[11C]phenytoin revisited: synthesis by [11C]CO carbonylation and first evaluation as a P-gp tracer in rats

Abstract

Background: At present, several positron emission tomography (PET) tracers are in use for imaging P-glycoprotein (P-gp) function in man. At baseline, substrate tracers such as R-[11C]verapamil display low brain concentrations with a distribution volume of around 1. [11C]phenytoin is supposed to be a weaker P-gp substrate, which may lead to higher brain concentrations at baseline. This could facilitate assessment of P-gp function when P-gp is upregulated. The purpose of this study was to synthesize [11C]phenytoin and to characterize its properties as a P-gp tracer.

Methods: [11C]CO was used to synthesize [11C]phenytoin by rhodium-mediated carbonylation. Metabolism and, using PET, brain pharmacokinetics of [11C]phenytoin were studied in rats. Effects of P-gp function on [11C]phenytoin uptake were assessed using predosing with tariquidar.

Results: [11C]phenytoin was synthesized via [11C]CO in an overall decay-corrected yield of 22 ± 4%. At 45 min after administration, 19% and 83% of radioactivity represented intact [11C]phenytoin in the plasma and brain, respectively. Compared with baseline, tariquidar predosing resulted in a 45% increase in the cerebral distribution volume of [11C]phenytoin.

Conclusions: Using [11C]CO, the radiosynthesis of [11C]phenytoin could be improved. [11C]phenytoin appeared to be a rather weak P-gp substrate.

Figure 1

Schematic overview of the synthesis unit used to synthesize [¹¹C]CO.

Scheme 1

The synthesis of azidodiphenylacetamide 3. (i) Triflyl azide, K₂CO₃, Cu(II)SO₄·5H₂O, CH₂Cl₂, CH₃OH, 16 hr, room temperature (RT), 75% yield. (ii) SOCl₂, CH₃CN, 90 min, reflux. (iii) NH₄, CH₃CN, 16 hr, RT, 76% yield.

Figure 2

Retention time of [¹¹C]CO₂on silica gel with varying mass percentages of water. The silica gel was prepared by drying for 16 h at 120°C, after which water was added.

Figure 3

Rhodium sources used: (Rh(cod)₂) and rhodium(II) acetate dimer.

Figure 4

Phosphine ligands used: triphenylphosphine, DPPE and DPPP.

Scheme 2

Radiolabeling of [¹¹C]phenytoin 4. (i) [¹¹C]CO, Rh(II) acetate dimer, DPPE, THF, 5 min, 250 bar, 120°C, 22 ± 4% overall yield.

Figure 5

Typical preparative HPLC chromatogram with UV absorption (upper panel) and radioactivity (lower panel) signals.

Figure 6

Summed (10 to 60 min) images of [¹¹C]phenytoin in the rat brain. Before (second image) and 15 min after (third image) intravenous administration of tariquidar (15 mg·kg⁻¹) in the rat. The brain is outlined in both images. Regions of interest used in the analysis are shown on an MR template image (left); the overlay image is shown on the right.

Figure 7

[¹¹C]phenytoin time-activity curves in selected rat brain regions. Baseline (open symbols) and after administration of tariquidar (closed symbols). The injected dose was 17.1 ± 2.1 MBq. Error bars represent standard deviation.