A Third Generation Potentially Bifunctional Trithiol Chelate, Its nat,1XXSb(III) Complex, and Selective Chelation of Radioantimony (119Sb) from Its Sn Target

Exploring a tristhione scorpionate ligand as a suitable chelator for the theranostic pair antimony-119 and antimony-117

BACKGROUND

The highly potent Auger electron emitter antimony-119 (119Sb) and the SPECT-isotope antimony-117 (117Sb) comprise a true theranostic pair particularly suitable for cancer theranostics. Harnessing this potential requires development of a chelator that can rapidly form a stable complex with radioactive antimony ions at the low concentrations typical of radiopharmaceutical preparations. Stable Sb(III) complexes of hydrotris(methimazolyl)borate (TMe) are known, prompting our investigation of this chelator. Additionally, the production of radioantimony was optimized and the SPECT imaging properties of 117Sb was investigated, in an attempt to move towards biomedical implementation of the theranostic isotope pair of antimony.

RESULTS

A method for rapid and effective labelling of TMe using 117Sb was developed, yielding high radiochemical purities of 98.5 ± 2.7% and high radionuclidic purities exceeding 99%. Radiolabelling yielded an Sb(III) complex directly from the acidic Sb(V) solution. [1XXSb]Sb-TMe in aqueous acidic solution showed high stability in the presence of cysteine, however, the stability of the radiocomplex at increased pH was significantly decreased. The production method of 117Sb was optimized, enabling a production yield of up to 19.6 MBq/µAh and the production of up to 564 MBq at end of bombardment, following irradiation of a thin 117Sn-enriched solid target. Preclinical SPECT/CT scanning of a mouse phantom containing purified 117Sb demonstrated excellent SPECT imaging properties of 117Sb with high spatial resolution comparable to that of technetium-99m.

CONCLUSION

We have explored the TMe chelator for complexation of radioantimony and devised a rapid chelation protocol suitable for the short half-life of 117Sb (T1/2 = 2.8 h). [1XXSb]Sb-TMe (1XXSb = 117Sb, 118mSb, 120mSb and 122Sb) demonstrated a high stability in presence of cysteine, although low stability was observed at pH > 4. We have achieved a production yield of 117Sb high enough for clinical applications and demonstrated the excellent SPECT-imaging properties of 117Sb. The results contribute valuable information for the development of suitable chelators for radioantimony and is a step further towards implementation of the antimony theranostic pair in biomedical applications.

Sustainable production of radionuclidically pure antimony-119

BACKGROUND

Radiopharmaceutical therapy (RPT) uses radionuclides that decay via one of three therapeutically relevant decay modes (alpha, beta, and internal conversion (IC) / Auger electron (AE) emission) to deliver short range, highly damaging radiation inside of diseased cells, maintaining localized dose distribution and sparing healthy cells. Antimony-119 (119Sb, t1/2 = 38.19 h, EC = 100%) is one such IC/AE emitting radionuclide, previously limited to in silico computational investigation due to barriers in production, chemical separation, and chelation. A theranostic (therapeutic/diagnostic) pair can be formed with 119Sb's radioisotopic imaging analogue 117Sb (t1/2 = 2.80 h, Eγ = 158.6 keV, Iγ = 85.9%, β+ = 262.4 keV, Iβ+ = 1.81%).

RESULTS

Within, we report techniques for sustainable and cost-effective production of pre-clinical quality and quantity, radionuclidically pure 119Sb and 117Sb, novel low energy photon measurement techniques for 119Sb activity determination, and physical yields for various tin target isotopic enrichments and thicknesses using (p, n) and (d, n) nuclear reactions. Additionally, we present a two-column separation providing a radioantimony yield of 73.1% ± 6.9% (N = 3) and tin separation factor of (6.8 ± 5.5) x 105 (N = 3). Apparent molar activity measurements for deuteron produced 117Sb using the chelator TREN-CAM were measured at 42.4 ± 25 MBq 117Sb/µmol (1.14 ± 0.68 mCi/µmol), and we recovered enriched 119Sn target material at a recycling efficiency of 80.2% ± 5.5% (N = 6) with losses of 11.6 mg ± 0.8 mg (N = 6) per production.

CONCLUSION

We report significant steps in overcoming barriers in 119Sb production, chemical isolation and purification, enriched target material recycling, and chelation, helping promote accessibility and application of this promising therapeutic radionuclide. We describe a method for 119Sb activity measurement using its low energy gamma (23.87 keV), negating the need for attenuation correction. Finally, we report the largest yet-measured 119Sb production yields using proton and deuteron irradiation of natural and enriched targets and radioisotopic purity > 99.8% at end of purification.

Chemistry of Antimony in Radiopharmaceutical Development: Unlocking the Theranostic Potential of Sb Isotopes

Antimony-119 (119Sb) holds promise for radiopharmaceutical therapy (RPT), emitting short-range Auger and conversion electrons that can deliver cytotoxic radiation on a cellular level. While it has high promise theoretically, experimental validation is necessary for 119Sb in vivo applications. Current 119Sb production and separation methods face robustness and compatibility challenges in radiopharmaceutical synthesis. Limited progress in chelator development hampers targeted experiments with 119Sb. This review compiles literature on the toxicological, biodistribution and redox properties of Sb, along with existing Sb complexes, evaluating their suitability for radiopharmaceuticals. Sb(III) is suggested as the preferred oxidation state for radiopharmaceutical elaboration due to its stability in vivo and lack of skeletal uptake. While Sb complexes with both hard and soft donor atoms can be achieved, Sb thiol complexes offer enhanced stability and compatibility with the desired Sb(III) oxidation state. For 119Sb to find application in RPT, scientists need to make discoveries and advancements in the areas of isotope production, and radiometal chelation. This review aims to guide future research towards harnessing the therapeutic potential of 119Sb in RPT.

Effects of clinically relevant radionuclides on the activation of a type I interferon response by radiopharmaceuticals in syngeneic murine tumor models

Radiopharmaceutical therapies (RPT) activate a type I interferon (IFN1) response in tumor cells. We hypothesized that the timing and amplitude of this response varies by isotope. We compared equal doses delivered by 90 Y, 177 Lu, and 225 Ac in vitro as unbound radionuclides and in vivo when chelated to NM600, a tumor-selective alkylphosphocholine. Response in murine MOC2 head and neck carcinoma and B78 melanoma was evaluated by qPCR and flow cytometry. Therapeutic response to 225 Ac-NM600+anti-CTLA4+anti-PD-L1 immune checkpoint inhibition (ICI) was evaluated in wild-type and stimulator of interferon genes knockout (STING KO) B78. The timing and magnitude of IFN1 response correlated with radionuclide half-life and linear energy transfer. CD8 + /Treg ratios increased in tumors 7 days after 90 Y- and 177 Lu-NM600 and day 21 after 225 Ac-NM600. 225 Ac-NM600+ICI improved survival in mice with WT but not with STING KO tumors, relative to monotherapies. Immunomodulatory effects of RPT vary with radioisotope and promote STING-dependent enhanced response to ICIs in murine models.

Teaser

This study describes the time course and nature of tumor immunomodulation by radiopharmaceuticals with differing physical properties.