The 18-membered macrocycle H macropa was investigated for Ac chelation in targeted alpha therapy (TAT). Radiolabeling studies showed that macropa, at submicromolar concentration, complexed all Ac (26 kBq) in 5 min at RT. [ Ac(macropa)] remained intact over 7 to 8 days when challenged with either excess La ions or human serum, and did not accumulate in any organ after 5 h in healthy mice. A bifunctional analogue, macropa-NCS, was conjugated to trastuzumab as well as to the prostate-specific membrane antigen-targeting compound RPS-070. Both constructs rapidly radiolabeled Ac in just minutes at RT, and macropa-Tmab retained>99 % of its Ac in human serum after 7 days. In LNCaP xenograft mice, Ac-macropa-RPS-070 was selectively targeted to tumors and did not release free Ac over 96 h. These findings establish macropa to be a highly promising ligand for Ac chelation that will facilitate the clinical development of Ac TAT for the treatment of soft-tissue metastases.

The synthesis of the octadentate bispidine ligand bearing two picolinic acid pendant arms (H bispa ), and its coordination chemistry with radionuclides relevant for nuclear medicine, namely indium(III) ( In), lutetium(III) ( Lu), and lanthanum(III) (as surrogate for Ac), are reported. The non-radioactive metal complexes of the N O -type bispa ligand were characterized by H and C NMR spectroscopy, elemental analysis, mass spectrometry and single-crystal X-ray analysis. Experimental structural data, computational analysis, complex stabilities determined by potentiometric titration, and "radiostabilities" determined by competition studies in the presence of human serum reveal complex stabilities of H bispa comparable to those of the macrocyclic "gold standard" DOTA. After an incubation time of 1 day, 86 and 87 % of [ Lu(bispa )] and [ Lu(DOTA)] , respectively, remain intact. Importantly, unlike DOTA, H bispa is radiolabeled quantitatively with In and Ac under ambient conditions, which is an essential aspect when working with heat-sensitive antibodies as targeting vectors. In the case of In , room temperature radiolabeling of H bispa yields molar activities as high as 70 MBq nmol within 10 minutes. These are promising results for radiopharmaceutical applications of H bispa .

Here, we present the synthesis and characterization of a new potentially nonadentate chelator H4pypa and its bifunctional analogue tBu4pypa-C7-NHS conjugated to prostate-specific membrane antigen (PSMA)-targeting peptidomimetic (Glu-urea-Lys). H4pypa is very functionally versatile and biologically stable. Compared to the conventional chelators (e.g., DOTA, DTPA), H4pypa has outstanding affinities for both 111In (EC, t 1/2 ≈ 2.8 days) and 177Lu (β–,γ, t 1/2 ≈ 6.64 days). Its radiolabeled complexes were achieved at >98% radiochemical yield, RT within 10 min, at a ligand concentration as low as 10–6 M, with excellent stability in human serum over at least 5–7 days (<1% transchelation). The thermodynamic stabilities of the [M­(pypa)]− complexes (M3+ = In3+, Lu3+, La3+) were dependent on the ionic radii, where the smaller In3+ has the highest pM value (30.5), followed by Lu3+ (22.6) and La3+ (19.9). All pM values are remarkably higher than those with DOTA, DTPA, H4octapa, H4octox, and H4neunpa. Moreover, the facile and versatile bifunctionalization enabled by the p-OH group in the central pyridyl bridge of the pypa scaffold (compound 14) allows incorporation of a variety of linkers for bioconjugation through easy nucleophilic substitution. In this work, an alkyl linker was selected to couple H4pypa to a PSMA-targeting pharmacophore, proving that the bioconjugation sacrifices neither the tumor-targeting nor the chelation properties. The biodistribution profiles of 111In- and 177Lu-labeled tracers are different, but promising, with the 177Lu analogue particularly outstanding.

Potentially nonadentate (NO) bifunctional chelator p-SCN-Bn-Hneunpa and its immunoconjugate Hneunpa-trastuzumab for In radiolabeling are synthesized. The ability of p-SCN-Bn-Hneunpa and Hneunpa-trastuzumab to quantitatively radiolabel InCl at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In, Bi, and La resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both In complexes to have high stability over 5 days. Mouse biodistribution of [In][In(p-NO-Bn-neunpa)], compared to that of [In][In(p-NH-Bn-CHX-A″-diethylenetriamine pentaacetic acid (DTPA))], at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates In-neunpa-trastuzumab andIn-CHX-A″-DTPA-trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of In-neunpa-trastuzumab compared to that ofIn-CHX-A″-DTPA-trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN-Bn-Hneunpa to be a promising chelator for In (and other radiometals) with high in vitro stability and also show Hneunpa-trastuzumab to be an excellent In chelator with promising biodistribution in mice.

Background Actinium-225 ( 225 Ac, t 1/2 = 9.9 d) is a promising candidate radionuclide for use in targeted alpha therapy (TAT), though the currently limited global supply has hindered the development of a suitable Ac-chelating ligand and 225 Ac-radiopharmaceuticals towards the clinic. We at TRIUMF have leveraged our Isotope Separation On-Line (ISOL) facility to produce 225 Ac and use the resulting radioactivity to screen a number of potential 225 Ac-radiopharmaceutical compounds. Results MBq quantities of 225 Ac and parent radium-225 ( 225 Ra, t 1/2 = 14.8 d) were produced and separated using solid phase extraction DGA resin, resulting in a radiochemically pure 225 Ac product in > 98% yield and in an amenable form for radiolabeling of ligands and bioconjugates. Of the many polydentate picolinic acid (“pa”) containing ligands evaluated (H 4 octapa [N 4 O 4 ], H 4 CHX octapa [N 4 O 4 ], p- NO 2 -Bn-H 4 neunpa [N 5 O 4 ], and H 6 phospa [N 4 O 4 ]), all out-performed the current gold standard, DOTA for 225 Ac radiolabeling ability at ambient temperature. Moreover, a melanocortin 1 receptor-targeting peptide conjugate, DOTA-modified cyclized α-melanocyte-stimulating hormone (DOTA-CycMSH), was radiolabeled with 225 Ac and proof-of-principle biodistribution studies using B16F10 tumour-bearing mice were conducted. At 2 h post-injection, tumour-to-blood ratios of 20.4 ± 3.4 and 4.8 ± 2.4 were obtained for the non-blocking (molar activity [M.A.] > 200 kBq/nmol) and blocking (M.A. = 1.6 kBq/nmol) experiment, respectively. Conclusion TRIUMF’s ISOL facility is able to provide 225 Ac suitable for preclinical screening of radiopharmaceutical compounds; [ 225 Ac(octapa)] − , [ 225 Ac( CHX octapa)] − , and [ 225 Ac(DOTA-CycMSH)] may be good candidates for further targeted alpha therapy studies. Electronic supplementary material The online version of this article (10.1186/s41181-019-0072-5) contains supplementary material, which is available to authorized users.