Novel approach to early detection of doxorubicin cardiotoxicity by gadolinium-enhanced cardiovascular magnetic resonance imaging in an experimental model

Abstract

Background: We sought to determine whether cardiovascular magnetic resonance measures of gadolinium (Gd) signal intensity (SI) within the left ventricular myocardium are associated with future changes in left ventricular ejection fraction (LVEF) after receipt of doxorubicin (DOX).

Methods and results: Forty Sprague-Dawley rats were divided into 3 groups scheduled to receive weekly intravenous doses of normal saline (n = 7), 1.5 mg/kg DOX (n = 19), or 2.5 mg/kg DOX (n = 14). Magnetic resonance determinations of LVEF and myocardial Gd-SI were performed before and at 2, 4, 7, and 10 weeks after DOX initiation. During treatment, animals were euthanized at different time points so that histopathologic assessments of the left ventricular myocardium could be obtained. Within-group analyses were performed to examine time-dependent relations between Gd-SI and primary events (deterioration in LVEF or an unanticipated death). Six of 19 animals receiving 1.5 mg/kg DOX and 10 of 14 animals receiving 2.5 mg/kg DOX experienced a primary event; no normal saline animals experienced a primary event. In animals with a primary event, histopathologic evidence of myocellular vacuolization occurred (P = 0.04), and the Gd-SI was elevated relative to baseline at the time of the event (P < 0.0001) and during the measurement period before the event (P = 0.0001). In all animals (including normal saline) without an event, measures of Gd-SI did not differ from baseline.

Conclusions: After DOX, low serial measures of Gd-SI predict an absence of an LVEF drop or unanticipated death. An increase in Gd-SI after DOX forecasts a subsequent drop in LVEF as well as histopathologic evidence of intracellular vacuolization consistent with DOX cardiotoxicity.

Figure 1. Left ventricular (LV) myocardial signal intensity

In the top left, a middle LV short axis image obtained 20 minutes after gadolinium contrast. As shown, the LV myocardial cavity is white and the LV myocardium is dark. On this short axis image, a region of interest was identified (top right) bounded by the LV endocardial surface (solid line) and the LV epicardial surface (dotted line). Within this region of interest the x, y, and z coordinates along with the signal intensity for all the boxes were recorded (bottom left). The number of voxels along with their intensity was plotted and the mean voxel intensity was determined (bottom right). This value was then subtracted from the background noise to obtain the mean voxel intensity.

Figure 2. Gadolinium CMR sample points and intervals

Longitudinal study design showing the multiple sample points and intervals for assessing the relationship between signal intensity and left ventricular ejection fraction measures. For animals not experiencing events, the last point in time sampled for the animal served as their final sample point (labeled D on the Figure).

Figure 3. Study Design

As shown 7, 19, and 14 animals were initiated into this study in groups receiving normal saline, 1.5 mg/kg/week of doxorubicin, and 2.5 mg/kg/week of doxorubicin, respectively. Upon animal sacrifice, 24 animals had not experienced a primary event and 16 animals experienced a primary event (13 with a drop in LVEF; 3 with sudden death)

Figure 4. Serial histograms and histopathology

On the top portion of the Figure, 4 week histograms of the number of pixels (y-axes) and intensities (x-axes) in individual animals after receipt of saline (top left), doxorubicin without (top middle) and with (top right) an ejection fraction (EF) drop. On the right are 40-power haematoxylin and eosin (H&E) histopatholigic images from the same animals. As shown, mean intensity increased in the animals that dropped their EF corresponding to vacuolization (arrows bottom right H&E).

Figure 5. Prediction of future primary events

Receiving operator characteristic curves to determine the cutoff point for: (1) the signal intensity (SI) of the point prior to a primary event (Gd-PRIOR to EVENT; dotted line); (2) the change in SI from the 2 examinations prior to the primary event (Gd-INTERMEDIATE; dashed line); and (3) the change in SI from baseline to the point prior to the primary event (Gd-BASELINE to PRIOR EVENT; dash/dot line). As shown, changes in SI early in the study predicted future events.

Figure 6. Myocardial morphology

Bar graph displaying the percentage of animals which developed myocellular necrosis, fibrosis, apoptosis, and vacuolar degeneration. Animals experiencing the primary endpoint of death (n=3) or drop in left ventricular ejection fraction (n=13) exhibited more vacuolization relative to animals that did not experience these primary endpoints.