Introduction: The current methods for diagnosing cardiac disease fail to detect the relevant pathological events at a time when intervention is possible. These methods include the detection of blood biomarkers and imaging irreversible tissue remodeling which are irremediable aspects of injury (1,2). The goal of this study is to image the early metabolic compensation that occurs in the heart in response to a pathological stimulus. This compensation precedes injury and tissue remodeling that eventually lead to heart failure (3). A method for assessing cardiometabolic compensation is essential for early diagnosis and intervention to ameliorate the progression of incipient cardiac disease.

The overworked heart can compensate by using alternative substrates, such as lactate, which is abundant in blood during exercise (4). Under pathological circumstances, the heart can also use other substrates, such as short-chain fatty acids (SCFA), to preserve cardiac output (5-7). This is the basis for using the SCFA analog, 2-[¹⁸F]fluoropropionic acid ([¹⁸F]FPA), to image this metabolic shift by PET. Thus, we hypothesize that [¹⁸F]FPA will accumulate in metabolically compensating hearts before the inception of hypertrophy in a manner that can be imaged by PET.

This hypothesis was tested by comparing the cardiac uptake of [¹⁸F]FPA in mice subjected to isoproterenol, a β-adrenergic agonist that increases cardiac workload, and control animals. In the experiments, [¹⁸F]FPA serves as a reporter of the change in cardiac fatty acid metabolism (i.e., reprogramming) and chronic isoproterenol treatment as a means of inducing hypertrophy. The aims of this study design are to document metabolic compensation in the overworked heart by PET imaging, optimize the administration and cardiac imaging protocols for [¹⁸F]FPA, and elucidate divergent mechanisms for how metabolism is affected in both applications.

Methods: Isoproterenol (10 mg/kg/day) or saline was administered subcutaneously for 14 days to male C57BL/6J mice. The isoproterenol-induced heart failure model has been studied extensively and is known to initiate cardiac hypertrophy within 14 days of repeated daily injections (8). A separate group of mice received a single injection of isoproterenol for 1 h before undergoing PET imaging. The mice were injected intravenously with a solution of  racemic [¹⁸F]FPA (9.25-11.1 MBq) in saline containing 5 mg/kg of monocarboxylate transporter 1 inhibitor AZD3965. Thirty-minute static PET acquisitions were then performed 20-30 min post-injection (p.i.) of [¹⁸F]FPA. Mice were euthanized after imaging and their hearts excised. Hypertrophy was determined by heart weights and heart weight to tibia length ratios (HW/TL). Cardiac [¹⁸F]FPA uptake was assessed by tissue gamma counts (%ID/g).

Results: A dose of 5 mg/kg AZD3965 improved cardiac image contrast when co-injected with [¹⁸F]FPA and was therefore applied in subsequent experiments. Isoproterenol induced significantly higher cardiac [¹⁸F]FPA uptake at 1 h p.i. (2.31 vs 8.04 %ID/g, p < .05), which indicates that the heart undergoes rapid metabolic compensation to support increased contractility. After 3 consecutive days of isoproterenol injections, cardiac [¹⁸F]FPA uptake was significantly elevated (~1.5-fold of controls, p < 0.05) despite the absence of hypertrophy. By contrast, cardiac [¹⁸F]FPA uptake significantly decreased after 15 days of successive isoproterenol injections (~0.5-fold of controls, p < 0.05, n = 4), at which point the mice experienced cardiac hypertrophy.

Conclusions: [¹⁸F]FPA reports on the metabolic reprogramming due to isoproterenol within an hour of the drug’s administration and prior to the onset of cardiac hypertrophy due to chronic isoproterenol treatment. Therefore, [¹⁸F]FPA could be a candidate PET agent for diagnosing the early metabolic compensation leading to cardiac disease.

Acknowledgements: This work was supported by awards to J.M. Kelly (R21CA246409) and J.A. Azcona (F32HL168948) from the National Cancer Institute and the National Heart, Lung, Blood Institute of the National Institutes of Health, respectively.

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