Biological activity of 1alpha,24-dihydroxycholecalciferol; a new synthetic analog of the hormonal form of vitamin D

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Assessment of the clinical effect of topical tacalcitol on ichthyoses with retentive hyperkeratosis

BACKGROUND

Topical tacalcitol (1alpha,24-dihydroxyvitamin D3) has been demonstrated to produce beneficial clinical effects on epidermo-hyperproliferative disorders such as psoriasis and pityriasis rubra pilaris. However, the efficacy of the agent has not been elucidated in retentive hyperkeratotic disorders.

OBJECTIVE

The object of the study was to assess the clinical efficacy of topical tacalcitol against ichthyoses with retentive hyperkeratosis; X-linked ichthyosis (XLI), ichthyosis vulgaris (IV), and acquired ichthyosis (AI).

METHODS

Tacalcitol was topically applied on 9 patients with the retention ichthyoses, using a single-blinded, bilaterally paired approach with right-left comparison of tacalcitol and the base.

RESULTS

Clinical improvement obtained by topical tacalcitol exclusively was not superior to that of the vehicle alone in any of the ichthyotic patients.

CONCLUSION

Topical tacalcitol therapy was ineffective against ichthyoses without epidermal hyperproliferation.

Production of a group-specific antibody to 1 alpha,25-dihydroxyvitamin D and its derivatives having the 1 alpha,3 beta-dihydroxylated A-ring structure

To obtain a group-specific antibody to 1 alpha,25-dihydroxyvitamin D and its derivatives having the 1 alpha,3 beta-dihydroxylated A-ring structure, 1 alpha-hydroxy-25,26,27-trinorvitamin D3-24-oic acid was synthesized as a haptenic derivative by a new convenient synthetic route. Three kinds of polyclonal antibodies were elicited in rabbits against the hapten conjugated with bovine serum albumin, and their specificity was examined by cross-reactivities in a radioimmunoassay procedure. One of the antibodies was found to show a suitable binding property by addition of a small amount of the plasma from vitamin D-deficient rats. Dose-response curves with measurable ranges applicable to biological specimens were obtained for 1 alpha,25-dihydroxyvitamin D3, 1 alpha,25-dihydroxyvitamin D2, (24R)-1 alpha,24,25-trihydroxyvitamin D3, (23S,25R)-1 alpha,25-dihydroxyvitamin D3 26,23-lactone, and 22-oxacalcitriol by the radioimmunoassay system using this improved antibody. The antibody will be widely useful for developing profile analyses of 1 alpha,25-dihydroxyvitamin D and its derivatives (metabolites or synthetic analogs) having the 1 alpha,3 beta-dihydroxylated A-ring, as well as immunoassays measuring each of these secosterols individually.

Comparison of calcipotriol with selected metabolites and analogues of vitamin D3: effects on cell growth regulation in vitro and calcium metabolism in vivo

Calcipotriol is a novel vitamin D3 analogue developed for topical treatment of psoriasis. Calcipotriol is believed to act via regulation of cell proliferation and differentiation. In this respect calcipotriol is as potent as 1 alpha, 25(OH)2D3, the physiologically active form of vitamin D3, but its calcaemic activity in vivo is 100 to 200 times lower. In the present investigation, the effects of calcipotriol on cell growth regulation in vitro and on calcium metabolism in vivo were compared to those exerted by a number of metabolites and analogues of vitamin D3. Besides 1 alpha, 25(OH)2D3, these included the two physiologically occurring metabolites 25(OH)D3 and 24,25(OH)2D3, and the two synthetic analogues 1 alpha (OH)D3 and 1 alpha, 24(OH)2D3. 25(OH)D3 and 24,25(OH)2D3 were shown to be inactive both in vitro and in vivo. 1 alpha (OH)D3 was found to have a low biological activity in vitro, but was highly calcaemic in vivo after biotransformation to 1 alpha, 25(OH)2D3. Calcipotriol, 1 alpha, 24(OH)2D3 and 1 alpha, 25(OH)2D3 were all three potent regulators of cell proliferation and differentiation in vitro. In vivo, only calcipotriol showed a greatly reduced calcaemic activity after both oral and intravenous administration. It is concluded that calcipotriol, with a reduced risk of inducing calcaemic side-effects upon absorption from the skin, possesses a favourable therapeutic profile for topical treatment of hyperproliferative diseases.

Protective effect of vitamin D3 analogues on endotoxin shock in mice

The effect of vitamin D3 analogues on endotoxin shock in mice was investigated. Male ICR mice were orally administered vitamin D3 analogues or vehicle, accompanied by an intraperitoneal injection of endotoxin (E. Coli lipopolysaccharide, LPS, 20 mg/kg). Endotoxin caused a decrease in survival rate in a time-dependent manner. Increases in plasma immunoreactive (i) eicosanoid and hepatic malondialdehyde (MDA) levels were also observed. Administration of 1 alpha-hydroxyvitamin D3 (1 alpha-OH-D3) improved the survival rate 24 to 48 h after endotoxin treatment. The effects were markedly observed at a dose of 20 ng/kg. In addition, 1 alpha-OH-D3 restored the plasma iTXB2 and hepatic MDA levels 8 h after endotoxin injection. However, it did not affect plasma iPGE2, i6-keto-PGF1 alpha and blood iLTB4 levels. At a dose of 20 ng/kg, both 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and 1,24(R)-dihydroxyvitamin D3 (1,24(R)-(OH)2D3) restored the survival rate, the plasma iTXB2 and hepatic MDA levels. These results suggest that vitamin D3 analogues may inhibit endotoxemia through regulation of the formation of TXA2 and free radicals.

Successful treatment of psoriasis with topical application of active vitamin D3 analogue, 1 alpha,24-dihydroxycholecalciferol

We treated 11 psoriatic patients with topical 1 alpha,24-dihydroxycholecalciferol, a new synthetic analogue of active vitamin D3. In 10 of 15 tests the lesions cleared completely within 1-4 weeks, although some relapses occurred shortly after cessation of treatment. There were no side-effects. We suggest that 1 alpha, 24(OH)2D3 merits further investigation as a potentially useful topical therapy for psoriasis.

Aspirin inhibition of 1 alpha-hydroxyvitamin D3 or parathyroid hormone induced hypercalcemia in vivo in rats. A mechanism independent of prostaglandin biosynthesis inhibition

The interactions of calcium-regulating hormones, active forms of vitamin D and parathyroid hormone, and aspirin were studied in rats. Aspirin, a prostaglandin biosynthesis inhibitor, abolished the hypercalcemia induced by 1 alpha-hydroxyvitamin D3 at 20, 50 and 100 mg/kg p.o. in parathyroidectomized or thyroparathyroidectomized rats with or without vitamin D deficiency, and in thyroparathyroidectomized plus nephrectomized rats. Aspirin did not affect the stimulation of intestinal calcium absorption by 1 alpha-hydroxyvitamin D3. By contrast, indomethacin, another prostaglandin biosynthesis inhibitor, did not affect hypercalcemia or stimulation of intestinal calcium absorption by 1 alpha-hydroxyvitamin D3. Aspirin also abolished the hypercalcemic action of parathyroid hormone in rats with or without intact thyroparathyroid glands. Moreover, aspirin alone caused hypocalcemia in rats with intact thyroparathyroid glands. Indomethacin had no effect in either of these systems. These data suggest that aspirin may inhibit bone resorption by the active form of vitamin D or parathyroid hormone via a mechanism independent of prostaglandin biosynthesis inhibition.

Phenotypic differentiation-linked growth inhibition in human leukemia cells by active vitamin D3 analogues

Active vitamin D3 induced phenotypic differentiation of the human promyelocytic leukemia cell line, HL-60, cultured in serum-free medium. Upon exposure to 10(-10)-10(-7) M 1,25-(OH)2D3, 1,24S-(OH)2D3 or 1,24R(OH)2D3, monocyte-granulocyte-associated plasma membrane antigens of HL-60 cells detected by monoclonal antibodies, OKMI, 63D3 and Mo2, quantitated by fluorescence-activated cell sorter analysis, were increased time- and dose-dependently. After expose to 1,25-(OH)2D3, promotion of this antigenic expression was detected within 16 h, and the induction of differentiation continued until 96 h. The number of cells bearing transferrin receptors recognized by the monoclonal antibody, OKT9, and its density on the surface of HL-60 cells were decreased symmetrically. These effects appeared in parallel with the inhibition of cell growth, poly(ADP-ribose) content and de novo DNA-RNA synthesis. These findings indicate that 1,24S-(OH)2D3 stimulates differentiating induction of HL-60 like 1,25-(OH)2D3 in vitro and that the decrease of transferrin receptor is apparently correlated with the inhibition of proliferation.

The 1 alpha-hydroxylation of 24-hydroxyvitamin D3 by chick kidney homogenates

Tritium-labeled 24(R)-hydroxyvitamin D3 and 24(S)-hydroxyvitamin D3 were chemically synthesized and the 1 alpha-hydroxylation of these compounds by chick kidney homogenates was studied. A marked stereospecific preference with regard to the orientation of the hydroxyl functionality on carbon-24 was noted: while the 24(R)-epimer could be 1 alpha-hydroxylated in readily detectable amounts, the 24(S)-epimer was not hydroxylated. Thus, 1.2 micrograms of 1 alpha,24(R)-dihydroxyvitamin D3 was isolated and its structure confirmed by mass spectrometry. The relative rate of 1 alpha-hydroxylation of 125 nM substrate tritiated 24(R)-hydroxyvitamin D3 and 25-hydroxyvitamin D3 (the presumed natural substrate for the renal 1 alpha-hydroxylase) was 1:6.7.

Synthesis and biological activity of vitamin D3 analogues

Many analogues of vitamin D metabolites including stereochemical isomers and fluorine substituents were synthesized, and their biological activities were determined by in vitro and in vivo assays. Among those analogues, 24,24-F2-1,25-(OH)2D3 and 26,26,26,27,27,27-F6-1,25-(OH)2D3 are more active than the most potent naturally occurring vitamin D3 metabolite, 1,25-(OH)2D3.

Studies on the mechanism of action of 1 alpha,24-dihydroxyvitamin D3. III. The specific binding of 1 alpha,24-dihydroxyvitamin D3 to the receptor of chick parathyroid gland

Three protein fractions of the cytosol of the chick parathyroid glands, which had the sedimentation constants of 2.5 S, 3.7 S and 5.5 S, were found to bind with 1 alpha,25-dihydroxyvitamin D3. Among these proteins, the 3.7 S protein was assumed to be the specific receptor protein. The 3.7 S receptor protein was also capable of binding to 1 alpha,24-dihydroxyvitamin D3 but not 25-hydroxyvitamin D3. The binding affinity of 1 alpha,24(R)-dihydroxyvitamin D3 to the 3.7 S receptor protein was estimated to be 1.2 times greater than that of 1 alpha,25-dihydroxyvitamin D3, while 1 alpha,25-dihydroxyvitamin D3 bound to the receptor protein about 10 times stronger than 1 alpha,24(S)-dihydroxyvitamin D3. The dissociation constant for the receptor-1 alpha,25-dihydroxyvitamin D3 complex at 0 degrees C was 2.7 x 10(-11) M, the dissociation constants were calculated to be 2.2 x 10(-11) M and 2.6 x 10(-10) M for the complexes with 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3.

Studies on the mechanism of action of 1 alpha, 24-dihydroxyvitamin D3. II. Specific binding of alpha, 24-dihydroxyvitamin D3 to chick intestinal receptor

The binding of vitamin D3 analogues to the chick intestinal cytosol receptor was studied. In intestinal cytosol fraction, receptor proteins having the sedimentation constant of 2.5 S and 3.7 S to which 1 alpha,25-dihydroxyvitamin D3 binds were present, and the latter was specific for the compound. The binding of 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3 to the receptor was also observed, while very weak binding was seen in the case of 24(R)25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3. The binding affinity of 1 alpha,24(R)-dihydroxyvitamin D3 to the 3.7 S receptor was 1.3 times as high as that of 1 alpha,25-dihydroxyvitamin D3, whereas those of 1 alpha,24(S)-dihydroxyvitamin D3, 1 alpha-hydroxyvitamin D3 and 25-hydroxyvitamin D3 were 10, 304 and 652 times lower than 1 alpha,25-dihydroxyvitamin D3, respectively. The dissociation constant of the receptor-1 alpha,25-dihydroxyvitamin D3 complex at 0 degrees C was 3.0 x 10(-11) M, and the dissociation constants were calculated to be 2.4 x 10(-11) M and 2.7 x 10(-10) M for the complexes with 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3, respectively.

1 alpha-hydroxy-25-fluorovitamin D3: a potent analogue of 1 alpha,25-dihydroxyvitamin D3

Chemically synthesized 1 alpha-hydroxy-25-fluorovitamin D3 was compared to 1,25-dihydroxyvitamin D3 for potency in the chick intestinal cytosol-binding protein assay, induction of intestinal calcium transport, mobilization of calcium from bone, and epiphyseal plate calcification in the rat. The 25-fluorinated analogue causes 50% displacement of 1,25-dihydroxy[23,24-3H]D3 at 1.8 X 10(-8) M in the competitive protein-binding assay, whereas only 5.6 X 10(-11) M of unlabeled 1,25-dihydroxyvitamin D3 is needed for equal competition. This 315-fold difference between and 1 alpha-hydroxy-25-fluorovitamin D3 indicates that the fluoro analogue is about equipotent with 1 alpha-hydroxyvitamin D3 in the protein-binding assay. However, 1 alpha-hydroxy-25-fluorovitamin D3 is 1/50 as active as 1,25-dihydroxyvitamin D3 in vivo in the stimulation of intestinal calcium transport and bone calcium mobilization in vitamin D deficient rats on a low-calcium diet. Likewise, 1 alpha-hydroxy-25-fluorovitamin D3 is about 40 times less active than 1,25-dihydroxyvitamin D3 in inducing endochondrial calcification in rachitic rats. No selective actions of 1alpha-hydroxy-25-fluorovitamin D3 were noted. Since the 25 position of the analogue is blocked by a fluorine atom, it appears that 25-hydroxylation of 1 alpha-hydroxylated vitamin D compounds in vivo is not an obligatory requirement for appreciable vitamin D activity.