Synthesis of 68Ga Core-doped Iron Oxide Nanoparticles for Dual Positron Emission Tomography /(T1)Magnetic Resonance Imaging

Summary

Here, we present a protocol to obtain ⁶⁸Ga core-doped iron oxide nanoparticles via fast microwave-driven synthesis. The methodology renders PET/(T₁)MRI nanoparticles with radiolabeling efficiencies higher than 90% and radiochemical purity of 99% in a 20-min synthesis.

Abstract

Here, we describe a microwave synthesis to obtain iron oxide nanoparticles core-doped with ⁶⁸Ga. Microwave technology enables fast and reproducible synthetic procedures. In this case, starting from FeCl₃ and citrate trisodium salt, iron oxide nanoparticles coated with citric acid are obtained in 10 min in the microwave. These nanoparticles present a small core size of 4.2 ± 1.1 nm and a hydrodynamic size of 7.5 ± 2.1 nm. Moreover, they have a high longitudinal relaxivity (r₁) value of 11.9 mM^-1·s^-1 and a modest transversal relaxivity value (r₂) of 22.9 mM^-1·s^-1, which results in a low r₂/r₁ ratio of 1.9. These values enable positive contrast generation in magnetic resonance imaging (MRI) instead of negative contrast, commonly used with iron oxide nanoparticles. In addition, if a ⁶⁸GaCl₃ elution from a ⁶⁸Ge/⁶⁸Ga generator is added to the starting materials, a nano-radiotracer doped with ⁶⁸Ga is obtained. The product is obtained with a high radiolabeling yield (> 90%), regardless of the initial activity used. Furthermore, a single purification step renders the nano-radiomaterial ready to be used in vivo.

Introduction

The combination of imaging techniques for medical purposes has triggered the quest for different methods to synthesize multimodal probes^1,2,3. Due to the sensitivity of positron emission tomography (PET) scanners and the spatial resolution of MRI, PET/MRI combinations seem to be one of the most attractive possibilities, providing anatomical and functional information at the same time⁴. In MRI, T₂-weighted sequences can be used, darkening the tissues in which they accumulate. T₁-weighted sequences may also be used, producing the br....

Protocol

1. Reagent Preparation

1. 0.05 M HCl
   1. Prepare 0.05 M HCl by adding 208 µL of 37% HCl to 50 mL of distilled water.
2. High-performance liquid chromatography eluent
   1. Prepare high-performance liquid chromatography (HPLC) eluent by dissolving 6.9 g of sodium dihydrogen phosphate monohydrate, 7.1 g of disodium hydrogen phosphate, 8.7 g of sodium chloride, and 0.7 g of sodium azide in 1 L of water. Mix well and check the pH. Pass the eluent through a 0.1-µm cutoff sterile filter and degas before use. Acceptance range: pH 6.2 - 7.0 (if not, adjust with NaOH [1 M] or HCl [5 M]).
   ....

Results

⁶⁸Ga-C-IONP were synthesized by combining FeCl₃, ⁶⁸GaCl₃, citric acid, water, and hydrazine hydrate. This mixture was introduced into the microwave for 10 min at 120 °C and 240 W under controlled pressure. Once the sample had cooled down to room temperature, the nanoparticles were purified by gel filtration to eliminate unreacted species (FeCl₃, citrate, hydrazine hydrate) and free ⁶⁸Ga (Figure 1

Discussion

Iron oxide nanoparticles are a well-established contrast agent for T₂-weighted MRI. However, due to the drawbacks of this type of contrast for the diagnosis of certain pathologies, T₁-weighted or bright contrast is many times preferred. The nanoparticles presented here not only overcome these limitations by offering positive contrast in MRI but also offer a signal in a functional imaging technique, such as PET, via ⁶⁸Ga incorporation in their core. Microwave technology enhances t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Iron (III) chloride hexahydrate	POCH	2317294
Citric acid, trisodium salt dihydrate 99%	Acros organics	227130010
Hydrazine hydrate	Aldrich	225819
Hydrochloric acid 37%	Fisher Scientific	10000180
Sodium dihydrogen phosphate monohydrate	Aldrich	S9638
Disodium phosphate dibasic	Aldrich	S7907
Sodium chloride	Aldrich	746398
Sodium Azide	Aldrich	S2002
Sodium dihydrogen phosphate anhydrous	POCH	799200119
68Ga Chloride	ITG Isotope Technologies Garching GmbH, Germany	68Ge/68Ga generator system
Microwave	Anton Paar	Monowave 300
Centrifuge	Hettich	Universal 320
Size Exclusion columns	GE Healthcare	PD-10