This is a Phase 1b, single center, dose-escalating study evaluating the safety, tolerability, and pharmacodynamic effect of RBT-3 in healthy volunteers and in subjects with stage 3-4 chronic kidney disease.

Inclusion Criteria:

1. Male and female subjects age 18 to 80 years (inclusive, at time of ICF).

2. Body weight <125 kg.

3. Able and willing to comply with all study procedures.

4. Female subjects must be either post-menopausal for at least 1 year or surgically sterile (tubal ligation, hysterectomy or bilateral oophorectomy) for at least 3 months, or if of childbearing potential, have a negative pregnancy test and agree to use dual methods of contraception, or abstain from sexual activity from Screening until 8 days after study drug administration.

   Male subjects with female partners of childbearing potential must agree to use a highly effective method of contraception from screening until 8 days after study drug administration. All fertile men with female partners of childbearing potential should be instructed to contact the Investigator immediately if they suspect their partner might be pregnant (e.g., missed or late menstrual period) at any time during study participation.

5. CKD as determined by estimated glomerular filtration rate (eGFR) between 15-59 ml/min as estimated using the CKD-EPI equation (CKD class 3-4).

Exclusion Criteria:

1. History of malignancy except carcinoma in situ in the cervix, early stage prostate cancer or non-melanoma skin cancers.
2. Use of investigational drugs or participation in another clinical trial within 30 days or 5 half-lives prior to screening, whichever is longer.
3. Serum ferritin > 500 ng/ml or who have received IV iron within 28 days of screening.
4. Women who are pregnant, breastfeeding, or planning to become pregnant while participating in the study.
5. Any significant acute or chronic diseases, excluding CKD (eGFR ≥15 ml/min).
6. Regular use of drugs of abuse and/or positive findings on urinary drug screening.
7. Current tobacco use and/or positive findings on urinary cotinine screening.
8. Subjects who are severely physically or mentally incapacitated and who, in the opinion of investigator, are unable to perform the subjects' tasks associated with the protocol.
9. Presence of any condition which, in the opinion of the investigator, places the subject at undue risk or potentially jeopardizes the quality of the data to be generated.
10. Known hypersensitivity or previous anaphylaxis to FeS.

8 days