New analogs of vitamin D3

Advances in mechanobiological pharmacokinetic-pharmacodynamic models of osteoporosis treatment - Pathways to optimise and exploit existing therapies

Osteoporosis (OP) is a chronic progressive bone disease which is characterised by reduction of bone matrix volume and changes in the bone matrix properties which can ultimately lead to bone fracture. The two major forms of OP are related to aging and/or menopause. With the worldwide increase of the elderly population, particularly age-related OP poses a serious health issue which puts large pressure on health care systems. A major challenge for development of new drug treatments for OP and comparison of drug efficacy with existing treatments is due to current regulatory requirements which demand testing of drugs based on bone mineral density (BMD) in phase 2 trials and fracture risk in phase 3 trials. This requires large clinical trials to be conducted and to be run for long time periods, which is very costly. This, together with the fact that there are already many drugs available for treatment of OP, makes the development of new drugs inhibitive. Furthermore, an increased trend of the use of different sequential drug therapies has been observed in OP management, such as sequential anabolic-anticatabolic drug treatment or switching from one anticatabolic drug to another. Running clinical trials for concurrent and sequential therapies is neither feasible nor practical due to large number of combinatorial possibilities. In silico mechanobiological pharmacokinetic-pharmacodynamic (PK-PD) models of OP treatments allow predictions beyond BMD, i.e. bone microdamage and degree of mineralisation can also be monitored. This will help to inform clinical drug usage and development by identifying the most promising scenarios to be tested clinically (confirmatory trials rather than exploratory only trials), optimise trial design and identify subgroups of the population that show benefit-risk profiles (both good and bad) that are different from the average patient. In this review, we provide examples of the predictive capabilities of mechanobiological PK-PD models. These include simulation results of PMO treatment with denosumab, implications of denosumab drug holidays and coupling of bone remodelling models with calcium and phosphate systems models that allows to investigate the effects of co-morbidities such as hyperparathyroidism and chronic kidney disease together with calcium and vitamin D status on drug efficacy.

Peroxygenase-Catalyzed Selective Synthesis of Calcitriol Starting from Alfacalcidol

Calcitriol is an active analog of vitamin D3 and has excellent physiological activities in regulating healthy immune function. To synthesize the calcitriol compound, the concept of total synthesis is often adopted, which typically involves multiple steps and results in an overall low yield. Herein, we envisioned an enzymatic approach for the synthesis of calcitriol. Peroxygenase from Agrocybe aegerita (AaeUPO) was used as a catalyst to hydroxylate the C-H bond at the C-25 position of alfacalcidol and yielded the calcitriol in a single step. The enzymatic reaction yielded 80.3% product formation in excellent selectivity, with a turnover number up to 4000. In a semi-preparative scale synthesis, 72% isolated yield was obtained. It was also found that AaeUPO is capable of hydroxylating the C-H bond at the C-1 position of vitamin D3, thereby enabling the calcitriol synthesis directly from vitamin D3.

Long-term outcomes and management considerations after parathyroidectomy in the dialysis patient

Parathyroidectomy (PTX) remains an important intervention for dialysis patients with poorly controlled secondary hyperparathyroidism (SHPT), though there are only retrospective and observational data that show a mortality benefit to this procedure. Potential consequences that we seek to avoid after PTX include persistent or recurrent hyperparathyroidism, and parathyroid insufficiency. There is considerable subjectivity in defining and diagnosing these conditions, given that we poorly understand the optimal PTH targets (particularly post PTX) needed to maintain bone and vascular health. While lowering PTH after PTX decreases bone turnover, long-term changes in bone activity have been poorly explored. High turnover bone disease, usually present at the time a PTX is considered, often swings to a state of low turnover in the setting of sufficiently low PTH levels. It remains unclear if all low bone turnover equate with disease. However, such changes in bone turnover appear to predispose to vascular calcification, with positive calcium balance after PTX being a potential contributor. We know little of how the post-PTX state resets calcium balance, how calcium and VDRA requirements change or what kind of adjustments are needed to avoid calcium loading. The current consensus cautions against excessive reduction of PTH although there is insufficient evidence-based guidance regarding the management of chronic kidney disease - mineral bone disease (CKD-MBD) parameters in the post-PTX state. This article aims to compile existing research, provide an overview of current practice with regard to PTX and post-PTX chronic management. It highlights gaps and controversies and aims to re-orient the focus to clinically relevant contemporary priorities in CKD-MBD management after PTX.

Medical and Surgical Treatment for Secondary and Tertiary Hyperparathyroidism

Prevention and treatment of secondary hyperparathyroidism (SHPT) in patients on chronic maintenance hemodialysis and of tertiary hyperparathyroidism (THPT) in patients after kidney transplantation is a challenge for the nephrologist and for the surgeon. Indication and results of medical and surgical therapy for SHPT and THPT have remained under discussion during the last decades. This review resumes the current medical and surgical strategies for patients with SHPT and THPT.

Modulation of the immune response to respiratory viruses by vitamin D

Background: Vitamin D deficiency has been shown to be independently associated with increased risk of viral acute respiratory infection (ARI) in a number of observational studies, and meta-analysis of clinical trials of vitamin D supplementation for prevention of ARI has demonstrated protective effects. Several cellular studies have investigated the effects of vitamin D metabolites on immune responses to respiratory viruses, but syntheses of these reports are lacking. Scope: In this article, we review the literature reporting results of in vitro experiments investigating immunomodulatory actions of vitamin D metabolites in human respiratory epithelial cells infected with respiratory viruses. Key findings: Vitamin D metabolites do not consistently influence replication or clearance of rhinovirus, respiratory syncytial virus (RSV) or influenza A virus in human respiratory epithelial cell culture, although they do modulate expression and secretion of type 1 interferon, chemokines including CXCL8 and CXCL10 and pro-inflammatory cytokines, such as TNF and IL-6. Future research: More studies are needed to clarify the effects of vitamin D metabolites on respiratory virus-induced expression of cell surface markers mediating viral entry and bacterial adhesion to respiratory epithelial cells.

Vitamin D receptor regulates TGF-β signalling in systemic sclerosis

BACKGROUND

Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily. Its ligand, 1,25-(OH)2D, is a metabolically active hormone derived from vitamin D3. The levels of vitamin D3 are decreased in patients with systemic sclerosis (SSc). Here, we aimed to analyse the role of VDR signalling in fibrosis.

METHODS

VDR expression was analysed in SSc skin, experimental fibrosis and human fibroblasts. VDR signalling was modulated by siRNA and with the selective agonist paricalcitol. The effects of VDR on Smad signalling were analysed by reporter assays, target gene analyses and coimmunoprecipitation. The effects of paricalcitol were evaluated in the models of bleomycin-induced fibrosis and fibrosis induced by overexpression of a constitutively active transforming growth factor-β (TGF-β) receptor I (TBRI(CA)).

RESULTS

VDR expression was decreased in fibroblasts of SSc patients and murine models of SSc in a TGF-β-dependent manner. Knockdown of VDR enhanced the sensitivity of fibroblasts towards TGF-β. In contrast, activation of VDR by paricalcitol reduced the stimulatory effects of TGF-β on fibroblasts and inhibited collagen release and myofibroblast differentiation. Paricalcitol stimulated the formation of complexes between VDR and phosphorylated Smad3 in fibroblasts to inhibit Smad-dependent transcription. Preventive and therapeutic treatment with paricalcitol exerted potent antifibrotic effects and ameliorated bleomycin- as well as TBRI(CA)-induced fibrosis.

CONCLUSIONS

We characterise VDR as a negative regulator of TGF-β/Smad signalling. Impaired VDR signalling with reduced expression of VDR and decreased levels of its ligand may thus contribute to hyperactive TGF-β signalling and aberrant fibroblast activation in SSc.

Initial parathyroid surgery in 606 patients with renal hyperparathyroidism

BACKGROUND

The aim of the present study was to evaluate the outcome of different surgical procedures for patients on permanent dialysis who underwent initial parathyroidectomy for renal hyperparathyroidism (rHPT).

METHODS

Out of a prospective database of patients who underwent parathyroid surgery for rHPT between 1976 and 2009, patients on permanent dialysis who underwent initial parathyroidectomy were further analyzed regarding perioperative biochemical changes and postoperative outcome.

RESULTS

A total of 606 patients were analyzed. Total parathyroidectomy with autotransplantation (group A) was performed in 504 patients, total parathyroidectomy without autotransplantation in 32 (group B), subtotal parathyroidectomy in 21 (group C), and incomplete parathyroidectomy in 49 (group D). After surgery, mean calcium levels dropped from 2.76 to 1.91 mmol/l in group A, from 2.67 to 2.11 mmol/l in group B, from 2.70 to 2.09 mmol/l in group C, and from 2.65 to 1.94 mmol/l in group D. The parathyroid hormone level dropped from 1,371.4 pg/ml to 28.8 pg/ml in group A, from 1,078.4 pg/ml to 27.0 pg/ml in group B, from 2,377.9 pg/ml to 61.4 pg/ml in group C, and from 1,010.2 pg/ml to 99.5 pg/ml in group D. Persistent rHPT occurred in 2/504 patients from group A (0.4%), 0/32 patients from group B (0%), 1/21 patients from group C (4.8%), and 2/49 patients from group D (4.1%). After a mean follow-up of 57.6 months, recurrent rHPT occurred in 27/504 patients from group A (5.4%), in 0/32 patients from group B (0%), in 2/21 patients from group C (9.5%), and in 3/49 patients from group D (6.1%).

CONCLUSIONS

Total parathyroidectomy with or without autotransplantation is a feasible and safe surgical procedure for patients on permanent dialysis with otherwise uncontrollable rHPT.

Coping with time scales in disease systems analysis: application to bone remodeling

In this study we demonstrate the added value of mathematical model reduction for characterizing complex dynamic systems using bone remodeling as an example. We show that for the given parameter values, the mechanistic RANK-RANKL-OPG pathway model proposed by Lemaire et al. (J Theor Biol 229:293-309, 2004) can be reduced to a simpler model, which can describe the dynamics of the full Lemaire model to very good approximation. The response of both models to changes in the underlying physiology and therapeutic interventions was evaluated in four physiologically meaningful scenarios: (i) estrogen deficiency/estrogen replacement therapy, (ii) Vitamin D deficiency, (iii) ageing, and (iv) chronic glucocorticoid treatment and its cessation. It was found that on the time scale of disease progression and therapeutic intervention, the models showed negligible differences in their dynamic properties and were both suitable for characterizing the impact of estrogen deficiency and estrogen replacement therapy, Vitamin D deficiency, ageing, and chronic glucocorticoid treatment and its cessation on bone forming (osteoblasts) and bone resorbing (osteoclasts) cells. It was also demonstrated how the simpler model could help in elucidating qualitative properties of the observed dynamics, such as the absence of overshoot and rebound, and the different dynamics of onset and washout.

Paricalcitol in secondary hyperparathyroidism and the survival benefit in patients with chronic kidney disease

Secondary hyperparathyroidism is a characteristic feature of chronic kidney disease, which develops early in the course of chronic kidney disease, often in a progressive way. It occurs as the renal function continues to decline and is encountered following a series of biochemical abnormalities, which are responsible for initiation and maintenance of increased parathyroid hormone (PTH) secretion. Several agents are used in the management of secondary hyperparathyroidism. Paricalcitol is a new generation selective vitamin D receptor activator that lowers PTH levels by exerting a less hypercalcaemic and hyperphosphataemic effect. In addition, there is emerging evidence of the benefit of paricalcitol in preventing intravascular calcification and proteinuria.

Vitamin D, vitamin D analogs (deltanoids) and prostate cancer

'Vitamin D' is a generic term for a family of secosteroids, members of which bind to the vitamin D receptor. Calcitriol, the active form of vitamin D, has antiproliferative effects on many tumor cells. However, clinical use of calcitriol in cancer prevention or therapy is limited because it induces hypercalcemia at the necessary supraphysiological doses. The anti-tumor effects of vitamin D analogs (deltanoids) have been researched extensively; more than 3000 deltanoids have now been described. Prostate cancer is more common in northern geographic regions; mortality decreases with exposure to sunlight. As UV light is necessary for vitamin D synthesis in the skin, it has long been dogma that vitamin D is involved. This review concerns deltanoids that have been assessed for use in treating or preventing prostate cancer.

Oxidative Stress and Inflammation in Chronic Kidney Disease: Role of Intravenous Iron and Vitamin D

Cardiovascular disease (CVD) is the leading cause of death among chronic kidney disease patients (CKD). The etiology of CVD in CKD is multifactorial and increasing evidence points to the important contribution of “nontraditional” risk factors including oxidative stress and inflammation. CKD is associated with a chronic imbalance of prooxidant and antioxidant factors that results in a state of chronic inflammation. Intravenous iron supplementation has been shown to induce oxidative stress and has been associated with lipid peroxidation and DNA damage. Conversely, treatment with vitamin D analogs has been associated with improved mortality in hemodialysis patients in 2 recent large cohort studies. These data suggest that vitamin D analogs may exert effects beyond their pharmacologic role in parathyroid hormone suppression. This article addresses the current data regarding the relative contributions of intravenous iron supplementation and vitamin D analog therapy on oxidative stress and inflammation in CKD patients.

Comparison of total parathyroidectomy without autotransplantation and without thymectomy versus total parathyroidectomy with autotransplantation and with thymectomy for secondary hyperparathyroidism: TOPAR PILOT-Trial

BACKGROUND

Secondary hyperparathyroidism (sHPT) is common in patients with chronic renal failure. Despite the initiation of new therapeutic agents, several patients will require parathyroidectomy (PTX). Total PTX with autotransplantation of parathyroid tissue (TPTX+AT) and subtotal parathyroidectomy (SPTX) are currently considered as standard surgical procedures in the treatment of sHPT. Recurrencerates after TPTX+AT or SPTX are between 10% and 12% (median follow up: 36 months). Recent retrospective studies demonstrated a lower rate of recurrent sHPT of 0-4% after PTX without autotransplantation and thymectomy (TPTX) with no higher morbidity when compared to the standard procedures. The observed superiority of TPTX is flawed due to different definitions of outcomes, varying follow up periods and different surgical treatment strategies (with and without thymectomy).

METHODS/DESIGN

Patients with sHPT (intact parathyroid hormone > 10 times above the upper limit of normal) on long term dialysis (>12 months) will be randomized either to TPTX or TPTX+AT and followed for 36 months. Outcome parameters are recurrence rates of sHPT, frequencies of reoperations due to refractory hypoparathyroidism or recurrent/persistent hyperparathyroidism, postoperative morbidity and mortality and quality of life. 50 patients per group will be randomized in order to obtain relevant frequencies of outcome parameters that will form the basis for a large scale confirmatory multicentred randomized controlled trial.

DISCUSSION

sHPT is a disease with a high incidence in patients with chronic renal failure. Even a small difference in outcomes will be of clinical relevance. To assess sufficient data about the rate of recurrent sHPT after both methods, a multicentred, randomized controlled trial (MRCT) under standardized conditions is mandatory. Due to the existing uncertainties the calculated number of patients necessary in each treatment arm (n > 4000) makes it impossible to perform this study as a confirmatory trial. Therefore estimates of different outcomes are performed using a pilot MRCT comparing 50 versus 50 randomized patients in order to establish a hypothesis that can be tested thereafter. If TPTX proves to have a lower rate of recurrent sHPT, no relevant disadvantages and no higher morbidity than TPTX+AT, current surgical practice may be changed.

TRIAL REGISTRATION

International Standard Randomized Controlled Trial Number Registration (ISRCTN86202793).

The Influences of Method of Calcium Correction and the Timing of Blood Collection on Application of The K/DOQI Clinical Practice Guidelines for Bone Metabolism and Disease in Japan

We investigated the treatment of renal osteodystrophy (ROD) in Japan and problems concerning the K/DOQI Guidelines. The subjects were 3698 hemodialysis patients (2328 males and 1370 females) with a mean age of 61.4 years. On average, they had been on hemodialysis for 8.34 years. The serum phosphorus level was <3.5 mg/dL in 5% of the subjects, 3.5-5.5 mg/dL in 49%, 5.6-7.0 mg/dL in 33%, and >7.0 mg/dL in 13%. The serum calcium level was <8.4 mg/dL in 16% of the subjects, 8.4-9.5 mg/dL in 47%, 9.5-10.2 mg/dL in 22%, and >10.2 mg/dL in 15%. The intact PTH level was <150 pg/mL in 57%, 150-300 pg/mL in 27%, and >300 pg/mL in 16% of the patients. The first problem is that correcting Ca is not always performed in clinical fields. The uncorrected calcium level was 9.14+/-0.92 mg/dL, while the corrected calcium level [Ca = Ca + 0.8 x (4-Alb)] was 9.26+/-0.93 mg/dL (P < 0.05). The second problem is that the timing of blood collection is not described in the K/DOQI Guidelines. Subjects with a serum phosphorus level >7.0 mg/dL at 3 days after the previous dialysis were selected for assessment. In these patients, the midweek serum phosphorus level (7.13+/-0.15 mg/dL) at was significantly lower than that (8.11+/-0.15 mg/dL) at the beginning of the next week (P < 0.001). These results suggest that it is necessary to specify the timing of measurement and the method of Ca correction when guidelines for management of ROD are established in the future.

Differential effects of Vitamin D analogs on bone formation and resorption

Deficiency in Vitamin D and its metabolites leads to a failure in bone formation primarily caused by dysfunctional mineralization, suggesting that Vitamin D analogs might stimulate osteoblastic bone formation and mineralization. In this study, we compare the effect of selected Vitamin D analogs and active metabolite, 1alpha,25-dihydroxyvitamin D(3), 19-nor-1alpha, 25-dihydroxyvitamin D(2), and 1alpha-hydroxyvitamin D(2) or 1alpha,25-dihydroxyvitamin D(2) on bone formation and resorption. In a mouse calvariae bone primary organ culture system, all Vitamin D analogs and metabolite tested-stimulated collagen synthesis in a dose-dependent manner and 19-nor-1alpha, 25-dihydroxyvitamin D(2) was the most efficacious among three. 19-nor-1alpha, 25-dihydroxyvitamin D(2) and 1alpha,25-dihydroxyvitamin D(2) showed similar potencies and 1alpha,25-dihydroxyvitamin D(3) was less potent than others. Osteocalcin was also up-regulated in a dose-dependent manner, suggesting that the three Vitamin D analogs have the equal potencies on bone formation. 25-Hydroxyvitamin D-24-hydroxylase expression was induced in a dose-dependent manner and 19-nor-1alpha, 25-dihydroxyvitamin D(2) was less potent than other two compounds. In a mouse calvariae organ culture, all induced a net calcium release from calvariae in a dose-dependent manner, but the potency is in the order of 1alpha,25-dihydroxyvitamin D(2) congruent with1alpha,25-dihydroxyvitamin D(3)>19-nor-1alpha, 25-dihydroxyvitamin D(2). In a Vitamin D/calcium-restricted rat model, all caused an elevation in serum calcium in a dose-dependent manner. There is no significant difference between 1alpha,25-dihydroxyvitamin D(3) and 1alpha-hydroxyvitamin D(2) in potencies, but 19-nor-1alpha, 25-dihydroxyvitamin D(2) is at least 10-fold less potent than the other two compounds. Our results suggest that Vitamin D analogs have direct effects on bone resorption and formation, and 19-nor-1alpha, 25-dihydroxyvitamin D(2) may be more effective than 1alpha,25-dihydroxyvitamin D(3) and 1alpha-hydroxyvitamin D(2) on stimulating anabolic bone formation.

Alternative promoters and renal cell-specific regulation of the mouse type IIa sodium-dependent phosphate cotransporter gene

The type IIa sodium-dependent phosphate cotransporter (NPT2a) expressed in renal proximal tubules represents an important determinant in maintaining inorganic phosphate (Pi) homeostasis. In the present study, we identified two variant transcripts of the mouse NPT2a gene, Npt2a-v1 and Npt2a-v2, characterized by the presence of alternative first exons (either exon 1A or exon 1B). The chromosomal structure analysis revealed that the Npt2a gene comprises of two promoters (promoters 1 and 2) and 14 exons, and spans approximately 17 kb. Quantitative PCR analysis showed that renal mRNA levels of both the variants markedly decreased in X-linked vitamin D-resistant hypophosphatemic rickets (Hyp) mice compared to normal littermates. Interestingly, transcriptional activity of a reporter gene, containing Npt2a promoters 1 and 2, was renal cell-specifically increased by 1alpha, 25(OH)2D3 and its analogs. The deletion analysis revealed that the CAAT box in the Npt2a promoter 2 is important for the 1alpha, 25(OH)2D3-dependent renal cell-specific activation of the reporter gene. These data suggested that two alternative promoters control the renal expression of Npt2a gene and both Npt2a variant transcripts are down regulated in Hyp mice.

Complications of chronic kidney disease: anemia, mineral metabolism, and cardiovascular disease

This article focuses on the importance of three major complications of chronic kidney disease: (1) anemia, (2) calcium-phosphorus regulation and bone disease, and (3) cardiovascular risk profiling and treatment. The arguments for early and effective intervention have been amply made with respect to these three complications. Substantive trial data are sorely need to provide the definitive evidence that effective treatment of these complications results in better outcomes.

Modeling the interactions between osteoblast and osteoclast activities in bone remodeling

We propose a mathematical model explaining the interactions between osteoblasts and osteoclasts, two cell types specialized in the maintenance of the bone integrity. Bone is a dynamic, living tissue whose structure and shape continuously evolves during life. It has the ability to change architecture by removal of old bone and replacement with newly formed bone in a localized process called remodeling. The model described here is based on the idea that the relative proportions of immature and mature osteoblasts control the degree of osteoclastic activity. In addition, osteoclasts control osteoblasts differentially depending on their stage of differentiation. Despite the tremendous complexity of the bone regulatory system and its fragmentary understanding, we obtain surprisingly good correlations between the model simulations and the experimental observations extracted from the literature. The model results corroborate all behaviors of the bone remodeling system that we have simulated, including the tight coupling between osteoblasts and osteoclasts, the catabolic effect induced by continuous administration of PTH, the catabolic action of RANKL, as well as its reversal by soluble antagonist OPG. The model is also able to simulate metabolic bone diseases such as estrogen deficiency, vitamin D deficiency, senescence and glucocorticoid excess. Conversely, possible routes for therapeutic interventions are tested and evaluated. Our model confirms that anti-resorptive therapies are unable to partially restore bone loss, whereas bone formation therapies yield better results. The model enables us to determine and evaluate potential therapies based on their efficacy. In particular, the model predicts that combinations of anti-resorptive and anabolic therapies provide significant benefits compared with monotherapy, especially for certain type of skeletal disease. Finally, the model clearly indicates that increasing the size of the pool of preosteoblasts is an essential ingredient for the therapeutic manipulation of bone formation. This model was conceived as the first step in a bone turnover modeling platform. These initial modeling results are extremely encouraging and lead us to proceed with additional explorations into bone turnover and skeletal remodeling.

Differential effects of acute administration of 19-Nor-1,25-dihydroxy-vitamin D2 and 1,25-dihydroxy-vitamin D3 on serum calcium and phosphorus in hemodialysis patients

BACKGROUND

Treatment of hyperparathyroidism includes the use of 1,25-dihydroxy-vitamin D3 (1,25D3) to suppress parathyroid hormone (PTH), but dosing of 1,25D3 is limited by the development of hypercalcemia and a high calcium x phosphorus (Ca x P) product because of gut absorption of calcium and phosphorus and enhanced bone resorption. The vitamin D analogue 19-nor-1,25(OH)2-vitamin D2 (19-Nor) causes less hypercalcemia and elevated Ca x P, whereas it still suppresses PTH in rats.

METHODS

To determine whether 19-Nor had similar effects in humans, we performed a prospective crossover study to assess bone mobilization. Ten hemodialysis patients on a low-calcium low-phosphorus diet were administered 20 microg of 1,25D3 and 120 and 160 microg of 19-Nor, and changes in calcium, phosphorus, and intact and whole PTH levels were measured over 36 hours.

RESULTS

Ca x P product increased more after 1,25D3 administration than after a six- or eightfold greater dose of 19-Nor and was significantly greater at 6, 12, and 24 hours. Ca x P product at 36 hours was 60.9 +/- 3.4 (4.91 +/- 0.27 mmol2/2) after 1,25D3 administration, 53.2 +/- 2.7 (4.29 +/- 0.22 mmol2/L2) after administration of 120 microg of 19-Nor, and 54.2 +/- 2.7 (4.37 +/- 0.22 mmol2/L2) after administration of 160 microg of 19-Nor. Suppression of intact PTH at 36 hours was similar after administration of 1,25D3 (54.1% +/- 6.0%) and 120 microg of 19-Nor (54.4% +/- 3.4%) and significantly greater after administration of 160 microg of 19-Nor (63.6% +/- 2.3%). The whole PTH assay yielded values approximately 25% to 30% lower than the intact PTH assay, and the percentage of suppression was virtually identical.

CONCLUSION

Consistent with animal studies, 19-Nor provides profound PTH suppression while stimulating bone resorption and/or intestinal absorption less than 1,25D3, resulting in less elevation of serum calcium and phosphorus levels.

The role of vitamin D in mild to moderate chronic kidney disease

Dietary vitamin D is a prohormone that is metabolized to the bioactive vitamin D hormone, 1 alpha, 25-dihydroxyvitamin D [1,25-(OH)2D]. 1,25-(OH)2D has been implicated in a variety of regulatory pathways that extend well beyond its traditional function in Ca2+ homeostasis. In uncovering these diverse functions, investigators have focused on the complex interaction between 1,25-(OH)2D and parathyroid hormone (PTH). Here, we present an overview of the functions of vitamin D hormone and PTH in the clinical context of secondary hyperparathyroidism. We discuss recent developments in treatment that address imbalances in vitamin D hormone and PTH levels, supporting the argument that early intervention can reduce the risk of metabolic complications caused by vitamin D hormone deficiency in patients with chronic kidney disease.

Medical and surgical management of hyperparathyroidism

Hypercalcemia is frequently encountered in healthy outpatients. Reliable measurements of the mediators of hypercalcemia have improved diagnostic certainty about the etiology in most patients. Hyperparathyroidism is overwhelmingly the most common cause. Medical evaluation of the patient with hyperparathyroidism requires an understanding of the complications of the disorder and the associated syndromes. At present decreased bone mineral density and nephrolithiasis are the major sequelae of hyperparathyroidism. Most cases of primary hyperparathyroidism are sporadic; however, hereditary forms can occur in patients with the multiple endocrine neoplasia syndromes. Surgery is the only curative therapy. Results are excellent when an experienced endocrine surgeon performs parathyroid surgery.

Parathyroid hormone-stimulated resorption in calvaria cultured in serum-free medium is enhanced by the calcium-mobilizing activity of 1,25-dihydroxyvitamin D(3)

1,25(OH)(2)D(3) enhances parathyroid hormone (PTH)-induced Ca(2+) signaling in osteoblasts by activating plasma membrane voltage-sensitive Ca(2+) channels (VSCCs). The ability of 1,25(OH)(2)D(3) or the VSCC-activating analog AT (25-hydroxy-16-ene-23-yne-D(3)) to enhance parathyroid hormone-stimulated (45)Ca(2+) release from cultured new-born rat calvaria was measured. Analog BT (1,24-dihydroxy-22-ene-24-cyclopropyl-D(3)), that does not mobilize Ca(2+), also was tested along with PTH. Control experiments were performed with and without PTH and with and without serum. Individual calvaria labeled in utero with (45)Ca(2+) were cultivated in serum-free medium on filters at the medium/air interface of 24-well culture plates and (45)Ca(2+) release followed over 72 h. The results demonstrated that 1,25(OH)(2)D(3) and the Ca(2+)-mobilizing analog, AT, but not the nuclear receptor-binding analog, BT, enhanced PTH-stimulated (45)Ca(2+) release under serum-free conditions. This enhancement effect of the seco-steroids was not evident in the presence of 10% fetal calf serum. The effect of analog AT was faster than that of 1,25(OH)(2)D(3). Nitrendipine, a specific L-type VSCC blocker, attenuated enhancement by vitamin D compounds, indicating that the high-threshold L-type VSCC is a molecular transducer of costimulation. These results emphasize the synergy between the calcitropic hormones 1,25(OH)(2)D(3) and PTH in cultures containing osteoblasts and osteoclasts, and suggest that the Ca(2+)-mobilizing activity of 1,25(OH)(2)D(3) enhances Ca(2+) release from bone.

Ligand affinity, homodimerization, and ligand-induced secondary structural change of the human vitamin d receptor

The intrinsic tryptophan fluorescence signal of the full-length nuclear receptor hVDR was used to directly determine the dissociation constants, K(d), of two ligands yielding K(d) = 32 nM for 1alpha,25(OH)(2)D(3) and K(d) = 322 nM for 25(OH)D(3). Ligand binding was accompanied by a conformational change in the alpha-helical part of hVDR as revealed by CD spectroscopy. In addition, the presence of calcitriol was found to be a necessary prerequisite for the homodimerisation of hVDR which was monitored using fluorescence anisotropy. We conclude that the observed ligand-induced structural change of hVDR is conditional for dimerisation of the protein.

Pathophysiological mechanisms of vascular calcification in end-stage renal disease

Vascular calcification has been clearly defined as a risk factor for cardiovascular mortality in the general population and is highly prevalent in end-stage renal disease (ESRD), where it is associated with a number of markers of increased mortality such as left ventricular hypertrophy. The pattern of calcification in ESRD is characterized by mineral deposition in the tunica media, in contrast to non-ESRD populations, where calcification of atheromatous plaque predominates. This difference may have important clinical implications. The pathophysiological mechanisms underlying both types of vascular calcification remain to be clarified; however, current evidence suggests that they are active processes rather than passive mineral precipitation, and the presence in the vasculature of cells expressing an osteoblastic phenotype may be of central importance. In ESRD, the presence of secondary and tertiary hyperparathyroidism, disordered calcium and phosphate homeostasis, and the use of vitamin D- and calcium-based treatments in its therapy may all contribute to vascular calcification. These issues and the impact on other current and future therapies have great importance for clinical nephrology, and a better understanding of vascular calcification through a focused research effort is essential.