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

Abstract

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 (64)Cu-labeled 2-(diphenylphosphoryl)ethyldiphenylphosphonium (TPEP) cations as novel PET radiotracers for tumor imaging. Biodistribution studies were performed 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 (64)Cu-labeled TPEP cations. Metabolism studies were carried out using normal athymic nude mice to determine the metabolic stability of four (64)Cu radiotracers. It was found that most (64)Cu radiotracers described in this study have significant advantages over (99m)Tc-Sestamibi for their high tumor/heart and tumor/muscle ratios. Both BFCs and linkers have significant impact on biological properties of (64)Cu-labeled TPEP cations. For example, (64)Cu(DO3A-xy-TPEP) has much lower liver uptake and better tumor/liver ratios than (64)Cu(DO3A-xy-TPP), suggesting that TPEP is a better mitochondrion-targeting molecule than TPP. Replacing DO3A with DO2A results in (64)Cu(DO2A-xy-TPEP) (+), which has a lower tumor uptake than (64)Cu(DO3A-xy-TPEP). Substitution of DO3A with NOTA-Bn leads to a significant decrease in tumor uptake for (64)Cu(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 (64)Cu(DOTA-Bn-xy-TPEP) (-) is not as good as that of (64)Cu(DO3A-xy-TPEP), probably due to the aromatic benzene ring in DOTA-Bn. Addition of an extra acetamido group in (64)Cu(DOTA-xy-TPEP) results in a lower liver uptake, but tumor/liver ratios of (64)Cu(DOTA-xy-TPEP) and (64)Cu(DO3A-xy-TPEP) are comparable due to a faster tumor washout of (64)Cu(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 (64)Cu(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 (64)Cu(DO3A-xy-TPEP), (64)Cu(DOTA-Bn-xy-TPEP)(-), and (64)Cu(NOTA-Bn-xy-TPEP). Considering both tumor uptake and T/B ratios (particularly tumor/heart, tumor/liver, and tumor/muscle), it was concluded that (64)Cu(DO3A-xy-TPEP) is a promising PET radiotracer for imaging the MDR-negative tumors.