Hyperphosphatemia in Chronic Kidney Disease on Dialysis

A total of 289 patients were treated with ferric citrate tablets and 149 patients were treated with active control (sevelamer carbonate and/or calcium acetate) during the 52-week, randomized, open-label, active control phase of a trial in patients on dialysis. A total of 322 patients were treated with ferric citrate tablets for up to 28 days in three short-term trials. Across these trials, 557 unique patients were treated with ferric citrate tablets; dosage regimens in these trials ranged from 210 mg to 2,520 mg of ferric iron per day, equivalent to 1 to 12 tablets of ferric citrate.

Adverse reactions reported in more than 5% of patients treated with ferric citrate tablets in these trials included diarrhea (21%), discolored feces (19%), nausea (11%), constipation (8%), vomiting (7%), and cough (6%).

During the 52-week, active-control period, 61 patients (21%) on ferric citrate tablets discontinued study drug because of an adverse reaction, as compared to 21 patients (14%) in the active control arm. Patients who were previously intolerant to any of the active control treatments (calcium acetate and sevelamer carbonate) were not eligible to enroll in the study. Gastrointestinal adverse reactions were the most common reason for discontinuing ferric citrate tablets (14%).

Iron Deficiency Anemia in Chronic Kidney Disease Not on Dialysis

Across two trials, 190 patients with CKD-NDD were treated with ferric citrate tablets. This included a study of 117 patients treated with ferric citrate tablets and 116 patients treated with placebo in a 16-week, randomized, double-blind period and a study of 75 patients treated with ferric citrate tablets and 73 treated with placebo in a 12-week randomized double-blind period. Dosage regimens in these trials ranged from 210 mg to 2,520 mg of ferric iron per day, equivalent to 1 to 12 tablets of ferric citrate.

Adverse reactions reported in at least 5% of patients treated with ferric citrate tablets in these trials are listed in Table 1.

During the 16-week, placebo-control trial, 12 patients (10%) on ferric citrate tablets discontinued study drug because of an adverse reaction, as compared to 10 patients (9%) in the placebo control arm. Diarrhea was the most common adverse reaction leading to discontinuation of ferric citrate tablets (2.6%).

Risk Summary

There are no available data on ferric citrate tablet use in pregnant women to inform a drug-associated risk of major birth defects and miscarriage. Animal reproduction studies have not been conducted using ferric citrate tablets. Skeletal and encephalic malformation was observed in neonatal mice when ferric gluconate was administered intraperitoneally to gravid dams on gestation days 7-9. However, oral administration of other ferric or ferrous compounds to gravid CD1-mice and Wistar-rats caused no fetal malformation.

An overdose of iron in pregnant women may carry a risk for spontaneous abortion, gestational diabetes and fetal malformation.

The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. Adverse outcomes in pregnancy occur regardless of the health of the mother or the use of medications. In the U.S. general population, the estimated background risks of major birth defects and miscarriage in clinically recognized pregnancies are 2 to 4% and 15 to 20%, respectively.

Clinical Considerations

The effect of ferric citrate tablets on the absorption of vitamins and other nutrients has not been studied in pregnant women. Requirements for vitamins and other nutrients are increased in pregnancy.

Risk Summary

There are no human data regarding the effect of ferric citrate in human milk, the effects on the breastfed child, or the effects on milk production. Data from rat studies have shown the transfer of iron into milk by divalent metal transporter-1 (DMT-1) and ferroportin-1 (FPN-1). Hence, there is a possibility of infant exposure when ferric citrate tablets are administered to a nursing woman. The development and health benefits of breastfeeding should be considered along with the mother’s clinical need for ferric citrate tablets and any potential adverse effects on the breastfed child from ferric citrate tablets or from the underlying maternal condition.

Juvenile Animal Toxicity Data

In animal studies, greater gastrointestinal toxicity was observed when ferric citrate was administered by gavage as compared to administration with solid food. Because ferric citrate tablets are recommended to be taken with meals and patients under 6 months of age are unlikely to be eating solid food, they may be at greater risk of gastrointestinal toxicity.

Hyperphosphatemia in Chronic Kidney Disease on Dialysis

Ferric iron binds dietary phosphate in the GI tract and precipitates as ferric phosphate. This compound is insoluble and is excreted in the stool. By binding phosphate in the GI tract and decreasing absorption, ferric citrate lowers the phosphate concentration in the serum.

Iron Deficiency Anemia in Chronic Kidney Disease Not on Dialysis

Ferric iron is reduced from the ferric to the ferrous form by ferric reductase in the GI tract. After transport through the enterocytes into the blood, oxidized ferric iron circulates bound to the plasma protein transferrin, and can be incorporated into hemoglobin.

Hyperphosphatemia in Chronic Kidney Disease on Dialysis

Ferric citrate tablets reduce serum phosphorus levels and has also been shown to increase serum iron parameters, including ferritin, iron and TSAT. In dialysis patients treated with ferric citrate tablets for hyperphosphatemia in a 52-week study in which intravenous iron could also be administered, mean (SD) ferritin levels rose from 593 (293) ng/mL to 895 (482) ng/mL, mean (SD) TSAT levels rose from 31% (11) to 39% (17) and mean (SD) iron levels rose from 73 (29) mcg/dL to 88 (42) mcg/dL. In contrast, in patients treated with active control, these parameters remained relatively constant [see Contraindications (4) and Warnings and Precautions 5.1)].

Iron Deficiency Anemia in Chronic Kidney Disease Not on Dialysis

Ferric citrate tablets may increase hemoglobin levels and has also been shown to reduce serum phosphorus levels. In chronic kidney disease patients not on dialysis treated with ferric citrate tablets for iron deficiency anemia in a 16-week placebo-controlled study, mean (SD) phosphorus levels decreased from 4.23 (0.91) mg/dL at baseline to 3.72 (0.60) mg/dL. In comparison, in patients treated with placebo control, mean (SD) phosphorus levels decreased from 4.12 (0.68) mg/dL at baseline to 3.87 (0.68) mg/dL.

Absorption and Distribution

Formal pharmacokinetic studies have not been performed with ferric citrate tablets. Examination of serum iron parameters has shown that there is systemic absorption of iron from ferric citrate tablets [see Contraindications (4), Warnings and Precautions (5.1) and Clinical Pharmacology (12.1)].

Drug Interaction Studies

Study KRX-0502-304 (NCT 01191255)

Study KRX-0502-304 was a long-term, randomized, controlled, safety and efficacy trial. After the 2-week washout period during which phosphate binders were held, patients with a mean serum phosphorus of 7.5 mg/dL during washout were randomized 2:1 to ferric citrate tablets (N=292) or active control (calcium acetate and/or sevelamer carbonate; N=149). The majority (>96%) of subjects were on hemodialysis. The starting dose of ferric citrate tablets was 6 tablets/day, divided with meals. The starting dose of active control was the patient’s dose prior to the washout period. The dose of phosphate binder was increased or decreased as needed to maintain serum phosphorus levels between 3.5 and 5.5 mg/dL, to a maximum of 12 tablets/day.

As shown in the figure below, serum phosphorus levels declined following initiation of therapy. The phosphorus lowering effect was maintained over 52 weeks of treatment.

Figure 1: Serum Phosphorus Control over 52 Weeks

Following completion of the 52-week active-controlled phase, ferric citrate tablets-treated patients were eligible to enter a 4-week placebo-controlled randomized withdrawal phase, in which patients were re-randomized in a 1:1 ratio to receive ferric citrate tablets (N=96) or placebo (N=96). During the placebo-controlled period, the serum phosphorus concentration rose by 2.2 mg/dL on placebo relative to patients who remained on ferric citrate tablets.

Study KRX-0502-305 (NCT 01074125)

Following a 1- to 2-week washout from all phosphate-binding agents, 154 patients with hyperphosphatemia (mean serum phosphorus of 7.5 mg/dL) and CKD on dialysis were randomized in a 1:1:1 ratio to 1, 6, or 8 tablets/day of ferric citrate for 4 weeks. Ferric citrate tablets were administered with meals; subjects receiving 1 tablet/day were instructed to take it with their largest meal of the day, and subjects on 6 or 8 tablets/day took divided doses in any distribution with meals. Dose-dependent decreases in serum phosphorus were observed by Day 7 and remained relatively stable for the duration of treatment. The demonstrated reductions from baseline to Week 4 in mean serum phosphorus were significantly greater with 6 and 8 tablets/day than with 1 tablet/day (p<0.0001). Mean reduction in serum phosphorus at Week 4 was 0.1 mg/dL with 1 tablet/day, 1.9 mg/dL with 6 tablets/day, and 2.1 mg/dL with 8 tablets/day.

Study KRX-0502-306 (NCT 02268994)

The efficacy of ferric citrate tablets for the treatment of iron deficiency anemia in adult patients with CKD not on dialysis was demonstrated in a 24-week study consisting of a 16-week, randomized, double-blind, placebo-controlled, efficacy period followed by an 8-week open-label safety extension period in which all patients remaining in the study, including the placebo group, received ferric citrate tablets. Patients with eGFR <60 mL/min/1.73m2, who were intolerant of or have had an inadequate therapeutic response to oral iron supplements, with Hgb ≥9.0 g/dL and ≤11.5 g/dL, serum ferritin ≤200 ng/mL and TSAT ≤25% were enrolled. Patients were randomized to treatment with either ferric citrate tablets (n=117) or placebo (n=117). Dosing with ferric citrate tablets or placebo was initiated at 3 tablets/day with meals. Dose titration could occur at Weeks 4, 8 and 12 during Randomized Period, and at Weeks 18 and 20 during Safety Extension Period based on Hgb response. Use of oral or intravenous iron, erythropoiesis stimulating agents (ESAs) was not permitted at any time during the study.

The mean age of the patients was 65 years (range 26 to 93); 63% were female, 69% Caucasian, 30% were African American and <2% were other races.

The main efficacy outcome measure was the proportion of subjects achieving an increase in Hgb of ≥1.0 g/dL at any time point between baseline and the end of the 16-week Randomized Period.

During the 16-week randomized period 49% of subjects in the ferric citrate tablet arm and 15% of subjects in the placebo arm (p <0.001) had a mean change in hemoglobin from baseline ≥0.75 g/dL over any 4-week time period provided that an increase of at least 1.0 g/dL had occurred during that 4-week period. Increases in mean hemoglobin (0.75 ± 0.09 g/dL), serum ferritin (163 ± 9 ng/mL) and transferrin saturation (18 ± 1%) were observed from baseline during the 16-week randomized period in the ferric citrate tablet arm.