Radiolabeling of poly(lactic-co-glycolic acid) (PLGA) nanoparticles with biotinylated F-18 prosthetic groups and imaging of their delivery to the brain with positron emission tomography

Abstract

The avidin-biotin interaction permits rapid and nearly irreversible noncovalent linkage between biotinylated molecules and avidin-modified substrates. We designed a biotinylated radioligand intended for use in the detection of avidin-modified polymer nanoparticles in tissue with positron emission tomography (PET). Using an F-18 labeled prosthetic group, [(18)F]4-fluorobenzylamine, and a commercially available biotin derivate, NHS-PEG4-biotin, [(18)F]-fluorobenzylamide-poly(ethylene glycol)4-biotin ([(18)F]NPB4) was prepared with high purity and specific activity. The attachment of the [(18)F]NPB4 radioligand to avidin-modified poly(lactic-co-glycolic acid) (PLGA) nanoparticles was tested by using PET imaging to measure the kinetics of convection-enhanced delivery (CED) of nanoparticles of varying size to the rat brain. PET imaging enabled the direct observation of nanoparticle delivery by measurement of the spatial volume of distribution of radiolabeled nanoparticles as a function of time, both during and after the infusion. This work thus validates new methods for radiolabeling PEG-biotin derivatives and also provides insight into the fate of nanoparticles that have been infused directly into the brain.

Figure 1

Synthesis of NPB4 (A) and [¹⁸F]NPB4 (B).

Figure 2

Analytical HPLC profile for [¹⁸F]NPB4 solution. In (A), radiochemical purity of [¹⁸F]NPB4 was 98.5 ± 1.2% (n = 10). In (B), the identity of [¹⁸F]NPB4 was confirmed by its coelution with NPB4. Conditions: Phenomenex Luna C18 column (5 μm, 4.6 × 250 mm) eluted with 25:75 (v/v) acetonitrile/0.1 M ammonium formate with 0.5% acetic acid at a flow rate of 2 mL/min, wavelength set at 264 nm. Gamma (red) and UV (black) traces are shown. X-axis is time (min).

Figure 3

Analytical HPLC profile for the [¹⁸F]NPB4-avidin complex. Near complete conjugation of [¹⁸F]NPB4 to avidin was observed. Conditions: Superose 12, 10/300 GL column (10 ± 2 μm) eluted with 10% MeCN and 90% 0.1 M tris-HCl with 1 M NaCl at a flow rate of 0.8 mL/min, wavelength set at 254 nm. Gamma (red) and UV (black) traces are shown.

Figure 4

PLGA nanoparticles were prepared for delivery to the rat striatum. SEM images of small and large nanoparticle fractions are shown in (A) and (B), respectively. (C) A guide cannula was implanted to enable insertion of an internal injector for imaging while the infusion was ongoing. The injection location for each rat was verified by comparing the location of the surface of the skull (identified in the transmission image) with the center of the infusion (identified in the first several frames of the emission image). Scale bars: 500 nm (A,B), 5 mm (C).

Figure 5

Spatial distribution of nanoparticles delivered to the brain via CED was measured with PET. An infusion volume of 20 μL was delivered over 30 min to the striatum. In (A), the spatial volume of distribution (V_d) of labeled nanoparticles is shown for a single subject that received small nanoparticles. Coronal images are shown in the direction parallel to the infusion track, and the scale bar is 5 mm. In (B), the ratio of V_d to the infusion volume (V_i) is plotted for 5 representative infusions.

Figure 6

Kinetics of nanoparticle delivery and subsequent clearance were measured by PET. In (A), the total fraction of nanoparticles delivered to the brain was measured over 30 min. Small nanoparticles were delivered steadily, with total activity that increased smoothly and reproducibly over time. However, large nanoparticles were not delivered smoothly. Noncontinuous jumps in the total delivered fraction were observed at various points during the infusion. Once the infusion was complete, total radioactivity decreased. Representative data for a small nanoparticle infusion are shown in (B). Clearance half-life of the radioactive signal was calculated by fitting these data to a first order elimination model (111 min in this example).