Targeted alpha therapy using short-lived alpha-particles and the promise of nanobodies as targeting vehicle
UNLABELLED Bisphosphonates were synthesized for use as carriers for astatine and iodine radioisotopes to target bone neoplasms. METHODS Radiohalogenated activated esters were coupled to the amino group in the side chain of the bisphosphonate. The bisphosphonate 3-amino-1-hydroxypropylidene bisphosphonate was combined with four different acylation agents: N-succinimidyl 3-[211At]astatobenzoate, N-succinimidyl 3-[131I]iodobenzoate, N-succinimidyl-5-[211At]astato-3-pyridinecarboxylate and N-succinimidyl-5-[131I]iodo-5-pyridinecarboxylate. The products, 3-[131I]iodobenzamide-N-3-hydroxypropylidene-3,3-bisphosphonate (IBPB), 3-[211At]astato-benzamide-N-3-hydroxypropylidene-3,3-bisphosphonat e (ABPB), 5-[131I]iodopyridine-3-amide-N-3-hydroxypropylidene-3,3-bisphospho nate (IPPB) and 5-[211At]astatopyridine-3-amide-N-3-hydroxypropylidene-3,3-bisphos phonate (APPB), were injected intravenously into Balb/c mice. MIRD and Monte Carlo methods were used on the basis of cumulated activity calculated from biodistribution data to estimate dose to organs and bone segments. RESULTS All 131I- and 211At-labeled analogs were strongly incorporated into osseous tissue and retained there at stable levels, while a rapid clearance from blood was observed. The bone uptake was found to be similar for 211At- and 131I-labeled bisphosphonate when compared in paired label experiments. Bone uptake and bone-to-tissue ratios were better for IBPB compared with IPPB, and ABPB compared with APPB. All four compounds appeared to be highly resistant to in vivo dehalogenation as indicated by low uptake of 131I/211At in the thyroid gland and stomach. According to dosimetric estimates, the bone surface-to-bone marrow ratio was three times higher with 211At than with 131I. CONCLUSION Both the beta-particle- and alpha-particle-emitting compounds showed high in vivo stability and excellent affinity for osseous tissue. Further preclinical evaluation is therefore warranted.