Parenteral iron therapy and phosphorus homeostasis: A review

Abstract Phosphorus has an essential role in cellular and extracellular metabolism; maintenance of normal phosphorus homeostasis is critical. Phosphorus homeostasis can be affected by diet and certain medications; some intravenous iron formulations can induce renal phosphate excretion and hypophosphatemia, likely through increasing serum concentrations of intact fibroblast growth factor 23. Case studies provide insights into two types of hypophosphatemia: acute symptomatic and chronic hypophosphatemia, while considering the role of pre‐existing conditions and comorbidities, medications, and intravenous iron. This review examines phosphorus homeostasis and hypophosphatemia, with emphasis on effects of iron deficiency and iron replacement using intravenous iron formulations.


| INTRODUCTION
Phosphorus plays a critical biochemical role through its involvement in cellular and extracellular metabolism, as an integral component of nucleic acids, cell membranes, high-energy compounds (eg, adenosine triphosphate [ATP]) utilized in metabolism, and through regulating the activity of many enzymes. Phosphorus is also an important component of the hydroxyapatite crystal, which provides mechanical strength to mineralized tissues and participates in maintaining the proper pH of extracellular fluids. 1,2 Hypophosphatemia is a common laboratory abnormality; however, hypophosphatemia is usually an incidental finding, delaying its diagnosis. 3 It is relevant to interpret serum phosphorus within age-dependent reference ranges for normal levels. In adults, normal serum phosphorus is defined as a serum phosphorus level of 2.5 mg/dL (0.8 mmol/L) or greater with the upper limit of normal of 4.5 mg/dL (1.45 mmol/L), but normal serum phosphorus levels are considerably higher in children (4.5-6.5 mg/dL [1.45-2.10 mmol/L]) and newborns (4.3-9.3 mg/dL [1.4-3.0 mmol/L]). 4,5 The prevalence of hypophosphatemia in the general population is difficult to ascertain because hypophosphatemia is usually asymptomatic. Moreover, serum phosphorus is not routinely measured. By contrast, hypophosphatemia has been documented in 2.2%-3.1% of hospitalized patients and 29%-34% of patients in intensive care. 6 Most cases of hypophosphatemia are the result of excessive renal loss of phosphorus, but in some patients hypophosphatemia is a consequence of inadequate gastrointestinal absorption or an insufficient amount in parenteral nutrition formulations. Some medications have also been associated with hypophosphatemia including specific formulations of intravenous iron used to treat iron deficiency, 3 the most common cause of anemia. 7 These iron formulations have been associated with transient hypophosphatemia, [8][9][10][11][12] which is usually asymptomatic, although some cases of severe or protracted hypophosphatemia have been reported. 13,14 This review focuses on the effects of iron deficiency and parenteral iron replacement on phosphorus homeostasis.

| Physiology of phosphorus homeostasis
Phosphorus is an abundant element with a widespread distribution.
Total body phosphorus in a 70 kg man is about 700 to 800 mg, most of which is within bones and teeth in a complex with calcium as hydroxyapatite; about 14% of phosphorus is in soft tissue in the form of phosphate. 1 Only 1% of phosphorus is in extracellular fluids 1,2 where it exists as inorganic phosphate (H 2  binding leads to decreases in membrane expression of NaPi2a and NaPi2c through activation of extracellular signal-related kinase 1/2 (ERK1/2) and serum/glucocorticoid-regulated kinase (SGK1) signaling cascades. 31,32 In distal renal tubules, activation of ERK1/2 and SGK1 signaling pathways stimulates the with-no-lysine kinase 1/4 complex, which results in increased membrane expression of calcium and sodium transporters and increased reabsorption of these minerals. 31 Production of intact FGF23 (iFGF23) involves posttranslational modifications that reduce furin-mediated peptide cleavage. 17 By contrast, biologically inactive C-terminal fragments (cFGF23) are generated by furin cleavage of the intact protein. 33 Both iFGF23 and F I G U R E 2 (A) Posttranslational cleavage of FGF23, which occurs primarily in mineralized tissue cells (eg, osteoblasts, odontoblasts and cementoblasts), is an important but not well understood regulatory mechanism that modulates the biological activity of FGF23 during iron deficiency and other pathological conditions. 21  inactive cFGF23 fragments are present in the circulation. Because commercially-available assays utilize antibodies that react with epitopes in the C-terminal region of FGF23, they detect biologically active iFGF23 as well as inactive cFGF23, which is the predominant immunoreactive form. 21 By contrast, many investigators use research assays that utilize antibodies which flank the furin cleavage site and react only with iFGF23, but these are not yet approved for clinical use The available FGF23 assays differ from each other in the epitopes targeted and in reference ranges, and harmonization among tests has not yet been undertaken. 35

| Diagnosis
Assays measuring inorganic phosphate in the blood are used to determine serum phosphorus, which is normally 2.5-4.5 mg/dL (0.80--1.45 mmol/L) in adults. 2 However, because only a very small proportion of total phosphorus is in circulation, the serum phosphorus concentration may not be a reliable indicator of total body phosphorus content. 2 Serum phosphorus varies with age, sex, time of day, and food intake 1,18 ; thus, it is important to use sex-related and age-related reference values, and it is best to collect fasting blood and urine samples in the morning. The serum level of phosphorus exhibits a circadian rhythm, with an estimated 30%-45% fluctuation between the lowest and highest levels over 24 hours ( Figure S1). 2 40 Chronic hypophosphatemia may also arise from causes independent of excess FGF23 or PTH, including primary defects in proximal tubule absorption of phosphorus (Table 2). 40 Fanconi syndrome (many causes including Dent disease), hereditary hypophosphatemic rickets with hypercalciuria, and other primary disorders of the renal proximal tubule are associated with low or suppressed serum concentrations of FGF23. 25 It has been shown that mutations in sodium-phosphate cotransporter genes involved in renal phosphate reabsorption, SLC34A1 and SLC34A3, cause hypophosphatemia in idiopathic infantile hypercalcemia and hereditary hypophosphatemic rickets with hypercalciuria, respectively. 43,44 Several medication classes are linked to hypophosphatemia ( Table 2, Table S1). 3,6,45 Diuretics, corticosteroids including glucocorticoids, bisphosphonates, and carbonic anhydrase inhibitors can lead to hypophosphatemia by increased renal secretion of phosphate; hypophosphatemia has also been associated with insulin therapy, agents affecting acid-base balance as well as parenteral iron formulations, as will be discussed further. Phosphate-binding medications can impair intestinal absorption and intake of phosphorus, while catecholamines and salicylate poisoning have been associated with hypophosphatemia following a transcellular shift of phosphate.
Iron deficiency is the most common cause of anemia, with an estimated global prevalence of 33%. 46 Additionally, iron deficiency anemia accounts for 50% of all anemia cases and affects an estimated 1.24 billion individuals worldwide, 47 predominantly women and children 7 and is often seen in combination with other nutritional disorders. Vitamin D affects iron homeostasis and erythropoiesis, and low vitamin D levels have been associated with iron deficiency in both adults and children. 48,49 Synthesis of FGF23 as well as proteolytic inactivation of iFGF23 is increased in proportion to the severity of iron deficiency in humans. 50 The increase in FGF23 results from an increase in FGF23 mRNA transcription, which has been demonstrated both in vivo (from a low-iron diet) and in cultured cells (from treatment with an iron chelator); this may be regulated by the transcription factor hypoxia inducible factor-1α. 34 The increase in FGF23 transcription and cleavage is stimulated by not only iron deficiency, but erythropoietin and inflammation as well. 34 Iron deficiency and inflammation affect an increase in transcription factor hypoxia inducible factor-1α, which is linked to increasing FGF23 levels with involvement of erythropoietin. 34 Moreover, FGF23 mRNA is increased in cells cultured under hypoxic conditions. 51 These studies suggest that bone-produced hypoxia inducible factor-1α may represent a novel therapeutic target to reduce FGF23 levels, for example in patients with CKD, to minimize the negative consequences associated with FGF23 excess. Taken in context, there is overlap in responses to iron deficiency and hypoxia, both of which are conditions that ultimately result in reduced oxygen delivery to cells.
Iron deficiency increases synthesis of FGF23 and also increases cleavage of iFGF23 into inactive cFGF23, which typically results in little or no change in the circulating level of iFGF23. Iron replacement actually decreases production of FGF23. 50,52 However, the conundrum is that certain iron formulations, such as ferric carboxymaltose (FCM), also seem to reduce cleavage of iFGF23. Hence, although total FGF23 production is decreased by FCM, the amount of iFGF23 that is secreted increases, leading to hypophosphatemia. 50,53 The exact mechanism for reduced cleavage of FGF23 is unknown, but may involve either posttranslational modification of FGF23 to make it less susceptible to cleavage or reduced production of enzymes (ie, furin) that process FGF23. 25 In ADHR, iron deficiency increases FGF23 synthesis but proteolytic processing is impaired resulting in high iFGF23 levels and consequent hypophosphatemia made worse by iron deficiency. 21,33 When tolerability and adherence to oral iron becomes an issue, intravenous iron infusion is commonly used as a treatment for iron deficiency anemia (IDA); infusion of some formulations of intravenous iron can increase circulating levels of iFGF23 and result in hypophosphatemia. 54 Although replacing iron per se appears to reduce production of FGF23, some iron formulations may inhibit proteolytic cleavage of FGF23, thereby increasing circulating levels of iFGF23.
Hypophosphatemia associated with intravenous iron supplementation is reported to be mostly asymptomatic and transient. 9,12,55,56 One retrospective study showed the median duration of hypophosphatemia to be 41 days following intravenous iron treatment, but that hypophosphatemia in some patients lasted greater than 2 months. 57 A multivariate model found independent risk factors for incident hypophosphatemia to include abnormal uterine bleeding associated with IDA, higher hemoglobin levels, and lower baseline serum phosphorus levels. 50 Other possible risk factors include concurrent or prior use of medications affecting proximal renal tubular function, hyperparathyroidism, vitamin D deficiency, a history of gastrointestinal disorders associated with malabsorption of fat-soluble vitamins or phosphate, and malnutrition. Resolution of most cases of hypophosphatemia occurs within three months. 58 The use of the following intravenous iron formulations has been associated with the occurrence of hypophosphatemia: ferric derisomaltose (formerly iron isomaltoside), 8,59,60 FCM,50,52,56,59 iron polymaltose, 10 and iron sucrose. 56 In clinical trials, the frequencies of a transient decline in serum phosphorus below 2 mg/dL ranged from 5%-20% of patients treated with ferric derisomaltose 1000 mg from the studies in patients with IDA of various etiologies and 1%-2% in studies of patients with CKD. 54,60,61 Nadir was in the first weeks. In one study of patients with IDA from various causes other than CKD, 3.9% of patients in the ferric derisomaltose 1000 mg group and 2.3% of patients in the iron sucrose 200 mg (≤ 5 injections; 1000 mg cumulative dose recommended) group had hypophosphatemia (serum phosphorus <2 mg/dL). 62 In two open-label, randomized clinical studies of patients with IDA without reduced kidney function, the incidence of hypophosphatemia was significantly lower (p < 0.001 in both studies) in patients treated with ferric derisomaltose 1000 mg (7.9%-8.1%) vs those treated with FCM 750 mg (73.7%-75.0%). 60 From the pivotal phase 3 IDA trials of FCM, the pooled incidence of serum phosphorus below 2 mg/dL in FCM-treated participants was 27%, and 2.1% was reported by study investigators to represent a treatment-emergent adverse event. 58 In these studies, hypophosphatemia was not associated with a serious adverse event. 63,64 Transient hypophosphatemia findings based solely on laboratory T A B L E 3 Characteristics of 12 case studies reporting hypophosphatemic osteomalacia after repeated courses of FCM

| CONCLUSIONS
Serum phosphorus should be monitored in patients with symptomatic hypophosphatemia and those at risk of hypophosphatemia.
Patients considered at risk for hypophosphatemia include those

DATA AVAILABILITY STATEMENT
Data sharing was not applicable to this article as no new data were generated or analyzed during the current study ORCID Tomas Ganz https://orcid.org/0000-0002-2830-5469