Non-invasive measures of atherosclerotic plaque composition in multiple vascular beds will dramatically improve the ability to identify patients at risk of life-threatening thromboembolic events and will aid in directing appropriate, patient-specific therapies. Gd-based MR contrast agents have been shown to be previously useful for aiding in determination of plaque composition 14–17
. However, this typically is limited to one vascular bed (or even one MR slice) due to the rapid wash-in/out plaque kinetics of currently approved agents. In this study we sought to determine the ability of GdF, an agent with a long plaque half-life, to report on plaque composition at multiple time points in a rabbit model of atherosclerosis. We present evidence showing that GdF administration improved the conspicuity of diseased vessel wall to muscle 24 hours following administration and that GdF accumulated more in diseased wall than normal wall. Importantly, we found that total GdF accumulation within diseased wall as measured by MRI (ΔSItotal-wall
) at both 1 and 24 hours positively correlated with a histological index of morphological features implicated in plaque instability, namely the plaque’s fibrocellular tissue composition and extent of intraplaque neovascularization. In contrast, accumulation within the same plaques during the peak enhancement phase using the conventional agent Gd-DTPA did not correlate with this measure. This suggests an advantage for using GdF over currently approved agents for quantifying these two plaque features as this agent can be used within a short time period after administration (currently an hour) but also widens the imaging window (up to 24 hours) potentially allowing multiple vascular beds to be examined.
The two primary features of advanced atherosclerotic plaques are the formation of a lipid-rich necrotic core with an overlying fibrous cap 5
. The composition of the cap and core is intrinsically linked to plaque stability. This includes the cap’s fibrocellular tissue composition and the degree of neovascularization in the core 6, 11–13, 15, 28
. Strong evidence suggests that plaques with a large number of macrophages at the surface and significant neovascularization are unstable and susceptible to rupture 6, 8, 11–13, 28
. Hence, an imaging agent that accumulates according to the plaque’s fibrocellular tissue composition and degree of neovascularization, such as GdF, would allow the staging of advanced plaque development and help identify plaques that are destabilizing at an accelerated rate.
As mentioned, Gd-based extracellular (EC) contrast agents have shown promise for identifying plaques with altered fibrocellular density and plaque neovascularization 14–16, 29
. While these are significant advances in in vivo
imaging of plaque composition, one potential drawback of using conventional agents is that the imaging window after injection of the agent is short (typically first 20 minutes). This will limit the use of these agents for imaging multiple plaques in a single vascular bed or multiple vascular beds in a single patient (or animal). Since atherosclerosis is considered a systemic disease affecting multiple vascular beds the use of an imaging agent capable of extending this imaging window would be of great benefit for clinical assessment of plaque vulnerability on a whole-body basis. GdF may be such an agent since it is retained in plaques for significant periods of time and its diffusion within the plaque is directly related to the plaque’s composition. Another added benefit of using GdF is its higher r1
relaxivity compared to conventional EC agents (17.4 mM−1
vs ~4 mM−1
at 37°C and 1.5T, respectively) 19, 25
and therefore should improve detection of these plaque features at similar doses, or may alternatively be used at lower doses.
Several groups have theorized about how GdF accumulates within plaques. One study showed that the amount of lipid within plaques correlated with the degree of enhancement of the plaques, and it was theorized that GdF binds to lipidic components of plaques via interactions with its hydrophobic tail 19
. This theory was tested in another study and it was shown that GdF enters plaques bound to albumin (hydrophobic interaction) and once in plaques it has a high affinity for other hydrophobic partners such as tenascin, proteoglycans and collagen found in more fibrous, not lipidic, regions of plaques 21
. These results suggest that GdF should accumulate in highly fibrotic regions of plaques. Clearly a discrepancy exists between the MR imaging results of the first study and the biochemical analysis of GdF affinity in the second study. Our study provides new evidence that helps to resolve this discrepancy. We have shown that while GdF does preferentially bind to collagenous (fibrous) material within plaques, consistent with the Meding study 21
, it also appears to accumulate more and penetrate deeper into plaques that contain a large number of lipid-rich macrophages (foam cells), consistent with the Sirol study 19
Two results in this study were in striking contrast to previously published results using GdF in atherosclerosis models. First, we found evidence of macrophage uptake of GdF, whereas cc-GdF was previously found to paralocalize to macrophages within plaques 21
. We believe this result is not unexpected for 3 reasons: 1) monocytes spontaneously phagocytose GdF in vitro 30
; 2) GdF enhancement of atherosclerotic wall can last for up to 2 months after a single injection, which may be due to GdF being taken up by phagocytes 20
; and 3) recent evidence has shown GdF colocalization with macrophages in optical nerves in rats with experimental autoimmune encephalomyelitis (EAE) rats 31
. Our second contrasting result to previous studies18, 19
is the significant enhancement of normal vessel wall using GdF. However, we are confident with our result since we also confirmed the presence of GdF within normal wall using both epifluorescent and confocal (not shown) microscopy. We believe that the discrepancy between our finding and previous literature is partially due to the fact that these previous studies used significantly lower resolution MR imaging protocols than ours, leading to partial voluming effects that likely limited their ability to detect the substantial accumulation of GdF in the very thin normal wall.
In summary we provide novel evidence for the use of the MR contrast agent GdF for non-invasive staging of plaque stability in a rabbit model of atherosclerosis. This agent allowed the combined effects of multiple plaque features related to plaque stability to be assessed simultaneously and is capable of doing so at multiple time points following administration, lessening the imaging timing restrictions typically required with currently approved Gd-based agents. We believe this agent could be useful for testing of the efficacy of current (eg. statins) or novel therapeutics (eg. anti-angiogenic or anti-inflammatory) aimed at affecting plaque stability in individual animals (or patients) over time. Finally, this agent would also allow plaques responsible for thromboembolic events, so-called ruptured or “culprit” plaques, to be localized non-invasively prior to these life-threatening events and guide clinicians to make appropriate medical or surgical decisions that could save lives.