Detection of inflamed atherosclerotic lesions with diadenosine-5′,5′′′-P1,P4-tetraphosphate (AP4A) and positron-emission tomography

Diadenosine-5', 5'-P-1, P-4-tetraphosphate (Ap(4)A) and its analog P-2,P-3-monochloromethylene diadenosine-5',5'-P-1,P-4-tetraphosphate (AppCHClppA) are competitive inhibitors of adenosine diphosphate-induced platelet aggregation, which plays a central role in arterial thrombosis and plaque formation. In this study, we evaluate the imaging capabilities of positron-emission tomography (PET) with P-2,P-3-[F-18]monofluoromethylene diadenosine-5',5'-P-1,P-4-tetraphosphate ([F-18]AppCHFppA) to detect atherosclerotic lesions in male New Zealand White rabbits. Three to six months after balloon injury to the aorta, the rabbits were injected with [F-18]AppCHFppA, and microPET imaging showed rapid accumulation of this radiopharmaceutical in the atherosclerotic abdominal aorta, with lesions clearly visible 30 min after injection. Computed tomographic images were coregistered with PET images to improve delineation of aortoiliac tracer activity. Plaque macrophage density, quantified by immunostaining with RAM11 against rabbit macrophages, correlated with PET measurements of [F-18]AppCHFppA uptake (r = 0.87, P < 0.0001), whereas smooth-muscle cell density, quantified by immunostaining with 1A4 against smooth muscle actin, did not. Biodistribution studies of [F-18]AppCHFppA in normal rats indicated typical adenosine dinucleotide behavior with insignificant myocardial uptake and fast kidney clearance. The accumulation of [F-18]AppCHFppA in macrophage-rich atherosclerotic plaques can be quantified noninvasively with PET. Hence, [F-18]AppCHFppA holds promise for the noninvasive characterization of vascular inflammation.