Department of Renal Medicine and Transplantation, Royal London Hospital, Whitechapel, London, UK
Introduction
The development of vitamin D analogues capable of effective parathyroid suppression whilst avoiding undesirable low bone turnover, hypercalcaemia and hyperphosphataemia has received considerable attention over the last decade [1]. Much in vitro and animal work has been undertaken with frustrating disparity emerging between therapeutic potential in the experimental setting and performance in available clinical trials. The biology and, in particular, gene regulatory properties of vitamin D are now more fully understood [2] and provide useful insights into the mechanisms underpinning the selective properties encountered experimentally. The formulation of designer compounds possessing distinct physiological characteristics that will afford them an advantage in the clinical arena remains an enticing prospect.
Vitamin D in uraemia
Vitamin D biology
Vitamin D and its metabolites are transported in the circulation by a specific binding protein, vitamin D binding protein (DBP), which is normally present in large excess. Active vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3 or calcitriol), is generated by hepatic 25-hydroxylation and renal 1
-hydroxylation of inactive precursors. The 25-hydroxyvitamin D-DBP complex gains access to 25(OH)D3-1
-hydroxylase in proximal tubular cells by binding to megalin located on their apical membrane [3]. Calcitriol alters gene expression by binding with high affinity to its intracellular receptor, the vitamin D receptor (VDR), which acts as a nuclear transcription factor. Liganded VDR undergoes conformational change and forms a heterodimer with a second protein, the retinoid X receptor (RXR). This, in turn, binds to DNA elements in the promoter regions of target genes described as vitamin D response elements (VDREs). Binding to the VDREs may promote transcription, as is the case with osteocalcin in the osteoblast, or inhibit transcription, as for parathyroid hormone (PTH) in the parathyroids, by either enhancing or repressing the activity of transcription machinery [4]. Additionally, 1,25(OH)2D3 appears to bind to one or more cell surface receptors that, through second messenger pathways, mediate certain non-genomic effects [5]. While the principal role of 1,25(OH)2D3 in mineral homeostasis is effected by its influence on classic targets, namely gut, bone and parathyroid glands, its actions extend much further [6]. This has helped to re-ignite interest in novel therapeutic applications and in vitamin D biology as a whole.
Vitamin D in the pathophysiology of secondary hyperparathyroidism and osteodystrophy
Decreased 1,25(OH)2D3 production by the failing kidney is of central importance to the development of secondary hyperparathyroidism [7]. Conversely, parathyroid hyperplasia, increased PTH synthesis and PTH release are all preventable, though not necessarily reversible, by replacement of the vitamin D hormone [8]. Much less clear is the direct impact of the lack of calcitriol on bonerenal osteodystrophy comprises a remarkably heterogeneous array of bone lesions, only some of which are likely to reflect calcitriol deficiency [9].
The vitamin D analogues
The limitations of vitamin D therapy centre around its excessive calcaemic and phosphataemic properties. This has heightened interest in structurally modified derivatives in the hope that these unwanted effects will be reduced. The number of analogues synthesized is large, but few have been evaluated in renal osteodystrophy. Selective attributes are certainly feasible; for example, analogues have been developed with enhanced or reduced affinity for the DBP and VDR. Other potential mechanisms include altered RXR dimerization characteristics, thereby changing the affinity or selectivity for different VDREs and modifying recruitment of transcriptional machinery. Non-genomic actions, some mediated by a specific membrane receptor, may also contribute to selectivity, but these mechanisms have yet to be elucidated fully [2,10].
22-oxacalcitriol (OCT) differs from 1,25(OH)2D3 by virtue of an oxygen atom replacing carbon-22 on the side chain. OCT has been shown to suppress PTH synthesis and secretion both in vitro and in vivo whilst, in experimental animals at least, eliciting much less calcaemia than its natural counterpart [11]. This selectivity may be a function of altered pharmacokinetics [12]. OCT has a low affinity for DBP allowing higher peak serum concentrations and facilitating rapid clearance from the circulation. This results in a more transient hit on target tissues so that rapid turnover cells such as intestinal epithelium (maturation time approximately 90 h) will be replaced quickly by cells naive to the analogue. Thus, in contrast with organs such as parathyroid and bone, the intestine is subject to a finite limit on the duration of vitamin D dependent gene expression following transient exposure to the ligand.
Paricalcitol is a vitamin D2 derived sterol lacking the carbon-19 methylene group found in all natural vitamin D metabolites. In vitro studies show that it is as effective as 1,25(OH)2D3 in abrogating PTH secretion and PTH mRNA production in isolated bovine parathyroid cell cultures [13]. In the uraemic rat it suppressed PTH synthesis and parathyroid gland growth without inciting hypercalcaemia or hyperphosphataemia [13,14]. The mechanism for the selectivity of paricalcitol is unknownin contrast to OCT it does not appear to reflect altered pharmacokinetics [15]. Experimentally, the acute administration of paricalcitol produces similar bone and intestinal mediated calcaemic responses to those of 1,25(OH)2D3 but it becomes less calcaemic with more prolonged administration [16]. The mechanism for this acquired resistance is unknown.
Doxercalciferol (1-hydroxyvitamin D2), like alfacalcidol (1
-hydroxyvitamin D3), is a pro-drug which is hydroxylated in the liver to 1
,25(OH)2D2. Unlike alfacalcidol, doxercalciferol is also 24-hydroxylated to produce 1
,24(S)-(OH)2D2, a metabolite with potent pro-differentiation actions and low calcaemic potency [17].
Dihydrotachysterol2 (DHT2), hydroxylated in vivo to 25(OH)DHT2 and 1,25(OH)2DHT2 is also of interest. Both of these metabolites powerfully suppress PTH gene transcription (unpublished observations) and, in addition, 1,25(OH)2DHT2 has been shown to augment the production of the osteotropic cytokine interleukin-6 (IL-6) by osteoblast-like cells at much lower concentrations than calcitriola property shared with OCT but not with paricalcitol [18].
Clinical experience with the new analogues
Recent studies with the new analogues must be viewed against the backdrop of historical data obtained with calcitriol and alfacalcidol. Both of these, which still comprise standard therapy in most countries, were shown to lower PTH effectively in patients with hyperparathyroidism, and also to alleviate hyperparathyroid bone disease. However, troublesome hypercalcaemia and increases in CaxP product often limit efficacy and may contribute to the vascular and other soft tissue calcification documented in end-stage renal disease (ESRD) patients [19]. New vitamin D analogues less prone to promote the cumulative CaxP burden would be highly desirable in regard to this worrying feature of ESRD.
In preliminary clinical studies OCT was shown to be effective in reducing serum PTH levels in haemodialysis patients over a six-month period, though at a disappointing cost of hypercalcaemia in many of those studied [20]. Although rarely performed, bone biopsy is an important marker of the success or failure of vitamin D therapy. OCT is the only new analogue whose effect in a patient group has been studied at a histological level. A recent report followed the effect of OCT injection on bone histomorphometry [21]. Fibrosis, mineralization and osteoid formation all improved significantly over a 24-week period in a small cohort of haemodialysis patients with severe biochemical and radiological secondary hyperparathyroidism. PTH suppression was effective if heterogeneous within the group, while elevations in serum calcium were frequent, particularly if initial PTH was low. None of the patients showed adynamic bone disease on second biopsy.
To date only placebo controlled trials involving paricalcitol have been reported. In a multicentre trial involving 78 haemodialysis patients with secondary hyperparathyroidism, paricalcitol achieved good PTH reduction but with increases in serum calcium, especially when PTH levels were low [22]. The treatment was well tolerated with minimal side effects. Comparative studies against existing therapies have been undertaken though the results remain unpublished. Two are filed with the US Food and Drugs Administration (FDA) and show no clinically relevant differences in calcium, CaxP or PTH [FDA No. 20-819; 1998]. No studies with histological data are available at the time of writing.
A multicentre study utilizing doxercalciferol confirmed earlier smaller studies showing that intermittent oral treatment effectively suppressed plasma PTH levels [23,24]. The study of 138 haemodialysis patients with moderate to severe secondary hyperparathyroidism, incorporated an 8-week washout period, 16 weeks of open-label 1(OH)D2 treatment and 8 weeks of randomized, double-blinded treatment with 1
(OH)D2 or placebo. Treated patients experienced small though significant increments in plasma calcium with an increased frequency of hypercalcaemic episodes requiring dose reduction of 1
(OH)D2 or calcium containing phosphate binders. Again, no comparative studies with alfacalcidol or calcitriol are available.
The fluorinated calcitriol analogue 26,27-hexafluorocalcitriol (falecalcitriol) has been compared with alfacalcidol in a crossover study [25]. It demonstrated superior PTH suppression but, at the doses used, a marked tendency to hypercalcaemia.
Conclusions
In the experimental setting, the newer generation of vitamin D analogues have considerable therapeutic potential. Unfortunately there remain discrepancies between animal/experimental work and available clinical data. None of these compounds has been shown to be truly non-calcaemic in the clinical setting and such descriptions of them are unhelpful and misleading. Currently available studies employ suppression of serum PTH as a primary end point, usually with safety related variables such as calcium, phosphorus and CaxP product as secondary end-points. Few bone histological data are available. The results of these studies, although showing impressive efficacy, are highly reminiscent of studies of calcitriol, and alfacalcidol undertaken in the 1970s. The only comparative studies conducted to date (paricalcitol vs intravenous calcitriol) have not been presented or published. However, data from these studies are available in the public domain via the FDA and show no convincing difference in efficacy or safety.
With other new treatments, notably calcimimetic agents, currently undergoing assessment it remains to be seen where the new vitamin D analogues will sit in the overall scheme. At the time of writing, there is no evidence to justify a wholesale move to the newer vitamin D therapies. At present, with the exception of one small study involving falecalcitriol, the new analogues have been shown to be efficacious in the context of placebo controlled studies only. Only when head to head studies against calcitriol or alfacalcidol demonstrate a clear clinical edge over established treatments will they be able to justify a more prominent role in the treatment of secondary hyperparathyroidism.
Notes
Correspondence and offprint requests to: Dr John Cunningham, Department of Renal Medicine and Transplantation, Royal London Hospital, Whitechapel, London E1 1BB, UK. Email: j.cunningham{at}thelondonclinic.co.uk
References