¹⁸⁶Re-HEDP or ¹⁵³Sm-EDTMP, subsequent therapy is recommended as long as blood cell counts are in the normal range. The activity administered and the extent of radiopharmaceutical uptake by bone are known determinants of the success of this treatment (4,8,13). Radiation doses of up to 3 Gy to bone and bone marrow and of up to 140 Gy to metastatic sites have been reported (3,7,8,14,15), strongly suggesting at least alterations of the local bone metabolism and, thus, uptake of radiolabeled bisphosphonates by bone. For example, reduced bone uptake of radiolabeled bisphosphonates can be observed within the irradiation fields after external radiotherapy (16) that applies center doses of up to 40 Gy. Although comparably high local doses are achieved by radionuclide therapy only in single metastases and not in larger bone areas, most of the skeleton-bound activity of ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP, that is, 20%–50% of the total activity administered (10), is absorbed by nonmetastatic bone tissue. In contrast, in vitro experiments with rat osteoblastlike cells showed inhibitory effects at doses of as low as 100 cGy (17). Similar results, such as a dose-dependent decrease in cellular differentiation and proliferation and a decrease in the production of transforming growth factor β1 and vascular endothelial growth factor as markers for radiation-induced cytokine profile alterations, were found by Dudziak et al. (18) for dose levels of 40, 400, and 800 cGy. Thus, even the lower irradiation of the bones may have a general effect on bone metabolism and on bone uptake of bisphosphonates. Therefore, it seems important to know about changes in bone uptake some 2–3 mo after radionuclide therapy.

In this animal study, skeletal uptake of ^99mTc-HDP was measured as an indicator of bone metabolism before and 8 wk after high-dose treatment with ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP to assess any post-therapeutic impairment of bone uptake. To measure skeletal uptake of the radiolabeled bisphosphonates used in this study, we used a recently introduced method that has been proven valid in patients in comparison with data from literature (9,10). This method is based on scintigraphic 3-phase whole-body imaging in combination with conventional ROI technique and allows calculation of kinetic data for the compartments assumed in this model: bone, soft tissue, and urinary excretion.

Significant alterations of ^99mTc-HDP uptake by bone 8 wk after radionuclide therapy with ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP could be excluded in this study. In animals that received ¹⁵³Sm-EDTMP, uptake by bone was somewhat lower after radionuclide therapy in 4 of 6 rabbits, whereas in all animals treated with ¹⁸⁶Re-HEDP, uptake was lower, although not significantly so. On account of follow-up studies in nonmetastatic-tumor patients undergoing repeated bone scintigraphy, we know that variations of up to ±10% in bone uptake are observed within 6 mo in many cases (W. Brenner et al., unpublished data, 2000). These variations are due mainly to the error in measurement of this method rather than to real changes in bone uptake. In this study, in 4 of 6 rabbits treated with ¹⁸⁶Re-HEDP and in 1 of 6 rabbits treated with ¹⁵³Sm-EDTMP, bone uptake values were more than 10% lower after radionuclide therapy, whereas in only 1 of these cases was bone uptake reduced by more than 20%. According to a potential error of measurement of about 10%, these differences were statistically not significant, nor do changes within this range seem crucial from the clinical point of view. Furthermore, these findings have been observed after high-dose treatment, that is, 400 MBq of either ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP per kilogram of body weight: The activities usually applied in patients are 37 and 15–20 MBq per kilogram of body weight for ¹⁵³Sm-EDTMP and ¹⁸⁶Re-HEDP, respectively. Thus, a significant impairment of bone uptake of radiolabeled bisphosphonates in patients undergoing a second radionuclide therapy within 2–3 mo after standard treatment with ¹⁵³Sm-EDTMP or ¹⁸⁶Re-HEDP is not to be expected according to the data provided in this study. Also, no significant changes in urinary excretion and soft-tissue retention of ^99mTc-HDP could be observed after administration of ¹⁵³Sm-EDTMP. Only the remainder soft-tissue activity of ^99mTc-HDP was statistically increased after therapy with ¹⁸⁶Re-HEDP; urinary excretion rate remained unchanged. High-dose treatment with ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP therefore did not significantly alter the biodistribution of radiolabeled bisphosphonates, suggesting no major impairment of bone uptake in general because of this therapeutic modality.

To justify the transfer of such a conclusion to patients, it seems necessary to check the validity of the animal model for both ¹⁵³Sm-EDTMP and ¹⁸⁶Re-HEDP. At 24 h after injection, kinetic data similar to those reported in patients undergoing radionuclide bone therapy were observed (5,8,10,19,20). The bone uptake values in patients at 24 h after injection were 47.7% ± 11.2% for ¹⁵³Sm-EDTMP, soft-tissue retention was 12.7% ± 4.7%, and the urinary excretion rate was 39.5% ± 13.8% (10). The corresponding data in rabbits—44.0% ± 6.5% for bone uptake, 9.8% ± 4.4% for remainder soft-tissue activity, and 46.3% ± 7.5% for urinary excretion—showed no significant differences. Similar results were obtained for ¹⁸⁶Re-HEDP, although bone uptake values in rabbits, at 31.2% ± 1.5%, were somewhat higher than the 21.8% ± 9.0% observed in patients. Nevertheless, the bone uptake values in rabbits were significantly higher for ¹⁵³Sm-EDTMP than for ¹⁸⁶Re-HEDP, clearly reflecting the same differences between the biodistribution pattern of these radiopharmaceuticals as already observed in patients (10). One point to be addressed, however, is the problem of a solely local change and reduction of bone uptake in metastases or the adjacent bone tissue, which may significantly hamper local-pain palliation in cases of retreatment. Because this point cannot be investigated by the model used in this study, a model with bone tumors or bone metastases is mandatory for further studies. For investigating the general effects of radionuclide bone therapy on bone uptake of bisphosphonates, however, the model used in this study—a normal nontumor model with animals large enough for scintigraphic bone scanning—seemed sufficient despite its limitations.

Furthermore, we could prove a transient decrease of about 50% and 60% in the leukocyte counts and platelet counts, respectively, of rabbits within 3 wk after radionuclide treatment. These findings are in line with the radiation-induced myelotoxic effects reported for patients treated with the activities recommended for clinical practice (4,21–23). Thus, the findings in rabbits concerning both the bone uptake and the biodistribution of ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP, as well as the radiation-induced myelotoxic effects, are comparable to the findings in patients. Therefore, one seems justified in expecting the same, that is, nonsignificant, effects on bone uptake in patients undergoing radionuclide therapy as are observed in rabbits.

CONCLUSION

In this study investigating the impact of radionuclide therapy with ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP on bone uptake of radiolabeled bisphosphonates in rabbits, no significant alterations of the bone uptake of ^99mTc-HDP could be observed 8 wk after high-dose treatment. Because the biokinetic data obtained for ¹⁸⁶Re-HEDP and ¹⁵³Sm-EDTMP and the myelotoxic effects in rabbits were similar to those in patients, it seems justified to expect the same results on bone uptake in patients as in rabbits. Thus, significant impairment of the bone uptake of radiolabeled bisphosphonates in patients undergoing a second radionuclide therapy within 2–3 mo after standard treatment with ¹⁸⁶Re-HEDP or ¹⁵³Sm-EDTMP is not to be expected according to the data obtained from this animal study.