13- and 14-membered macrocyclic ligands containing methylcarboxylate or methylphosphonate pendant arms: chemical and biological evaluation of their (153)Sm and (166)Ho complexes as potential agents for therapy or bone pain palliation

The stability constants of La(3+), Sm(3+) and Ho(3+) complexes with 13- and 14-membered macrocycles having methylcarboxylate (trita and teta) or methylphosphonate (tritp and tetp) arms were determined. All the ligands were labelled with (153)Sm and (166)Ho in order to evaluate the effect of the macrocyclic cavity size and type of appended arms on their in vitro and in vivo behaviour. The radiolabelling efficiency was found to be higher than 98% for all the complexes, except for those of tetp. All radiocomplexes studied are hydrophilic with an overall negative charge and low plasmatic protein binding. Good in vitro stability in physiological media and human serum was found for all complexes, except the (153)Sm/(166)Ho-teta, which are unstable in phosphate buffer (pH 7.4). In vitro hydroxyapatite (HA) adsorption studies indicated that (153)Sm/(166)Ho-tritp complexes bind to HA having the (166)Ho complex the highest degree of adsorption (>80%, 10 mg). Biodistribution studies in mice demonstrated that (153)Sm/(166)Ho-trita complexes have a fast tissue clearance with more than 95% of the injected activity excreted after 2 h, value that is comparable to the corresponding dota complexes. In contrast, the (153)Sm-teta complex has a significantly lower total excretion. (153)Sm/(166)Ho-tritp complexes are retained by the bone, particularly (166)Ho-tritp that has 5-6% (% I.D./g) bone uptake and also a high rate of total excretion. Thus, these studies support the potential interest of (153)Sm/(166)Ho-trita complexes for therapy when conjugated to a biomolecule and the potential usefulness of the (166)Ho-tritp complex in bone pain palliation.