[¹¹C]Rhodamine-123: synthesis and biodistribution in rodents

Abstract

Introduction: Rhodamine-123 is a known substrate for the efflux transporter, P-glycoprotein (P-gp). We wished to assess whether rhodamine-123 might serve as a useful substrate for developing probes for imaging efflux transporters in vivo with positron emission tomography (PET). For this purpose, we aimed to label rhodamine-123 with carbon-11 (t(1/2)=20.4min) and to study its biodistribution in rodents.

Methods: [¹¹C]Rhodamine-123 was prepared by treating rhodamine-110 (desmethyl-rhodamine-123) with [¹¹C]methyl iodide. The biodistribution of this radiotracer was studied with PET in wild-type mice and rats, in efflux transporter knockout mice, in wild-type rats pretreated with DCPQ (an inhibitor of P-gp) or with cimetidine (an inhibitor of organic cation transporters; OCT), and in P-gp knockout mice pretreated with cimetidine. Unchanged radiotracer in forebrain, plasma and peripheral tissues was also measured ex vivo at 30min after radiotracer administration to wild-type and efflux transporter knockout rodents.

Results: [(¹¹C]Rhodamine-123 was obtained in 4.4% decay-corrected radiochemical yield from cyclotron-produced [¹¹C]carbon dioxide. After intravenous administration of [¹¹C]rhodamine-123 to wild-type rodents, PET and ex vivo measurements showed radioactivity uptake was very low in brain, but relatively high in some other organs such as heart, and especially liver and kidney. Inhibition of P-gp increased uptake in brain, heart, kidney and liver, but only by up to twofold. Secretion of radioactivity from kidney was markedly reduced by OCT knockout or pretreatment with cimetidine.

Conclusions: [¹¹C]Rhodamine-123 was unpromising as a PET probe for P-gp function and appears to be a strong substrate of OCT in kidney. Cimetidine appears effective for blocking OCT in kidney in vivo.

Figure 1

Synthesis of [¹¹C]rhodamine-123.

Figure 2

pH-dependence of logD_cyclo for [¹¹C]rhodamine-123. Values are mean ± SD for n = 3.

Figure 3

PET measures of the decay-corrected time-curves for radioactivity uptake in heart and brain of rat and mouse after intravenous administration of [¹¹C]rhodamine-123. The effects of pretreatment of rats with the P-gp inhibitor, DCPQ (32 mg/kg, i.v.), are shown in the left panel.

Figure 4

PET measures of the decay-corrected time-curves of radioactivity uptake in the kidneys and liver of rat and mouse after intravenous administration of [¹¹C]rhodamine-123. The effects of pretreatment of rats with the P-gp inhibitor, DCPQ (32 mg/kg i.v.), are shown in the left panel.

Figure 5

PET measures of the decay-corrected time-courses of radioactivity uptake in kidney, heart and brain after intravenous administration of [¹¹C]rhodamine-123 to rats. The effects of pretreatment of rats with the OCT inhibitor, cimetidine (30 mg/kg, i.v.), are shown. Error bars represent 1 SD, and where they are not visible are within the symbol size.

Figure 6

PET measures of the decay-corrected time-courses of radioactivity in kidney after intravenous administration of [¹¹C]rhodamine-123 to four groups of mice: wild-type, P-gp knockout, cimetidine (30 mg/kg, i.v.) treated P-gp knockout, and Oct1/2 knockout mice. Error bars represent 1 SD.

Figure 7

Ratios of unchanged [¹¹C]rhodamine-123 in mouse tissue to that in plasma at 30 min after i.v. injection of [¹¹C]rhodamine-123 to wild-type and efflux transporter knockout mice.

Chart 1

Structures of rhodamine-123 and fluoroethylrhodamine B.