Introduction
Multiple myeloma is a plasma cell malignancy, mainly involving bone marrow, with the 5-year survival rate of <30%.Reference Alexanian and Dimopoulos 1 – Reference Cascini, Cuccurullo and Tamburrini 3 However, chemotherapy is known as the standard method of treatment;Reference Cavo, Gobbi and Tura 4 radiation therapy and stem cell transplant are used as the other treatment methods for disease control. 5 Stem cell transplant is the common procedure in multiple myeloma treatment that is used following radiation or chemotherapy.
In the technique called bone marrow ablation, all stem cells of the haematopoietic system are destroyed to follow the bone marrow transplantation for patients with multiple myeloma.Reference Bayouth, Macey and Kasi 6 Recently, some bone-seeking radiopharmaceuticals using bisphosphonates are developed for bone marrow ablation, offering a beneficial therapeutic option in patients with multiple myeloma.Reference Breitz, Wendt and Stabin 7 , Reference Wiesmann, Einsele, Kanz and Dohmen 8
However, bisphosphonates are known as the effective drugs for transportation of the diagnostic and therapeutic radionuclides to the bone site.Reference Rabie, Enayati and Yousefnia 9 New generation of bisphosphonates such as (4-{[(bis(phosphonomethyl))carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododec-1-yl)acetic acid (BPAMD) are developed solving some requirements and restrictions of the first-generation phosphonates.Reference Fellnera, Biesalski and Bausbacher 10 Lately, the radiolabelled compounds of BPAMD have shown excellent biodistribution in animal model or human studies.Reference Fellner, Baum and Kubícek 11 , Reference Yousefnia, Zolghadri and Sadeghi 12
Among the different therapeutic radionuclides, Samarium-153 [153Sm]Reference Macfarlane, Durrant and Bartlett 13 and Holmiun-166 [166Ho]Reference Breitz, Wendt and Stabin 7 with their high-energy β-particles have been used for bone marrow ablation in the patients with multiple myeloma. 166Ho-DOTMPReference Breitz, Wendt and Stabin 7 is used as the main radiopharmaceutical for this purpose. Yttrium-90 [90Y] (Eβ − max=2·25 MeV, T 1/2=2·7 d) is of enormous interest owing to its favourable decay characteristics and its availability in the form of 90Sr–90Y generator system, resulting in the preparation of radiopharmaceuticals with greater specific activity. 90Y seems to be a better candidate for bone marrow ablation compared with 166Ho, according to its high-energy β-particles.
90Y is believed to be the most useful among the radionuclides that have been used for radiotherapeutic purposes.Reference Vanhemert, Sloof and Schimmel 14 90Y-radiopharmaceuticals are widely used for the treatment and management of different cancer types.Reference Horovitz 15 However, some radiolabelled compounds of this radionuclide, including 90Y-citrate and 90Y-EDTMP, are used in clinical applications.Reference Ogawa, Kawashima and Shiba 16 , Reference Környei, Antalffy and Baranyai 17 These complexes suffer from the low stability in human serum resulting in the liver toxicity. Recently, 90Y-BPAMD was prepared that showed interesting characteristics compared with the other 90Y bone-seeking agents and even 166Ho-BPAMD.Reference Rabiei, Shamsaei and Yousefnia 18
In this study, with respect to the importance of absorbed dose in the bone marrow and other non-target organs, as a first step to evaluate the possibility of its usage, the absorbed dose to human organs after injection of 90Y-BPAMD was estimated based on the biodistribution of rats data by radiation-absorbed dose-assessment resource (RADAR) method. The resulting data were compared with 166Ho-DOTMP as the only clinically used 166Ho bone marrow ablative agent.
Materials and Methods
An electrochemical 90Sr–90Y generator “Kamadhenu” system from Isotope Technologies Dresden, ITD, Germany was used for radiolabelling purpose. BPAMD was obtained from ABX (Radeberg, Germany). The other chemical reagents and Whatman No. 2 paper were purchased from Merck (Darmstadt, Germany) and Whatman (Buckinghamshire, UK). A high-purity germanium (HPGe) detector coupled with a Canberra™ (model GC1020-500SL) multichannel analyser and a Wallac 1220 Quantulus, Perkin-Elmer, ultra-low-level liquid scintillation spectrometer (Turku, Finland) were used for the assessment of γ and β impurities. Radio-chromatography was performed by using a Bioscan AR-2000 radio thin layer chromatography scanner instrument (Bioscan, Washington, DC, USA). The male Syrian rats weighing 180–220 g kept at routine day/night light program and under common rodent diet pellets were used for biodistribution studies.
Preparation and quality control of 90Y-BPAMD
An electrochemical generator based on the secular equilibrium of 90Sr–90Y was used to attain 90YCl3 for radiolabelling purpose. The amounts of 90Sr in 90YCl3 solution was specified by extraction paper chromatography technique according to the previous research.Reference Pandey, Dhami and Jagesia 19 Radionuclidic purity and radiochemical purity of the solution were checked by an HPGe detector and instant thin layer chromatography (ITLC) method, respectively.
90Y-BPAMD was prepared according to the previously reported literature.Reference Rabiei, Shamsaei and Yousefnia 18 Briefly, 1 mg of BPAMD was dissolved in 1 mL pure water and the aqueous solution was used for labelling studies. 60 μL (105 nmol) of the stock solution was added to the vial containing 370 MBq of 90YCl3. The pH of the reaction mixture was adjusted to 5 and the mixture was incubated for 45 minutes at 90–100°C. To remove the non-complexed 90Y from the radiolabelled compound, the mixture was passed over the strong cation exchanger (Strata-X-C 60 mg). The radiochemical purity was checked by ITLC method using the NH4OH:MeOH:H2O (0·2:2:4) solvent system as the mobile phase.
Biodistribution assessment of the radiolabelled complex in male Syrian rats
Final 90Y-BPAMD solution with approximately 5·55 MBq radioactivity was injected intravenously into the male Syrian rats through their tail veins. The animals were sacrificed at selected times after injection using the animal care protocols. The percentage of injected dose per gram (%ID/g) for different organs was calculated by dividing the activity amount of each tissue (A) to the injected activity and the mass of each organ.
Statistical analysis
Five mice were sacrificed for each time interval. All values were expressed as mean±SD and the data were compared using Student’s t-test. Statistical significance was defined as p<0·05.
Calculation of accumulated activity in human organs
The accumulated activity in human organs was determined according to the previous research performed by Yousefnia et al.Reference Yousefnia, Zolghadri, Jalilian and Naseri 20 In summary, the non-decay corrected percentage-injected dose versus time was plotted for each animal organ. Although linear approximation was used between the two experimental points of times and the curves were extrapolated to infinity by fitting the tail of each curve to a monoexponential curve, the area under the curves were counted as the accumulated activity of animal organs. Then, the accumulated activity for human organs were determined by the extrapolation of this amount for animal organs using the proposed method of Sparks and Aydogan.Reference Sparks and Aydogan 21
Equivalent and effective absorbed dose calculation
The absorbed dose in human organs was calculated by RADAR formalism based on biodistribution data of the ratsReference Stabin and Siegel 22 and in accordance with the other previously reported research.Reference Sparks and Aydogan 21 Briefly, the calculated accumulated activity was multiplied by the dose factors of source organs taken from the amount presented in OLINDA/EXM software.Reference Stabin, Sparks and Crowe 23
The effective absorbed dose for each organ was computed by Equation (1):
$$E{\equals}\mathop{\sum}\limits_T {W_{T} H_{T} } $$
where H T is the equivalent absorbed dose for each organ and W T is the tissue-weighting factor that represents a subjective balance between the different stochastic health risks.Reference Brenner 24 W T was obtained from the reported value in ICRP-103. 25
Results and Discussion
Preparation and quality control of 90Y-BPAMD
The radionuclidic purity and radiochemical purity of 90YCl3 solution were calculated to be more than 99·99 and 99%, respectively. The amount of 90Sr contamination in the solution was estimated to be equal to 1·05 ppm (Figure 1), which is within the permissible limit. Radiochemical purity of higher than 98% was observed for 90Y-BPAMD complex (Figure 2).
Figure 1 Extraction paper chromatography of the 90Y solution used for radiolabelling.
Figure 2 ITLC of 90YCl3 (a) and 90Y-BPAMD (b) in NH4OH:MeOH:H2O (0·2:2:4) solution on Whatman No. 2 papers.
Biodistribution assessment of the radiolabelled complex in rats
Biodistribution of 90Y-BPAMD in the rats was investigated up to 72 h post injection. The non-decay corrected %ID/g for the rat organs after the injection of the complex was demonstrated in Table 1. The biodistribution of the complex showed that the radioactivity would remove from blood circulation very fast and significantly excreted from the kidneys. No considerable accumulation was observed in the liver. Based on the obtained results, it is clearly concluded that the major portion of the injected activity of the complex is accumulated in the bones.
Table 1 The non-decay corrected %ID/g values after intravenously injection of 90Y-BPAMD to the rats
Dosimetric studies
Dosimetric evaluation of the complex in human organs was carried out by RADAR method based on biodistribution data in the rat organs. The equivalent and effective absorbed dose in human organs after intravenous injection were presented in Table 2.
Table 2 Equivalent and effective absorbed dose delivered into human organs after injection of 90Y-BPAMD
Note: aTissue weighting factors according to international commission on radiological protection, ICRP-103 (2007).
Abbreviations: GB, gallbladder; LLI, lower large intestine; int, intestine; ULI, upper large intestine; UB Wall, urinary bladder wall.
Owing to the direct relationship between the absorbed dose and response in terms of cell killing/survival, this study was done with the aim of estimating the radiation absorbed dose in human organs after injection of 90Y-BPAMD. For this purpose, the biodistribution data of male Syrian rats were used. However, it should be noted that the actual percentage of the administered dose of radiation that reaches the bone/bone marrow necessarily varies from subject to subject.
Besides, extrapolation between animals and humans may lead to overestimation or underestimation of the absorbed dose, but the previous studies have demonstrated the usefulness of animal biodistribution as a model for the absorbed dose estimations in humans.Reference Bélanger, Krause and Ryan 26 , Reference Kesner, Hsueh and Czernin 27 Estimation of the absorbed dose from small animals such as mice and rats is probably the most accurate procedure that has been used since the earliest days of nuclear medicine.Reference Williams 28
As expected, the highest amounts of absorbed dose are observed in the bone surface and the bone marrow with 2·52 and 2·29 mGy/MBq, respectively, whereas the other organs receive insignificant absorbed dose. The bone marrow to other tissue absorbed dose ratios for 90Y-BPAMD were calculated and presented in Table 3. For better comparison, the calculated ratios after injection of 166Ho-DOTMP, as the only clinically used 166Ho bone marrow ablative agent, to the patients with multiple myeloma are also given in Table 3.
Table 3 Bone marrow to non-target absorbed dose ratios for 90Y-BPAMD and 166Ho-DOTMP
Note: aN.D. refers to not defined.
Abbreviations: GB, gallbladder; LLI, lower large intestine; int, intestine; ULI, upper large intestine; UB Wall, urinary bladder wall.
As can be seen in Table 3, the red marrow to the most organ absorbed dose ratios for 90Y-BPAMD are much higher than these ratios for 166Ho-DOTMP. This means for a given dose to the bone marrow as the target organ, total body and other organs such as spleen, bone surface, brain etc. would receive lesser absorbed dose in the case of 90Y-BPAMD utilisation. The absorbed dose ratios of red marrow:liver and red marrow:lung are greater for 166Ho-DOTMP, rather than 90Y-BPAMD. Therefore, for delivering a certain dose to the bone marrow, these two organs would receive lesser absorbed dose in the case of 166Ho-DOTMP usage.
Conclusion
The radiolabelled 90Y complex was prepared with high radiochemical purity (>98%, ITLC). The biodistribution of the complex in normal rats up to 72 h post injection indicated considerable accumulation in the bones. The highest amounts of absorbed dos are observed in the bone surface and the bone marrow with 2·52 and 2·29 mGy/MBq, respectively, while the other organs receive insignificant absorbed dose. The results indicate that for a given dose to the bone marrow as the target organ, the most non-target organs would receive lesser absorbed dose in the case of 90Y-BPAMD utilisation rather than 166Ho-DOTMP. Generally, the dosimetric data of 90Y-BPAMD showed high red marrow to non-target organs absorbed dose ratios and therefore can be considered as a potential agent for bone marrow ablative therapy; however, further biological studies are still needed.
Acknowledgements
None.
Ethical standards
All procedures contributing to this work comply with the ethical standards of the relevant national guidelines on the care and use of laboratory animals and has been approved by the NSTRI Committee.
