Ferric citrate (FC) is an approved therapy for chronic kidney disease (CKD) patients as a phosphate (Pi) binder for dialysis‐dependent CKD, and for iron deficiency anemia (IDA) in non‐dialysis CKD. Elevated Pi and IDA both lead to increased FGF23, however, the roles of iron and FGF23 during CKD remain unclear. To this end, iron and Pi metabolism were tested in a mouse model of CKD (0.2% adenine) ± 0.5% FC for 6 weeks, with and without osteocyte deletion of Fgf23 (flox‐Fgf23/Dmp1‐Cre). Intact FGF23 (iFGF23) increased in all CKD mice but was lower in Cre+ mice with or without FC, thus the Dmp1‐Cre effectively reduced FGF23. Cre+ mice fed AD‐only had higher serum Pi than Cre− pre‐ and post‐diet, and the Cre+ mice had higher BUN regardless of FC treatment. Total serum iron was higher in all mice receiving FC, and liver Tfrc, Bmp6, and hepcidin mRNAs were increased regardless of genotype; liver IL‐6 showed decreased mRNA in FC‐fed mice. The renal 1,25‐dihydroxyvitamin D (1,25D) anabolic enzyme Cyp27b1 had higher mRNA and the catabolic Cyp24a1 showed lower mRNA in FC‐fed mice. Finally, mice with loss of FGF23 had higher bone cortical porosity, whereas Raman spectroscopy showed no changes in matrix mineral parameters. Thus, FC‐ and FGF23‐dependent and ‐independent actions were identified in CKD; loss of FGF23 was associated with higher serum Pi and BUN, demonstrating that FGF23 was protective of mineral metabolism. In contrast, FC maintained serum iron and corrected inflammation mediators, potentially providing ancillary benefit.

The adenine diet is widely used in animal models to produce a tubulointerstitial fibrosis and inflammation that mimics human CKD in many aspects. These include the biochemical manifestations hyperphosphatemia and anemia, as well as endocrine dysfunction with elevated FGF23 and hyperparathyroidism. Male rodents are known to be less tolerant of adenine diet regimen than females, however the underlying mechanisms driving the sex differences remain unclear. Additionally, much of the data for adenine studies arises from rats, whereas mice are more commonly used in laboratory settings and are far easier to manipulate genetically. To this end, as part of a larger study to test the effects of iron-handling in CKD, we assessed the biochemical, molecular, and physical differences between male and female mice receiving an adenine diet to induce CKD. Flox-Fgf23 mice (8 weeks of age, n=4-6/group; mice were Cre negative, thus phenotypically wild type) were placed on a 0.2% adenine-containing diet (CKD); a matching casein-based diet served as control. After 6 weeks, mice were euthanized, and blood and tissues were collected for analysis. As expected, body weight at baseline was initially higher in males than in females, however males lost significantly more weight. Serum BUN was also elevated in both sexes receiving adenine, although males were higher (1.2 fold; p<0.01). Males also had elevated creatinine and lower total serum iron from baseline whereas females had no significant changes. FGF23 was elevated in all mice, with no significant differences between sexes. Kidney fibrosis and inflammation markers were elevated in the CKD mice, with males having higher expression of Col1a1 and -3a1 versus females (3.5/1.5 fold; p<0.001) and TNFα mRNA (2 fold; p<0.001). Renal expression of the anabolic vitamin D metabolizing enzyme Cyp27b1 (1α-hydroyxlase) and early growth response 1 (Egr1) were increased in CKD mice, with males having higher expression over females. Conversely, CKD males had lower kidney Klotho mRNA expression, and both sexes fed adenine expressed significantly lower NPT2a (sodium- phosphate co-transporter2a) mRNA. Liver expression of ferritin (Fth1) was elevated in male CKD mice compared to diet controls, whereas female mice had no differences. Elevated FGF23 has been linked to ventricular hypertrophy, and CKD males had significantly higher heart weight to femur ratio at completion of the study. Our results support that male mice succumb more rapidly than females to adenine diet mediated CKD phenotypes, potentially enhanced by fibrosis and inflammation. It remains to be determined whether the more rapid onset of defects in iron handling parameters accelerate the severe male CKD phenotype.