Preclinical evaluation of MC1R targeting theranostic pair [¹⁵⁵Tb]Tb-crown-αMSH and [¹⁶¹Tb]Tb-crown-αMSH

Targeted radionuclide therapy is established as a highly effective strategy for the treatment of metastatic tumors; however, the co-development of suitable imaging companions to therapy remains significant challenge. Theranostic isotopes of terbium (¹⁴⁹Tb, ¹⁵²Tb, ¹⁵⁵Tb, ¹⁶¹Tb) have the potential to provide chemically identical radionuclidic pairs, which collectively encompass all modes of nuclear decay relevant to nuclear medicine. Herein, we report the first radiochemistry and preclinical studies involving ¹⁵⁵Tb- and ¹⁶¹Tb-labeled crown-αMSH, a small peptide-based bioconjugate suitable for targeting melanoma.

¹⁵⁵Tb was produced via proton induced spallation of Ta targets using the isotope separation and acceleration facility at TRIUMF with isotope separation on-line (ISAC/ISOL). The radiolabeling characteristics of crown-αMSH with ¹⁵⁵Tb and/or ¹⁶¹Tb were evaluated by concentration-dependence radiolabeling studies, and radio-HPLC stability studies. LogD_7.4 measurements were obtained for [¹⁶¹Tb]Tb-crown-αMSH. Competitive binding assays were undertaken to determine the inhibition constant for [^natTb]Tb-crown-αMSH in B16-F10 cells. Pre-clinical biodistribution and SPECT/CT imaging studies of ¹⁵⁵Tb and ¹⁶¹Tb labeled crown-αMSH were undertaken in male C57Bl/6 J mice bearing B16-F10 melanoma tumors to evaluate tumor specific uptake and imaging potential for each radionuclide.

Quantitative radiolabeling of crown-αMSH with [¹⁵⁵Tb]Tb³⁺ and [¹⁶¹Tb]Tb³⁺ was demonstrated under mild conditions (RT, 10 min) and low chelator concentrations; achieving high molar activities (23-29 MBq/nmol). Radio-HPLC studies showed [¹⁶¹Tb]Tb-crown-αMSH maintains excellent radiochemical purity in human serum, while gradual metabolic degradation is observed in mouse serum. Competitive binding assays showed the high affinity of [^natTb]Tb-crown-αMSH toward MC1R. Two different methods for preparation of the [¹⁵⁵Tb]Tb-crown-αMSH radiotracer were investigated and the impacts on the biodistribution profile in tumor bearing mice is compared. Preclinical in vivo studies of ¹⁵⁵Tb- and ¹⁶¹Tb- labeled crown-αMSH were performed in parallel, in mice bearing B16-F10 tumors; where the biodistribution results showed similar tumor specific uptake (6.06-7.44 %IA/g at 2 h pi) and very low uptake in nontarget organs. These results were further corroborated through a series of single-photon emission computed tomography (SPECT) studies, with [¹⁵⁵Tb]Tb-crown-αMSH and [¹⁶¹Tb]Tb-crown-αMSH showing comparable uptake profiles and excellent image contrast.

Collectively, our studies highlight the promising characteristics of [¹⁵⁵Tb]Tb-crown-αMSH and [¹⁶¹Tb]Tb-crown-αMSH as theranostic pair for nuclear imaging (¹⁵⁵Tb) and radionuclide therapy (¹⁶¹Tb).