Secondary hyperparathyroidism (SH) is a universal complication of chronic renal failure. In early renal failure, alteration in vitamin D metabolism plays a key role in the development of SH1,2,3,4. Low levels of 1,25(OH)2D3 and decreased repression of the PTH gene transcription may allow a greater synthesis and secretion of PTH. As renal disease progresses, the number of vitamin D receptors (VDR) in the parathyroid glands (PTG) decrease5,6,7,8; thus, the PTG becomes resistant to the action of 1,25(OH)2D3. In addition, "uremic toxins" may further decrease the suppressive effect of 1,25(OH)2D39. Concomitantly with the above-described alterations, hyperplasia of the PTG develops and a decreased number of calcium receptors (CaR)10,11 further increases the resistance of the PTG to serum ICa. Thus, higher serum calcium is necessary to suppress SH. Recently, investigators examined the clonality of hyperplastic tumors using X-chromosome inactivation analysis12. In about two thirds of uremic patients with refractory S.H. harbored at least one monoclonal parathyroid tumor. Phosphate independent of serum Ca and 1,25(OH)2D3 increases PTH synthesis and secretion by a post-transcriptional mechanism13,14,15,16,17,18,19. Dietary phosphate also regulates parathyroid growth20. Low phosphate diet increases p21, a repressor of the cell cycle and inhibitor of parathyroid gland hyperplasia, while high phosphate enhances transforming growth factor alpha (TGF
) and the epidermal growth factor receptor (EGFR) known to play an important role on cell proliferation.
The vitamin D hormone, 1,25(OH)2D3 (calcitriol), the most active metabolite of vitamin D, controls parathyroid gland growth and suppresses the synthesis and secretion of parathyroid hormone. Because of its effects on PTH suppression, calcitriol has been successfully used in the treatment of secondary hyperparathyroidism that almost always accompanies chronic renal failure21,22. The efficacy of intravenous calcitriol in suppressing PTH in patients with secondary hyperparathyroidism is well established23,24. However, because of its potent effects on intestinal calcium and phosphate absorption and bone calcium and phosphate mobilization, calcitriol can induce hypercalcemia and hyperphosphatemia, often precluding its use at therapeutic doses. Therefore, an analog of calcitriol that retains the therapeutic effects but has minor effects on calcium and phosphate metabolism would be an ideal tool for the treatment of secondary hyperparathyroidism.
VITAMIN D ANALOGS
The biological actions of calcitriol are mediated by a nuclear VDR. At present, there is evidence for only a single form of the VDR. Thus, the same VDR mediates both the calcemic actions and the non-classical potentially therapeutic actions of calcitriol. The novel aspect of recently developed analogs is their differential actions, compared to calcitriol in vivo. In fact, as these analogs have a relatively high affinity for the vitamin D receptor, usually within one order of magnitude, it is not unexpected that they are able to mimic many of the actions of calcitriol in vivo. The unique feature of therapeutically useful analogs is their ability to efficiently support some but not all calcitriol associated activities. The potential mechanisms through which this selectivity could be achieved are summarized in Figure 1. Most commonly, the analogs display decreased potency in enhancing intestinal absorption or bone mobilization of calcium and phosphate. The selectivity is not always cell or tissue specific but can be gene or process specific within the same tissue.
Figure 1.
Potential sites of differential actions of 1,25(OH)2D3 and its analogs. The possible steps in the vitamin D activation pathways at which differences in vitamin D analog action could lead to selective activities in vivo are shown. The steps diagrammed include: 1) DBP affinity, 2) interaction with other serum proteins including lipoproteins, 3) cellular uptake, 4) conversing to active metabolites, 5) catabolic inactivation, 6) activation of the nongenomic pathway through a membrane vitamin D receptor (mVDR), 7) interaction with the nuclear vitamin D receptor (VDR), 8) formation of the VDR-RXR complex, 9) binding to the activated complex to DNA, and 10) formation of the preinitiating complex RNA polymerase II (RNApol) (reproduced from40; used with permission).
Full figure and legend (49K)The structure-activity relationship for ligand-mediated transcriptional regulation has been studied in detail25. The A-ring structure is most crucial, especially the hydroxyl groups, for binding to the VDR. Modification of the D-ring or side chain does not greatly affect VDR binding, but can influence biological potency by altering the pharmacokinetics or catabolism. Analogs can also produce distinct conformational changes in the VDR that may produce gene-specific actions. A combination of structural modifications can produce analogs with diverse biological profiles.
Recruitment of coactivators or co-repressors also can play an important role on vitamin D analogs transcription-induced biological actions. Work from Takeyema et al26 demonstrated that calcitriol can recruit binding of several coactivators to the VDR that may enhance the activation of transcription, whereas 22 oxa-calcitriol recruits only a subset of these, which could potentially produce biological effects distinct from those of calcitriol.
22-OXACALCTRIOL (OCT)
OCT differs from calcitriol only by a substitution of an oxygen in place of carbon 22 in the side chain Figure 2. The affinity of 22-oxacalcitriol for the VDR is about 8 times lower than that calcitriol, consistent with its lower activity in suppressing PTH. Studies in animals demonstrated that OCT is rapidly cleared from the circulation; the short half life may be secondary to diminished affinity for DBP that is approximately 400–500 times less than of calcitriol27,28,29,30. The low calcemic and phosphatemic effect of OCT may be secondary to low affinity for DBP. Currently the mechanism for the differences in the duration of the effects in the parathyroid glands versus the intestine and bone are not completely understood, but the findings indicated that stimulation of intestinal calcium absorption and bone resorption are short-lived responses that require continuous exposure to vitamin D compounds. On the other hand, even a short exposure of the parathyroid gland to OCT leads to a prolonged suppression of PTH. The mechanism of PTH suppression by OCT was similar to that of calcitriol in that the analog decreased PTH mRNA, suggesting that it was also acting at the level of gene transcription31. Studies in animal models with experimental renal failure demonstrated that OCT was able to suppress PTH over a wide dose range with no change in serum calcium. In contrast, doses of calcitriol just above those that suppress PTH produced a significant increase in serum calcium. The effect of OCT on renal osteodystrophy was examined by Monier-Faugere et al32 in dogs made uremic by subtotal nephrectomy. OCT significantly decreased PTH levels. The analog reversed abnormalities in bone formation, including woven osteoid and fibrosis. However, no change in the rate of bone turnover was observed. While hypercalcemic episodes occurred, OCT did not induce low turnover bone disease. OCT significantly reduced bone marrow fibrosis and decreased markers of bone turnover in patients with end stage renal failure33.
Figure 2.
Chemical structure of calcitriol and several vitamin D analogs.
Full figure and legend (34K)19-NOR 1,25(OH)2D2 (19-NOR)
This vitamin analog lacks the exocyclic carbon 19 and has a vitamin D2 side chain (double bone in carbon 22 and extra carbon in 28 position) Figure 2. We demonstrated that 19-nor 1,25(OH)2D2 suppress parathyroid hormone secretion in primary cultures of bovine parathyroid cells as potently as calcitriol34. In addition, this compound can suppress pre-pro PTH messenger RNA and PTH secretion without inducing hypercalcemia or hyperphosphatemia.
Daily administration of 19-nor 1,25(OH)2D2 to parathyroidectomized rats fed either a calcium or a phosphorus-deficient diet for 9 days produced smaller increases in plasma calcium and phosphate than calcitriol. Dose-response studies demonstrated that 19-nor 1,25(OH)2D2 is approximately 10 times less active than calcitriol in mobilizing calcium and phosphate from bone Figure 335. Moreover, in contrast to calcitriol, which up regulates the VDR in the intestine, 19-nor-1,25(OH)2D2 has the opposite effect36 Figure 4.
Figure 3.
Effects of 1,25(OH)2D3 and 19-nor 1,25(OH)2D2 on plasma ionized calcium levels in parathyroidectomized rats fed a calcium-deficient diet. Rats were given daily injections of vehicle (white bar), 1,25(OH)2D3, or 19-nor-1,25(OH)2D2 for 9 days.
Full figure and legend (16K)Figure 4.
Effects of 1,25(OH)2D3 and 19-nor 1,25(OH)2D2 on intestinal 1,25(OH)2D3- VDR binding in uremic rats. Rats were treated with vehicle, 1,25(OH)2D3 (2 or 6 ng), or 19-nor-1,25(OH)2D2 (25 ng or 100 ng) three times a week for 8 weeks. All data are mean 
SEM. N = 11 to 15 rats per group. Asterisk and double asterisk indicate P < 0.01 an P < 0.05 versus uremic + 1,25(OH)2D3–6 ng, respectively (reproduced from36; used with permission).
The efficacy of 19-nor 1,25(OH)2D2 in renal failure patients was demonstrated in a recent study37 in 78 patients. Placebo was given to approximately one third of the patients and 19-nor 1,25(OH)2D2 was administered to the other two-thirds The dose was initially 0.04 
g/kg and rose to an average of 0.12 g/kg during the course of the 7 week study. Serum PTH levels dropped an average of approximately 60% Figure 5 with only a slight increase in serum calcium, from 9.24 0.12 to 9.5 0.15 Figure 6.
Figure 5.
Changes in the levels of intact PTH expressed as a percentage of change from baseline values during the study period in placebo-treated (open circle) and paricalcitriol-treated (closed circle) groups. The bars depict the doses of paricalcitriol that increase according to protocol. (Reproduced from37; used with permission).
Full figure and legend (29K)Figure 6.
The values for normalized serum calcium (upper lines) and serum phosphorus (lower lines) during the 12 weeks of study in placebo (open circle) and paricalcitriol (closed circle) groups. *P < 0.05. (Reproduced from37; used with permission).
Full figure and legend (26K)1
(OH)D2
This vitamin D analog is a pro-hormone and must be converted by the liver to 1,25(OH)2D2 before it becomes an active compound Figure 2. The basis for the low calcemic activity is much less understood. Early studies with 1(OH)D2 show that it is less toxic than 1(OH)D3 when the compounds were administered chronically38. Paradoxically, the stimulation of calcium transport and bone mobilization by 1(OH)D2 and 1(OH)D3 were not different38. Recently Maung et al39 reviewed their experience with 1(OH)D2. The investigators found, in patients with renal failure, that both oral and intravenous preparations of 1(OH)D2 were effective in controlling secondary hyperparathyroidism. They also noted smaller increments in serum calcium and phosphorus levels with intravenous administration compared with oral therapy. However, the prevalence of hypercalcemia and hyperphosphatemia still remains high with the IV therapy. Serum calcium increased above 10.5 mg/dl in 8.4% of patient and serum phosphorus above 6.8 mg/dl in 13.5% of patients.
CONCLUSION
Vitamin D analogs with improved specificity are now available for treatment of secondary hyperparathyroidism. Studies in uremic rats have demonstrated that OCT and 19-nor 1,25(OH)2D2 have a wider therapeutic window than 1,25(OH)2D3 due to their lower calcemic and phosphatemic effects on the intestine and bone. Studies in humans have shown their analogs to be less calcemic and phosphatemic than calcitriol but further studies will be necessary to confirmed their parathyroid selectivity in renal patients.
The mechanisms responsible for the lower calcemic and phosphatemic activities of these analogs vary. The reduced effect of OCT on intestine and bone is attributed to its altered pharmacokinetics and possibly to restricted coactivator recruitment by the OCT-VDR complex. The molecular basis for the lower calcemic and phosphatemic effect of 19-nor 1,25(OH)2D2 and 1(OH)D2 is under investigation. A clearer understanding of how these analogs exert their selectivity may allow the design of the future analogs with a greater specificity for suppressing secondary hyperparathyroidism in renal failure.
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Acknowledgments
This work was supported in part by a grant provided by Abbott Pharmaceutical Company and Research in Renal Diseases, Washington University.
