Conceptually new 20-epi-22-oxa sulfone analogues of the hormone 1alpha,25-dihydroxyvitamin D(3): synthesis and biological evaluation

Total synthesis of biologically active 20S-hydroxyvitamin D3

A total synthetic strategy of 20S-hydroxyvitamin D3 [20S-(OH)D3] involving modified synthesis of key intermediates 7 and 12, Grignard reaction to stereoselectively generate 20S-OH and Wittig-Horner coupling to establish D3 framework, was completed in 16 steps with an overall yield of 0.4%. The synthetic 20S-(OH)D3 activated vitamin D receptor (VDR) and initiated the expression of downstream genes. In addition, 20S-(OH)D3 showed similar inhibitory potency as calcitriol [1,25(OH)2D3] on proliferation of melanoma cells.

Removal of the 20-methyl group from 2-methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D(3) (2MD) selectively eliminates bone calcium mobilization activity

The 18-nor (7), 21-nor (8) and 18,21-dinor (9) analogs of (20S)-1alpha,25-dihydroxy-2-methylene-19-norvitamin D(3) (6, 2MD) were prepared by convergent syntheses. The known phosphine oxide 10 was coupled by the Wittig-Horner process with the corresponding C,D-fragments (13-15), obtained by a multi-step procedure from commercial vitamin D(2). The goal of our studies was to examine the influence of removal of the methyl groups located at carbons 13 and 20 on the biological potency of 2MD in the hope of finding analogs with improved therapeutic profiles. Replacement of the 20-methyl with hydrogen in 2-methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D(3) (2MD) did not affect binding to the rat vitamin D receptor and had little effect on transcription activity and on HL-60 differentiation. However, the mobilization of calcium from bone was largely eliminated while intestinal calcium transport remained strong. Curiously, removal of both the C-13-methyl and 20-methyl restored slightly the bone calcium mobilizing activity. Thus, the 21-nor analog of 2MD may provide a potent analog with a greater margin of safety than 2MD.

Antiproliferative, low-calcemic, fluorinated sulfone analogs of 1alpha,25-dihydroxyvitamin D3: chemical synthesis and biological evaluation

Novel fluorinated sulfone analogs of the hormone 1alpha,25-dihydroxyvitamin D(3) have been designed and synthesized in order to study the biological effects of fluorine incorporation at the terminus of the C,D-ring side chain. Although biologically active 26,27-hexafluorinated 1alpha,25-dihydroxyvitamin D(3) analogs have been synthesized previously, this investigation reports the first successful fluorinated series in which trifluoromethyl sulfone analogs present a favorable biological profile. This study shows that two new analogs featuring incorporation of a synthetically simple single trifluoromethyl sulfone group have significantly increased antiproliferative activity while causing desirably low in vivo calciuria relative to that of calcitriol. Incorporation of additional fluorines, as in a perfluorobutyl analog, results in a loss of antiproliferative activity.

A 24-phenylsulfone analog of vitamin D inhibits 1alpha,25-dihydroxyvitamin D(3) degradation in vitamin D metabolism-competent cells

The antimitotic, prodifferentiating, and proapoptotic steroid hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)], at supraphysiological levels has potential for tumor therapy. However, epithelial cells from tumor-prone organs such as colon, prostate, and breast express not only the vitamin D receptor, but also vitamin D hydroxylases. In contrast to normal cells, malignant cells have high basal levels of the hydroxylase 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) and, in addition, have the potential to induce CYP24 in response to 1alpha,25-(OH)(2)D(3). Because 24-hydroxylation by CYP24 would rapidly degrade the steroid hormone in the course of therapy, the enzyme activity in tumor cells should be inhibited. We demonstrate that a 24-phenylsulfone analog of 1alpha,25-(OH)(2)D(3), KRC-24SO(2)Ph-1 (S-4a), rapidly and potently inhibits 24-hydroxylase activity in human tumor cells derived from colon, prostate, and mammary gland. Although enzymatic inhibition is a consequence of direct interaction, S-4a as a vitamin D analog apparently binds to the vitamin D receptor and induces CYP24 mRNA, which, however, is not translated into increased enzymatic activity. 25-Hydroxyvitamin D(3)-1alpha-hydroxylase expression is not affected at all by S-4a. When both 1alpha,25-(OH)(2)D(3) and S-4a are added to the cell culture, transcription of CYP24 is increased, possibly because of an increase in the half-life of the hormone. The colon cell line COGA-13 has very high levels of CYP24 and is, therefore, resistant to the action of vitamin D. Yet, S-4a imparts antimitotic activity to 1alpha,25-(OH)(2)D(3) and may therefore constitute a therapeutic to stimulate the antiproliferative potential of vitamin D-based antitumor activity.

Novel A-ring analogs of the hormone 1α,25-dihydroxyvitamin D3: synthesis and preliminary biological evaluation

Prepared from a commercial prostaglandin building block, novel vitamin D3 analogs with a contracted five-membered A-ring were designed and synthesized to mimic the A-ring diol structure of the natural hormone 1alpha,25-dihydroxyvitamin D3. Prepared from commercial 1,4-cyclohexanedione, a structurally simplified analog was designed and synthesized in which a suitably oriented primary allylic hydroxyl group at the C-2 position might be a surrogate for the biologically important 1alpha-OH in the natural hormone.

Potent, selective and low-calcemic inhibitors of CYP24 hydroxylase: 24-sulfoximine analogues of the hormone 1alpha,25-dihydroxyvitamin D(3)

A dozen 24-sulfoximine analogues of the hormone 1alpha,25-dihydroxyvitamin D(3) were prepared, differing not only at the stereogenic sulfoximine stereocenter but also at the A-ring. Although these sulfoximines were not active transcriptionally and were only very weakly antiproliferative, some of them are powerful hydroxylase enzyme inhibitors. Specifically, 24-(S)-NH phenyl sulfoximine 3a is an extremely potent CYP24 inhibitor (IC(50) = 7.4 nM) having low calcemic activity. In addition, this compound shows high selectivity toward the CYP24 enzyme in comparison to CYP27A1 (IC(50) > 1000 nM) and CYP27B (IC(50) = 554 nM).

Potent, low-calcemic, selective inhibitors of CYP24 hydroxylase: 24-sulfone analogs of the hormone 1alpha,25-dihydroxyvitamin D3

The new 24-phenylsulfone 4a, a low-calcemic analog of the natural hormone 1alpha,25-dihydroxyvitamin D(3), is a potent (IC(50) = 28nM) and highly selective inhibitor of the human 24-hydroxylase enzyme CYP24.

Conceptually new low-calcemic oxime analogues of the hormone 1 alpha,25-dihydroxyvitamin D(3): synthesis and biological testing

New chemical entities 16-ene-25-ketone 2b and the corresponding oxime 3b and oxime ether 4b, analogues of natural calcitriol (1), were rationally designed and synthesized on a milligram scale. Chemical introduction of the oxime ether functionality in analogue 4b was successful via direct oximation of an intact vitamin D conjugated triene system. Even though all three analogues are at least as antiproliferative in vitro as calcitriol (1) even at physiologically relevant low nanomolar concentrations, only side chain ketone 2b is more transcriptionally potent than calcitriol (1). Although oxime O-methyl ether 4b lacks the traditional side chain hydrogen bond-donating OH group of the natural hormone and lacks also the oxime-NOH group of analogue 3b, surprisingly, oxime ether 4b retains 20% of the transcriptional potency of natural calcitriol (1). In terms of in vivo toxicity (hypercalcemia), ketone 2b is strongly calcemic in rats, whereas oxime 3b and oxime ether 4b are considerably less calcemic (i.e., safer) than calcitriol (1).