Optical coherence tomography to detect macrophages in atheroma in vivo
Introduction. Macrophages play a key role in atherosclerotic plaque instability, they engender plaque rupture, leading to thrombosis and myocardial infarction. Their detection in coronary artery plaques would enable stratification of patients for better treatment of “vulnerable” phenotypes. Intra-coronary Optical Coherence Tomography (OCT) enables visualisation of vulnerable plaque morphology and has the potential to detect the signal variance created by increased numbers of macrophages. Aims. Our aim was to enhance the detection of macrophages using OCT in combination with intracellular contrast agents; and to demonstrate it in-vitro, ex-vivo and in-vivo. This was first endeavoured using clinically approved ultra-small superparamagnetic particles of iron oxide (USPIOs): an MRI contrast agent which is readily uptaken by macrophages in coronary plaques. The second approach was to design a novel OCT contrast agent based on different lanthanide ions with either a carbonate or fluoride structure. Methods. A “phantom” artery model was designed and 3D printed for standardised in-vitro imaging with intra-coronary OCT. Particle-laden macrophages were tested in this system to determine OCT contrast enhancement. Cell viability assays were performed to determine toxicity of novel lanthanide particles. Aortic plaques of Apo-lipoprotein E knockout mice, treated with USPIO or saline were OCT imaged and sectioned for histological analysis. Patients presenting with acute coronary artery disease were randomised to receive USPIO or saline treatment, followed by OCT imaging of the culprit plaque and non-culprit control. The local pixel Standard Deviation (SD) of OCT images were processed in Matlab as a metric of contrast enhancement. A local SD hotspot visualisation tool was developed in Matlab for use with the phantom, murine and human OCT images. Results. In all 7 phantoms, USPIO-laden macrophages significantly increased the mean SD compared to untreated macrophages (p<0.001). USPIO treated mouse plaques (n=3) showed significantly higher mean SD than control plaques (n=3) (p<0.05). USPIO treated patients (n=8) demonstrated a significantly increased mean SD compared to control patients (n=10) in culprit and non-culprit plaques (p<0.05). Within patients the mean SD was significantly higher for culprit compared to non-culprit plaques in both USPIO and control groups. The initial OCT screening of 10 novel lanthanide particles revealed the carbonate structures to be most efficacious for contrast enhancement compared to the fluoride ones. Phantoms made with particle-laden macrophages revealed the Gadolinium carbonate particle with Polyacrylic acid coating to have the highest mean SD increase compared to untreated macrophages. This particle also showed low levels of cell toxicity. Conclusions. USPIO administration was associated with a significant increase in OCT contrast in a novel in-vitro phantom model, an ex-vivo atherosclerotic mouse study and in a human clinical trial. USPIO enhances the detection of macrophages and may facilitate patient stratification. Additionally, novel OCT contrast agents have been made which show great potential for further enhancing macrophage detection in this same context.