64Cu-Labeled 2-(Diphenylphosphoryl)ethyldiphenylphosphonium Cations as Highly Selective Tumor Imaging Agents: Effects of Linkers and Chelates on Radiotracer Biodistribution Characteristics

Radiolabeled organic cations, such as triphenylphosphonium (TPP), represents a new class of radiotracers for imaging cancers and the transport function of multidrug resistance P-glycoproteins (particularly MDR1 Pgp) by single photon emission computed tomography (SPECT) or positron emission tomography (PET). This report presents the synthesis and biological evaluation of Cu-64-labeled 2-(diphenylphosphoryl)ethyldiphenylphosphonium (TPEP) cations as novel PET radiotracers; for tumor imaging. Biodistribution studies were per-formed using the athymic nude mice bearing subcutaneous U87MG human glioma xenografts to explore the impact of linkers, bifunctional chelators (BFCs), and chelates on biodistribution characteristics of the Cu-64-labeled TPEP cations. Metabolism studies were carried out using normal athymic nude mice to determine the metabolic stability of four Cu-64 radiotracers. It was found that most Cu-64 radiotracers described in this study have significant advantages over Tc-99m-Sestamibi for their high tumor/heart and tumor/muscle ratios. Both BFCs and linkers have significant impact on biological properties of Cu-64-labeled TPEP cations. For example, Cu-64(DO3A-xy-TPEP) has much lower liver uptake and better tumor/liver ratios than Cu-64(DO3A-xy-TPP), suggesting that TPEP is a better mitochondrion-targeting molecule than TPP. Replacing DO3A with DO2A results in Cu-64(DO2A-xy-TPFP)(+), which has a lower tumor uptake than Cu-64(DO3A-xy-TPEP). Substitution of DO3A with NOTA-Bn leads to a significant decrease in tumor uptake for Cu-64(NOTA-Bn-xy-TPEP). The use of DOTA-Bn to replace DO3A has little impact on the tumor uptake, but the tumor/liver ratio of Cu-64(DOTA-Bn-xy-TPEP)(-) is not as good as that of Cu-64(DO3A-xy-TPEP), probably due to the aromatic benzene ring in DOTA-Bn. Addition of an extra acetamido Cu-64(DOTA-xy-TPEP) results in a lower liver uptake, but tumor/liver ratios of Cu-64(DOTA-xy-TPEP) group in (and Cu-64(DO3A-xy-TPEP) are comparable due to a faster tumor washout of Cu-64(DOTA-xy-TPEP). Substitution of xylene with the PEG(2) linker also leads to a significant reduction in both tumor and liver uptake. MicroPET imaging studies on Cu-64(DO3A-xy-TPEP) in athymic nude mice bearing U87MG glioma xenografts showed that the tumor was clearly visualized as early as 1 h postinjection with very high T/B contrast. There was very little metabolite (<2%) detectable in the urine and feces samples for Cu-64(DO3A-xy-TPEP), Cu-64(DOTA-Bn-xy-TPEP)(-), and Cu-64(NOTA-Bn-xy-TPEP). Considering both tumor uptake and T/B ratios (particularly tumor/heart, tumor/liver, and tumor/muscle), it was concluded that Cu-64(DO3A-xy-TPEP) is a promising PET radiotracer for imaging the MDR-negative tumors.