A Study to Characterize the Biodistribution of 124I-Labeled AT-03 in Patients With Systemic Amyloidosis

1. Understands the study procedures and capable of giving signed informed consent, as described in Appendix 1, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol.
2. Male or female ≥18 years of age.

3. Has a confirmed diagnosis of Amyloid Light-Chain (AL), Transthyretin Amyloidosis (ATTR), or Leukocyte chemotactic factor 2 amyloidosi (ALECT2), or any other forms of systemic amyloidosis based on any one of the following:

   1. a histologic confirmation with a biopsy containing deposits of apple-green birefringent, congophilic material;
   2. genetic screening with presence of amyloid-related pathology; and/or
   3. amyloid-specific imaging study.
4. Has known involvement of at least one thoracoabdominal organ (excluding peripheral nervous system) by clinical history (e.g., imaging consistent with amyloid deposition, organ biopsy, elevated amyloid-related biomarkers, etc.).

5. For women of childbearing potential: agreement to remain as abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of \<1% per year during the treatment period and for at least 90 days after the last dose of study intervention.

   1. A woman is considered of childbearing potential if she is postmenarchal, has not reached a postmenopausal state (≥ 12 months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). The definition of childbearing potential may be adapted for alignment with local guidelines or requirements.
   2. Examples of contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, established, proper use of hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices and copper intrauterine devices.
   3. Contraception methods that do not result in a failure rate of \<1% per year such as cap, diaphragm, or sponge with spermicide, or male or female condom with or without spermicide, are not acceptable.
   4. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the subject. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.

6. For men: agreement to remain abstinent or use contraceptive measures and agreement to refrain from donating sperm, as defined below:

   1. With female partners of childbearing potential, men must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of \< 1% per year during the treatment period and for at least 30 days plus 90 days (a spermatogenesis cycle) after the last dose of study intervention. Men must refrain from donating sperm during this same time period.

Exclusion Criteria:

1. Is pregnant or breast-feeding.
2. Is mentally or legally incapacitated, has significant emotional problems at the time of the study, or has a history of psychosis.
3. Has participated in another nuclear medicine amyloid imaging clinical trial protocol and received tracer injection in the 4 months prior to Screening.
4. Has a significant co-morbidity (e.g., ECOG score of 3 or greater), NYHA Class IV heart failure, uncontrolled infection, or other ongoing serious illness.
5. Has a known allergy to iOSAT iodine treatment.
6. Has end-stage renal disease and is receiving hemodialysis or peritoneal dialysis.
7. Has received an investigational agent within five half-lives of the agent or 30 days, whichever is longer, prior to Screening.
8. Has any illness that, in the opinion of the Investigator, might confound the results of the study or pose additional risk to the subject.