11C-Labeled rhodamine-123

Excerpt

The P-glycoprotein (P-gp; also known as MDR1 or ABCB1) is a member of the ATP-binding cassette transporter family of proteins (the multi-drug resistance (MDR) protein and the breast cancer resistance protein (BCRP) are the other two members of this group of proteins) that is responsible for the rapid transportation of drugs across the cell membrane (uptake and efflux) (1). The structure, functions, and activities of P-gp have been discussed in detail by Giacomini et al. (2) and Sharom (3). Overexpression of these transporters, particularly P-gp, affects the distribution of drugs in various parts of the body, including the central nervous system (CNS), and is responsible for the development of drug resistance in cancer cells (4). In addition, reduced function and expression of P-gp have been suggested to cause slow or reduced clearance of neurotoxic peptides such as the amyloid-β peptide from the neuronal cells, and this has been hypothesized to contribute to the development of Alzheimer’s disease, Parkinson’s disease, or other such neurological conditions (5). There is much interest in studying the role of P-gp at the blood–brain barrier (BBB) with positron emission tomography (PET), using P-gp substrates such as [¹¹C]verapamil and [¹¹C]desmethyl-loperamide (6). These tracers can measure the decrease in transport function of the P-gp because this decrease results in the accumulation of the label in the brain; however, the uptake of these radiolabeled compounds in normal brain cells as such is very low, and an increase in P-gp function cannot be determined in the tissue with these labeled compounds. Therefore, it was hypothesized that radiolabeled substrates that have a low to moderate affinity for P-gp will probably accumulate in the brain and likely can be used to determine the expression level of this transporter in the tissue or organ (6).

Rhodamine-123 is a fluorescent dye that is used to measure the expression of P-gp in drug-resistant cells, and the National Cancer Institute of the United States uses it to screen for drugs that serve as substrates for MDR1 (6). This fluorescent dye has also been used to investigate the P-gp function in the blood–brain barrier of wild-type (WT) and knockout (KO) mice (mdr 1 a(-/-); these animals lack the P-gp transporter), and the KO rodents were shown to accumulate up to a 4-fold higher amount of rhodamine-123 in the brain compared with the WT animals (6). Similar results were obtained with the WT rodents when they were intravenously infused with rhodamine-123 in the presence of cyclosporine A, an inhibitor of P-gp (6). From these studies, it was concluded that rhodamine-123 was an in vivo substrate for P-gp in mice. The biodistribution of ¹³¹I-labeled rhodamine-123 has been studied in mice (7). On the basis of information reported in the studies mentioned above, rhodamine-123 was labeled with ¹¹C ([¹¹C]rhodamine-123; half-life of ¹¹C is 20.4 min) and used to study the efflux transporters in WT and various efflux transporter KO mice (organic cation transporter (OCT) 1 and 2 (OCT1/2) KO mice, BCRP KO mice, and MDR-associated protein 1 (MRP1) KO mice; MRP1 is responsible for the cellular efflux of various anti-cancer drugs and contributes to the development of drug resistance) (6). In addition, the biodistribution of [¹¹C]rhodamine-123 was investigated with PET in rats and mice (6).