Metabolism of a cyclopropane-ring-containing analog of 1 alpha-hydroxyvitamin D3 in a hepatocyte cell model. Identification of 24-oxidized metabolites

HIV-protease inhibitors impair vitamin D bioactivation to 1,25-dihydroxyvitamin D

BACKGROUND

A high prevalence of bone demineralization occurs in people living with HIV/AIDS. The contribution of HIV itself and its treatment is still unclear. Protease inhibitors (PIs) are potent inhibitors of the cytochrome p450 enzyme system. Three cytochrome p450 mixed function oxygenases control serum levels of 1,25-dihydroxyvitamin D (1,25(OH) D ), which is responsible for vitamin D actions in target tissues including bone. The 25- and 1alpha-hydroxylases regulate 1,25(OH) D synthesis and 24-hydroxylase 1,25(OH) D catabolism.

OBJECTIVE

To assess whether HIV-protease inhibitors (ritonavir, indinavir, nelfinavir) impair the activity of the main enzymes involved in 1,25(OH) D homeostasis.

DESIGN AND METHODS

Studies were conducted in the human hepatocyte (H3B)- and monocyte (THP-1) cell lines, expressing 25-hydroxylase and 1alpha-hydroxylase, respectively. The 24-hydroxylase expression was induced in macrophages by exposure to 1,25(OH) D. Conversion rates of vitamin D to 25-hydroxyvitamin D [25(OH)D ]; 25(OH)D to 1,25(OH) D or 24,25(OH) D, and 1,25(OH) D degradation were quantified in untreated and HIV-PI-treated cells after C -cartridge extraction and high-performance liquid chromatography purification of 25(OH)D - 24,25(OH) D - and 1,25(OH) D fractions.

RESULTS

The PIs impair hepatocyte 25(OH)D - and macrophage 1,25(OH) D synthesis in a reversible, dose-dependent manner. Furthermore, PIs inhibit 1,25(OH) D -degradation in macrophages with lower potency than that elicited on 1alpha-hydroxylase. Thus, reduced macrophage 1,25(OH) D production is the net effect of PIs action.

CONCLUSIONS

In intact cells, HIV-PIs markedly suppress the activities of 25- and 1alpha-hydroxylase, which are critical in 1,25(OH) D synthesis, while exerting mild inhibition of 24-hydroxylase, responsible for 1,25(OH) D catabolism. If PIs elicit a similar potency in inhibiting these critical steps for 1,25(OH) D homeostasis, defective 1,25(OH) D production could contribute to the bone demineralization in HIV patients.

Use of vitamin D(4) analogs to investigate differences in hepatic and target cell metabolism of vitamins D(2) and D(3)

In this study, we used molecules with either of the structural differences in the side chains of vitamin D(2) and vitamin D(3) to investigate which feature is responsible for the significant differences in their respective metabolism, pharmacokinetics and toxicity. We used two cell model systems-HepG2 and HPK1A-ras-to study hepatic and target cell metabolism, respectively. Studies with HepG2 revealed that the pattern of 24- and 26-hydroxylation of the side chain reported for 1alpha-hydroxyvitamin D(2) (1alpha-OH-D(2)) but not for 1alpha-OH-D(3) is also observed in both 1alpha-OH-D(4) and Delta(22)-1alpha-OH-D(3) metabolism. This suggests that the structural feature responsible for targeting the enzyme to the C24 or C26 site could be either the C24 methyl group or the 22-23 double bond. In HPK1A-ras cells, the pattern of metabolism observed for the 24-methylated derivative, 1alpha,25-(OH)(2)D(4), was the same pattern of multiple hydroxylations at C24, C26 and C28 seen for vitamin D(2) compounds without evidence of side chain cleavage observed for vitamin D(3) derivatives, suggesting that the C24 methyl group plays a major role in this difference in target cell metabolism of D(2) and D(3) compounds. Novel vitamin D(4) compounds were tested and found to be active in a variety of in vitro biological assays. We conclude that vitamin D(4) analogs and their metabolites offer valuable insights into vitamin D analog design, metabolic enzymes and maybe useful clinically.

Metabolism of a 20-methyl substituted series of vitamin D analogs by cultured human cells: apparent reduction of 23-hydroxylation of the side chain by the 20-methyl group

We describe here for the first time the effect of introducing a 20-methyl group on the side-chain metabolism of the vitamin D molecule. Using a series of 20-methyl-derivatives of 1alpha,25-(OH)2D3 incubated with two different cultured human cell lines, HPK1A-ras and HepG2, previously shown to metabolize vitamin D compounds, we obtained a series of metabolic products that were identified by comparison to chemically synthesized standards on HPLC and GC-MS. 24-Hydroxylated-, 24-oxo-hydroxylated-, and 24-oxo-23-hydroxylated products of 20-methyl-1alpha,25-(OH)2D3 were observed, but the efficiency of 23-hydroxylation was low as compared with that of the natural hormone and, in contrast to 1alpha,25-(OH)2D3, no truncated 23-alcohol was formed from the 20-methyl analog. These data, taken together with results from other analogs with changes in the vicinity of the C17-C20 positions, lead us to speculate that such changes must alter the accessibility of the C-23 position to the cytochrome P450 involved. Using the HepG2 cell line, we found evidence that the 24S-hydroxylated product of 20-methyl-1alpha,25-(OH)2D3 predominates, implying that the liver cytochrome involved in metabolism is a different isoform. Studies with a more metabolically resistant analog of the series, 20-methyl-Delta(23)-1alpha,25-(OH)2D3, gave the expected block in 23- and 24-hydroxylation, and evidence of an alternative pathway, namely 26-hydroxylation. 20-Methyl-Delta(23)-1alpha,25-(OH)2D3 was also more potent in biological assays, and the metabolic studies reported here help us to suggest explanations for this increased potency. We conclude that the 20-methyl series of vitamin D analogs offers new perspectives into vitamin D analog action, as well as insights into the substrate preferences of the cytochrome(s) P450 involved in vitamin D catabolism.

In Vitro Metabolism of 19-Nor-1α,25-(OH)2D2 in Cultured Cell Lines: Inducible Synthesis of Lipid- and Water-Soluble Metabolites

The active vitamin D analog, 19-nor-1alpha,25-dihydroxyvitamin D2 (19-nor-1alpha,25-(OH)2D2), has a similar structure to the natural vitamin D hormone, 1a,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3), but lacks the C10-19 methylene group and possesses an ergosterol/ vitamin D2 rather than a cholesterol/vitamin D3 side chain. We have used this analog to investigate whether any of these structural features has any effect upon the type and rate of in vitro metabolism observed. Using a vitamin D-target cell, the human keratinocyte, HPK1A-ras, we observed formation of a number of metabolites, three of which were purified by extensive HPLC and conclusively identified by a combination of GC-MS and chemical derivatization as 19-nor-1alpha,24,25-(OH) 3D2, 19-nor-1alpha,24,25,26-(OH) 4D2, and 19-nor-1alpha,24,25,28-(OH)4,D2. The first metabolite is probably a product of the vitamin D-inducible cytochrome P450, P450cc24 (CYP24), while the latter two metabolites are likely to be further metabolic products of 19-nor-1alpha,24,25-(OH)3D2. These hydroxylated metabolites resemble those identified by other workers as products of the metabolism of 1alpha,25-(OH)2D2 in the perfused rat kidney. It therefore appears from the similar metabolic fate of 19-nor-1alpha,25-(OH)2D2 and 1alpha,25-(OH)2D2 that the lack of the C10-19 methylene group has little effect upon the nature of the lipid-soluble metabolic products and the rate of formation of these products seems to be comparable to that of products of 1alpha,25-(OH)2D3 in vitamin D-target cells. We also found extensive metabolism of 19-nor-1alpha,25(OH)2D2 to water-soluble metabolites in HPK1A-ras, metabolites which remain unidentified at this time. When we incubated 19-nor-1alpha,25-(OH)2D2 with the liver cell line HepG2, we obtained only 19-nor-1alpha,24,25-(OH)3D2. We conclude that 19-nor-1alpha,25-(OH)2D2 is efficiently metabolized by both vitamin D-target cells and liver cells.

The vitamin D analog, KH1060, is rapidly degraded both in vivo and in vitro via several pathways: principal metabolites generated retain significant biological activity

Vitamin D analogs are valuable drugs with established and potential uses in hyperproliferative disorders. Lexacalcitol (KH1060) is over 100 times more active than 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], as judged by in vitro antiproliferative and cell differentiating assays. The underlying biochemical reasons for the increased biological activity of KH1060 are unknown, but are thought to include 1) metabolic considerations in addition to explanations based upon 2) enhanced stability of KH1060-liganded transcriptional complexes. In this study we explored the in vivo and in vitro metabolism of KH1060. We established by physicochemical techniques the existence of multiple side-chain hydroxylated metabolites of KH1060, including 24-, 24a-, 26-, and 26a-hydroxylated derivatives as well as side-chain truncated forms. KH1060 metabolism could be blocked by the cytochrome P450 inhibitor, ketoconazole. KH1060 was not an effective competitor of C24 oxidation of 1alpha,25-(OH)2D3. Certain hydroxylated metabolites of KH1060 retained significant biological activity in vitamin D-dependent reporter gene systems (chloramphenicol acetyltransferase). Likewise, those metabolites accumulating in the target cell culture models in metabolism studies, particularly 24a-hydroxy-KH1060 and 26-hydroxy-KH1060, retained biological activities superior to those of 1alpha,25-(OH)2D3 in native gene expression systems in vitamin D target cells (osteopontin and P450cc24). We conclude that KH1060 is rapidly metabolized by a variety of cytochrome P450-mediated enzyme systems to products, many of which retain significant biological activity in vitamin D-dependent assay systems. These results provide an explanation for the considerable biological activity advantage displayed by KH1060 compared with 1alpha,25-(OH)2D3 in various in vitro assay systems.

In vitro metabolism of the vitamin D analog, 22-oxacalcitriol, using cultured osteosarcoma, hepatoma, and keratinocyte cell lines

Using four cultured cell models representing liver, keratinocyte, and osteoblast, we have demonstrated that the vitamin D analog, 22-oxacalcitriol is degraded into a variety of hydroxylated and side chain truncated metabolites. Four of these metabolic products have been rigorously identified by high pressure liquid chromatography, diode array spectrophotometry, and gas chromatography-mass spectrometry analysis as 24-hydroxylated and 26-hydroxylated derivatives as well as the cleaved molecules, hexanor-1alpha,20-dihydroxyvitamin D3 and hexanor-20-oxo-1alpha-hydroxyvitamin D3. Comparison with chemically synthesized standards has revealed the stereochemistry of the biological products. Although differences exist in the amounts of products formed with the different cell types, it is apparent that 22-oxacalcitriol is subject to metabolism by both vitamin D-inducible and noninducible enzymes. Time course studies suggest that the truncated 20-alcohol is derived from a side chain hydroxylated molecule via a hemiacetal intermediate and the 20-oxo derivative is likely formed from the 20-alcohol. Biological activity measurements of the metabolites identified in our studies are consistent with the view that these are catabolites and that the biological activity of 22-oxacalcitriol is due to the parent compound. These results are also consistent with recent findings of others that the biliary excretory form of 22-oxacalcitriol is a glucuronide ester of the truncated 20-alcohol.

1 alpha,24(S)-dihydroxyvitamin D2: a biologically active product of 1 alpha-hydroxyvitamin D2 made in the human hepatoma, Hep3B

A major metabolite of the vitamin D analogue 1 alpha-hydroxyvitamin D2 in human liver cells in culture has been identified as 1 alpha,24(S)-dihydroxyvitamin D2 [1 alpha,24(S)-(OH)2D2]. 1 alpha-Hydroxyvitamin D3 incubated with the same cells gives rise to predominantly 25- and 27-hydroxylated products. Our identification of 1 alpha,24(S)-dihydroxyvitamin D2 is based on comparisons of the liver cell metabolite with chemically synthesized 1 alpha,24(S)-(OH)2D2 and 1 alpha,24(R)-(OH)2D2 by using HPLC, GC and GC-MS techniques. The stereochemical orientation of the 24-hydroxyl group was inferred after X-ray-crystallographic analysis of the 24(R)-OH epimer. 1 alpha,24(S)-Dihydroxyvitamin D2 binds strongly to the vitamin D receptor and is biologically active in growth hormone and chloramphenicol acetyltransferase reporter gene expression systems in vitro, but binds poorly to rat vitamin D-binding globulin, DBP. We suggest that this metabolite, 1 alpha,24(S)-(OH)2D2, possesses the spectrum of biological properties to be useful as a drug in the treatment of psoriasis, metabolic bone disease and cancer.

Different mechanisms of hydroxylation site selection by liver and kidney cytochrome P450 species (CYP27 and CYP24) involved in vitamin D metabolism

A series of homologated 1 alpha-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 molecules with one to three extra carbons in the side chain were used to examine the substrate preferences and hydroxylation site selection mechanisms of the liver vitamin D3-25-hydroxylase (CYP27) and the target cell 25-hydroxyvitamin D3-24-hydroxylase (CYP24). Cultured and transfected cell models, used as sources of these hydroxylases, gave 23-, 24-, 25-, and 27-hydroxylated metabolites which were identified by their high performance liquid chromatography and GC-MS characteristics. Lengthening the side chain is tolerated by each cytochrome P450 isoform such that 25-hydroxylation or 24-hydroxylation continues to occur at the same rate as in the native side chain, while the site of hydroxylation remains the same for the liver enzyme in that CYP27 continues to hydroxylate at C-25 and C-27 (minor) despite the two-carbon-atom extension. Somewhat surprising is the finding that C-24 and C-23 (minor) hydroxylations also do not change as the side chain is extended by as much as three carbons. We conclude that CYP24 must be directed to its hydroxylation site(s) by the distance of carbon 24 from the vitamin D ring structure and not as in CYP27 by the distance of the hydroxylation site from the end of the side chain.

Increased biological activity of 20-epi-1,25-dihydroxyvitamin D3 is due to reduced catabolism and altered protein binding

The 20-epi series of vitamin D3 analogs has been shown to be made up of more potent inducers of cell differentiation than calcitriol in vitro. Using 20-epi-1 alpha,25-dihydroxyvitamin D3 (MC 1288), we attempted to rationalize this increased biological activity by examining several parameters including the binding affinity of the analog for the plasma binding globulin (DBP) and the target cell vitamin D receptor (VDR), as well as attempting to measure the rate at which MC 1288 is metabolized. MC 1288 was found to be metabolized 36 times more slowly than its epimer 1,25-dihydroxy vitamin D3 (1,25-(OH)2D3), forming several metabolites which were analogous to metabolites of 1,25-(OH)2D3 formed in the side chain oxidation pathway. Bovine thymus VDR bound MC 1288 with five times greater affinity than calcitriol, while rat plasma DBP did not bind MC 1288 even at a concentration of 50 microM, 5000 times the B50 of 25-OH-D3, the ligand used in the assay. Using a vitamin D-inducible growth hormone gene reporter system we were able to demonstrate that MC 1288 induces human growth hormone (hGH) activity 30-fold more efficiently than 1,25-(OH)2D3 in the presence of fetal calf serum (FCS), while the analog is only 10 times more efficient than 1,25-(OH)2D3 in the absence of FCS. We therefore conclude that MC 1288 is more biologically active than calcitriol in vitro due to a combination of factors: the increased VDR binding affinity, the decreased DBP binding affinity, and the decreased rate of metabolism. As with other analogs of vitamin D, the altered protein binding and decreased catabolism of MC 1288 may be important in pharmaceutical applications such as a topical treatment for psoriasis or skin cancer.

The metabolism of dihydrotachysterols: renal side chain and non-renal nuclear hydroxylations in vivo and in vitro

The metabolism of dihydrotachysterol (DHT), a hydrogenated analogue of vitamin D, has been studied in vivo using man and rat and in vitro using the perfused rat kidney, and hepatoma (3B) and osteosarcoma (UMR-106) cell lines. In vivo a large number of metabolites appeared in the plasma of rats given DHT2 and DHT3. Of particular interest was a compound more polar than 25-hydroxy-DHT, which has been designated compound H. Further study of this compound showed that it was composed of two components, one (Ha) being in much lower concentration than the other (Hb). The production of T2/H (peak H from DHT2) was demonstrated in human plasma after administration of oral DHT2. Comparison of the metabolites formed in vivo with those isolated from the rat kidney perfused with 25-hydroxy-DHT3 in vitro showed that 25-hydroxy-DHT3 was metabolized along two metabolic pathways previously described for vitamin D, culminating in the production of 25-hydroxy-DHT3-23,26-lactone and 23,25-dihydroxy-24-oxo-DHT3. The osteosarcoma cell line metabolized 25-OH-DHT3 in vitro along the same two metabolic pathways already demonstrated in the perfused rat kidney. More polar metabolites than compound H seen in rat plasma in vivo were shown to be metabolites of compound H and similar metabolites were also produced in the osteosarcoma cell line from chemically synthesized 1 alpha,25-dihydroxy-DHT3. The hepatoma cell line 25-hydroxylated DHT and no feed-back inhibition was observed. Use of the hepatoma cell to 25-hydroxylate a number of chemically synthesized 1-hydroxy-DHTs indicated that compound Ha was indistinguishable from 1 alpha,25-dihydroxy-DHT whereas compound Hb is possibly 1 beta,25-dihydroxy-DHT. Studies with the VDR in both chick gut and calf thymus indicated that 1 alpha,25-dihydroxy-DHT is very effective in displacing radiolabelled 1 alpha,25-dihydroxyvitamin-D3 and is thus most likely to be the calcaemic metabolite of DHT.

Actions of calcipotriol (MC 903), a novel vitamin D3 analog, on human bone-derived cells: comparison with 1,25-dihydroxyvitamin D3

The actions of a novel vitamin D3 analog calcipotriol (MC 903), on human bone-derived cells were compared to those of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Both calcipotriol and 1,25-(OH)2D3 inhibited the proliferation of human osteoblast-like cells in a dose-dependent manner (10(-10)-10(-6) M), an effect observed at different cell densities. Lower concentrations of either agent exerted no marked effect on the growth of the cells compared to untreated cultures. Calcipotriol and 1,25-(OH)2D3 were equipotent in stimulating the activity of alkaline phosphatase and the synthesis of osteocalcin in human osteoblast-like cells. The stimulation of alkaline phosphatase activity and osteocalcin synthesis by both compounds was evident by 24 h and was increased progressively up to 96 h in a dose-dependent manner over the concentration range of 10(-10)-10(-6) M. The increment in both proteins was dependent on cell density and was attenuated at higher cell densities. In contrast to these actions, neither calcipotriol nor 1,25-(OH)2D3 (10(-14)-10(-6) M) affected the synthesis of prostaglandin E2. These studies indicate that calcipotriol and 1,25-(OH)2D3 exhibit a similar spectrum of activity on human osteoblast-like cells in vitro.