Nitric oxide (NO) is an important and unique mediator of a variety of physiological and pathological processes. NO is generated from the oxidation of L-arginine to L-citrulline in a two-step process by nitric oxide synthase (NOS) enzymes. In the NOS family, there are two constitutive isozymes of NOS, neuronal NOS (nNOS) and endothelial NOS (eNOS), and one inducible isozyme (iNOS). The three isozymes of NOS are expressed in different tissues to generate NO for specific physiological roles. nNOS generates NO as a neurotransmitter and neuromodulator, mainly in brain and peripheral nerve cells; eNOS regulates blood pressure, and blood flow primarily in vascular endothelial cells. The induction of iNOS occurs by various inflammatory stimuli (e.g., endotoxin) in activated macrophages and other types of cells and plays a crucial role in the host defense and the inflammatory processes.
Normally, the basal level of NO in all parts of the body is very low, mainly due to the constitutive nNOS and eNOS. In contrast, once expressed, iNOS can continue to generate NO in large amounts (up to μM concentrations) for a prolonged period of time. Studies have shown that production of NO by iNOS is implicated in a variety of acute and chronic inflammatory diseases (e.g., sepsis, septic shock, organ transplant rejection, vascular dysfunction in diabetes, asthma, arthritis, multiple sclerosis, and inflammatory diseases of the gut). iNOS activity has also been found in many tumors. Because of the central role of iNOS in NO-related diseases, numerous efforts have been made to develop iNOS inhibitors as pharmaceuticals ranging from the nonselective L-arginine analogues to the selective inhibitors reported recently. Some inhibitors of iNOS have shown promising results in animal models of sepsis, lung inflammation, arthritis, and autoimmune diabetes. Therefore, the development of a radiolabeled iNOS inhibitor for probing iNOS expression in vivo using noninvasive positron emission tomography (PET) imaging will be of tremendous value to the study and treatment of NO-related diseases.
Acute allograft rejection is the major contributor to mortality in patients receiving orthotopic heart transplantation (OHT). Specifically, iNOS has been thought to be the main NOS involved in producing NO that is active in acute cardiac allograft rejection. Up-regulation of iNOS occurs in macrophage cellular infiltrates and later within the graft parenchymal cells. In human cardiac transplantation a positive correlation has shown between iNOS expression and left ventricular contractile dysfunction measured by echocardiography and Doppler techniques. We have recently developed a novel PET radiotracer, \[18F\](+/-)NOS, designed to measure cellular iNOS activity. This study evaluates the feasibility of the method in OHT patients undergoing surveillance endomyocardial biopsy as part of their normal post-transplant evaluation for potential allograft rejection. More specifically, it will compare the myocardial kinetics of this radiotracer measured by PET with tissue measurements of iNOS measured by immunohistochemistry.
