In vitro and in vivo characterization of [${}^{64}$Cu][Cu(elesclomol)] as a novel theranostic agent for hypoxic solid tumors

Hypoxic tumors are associated with therapy resistance and poor cancer prognosis, but methods to detect and counter tumor hypoxia remain insufficient. Our purpose was to investigate ${}^{64}$Cu(II)-elesclomol ([${}^{64}$Cu][Cu(ES)]) as a novel theranostic agent for hypoxic tumors, by implementing an improved production method and assessing its therapeutic and diagnostic potential compared to the established Cu-64 radiopharmaceuticals [${}^{64}$Cu]CuCl${}_2$ and [diacetyl-bis(N4-methylthiosemicarbazone) [${}^{64}$Cu][Cu(ATSM)]. Cu-64 was produced using a biomedical cyclotron at 12 MeV with the reaction ${}^{64}$Ni(p,n)${}^{64}$Cu, followed by synthesis of [${}^{64}$Cu]CuCl${}_2$, [${}^{64}$Cu][Cu(ATSM)], and [${}^{64}$Cu][Cu(ES)]. In vitro therapeutic effects were assessed in both normoxic and hypoxic cells (22Rv1 and PC3 prostate cancer cells, and U-87MG glioblastoma cells) using the clonogenic assay and analyzing cellular uptake and internalization. In vivo therapeutic effects were assessed in 22Rv1 xenografts in BALB/cAnN-Foxn1nu/nu/Rj mice receiving a single or multiple doses of radiopharmaceutical, before their feasibility to detect tumor hypoxia was assessed by positron emission tomography (PET) in 22Rv1 and U-87MG xenografts. In vitro and in vivo studies demonstrated that [${}^{64}$Cu][Cu(ES)] reduced cell survival and inhibited tumor growth more effectively than [${}^{64}$Cu][Cu(ATSM)] and [${}^{64}$Cu]CuCl${}_2$. Hypoxia increased the cellular uptake and internalization of [${}^{64}$Cu][Cu(ES)] and [${}^{64}$Cu][Cu(ATSM)]. [${}^{64}$Cu][Cu(ES)]-PET tumor hypoxia detection was feasible and also revealed an unexpected finding of uptake in the brain. To the best of our knowledge, this is the first time that ES is radiolabeled with [${}^{64}$Cu]CuCl${}_2$ to [${}^{64}$Cu][Cu(ES)]. We demonstrated superior therapeutic effects of [${}^{64}$Cu][Cu(ES)] compared to [${}^{64}$Cu][Cu(ATSM)] and [${}^{64}$Cu]CuCl${}_2$ and that [${}^{64}$Cu][Cu(ES)]-PET is feasible. [${}^{64}$Cu][Cu(ES)] is a promising theranostic agent for hypoxic solid tumors.