Towards a nanoscale mammographic contrast agent: development of a modular pre-clinical dual optical/x-ray agent

Abstract

Contrast-enhanced digital mammography (CEDM) can provide improved breast cancer detection and characterization compared to conventional mammography by imaging the effects of tumour angiogenesis. Current small-molecule contrast agents used for CEDM are limited by a short plasma half-life and rapid extravasation into tissue interstitial space. To address these limitations, nanoscale agents that can remain intravascular except at sites of tumour angiogenesis can be used. For CEDM, this agent must be both biocompatible and strongly attenuate mammographic energy x-rays. Nanoscale perfluorooctylbromide (PFOB) droplets have good x-ray attenuation and have been used in patients for other applications. However, the macroscopic scale of x-ray imaging (50-100 µm) is inadequate for direct verification that PFOB droplets localize at sites of breast tumour angiogenesis. For efficient pre-clinical optimization for CEDM, we integrated an optical marker into PFOB droplets for microscopic assessment (≪50 µm). To develop PFOB droplets as a new nanoscale mammographic contrast agent, PFOB droplets were labelled with fluorescent quantum dots (QDs). The droplets had mean diameters of 160 nm, fluoresced at 635 nm and attenuated x-ray spectra at 30.5 keV mean energy with a relative attenuation of 5.6 ± 0.3 Hounsfield units (HU) mg(-1) mL(-1) QD-PFOB. With the agent loaded into tissue phantoms, good correlation between x-ray attenuation and optical fluorescence was found (R(2) = 0.96), confirming co-localization of the QDs with PFOB for quantitative assessment using x-ray or optical methods. Furthermore, the QDs can be removed from the PFOB agent without affecting its x-ray attenuation or structural properties for expedited translation of optimized PFOB droplet formulations into patients.

Methods of Generating Submicrometer Phase-Shift Perfluorocarbon Droplets for Applications in Medical Ultrasonography

“…Probe or tip sonication is a general homogenization technique and a common and simple method to produce different types of nanoscale PFC droplets [19], [46], [47]. In this method, a sonication tip is placed directly into a vial containing the components of the droplets (PFC and encapsulation material) and the continuous aqueous phase (typically water or saline).…”

“…Furthermore, since the system is closed with less intrinsic loss in the components of the emulsion during synthesis, relative droplet concentrations can be better quantified compared with other methods that are based on flow techniques where material can be lost during processing. It is also simple to incorporate other agents (e.g., solid nanoparticles or liquid agents [46], [47]) into PFC droplets using tip sonication. This is more difficult using other synthesis methods as even very small aggregates can block the membrane filters used in extrusion or the channels/orifice in microfluidics.…”

“…A sample protocol based on the literature is given here [46], [47]. Like all methods, it is easiest to start with a simple formulation [e.g., containing only a single fluorosurfactant as the emulsifier and perfluorocarbon with high thermal stability (e.g., PFH)] to form stable and consistent PFC nanodroplets as a control for new method development.…”

Perfluorocarbon nanodroplets can reoxygenate hypoxic tumors in vivo without carbogen breathing

“…For example, various PFC nanodroplets have been used to label cells for tracking by 19 F-MRI 10,11, and droplets with metastable PFC cores have been formulated as droplet-to-bubble phase-shift agents for photoacoustic and ultrasound imaging 8,12-14 or for image-guided drug delivery 15,16. Heteroatoms (e.g., bromide) present in PFC molecules offers X-ray attenuation for contrast-enhanced CT imaging 17,18. For therapy, dispersed PFC nanodroplets have been used in combination with MRI-guided focused ultrasound and laser light for targeted drug delivery 4,5,19,20, vascular disruption 21,22 and tumor ablation 10,23-26.…”

Perfluorooctylbromide nanoparticles for ultrasound imaging and drug delivery

“…PFOB emulsions used in the clinic are marketed as LiquiVent (Alliance Pharmaceutical Corporation, San Diego, CA, USA), 10 an oxygen-carrying liquid drug, and Oxygent™ (Alliance Pharmaceutical Corporation, San Diego, CA, USA), 11 a blood substitution agent. In addition, the use of PFOB NPs in various medical applications, including 19 F or 1 H magnetic resonance imaging (MRI) tracers, [12][13][14][15][16][17][18][19] cell-tracking agents, [20][21][22] X-ray contrast agents 21,23 and computed tomography (CT) tracers, has been explored. 24 US imaging is a clinical diagnostic technique that is frequently used because of its advantages, namely its real-time monitoring capability, low cost, high safety, convenience and portability.…”