Clinically translatable quantitative molecular photoacoustic imaging with liposome-encapsulated ICG J-aggregates

Cayla A Wood,Kimberly A Homan,Yunfei Wen,Diego R T Sampaio,Sangheon Han,Stanislav Y Emelianov,Konstantin V Sokolov,Chang Soo Kim,Jody L Swain,Justin T Harris,Richard R Bouchard,Jason R Cook,Anil K Sood

doi:10.1038/s41467-021-25452-3

Abstract

Photoacoustic (PA) imaging is a functional and molecular imaging technique capable of high sensitivity and spatiotemporal resolution at depth. Widespread use of PA imaging, however, is limited by currently available contrast agents, which either lack PA-signal-generation ability for deep imaging or their absorbance spectra overlap with hemoglobin, reducing sensitivity. Here we report on a PA contrast agent based on targeted liposomes loaded with J-aggregated indocyanine green (ICG) dye (i.e., PAtrace) that we synthesized, bioconjugated, and characterized to addresses these limitations. We then validated PAtrace in phantom, in vitro, and in vivo PA imaging environments for both spectral unmixing accuracy and targeting efficacy in a folate receptor alpha-positive ovarian cancer model. These study results show that PAtrace concurrently provides significantly improved contrast-agent quantification/sensitivity and SO2 estimation accuracy compared to monomeric ICG. PAtrace’s performance attributes and composition of FDA-approved components make it a promising agent for future clinical molecular PA imaging.

Highlights

Photoacoustic (PA) imaging is a functional and molecular imaging technique capable of high sensitivity and spatiotemporal resolution at depth
Spectral characterization was performed on both intact PAtrace (i.e., J-aggregated form) and indocyanine green (ICG) released from PAtrace with UV–Vis–near-infrared (NIR) spectrophotometry; for this comparison, PAtrace was mixed with Tween 20 at 5% concentration to disrupt the liposomal coating and to dissociate ICG J-aggregates into monomeric ICG molecules
Our data show that PAtrace solution at 1 optical density (OD) has ~8.88 × 108 PAtrace nanoparticles/mL and ~1.37 × 1015 ICG molecules/mL

Summary

Introduction

Photoacoustic (PA) imaging is a functional and molecular imaging technique capable of high sensitivity and spatiotemporal resolution at depth. In a heterogeneous in vivo environment, local fluence variation introduces complex spectral coloring and affects PA-based estimates of chromophore distributions, making robust PA imaging at depths greater than 2 cm a challenge[15]. Such (unknown) fluence variations already make accurate spectral unmixing difficult[16], so the introduction of an exogenous agent with identifying spectral features that overlap with those of hemoglobin further confounds these methods. Clinically translatable PA contrast agents should provide high contrast at depth after spectral unmixing from background hemoglobin, maintain molecular specificity to cellular targets, afford sufficient stability during imaging, and have a composition amenable to translation (i.e., biocompatible and scalable for clinical production)[17,18,19]. While many of these agents have demonstrated impressive phantom and preclinical imaging results, they tend to be composed of materials that lack FDA approval, making the barrier to clinical translation relatively high

Methods

Results

Conclusion