Post-transplant hypophosphatemia

Fibroblast growth factor 23-Klotho and mineral metabolism in the first year after pediatric kidney transplantation: A single-center prospective study

BACKGROUND

The role of fibroblast growth factor 23 (FGF23) levels in mineral metabolism before and after kidney transplantation in pediatric patients is poorly understood.

METHODS

We prospectively evaluated 24 patients under 18 years of age (4.5 [3.3-9.8] years) who underwent living kidney transplantation between July 2016 and March 2018, and measured intact FGF23 and serum αKlotho levels, and other parameters of mineral metabolism before and after transplantation (Day 7, 1 and 4 months, and 1 year). Relationships between parameters were examined by linear analysis.

RESULTS

FGF23 level was 440.8 [63.4-5916.3] pg/ml pre-transplant and decreased significantly to 37.1 [16.0-71.5] pg/ml at Day 7 post-transplant (-91.6%, p < .001). Thereafter, it remained at normal levels until 1 year. αKlotho level was 785 [568-1292] pg/ml pre-transplant and remained low at Day 7 and 1 month post-transplant, with an increasing trend at 4 months. Post-transplant phosphorus levels were significantly decreased compared with pre-transplant, with a lowest level of 1.7 [1.3-2.9] mg/dl, -5.7 [-6.8, -3.8] SD at Day 4, followed by gradual recovery. Phosphorus levels and the ratio of tubular maximum phosphate reabsorption were significantly and negatively associated with pre-transplant FGF23 until 4 months of post-transplant. Pre-transplant αKlotho was negatively associated with pre-transplant FGF23 but not FGF23 or other parameters after transplantation.

CONCLUSION

FGF23 in pediatric kidney transplant patients decreased rapidly after transplantation and associated with post-transplant hypophosphatemia and increased phosphorus excretion. Post-transplant αKlotho was low early post-transplant but tended to increase subsequently. Post-transplant αKlotho was unaffected by pre-transplant FGF23 or other factors, suggesting pre-transplant chronic kidney disease status has no effect.

Lessons from effect of etelcalcetide on left ventricular hypertrophy in patients with end-stage kidney disease

PURPOSE OF REVIEW

Patients with end-stage kidney disease (ESKD) frequently develop left ventricular hypertrophy (LVH), which is associated with an exceptionally high risk of cardiovascular events and mortality. This review focuses on interventional studies that modify levels of fibroblast growth factor 23 (FGF23) and examine effects on myocardial hypertrophy, cardiovascular events and mortality.

RECENT FINDINGS

Quantitative evaluations of trials of calcimimetics found no effects on cardiovascular events and cardiovascular and all-cause mortality when compared with placebo. However, a recent randomized, controlled trial of etelcalcetide versus alfacalcidol showed that etelcalcetide effectively inhibited the progression of LVH in comparison to vitamin D in patients on haemodialysis after 1 year of treatment. Prior to that, oral calcimimetic treatment has already been shown to reduce left ventricular mass in patients on haemodialysis, whereas treatment with active vitamin D or mineralocorticoids was ineffective in patients with ESKD.

SUMMARY

Data from a recent trial of etelcalcetide on LVH suggest that FGF23 may be a possible therapeutic target for cardiac risk reduction in patients on haemodialysis. If these findings are confirmed by further research, it might be speculated that a treatment shift from active vitamin D towards FGF23-lowering therapy may occur in patients on haemodialysis.

Natural History of Bone Disease following Kidney Transplantation

BACKGROUND

Knowledge of the effect of kidney transplantation on bone is limited and fragmentary. The aim of this study was to characterize the evolution of bone disease in the first post-transplant year.

METHODS

We performed a prospective, observational cohort study in patients referred for kidney transplantation under a steroid-sparing immunosuppressive protocol. Bone phenotyping was done before, or at the time of, kidney transplantation, and repeated at 12 months post-transplant. The phenotyping included bone histomorphometry, bone densitometry by dual-energy x-ray absorptiometry, and biochemical parameters of bone and mineral metabolism.

RESULTS

Paired data were obtained for 97 patients (median age 55 years; 72% male; 21% of patients had diabetes). Bone turnover remained normal or improved in the majority of patients (65%). Bone histomorphometry revealed decreases in bone resorption (eroded perimeter, mean 4.6% pre- to 2.3% post-transplant; P<0.001) and disordered bone formation (fibrosis, 27% pre- versus 2% post-transplant; P<0.001). Whereas bone mineralization was normal in all but one patient pretransplant, delayed mineralization was seen in 15% of patients at 1 year post-transplant. Hypophosphatemia was associated with deterioration in histomorphometric parameters of bone mineralization. Changes in bone mineral density were highly variable, ranging from -18% to +17% per year. Cumulative steroid dose was related to bone loss at the hip, whereas resolution of hyperparathyroidism was related to bone gain at both spine and hip.

CONCLUSIONS

Changes in bone turnover, mineralization, and volume post-transplant are related both to steroid exposure and ongoing disturbances of mineral metabolism. Optimal control of mineral metabolism may be key to improving bone quality in kidney transplant recipients.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Evolution of Bone Histomorphometry and Vascular Calcification Before and After Renal Transplantation, NCT01886950.

Clinical factors associated with severe hypophosphataemia after kidney transplant

BACKGROUND

The mechanism by which hypophosphataemia develops following kidney transplantation remains debated, and limited research is available regarding risk factors. This study aimed to assess the association between recipient and donor variables, and the severity of post-transplantation hypophosphataemia.

METHODS

We performed a single-centre retrospective observational study. We assessed the association between demographic, clinical and biochemical variables and the development of hypophosphataemia. We used linear regression analysis to assess association between these variables and phosphate nadir.

RESULTS

87.6% of patients developed hypophosphataemia. Patients developing hypophosphataemia were younger, had a shorter time on renal replacement therapy, were less likely to have had a parathyroidectomy or to experience delayed graft function, were more likely to have received a living donor transplant, from a younger donor. They had higher pre-transplantation calcium levels, and lower alkaline phosphatase levels. Receipt of a living donor transplant, lower donor age, not having had a parathyroidectomy, receiving a transplant during the era of tacrolimus-based immunosuppression, not having delayed graft function, higher pre-transplantation calcium, and higher pre-transplantation phosphate were associated with lower phosphate nadir by multiple linear regression.

CONCLUSIONS

This analysis demonstrates an association between variables relating to better graft function and hypophosphataemia. The links with biochemical measures of mineral-bone disease remain less clear.

Association of time-updated plasma calcium and phosphate with graft and patient outcomes after kidney transplantation

Disturbances in calcium-phosphate homeostasis are common after kidney transplantation. We aimed to assess the relationship between deregulations in plasma calcium and phosphate over time and mortality and death-censored graft failure (DCGF). In this prospective cohort study, we included kidney transplant recipients with ≥2 plasma calcium and phosphate measurements. Data were analyzed using time-updated Cox regression analyses adjusted for potential confounders including time-updated kidney function. We included 2769 patients (mean age 47 ± 14 years, 42.3% female) with 138 496 plasma calcium and phosphate levels (median [IQR] 43 [31-61] measurements per patient). During follow-up of 16.3 [8.7-25.2] years, 17.2% developed DCGF and 7.9% died. Posttransplant hypercalcemia was associated with an increased risk of mortality (1.63 [1.31-2.00], p < 0.0001), but not with DCGF. Hyperphosphatemia was associated with both DCGF (2.59 [2.05-3.27], p < .0001) and mortality (3.14 [2.58-3.82], p <  .0001). Only the association between hypercalcemia and mortality remained significant in sensitivity analyses censored by a simultaneous eGFR <45 mL/min/1.73 m2 . Hypocalcemia and hypophosphatemia were not consistently associated with either outcome. Posttransplant hypercalcemia, even in the presence of preserved kidney function, was associated with an increased mortality risk. Associations of hyperphosphatemia with DCGF and mortality may be driven by eGFR.

Bone Mineral Disease After Kidney Transplantation

Chronic kidney disease-mineral bone disorder (CKD-MBD) after kidney transplantation is a mix of pre-existing disorders and new alterations. The final consequences are reflected fundamentally as abnormal mineral metabolism (hypercalcemia, hypophosphatemia) and bone alterations [high or low bone turnover disease (as fibrous osteitis or adynamic bone disease), an eventual compromise of bone mineralization, decrease bone mineral density and bone fractures]. The major cause of post-transplantation hypercalcemia is the persistence of severe secondary hyperparathyroidism, and treatment options include calcimimetics or parathyroidectomy. On turn, hypophosphatemia is caused by both the persistence of high blood levels of PTH and/or high blood levels of FGF23, with its correction being very difficult to achieve. The most frequent bone morphology alteration is low bone turnover disease, while high-turnover osteopathy decreases in frequency after transplantation. Although the pathogenic mechanisms of these abnormalities have not been fully clarified, the available evidence suggests that there are a number of factors that play a very important role, such as immunosuppressive treatment, persistently high levels of PTH, vitamin D deficiency and hypophosphatemia. Fracture risk is four-fold higher in transplanted patients compared to general population. The most relevant risk factors for fracture in the kidney transplant population are diabetes mellitus, female sex, advanced age (especially > 65 years), dialysis vintage, high PTH levels and low phosphate levels, osteoporosis, pre-transplant stress fracture and high doses or prolonged steroids therapy. Treatment alternatives for CKD-MBD after transplantation include minimization of corticosteroids, use of calcium and vitamin D supplements, antiresorptives (bisphosphonates or Denosumab) and osteoformers (synthetic parathyroid hormone). As both mineral metabolism and bone disorders lead to increased morbidity and mortality, the presence of these changes after transplantation has to be prevented (if possible), minimized, diagnosed, and treated as soon as possible.

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.

Calcium and phosphate levels after kidney transplantation and long-term patient and allograft survival

Background

Non-traditional cardiovascular risk factors, including calcium and phosphate derangement, may play a role in mortality in renal transplant. The data regarding this effect are conflicting. Our aim was to assess the impact of calcium and phosphate derangements in the first 90 days post-transplant on allograft and recipient outcomes.

Methods

We performed a retrospective cohort review of all-adult, first renal transplants in the Republic of Ireland between 1999 and 2015. We divided patients into tertiles based on serum phosphate and calcium levels post-transplant. We assessed their effect on death-censored graft survival and all-cause mortality. We used Stata for statistical analysis and did survival analysis and spline curves to assess the association.

Results

We included 1525 renal transplant recipients. Of the total, 86.3% had hypophosphataemia and 36.1% hypercalcaemia. Patients in the lowest phosphate tertile were younger, more likely female, had lower weight, more time on dialysis, received a kidney from a younger donor, had less delayed graft function and better transplant function compared with other tertiles. Patients in the highest calcium tertile were younger, more likely male, had higher body mass index, more time on dialysis and better transplant function. Adjusting for differences between groups, we were unable to show any difference in death-censored graft failure [phosphate = 1.14, 95% confidence interval (CI) 0.92–1.41; calcium = 0.98, 95% CI 0.80–1.20] or all-cause mortality (phosphate = 1.10, 95% CI 0.91–1.32; calcium = 0.96, 95% CI 0.81–1.13) based on tertiles of calcium or phosphate in the initial 90 days.

Conclusions

Hypophosphataemia and hypercalcaemia are common occurrences post-kidney transplant. We have identified different risk factors for these metabolic derangements. The calcium and phosphate levels exhibit no independent association with death-censored graft failure and mortality.

Pathogenesis and treatment of electrolyte problems post transplant

PURPOSE OF REVIEW

Electrolyte abnormalities posttransplant are common occurrences that can have significant short-term and long-term effects on graft outcome and patient quality of life. Understanding the pathophysiology of these electrolyte derangements can help guide management to optimize bone health and minimize cardiovascular disease. This review explores the pathogenesis of the most common postrenal transplant electrolytes abnormalities as well as current treatment options.

RECENT FINDINGS

Clarifications of the role of FGF-23 has improved our understanding of posttransplant bone disease in addition to the known roles of hyperparathyroidism and vitamin D. The mechanisms of renal electrolyte wasting by immunosuppressive agents give insight into potential treatment options for hyperkalemia and hypomagnesemia.

SUMMARY

Understanding the pathogenesis of the common electrolyte abnormalities found post renal transplant may lead to targeted treatment options that in turn may improve transplant complications. Further studies are required to evaluate the effects on long-term outcomes of renal allografts.

Uremic Toxins and Clinical Outcomes: The Impact of Kidney Transplantation

Non-transplanted and transplanted patients with chronic kidney disease (CKD) differ in terms of mortality and the risk of clinical events. This difference is probably due to the difference of both traditional and non-traditional risk factors. Uremic retention solutes may constitute important non-traditional risk factors in this population. In the present review, we selected a set of uremic toxins that have been associated with harmful effects, and are an appealing target for adjuvant therapy in CKD. For each toxin reviewed here, relevant studies were selected and the relationship with hard clinical outcomes of uremic toxins were compared between non-transplanted CKD patients and transplanted patients taking into account the level of glomerular filtration rate in these two situations.

FGF23 in kidney transplant: the strange case of Doctor Jekyll and Mister Hyde

During the last decade, a new view into the molecular mechanisms of chronic kidney disease-mineral bone disorder (CKD-MBD) has been proposed, with fibroblast growth factor 23 (FGF23) as a novel player in the field. Enhanced serum FGF23 levels cause a reduction in serum phosphate, together with calcitriol suppression and consequent hyperparathyroidism (HPT). In contrast, reduced serum FGF23 levels are associated with hyperphosphatemia, higher calcitriol levels and parathyroid hormone (PTH) suppression. In addition, serum FGF23 levels are greatly increased and positively correlated with serum phosphate levels in CKD patients. In this population, high serum FGF23 concentration seems to predict the occurrence of refractory secondary HPT and to be associated with higher mortality risk in incident haemodialysis patients. In living-donor kidney transplant recipients, a faster normalization of FGF23 and phosphate levels with a lower prevalence of HPT, may be considered a major pathway to investigate.

FGF23 is associated with early post-transplant hypophosphataemia and normalizes faster than iPTH in living donor renal transplant recipients: a longitudinal follow-up study

Background

We aimed to longitudinally analyse changes in the levels of serum fibroblast growth factor 23 (FGF23), intact parathyroid hormone (iPTH) and associated minerals in patients undergoing renal transplantation.

Methods

Sixty-three patients with end-stage renal disease (ESRD) who underwent living donor transplantation were recruited. Serum FGF23, iPTH, uric acid, inorganic phosphorous (iP), blood urea nitrogen and serum creatinine were measured pre-transplant and at 1 (M1), 3 (M3) and 12 months (M12) post-transplantation.

Results

FGF23 levels were decreased at M1, M3 and M12 by 93.81, 96.74 and 97.53%, respectively. iPTH levels were decreased by 67.95, 74.95 and 84.9%, respectively. The prevalence of hyperparathyroidism at M1, M3 and M12 post-transplantation was 63.5, 42.9 and 11.1%, respectively. FGF23 and iP levels remained above the normal range in 23 (36.5%) and 17 (27%) patients at M1, 10 (15.9%) and 5 (8%) at M3 and in none at M12 post-transplantation, respectively. A multivariate regression model revealed that, pre-transplant, iP was positively associated with iPTH (P = 0.016) but not with FGF 23; however, post-transplant, iP level was negatively associated with FGF23 (P < 0.001) but not with iPTH.

Conclusions

Post-transplant FGF23 levels settle faster than those of iPTH. However, 11% of patients continued to have hyperparathyroidism even after 12 months.

Phosphate and FGF-23 homeostasis after kidney transplantation

Dysregulated phosphate metabolism is a common consequence of chronic kidney disease, and is characterized by a high circulating level of fibroblast growth factor (FGF)-23, hyperparathyroidism, and hyperphosphataemia. Kidney transplantation can elicit specific alterations to phosphate metabolism that evolve over time, ranging from severe hypophosphataemia (<0.5 mmol/l) to hyperphosphataemia (>1.50 mmol/l) and high FGF-23 levels. The majority of renal transplant recipients develop hypophosphataemia during the first 3 months after transplantation as a consequence of relatively slow adaptation of FGF-23 and parathyroid hormone levels to restored renal function, and the influence of immunosuppressive drugs. By 3-12 months after transplantation, phosphate homeostasis is at least partially restored in the majority of recipients, which is paralleled by a substantially reduced risk of cardiovascular-associated morbidity and mortality compared with the pre-transplantation setting. Many renal transplant recipients, however, exhibit persistent abnormalities in phosphate homeostasis, which is often due to multifactorial causes, and may contribute to adverse outcomes on the cardiovascular system, kidney, and bone. Dietary and pharmacologic interventions might improve phosphate homeostasis in renal transplant recipients, but additional insight into the pathophysiology of transplantation-associated abnormalities in phosphate homeostasis is needed to further optimize disease management and improve prognosis for renal transplant recipients.

Long-term evolution of the mineral metabolism after renal transplantation: a prospective, single-center cohort study

BACKGROUND

Abnormalities in bone and mineral metabolism are common after renal transplantation (RT) but information on their long-term time course is scarce.

OBJECTIVES

(1) Evaluate the time course of biochemical parameters of bone and mineral metabolism over 60 months after RT; (2) identify predictors for persistent hyperparathyroidism (HPT).

DESIGN

Prospective, longitudinal, single-center cohort study.

METHODS

We determined serum levels (mean values ± standard deviations) of intact parathyroid hormone (iPTH), calcium (Ca), phosphorus (P), magnesium (Mg), alkaline phosphatase (APh), calcitriol, and creatinine (Cr) preoperatively as well as 6, 12, 24, 36, 48, and 60 months after cadaveric RT in 49 patients. We in addition recorded demographic, clinical, and therapeutic data.

RESULTS

Pretransplantation iPTH stabilized from 194.2 ± 273.5 to 71.5 ± 50.7 ng/L at 6 months. Serum Ca (9.5 ± 1.1 mg/dL) and APh (81.9 ± 42.1 U/L) did not change. Baseline serum P (5.7 ± 1.8 mg/dL) and serum Mg (2.4 ± 0.4 mg/dL) decreased to normal ranges from month 6 onward. Low baseline calcitriol (22.4 ± 21.8 pmol/L) normalized slowly by 12 months (95.4 ± 46.7 pmol/L). Stable graft function (53.2 ± 15.8 mL/min) was achieved from 6 months onward. By 60 months, 26.5% of patients had a serum Ca above 9.8 mg/dL and serum P below 2.7 mg/dL; 22.4%, an Mg below 1.7 mg/dL and 8.2%, a serum iPTH more than 2.5-fold the upper limit of normal. Upon multiple regression analyses the iPTH at 60 months was influenced by a dialysis duration ≥ 2 years (β = 0.259, P = .003), body mass index > 25 kg/m(2) (β = 0.257, P = .006), baseline iPTH (β = 0.182, P = .036), serum Cr (β = 0.268, P = .002) and Mg (β = -0.242, P = .006).

CONCLUSIONS

Hypercalcemia, hypophosphatemia, hypomagnesemia, and elevated iPTH persist in a subset of post-RT patients. Pretransplantation iPTH and obesity, dialysis duration, and posttransplant serum creatininemia and hypomagnesemia independently contribute to persistent HPT.

Systemic and nonrenal adverse effects occurring in renal transplant patients treated with mTOR inhibitors

The mammalian target of rapamycin inhibitors (mTOR-I), sirolimus and everolimus, are immunosuppressive drugs largely used in renal transplantation. The main mechanism of action of these drugs is the inhibition of the mammalian target of rapamycin (mTOR), a regulatory protein kinase involved in lymphocyte proliferation. Additionally, the inhibition of the crosstalk among mTORC1, mTORC2, and PI3K confers the antineoplastic activities of these drugs. Because of their specific pharmacological characteristics and their relative lack of nephrotoxicity, these inhibitors are valid option to calcineurine inhibitors (CNIs) for maintenance immunosuppression in renal transplant recipients with chronic allograft nephropathy. However, as other immunosuppressive drugs, mTOR-I may induce the development of several adverse effects that need to be early recognized and treated to avoid severe illness in renal transplant patients. In particular, mTOR-I may induce systemic nonnephrological side effects including pulmonary toxicity, hematological disorders, dysmetabolism, lymphedema, stomatitis, cutaneous adverse effects, and fertility/gonadic toxicity. Although most of the adverse effects are dose related, it is extremely important for clinicians to early recognize them in order to reduce dosage or discontinue mTOR-I treatment avoiding the onset and development of severe clinical complications.

Serum level of fibroblast growth factor 23 in maintenance renal transplant patients

BACKGROUND

The discovery of fibroblast growth factor 23 (FGF23) provides a new conceptual framework that improves our understanding of the pathogenesis of post-transplant bone disease. Excess FGF23 is produced in the early post-transplant period; levels return to normal in the months following transplant. However, few manuscripts discuss FGF23 levels in stable long-term renal transplant recipients.

METHODS

We performed a cross-sectional observational study of 279 maintenance kidney recipients with chronic kidney disease (CKD) Stages 1-4 and stable allograft function who had received their transplant at least 12 months previously. We calculated the estimated GFR (eGFR) using the MDRD4 equation.

RESULTS

FGF23, parathyroid hormone (PTH) and phosphorus values were higher in more advanced stages, while the serum calcitriol levels and the phosphate reabsorption rate were lower. A significant inverse correlation was found between eGFR and FGF23 (r = -0.487; P < 0.001), PTH (r = -0.444; P < 0.001), serum phosphate levels (r = -0.315; P < 0.001) and fractional excretion of magnesium (r = -0.503; P < 0.001). Multivariable analysis showed that increased time on corticosteroids (P < 0.001), PTH (P < 0.001), serum phosphate (P = 0.003), decreased serum calcitriol (P = 0.049) and estimated glomerular filtration (P = 0.003) rate were associated with high FGF23 levels. In contrast with pre-transplant patients and first year post-transplant patients, higher FGF23 values were not correlated with increased phosphate excretion. An elevated phosphate reabsorption rate was associated with decreased PTH (P < 0.001) and calciuria (P = 0.028) and increased serum calcitriol (P = 0.009), plasma bicarbonate (P = 0.024) and estimated glomerular filtration (P = 0.003).

CONCLUSIONS

Serum FGF23 concentrations remain increased in long-term kidney graft recipients, even in the early stages of CKD. It remains to be seen whether measures aimed at reducing serum levels of PTH and phosphate and/or corticosteroid doses might help to lower serum FGF23 and whether this will improve kidney recipient outcomes.

Metabolic bone diseases in kidney transplant recipients

Metabolic bone disease after kidney transplantation has a complex pathophysiology and heterogeneous histology. Pre-existing renal osteodystrophy may not resolve completely, but continue or evolve into a different osteodystrophy. Rapid bone loss immediately after transplant can persist, at a lower rate, for years to come. These greatly increase the risk of bone fracture and vertebral collapse. Each patient may have multiple risk factors of bone loss, such as steroids usage, hypogonadism, persistent hyperparathyroidism (HPT), poor allograft function, metabolic acidosis, hypophosphatemia, vitamin D deficiency, aging, immobility and chronic disease. Clinical management requires a comprehensive approach to address the underlying and ongoing disease processes. Successful prevention of bone loss has been shown with vitamin D, bisphosphonates, calcitonin as well as treatment of hypogonadism and HPT. Novel approach to restore the normal bone remodeling and improve the bone quality may be needed in order to effectively decrease bone fracture rate in kidney transplant recipients.

Management of mineral and bone disorder after kidney transplantation

PURPOSE OF REVIEW

Mineral and bone disorders (MBDs), inherent complications of moderate and advanced chronic kidney disease, occur frequently in kidney transplant recipients. However, much confusion exists about the clinical application of diagnostic tools and preventive or treatment strategies to correct bone loss or mineral disarrays in transplanted patients. We have reviewed the recent evidence about prevalence and consequences of MBD in kidney transplant recipients and examined diagnostic, preventive and therapeutic options to this end.

RECENT FINDINGS

Low turnover bone disease occurs more frequently after kidney transplantation according to bone biopsy studies. The risk of fracture is high, especially in the first several months after kidney transplantation. Alterations in minerals (calcium, phosphorus and magnesium) and biomarkers of bone metabolism (parathyroid hormone, alkaline phosphatase, vitamin D and FGF-23) are observed with varying impact on posttransplant outcomes. Calcineurin inhibitors are linked to osteoporosis, whereas steroid therapy may lead to both osteoporosis and varying degrees of osteonecrosis. Sirolimus and everolimus might have a bearing on osteoblast proliferation and differentiation or decreasing osteoclast-mediated bone resorption. Selected pharmacologic interventions for the treatment of MBD in transplant patients include steroid withdrawal, and the use of bisphosphonates, vitamin D derivatives, calcimimetics, teriparatide, calcitonin and denosumab.

SUMMARY

MBD following kidney transplantation is common and characterized by loss of bone volume and mineralization abnormalities, often leading to low turnover bone disease. Although there are no well established therapeutic approaches for management of MBD in renal transplant recipients, clinicians should continue individualizing therapy as needed.

Elevated fibroblast growth factor 23 levels as a cause of early post-renal transplantation hypophosphatemia

BACKGROUND

Hypophosphatemia is a common complication after renal transplantation. Hyperparathyroidism has long been thought to be the cause, but hypophosphatemia can persist after high parathyroid hormone (PTH) levels normalize. Furthermore, calcitriol levels remain inappropriately low after transplantation, suggesting that mechanisms other than PTH contribute. Fibroblast growth factor 23 (FGF-23) induces phosphaturia, inhibits calcitriol synthesis, and accumulates in chronic kidney disease. We performed prospective study to investigate if FGF-23 early after renal transplantation contributes to hypophosphatemia.

METHODS

We measured FGF-23 levels before and at 1, 2, 4, and 12 weeks after transplantation in 20 renal transplant recipients. Serum creatinine, calcium (Ca), phosphate (Pi), intact PTH (PTH), and 1,25-dihydroxy vitamin D (1,25(OH)(2)VitD) were measured at the same time.

RESULTS

FGF-23 levels decreased by 97% at 4 weeks after renal transplantation (PRT) (7,471 ± 11,746 vs 225 ± 295 pg/mL; P < .05) but were still above normal. PTH and Pi levels also decreased significantly after renal transplantation, and Ca and 1,25(OH)(2)VitD slightly increased. PRT hypophosphatemia of <2.5 mg/dL developed in 15 (75%) and 12 (60%) patients at 4 weeks and 12 weeks respectively. Compared with nonhypophosphatemic patients, the levels of FGF-23 of hypophosphatemic patients were higher (303 ± 311 vs 10 ± 6.9 pg/mL; P = .02) at 4 weeks PRT. FGF-23 levels were inversely correlated with Pi (r(2) = 0.406; P = .011); PTH was not independently associated with Pi (r(2) = 0.132; P = .151).

CONCLUSIONS

FGF-23 levels decrease dramatically after renal transplantation. During the early PRT period, Pi rapidly decreased, suggesting that FGF-23 is cleared by the kidney, but residual FGF-23 may contribute to the PRT hypophosphatemia. FGF-23, but not PTH levels, was independently associated with PRT hypophosphatemia.

Clinical manifestations of hypercalcemia and hypophosphatemia after kidney transplantation

INTRODUCTION

Abnormalities of calcium and phosphorus metabolism in end-stage renal disease patients can persist after transplantation. We investigated their natural courses after transplantation, their risk factors for posttransplantation hypercalcemia and hypophosphatemia, and their impacts on allograft outcomes.

METHODS

We retrospectively analyzed a total of 490 adult patients who underwent kidney transplantations between 2000 and 2009.

RESULTS

The serum calcium continued to increase, and reaching a plateau at around 3 months after transplantation. Thereafter it decreased, reaching a stable level by 2 years. Forty-four patients (9.0%) displayed hypercalcemia within 1 year; it persisted longer than that in 23 subjects (4.7%). Both longer dialysis duration (odds ratio [OR] 1.423; 95% confidence interval [CI], 1.192-1.699) and high intact serum parathyroid hormone (iPTH) level before transplantation (OR 1.002; 95% CI, 1.000-1.003) increased the risk for posttransplantation hypercalcemia. After a significant decrease during the first week, the serum phosphorus level increased, becoming stable between 1 and 6 months after transplantation. Hypophsphatemia occurred in 379 patients (77.3%) with 336 patients displaying hypophosphatemia without hypercalcemia. However, neither hypercalcemia nor hypophosphatemia influenced graft outcomes. Eight patients underwent pretransplantation parathyroidectomy, whereas 4 patients underwent posttransplantation parathyroidectomy. Neither group of patients experienced posttransplantation hypercalcemia.

CONCLUSIONS

Both hypercalcemia and hypophosphatemia are common after renal transplantation, especially among patients with a long history of dialysis before transplantation. Strict control of hyperparathyroidism including parathyroidectomy before transplantation may be the appropriate approach to these abnormalities.

FGF23 and syndromes of abnormal renal phosphate handling

Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and circulating phosphate and possibly PTH. FGF23 was discovered as the humoral factor in tumors that causes hypophosphatemia and osteomalacia and through the identification of a mutant form of FGF23 that leads to autosomal dominant hypophosphatemic rickets (ADHR), a rare genetic disorder. FGF23 appears to be mainly secreted by osteocytes where its expression is up-regulated by 1,25(OH)(2)D and probably by increased serum phosphate levels. Its synthesis and secretion is reduced through yet unknown mechanisms that involve the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), dentin matrix protein 1 (DMP1) and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Consequently, loss-of-function mutations in these genes underlie hypophosphatemic disorders that are either X-linked or autosomal recessive. Impaired O-glycosylation of FGF23 due to the lack of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase 3 (GALNT3) or due to certain homozygous FGF23 mutations results in reduced secretion of intact FGF23 and leads to familial hyperphosphatemic tumoral calcinosis. FGF23 acts through FGF-receptors and the coreceptor Klotho to reduce 1,25(OH)(2)D synthesis in the kidney and probably the synthesis of parathyroid hormone (PTH) by the parathyroid glands. It furthermore synergizes with PTH to increase renal phosphate excretion by reducing expression of the sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Loss-of-function mutations in these two transporters lead to autosomal recessive Fanconi syndrome or to hereditary hypophosphatemic rickets with hypercalciuria, respectively.

Association of pretransplant serum phosphorus with posttransplant outcomes

BACKGROUND AND OBJECTIVES

Serum phosphorus levels are associated with mortality, cardiovascular disease, and renal function loss in individuals with and without chronic kidney disease. The association of pretransplant serum phosphorus levels with transplant outcomes is not clear.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Data of the Scientific Registry of Transplant Recipients (SRTR) up to June 2007 were linked to the database (2001 through 2006) of one of the U.S.-based large dialysis organizations (DaVita). The selected 9384 primary kidney recipients were divided into five groups according to pretransplant serum phosphorus levels (mg/dl): <3.5, 3.5 to <5.5 (reference group), 5.5 to <7.5, 7.5 to <9.5, and ≥9.5. Unadjusted and multivariate adjusted risks for transplant outcomes were compared.

RESULTS

Patients were 48 ± 14 years old and included 37% women and 27% African Americans. After multivariate adjustment, all-cause and cardiovascular death hazard ratios were 2.44 (95% confidence interval: 1.28 to 4.65) and 3.63 (1.13 to 11.64), respectively, in recipients in the ≥9.5 group; allograft loss hazard ratios were 1.42 (1.04 to 1.95) and 2.36 (1.33 to 4.17) in recipients with 7.5 to >9.5 and ≥9.5, respectively. No significant association with delayed graft function was found.

CONCLUSIONS

Pretransplant phosphorus levels 7.5 to <9.5 mg/dl and ≥9.5 mg/dl were associated with increased risk of functional graft failure and increased risk of all-cause and cardiovascular deaths, respectively, when compared with 3.5 to <5.5 mg/dl. Additional studies are needed to examine whether more aggressive control of pretransplant serum phosphorus may improve posttransplant outcomes.

Therapeutic management of post-kidney transplant hyperparathyroidism

Left uncontrolled, persistent post-kidney transplant hyperparathyroidism (HPT) may lead to or exacerbate pre-existing bone and cardiovascular disease. Parathyroidectomy has long been the primary treatment option for long-term uncontrolled HPT in post-kidney transplant patients. However, patients with contraindications for surgery and parathyroidectomy-associated complications, including graft loss, highlight the need for other approaches. Conventional medical therapies have limited impact on serum calcium (Ca) and parathyroid hormone (PTH) levels. Bisphosphonates and calcitonin, used to spare bone loss, and phosphorus supplementation, to correct hypophosphatemia, do not directly regulate PTH or Ca. Although vitamin D supplementation can reduce PTH, it is often contraindicated because of hypercalcemia. Studies of the calcimimetic cinacalcet in patients with post-kidney transplant HPT suggest that it can rapidly reduce serum PTH and Ca concentrations while increasing serum phosphorus concentrations toward the normal range. Although the clearest application for cinacalcet is the non-surgical treatment of hypercalcemic patients with persistent HPT, current indications for other transplant patients are as yet uncertain. Further studies are needed to determine the utility of cinacalcet in patients with spontaneous resolution of HPT or low bone turnover. This review discusses the pathophysiology of post-kidney transplant HPT, associated complications, and current options for clinical management.

Clinical practice. Fibroblast growth factor (FGF)23: a new hormone

Until a decade ago, two main hormones were recognized as directly affecting phosphate homeostasis and, with that, bone metabolism: parathyroid hormone and 1,25(OH)(2) vitamin D (calcitriol). It was only a decade ago that the third major player hormone was found, linking gut, bone, and kidney. The physiologic role of fibrinogen growth factor (FGF)23 is to maintain serum phosphate concentration within a narrow range. Secreted from osteocytes, it modulates kidney handling of phosphate reabsorption and calcitriol production. Genetic and acquired abnormalities in FGF23 structure and metabolism cause conditions of either hyper-FGF23-manifested by hypophosphatemia, low serum calcitriol, and rickets/osteomalacia-or hypo-FGF23, expressed by hyperphosphatemia, high serum calcitriol, and extra-skeletal calcifications. In patients with chronic renal failure, FGF23 levels increase as kidney functions deteriorate and are under investigation to learn if the hormone actually participates in the pathophysiology of the deranged bone and mineral metabolism typical for these patients and, if so, whether it might serve as a therapeutic target. This review addresses the physiology and pathophysiology of FGF23 and its clinical applications.

Early post-transplantation hypophosphatemia is associated with elevated FGF-23 levels

BACKGROUND

We examined the hypothesis that high FGF-23 levels early after transplantation contribute to the onset of hypophosphatemia, independently of parathyroid hormone (PTH) and other factors regulating phosphate metabolism.

METHODS

We measured serum phosphate levels (sPi), renal tubular reabsorption of Pi (TmPi/GFR), estimated GFR (eGFR), intact PTH (iPTH), calcitriol, intact (int) and C-terminal (Cter) FGF-23, dietary Pi intake and cumulative doses of glucocorticoids in 69 patients 12 days (95% confidence interval, 10-13) after renal transplantation.

RESULTS

Hypophosphatemia was observed in 43 (62%) of the patients 12 days after transplantation. Compared with non-hypophosphatemic subjects, their post-transplantation levels of intact and CterFGF-23 were higher (195 (108-288) vs 48 (40-64) ng/l, P<0.002 for intFGF-23; 205 (116-384) vs 81 (55-124) U/ml, P<0.002, for CterFGF-23). In all subjects, Cter and intFGF-23 correlated inversely with sPi (r=-0.35, P<0.003; -0.35, P<0.003, respectively), and TmPi/GFR (r=-0.50, P<0.001; -0.54, P<0.001, respectively). In multivariate models, sPi and TmPi/GFR were independently associated with FGF-23, iPTH and eGFR. Pre-transplant iPTH levels were significantly higher in patients developing hypophosphatemia after renal transplantation. Pre-transplant levels of FGF-23 were not associated with sPi at the time of transplantation.

CONCLUSION

In addition to PTH, elevated FGF-23 may contribute to hypophosphatemia during the early post-renal transplant period.

Hypophosphatemia in kidney transplant recipients: report of acute phosphate nephropathy as a complication of therapy

Hypophosphatemia is a common complication after kidney transplant, affecting >90% of patients. However, no specific recommendations for phosphate repletion exist for transplant recipients. We report a case of a 70-year-old highly sensitized woman with end-stage renal disease caused by diabetic nephropathy who underwent deceased donor kidney transplant. Four weeks later, she was noted to have hypophosphatemia with undetectable serum phosphate levels, and she reported mild diarrhea. She was started on oral phosphate supplementation. On a routine visit 2 weeks later, she was found to have an acute increase in serum creatinine level and kidney biopsy was performed. We discuss the causes, management, and complications of hypophosphatemia in kidney transplant.

The relative role of fibroblast growth factor 23 and parathyroid hormone in predicting future hypophosphatemia and hypercalcemia after living donor kidney transplantation: a 1-year prospective observational study

BACKGROUND

Kidney transplantation (KTx) restores many of the disorders accompanying end-stage renal failure. However, hypercalcemia and hypophosphatemia are both common complications after renal transplantation. Prospective observation of these complications has not been well described and pre-transplant predictors also remain unknown. This prospective observational cohort study was carried out to clarify pre-transplant risk factors of persistent hypophosphatemia and/or hypercalcemia at 12 months after transplantation.

METHODS

Consecutive living donor KTx recipients (n = 39) at Tokyo Women's Medical University were prospectively recruited. Parameters of bone and mineral metabolism including intact parathyroid hormone (iPTH) and full-length fibroblast growth factor (FGF) 23 were followed.

RESULTS

FGF23 decreased to comparable levels for renal function while hyperparathyroidism persisted at 12 months after transplantation. Multivariate linear regression analysis revealed that pre-transplant iPTH correlated with hypercalcemia at 12 months and pre-transplant FGF23 was the best pre-transplant predictor of persistent hypophosphatemia at 12 months.

CONCLUSIONS

It is intriguing that although FGF23 is not a causal factor for hypophosphatemia at 12 months post-transplantation, it is a significant predictor of this common complication.

Tertiary excess of fibroblast growth factor 23 and hypophosphatemia following kidney transplantation

Hypophosphatemia caused by inappropriate urinary phosphate wasting is a frequent metabolic complication of the early period following kidney transplantation. Although previously considered to be caused by tertiary hyperparathyroidism, recent evidence suggests a primary role for persistently elevated circulating levels of the phosphorus-regulating hormone, FGF23. In the setting of a healthy renal allograft, markedly increased FGF23 levels from the dialysis period induce renal phosphate wasting and inhibition of calcitriol production, which contribute to hypophosphatemia. While such tertiary FGF23 excess and resultant hypophosphatemia typically abates within the first few weeks to months post-transplant, some recipients manifest persistent renal phosphate wasting. Furthermore, increased FGF23 levels have been associated with increased risk of kidney disease progression, cardiovascular disease, and death outside of the transplant setting. Whether tertiary FGF23 excess is associated with adverse transplant outcomes is unknown. In this article, we review the physiology of FGF23, summarize its relationship with hypophosphatemia after kidney transplantation, and speculate on its potential impact on long-term outcomes of renal allograft recipients.

Post-renal transplantation hypophosphatemia

An understanding of the pathophysiologic mechanisms of post-renal transplant (PRT) bone disease is of important clinical significance. Although bone disease occurs after all solid organ transplantation, the cumulative skeletal fracture rate remains high in PRT subjects while reaching a plateau with other transplantations. One major difference in the pathophysiology of PRT bone disease is, perhaps, due to persistent renal phosphorus (Pi) wasting. Novel phosphaturic agents have recently been suggested to participate in the development of bone disease in PRT subjects. However, it is unclear as of yet whether these factors alone or in conjunction with excess parathyroid hormone (PTH) secretion play a key role in the development of negative Pi balance and consequent bone disease in this population. In this review, I present a natural history of PRT hypophosphatemia and persistent renal Pi leak, provide pathophysiologic insight into these developments, and discuss the difficulty in diagnosing these phenotypes in both adult and pediatric populations.

FGF23-parathyroid interaction: implications in chronic kidney disease

Over the past few years there have been considerable advances in our understanding of the physiological regulation of mineral homeostasis. One of the most important breakthroughs is the identification of fibroblastic growth factor 23 (FGF23) and its role as a key regulator of phosphate and 1,25-dihydroxyvitamin D metabolism. FGF23 exerts its biological functions by binding to its cognate receptor in the presence of Klotho as a cofactor. FGF23 principally acts on the kidney to induce urinary phosphate excretion and suppresses 1,25-dihydroxyvitamin D synthesis, thereby indirectly modulating parathyroid hormone secretion. FGF23 also acts directly on the parathyroid to decrease parathyroid hormone synthesis and secretion. In patients with chronic kidney disease, FGF23 levels increase progressively to compensate for phosphate retention, but these elevated FGF23 levels fail to suppress the secretion of parathyroid hormone, particularly in the setting of uremia. Recent data suggest that this parathyroid resistance to FGF23 may be caused by decreased expression of Klotho-FGFR1 complex in hyperplastic parathyroid glands. This review summarizes recent insights into the role of FGF23 in mineral homeostasis and discusses the involvement of its direct and indirect interaction with the parathyroid gland, particularly focusing on the pathophysiology of secondary hyperparathyroidism in chronic kidney disease.

The effect of low-dose cholecalciferol and calcium treatment on posttransplant bone loss in renal transplant patients: a prospective study

BACKGROUND/AIM

Posttransplant steroid doses have been reduced with the use of new and potent immunosuppressive agents. However, posttransplant osteoporosis is still a serious problem. Our aim in this study was to investigate the effect of low-dose cholecalciferol and calcium supplementation on bone loss after transplantation in renal transplant patients.

METHODS

Fifty-eight renal transplantation patients were included in the study. Fourteen newly transplanted patients (group 1) and 44 renal transplantation patients with a graft age of at least six months (group 2) were involved. All patients received 400 IU/day orally cholecalciferol (vitamin D3) and 600 mg/day orally calcium replacement starting from the second day posttransplantation. All patients baseline serum and urine biochemistry, serum 25-hydroxy vitamin D3 (25 (OH)D3), and bone mineral density (BMD) tests were performed. Also, the same measurements were performed at the 12th month in group 1.

RESULTS

After one year of treatment, BMDs were improved in group 1. Patients in group 1 had a nonsignificant increase of lumbar spine (8.12 +/- 18.64% of baseline BMD) and femoral total (7.10 +/- 13.48% of baseline BMD) BMD at the end of the first year. On the other hand, there was a significant increase in femoral neck (10.06 +/- 15.70% of baseline BMD, p < 0.05) measurements. The baseline results of group 2 were similar to group 1. In group 1, 25 (OH)D3 levels were increased while PTH levels were decreased at the end of the year.

CONCLUSION

In renal transplant patients who use low-dose metilprednisolon and new immunosuppressive agents together, low doses of vitamin D3 and calcium replacement for one year provides a reduction in lumbar spine, femoral neck, and femoral total bone loss and prevents bone loss in group 2. In addition, it contributed to the normalization of PTH levels.

Disorders of phosphate homeostasis and tissue mineralisation

Phosphate is absorbed from the diet in the gut, stored as hydroxyapatite in the skeleton, and excreted with the urine. The balance between these compartments determines the circulating phosphate concentration. Fibroblast growth factor 23 (FGF23) has recently been discovered and is part of a previously unrecognised hormonal bone-kidney axis. Phosphate-regulating gene with homologies to endopeptidases on the X chromosome, and dentin matrix protein 1 regulate the expression of FGF23 in osteocytes, which then is O-glycosylated by UDP-N-acetyl-alpha-D-galactosamine: polypeptide N-acetylgalactosaminyl-transferase 3 and secreted into the circulation. FGF23 binds with high affinity to fibroblast growth factor receptor 1c in the presence of its co-receptor Klotho. It inhibits, either directly or indirectly, reabsorption of phosphate and the synthesis of 1,25-dihydroxy-vitamin-D by the renal proximal tubule and the secretion of parathyroid hormone by the parathyroid glands. Acquired or inborn errors affecting this newly discovered hormonal system can lead to abnormal phosphate homeostasis and/or tissue mineralisation. This chapter will provide an update on the current knowledge of the pathophysiology, the clinical presentation, diagnostic evaluation and therapy of the disorders of phosphate homeostasis and tissue mineralisation.

Recovery of hyperphosphatoninism and renal phosphorus wasting one year after successful renal transplantation

BACKGROUND AND OBJECTIVES

In the first months after successful kidney transplantation, hypophosphatemia and renal phosphorus wasting are common and related to inappropriately high parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23) levels. Little is known about the long-term natural history of renal phosphorus homeostasis in renal transplant recipients.

DESIGN, SETTING, PARTICIPANTS

We prospectively followed parameters of mineral metabolism (including full-length PTH and FGF-23) in 50 renal transplant recipients at the time of transplantation (Tx), at month 3 (M3) and at month 12 (M12). Transplant recipients were (1:1) matched for estimated GFR with chronic kidney disease (CKD) patients.

RESULTS

FGF-23 levels (Tx: 2816 [641 to 10665] versus M3: 73 [43 to 111] versus M12: 56 [34 to 78] ng/L, median [interquartile range]) and fractional phosphorus excretion (FE(phos); M3: 45 +/- 19% versus M12: 37 +/- 13%) significantly declined over time after renal transplantation. Levels 1 yr after transplantation were similar to those in CKD patients (FGF-23: 47 [34 to 77] ng/L; FE(phos) 35 +/- 16%). Calcium (9.1 +/- 0.5 versus 8.9 +/- 0.3 mg/dl) and PTH (27.2 [17.0 to 46.0] versus 17.5 [11.7 to 24.4] ng/L) levels were significantly higher, whereas phosphorus (3.0 +/- 0.6 versus 3.3 +/- 0.6 mg/dl) levels were significantly lower 1 yr after renal transplantation as compared with CKD patients.

CONCLUSIONS

Data indicate that hyperphosphatoninism and renal phosphorus wasting regress by 1 yr after successful renal transplantation.

Evaluation and management of bone disease and fractures post transplant

Bone disease is common in recipients of kidney, liver, heart, and lung transplants and results in fractures in 20-40% of patients, a rate much higher than expected for age. Fractures occur because of the presence of bone disease as well as other factors such as neuropathy, poor balance, inactivity, and low body or muscle mass. Major contributors to bone disease include both preexisting bone disease and bone loss post transplant, which is greatest in the first 6-12 months when steroid doses are highest. Bone disease in kidney transplant recipients should be considered different from that which occurs in other solid organ transplant recipients for several reasons including the presence of renal osteodystrophy, which contributes to low bone mineral density in these patients; the location of fractures (more common in the legs and feet in these patients than in spine and hips as in other solid organ recipients); and the potential danger in using bisphosphonate therapy, which may cause more harm than good in kidney transplant recipients with low bone turnover. Evaluation in all patients should preferably occur in the pretransplant period or early post transplant and should include assessment of fracture risk as well as metabolic factors that can contribute to bone disease. Bone mineral density measurement is recommended in all patients even if its predictive value for fracture risk in the transplant population is unproven. Management of bone disease should be directed toward decreasing fracture risk as well as improving bone density. Pharmacologic and nonpharmacologic treatment strategies are discussed in this review. Although there have been many studies describing a beneficial effect of bisphosphonates and vitamin D analogues on bone density, none have been powered to detect a decrease in fracture rate.

Management of metabolic bone disease in kidney transplant recipients

Bone disease after kidney transplantation has a complex pathophysiology and heterogeneous histology. Pre-existing renal osteodystrophy may not resolve completely, but continue or evolve into a different osteodystrophy. Rapid bone loss immediately after transplant can persist, at a lower rate, for years to come. These greatly increase the risk of bone fracture and vertebral collapse. Hypovitaminosis D, hyperparathyroidism and hyperaluminemia may resolve after kidney transplant, but many patients have other risk factors of bone loss, such as steroids usage, hypogonadism, persistent hyperparathyroidism, poor allograft function, aging, and chronic diseases. Clinical management requires a comprehensive approach to address the underlying and ongoing disease processes. Successful prevention of bone loss has been shown with vitamin D analogues, bisphosphonates and calcitonin. Novel approaches to restore the normal bone remodeling and improve the bone quality may be needed in order to effectively decrease bone fractures in kidney transplant recipients.

Calcium and bone metabolism pre- and post-kidney transplantation

Chronic kidney disease (CKD) is associated with significant disturbances in bone and mineral metabolism, the manifestations of which are heterogeneous in their expression and clinical impact. Over the last 2 decades, advances in the management of CKD and improved outcomes of kidney transplantation have led to the emergence of post-transplantation bone disease as a serious cause of morbidity in long-term survivors. The management of post-kidney transplantation bone disease represents a difficult challenge because of its complex pathophysiology and the paucity of clinical data on effective therapies. The optimal management of disturbances of bone and mineral metabolism before kidney transplantation forms the cornerstone of their successful management after transplantation. Therapeutic strategies to effectively and safely decrease skeletal morbidity after kidney transplantation are not yet clearly established.

Persistent high level of fibroblast growth factor 23 as a cause of post-renal transplant hypophosphatemia

Post-transplant hypophosphatemia is a highly prevalent problem, and fibroblast growth factor 23, a newly discovered phosphatonin, has recently been reported to be involved in its pathogenesis. We report a 52-year-old Japanese woman who received a living-related kidney transplant and showed severe hypophosphatemia immediately after transplantation. We suspected that fibroblast growth factor 23 was the main cause of this hypophosphatemia and investigated its levels longitudinally after the transplantation. The patient showed persistently high levels of fibroblast growth factor 23, with suppressed 1,25-dihydroxyvitamin D and parathyroid hormone. She recovered from the hypophosphatemia when fibroblast growth factor returned to its reference level half a year after the transplantation. We conclude that a persistently high level of fibroblast growth factor 23 is an important cause of post-transplant hypophosphatemia, other than hyperparathyroidism, a previously noted cause.

Hypophosphatemia: clinical consequences and management

Current evidence regarding the clinical consequences of hypophosphatemia is not straightforward. Given the potentially different implications of hypophosphatemia among various patient groups, this commentary touches on patients with low serum phosphate after acute hospitalization, those with chronic ambulatory hypophosphatemia, and those with hypophosphatemia in the setting of advanced renal disease. Finally, this commentary examines the evidence regarding how best to replete phosphorous in the hypophosphatemic patient.

Post-transplant hypophosphatemia: Tertiary 'Hyper-Phosphatoninism'?

Hypophosphatemia is a common complication of kidney transplantation. Tertiary hyperparathyroidism has long been thought to be the etiology, but hypophosphatemia can occur despite low parathyroid hormone (PTH) levels and can persist after high PTH levels normalize. Furthermore, even in the setting of normal allograft function, hypophosphatemia, and hyperparathyroidism, calcitriol levels remain inappropriately low following transplantation, suggesting that mechanisms other than PTH contribute. Fibroblast growth factor-23 (FGF-23) induces phosphaturia, inhibits calcitriol synthesis, and accumulates in chronic kidney disease. We performed a prospective, longitudinal study of 27 living donor transplant recipients to test the hypotheses that excessive FGF-23 accounts for hypophosphatemia and decreased calcitriol levels following kidney transplantation. Hypophosphatemia <2.5 mg/dl developed in 85% of subjects, including one who had previously undergone parathyroidectomy; 37% developed phosphate < or =1.5 mg/dl. The mean pre-transplant FGF-23 level was 1,218+/-542 RU/ml. Within the first week following transplantation, mean levels decreased to 557+/-579 RU/ml, which were still above normal. FGF-23 was independently associated with serum phosphate (P < 0.01), urinary excretion of phosphate (P < 0.01), and calcitriol levels (P < 0.01); PTH was not independently associated with any of these parameters. We calculated area under the curve for FGF-23 and PTH between the pre- and first post-transplant levels as a summary measure of early exposure to these phosphaturic hormones. An area under the FGF-23 curve greater than the median was associated with a relative risk of developing hypophosphatemia < or =1.5 mg/dl of 5.3 (P = 0.02) compared with lower levels. Increased area under the PTH curve was not associated with greater risk of hypophosphatemia. Excessive FGF-23 exposure in the early post-transplant period appears to be more strongly associated with post-transplant hypophosphatemia than PTH.

Bone disease after renal transplantation

It has been well established that a rapid decrease in bone mineral density (BMD) occurs in the first 6 to 12 mo after a successful renal transplantation and persists, albeit at a lower rate, for many years. This rapid BMD loss significantly increases the fracture risk of these patients to levels that are even higher than those of patients who have chronic kidney disease stage 5 and are on dialysis. The presence of low BMD in renal transplant patients as a predictor of risk fracture is controversial. Indeed, as has been suggested also for patients with postmenopausal osteoporosis, there is not a compelling correlation between the decline in BMD and skeletal fractures. However, bone disease after renal transplantation probably represents a unique bone disorder that must encompass underlying renal osteodystrophy. In fact, this syndrome results from multiple factors that include pretransplantation bone status, use of glucocorticoids and other immunosuppressive drugs, hypophosphatemia, and alterations of the calcium-vitamin D axis. Recent studies have demonstrated decreased osteoblast number, reduced bone formation rate, delayed mineralization, and increased osteoblast and osteocyte apoptosis. Bisphosphonates and vitamin D metabolites may be valuable in preventing or diminishing early bone loss. However, clinicians should be careful with the use of bisphosphonates and oversuppression of bone, especially in patients with low bone turnover. New prospective, controlled trials are required to confirm the real efficacy of these drugs, particularly in long-term renal transplant patients.

FGF-23 and sFRP-4 in chronic kidney disease and post-renal transplantation

BACKGROUND

The phosphatonins fibroblast growth factor-23 (FGF-23) and FRP-4 are inhibitors of tubular phosphate reabsorption that may play a role in the hyperphosphatemia associated with chronic kidney disease (CKD) or in the hypophosphatemia associated with renal transplants.

METHODS

Plasma FGF-23, FRP-4, phosphorus and parathyroid hormone were measured in patients at all stages of CKD. Phosphate regulation of FGF-23 and secreted frizzled related protein-4 (sFRP-4) was examined in end-stage renal disease patients in the presence and absence of therapeutic phosphate binder usage. In renal transplant patients, plasma FGF-23, sFRP-4 and phosphorus concentrations were determined before and 4-5 days after transplantation.

RESULTS

Plasma FGF-23 correlated with creatinine clearance (r2 = -0.584, p < 0.0001) and plasma phosphorus (r2 = 0.347, p < 0.001) in CKD patients and with plasma phosphorus (r2 = 0.448, p < 0.001) in end-stage renal disease patients. Phosphate binder withdrawal increased FGF-23 levels. In kidney transplant patients, dramatic decreases in FGF-23 (-88.8 +/- 5.4%) and phosphorus (-64 +/- 10.2%) were observed by 4-5 days post-transplantation. In patients with post-transplant hypophosphatemia, FGF-23 levels correlated inversely with plasma phosphorus (r2 = 0.661, p < 0.05). sFRP-4 levels did not change with creatinine clearance or hyperphosphatemia in CKD or end-stage renal disease patients, and no relation was noted between post-transplant sFRP-4 levels and hypophosphatemia.

CONCLUSIONS

In CKD, FGF-23 levels rose with decreasing creatinine clearance rates and increasing plasma phosphorus levels, and rapidly decreased post-transplantation suggesting FGF-23 is cleared by the kidney. Residual FGF-23 may contribute to the hypophosphatemia in post-transplant patients.

Improvement of adynamic bone disease after renal transplantation

Low bone remodeling and relatively low serum parathyroid hormone (PTH) levels characterize adynamic bone disease (ABD). The impact of renal transplantation (RT) on the course of ABD is unknown. We studied prospectively 13 patients with biopsy-proven ABD after RT. Bone histomorphometry and bone mineral density (BMD) measurements were performed in the 1st and 12th months after RT. Serum PTH, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and osteocalcin were measured regularly throughout the study. Serum PTH levels were slightly elevated at transplantation, normalized at the end of the third month and remained stable thereafter. Bone biopsies performed in the first month after RT revealed low bone turnover in all patients, with positive bone aluminum staining in 5. In the 12th month, second biopsies were performed on 12 patients. Bone histomorphometric dynamic parameters improved in 9 and were completely normalized in 6, whereas no bone mineralization was detected in 3 of these 12 patients. At 12 months post-RT, no bone aluminum was detected in any patient. We also found a decrease in lumbar BMD and an increase in femoral BMD. Patients suffering from ABD, even those with a reduction in PTH levels, may present partial or complete recovery of bone turnover after successful renal transplantation. However, it is not possible to positively identify the mechanisms responsible for the improvement. Identifying these mechanisms should lead to a better understanding of the physiopathology of ABD and to the development of more effective treatments.

Renal Function and Tubular Phosphate Handling in Long-Term Cyclosporine- and Tacrolimus-Based Immunosuppression in Kidney Transplantation

The aim of the study was to assess impaired tubular phosphate reabsorption and renal function among patients on cyclosporine- or tacrolimus-based immunosuppression for 2 years after kidney transplantation. Among 60 cadaveric kidney allograft recipients observed for 48 months, 40 received cyclosporine, azathioprine, and prednisone (group A and B). Group C consisted of 20 patients receiving tacrolimus with steroid withdrawal at 3 months after transplantation. Renal function and calcium-phosphate metabolism-iPTH, 25-OHD, 1,25(OH)(2)D concentration, phosphate reabsorption (TRP; mmol/L), and tubular maximum phosphate reabsorption per glomerular filtration rate (TmPO(4)/GFR; mmol/L)-were assessed at 1, 6, 12, 18, and 24 months (groups A and C) or 24, 30, 36, 42, and 48 months (group B). Renal function after 24 months of observation was significantly better among tacrolimus-treated patients (serum creatinine concentration mumol/L; C: 94.6 +/- 16.8 vs A: 110.5 +/- 22.1 vs B: 121.1 +/- 30.9; P < .05). Among tacrolimus-treated recipients, TRP and TmPO(4)/GFR remained within normal values during the whole observation period. In groups A and B, TRP improved during the first year of observation; after 2 years it reached values observed in group C (TRP: A: 0.67 +/- 0.1; B: 0.72 +/- 0.13; C: 0.76 +/- 0.07; P = NS), whereas TmPO(4)/GFR remained low in group A after 2 years (A: 0.78 +/- 0.19; B: 0.91 +/- 0.25; C: 0.94 +/- 0.15; P < .05). Tacrolimus-treated patients exhibit significantly faster recovery from tubular phosphate reabsorption impairment compared with cyclosporine-treated recipients. Tacrolimus-based immunosuppression led to better kidney allograft function during 2-year observation.