We will conduct a 12-month, double-blind, randomized, placebo-controlled trial to assess the effects of therapy with ferric citrate (FC) on changes in intact FGF23 levels (iFGF23, primary endpoint) in 160 pediatric patients (80 in each of the two arms) aged 6-18 years of either sex with chronic kidney disease (CKD) stages 3-4 and age-appropriate normal serum phosphate levels. Participants will be randomized to one of the two groups: 1) FC or 2) FC placebo. Participants will be recruited from 20 core clinical sites.

We will conduct a double-blind, randomized, placebo-controlled trial to assess the effects of therapy with ferric citrate (FC) on changes in intact FGF23 levels (iFGF23, primary endpoint) aged 6-18 years of either sex with chronic kidney disease (CKD) stages 3-4 and age-appropriate normal serum phosphate levels. Participants will be randomized to one of the two groups: 1) FC or 2) FC placebo. Participants will be recruited from 20 core clinical sites.

Schedule of Intervention: During the 12-month trial, participants will be given a daily fixed weight-based dose of FC.

Schedule for data collection/analyses to be performed:

Blood for primary outcome assessments will be collected at screening, baseline and at months 3, 6, 9, 12. Blood for safety assessments will be collected at the the months 1, 2, 3, 6, 9, 12.

The primary analyses for this 2-arm trial will compare log-transformed iFGF23 values over 12 months between the treatment and the placebo arms. The analysis will use a linear mixed-effects model, including stratification factors CKD stage and urine protein to creatinine ratio, with random participant effects accounting for repeated measurements, and a fixed treatment effect, which interacts with a time indicator (Months 3-12 vs. Baseline/Screening).

Primary objectives:

• To assess the effects of therapy with FC on iFGF23 levels
• To determine safety and tolerability of FC.

Secondary objectives:

• To assess the effects of FC on anemia and indices of mineral and bone metabolism.

Primary Endpoint:

• iFGF23 level

Safety and Tolerability Endpoints:

• Ability to safely tolerate FC

Secondary Endpoints:

• Anemia
• Indices of mineral and bone metabolism

This is a Phase 2 study with participation from 20 sites that will take 36 months to complete enrollment and a total of 48 months to complete data collection with each participant being part of the study for 12 months.

Study website: fit4kid.dgsom.ucla.edu

Inclusion Criteria:

1. Ages 6 to 18 years (inclusive);
2. Estimated Glomerular Filtration Rate (GFR) of 15-59 ml/min per 1.73 m2 by modified Chronic Kidney disease in Children (CKiD) under 25 (U25) formula;56
3. Serum phosphate <=5.9 mg/dl;
4. Serum ferritin <500 ng/ml and TSAT <50%;
5. For those patients treated with growth hormone, calcitriol, nutritional vitamin D, iron, and/or erythropoiesis-stimulating agents (ESAs) such treatments must have stable dosing for at least 2 weeks prior to screening;
6. Able to swallow tablets;
7. Able to eat at least two meals a day;
8. In the opinion of the investigator, willing and able to follow the study treatment regimen and comply with the site investigator's recommendations.

Exclusion Criteria:

1. Patients currently treated with phosphate binders.
2. History of allergy to all ingredients (including non-medical ingredients) in both products (i.e. investigational product and placebo)
3. Current intestinal malabsorption, documented in the medical record; disease, inflammatory bowel syndrome, and/or Crohn's Disease.
4. Anticipated initiation of dialysis or kidney transplantation within 6 months
5. Current or planned future systemic immunosuppressive therapy
6. Prior solid organ transplantation
7. Receipt of bone marrow transplant within two years of screening
8. Current pregnancy, lactation or female subjects who have reached menarche, unless using highly-effective contraception as outlined in section 7.1.1 of Protocol
9. Patients participating in other interventional study (observational study participation permitted)
10. Poor adherence to medical treatments in the opinion of the investigator
11. Cystinosis
12. Fanconi syndrome
13. Hemochromatosis or laboratory tests indicating possible hemochromatosis or other iron overload (primary or secondary) syndrome

All de-identified clinical data, including study outcomes and participant characteristics will be submitted to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Central Repository at the completion of the study. De-identified research samples blood and urine will also be provided to the NIDDK central reporsitory

the IPD will become available 12 months after study completion.

Access will be provided by National Institute of Health and any researcher will be able to request access from NIH once the data becomes available.