Saccharated ferric oxide (SFO)-induced osteomalacia: in vitro inhibition by SFO of bone formation and 1,25-dihydroxy-vitamin D production in renal tubules

Drug-related hypophosphatemia: Descriptive study and case/non-case analysis of the French national pharmacovigilance database

Phosphorus is an essential element for all living organisms and is involved in various biological pathways. A severe hypophosphatemia can lead to serious complications (acute heart or respiratory failure, rhabdomyolysis, hemolysis…) and increases mortality in patients at risk. Various drugs are known to induce hypophosphatemia through various mechanisms. The aim of this study was to highlight the main drugs associated with hypophosphatemia and to deduce the underlying mechanisms based on a descriptive analysis and a case/non-case analysis using the cases of drug-induced hypophosphatemia reported to the French Pharmacovigilance Network. A total of 368 cases of hypophosphatemia were included in the study. Patients' mean age was 52±18 years. One hundred and ninety-one cases (52%) were serious including 131 (36%) hospitalizations. The median value of serum phosphorus level was 0.54mmol/L [0.40-0.66] (n=309). Those 368 cases corresponded to 185 different suspected substances among which the most frequent drugs were tenofovir disoproxil (n=175; 48%), ferric carboxymaltose (n=29; 8%), denosumab (n=16; 4%), zoledronic acid (n=14; 4%) and hydrochlorothiazide (n=10; 3%). For these five drugs, a significant disproportionality was found. Tenofovir-disoproxil related hypophosphatemia occurred more than one year after its introduction, and a renal tubulopathy (Fanconi's syndrome) was reported in 44 cases (25%). Hypophosphatemia related to iron carboxymaltose occurred within a median of 20 days after injection and was mostly severe. Mechanism included the fibroblast growth factor 23 which can be measured to confirm drug origin. Concerning anti-osteoporosis treatments, hypophosphatemia could be explained by their mechanism of action (abrupt increase of parathormone induced by hypocalcemia) but the patient history (malignancy condition) was a major bias. For hydrochlorothiazide, hyphosphatemia was often moderate, associated with other electrolytic disturbances and occurred during a long-term treatment. Awareness of healthcare professionals is essential to detect as soon as possible hypophosphatemia and its complications related to these drugs.

High-Dose Intravenous Iron with Either Ferric Carboxymaltose or Ferric Derisomaltose: A Benefit-Risk Assessment

The intravenous iron formulations ferric carboxymaltose (FCM) and ferric derisomaltose (FDI) offer the possibility of administering a large amount of iron in one infusion. This results in faster correction of anemia and the formulations being better tolerated than oral iron formulations. This triad of logistic advantages, improved patient convenience, and fast correction of anemia explains the fact that intravenous iron formulations nowadays are frequently prescribed worldwide in the treatment of iron deficiency anemia. However, these formulations may result in hypophosphatemia by inducing a strong increase in active fibroblast growth factor-23 (FGF-23), a hormone that stimulates renal phosphate excretion. This effect is much more pronounced with FCM than with FDI, and therefore the risk of developing hypophosphatemia is remarkably higher with FCM than with FDI. Repeated use of FCM may result in severe osteomalacia, which is characterized by bone pain, Looser zones (pseudofractures), and low-trauma fractures. Intravenous iron preparations are also associated with other adverse effects, of which hypersensitivity reactions are the most important and are usually the result of a non-allergic complement activation on nanoparticles of free labile iron-Complement Activation-Related Pseudo-Allergy (CARPA). The risk on these hypersensitivity reactions can be reduced by choosing a slow infusion rate. Severe hypersensitivity reactions were reported in < 1% of prospective trials and the incidence seems comparable between the two formulations. A practical guideline has been developed based on baseline serum phosphate concentrations and predisposing risk factors, derived from published cases and risk factor analyses from trials, in order to establish the safe use of these formulations.

Osteomalacia as a Complication of Intravenous Iron Infusion: A Systematic Review of Case Reports

Randomized control trials (RCTs) have shown that certain intravenous iron preparations can induce high levels of fibroblast growth factor 23 (FGF-23) and persistent hypophosphatemia. Repeated iron infusions may lead to prolonged hypophosphatemia and osteomalacia events not captured by RCTs. Several previous case reports have described skeletal adverse effects after repeated iron infusions. To characterize these effects, we conducted a systematic review of case reports. MEDLINE, Embase, Web of Science, and Cochrane databases were searched in March 2021. We selected case reports of patients ≥16 years old. Study quality was assessed using the tool from Murad and colleagues. We report the results in a narrative summary. We identified 28 case reports, reporting 30 cases. Ages ranged from 28 to 80 years (median 50 years). Most patients (n = 18) received ferric carboxymaltose (FCM), whereas 8 received saccharated ferric oxide (SFO) and 3 received iron polymaltose (IPM). All but 2 cases had more than five infusions (range 2 to 198, median 17). The lowest phosphate levels ranged from 0.16 to 0.77 mmol/L (median 0.36 mmol/L). Intact FGF-23 (iFGF-23) was high when measured. Serum 25OH vitamin D was low in 10 of 21 cases measured and 1,25(OH)₂ vitamin D in 12 of 18. Alkaline phosphatase was high in 18 of 22 cases. Bone or muscle pain was reported in 28 of the 30 cases. Twenty patients had pseudofractures, 9 had fractures, and 6 patients had both. All 15 available bone scans showed focal isotope uptake. Case reports tend to report severe cases, so potential reporting bias should be considered. Osteomalacia is a potential complication of repeated iron infusion, especially in patients with gastrointestinal disorders receiving prolonged therapy. Pain and fractures or pseudofractures are common clinical findings, associated with low phosphate, high iFGF-23, high alkaline phosphatase, and abnormal isotope bone scan. Discontinuing or switching the iron formulation was an effective intervention in most cases. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Intravenous Iron-Induced Hypophosphatemia: An Emerging Syndrome

Some, but not all, intravenous iron formulations have been recognized to induce renal phosphate wasting syndrome. Most commonly this has been reported following treatment of iron deficiency anemia (IDA) with ferric carboxymaltose (FCM). A search of PubMed identified relevant randomized controlled trials (RCTs), and case studies evaluating hypophosphatemia (HPP) resulting from intravenous iron treatment. While more recent larger comparative RCTs have confirmed that the majority of patients receiving FCM, especially those with normal renal function, may experience severe HPP, complete documentation is hampered by inconsistent reporting of serum phosphate in such trials. Similarly, while case series and RCTs have documented the persistence of HPP for several weeks or even months, the lack of studies lasting beyond 5–6 weeks has constrained full understanding of the duration of effect. Clinical trials have established that the mechanism involves the bone/metabolic axis with the elevation of intact fibroblast growth factor 23 playing the central role. Reports continue to accumulate of the clinical consequences of severe HPP which are, most commonly, bone abnormalities following repetitive dosing. Case reports and studies, however, have also shown that symptomatic hypophosphatemia can occur after a single FCM dose. The frequency of such events remains unknown, in part due to lack of awareness of hypophosphatemia coupled with the fact that the most common acute symptoms of HPP (fatigue and weakness) are the same for IDA and for many of the chronic diseases that cause IDA. Changes to US and European prescribing information for FCM should raise awareness of the potential for HPP and need to monitor patients at risk for it.

New Therapies for Hypophosphatemia-Related to FGF23 Excess

FGF23 is a hormone produced by osteocytes in response to an elevation in the concentration of extracellular phosphate. Excess production of FGF23 by bone cells, or rarely by tumors, is the hormonal basis for several musculoskeletal syndromes characterized by hypophosphatemia due to renal phosphate wasting. FGF23-dependent chronic hypophosphatemia causes rickets and osteomalacia, as well as other skeletal complications. Genetic disorders of FGF23-mediated hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR), fibrous dysplasia of bone, McCune-Albright syndrome, and epidermal nevus syndrome (ENS), also known as cutaneous skeletal hypophosphatemia syndrome (CSHS). The principle acquired form of FGF23-mediated hypophosphatemia is tumor-induced osteomalacia (TIO). This review summarizes current knowledge about the pathophysiology and clinical presentation of the most common FGF23-mediated conditions, with a focus on new treatment modalities. For many decades, calcitriol and phosphate supplements were the mainstay of therapy. Recently, burosumab, a monoclonal blocking antibody to FGF23, has been approved for treatment of XLH in children and adults, and an active comparator trial in children has shown good efficacy and safety for this drug. The remainder of FGF23-mediated hypophosphatemic disorders continue to be treated with phosphate and calcitriol, although ongoing trials with burosumab for treatment of tumor-induced osteomalacia show early promise. Burosumab may be an effective treatment for the remainder of FGF23-mediated disorders, but clinical trials to support that possibility are at present not available.

Comparative safety of intravenous ferumoxytol versus ferric carboxymaltose in iron deficiency anemia: A randomized trial

Few trials have examined rates of hypersensitivity reactions (HSRs) with intravenous iron formulations used to treat iron deficiency anemia (IDA). This randomized, multicenter, double‐blind clinical trial compared the safety, and efficacy of ferumoxytol versus ferric carboxymaltose (FCM), focusing on rates of HSRs and hypotension as the primary end point. Patients with IDA of any etiology in whom oral iron was unsatisfactory or intolerable received ferumoxytol (n = 997) or FCM (n = 1000) intravenously over ≥15 minutes on days 1 and 8 or 9 for total respective doses of 1.02 g and 1.50 g. Composite incidences of moderate‐to‐severe HSRs, including anaphylaxis, or moderate‐to‐severe hypotension from baseline to week 5 (primary safety end point) were 0.6% and 0.7% in the ferumoxytol and FCM groups, respectively, with ferumoxytol noninferior to FCM. No anaphylaxis was reported in either group. The secondary safety end point of incidences of moderate‐to‐severe HSRs, including anaphylaxis, serious cardiovascular events, and death from baseline to week 5 were 1.3% and 2.0% in the ferumoxytol and FCM groups, respectively (noninferiority test P < .0001). Least‐squares mean changes in hemoglobin at week 5 were 1.4 g/dL and 1.6 g/dL in the ferumoxytol and FCM groups, respectively (noninferiority test P < .0001). Incidence of hypophosphatemia was 0.4% for ferumoxytol and 38.7% for FCM.

Associations Between Intravenous Iron, Inflammation and FGF23 in Non-Dialysis Patients with Chronic Kidney Disease Stages 3-5

BACKGROUND/AIMS

Both iron deficiency and chronic inflammation are highly prevalent in patients with chronic kidney disease (CKD). The effect of intravenous iron infusion on mineral metabolism in CKD may be modified by inflammation. Intravenous iron theraphy may reduce peripheral degradation, secretion, clearence of iFGF23 and lead to hypophosphatemia. The aim of the study was to evaluate the effect of intravenous iron on mineral metabolism in CKD patients.

METHODS

35 non-dialysis patients with CKD stages 3-5. received 100 mg/24h of ferric oxide saccharated solution for 5 days. Serum calcium (Ca), phosphorus (P), parathormone (PTH), intact-FGF23 (iFGF23), C-terminal-FGF23 (cFGF23), bone alkaline phosphatase (BAP) and high-sensitive CRP were assessed on day 1 and 3 at baseline and 2 hours after each dose administration and once on day 6. Plasma iFGF23 and cFGF23, as well as serum BAP were measured with ELISA and other parameters with standard automated laboratory methods.

RESULTS

Serum iFGF23 increased after iv iron on day 1 and 6 (from 268.9±446.5 to 326.3±529.9 on day 1; p=0.05 and to 451.4±601 pg/mL on day 6; p=0.03). cFGF23 was reduced only on day 1 (from 654.3±441.3 to 473.6±414 RU/mL; p=0.016). P concentration decreased significantly two hours after the first iron infusion (from 1.69±0.5 to 1.54±0.35 mmol/l; p=0.003). In following days the changes of cFGF23, P and of other calcium-phosphate metabolism were not significant. Serum CRP correlated neither with iFGF-23 nor cFGF-23.

CONCLUSION

Intravenous iron supplementation may only transiently affect the production and degradation of FGF23 resulting in hypophosphatemia at the commencement of iron therapy. Chronic low-grade inflammation does not seem to play a role in that mechanism.

Comparative safety of intravenous Ferumoxytol versus Ferric Carboxymaltose for the Treatment of Iron Deficiency Anemia: rationale and study design of a randomized double-blind study with a focus on acute hypersensitivity reactions

Background

Intravenous (IV) iron is often used to treat iron deficiency anemia in patients who are unable to tolerate or are inadequately managed with oral iron. However, IV iron treatment has been associated with acute hypersensitivity reactions. The comparative risk of adverse events (AEs) with IV iron preparations has been assessed by a few randomized controlled trials, which are most often limited by small patient numbers, which lack statistical power to identify differences in low-frequency AE such as hypersensitivity reactions.

Materials and methods

Ferumoxytol versus Ferric Carboxymaltose for the Treatment of Iron Deficiency Anemia (FIRM) is a randomized, double-blind, international, multicenter, Phase III study designed to compare the safety of ferumoxytol and ferric carboxymaltose (FCM). The study includes adults with hemoglobin <12.0 g/dL (females) or <14.0 g/dL (males), transferrin saturation ≤20% or ferritin ≤100 ng/mL within 60 days of dosing, and a history of unsatisfactory or nontolerated oral iron therapy or in whom oral iron therapy is inappropriate. Patients are randomized (1:1) to ferumoxytol 510 mg or FCM 750 mg, each given intravenously on days 1 and 8. Primary end points are the incidence of moderate-to-severe hypersensitivity reactions, including anaphylaxis, and moderate-to-severe hypotension. All potential hypersensitivity and hypotensive reactions will be adjudicated by a blinded, independent Clinical Events Committee. A secondary safety end point is the composite frequency of moderate-to-severe hypersensitivity reactions, including anaphylaxis, serious cardiovascular events, and death. Secondary efficacy end points include mean change in hemoglobin and mean change in hemoglobin per milligram of iron administered from baseline to week 5. Urinary excretion of phosphorus and the occurrence of hypophosphatemia after IV iron administration will be examined as well as the mechanisms of such hypophosphatemia in a substudy.

Conclusion

FIRM will provide data on the comparative safety of ferumoxytol and FCM, two IV iron preparations with similar dosing schedules, focusing on moderate-to-severe hypersensitivity reactions, including anaphylaxis, and moderate-to-severe hypotension. The study plans to enroll 2000 patients and is expected to complete in 2017.

Beneficial Effects of Oral Iron in Japanese Patients on Hemodialysis

Objective Iron deficiency anemia (IDA) has become important with regard to mortality in hemodialysis (HD) patients. Therefore, it is necessary to optimize the treatment of these patients. Methods IDA in end-stage renal disease patients on HD was observed in 42 (33.6%) of 125 patients. We examined the influence of daily orally iron [sodium ferrous citrate (SFC) iron/tablet 50 mg, 1-2 tablets] on the renal function markers, anemia and iron data for about 6 months. Results The hematocrit and hemoglobin levels were significantly increased in the patients treated with SFC [hematocrit: before 28.5%±2.1% (mean ± standard deviation), 1st month 30.0%±2.3%, p<0.05; 3rd month 32.4%±2.9%, p<0.05; 6th month 31.3%±3.4%, p<0.05; and hemoglobin: before 9.25±0.70, 1st month 9.72±0.71, p<0.05; 3rd month 10.54±0.96, p<0.05; 6th month 10.25±1.21 g/dL, p<0.05]. The transferrin saturation (TSAT) and serum ferritin levels were significantly increased in the patients treated with SFC (TSAT: before 21.5%±10.0%, 1st-3rd month, 34.1%±15.1%, p<0.05; 6-8th month 34.7%±11.9%, p<0.05; and ferritin: before 38.2±37.1, 6-8th month 67.5±44.0 ng/mL, p<0.05). The present findings clearly indicate that oral iron is an effective route of iron supplementation in HD patients, and no adverse effects associated with SFC occurred during the treatment and follow-up period. Conclusion Our results clearly indicate that oral iron delivered via SFC is a well-tolerated and effective form of iron supplementation in long-term HD and IDA patients in Japan.

A case of osteomalacia due to deranged mineral balance caused by saccharated ferric oxide and short-bowel syndrome: A case report

RATIONALE

Saccharated ferric oxide has been shown to lead to elevation of fibroblast growth factor 23, hypophosphatemia, and, consequently, osteomalacia. Moreover, mineral imbalance is often observed in patients with short-bowel syndrome to some degree.

PATIENT CONCERNS

A 62-year-old woman with short-bowel syndrome related with multiple resections of small intestines due to Crohn disease received regular intravenous administration of saccharated ferric oxide. Over the course of treatment, she was diagnosed with tetany, which was attributed to hypocalcemia. Additional assessments of the patient revealed not only hypocalcemia, but also hypophosphatemia, hypomagnesemia, osteomalacia, and a high concentration of fibroblast growth factor 23 (314 pg/mL).

DIAGNOSES

We diagnosed her with mineral imbalance-induced osteomalacia due to saccharated ferric oxide and short-bowel syndrome.

INTERVENTIONS

Magnesium replacement therapy and discontinuation of saccharated ferric oxide alone.

OUTCOMES

These treatments were able to normalize her serum mineral levels and increase her bone mineral density.

LESSONS

This case suggests that adequate evaluation of serum minerals, including phosphate and magnesium, during saccharated ferric oxide administration may be necessary, especially in patients with short-bowel syndrome.

A Case of an Insufficiency Fracture of the Medial Proximal Tibia Secondary to Osteomalacia Associated with Long-Term Saccharated Ferric Oxide Administration

This article presents a case of insufficiency fracture of medial proximal tibia caused by long-term administration of saccharated ferric oxide (SFO) in a 77-year-old female. In this case, 2-year administration of SFO for iron deficit anemia induced hypophosphatemic osteomalacia and finally resulted in an insufficiency fracture of medial proximal tibia. Hypophosphatemia and pain due to the insufficiency fracture were recovered promptly by withdrawing SFO administration and rest. This case represented varus deformity of the knee associated with osteoarthritis, which may also cause the insufficiency fracture of medial proximal tibia in addition to osteomalacia due to long-term administration of SFO. Long-term SFO administration should be avoided because of a definitive risk of osteomalacia and fragile fracture.

Novel Faces of Fibroblast Growth Factor 23 (FGF23): Iron Deficiency, Inflammation, Insulin Resistance, Left Ventricular Hypertrophy, Proteinuria and Acute Kidney Injury

FGF23 is a hormone that appears as the core regulator of phosphate metabolism. Great deal of data has accumulated to demonstrate increased FGF23 secretion from the bone to compensate for even subtle increases in serum phosphorus long before intact PTH. However, recent evidence points to the fact that actions and interactions of FGF23 are not limited solely to phosphate metabolism. FGF23 may be implicated in iron metabolism and erythropoiesis, inflammation, insulin resistance, proteinuria, acute kidney injury and left ventricular hypertrophy. In this review, we will summarize latest experimental and clinical data examining impact of FGF23 on aforementioned pathophysiologic pathways/disorders.

Efficacy and safety of iron isomaltoside (Monofer(®)) in the management of patients with iron deficiency anemia

New intravenous (IV) iron preparations should ideally be capable of delivering a wide dosing range to allow iron correction in a single or low number of visits, a rapid infusion (doses up to 1,000 mg must be administered over more than 15 minutes and doses exceeding 1,000 mg must be administered over 30 minutes or more), and minimal potential side effects including low catalytic/labile iron release with minimal risk of anaphylaxis. Furthermore, they should be convenient for the patient and health-care professional, and cost effective for the health-care system. The intention behind the development of iron isomaltoside (Monofer(®)) was to fulfill these requirements. Iron isomaltoside has been shown to be effective in treating iron deficiency anemia across multiple therapeutic patient groups and compared to placebo, IV iron sucrose, and oral iron. Iron isomaltoside consists of iron and a carbohydrate moiety where the iron is tightly bound in a matrix structure. It has a low immunogenic potential, a low potential to release labile iron, and does not appear to be associated with clinically significant hypophosphatemia. Due to the structure of iron isomaltoside, it can be administered in high doses with a maximum single dosage of 20 mg/kg body weight. Clinical trials and observational studies of iron isomaltoside show that it is an effective and well-tolerated treatment of anemia across different therapeutic areas with a favorable safety profile.

Low serum levels of zinc, copper, and iron as risk factors for osteoporosis: a meta-analysis

Zinc (Zn), copper (Cu), and iron (Fe) are essential trace elements for the growth, development, and maintenance of healthy bones. However, there are conflicting reports as to the relationship between serum level of Zn, Cu, or Fe and osteoporosis (OP). The purpose of the present study is to clarify the relationship between serum Zn, Cu, or Fe and OP using a meta-analysis approach. We searched all articles indexed in PubMed published up to May 2014 concerning the association between serum level of Zn, Cu, or Fe and OP. Eight eligible articles involving 2,188 subjects were identified. Overall, pooled analysis indicated that patients with OP had a lower serum level of Zn, Cu, or Fe than the healthy controls (Zn standardized mean difference (SMD) = -1.396, 95% confidence interval (CI) = [-2.129, -0.663]; Cu SMD = -0.386, 95% CI = [-0.538, -0.234]; Fe SMD = -0.22, 95% CI = [-0.30, -0.13]). Further subgroup analysis found that geographical location and gender had an influence on the serum level of Zn in OP and healthy controls, but not on the serum level of Cu or Fe. No evidence of publication bias was observed. In conclusion, this meta-analysis suggests that low serum levels of Zn, Cu, and Fe seem to be important risk factors for OP and well-designed studies with adequate control for confounding factors are required in future investigations.

A randomized, open-label, non-inferiority study of intravenous iron isomaltoside 1,000 (Monofer) compared with oral iron for treatment of anemia in IBD (PROCEED)

OBJECTIVES

In the largest head-to-head comparison between an oral and an intravenous (IV) iron compound in patients with inflammatory bowel disease (IBD) so far, we strived to determine whether IV iron isomaltoside 1,000 is non-inferior to oral iron sulfate in the treatment of iron deficiency anemia (IDA).

METHODS

This prospective, randomized, comparative, open-label, non-inferiority study was conducted at 36 sites in Europe and India. Patients with known intolerance to oral iron were excluded. A total of 338 IBD patients in clinical remission or with mild disease, a hemoglobin (Hb) <12 g/dl, and a transferrin saturation (TSAT) <20% were randomized 2:1 to receive either IV iron isomaltoside 1,000 according to the Ganzoni formula (225 patients) or oral iron sulfate 200 mg daily (equivalent to 200 mg elemental iron; 113 patients). An interactive web response system method was used to randomize the eligible patient to the treatment groups. The primary end point was change in Hb from baseline to week 8. Iron isomaltoside 1,000 and iron sulfate was compared by a non-inferiority assessment with a margin of -0.5 g/dl. The secondary end points, which tested for superiority, included change in Hb from baseline to weeks 2 and 4, change in s-ferritin, and TSAT to week 8, number of patients who discontinued study because of lack of response or intolerance of investigational drugs, change in total quality of life (QoL) score to weeks 4 and 8, and safety. Exploratory analyses included a responder analysis (proportion of patients with an increase in Hb ≥2 g/dl after 8 weeks), the effect of regional differences and total iron dose level, and other potential predictors of the treatment response.

RESULTS

Non-inferiority in change of Hb to week 8 could not be demonstrated. There was a trend for oral iron sulfate being more effective in increasing Hb than iron isomaltoside 1,000. The estimated treatment effect was -0.37 (95% confidence interval (CI): -0.80, 0.06) with P=0.09 in the full analysis set (N=327) and -0.45 (95% CI: -0.88, -0.03) with P=0.04 in the per protocol analysis set (N=299). In patients treated with IV iron isomaltoside 1,000, the mean change in s-ferritin concentration was higher with an estimated treatment effect of 48.7 (95% CI: 18.6, 78.8) with P=0.002, whereas the mean change in TSAT was lower with an estimated treatment effect of -4.4 (95% CI: -7.4, -1.4) with P=0.005, compared with patients treated with oral iron. No differences in changes of QoL were observed. The safety profile was similar between the groups. The proportion of responders with Hb ≥2 g/dl (IV group: 67%; oral group: 61%) were comparable between the groups (P=0.32). Iron isomaltoside 1,000 was more efficacious with higher cumulative doses of >1,000 mg IV. Significant predictors of Hb response to IV iron treatment were baseline Hb and C-reactive protein (CRP).

CONCLUSIONS

We could not demonstrate non-inferiority of IV iron isomaltoside 1,000 compared with oral iron in this study. Based on the dose-response relationship observed with the IV iron compound, we suggest that the true iron demand of IV iron was underestimated by the Ganzoni formula in our study. Alternative calculations including Hb and CRP should be explored to gauge iron stores in patients with IBD.

Effects of iron deficiency anemia and its treatment on fibroblast growth factor 23 and phosphate homeostasis in women

Fibroblast growth factor 23 (FGF23) is an osteocyte-derived hormone that regulates phosphate and vitamin D homeostasis. Through unknown mechanisms, certain intravenous iron preparations induce acute, reversible increases in circulating FGF23 levels that lower serum phosphate in association with inappropriately low levels of calcitriol, similar to genetic diseases of primary FGF23 excess. In contrast, studies in wild-type mice suggest that iron deficiency stimulates fgf23 transcription but does not result in hypophosphatemia because FGF23 is cleaved within osteocytes by an unknown catabolic system. We tested the association of iron deficiency anemia with C-terminal FGF23 (cFGF23) and intact FGF23 (iFGF23) levels in 55 women with a history of heavy uterine bleeding, and assessed the longitudinal biochemical response over 35 days to equivalent doses of randomly-assigned, intravenous elemental iron in the form of ferric carboxymaltose (FCM) or iron dextran. Iron deficiency was associated with markedly elevated cFGF23 (807.8 ± 123.9 relative units [RU]/mL) but normal iFGF23 (28.5 ± 1.1 pg/mL) levels at baseline. Within 24 hours of iron administration, cFGF23 levels fell by approximately 80% in both groups. In contrast, iFGF23 transiently increased in the FCM group alone, and was followed by a transient, asymptomatic reduction in serum phosphate <2.0 mg/dL in 10 women in the FCM group compared to none in the iron dextran group. Reduced serum phosphate was accompanied by increased urinary fractional excretion of phosphate, decreased calcitriol levels, and increased parathyroid hormone levels. These findings suggest that iron deficiency increases cFGF23 levels, and that certain iron preparations temporarily increase iFGF23 levels. We propose that intravenous iron lowers cFGF23 in humans by reducing fgf23 transcription as it does in mice, whereas carbohydrate moieties in certain iron preparations may simultaneously inhibit FGF23 degradation in osteocytes leading to transient increases in iFGF23 and reduced serum phosphate.

The effects of iron on FGF23-mediated Ca-P metabolism in CKD patients

BACKGROUND

Fibroblast growth factor 23 (FGF23) is an important counterregulatory hormone for phosphate homeostasis. Since it has been reported that iron administration induces hypophosphatemic osteomalacia by triggering FGF23 synthesis, we hypothesized that iron administration might lead to a further increase in FGF23, resulting in alterations to Ca-P metabolism in a stage 5 CKD population.

METHODS

This cross-sectional study was performed in a single center, and involved 73 hemodialysis patients (47.7 ± 15.74 years old, 68.5% men), 29 peritoneal dialysis patients (44.55 ± 15.05 years old, 62.1% men), and 55 healthy (43.57 ± 14.36 years old, 55.6% men) subjects. The dialysis group was subcategorized according to iron therapy administration into users and nonusers.

RESULTS

The median iFGF23 level was significantly higher in the dialysis population than in the healthy controls [88.050 (25.2-1038.3) pg/ml versus 46.95 (2.4-356) pg/ml (p < 0.001)]. In the dialysis population, a significantly lower median iFGF23 level was observed in iron therapy users than in nonusers [87.6 (25.2-1038.3) versus 119 (51.6-1031); respectively, p = 0.045]. A significant negative association between iron administration and iFGF23 level was revealed by both univariate (r = -0.237, p = 0.016) and multivariate (β = -0.221, p = 0.032) analysis. No association was found between iFGF23 and serum ferritin and iron levels. Also, there was no association between iron therapy and serum phosphate level.

CONCLUSION

In contrast to what is seen for the general population, this study showed that there was a negative relationship between iron administration and serum iFGF23 level in a dialysis population. We can therefore conclude that if high levels of FGF23 are harmful, iron therapy may have a beneficial effect on bone metabolism by reducing FGF23 levels in a dialysis population.

Tumor-induced osteomalacia: an important cause of adult-onset hypophosphatemic osteomalacia in China: Report of 39 cases and review of the literature

Tumor-induced osteomalacia (TIO) is an acquired form of hypophosphatemia. Tumor resection leads to cure. We investigated the clinical characteristics of TIO, diagnostic methods, and course after tumor resection in Beijing, China, and compared them with 269 previous published reports of TIO. A total of 94 patients with adult-onset hypophosphatemic osteomalacia were seen over a 6-year period (January, 2004 to May, 2010) in Peking Union Medical College Hospital. After physical examination (PE), all patients underwent technetium-99m octreotide scintigraphy ((99) Tc(m) -OCT). Tumors were removed after localization. The results demonstrated that 46 of 94 hypophosphatemic osteomalacia patients had high uptake in (99) Tc(m) -OCT imaging. Forty of them underwent tumor resection with the TIO diagnosis established in 37 patients. In 2 patients, the tumor was discovered on PE but not by (99) Tc(m) -OCT. The gender distribution was equal (M/F = 19/20). Average age was 42 ± 14 years. In 35 patients (90%), the serum phosphorus concentration returned to normal in 5.5 ± 3.0 days after tumor resection. Most of the tumors (85%) were classified as phosphaturic mesenchymal tumor (PMT) or mixed connective tissue variant (PMTMCT). Recurrence of disease was suggested in 3 patients (9%). When combined with the 269 cases reported in the literature, the mean age and sex distribution were similar. The tumors were of bone (40%) and soft tissue (55%) origins, with 42% of the tumors being found in the lower extremities. In summary, TIO is an important cause of adult-onset hypophosphatemia in China. (99) Tc(m) -OCT imaging successfully localized the tumor in the overwhelming majority of patients. Successful removal of tumors leads to cure in most cases, but recurrence should be sought by long-term follow-up.

Iron deficiency drives an autosomal dominant hypophosphatemic rickets (ADHR) phenotype in fibroblast growth factor-23 (Fgf23) knock-in mice

Autosomal dominant hypophosphatemic rickets (ADHR) is unique among the disorders involving Fibroblast growth factor 23 (FGF23) because individuals with R176Q/W and R179Q/W mutations in the FGF23 (176)RXXR(179)/S(180) proteolytic cleavage motif can cycle from unaffected status to delayed onset of disease. This onset may occur in physiological states associated with iron deficiency, including puberty and pregnancy. To test the role of iron status in development of the ADHR phenotype, WT and R176Q-Fgf23 knock-in (ADHR) mice were placed on control or low-iron diets. Both the WT and ADHR mice receiving low-iron diet had significantly elevated bone Fgf23 mRNA. WT mice on a low-iron diet maintained normal serum intact Fgf23 and phosphate metabolism, with elevated serum C-terminal Fgf23 fragments. In contrast, the ADHR mice on the low-iron diet had elevated intact and C-terminal Fgf23 with hypophosphatemic osteomalacia. We used in vitro iron chelation to isolate the effects of iron deficiency on Fgf23 expression. We found that iron chelation in vitro resulted in a significant increase in Fgf23 mRNA that was dependent upon Mapk. Thus, unlike other syndromes of elevated FGF23, our findings support the concept that late-onset ADHR is the product of gene-environment interactions whereby the combined presence of an Fgf23-stabilizing mutation and iron deficiency can lead to ADHR.

Recent advances in renal phosphate handling

Phosphate is critical for the maintenance of skeletal integrity, is a necessary component of important biomolecules, and is central to signal transduction and cell metabolism. It is becoming clear that endocrine communication between the skeleton, kidney, and the intestine is involved in maintaining appropriate serum phosphate concentrations, and that the kidney is the primary site for minute-to-minute regulation of phosphate levels. The identification of genetic alterations in Mendelian disorders of hypophosphatemia and hyperphosphatemia has led to the isolation of novel genes and the identification of new roles for existing proteins--such as fibroblast growth factor 23 and its processing systems, the co-receptor alpha-klotho, and phosphate transporters--in the control of renal phosphate handling. Recent findings also indicate that fibroblast growth factor 23 has feedback mechanisms involving parathyroid hormone and vitamin D that control phosphate homeostasis. This Review will highlight genetic, in vitro and in vivo findings, and will discuss how these clinical and experimental discoveries have uncovered novel aspects of renal phosphate handling and opened new research and therapeutic avenues.

FGF23 elevation and hypophosphatemia after intravenous iron polymaltose: a prospective study

CONTEXT

Parenteral iron administration has been associated with hypophosphatemia. Fibroblast growth factor 23 (FGF23) has a physiological role in phosphate homeostasis via suppression of 25-hydroxyvitamin D [25(OH)D] activation and promotion of phosphaturia. We recently reported a case of iron-induced hypophosphatemic osteomalacia associated with marked FGF23 elevation.

OBJECTIVE

Our objective was to prospectively investigate the effect of parenteral iron polymaltose on phosphate homeostasis and to determine whether any observed change was related to alterations in circulating FGF23.

DESIGN, SETTING, AND PARTICIPANTS

Eight medical outpatients prescribed iv iron polymaltose were recruited. Plasma phosphate, 25(OH)D, 1,25-dihydroxyvitamin D [1,25(OH)(2)D], PTH, FGF23, and urinary tubular reabsorption of phosphate were measured prior to iron administration and then weekly for a minimum of 3 wk.

RESULTS

Plasma phosphate fell from 3.4 +/- 0.6 mg/dl at baseline to 1.8 +/- 0.6 mg/dl at wk 1 (P < 0.0001) associated with a fall in percentage tubular reabsorption of phosphate (90 +/- 4.8 to 68 +/- 13; P < 0.001) and 1,25(OH)(2)D (54 +/- 25 to 9 +/- 8 pg/ml; P < 0.001). These indices remained significantly suppressed at wk 2 and 3. 25(OH)D levels were unchanged. FGF23 increased significantly from 43.5 pg/ml at baseline to 177 pg/ml at wk 1 (P < 0.001) with levels correlating with both serum phosphate (R = -0.74; P <0.05) and 1,25(OH)(2)D (R = -0.71; P < 0.05).

CONCLUSION

Parenteral iron suppresses renal tubular phosphate reabsorption and 1alpha-hydroxylation of vitamin D resulting in hypophosphatemia. Our data suggest that this is mediated by an increase in FGF23.

Iron polymaltose-induced FGF23 elevation complicated by hypophosphataemic osteomalacia

Iron-induced renal phosphate wasting, hypophosphataemia and osteomalacia have previously been reported in a small number of Japanese patients receiving parenteral iron sucrose. We report the case history of a European male who, as a result of regular intravenous iron polymaltose, developed prolonged hypophosphataemia complicated by widespread insufficiency fractures. The pathogenesis of this complication remains unknown however our novel finding of a marked elevation in fibroblast growth factor 23 (FGF23), which normalized after ceasing parenteral iron, suggests an important and previously unreported effect of iron on FGF23 homeostasis.

Role of iron metabolism and oxidative damage in postmenopausal bone loss

It has been suggested that iron-deficient rats have lower bone mass than iron-replete animals, but a clear association between bone and iron repletion has not been demonstrated in humans. A growing body of evidences also suggests a relation between lipid oxidation and bone metabolism and between iron metabolism and LDL oxidation. Iron availability to cells also depends on haptoglobin (Hp) phenotypes. Hp has also important antioxidant properties according to its phenotype, hence we evaluate whether Hp phenotype could influence bone density, iron metabolism and lipid oxidation. This cross-sectional study enrolled 455 postmenopausal women affected by osteoporosis (260) or not (195). Bone mineral density, markers of bone and iron metabolism, levels of oxidized LDL (oxLDL) and Hp phenotype were measured in all the subjects. Hp 1.1 and 2.2 frequency was higher and Hp 2.1 was lower in the patients with fragility fractures (80) compared with the controls. We therefore evaluate different Hp phenotypes as risk or protective factors against fragility fracture: Hp 2.1 is a protective factor against fracture while 1.1 is an important and 2.2 a moderate risk factor for fragility fractures. Lower serum iron was associated with elevated transferrin in patients with Hp 1.1; moreover patients had relative iron deficiency compared with the controls and fractured patients had higher level of oxLDL. We found that both iron metabolism and oxLDL varies according to Hp phenotypes and are predictive of bone density. Our data indicate that Hp 2.1 is a protective factor for fragility fractures, depending on its role on iron metabolism and its antioxidant properties.

Effect of saccharated ferric oxide and iron dextran on the metabolism of phosphorus in rats

As a means of investigating the physiologic damage to and histologic deterioration of the kidney caused by saccharated ferric oxide (SFO) and iron dextran (ID), we administered these substances intraperitoneally to rats. The rats were divided into 3 groups. Group 1 rats ( n = 7) were given SFO, 28 mg/kg for 9 days and 20 mg/kg for 19 days. Group 2 rats ( n = 5) were given ID, 28 mg/kg for 9 days and 20 mg/kg for 19 days. Group 3 rats (control, n = 9) were given normal saline solution. After 28 days, serum calcium, total protein, and albumin concentrations were significantly lower in the SFO group than in the ID group. Serum phosphorus concentrations were significantly lower in the SFO group than in the control group. Serum iron concentrations were significantly higher in the ID group than in the SFO group or the control, and the fractional excretion of sodium was significantly lower in the ID group than in the control group. The percent tubular reabsorption of phosphorus was significantly greater in the ID group than in the SFO group or the control group, and the theoretical threshold concentration of phosphorus was also significantly lower in the SFO group than in the ID or the control group. Histologic examination of the kidney after treatment revealed neither iron in the tubules nor any structural damage to the tubules. ID was not found to induce hypophosphatemia, whereas SFO did. We believe that the cause of such hypophosphatemia is impaired tubular reabsorption.

Iron lactate-induced osteomalacia in association with osteoblast dynamics

Osteomalacia was induced in rats fed a diet containing 50,000 ppm (5%) iron lactate for 13 weeks. The histopathology and histomorphometrical dynamics of osteoblasts under this condition were examined. Bone histomorphometry of the proximal tibial metaphysis revealed that the osteoblast surface, osteoid volume, osteoid surface and labeled surface ratio, which are the parameters of bone formation had increased. The blood chemistry revealed the greatest elevation in the osteocalcin level; however, the parathyroid hormone (PTH) secretion and inorganic phosphorus level were very low. From the serum biochemical, histopathological and histomorphometrical findings, the bone lesion in iron lactate-overloaded rats was considered to be similar to low turnover osteomalacia showing decreased trabeculae in secondary spongiosa and increased lamellar osteoid. Furthermore, an iron-positive reaction was detected at the interface between osteoid and mineralized bone. In the bone lesions induced by chronic iron overload, osteoblast recruitment exceeded that of mineralization or, alternatively, the iron within osteoblasts along the trabecular bone suppressed the remodeling and led to an increase in osteoid thickness.