Synthesis and Stability Studies of ²²⁵Actinium Tin Colloid Radiopharmaceutical

Synthesis of novel ²²⁵Ac-Sn particles was described for the first time. Detailed experimental and stability studies were successfully exhibited. Treatment of excess amount of SnCl ₂ with 0.2 mCi ²²⁵Ac furnished highly stable ²²⁵Ac tin colloid with 90% of radiochemical yield (RCY) at optimized reaction condition. R-TLC analyses indicated 95% of radiochemical purity (RCP). Stability studies showed that colloidal structure also retained free daughter radionuclides formed by the ²²⁵Ac decay chain. ²²⁵Ac tin colloids could be ideal nanocarriers for localized cell killing due to high linear energy transfer and prevention of free radioisotope daughters.

• ²²⁵Actinium Tin Colloid
• Radiosynovectomy
• Stability
• Targeted Alpha-Particle Therapy

1. Introduction

Radiosynovectomy (RSV) is a kind of local radiotherapy for joint synovitis and synovial processes [1] [2] [3]. Considerable attention has been afforded to RSV application due to the cost effective, lack of surgial risk, low radiation dose, 70% - 80% of response rate and local treatment option without side effects [4] [5]. The main concept of RSV is that colloidal particles supported radionuclides easily undergo in the inflamed synovial membrane by intra-articular process. Recently beta particle emitters such as ⁹⁰Y [6], ¹⁸⁸Re [7] and ¹⁶⁹Er [8] have been frequently utilized for the treatment of different types of arthritis approved by European authorities [9]. The administration of those radiactive colloids depends on the size of the joints and amount of inflamation. For example, ⁹⁰Y colloid is mainly administered to larger size joint such as knee with approximately 5 - 6 mCi, whereas ¹⁸⁶Re colloid is suitable for hip, shoulder, elbow, wrist, ankle and subtalar joints with 1 - 5 mCi and ¹⁶⁹Er colloid is injected for small joints of fingers and toes with small amount of activity (0.3 - 1.0 mCi). ¹⁷⁷Lu tin colloid has also attracted interest for use in RSV due to the ideal decay characteristics and palliative treatment (T1/2 = 6.73 days, Eβ_max = 497 keV; Eγ = 113, 208 keV) [10].

Targeted alpha-particle therapy (TAT) is of great importance for cancer treatment [11] [12] [13] [14]. Alpha particles are more effective than beta particles for elimination of solid tumors due to the sufficient shorter range, greater linear energy transfer and higher cytotoxicity. Those potentials provided more advantageous about destroying of tumor cells with specific target with minimum toxicity. 15 Numerous alpha emitter radionuclides such as ²²³Ra [16] [17], ²¹¹At [18], ²¹²Pb [19], ²¹³Bi [20] and ²²⁵Ac [21] have been administered to patients for various cancer treatments with desirable positive response. Nowadays, ²²⁵Ac radionuclide has become more popular among them for alpha radiotherapy which emits four alpha particles with sufficient energies (from 5.8 to 8.4 MeV) with a long- lived high life of 9.9 days (Figure 1) [22] [23]. Even ²²⁵Ac based radiopharmaceuticals have been frequently employed in various vitro and vivo studies, its chemistry about RSV has not been worked yet.

In this study, detailed chemistry studies of ²²5Ac tin colloid have been discussed for the first time. Radiochemical yield and labeling efficiency were also examined. The vitro stability of ²²⁵Ac-Sn particles has been monitored in synovial fluid up to 5 days after preparation.

2. Materials and Methods

2.1. Materials

²²⁵Ac (0.2 mCi) was obtained from Polatom, tin(II) chloride dihydrate (purity 98%) and L-ascorbic acid (purity 99.7%) were purchased from Sigma-Aldrich. di-Sodium hydrogen phosphate dihydrate (purity 99.5%) and sodium dihydrogen phosphate dihydrate (purity 99.0%) were supplied from Isolab for preparation of 0.5 M sodium phosphate buffer (PBS) solution. Synovial fluid was obtained from Semical Biosurgery. Other chemicals and materials were obtained from Merck and Waters.

2.2. Synthesis of 225 Ac-Tin Colloid

50.0 mg of SnCl₂ and 10.0 mg of L-ascorbic acid were dissolved in 1.0 ml of 0.1 M HCI(aq) solution respectively. These solutions were transferred to reaction vial and 200 µCi of ²²⁵Ac was added to this solution. The reaction was continued at 98˚C for 100 minutes and reaction vial was removed from the heater and allowed to be cooled for 20 minutes. Then, colloidal structure was successfully obtained by adding 2 mL of PBS solution (pH: 7.8) followed by centrifugation at 3500 rpm for 10 minutes for the precipitation of particles. The supernatant was carefully decanted and residue was washed by ultrapure water with three times. Finally, the colloids were diluted with 1.0 ml of saline for both quality control and stability studies (Table 1).

Table 1

Amount of activity studies on 225 Ac-tin colloid

Entry	SnCl2 (mg)	Ascorbic Acid (mg)	Activity of free 225Ac (µCi) (before reaction)	Activity of 225Ac tin Colloid (µCi)	Activity of supernatant (free 225Ac, µCi)	Radiochemical Yield (RCY) (%)
1	5	2	200	97	85	49
2	10	4	200	116	71	58
3	30	6	200	175	22	87.5
4	50	10	200	180	17	90

2.3. Characterization Methods

TLC analyses were performed by Whatman 3 mm, ITLC-SG Agilent TLC plates and Eckert & Ziegler TLC Scan device. The amount of radioactivity of residue and supernatant were measured using a dose calibrator, Capintec Inc., New Jersey, USA.

2.3.2. Stability Experiments of 225 Ac Tin Colloid

²²⁵Ac tin colloid was dissolved in the mixture of 1 ml of synovial fluid and 1 ml of PBS and it was incubated at 37˚C at 5 days (Table 2).

Table 2

Stability studies on 225 Ac tin colloid (Entry 4, Table 1 ) (T 0 = end of the synthesis)

Time	Total Activity (µCi)	Activity of 225Ac tin Colloid (µCi)	Activity of supernatant (free 225Ac, µCi)
T0	200	180	17
1 d	162	154	8
2 d	146	136	10
5 d	114	105	8

3. Results and Discussion

Recently, nanoparticle supported radionuclides have attracted interest as promising alternatives due to the increasing the therapeutic efficacy and reducing undesired side effects with strong multivalent interactions [24] [25] [26]. More recently, Cędrowska et al. [27] has described a highly stable functionalized TiO₂- decorated ²²⁵Ac nanoparticles for TAT application. Woodward et al. [28] and Kruijff et al. [29] also emphasized that nanoparticles could retain free daughter radionuclides emerged during the ²²⁵Ac decay chain in addition to the more efficiency for radiotherapy. McLaughlin et al. [30] showed that multilayered nanoparticles (NPs) {La_0.5Gd_0.5}PO₄@GdPO₄@Au supported ²²⁵Ac provided to retain 99% of the ²²⁵Ac within three weeks. In the light of those previously published articles, we have focused on the chemistry of ²²⁵Ac-Sn particles with experimental studies.

Optimization of reaction medium and stability studies are of critical cases for exact synthesis of ²²⁵Ac tin colloid. In our experimental studies, temperature was kept constant since the reaction of ²²⁵Actinium with various precursors was well performed between 90˚C - 100˚C [15] [21] [31]. Recently, Arora et al. [32] has successfully demonstrated that optimum labeling efficiency of ¹⁷⁷Lu tin colloid was obtained within 120 - 150 minutes. Therefore, the reaction of ²²⁵Ac tin colloid was carried out in two hours without any different reaction time formulations. The effect of the amount of SnCl₂ and ascorbic acid are important for optimization protocol. 0.2 mCi ²²⁵AcCl₃ was treated with different amount of tin(II) chloride. Table 1 indicated that amount of thin(II) chloride dramatically affected RCY of ²²⁵Ac tin colloid. 0.2 mCi ²²⁵Actinium and 50.0 mg of thin(II) chloride afforded high RCY of colloidal structure (Entry 4, Table 1). We assume that 0.18 mCi could be quite high amount of activity for the possible applications of further preclinical/clinical trials. However, we wanted to work with high

amount activity to check the stability and product yield. Table 2 also summarized the stability studies of ²²⁵Ac tin colloid. Colloid structure was dissolved in synovial fluid and PBS mixture and was exposed to incubation process at 37˚C within 5 days. In the light of the experiments, it was observed that there was no significant loss of activity after five days. Even ²²⁵Actinium has more daughter radionuclides 13 as illustrated in Figure 1, stability experiments exhibited that there was no significant deviation from colloidal structure.

4. Conclusion

Synthesis procedure, stability studies of ²²⁵Ac tin colloid have been well described for the first time. The colloidal structure was obtained with high RCY with more than 95% RCP. Stability studies at 37˚C in synovial fluid were also discussed in details to demonstrate the formation of highly stable ²²⁵Ac tin colloids without any free daughter radionuclides. These promising results could lead to the preclinical/clinical trials in the future for RSV applications and four alpha particles formed by each of ²²⁵Ac decay, could lead to very effective towards treatment of arthritis.

Acknowledgements

Moltek Radiopharmaceutical Company financially contributed to this work. We greatly acknowledge Kurtulus Eryilmaz for valuable discussions concerning stability experiments.

Conflicts of Interest

The authors declare no conflicts of interest

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