Liarozole fumarate inhibits the metabolism of 4-keto-all-trans-retinoic acid

Combining phenotypic profiling and targeted RNA-Seq reveals linkages between transcriptional perturbations and chemical effects on cell morphology: Retinoic acid as an example

The United States Environmental Protection Agency has proposed a tiered testing strategy for chemical hazard evaluation based on new approach methods (NAMs). The first tier includes in vitro profiling assays applicable to many (human) cell types, such as high-throughput transcriptomics (HTTr) and high-throughput phenotypic profiling (HTPP). The goals of this study were to: (1) harmonize the seeding density of U-2 OS human osteosarcoma cells for use in both assays; (2) compare HTTr- versus HTPP-derived potency estimates for 11 mechanistically diverse chemicals; (3) identify candidate reference chemicals for monitoring assay performance in future screens; and (4) characterize the transcriptional and phenotypic changes in detail for all-trans retinoic acid (ATRA) as a model compound known for its adverse effects on osteoblast differentiation. The results of this evaluation showed that (1) HTPP conducted at low (400 cells/well) and high (3000 cells/well) seeding densities yielded comparable potency estimates and similar phenotypic profiles for the tested chemicals; (2) HTPP and HTTr resulted in comparable potency estimates for changes in cellular morphology and gene expression, respectively; (3) three test chemicals (etoposide, ATRA, dexamethasone) produced concentration-dependent effects on cellular morphology and gene expression that were consistent with known modes-of-action, demonstrating their suitability for use as reference chemicals for monitoring assay performance; and (4) ATRA produced phenotypic changes that were highly similar to other retinoic acid receptor activators (AM580, arotinoid acid) and some retinoid X receptor activators (bexarotene, methoprene acid). This phenotype was observed concurrently with autoregulation of the RARB gene. Both effects were prevented by pre-treating U-2 OS cells with pharmacological antagonists of their respective receptors. Thus, the observed phenotype could be considered characteristic of retinoic acid pathway activation in U-2 OS cells. These findings lay the groundwork for combinatorial screening of chemicals using HTTr and HTPP to generate complementary information for the first tier of a NAM-based chemical hazard evaluation strategy.

Liarozole inhibits transforming growth factor-β3--mediated extracellular matrix formation in human three-dimensional leiomyoma cultures

OBJECTIVE

To investigate the impact of liarozole on transforming growth factor-β3 (TGF-β3) expression, TGF-β3 controlled profibrotic cytokines, and extracellular matrix formation in a three-dimensional (3D) leiomyoma model system.

DESIGN

Molecular and immunohistochemical analysis in a cell line evaluated in a three-dimensional culture.

SETTING

Laboratory study.

PATIENT(S)

None.

INTERVENTION(S)

Treatment of leiomyoma and myometrial cells with liarozole and TGF-β3 in a three-dimensional culture system.

MAIN OUTCOME MEASURE(S)

Quantitative real-time reverse-transcriptase polymerase chain reaction and Western blotting to assess fold gene and protein expression of TGF-β3 and TGF-β3 regulated fibrotic cytokines: collagen 1A1 (COL1A1), fibronectin, and versican before and after treatment with liarozole, and confirmatory immunohistochemical stains of treated three-dimensional cultures.

RESULT(S)

Both TGF-β3 gene and protein expression were elevated in leiomyoma cells compared with myometrium in two-dimensional and 3D cultures. Treatment with liarozole decreased TGF-β3 gene and protein expression. Extracellular matrix components versican, COL1A1, and fibronectin were also decreased by liarozole treatment in 3D cultures. Treatment of 3D cultures with TGF-β3 increased gene expression and protein production of COL1A1, fibronectin, and versican.

CONCLUSION(S)

Liarozole decreased TGF-β3 and TGF-β3-mediated extracellular matrix expression in a 3D uterine leiomyoma culture system.

Therapeutic potential of the inhibition of the retinoic acid hydroxylases CYP26A1 and CYP26B1 by xenobiotics

Retinoic acid (RA), the active metabolite of vitamin A, is an important endogenous signaling molecule regulating cell cycle and maintenance of epithelia. RA isomers are also used as drugs to treat various cancers and dermatological diseases. However, the therapeutic uses of RA isomers are limited due to side effects such as teratogenicity and resistance to treatment emerging mainly from autoinduction of RA metabolism. To improve the therapeutic usefulness of retinoids, RA metabolism blocking agents (RAMBAs) have been developed. These inhibitors generally target the cytochrome P450 (CYP) enzymes because RA clearance is predominantly mediated by P450s. Since the initial identification of inhibitors of RA metabolism, CYP26 enzymes have been characterized as the main enzymes responsible for RA clearance. This makes CYP26 enzymes an attractive target for the development of novel therapeutics for cancer and dermatological conditions. The basic principle of development of CYP26 inhibitors is that endogenous RA concentrations will be increased in the presence of a CYP26 inhibitor, thus, potentiating the activity of endogenous RA in a cell-type specific manner. This will reduce side effects compared to administration of RA and allow for more targeted therapy. In clinical trials, inhibitors of RA metabolism have been effective in treatment of psoriasis and other dermatological conditions as well as in some cancers. However, no CYP26 inhibitor has yet been approved for clinical use. This review summarizes the history of development of RAMBAs, the clinical and preclinical studies with the various structural series and the available knowledge of structure activity relationships of CYP26 inhibitors.

Retiferols - synthesis and biological activity of a conceptually novel class of vitamin D analogs

INTRODUCTION

The hypothesis that retiferols are a novel class of vitamin D analogs with therapeutic potential has been recently proved. The CD-ring of vitamin D, originated from a steroid precursor, is not necessary for biological activity. The retiferol, disubstituted at C-13, was bound to the ligand-binding domain (LBD) of vitamin D receptor (VDR) just like the vitamin D hormone [1,25-(OH)2D3]. This finding opens the way for retiferols as a novel class of vitamin D therapeutics.

AREAS COVERED

This review presents the concept of retiferols and their structure evolution. Medicinal chemistry and therapeutic perspective of retiferols are reviewed showing how these vitamin D analogs became a source of potential therapeutics.

EXPERT OPINION

Docking experiments and molecular modeling have shown that positioning of vitamin D analog at the LBD of VDR is not disturbed by deletion of a large portion of the vitamin D, exactly as hypothesized. Twenty years of structural modifications have shown that removal of the CD-ring fragment and regioselective methylation results in an almost complete loss of the undesired calcemic activity of retiferol while gaining the agonistic activity comparable to that of 1,25-(OH)2D3.

Fenretinide metabolism in humans and mice: utilizing pharmacological modulation of its metabolic pathway to increase systemic exposure

BACKGROUND AND PURPOSE

High plasma levels of fenretinide [N-(4-hydroxyphenyl)retinamide (4-HPR)] were associated with improved outcome in a phase II clinical trial. Low bioavailability of 4-HPR has been limiting its therapeutic applications. This study characterized metabolism of 4-HPR in humans and mice, and to explore the effects of ketoconazole, an inhibitor of CYP3A4, as a modulator to increase 4-HPR plasma levels in mice and to increase the low bioavailability of 4-HPR.

EXPERIMENTAL APPROACH

4-HPR metabolites were identified by mass spectrometric analysis and levels of 4-HPR and its metabolites [N-(4-methoxyphenyl)retinamide (4-MPR) and 4-oxo-N-(4-hydroxyphenyl)retinamide (4-oxo-4-HPR)] were quantified by high-performance liquid chromatography (HPLC). Kinetic analysis of enzyme activities and the effects of enzyme inhibitors were performed in pooled human and pooled mouse liver microsomes, and in human cytochrome P450 (CYP) 3A4 isoenzyme microsomes. In vivo metabolism of 4-HPR was inhibited in mice.

KEY RESULTS

Six 4-HPR metabolites were identified in the plasma of patients and mice. 4-HPR was oxidized to 4-oxo-4-HPR, at least in part via human CYP3A4. The CYP3A4 inhibitor ketoconazole significantly reduced 4-oxo-4-HPR formation in both human and mouse liver microsomes. In two strains of mice, co-administration of ketoconazole with 4-HPR in vivo significantly increased 4-HPR plasma concentrations by > twofold over 4-HPR alone and also increased 4-oxo-4-HPR levels.

CONCLUSIONS AND IMPLICATIONS

Mice may serve as an in vivo model of human 4-HPR pharmacokinetics. In vivo data suggest that the co-administration of ketoconazole at normal clinical doses with 4-HPR may increase systemic exposure to 4-HPR in humans.

Study on cytochrome p-450 dependent retinoic Acid metabolism and its inhibitors as potential agents for cancer therapy

The relative lack of clinical success with conventional anticancer agents may be due in part to the traditional concept of cancer being a biological state rather than a dynamic process. Redefining cancer as a dynamic disease commencing with carcinogenesis introduces the possibility of chemoprevention. Retinoids offer the promise of a therapeutic option based on differentiation of premalignant as well as malignant cells. Research to date has concentrated on the use of exogenous retinoids in cancer. Although this research continues with new retinoid derivatives, an alternative approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450- mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs which increase the level of endogenous retinoic acid (RA) within the tumor cells by blocking their metabolism. This approach presents several theoretic advantages.

In the present study a wide range of established P-450 inhibitors has been screened to examine their inhibitory activity on all-trans-Retinoic acid (ATRA) metabolism. Forty-one known P450 inhibitors were tested for their inhibitory activity against RA metabolism. Most of them are nitrogen-containing compounds. The results showed that among these compounds only six compounds (N-benzyl-2-phenylethanamine, itraconazole, chlorpromazine, 5-chloro-1,3-benzoxazol-2-amine, proadifen and furazolidone) showed inhibition of RA metabolism which was > 50%. Ketoconazole and liarozole were also screened as standard potent inhibitors in the same system and gave 87.5% and 89% inhibition, respectively. The results indicate that mostly azoles with substituents in positions other than the 1-position on the ring are very weak inhibitors of RA metabolism. The most effective inhibitors (ketoconazole, itraconazole, bifonazole and clotrimazole) are 1-substituted and possess relatively large aromatic groups in the molecule. 1-Substituted imidazoles bind to cytochrome P-450 with a very high affinity but substitution in the other position of the imidazole decreases the binding affinity.

Effects of liarozole fumarate (R85246) in combination with tamoxifen on N-methyl-N-nitrosourea (MNU)-induced mammary carcinoma and uterus in the rat model

Background

Liarozole fumarate (liarozole – R85246) is a novel compound with characteristics of both aromatase inhibitor (AI) and a retinoic acid metabolism blocking agent (RAMBA). Our objective was to determine the effects of liarozole alone or in combination with tamoxifen on the N-methyl-N-nitrosourea (MNU)-induced rat mammary carcinoma model, as well as on the uterus in ovariectomized immature rats.

Methods

(1) Tumor burden experiments: Animals bearing one or more tumors greater than 10 mm in diameter were treated for 56 consecutive days with 20 mg/kg or 80 mg/kg of liarozole by oral gavage, tamoxifen 100 μg/kg by subcutaneous injection, or a combination of liarozole and tamoxifen. At the end of the treatment period, total cumulative tumor volume as well as retinoic acid levels were measured. (2) Uterotrophic assay and proliferation experiments: 21-day-old ovariectomized (OVX) Sprague-Dawley rats were treated with 20 mg/kg or 80 mg/kg of liarozole by oral gavage, tamoxifen 1 mg/kg by subcutaneous injection, and combination of both for 4 consecutive days. At the end of the treatment period, uterine weight, epithelial lining cell height and indices of proliferation cell nuclear antigen (PCNA) were measured.

Results

The tumor burden experiments in rats bearing estrogen receptor (ER) positive mammary tumours showed that liarozole has a marked anti-tumour effect. In combination with tamoxifen, liarozole had neither an additive nor an antagonistic effect. However, liarozole markedly reduced the uterotrophic effects induced by tamoxifen.

Conclusion

Liarozole's antitumor effects on ER positive mammary tumors and its protective effect on the uterus merit further studies to confirm its clinical value in combination with tamoxifen in ER positive postmenopausal breast cancer. Liarozole and other retinomimetics might also be suitable chemoprevention drugs in combination with tamoxifen because of their favorable toxicity profile.

Vitamin A metabolism in cultured somatic cells from rat testis

Sertoli and peritubular myoid cells, the somatic cells of the seminiferous tubule, support growth and differentiation of developing germ cells. This action strictly depends on the availability of in situ synthesized retinoic acid and we have previously documented the ability of Sertoli, but not peritubular cell extracts, to support the oxidation of retinol to retinoic acid. Using primary cultures of somatic cells treated with a physiological concentration of free retinol, we show here that the same is essentially true also for whole cultured cells. Sertoli cells are capable of producing not only retinoic acid, but are also the major site of retinyl ester (mainly, retinyl palmitate) formation. Compared with retinyl palmitate accumulation, retinoic acid synthesis was both faster and positively influenced by prior exposure to retinol. This increase in retinoic acid synthesis was further augmented by treatment with the retinoic acid catabolic inhibitor liarozole, thus indicating that enhanced synthesis, rather than reduced catabolism, is responsible for such an effect. Myoid cells had a higher capacity to incorporate exogenously supplied retinol, yet retinoic acid synthesis, and even more so retinyl palmitate formation, were considerably lower than in Sertoli cells. Retinoic acid synthesis in myoid cells was not only depressed, but also very little influenced by prior retinol exposure and totally insensitive to liarozole. These data further support the view that myoid cells are involved in retinol uptake from the blood and its transfer to other cells, rather than in metabolic interconversion or long-term storage of vitamin A, two processes that mainly take place in Sertoli cells.

Inhibition of all-TRANS-retinoic acid metabolism by R116010 induces antitumour activity

All-trans-retinoic acid is a potent inhibitor of cell proliferation and inducer of differentiation. However, the clinical use of all-trans-retinoic acid in the treatment of cancer is significantly hampered by its toxicity and the prompt emergence of resistance, believed to be caused by increased all-trans-retinoic acid metabolism. Inhibitors of all-trans-retinoic acid metabolism may therefore prove valuable in the treatment of cancer. In this study, we characterize R116010 as a new anticancer drug that is a potent inhibitor of all-trans-retinoic acid metabolism. In vitro, R116010 potently inhibits all-trans-retinoic acid metabolism in intact T47D cells with an IC(50)-value of 8.7 nM. In addition, R116010 is a selective inhibitor as indicated by its inhibition profile for several other cytochrome P450-mediated reactions. In T47D cell proliferation assays, R116010 by itself has no effect on cell proliferation. However, in combination with all-trans-retinoic acid, R116010 enhances the all-trans-retinoic acid-mediated antiproliferative activity in a concentration-dependent manner. In vivo, the growth of murine oestrogen-independent TA3-Ha mammary tumours is significantly inhibited by R116010 at doses as low as 0.16 mg kg(-1). In conclusion, R116010 is a highly potent and selective inhibitor of all-trans-retinoic acid metabolism, which is able to enhance the biological activity of all-trans-retinoic acid, thereby exhibiting antitumour activity. R116010 represents a novel and promising anticancer drug with an unique mechanism of action.

Ethanol enhances retinoic acid metabolism into polar metabolites in rat liver via induction of cytochrome P4502E1

BACKGROUND & AIMS

Long-term and excessive ethanol intake results in decreased plasma and hepatic levels of retinoic acid (RA), the most active derivative of vitamin A. The decrease of RA by ethanol treatment has been proposed to be a cytochrome P450 enzyme (CYP)-dependent process. However, the role of the major ethanol-induced CYP, CYP2E1, in the metabolism of RA has not been defined.

METHODS

In vitro incubations of RA with microsomal fractions of liver tissue containing CYPs from either ethanol-exposed or non-ethanol-exposed rats were carried out using chemical inhibitors and antibodies against various CYPs. In vivo, both ethanol-exposed and non-ethanol-exposed rats were treated with or without chlormethiazole, a specific CYP2E1 inhibitor, for 1 month. RA and its catabolic metabolites were analyzed by high-performance liquid chromatography and spectral analysis.

RESULTS

Incubation of RA with the liver microsomal fraction from ethanol-exposed rats resulted in greater disappearance of RA and increased appearance of 18-hydroxy-RA and 4-oxo-RA compared with control rat liver microsomal fractions. The enhancement of RA catabolism by ethanol was inhibited by both CYP2E1 antibody and specific inhibitors (allyl sulfide and chlormethiazole) in a dose-dependent fashion, whereas the metabolism of RA into polar metabolites was abolished completely by nonspecific CYP inhibitors (disulfiram and liarozole). Furthermore, treatment with chlormethiazole in ethanol-fed rats in vivo restored both hepatic and plasma RA concentrations to normal levels.

CONCLUSIONS

Ethanol-induced CYP2E1 plays a major role in the degradation of RA, which may provide a possible biochemical mechanism for chronic and excessive ethanol intake as a risk for both hepatic and extrahepatic cell proliferation and carcinogenesis.

Liarozole fumarate (R85246): in the treatment of ER negative, tamoxifen refractory or chemotherapy resistant postmenopausal metastatic breast cancer

Three phase II studies were conducted to determine the efficacy and tolerability of liarozole fumarate (R85246; liarozole), a retinoic acid metabolism blocking agent (RAMBA) and aromatase inhibitor. Additionally, animal experiments in the MNU-induced rat mammary tumor model and in immature ovariectomized rats were conducted to further elucidate liarozole's mechanisms of action. Patients were postmenopausal with either: ER negative disease in first relapse (Group 1: 1n = 16); ER positive or unknown disease refractory to tamoxifen (Group 2; n = 16); ER positive, negative or unknown disease resistant or refractory to chemotherapy (Group 3; n = 27). Treatment was liarozole (150-300mg) twice daily orally until disease progression. Response rates were: 25% in group 1 (95% CI 11.0-52.3%: median duration (MD) 20 months; range 2-36.5); 25% in group 2 (95% CI 11.0-52.3%; MD 6.5 months: range 3.5-38): 11% in group 3 (95% CI 4.2-29.2%; MD 7 months; range 3-8.5). No significant improvement in quality of life scores (FLI-C) was noted. Toxicities observed were predominantly dermatological (skin disorders: 88%; dry mouth/eyes/lips: 69%). Plasma estradiol decreased from mean pre-treatment levels of 72.7 pM (9.1-1,839 pM) to below detection (9.2 pM) after 1 month. Liarozole, but not vorozole, partially inhibited estradiol induced uterine hypertrophy and demonstrated dose-dependent anti-tumor effects in the rats, only partially overcome by coadministration of estradiol. The clinical responses observed, together with our preclinical results, confirm liarozole's dual mechanism of action and provide a rationale for further evaluation of RAMBAs in the treatment of breast cancer.

In-vitro metabolism of retinoic acid by different tissues from male rats

Significant differences between the metabolism of retinoic acid by different tissues might be an important determinant of the effectiveness of a systemically administered inhibitor at a particular tissue site. Here the metabolism of retinoic acid has been studied in microsomal fractions from different tissues (liver, kidney, intestinal mucosa, lung, skin, brain) of the male rat to determine their relative metabolic activity. Kinetic analysis revealed major differences between the activity of different tissue microsomes. This is shown by the Vmax values for the metabolism of retinoic acid-liver (102+/-39.0 pmol (mg protein)(-1) min(-1)) was 100 times more active than the lung (1+/-0.03 pmol (mg protein)(-1) min(-1)), which was the least active. The range of Km values for microsomes from the different tissues was narrow (0.48-1.40 microM). Taking into account the mass of the tissue, the gross activity ranking for metabolism of retinoic acid was liver >> skin = kidney > brain > intestinal mucosa >> lung. It is concluded that metabolism of administered retinoic acid occurs mainly in the liver but that cellular retinoic acid levels in some other tissues (skin, kidney, brain) could be reduced (metabolized) to such an extent that higher levels might be observed after the use of inhibitors of retinoic acid metabolism.

Effect of P450 isozyme-selective inhibitors on in-vitro metabolism of retinoic acid by rat hepatic microsomes

Cytochrome P450-mediated 4-hydroxylation of retinoic acid is an important pathway in the termination of its biological action and the activity of certain P450 isozymes has been studied in non-induced male rat hepatic microsomes using isozyme-selective inhibitors. The importance of the activity of the isozyme to retinoic acid metabolism was, 2A6 (diethyl dithiocarbamate as selective inhibitor) > 1A1/1A2 (7,8-benzoflavone) >> 1A1 (ellipticine) > 3A4 (naringenin, ketoconazole) as shown by the respective apparent IC50 values of 0.12, 0.34, 2.7, 9.25 and 13.5 microM with 2C8-10, 2D6 and 2E1 having little effect on metabolism. It is concluded that although the P450 3A family normally constitutes half the total rat hepatic P450 activity, other hepatic isozymes (1A1, 1A2 and 2A6) are also involved in retinoic acid metabolism. This suggests that the horizons for the design of potential anticancer agents acting through inhibition of retinoic acid metabolism may be widened to include structures which do not resemble the established hetereocyclic base P450 3A4 inhibitors.

Liarozole fumarate (R85246): a novel imidazole in the treatment of receptor positive postmenopausal metastatic breast cancer

This phase II study of liarozole fumarate (R85246, liarozole), a novel imidazole with retinomimetic and aromatase inhibitory effects, was designed to determine the efficacy and tolerability in postmenopausal women with advanced breast cancer in progression, to correlate these effects with hormonal levels, and to evaluate quality of life. Twenty-nine women with ER-positive or unknown metastatic disease who received > or = 2 prior hormonal therapies were treated with 150-300 mg liarozole twice daily until disease progression. All patients were evaluable for toxicity and 25 for response. Four patients (16.0%, 95% CI 5.3-37.4%) had partial remission (PR) of their disease for a median of 7.4 months (range 1.2-12.9) and 7 (28%) had disease stabilization for a median of 4.8 months (1.6-16.0). Estradiol decreased from pre-treatment levels of 9.2-52 pM (mean 17.1) to below detection (9.2 pM, p = 0.0005) after 1 month. Similarly estrone levels fell from 14-307 pM (mean 92.7) to below detection (9.2 pM, p = 0.0001). The most common toxicity was dermatological (96.6%) with features compatible with hypervitaminosis A syndrome such as rash, pruritus, dry skin, and brittle nails. The majority of these were mild to moderate in severity. No significant improvement in quality of life scores (FLI-C) were noted. Liarozole is an active new treatment for breast cancer in patients heavily pre-treated with hormone therapies. Further studies are needed to confirm its relative efficacy in both receptor positive and negative postmenopausal breast cancer.

Embryonal carcinoma cell lines stably transfected with mRARbeta2-lacZ: sensitive system for measuring levels of active retinoids

Embryonal carcinoma cell lines (F9 EC and P19 EC) were stably transfected with 1.8 kb promoter sequence of RARbeta2 coupled to the lacZ gene as a system for measuring active retinoids. These stable transfectants, designated F9-1.8 and P19-1.8, were used as reporter cell lines to investigate different retinoids for their ability to activate the reporter gene. F9-1.8 cells showed similar EC(50) values for the acidic retinoids all-trans retinoic acid (RA), 4-oxo RA, 9-cis RA, and 13-cis RA, in the range of 1-7 nM, while P19-1.8 cells were less sensitive. Retinal showed decreased activity compared to the RA isomers in both lines. However, P19-1.8 cells hardly showed beta-gal activity after treatment with retinol, while the lacZ reporter in F9-1.8 cells was still inducible by this retinoid. In addition, the reporter system was used to investigate RA metabolism and its inhibition by P450 inhibitors. A combination of RA and liarozole showed a 10 times greater induction of the RARbeta2-lacZ reporter in P19-1.8 cells, but not in F9-1.8 cells. The EC(50) value for 4-oxo RA, however, was not altered, indicating that metabolic conversion of RA to 4-oxo RA is the target for inhibition by liarozole in P19-1.8 cells. HPLC analysis revealed nearly complete inhibition of RA metabolism after liarozole treatment in P19-1.8 cells, resulting in higher levels of RA. Finally, the F9-1.8 cells were used to detect active retinoids during different stages of chick limb bud development, demonstrating that it is the limb bud mesenchyme which generates RA and not the epidermis, with a twofold higher level of RA in the posterior half than in the anterior half.

The emerging role of retinoids and retinoic acid metabolism blocking agents in the treatment of cancer

BACKGROUND

Although significant advances have been made in the treatment of some malignancies, the prognosis of patients with metastatic tumors remains poor. Differentiating agents redirect cells toward their normal phenotype and therefore may reverse or suppress evolving malignant lesions or prevent cancer invasion. In addition, they offer a potential alternative to the classic cytostatic drugs.

METHODS

The purpose of this review was to examine the current and potential future roles of differentiating agents in the treatment of cancer.

RESULTS

Initial studies with differentiating agents focused on retinoid therapy. Although retinoids have shown some clinical success, their widespread use has been limited by resistance and, in the chemopreventive setting, toxicity. This has led to the synthesis of a number of new retinoids that currently are undergoing clinical investigation. A further approach to overcoming the drawbacks associated with exogenous retinoids has been to increase the levels of endogenous retinoic acid (RA) by inhibiting the cytochrome P450-mediated catabolism of RA using a novel class of agents known as retinoic acid metabolism blocking agents (RAMBAs). Liarozole, the first RAMBA to undergo clinical investigation, preferentially increases intratumor levels of endogenous RA resulting in antitumor activity.

CONCLUSIONS

Although studies using exogenous retinoids in this setting have not yet fulfilled their initial promise, studies with a growing set of synthetic retinoids are ongoing. Furthermore, modulation of endogenous retinoids may offer a significant new potential treatment for cancer.

The antiproliferative activity of all-trans-retinoic acid catabolites and isomers is differentially modulated by liarozole-fumarate in MCF-7 human breast cancer cells

The clinical use of all-trans-retinoic acid (ATRA) in the treatment of cancer is significantly hampered by the prompt emergence of resistance, believed to be caused by increased ATRA catabolism. Inhibitors of ATRA catabolism may therefore prove valuable for cancer therapy. Liarozole-fumarate is an anti-tumour drug that inhibits the cytochrome P450-dependent catabolism of ATRA. ATRA, but also its naturally occurring catabolites, 4-oxo-ATRA and 5,6-epoxy-ATRA, as well as its stereoisomers, 9-cis-RA and 13-cis-RA, show significant antiproliferative activity in MCF-7 human breast cancer cells. To further elucidate its mechanism of action, we investigated whether liarozole-fumarate was able to enhance the antiproliferative activity of ATRA catabolites and isomers. Liarozole-fumarate alone up to a concentration of 10(-6) M had no effect on MCF-7 cell proliferation. However, in combination with ATRA or the ATRA catabolites, liarozole-fumarate (10(-6) M) significantly enhanced their antiproliferative activity. On the contrary, liarozole-fumarate (10(-6) M) was not able to potentiate the antiproliferative activity of the ATRA stereoisomers, most probably because of the absence of cytochrome P450-dependent catabolism. Together, these findings show that liarozole-fumarate acts as a versatile inhibitor of retinoid catabolism in that it not only blocks the breakdown of ATRA, but also inhibits the catabolic pathway of 4-oxo-ATRA and 5,6-epoxy-ATRA, thereby enhancing their antiproliferative activity.

Effects of systemic treatment with liarozole on cutaneous inflammation, epidermal proliferation and differentiation in extensive plaque psoriasis

Liarozole is a novel inhibitor of the enzyme cytochrome P450 which has inhibitory effects on the 4-hydroxylation of retinoic acid. Previous studies have shown that liarozole is effective in the treatment of psoriasis. We performed an immunohistochemical study on the lesional skin from 7 patients with extensive plaque psoriasis, who were treated with systemic liarozole 75 mg BID for a period of at least 2 months. The effects of liarozole treatment on clinical and histological parameters were investigated. In particular, the effect of liarozole on the integrin markers CD11b and CD18 was studied. For immunohistochemistry, three consecutive biopsies were taken: before treatment, after 4, and after 8 weeks of treatment. Clinical scores and side effects were recorded before and during treatment. The medication was well tolerated and only mild side effects were reported, which were comparable with hypervitaminosis A. After 2 months of treatment a statistically significant decrease of the extent of body involvement was observed. In the psoriatic plaque, markers for epidermal proliferation and cutaneous inflammation decreased, and markers for epidermal differentiation increased to values comparable to normal skin. The first therapeutic effects in the psoriatic plaque occurred after 4 weeks of treatment, and consisted of a decreased induration, accompanied by a decrease of the total number of inflammatory infiltrate cells and a decreased epidermal ICAM-1 expression. Already after 4 weeks of treatment, a decrease of CD11b-positive cells was observed. Subsequently, after 8 weeks of treatment recruitment of cycling epidermal cells and the number of involucrin-positive cell layers decreased. The present study demonstrates that liarozole treatment of psoriasis results in a reduction of aspects of cutaneous inflammation and subsequently a reduction of epidermal proliferation and promotion of differentiation. After 4 weeks of treatment, effects are observed on the epidermal ICAM-1 expression and on the CD11b-positive cell population.

Synthesis of LIAZAL, a retinoic acid metabolism blocking agent (RAMBA) with potential clinical applications in oncology and dermatology

The synthesis of LIAZAL (compound 9, R085246) is described. LIAZAL inhibits all-trans-retinoic acid metabolism and thereby exerts retinoid-like effects in vivo.

All-trans-retinoic acid metabolites significantly inhibit the proliferation of MCF-7 human breast cancer cells in vitro

All-trans-retinoic acid (ATRA) is well known to inhibit the proliferation of human breast cancer cells. Much less is known about the antiproliferative activity of the naturally occurring metabolites and isomers of ATRA. In the present study, we investigated the antiproliferative activity of ATRA, its physiological catabolites 4-oxo-ATRA and 5,6-epoxy-ATRA and isomers 9-cis-RA and 13-cis-RA in MCF-7 human breast cancer cells by bromodeoxyuridine incorporation. MCF-7 cells were grown in steroid- and retinoid-free medium supplemented with growth factors. Under these culture conditions, ATRA and its naturally occurring catabolites and isomers showed significant antiproliferative activity in MCF-7 cells in a concentration-dependent manner (10[-11] M to 10[-6] M). The antiproliferative activity of ATRA catabolites and isomers was equal to that of the parent compound ATRA at concentrations of 10(-8) M and 10(-7) M. Only at 10(-6) M were the catabolites and the stereoisomer 13-cis-RA less potent. The stereoisomer 9-cis-RA was as potent as ATRA at all concentrations tested (10[-11] M to 10[-6] M). In addition, we show that the catabolites and isomers were formed from ATRA to only a limited extent. Together, our findings suggest that in spite of their high antiproliferative activity the catabolites and isomers of ATRA cannot be responsible for the observed growth inhibition induced by ATRA.

Autoinduction of retinoic acid metabolism to polar derivatives with decreased biological activity in retinoic acid-sensitive, but not in retinoic acid-resistant human breast cancer cells

Previous studies have shown that all-trans-retinoic acid (RA) inhibits in vitro proliferation of hormone-dependent human breast cancer cells but not the growth of hormone-independent cells. Here we report on RA metabolism in breast cancer cells as examined by high performance liquid chromatography analysis and found a correlation with sensitivity to growth inhibition by RA. RA-sensitive T-47D and MCF-7 cells exhibited high rate metabolism to polar metabolites, whereas RA-resistant MDA-MB-231 and MDA-MB-468 cells metabolized RA to a much lesser extent, and almost no polar metabolites could be detected. The high metabolic rate in RA-sensitive cells appears to be the result of autoinduction of RA metabolism, whereas RA-resistant cells showed no such induction of metabolism. We observed furthermore that transfection with retinoic acid receptor-alpha expression vectors in RA-resistant MDA-MB-231 cells resulted in increased RA metabolism and inhibition of cell proliferation. Metabolism of RA, however, seems not to be required to confer growth inhibition of human breast cancer cells. The biological activity of the polar metabolites formed in RA-sensitive cells was found to be equal or lower than that of RA, indicating that RA itself is the most active retinoid in these cells. Together our data suggest that RA-sensitive cells contain mechanisms to activate strongly the catabolism of RA probably to protect them from the continuous exposure to this active retinoid.

Induction of the oxidative catabolism of retinoid acid in MCF-7 cells

Cytochrome P450-dependent oxidation is a pathway for all-trans-retinoic acid (all-trans-RA) catabolism. Induction of this catabolic pathway was studied in MCF-7 breast cancer cells. MCF-7 cells showed low constitutive all-trans-RA catabolism. Concentration-dependent induction was obtained by preincubation of the cells with all-trans-RA (10(-9) to 10(-6) M). Onset of induction was fast, being detectable within 60 min, with maximal induction (45-fold) obtained after 16 h. Enzymatic characterization of induced all-trans-RA catabolism showed an estimated Km value (Michaelis-Menten constant) of 0.33 microM and a Vmax value (maximal velocity of an enzyme-catalysed reaction) of 54.5 fmol polar all-trans-RA metabolites 10(6) cells(-1) h(-1). These kinetic parameters represent the overall formation of polar metabolites from all-trans-RA. Induction of all-trans-RA catabolism was also obtained with other retinoids, CH55 >> 13-cis-RA = all-trans-RA > 9-cis-RA > 4-keto-all-trans-RA > 4-keto-13-cis-RA > retinol. The potency of the retinoids to induce all-trans-RA catabolism was correlated to their retinoic acid receptor affinity (Crettaz et al, 1990; Repa et al, 1990; Sani et al, 1990). Induction of all-trans-RA catabolism was inhibited by actinomycin D. Furthermore, all-trans-RA did not increase cytosolic retinoic acid-binding protein (CRABP) mRNA levels. These data suggest that induction of all-trans-RA catabolism in MCF-7 cells is a retinoic acid receptor-mediated gene transcriptional event. Induced all-trans-RA catabolism was inhibited by various retinoids with decreasing potency in the order: all-trans-RA > 4-keto-all-trans-RA > 13-cis-RA > 9-cis-RA > 4-keto-13-cis-RA > retinol > CH55. The antitumoral compound liarozole-fumarate inhibited all-trans-RA catabolism with a potency similar to that of all-trans-RA.

Analysis of the oxidative catabolism of retinoic acid in rat Dunning R3327G prostate tumors

We studied the enzymatic characteristics of the oxidative catabolism of retinoic acid (RA) and its inhibition by liarozole-fumarate in homogenates of rat Dunning R3327G prostate tumors. Homogenates of rat liver were used as reference material. Both tumor and liver homogenates were able to catabolize retinoic acid. HPLC analysis revealed only very polar metabolites in tumors, while in the liver both metabolites with intermediate polarity and more polar metabolites were found. Kinetic analysis of retinoic acid catabolism showed a K(m) of 1.7 +/- 0.7 microM and a Vmax of 4.2 +/- 4.4 pmol polar RA metabolites/mg protein/hr for Dunning G tumor homogenates. In liver homogenates a K(m) value of 4.3 +/- 0.5 microM and a Vmax value of 290 +/- 120 pmol polar RA metabolites/mg protein/hr were obtained. Liarozole-fumarate inhibited retinoic acid catabolism in Dunning tumors and liver with IC50 values of 0.26 +/- 0.16 microM and 0.14 +/- 0.05, respectively. The results suggest that rat Dunning R3327G tumors are able to metabolize retinoic acid in a manner similar to that found in rat liver but with a lower metabolizing capacity.

An analysis of retinoic acid-induced gene expression and metabolism in AB1 embryonic stem cells

Murine embryonic stem cells such as the AB1 cell line undergo differentiation in the presence of retinoic acid (RA) into an extraembryonic epithelial cell type. This results in the activation of genes such as Hoxa-1, Hoxb-1, laminin, collagen IV(alpha1), tissue plasminogen activator, RARbeta, and CRABPII. The CRABPI gene is regulated in an unusual fashion; CRABPI message and protein levels are induced at low concentrations of RA, but induction is diminished at higher concentrations. AB1 cells take up RA rapidly from the medium, and the addition of low, exogenous concentrations of RA to the culture medium results in very high intracellular RA concentrations. For example, AB1 stem cells cultured in 5 nM [3H]RA have an internal [3H]RA concentration of 1-2 microM within the first hour. AB1 cells also metabolize [3H]RA to more polar RA derivatives. The half-life of RA in AB1 cells not previously exposed to RA is about 2-2.5 h versus 40-45 min in cells cultured for 2-3 days in 1 microM exogenous RA. Thus, the enzyme(s) which metabolize RA are induced or activated by RA. Furthermore, the local concentration of RA required to elicit some biological responses may be higher than previously thought.

P450-dependent enzymes as targets for prostate cancer therapy

Metastatic prostate adenocarcinoma is a leading cause of cancer-related deaths among men. First line treatment is primarily aimed at blocking the synthesis and action of androgens. As primary endocrine treatment, androgen deprivation is usually achieved by orchidectomy or LHRH analogues, frequently combined with androgen receptor antagonists in order to block the residual adrenal androgens. However, nearly all the patients will eventually relapse. Available or potential second line therapies include, among others, alternative endocrine manipulations and chemotherapy. Cytochrome P450-dependent enzymes are involved in the synthesis and/or degradation of many endogenous compounds, such as steroids and retinoic acid. Some of these enzymes represent suitable targets for the treatment of prostate cancer. In first line therapy, inhibitors of the P450-dependent 17,20-lyase may achieve a maximal androgen ablation with a single drug treatment. Ketoconazole at high dose blocks both testicular and adrenal androgen biosynthesis but its side-effects, mainly gastric discomfort, limit its widespread use. A series of newly synthesized, more selective, steroidal 17,20-lyase inhibitors related to 17-(3-pyridyl)androsta-5,16-dien-3beta-ol, may open new perspectives in this field. In prostate cancer patients who relapse after surgical or medical castration, therapies aiming at suppressing the remaining adrenal androgen biosynthesis (ketoconazole) or producing a medical adrenalectomy (aminoglutethimide+hydrocortisone) have been used, but are becoming obsolete with the generalization of maximal androgen blockade in first line treatment. The role of inhibition of aromatase in prostate cancer therapy, which was postulated for aminoglutethimide, could not be confirmed by the use of more selective aromatase inhibitors, such as formestane. An alternative approach is represented by liarozole fumarate (LIA), a compound that blocks the P450-dependent catabolism of retinoic acid (RA). In vitro, it enhances the antiproliferative and differentiation effects of RA in cell lines that express RA metabolism, such as F9 teratocarcinoma and MCF-7 breast carcinoma cells. In vivo, monotherapy with LIA increases RA plasma levels and, to a greater extent, endogenous tissue RA levels leading to retinoid-mimetic effects. In the rat Dunning prostate cancer models, it inhibits the growth of androgen-independent as well as androgen-dependent carcinomas relapsing after castration. Concurrently, changes in the pattern of cytokeratins characteristic of increased differentiation were observed. Early clinical trials show that LIA, in second or third line therapy in metastatic prostate cancer, induces PSA responses in about 30% of unselected patients. In some patients regression of soft tissue metastasis ha been observed. In a subgroup of patients, an important relief of metastatic bone pain was also noted.