Biochemical Characteristics of Bone Mineral Metabolism before and throughout the First Year after Kidney Transplantation, Persistent Hyperparathyroidism, and Risk Factors in a Latin Population

Bone mineral metabolism disease, which included persistent hyperparathyroidism, is common after successful kidney transplantation (KT) and is related with negative outcomes in kidney transplant recipients. There is a lack of information about bone mineral metabolism, persistent hyperparathyroidism, and its risk factors in Latin kidney transplant recipients (KTRs). Material and Methods: A retrospective study was conducted in 74 patients aged 18–50 years with evolution of 12 months after KT and estimated glomerular filtration rate (eGFR) >60 ml/min; biochemical data of bone mineral metabolism before and at 1, 3, 6, and 12 months of KT were registered. Results. Age was 33 (IQR 27–37) years; 54% (n = 40) were men. Before KT, all patients had hyperparathyroidism, 40% (n = 30) hypocalcemia, 86% (n = 64) hyperphosphatemia, and 42% (n = 31) hyperphosphatasemia. After KT, an increase of calcium and a diminution of PTH, phosphorus, and alkaline phosphatase were corroborated (p=0.001). All patients had hypovitaminosis D (deficiency: 91% (n = 67); insufficiency: 9% (n = 7)); 40% (n = 30) had persistent hyperparathyroidism at 12 months. Hyperphosphatasemia before KT (OR = 4.17 (95% CI: 1.21–14.44); p=0.04), hyperparathyroidism at 6 months (OR = 1.84 (95% CI; 1.67–2.06); p=0.02), hypovitaminosis D at 6 months (OR = 3.94 (95% CI: 1.86–17.9); p=0.01), and hyperphosphatasemia at 6 months (OR = 1.47 (95% CI: 1.07–2.86); p=0.03) were risk factors for persistent hyperparathyroidism at 12 months after KT. Conclusion. Persistent hyperparathyroidism at 6 months, hypovitaminosis D, and hyperphosphatasemia are risk factors for persistent hyperparathyroidism at 1 year of KT in Latin population.


Introduction
Disorders of mineral metabolism and bone disease are common in chronic kidney disease (CKD) patients, conditioning an increase of morbidity, diminution of quality of life, and increased cardiovascular mortality. e term "chronic kidney disease-mineral bone disorder" (CKD-MBD) has been used to describe a broader clinical syndrome that develops as a systemic disorder of mineral and bone metabolism due to CKD, which is manifested by abnormalities in bone and mineral metabolism and/or extraskeletal calcification. CKD-MBD includes three aspects: (1) laboratory abnormalities of calcium, phosphorus, parathormone (PTH), or vitamin D, (2) bone abnormalities in turnover, mineralization, volume, linear growth, or strength (osteodystrophy, osteopenia, osteoporosis, and low-mineral density), and (3) calcification of the vasculature or other soft tissues [1]. KT is the most effective treatment for end-stage renal disease. Many of complications of CKD may be reversed by transplantation; however, bone mineral metabolism disease (BMMD) may persist. Changes in mineral bone metabolism after KT have been described; however, there is a lack of information about bone mineral metabolism changes at long-term in Latin KTR. BMMD is common after KT and is influenced by factors such as preexisting renal osteodystrophy, immunosuppressive therapy, or kidney transplant dysfunction [1,2]. Persistent hyperparathyroidism has been reported after KT, representing a related-factor with negative outcomes in KTR. e aims of our study were to describe the biochemical characteristics of bone mineral metabolism before and at 1, 3, 6, and 12 months after KT in Latin KTR and the frequency of persistent hyperparathyroidism at 1-year and its associated-factors.

Patients.
We conducted a retrospective study at a tertiary care center. Eligible patients were men and women aged 18-50 years with 12 months of KT evolution, eGFR >60 ml/ min, and no allograft dysfunction or rejection, enrolled in the clinic of bone mineral metabolism at Hospital de Especialidades, Centro Médico Nacional Siglo XXI in Mexico City. e biochemical evaluation of bone mineral metabolism involved the measurement of serum intact parathormone (PTH), serum 25-hydroxyvitamin D (25OHD), serum calcium, magnesium, phosphorus, albumin, alkaline phosphatase, 24-hour urinary calcium, and 24-hour urinary phosphorus; the biochemical data of bone mineral metabolism before TR and at 1, 3, 6, and 12 months of KT were registered for analysis. e eGFR was calculated by CKD-EPI. Hypovitaminosis D was established with concentrations of 25OHD below 30 ng/mL and classified under deficiency (below 20 ng/mL) or insufficiency (21-29 mg/mL); treatment was established according to the Endocrine Society's guidelines (cholecalciferol 6000-10,000 IU/d to obtain a 25OHD above 30 ng/ml, followed by maintenance therapy of 3000-6000 IU/d) [3]. Persistent hyperparathyroidism was defined as the presence of high concentrations of PTH at the first year despite the successful KT and 25OHD above 30 ng/mL. Tertiary hyperparathyroidism was diagnosed in KTR with persistent hyperparathyroidism and hypercalcemia and immunosuppression. Patients with living-donor KT received induction therapy with basiliximab, and patients with deceased donor KT received thymoglobulin. All patients had maintenance immunosuppressive therapy with corticosteroid (prednisone) associated with mycophenolate mofetil and tacrolimus.

Statistical Analysis.
e continuous variables were described as mean ± standard deviation (SD) or median and interquartile range (IQR) according to their distribution. For the categorical variables, proportions were used (expected frequency, prevalence). To establish the association between the continuous variables, the Student t test, Mann-Whitney test, or Wilcoxon signed-rank test were used, and for the categorical variables, the χ 2 test was used.
e Kruskal-Wallis test was performed to analyze the differences between groups. Correlations of quantitative variables were performed by Spearman's rank test. Multivariable logistic regression was used to identify risk factors for persistent hyperparathyroidism. All statistical tests were two-tailed; p < 0.05 was considered statistically significant. We used IBM SPSS Statistics V25.0 (IBM SPSS ® , EEUU) and STATA V14 (StataCorp ® , EEUU) as statistical software.

Ethical Approval.
e study was conducted in accordance with the ethical principles specified in the Declaration of Helsinki, Good Clinical Practices, and regulatory requirements. Institutional scientific and ethics committees approved the study protocol. All patients provided written informed consent before participation.

Bone Mineral Metabolism before Kidney Transplantation.
Before KT, all patients had secondary hyperparathyroidism; no patient had parathyroidectomy. Hypocalcemia was evidenced in 40% (n � 30) and hyperphosphatemia in 86% (n � 64); 42% (n � 31) had hyperphosphatasemia. No patient had hyper-or hypomagnesaemia, hypercalcemia, or hypophosphatemia. No patients received calcimimetics after KT. e biochemical characteristics of bone mineral metabolism are shown in Table 2.

Discussion
Several studies had reported the changes in bone mineral metabolism after KT; however, there is a lack of information International Journal of Endocrinology 3 of these modifications in Latin KTR. In this study, we evaluated the biochemical characteristics of bone mineral metabolism, before and throughout the first year after KT in a Latin population. Bone mineral metabolism presents different changes before and after KT; in patients with CKD, the declining kidney function results in decreased phosphorus excretion and reduced 1,25-dihydroxyvitamin D production, with a consequent reduction of serum calcium levels and increased PTH secretion. e elevation of FGF-23 in CKD is related with hyperphosphatemia and the   International Journal of Endocrinology impacts in bone metabolism and immune system, having a relationship with dysfunction of kidney allograft, cardiovascular disease, osteoporosis, factures, cancer, and infectious diseases [5,6]. Due to the different changes in bone mineral metabolism, the Kidney Disease Improving Global Outcomes (KDIGO) recommends the early and routine monitoring of calcium-parathyroid hormone-vitamin D in KTR [7]. In our series, we evidenced a progressive reduction of PTH and alkaline phosphatase during the first year after KT; hypercalcemia was observed during the first 3 months, with a low frequency at 1st year; hypophosphatemia showed predominance during the first 3 months, with remission at 1 year; hypovitaminosis was found in all KTRs, with a requirement of high doses of cholecalciferol and longer time for achieve normalization of 25OHD. e improvement of bone mineral metabolism after successful KT is expected; however, KTR has multiple associated factors of BMMD.
BMMD after KT is characterized by changes in bone quality and calcium-parathyroid hormone-vitamin D axis alterations and is related with a low quality of life and a high risk of fracture, morbidity, and mortality [8]. Preexisting renal osteodystrophy at time of KT, effects and consequences of transplantation-specific therapies on bone (immunosuppression), and the effects of reduced renal function after KT are the major associated factors [9]. Secondary and tertiary hyperparathyroidism, hypovitaminosis D, osteopenia, osteoporosis, osteomalacia, and pathologic fractures are included in BMMD [7][8][9].
After KT, PTH decreases, especially, during the first three months; high PTH concentrations after 3 months have been related with persistent hyperparathyroidism, that may or may not improve over time. At 6 months after KT, PTH remains above normal in approximately one-third of patients. Spontaneous resolution of persistent hyperparathyroidism occurs within the first year in approximately 50% of KTR; however, a 30% to 60% can persist after 1 year, even 21% after 15 years [4,10,11]. Persistent hyperparathyroidism after KT is associated with negative outcomes, including low bone density, fractures, vascular calcification, cardiovascular disease, nephrocalcinosis, allograft dysfunction, graft loss, and all-cause mortality [10,11]. Long dialysis duration, high PTH prior to KT, high calcium or high alkaline phosphatase after KT, impaired kidney function after KT, parathyroid hyperplasia, older age, large maximum parathyroid gland size before KT, and monoclonal transformation (nodular hyperplasia) of parathyroid glands have been reported as risk factors of persistent hyperparathyroidism [9][10][11].
We found a similar frequency of persistent hyperparathyroidism at 1-year (40%) in Latin KTR, evidencing its association with high alkaline phosphatase before and after KT, hyperparathyroidism before and at 6 months after KT, and low concentrations of 25OHD. Hyperphosphatasemia before KTand at 6 months, hyperparathyroidism at 6 months, and vitamin D deficiency at 6 months were risk factors for persistent hyperparathyroidism in our population. ese findings lead a timely diagnosis and treatment of associated-factors to avoid the presence and the deleterious consequences of persistent hyperparathyroidism [2-4, 10, 11]. An important point to emphasize is the evidence that changes in bone mineral metabolism are independent of gender, etiology of nephropathy, type of dialysis, or precedence of KT; these data show that the assessment of bone status is essential in all KTRs.
e main strength of our study is the evaluation of biochemical parameters of bone mineral metabolism both before and after KT in Latin KTR, which represents the first published study that evaluates bone mineral metabolism at long term in this specific population. e limitations include the design of study and the lack of resources to complete bone mineral metabolism evaluation, including vitamin D status, before KT. We propose the establishment of strategies for prevention, diagnosis, and treatment of BMMD before and after KT, with the development of longitudinal studies in this population, to overcome these limitations.

Conclusion
Successful KT improves the biochemical parameters of bone mineral metabolism throughout the first year; however, persistent hyperparathyroidism can be found in KTR. Persistent hyperparathyroidism at 6 months after KT, hypovitaminosis D, and hyperphosphatasemia (before and after KT) are risk factors for persistent hyperparathyroidism at 1 year of KT.

Data Availability
All data generated or analyzed during this study are included in this published article. e database generated during the current study is available with the corresponding author on reasonable request.
Ethical Approval e present study protocol was reviewed and approved by the Institutional Review Board of Hospital de Especialidades Centro Médico Nacional Siglo XXI, IMSS (approval no. R-2017-3601-204).

Consent
Informed consent was obtained from all subjects when they were enrolled. 6 International Journal of Endocrinology