The a series of clinical studies of [18F]FSPG PET/CT showed that [18F]FSPG is safe and promising PET tracer for detecting inflammation with favorable biodistribution and pharmacokinetics in patients. By using [18F]FSPG, which targets system xCˉ, a key player in the active phase of inflammation, the goal of this phase 2 study is to evaluate diagnostic validity of [18F]FSPG PET/CT to detect allograft rejection after heart and liver allograft, where conventional imaging has limitations. Diagnostic efficacy of [18F]FSPG PET/CT will be determined in comparison with histologic evaluation of allograft biopsy. Other critical questions including whether quantitative measures of [18F]FSPG uptake is associated with the severity of rejection, inter-reader variability of [18F]FSPG PET/CT, and safety assessment will be also evaluated.

This open label, non-randomized, single center, single-dose exploratory study is designed to obtain imaging assessments using PET/CT with [18F]FSPG in patients with heart transplantation (HT, n = 10) and liver transplantation (LT, n = 10). Subjects who underwent or scheduled to undergo biopsy for histologic diagnosis of acute rejection will be enrolled.

The study comprises 3 periods: screening, treatment, and follow-up. The screening period starts with the subject's signature on the informed consent form and ends with assignment to treatment that is inclusion of the subject for treatment. The treatment period starts with baseline measurements, and ends with the last measurement/procedure on the day following injection of 200 MBq of [18F]FSPG. The follow-up period contains the end-of-study interview. Key measurement is the PET/CT image acquisition about one hour after the single injection of [18F]FSPG. For evaluation of safety, adverse events (AE) will be monitored and recorded during the study period.

This study will be monitored regularly by a clinical research associate from the sponsor or a designated contract research organization. Data quality and study integrity will be assessed. The investigator will document the items that are required to evaluate the study in the subject files. Data required according to this protocol are to be recorded on the case report forms provided by the sponsor as soon as possible.

Inclusion Criteria:

• Subject is aged between 19 and 79 years and male or female of any race/ethnicity.
• Subject received heart transplantation or liver transplantation at least 30 days prior to screening
• Subject underwent or is scheduled to undergo biopsy for histologic diagnosis of acute rejection within 7 days prior to or after the planned [18F]FSPG PET/CT administration.

Exclusion Criteria:

• Subject or subject's legally acceptable representative does not provide written informed consent.
• Dose escalation of the current immunosuppressant drugs or starting a new immunosuppressant is scheduled from the study enrollment to the scheduled day of biopsy, or 24 hours after [18F]FSPG administration
• Female subject is pregnant or nursing. Exclusion of the possibility of pregnancy is made by one of the following: 1) Woman is physiologically post-menopausal (cessation of menses for more than 2 years), 2) woman is surgically sterile (has had a documented bilateral oophorectomy and/or documented hysterectomy, or 3) if the woman is of childbearing potential, a serum or urine pregnancy test performed within 24 hours immediately prior to administration of [18F]FSPG has to be negative and the women is advised to apply contraceptive measures during her participation in this study.
• Subject has concurrent severe and/or uncontrolled and/or unstable medical disease (e.g. congestive heart failure, acute myocardial infarction, severe pulmonary disease, chronic renal or hepatic disease which could compromise participation in the study) in the judgment of the investigator.
• Subject is a relative of the investigator, student of the investigator or otherwise dependent.
• Subject has received any investigational drugs or devices within four weeks prior to the study enrollment or within 24 hours after administration of [18F]FSPG.
• Subject has been previously included in this study.
• Subject has any other condition or personal circumstances that, in the judgment of the investigator, might make collection of complete data difficult or impossible.