iNOS With Positron Emission Tomography (PET) in Cellular Inflammation.

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.

Inclusion criteria

• The OHT patients will be undergoing surveillance endomyocardial biopsy and will Patients 21 years of age or older of either sex, who are status-post OHT and normal healthy volunteers (2 women and 2 men) will be enrolled.be on standard immunosuppressive therapy and anti-hyperlipidemic, anti-hypertensive and anti-diabetic therapies as needed. "Healthy volunteer" is someone who has volunteered to be imaged and who, based on physical exam and baseline electrocardiogram, has no evidence of cardiovascular disease, is not on medication, such as steroids, that will interfere with the accuracy of measuring [18F](+/-)NOS activity and is not under the care of a physician for any active medical conditions.
• Able to give informed consent.
• Not currently pregnant or nursing: Female subjects must be either: surgically sterile (has had a documented bilateral oophorectomy and/or documented hysterectomy), post menopausal (cessation of menses for more than 1 year), or of childbearing potential for whom a urine pregnancy test (with the test performed within the 24 hour period immediately prior to administration of [18F](+/-)NOS) is negative.

Exclusion criteria

• Patients with an unstable cardiovascular (e.g., severe rejection) or other clinical condition (e.g., active severe infection) that in the opinion of the Principal Investigator or designee or Dr. Ewald precludes participation in the study.
• Unable to tolerate 60-90 mins of PET imaging or is claustrophobic.
• Normal volunteers with evidence of cardiovascular disease or other diseases based on clinical evaluation and/or blood laboratory tests, which are judged by the Principal Investigator or designee to interfere with accurate determination of the of [18F](+/-)NOS on such measures.