In vivo and in vitro properties of STX2484: a novel non-steroidal anti-cancer compound active in taxane-resistant breast cancer

BACKGROUND

STX2484 is a novel non-steroidal compound with potent anti-proliferative activity. These studies aimed to identify STX2484's mechanism of action, in vivo efficacy and activity in taxane-resistant breast cancer models.

METHODS

Effects of STX2484 and paclitaxel on proliferation, cell cycle and apoptosis were assessed in vitro in drug-resistant (MCF-7(DOX)) and non-resistant cells (MCF-7(WT)). STX2484 efficacy in βIII tubulin overexpression in MCF-7 cells was also determined. Anti-angiogenic activity was quantified in vitro by a co-culture model and in vivo using a Matrigel plug assay. An MDA-MB-231 xenograft model was used to determine STX2484 efficacy in vivo.

RESULTS

STX2484 is a tubulin disruptor, which induces p53 expression, Bcl2 phosphorylation, caspase-3 cleavage, cell cycle arrest and apoptosis. In addition, STX2484 is a potent anti-angiogenic agent in vitro and in vivo. In breast cancer xenografts, STX2484 (20 mg kg(-1) p.o.) suppressed tumour growth by 84% after 35 days of daily dosing, with limited toxicity. In contrast to paclitaxel, STX2484 efficacy was unchanged in two clinically relevant drug-resistant models.

CONCLUSIONS

STX2484 is an orally bioavailable microtubule-disrupting agent with in vivo anti-angiogenic activity and excellent in vivo efficacy with no apparent toxicity. Crucially, STX2484 has superior efficacy to paclitaxel in models of clinical drug resistance.

A novel inhibitor of the PI3K/Akt pathway based on the structure of inositol 1,3,4,5,6-pentakisphosphate

BACKGROUND

Owing to its role in cancer, the phosphoinositide 3-kinase (PI3K)/Akt pathway is an attractive target for therapeutic intervention. We previously reported that the inhibition of Akt by inositol 1,3,4,5,6-pentakisphosphate (InsP(5)) results in anti-tumour properties. To further develop this compound we modified its structure to obtain more potent inhibitors of the PI3K/Akt pathway.

METHODS

Cell proliferation/survival was determined by cell counting, sulphorhodamine or acridine orange/ethidium bromide assay; Akt activation was determined by western blot analysis. In vivo effect of compounds was tested on PC3 xenografts, whereas in vitro activity on kinases was determined by SelectScreen Kinase Profiling Service.

RESULTS

The derivative 2-O-benzyl-myo-inositol 1,3,4,5,6-pentakisphosphate (2-O-Bn-InsP(5)) is active towards cancer types resistant to InsP(5) in vitro and in vivo. 2-O-Bn-InsP(5) possesses higher pro-apoptotic activity than InsP(5) in sensitive cells and enhances the effect of anti-cancer compounds. 2-O-Bn-InsP(5) specifically inhibits 3-phosphoinositide-dependent protein kinase 1 (PDK1) in vitro (IC(50) in the low nanomolar range) and the PDK1-dependent phosphorylation of Akt in cell lines and excised tumours. It is interesting to note that 2-O-Bn-InsP(5) also inhibits the mammalian target of rapamycin (mTOR) in vitro.

CONCLUSIONS

InsP(5) and 2-O-Bn-InsP(5) may represent lead compounds to develop novel inhibitors of the PI3K/Akt pathway (including potential dual PDK1/mTOR inhibitors) and novel potential anti-cancer drugs.

BCRP expression does not result in resistance to STX140 in vivo, despite the increased expression of BCRP in A2780 cells in vitro after long-term STX140 exposure

The anti-proliferative and anti-angiogenic properties of the endogenous oestrogen metabolite, 2-methoxyoestradiol (2-MeOE2), are enhanced in a series of sulphamoylated derivatives of 2-MeOE2. To investigate possible mechanisms of resistance to these compounds, a cell line, A2780.140, eightfold less sensitive to the 3,17-O,O-bis-sulphamoylated derivative, STX140, was derived from the A2780 ovarian cancer cell line by dose escalation. Other cell lines tested did not develop STX140 resistance. RT-PCR and immunoblot analysis demonstrated that breast cancer resistance protein (BCRP) expression is dramatically increased in A2780.140 cells. The cells are cross-resistant to the most structurally similar bis-sulphamates, and to BCRP substrates, mitoxantrone and doxorubicin; but they remain sensitive to taxol, an MDR1 substrate, and to all other sulphamates tested. Sensitivity can be restored using a BCRP inhibitor, and this pattern of resistance is also seen in a BCRP-expressing MCF-7-derived cell line, MCF-7.MR. In mice bearing wild-type (wt) and BCRP-expressing tumours on either flank, both STX140 and mitoxantrone inhibited the growth of the MCF-7wt xenografts, but only STX140 inhibited growth of the MCF-7.MR tumours. In conclusion, STX140, a promising orally bioavailable anti-cancer agent in pre-clinical development, is highly efficacious in BCRP-expressing xenografts. This is despite an increase in BCRP expression in A2780 cells in vitro after chronic dosing with STX140.

The in vivo properties of STX243: a potent angiogenesis inhibitor in breast cancer

The steroidal-based drug 2-ethyloestradiol-3,17-O,O-bis-sulphamate (STX243) has been developed as a potent antiangiogenic and antitumour compound. The objective of this study was to ascertain whether STX243 is more active in vivo than the clinically relevant drug 2-methoxyoestradiol (2-MeOE2) and the structurally similar compound 2-MeOE2-3,17-O,O-bis-sulphamate (STX140). The tumour growth inhibition efficacy, antiangiogenic potential and pharmacokinetics of STX243 were examined using four in vivo models. Both STX243 and STX140 were capable of retarding the growth of MDA-MB-231 xenograft tumours (72 and 63%, respectively), whereas no inhibition was observed for animals treated with 2-MeOE2. Further tumour inhibition studies showed that STX243 was also active against MCF-7 paclitaxel-resistant tumours. Using a Matrigel plug-based model, in vivo angiogenesis was restricted with STX243 and STX140 (50 and 72%, respectively, using a 10 mg kg(-1) oral dose), thereby showing the antiangiogenic activity of both compounds. The pharmacokinetics of STX243 were examined at two different doses using adult female rats. The compound was orally bioavailable (31% after a single 10 mg kg(-1) dose) and resistant to metabolism. These results show that STX243 is a potent in vivo drug and could be clinically effective at treating a number of oncological conditions.

2-Methoxyoestradiol-3,17-O,O-bis-sulphamate and 2-deoxy-D-glucose in combination: a potential treatment for breast and prostate cancer

Drug combination therapy is a key strategy to improve treatment efficacy and survival of cancer patients. In this study the effects of combining 2-methoxyoestradiol-3,17-O,O-bis-sulphamate (STX140), a microtubule disruptor, with 2-deoxy-D-glucose (2DG) were assessed in MCF-7 (breast) and LNCaP (prostate) xenograft models in vivo. In mice bearing MCF-7 xenografts, daily p.o. administration of STX140 (5 mg kg⁻¹) resulted in a 46% (P<0.05) reduction of tumour volume. However, the combination of STX140 (5 mg kg⁻¹ p.o.) and 2DG (2 g kg⁻¹ i.p.) reduced tumour volume by 76% (P<0.001). 2-Methoxyoestradiol-3,17-O,O-bis-sulphamate also reduced tumour vessel density. 2-Deoxy-D-glucose alone had no significant effect on tumour volume or vessel density. A similar benefit of the combination treatment was observed in the LNCaP prostate xenograft model. In vitro the degree of inhibition of cell proliferation by STX140 was unaffected by oxygen concentrations. In contrast, the inhibition of proliferation by 2DG was enhanced under hypoxia by 20 and 25% in MCF-7 and LNCaP cells, respectively. The combination of STX140 and 2DG in LNCaP cells under normoxia or hypoxia inhibited proliferation to a greater extent than either compound alone. These results suggest that the antiangiogenic and microtubule disruption activities of STX140 may make tumours more susceptible to inhibition of glycolysis by 2DG. This is the first study to show the benefit of combining a microtubule disruptor with 2DG in the two most common solid tumours.

Inhibition of steroid sulphatase activity via the percutaneous route: a new option for breast cancer therapy

Steroid sulphatase (STS) inhibitors have been developed primarily for the treatment of hormone-dependent breast cancer, but may also have utility for the treatment of a number of androgen-dependent skin conditions. STS regulates the hydrolysis of steroid sulphates, such as oestrone sulphate (E1S) and dehydroepiandrosterone sulphate, (DHEAS). Liberated oestrone (E1) can be converted to biologically active oestradiol (E2) while dehydroepiandrosterone (DHEA) can undergo reduction to testosterone or aromatisation to E1. In this study the ability of the STS inhibitor STX64 (BN83495) and its N,N-dimethyl analogue (STX289) to inhibit liver and skin STS when applied orally or topically to nude mice was examined. Oral administration at 1 and 10 mg/kg resulted in almost complete inhibition of skin and liver STS. When applied topically to the dorsal neck region at 1.0 and 10 mg/kg not only skin but, unexpectedly, also liver STS was effectively inhibited. An investigation into the metabolism of these two compounds by HepG2 liver carcinoma cells, with high-performance liquid chromatography (HPLC) analysis, was also undertaken. In the presence of HepG2 cells a similar degree of desulphamoylation of STX64 (68%) or de-N, N-dimethylsulphamoylation of STX289 (66%) occurred over a 3h period. In the absence of cells, however, STX289 was resistant to de-N, N-dimethylsulphamoylation whereas STX64 was completely desulphamoylated, demonstrating the more favourable pharmaceutical profile of STX289 for development for topical application. It is concluded that both STX64 and STX289 are not only effective inhibitors of skin STS, but also liver STS when applied topically. These findings suggest that it may be possible to develop a formulation for the percutaneous administration of STS inhibitors, but also that this class of compound may have therapeutic potential for the treatment of a number of skin disorders.

Inhibition of steroid sulphatase activity in endometriotic implants by 667 COUMATE: a potential new therapy

BACKGROUND

Local biosynthesis of estrogens is thought to be important for the maintenance and growth of endometriotic implants. In addition to the formation of estrogen via the aromatase pathway, steroid sulphatase (STS), which is responsible for the hydrolysis of estrogen sulphates, may be an important source of estrogens in endometriosis.

METHODS

Eutopic and ectopic endometrial samples from 14 women with minimal or mild (MM) endometriosis and from 13 women with moderate to severe (MS) endometriosis were analysed for aromatase and STS activities.

RESULTS

Aromatase and STS activity were detected in all samples. STS enzyme activity in both eutopic and ectopic endometrium was considerably higher and less variable than aromatase activity. Moreover, STS, but not aromatase, activity in endometriotic implants correlated with the severity of the disease (mean +/- SEM: 203 +/- 38 nmol/4 h/g wet weight tissue in MM disease versus 423 +/- 44 nmol/4 h/g wet weight tissue in MS endometriosis, P < 0.001). The STS inhibitor 667 COUMATE almost completely blocked STS activity (>99%) in both eutopic and ectopic tissues.

CONCLUSIONS

The high levels of STS activity detected in ectopic endometrium and the correlation with severity of disease suggest that STS inhibitors could be useful for the treatment of endometriosis.

The therapeutic potential of a series of orally bioavailable anti-angiogenic microtubule disruptors as therapy for hormone-independent prostate and breast cancers

Therapies for hormone-independent prostate and breast cancer are limited, with the effectiveness of the taxanes compromised by toxicity, lack of oral bioavailability and drug resistance. This study aims to identify and characterise new microtubule disruptors, which may have improved efficacy relative to the taxanes in hormone-independent cancer. 2-Methoxy-3-O-sulphamoyl-17β-cyanomethyl-oestra-1,3,5(10)-triene (STX641), 2-methoxy-3-hydroxy-17β-cyanomethyl-oestra-1,3,5(10)-triene (STX640) and 2-methoxyoestradiol-3,17-O,O-bis-sulphamate (STX140) were all potent inhibitors of cell proliferation in a panel of prostate and breast cancer cell lines. STX641 and STX640 significantly inhibited tumour growth in the MDA-MB-231 xenograft model. STX641 inhibited both in vitro and in vivo angiogenesis. Despite good in vivo activity, STX641 was not as potent in vivo as STX140. Therefore, STX140 was evaluated in the prostate hormone-independent PC-3 xenograft model. STX140 had superior efficacy to docetaxel, 2-MeOE2 and bevacizumab. In contrast to vinorelbine, no significant toxicity was observed. Furthermore, STX140 could be dosed daily over a 60-day period leading to tumour regression and complete responses, which were maintained after the cessation of dosing. This study demonstrates that STX641 and STX140 have considerable potential for the treatment of hormone-independent breast and prostate cancer. In contrast to the taxanes, STX140 can be dosed orally, with no toxicity being observed even after prolonged daily dosing.

In vivo inhibition of angiogenesis by sulphamoylated derivatives of 2-methoxyoestradiol

Drugs that inhibit growth of tumours and their blood supply could have considerable therapeutic potential. 2-Methoxyoestradiol-3,17-O,O-bis-sulphamate (2-MeOE2bisMATE) has been shown to inhibit the proliferation of MCF-7 (ER+) breast cancer cells and angiogenesis in vitro. 2-MeOE2bisMATE and its analogue, 17-Cym-2-MeOE2MATE, were investigated for their ability to inhibit in vivo angiogenesis and tumour growth. The mouse Matrigel plug assay for angiogenesis was used to investigate the effect of compounds on neovascularisation and was quantified using a FITC-dextran injection technique. Nude mice bearing tumours derived from MCF-7 cells were used to assess efficacy on tumour growth. Tumour sections were stained for VEGFR-2 and Ki67 to assess tumour angiogenesis and cell proliferation respectively. Matrigel plugs supplemented with basic fibroblast growth factor resulted in increased neovascularisation over 7 days. Oral administration of 2-MeOE2bisMATE for 7 days at 10 or 50 mg kg(-1) significantly reduced neovascularisation to or below control levels respectively. 17-Cym-2-MeOE2MATE at 20 mg kg(-1) was equally effective. 2-MeOE2bisMATE, dosed daily for 21 days, caused a 52% reduction in tumour growth at 5 mg kg(-1) and 38% regression at 20 mg kg(-1). 17-Cym-2-MeOE2MATE (20 mg kg(-1)) reduced tumour growth by 92%. Immunohistochemistry revealed a reduction in angiogenesis and proliferation. Matrigel plug and tumour imaging after FITC-dextran injection indicated that 2-MeOE2bisMATE caused a marked disruption of vasculature. These sulphamoylated oestrogen derivatives have been shown to be potent inhibitors of angiogenesis in vivo. This, together with their ability to inhibit tumour growth, indicates the potential of this new class of drugs for further development for cancer therapy.

Cellular effects and metabolic stability of N1-cyclic inosine diphosphoribose and its derivatives

BACKGROUND AND PURPOSE

Recently, a number of mimics of the second messenger cyclic ADP-ribose (cADPR) with replacement of adenosine by inosine were introduced. In addition, various alterations in the molecule ranging from substitutions at C8 of the base up to full replacement of the ribose moieties still retained biological activity. However, nothing is known about the metabolic stability and cellular effects of these novel analogues.

EXPERIMENTAL APPROACH

cADPR and the inosine-based analogues were incubated with CD38, ADP-ribosyl cyclase and NAD-glycohydrolase and metabolism was analysed by RP-HPLC. Furthermore, the effect of the analogues on cytokine expression and proliferation was investigated in primary T-lymphocytes and T-lymphoma cells.

KEY RESULTS

Incubation of cADPR with CD38 resulted in degradation to adenosine diphosphoribose. ADP-ribosyl cyclase weakly catabolised cADPR whereas NAD-glycohydrolase showed no such activity. In contrast, N1-cyclic inosine 5'-diphosphoribose (N1-cIDPR) was not hydrolyzed by CD38. Three additional N1-cIDPR analogues showed a similar stability. Proliferation of Jurkat T-lymphoma cells was inhibited by N1-cIDPR, N1-[(phosphoryl-O-ethoxy)-methyl]-N9-[(phosphoryl-O-ethoxy)-methyl]-hypoxanthine-cyclic pyrophosphate (N1-cIDP-DE) and N1-ethoxymethyl-cIDPR (N1-cIDPRE). In contrast, in primary T cells neither proliferation nor cytokine expression was affected by these compounds.

CONCLUSIONS AND IMPLICATIONS

The metabolic stability of N1-cIDPR and its analogues provides an advantage for the development of novel pharmaceutical compounds interfering with cADPR mediated Ca2+ signalling pathways. The differential effects of N1-cIDPR and N1-cIDPRE on proliferation and cytokine expression in primary T cells versus T-lymphoma cells may constitute a starting point for novel anti-tumor drugs.

The regulation and inhibition of 17beta-hydroxysteroid dehydrogenase in breast cancer

17Beta-hydroxysteroid dehydrogenase Type 1 (17beta-HSD1) has a pivotal role in regulating the synthesis of oestradiol (E2) within breast tumours. In whole body studies in postmenopausal women with breast cancer the conversion of oestrone (E1) to E2 (4.4+/-1.1%) was much lower than the inactivation of E2 to E1 (17.3+/-5.0%). In contrast, an examination of in vivo oestrogen metabolism within breast tumours revealed that whereas little metabolism of E2 occurred, E1 was converted to E2 to a much greater extent in malignant (48+/-14%) than in normal (19+/-6%) breast tissue. Findings from these studies originally suggested that oestrogen metabolism within breast tumours may differ from the mainly oxidative direction found in most other body tissues and that the activity of 17beta-HSD1 might be regulated by tumour-derived factors. Several growth factors (e.g. IGF-I, IGF-II) and cytokines (e.g. IL-6, TNFalpha) have now been identified which can markedly stimulate the activity of 17beta-HSD1 and such a mechanism may account for the high concentrations of E2 found in most breast tumours. Cells of the immune system, which can infiltrate breast tumours, are thought to be a major source of the growth factors and cytokines which can modulate 17beta-HSD1 activity. Given the central role that 17beta-HSD1 has in regulating breast tumour E2 concentrations the development of potent inhibitors of this enzyme has recently attracted considerable attention. Our initial studies in this area explored the use of derivatives of E1 as inhibitors, with 2-ethyl- and 2-methoxy E1 being found to inhibit 17beta-HSD1 activity in T-47D breast cancer cells by 96+/-2 and 91+/-1% respectively at 10 microM, but with a lack of specificity. Using the E1 scaffold a number of potent, selective 17beta-HSD1 inhibitors have now been identified including E1- and 2-ethyl-E1 containing a side chain with a m-pyridylmethylamidomethyl functionality extending from the 16beta position of the steroid nucleus. At 10 microM these compounds both inhibited 17beta-HSD1 activity by >90%, however some inhibition of 17beta-HSD2 activity was exhibited by the E1 derivative (25%) but not the 2-ethyl analogue. It is now apparent that 17beta-HSD1 activity contributes to the high E2 concentrations found in most breast tumours. The identification of potent, selective novel 17beta-HSD1 inhibitors will allow their efficacy to be tested in in vitro and in vivo studies.

2-MeOE2bisMATE induces caspase-dependent apoptosis in CAL51 breast cancer cells and overcomes resistance to TRAIL via cooperative activation of caspases

2-Methoxyoestradiol (2-MeOE2) is an endogenous oestrogen metabolite which inhibits tubulin polymerisation and has anti-tumour and anti-angiogenic activity. 2-MeOE2 induces apoptosis in a wide range of cancer cell types and has recently been demonstrated to cooperate with TRAIL to induce apoptosis in breast cancer cells. 2-Methoxyoestradiol-3,17-bis-O,O-sulphamate (2-MeOE2bisMATE) is a sulfamoylated derivative of 2-MeOE2 with enhanced activity and improved pharmacokinetic properties, and 2-MeOE2bisMATE is a promising candidate for early clinical trials. It is important, therefore, to understand the mechanisms by which 2-MeOE2bisMATE acts, and whether it retains the ability to cooperate with TRAIL. We demonstrate that 2-MeOE2bisMATE-induced apoptosis of CAL51 breast cancer cells was associated with rapid activation of caspase 3 and 9, but not caspase 8 (as measured by BID cleavage) and was completely prevented by the caspase inhibitor zVADfmk. Interfering with Fas- or TRAIL-receptor function did not prevent 2-MeOE2bisMATE-induced apoptosis. Whereas CAL51 cells were resistant to TRAIL-induced apoptosis, 2-MeOE2bisMATE and TRAIL cooperated to induce cell death. This apoptosis was associated with enhanced activation of caspases, but not increased expression of the DR5 TRAIL receptor, previously demonstrated to be induced by 2-MeOE2. Therefore, 2-MeOE2bisMATE-induced apoptosis is dependent on caspases and like 2-MeOE2, 2-MeOE2bisMATE can overcome resistance to TRAIL by stimulating activation of downstream caspases. Our results suggest that 2-MeOE2bisMATE and TRAIL might be a particularly effective combination of anti-cancer agents.

Steroid sulfatase: molecular biology, regulation, and inhibition

Steroid sulfatase (STS) is responsible for the hydrolysis of aryl and alkyl steroid sulfates and therefore has a pivotal role in regulating the formation of biologically active steroids. The enzyme is widely distributed throughout the body, and its action is implicated in physiological processes and pathological conditions. The crystal structure of the enzyme has been resolved, but relatively little is known about what regulates its expression or activity. Research into the control and inhibition of this enzyme has been stimulated by its important role in supporting the growth of hormone-dependent tumors of the breast and prostate. STS is responsible for the hydrolysis of estrone sulfate and dehydroepiandrosterone sulfate to estrone and dehydroepiandrosterone, respectively, both of which can be converted to steroids with estrogenic properties (i.e., estradiol and androstenediol) that can stimulate tumor growth. STS expression is increased in breast tumors and has prognostic significance. The role of STS in supporting tumor growth prompted the development of potent STS inhibitors. Several steroidal and nonsteroidal STS inhibitors are now available, with the irreversible type of inhibitor having a phenol sulfamate ester as its active pharmacophore. One such inhibitor, 667 COUMATE, has now entered a phase I trial in postmenopausal women with breast cancer. The skin is also an important site of STS activity, and deficiency of this enzyme is associated with X-linked ichthyosis. STS may also be involved in regulating part of the immune response and some aspects of cognitive function. The development of potent STS inhibitors will allow investigation of the role of this enzyme in physiological and pathological processes.

Regulation of aromatase activity by cytokines, PGE2 and 2-methoxyoestrone-3-O-sulphamate in fibroblasts derived from normal and malignant breast tissues

Synthesis of oestrone from androstenedione within tumours, by the aromatase enzyme complex, is an important source of oestrogen that is available to support the growth of hormone-dependent breast tumours. In view of the central role that the aromatase enzyme has in oestrogen synthesis there has been considerable interest in understanding its regulation and developing inhibitors to block its action. In the present study we have derived fibroblasts from breast tumours (TFs), tissue proximal to tumours (PFs) and reduction mammoplasty tissue (RMFs) and used them to investigate the regulation of aromatase activity by PGE(2), IL-6 plus its soluble receptor (SR) or TNFalpha. In addition we have examined the ability of 2-methoxyoestrone sulphamate (2-MeOEMATE), a compound which alters microtubule stability, to block the stimulation of aromatase activity by these factors. Basal aromatase activity in PFs was significantly higher (p<0.001) than in TFs or RMFs. The combination of IL-6 plus SR or TNFalpha produced the greatest stimulation of aromatase activity in TFs (up to 61-fold) while having a much lower stimulatory effects on aromatase activity in PFs (up to 60% increase) or RMFs (up to 192% increase). 2-MeOEMATE reduced basal aromatase activity in TFs by 87% and completely abrogated the ability of PGE(2), IL-6 plus SR or TNFalpha to stimulate aromatase activity in these fibroblasts. Results from these studies indicate that while PFs have the highest level of non-stimulated aromatase activity, aromatase activity in TFs shows the greatest response to cytokines. These findings suggest that intrinsic difference may exist for the different types of fibroblasts in the way in which they respond to regulatory factors. The ability of 2-MeOEMATE to block cytokine stimulated aromatase activity suggests that, in addition to its other anti-cancer properties, this compound may also act to inhibit cytokine-stimulated aromatase activity in breast tumours.

The effects of 2-methoxyoestrogen sulphamates on the in vitro and in vivo proliferation of breast cancer cells

2-Methoxyoestrogen sulphamates are a new class of compounds, which inhibit breast cancer cell proliferation and are also potent inhibitors of steroid sulphatase (STS) activity. In the present study, we have used two cell proliferation assays (MTS and AB) to identify potent new compounds in this class. Similar IC(50) values were obtained using these assays with two of the most potent compounds identified being 2-methoxyoestradiol-bis-sulphamate (2-MeOE2bisMATE) and 2-methoxyoestradiol-17beta-cyanomethyl-3-O-sulphamate (2-MeOE2CyMATE). Both compounds inhibited the proliferation of MCF-7 (ER+) and MDA-MB-231 (ER-) breast cancer cells. Using the AB assay, which allows repeat measurements of cell proliferation without killing cells, both compounds were shown to inhibit cell proliferation in an irreversible manner. As STS may be involved in the removal of the sulphamoyl moiety of these compounds, which could reduce their potency, their ability to inhibit the proliferation of MCF-7 cells transfected with the cDNA for STS was also examined. Although the STS activity was 20-fold higher in these cells than in non-transfected MCF-7 cells, no decrease in the ability of these compounds to inhibit cell proliferation was detected. To test the efficacy of these compounds in vivo, nude mice were inoculated with MCF-7 cells in Matrigel and stimulated to grow with oestradiol. Three weeks after the oral administration of 2-MeOE2bisMATE or 2-MeOE2CyMATE (20mg/kg/day, 5 days/week) tumour volumes had regressed by 52% and 22%, respectively. Both compounds also inhibited liver and tumour STS activity by >90%. The potent anti-proliferative effects of these compounds, and their ability to inhibit tumour growth and STS activity in vivo, indicates that they are suitable for development as novel therapeutic agents, which should be active against a wide range of cancers.

The nature of inhibition of steroid sulphatase activity by tibolone and its metabolites

Tibolone is used for hormone replacement therapy and acts in a tissue-specific manner being oestrogenic on CNS and bone but not on breast tissues or endometrium. The ability of tibolone and its metabolites to inhibit steroid sulphatase (STS) activity has a crucial role in regulating its tissue-specific effects. In this study, we have examined the ability of tibolone and its non-sulphated and sulphated metabolites to inhibit STS activity in different enzyme preparations and in intact cells. For this, we have used an 'extracellular' method, which measures the amount of product released into culture medium, and an 'intracellular' method, which assesses the extent of product formation within cells. In addition, the nature by which tibolone and some of its metabolites inhibit STS activity was investigated using intact cells and an enzyme kinetic method. In MCF-7 and T47D breast cancer cells and JEG-3 choriocarcinoma cells, which have high STS activity, tibolone and its metabolites were relatively potent inhibitors of STS activity (33-57% inhibition at 10 microM) using the extracellular assay method. In HOS-TE-85 osteoblast-like cells, tibolone and its Delta-4 metabolite were relatively inactive whereas the 3alpha/3beta-hydroxy metabolites and their sulphated conjugates inhibited activity by 39-55%. When STS activity was assessed in HOS-TE-85 cells using an 'intracellular' method tibolone and its 3beta-hydroxy metabolite were inactive. Pre-treatment of breast cancer cells and JEG-3 cells, and removal of drugs prior to assaying for STS activity, revealed that in these cells tibolone and its metabolites were acting mainly as reversible inhibitors. This finding was confirmed in an enzyme kinetic study to measure concentration-dependent STS inhibition. In HOS-TE-85 cells, pre-treatment of cells and removal of compounds before assaying for remaining STS activity indicated that some tibolone metabolites appeared to stimulate STS activity. Possible mechanisms by which this might occur are discussed but, if confirmed, this could contribute to the positive oestrogenic effects that tibolone has on bone.

Pharmacokinetics of the nonsteroidal steroid sulphatase inhibitor 667 COUMATE and its sequestration into red blood cells in rats

Breast cancer is a major cause of mortality in Western countries and there is an urgent requirement for novel treatment strategies. The nonsteroidal sulphatase inhibitor 667 COUMATE inhibits hepatic steroid sulphatase and growth of oestrone sulphate stimulated tumours in the nitrosomethylurea-induced rat mammary model. Other compounds that contain an aryl sulphamate moiety, for example, oestrone-3-O-sulphamate, are sequestered into red blood cells (RBCs). The aims of this study were to determine the pharmacokinetics of 667 COUMATE and to investigate its sequestration into RBCs. We administered a single p.o. or i.v. dose (10 mg kg⁻¹) of 667 COUMATE to rats and used a high-performance liquid chromatography method to measure the levels of the agent and its putative metabolites in plasma. 667 COUMATE had a bioavailability of 95% and could be detected in plasma for up to 8 h. Using two independent analytical methods, we demonstrated that 667 COUMATE is sequestered by RBCs both ex vivo and in vivo. Previous investigations have revealed that 667 COUMATE is rapidly degraded in plasma ex vivo. In this study, we demonstrate that 667 COUMATE is stabilised due to its sequestration into RBCs. In conclusion, the pharmacological efficacy and high oral bioavailability of 667 COUMATE may be partly a consequence of the ability of RBCs to both protect the agent from metabolic degradation and facilitate its transport to tissues. These data support the further clinical evaluation of this novel endocrine therapeutic agent.

Pharmacokinetics and efficacy of 2-methoxyoestradiol and 2-methoxyoestradiol-bis-sulphamate in vivo in rodents

2-Methoxyoestradiol (2-MeOE2) is an endogenous oestrogen metabolite that inhibits the proliferation of cancer cells in vitro, and it is also antiangiogenic. In vivo 2-MeOE2, when administered at relatively high doses, inhibits the growth of tumours derived from breast cancer cells, sarcomas and melanomas. Sulphamoylated derivatives of 2-MeOE2 are more potent inhibitors of in vitro breast cancer cell growth than 2-MeOE2. In the present study, we have compared the pharmacokinetic profiles and metabolism of 2-MeOE2 and its sulphamoylated derivative, 2-methoxyoestradiol-bis-sulphamate (2-MeOE2bisMATE), in adult female rats. Their ability to inhibit tumour growth was compared in nude mice bearing xenografts derived from MDA-MB-435 (oestrogen receptor negative) melanoma cancer cells. After a single oral 10 mg kg⁻¹ dose of 2-MeOE2bisMATE, significant concentrations of this compound were still detectable at 24 h. In contrast, no 2-MeOE2 or metabolites were detected in plasma at any time after a 10 mg kg⁻¹ oral dose. Thus, the bioavailability of 2-MeOE2 is very low, whereas for 2-MeOE2bisMATE it was 85%. No significant metabolites of 2-MeOE2bisMATE were detected in plasma after oral or intravenous dosing, showing that this drug is resistant to metabolism. In the tumour efficacy model, oral administration of 2-MeOE2bisMATE, at 20 mg kg⁻¹ day⁻¹ daily for 28 days, almost completely inhibited tumour growth. Inhibition of tumour growth was maintained for a further 28 days after the cessation of dosing. At this dose level, 2-MeOE2 did not inhibit tumour growth. The resistance to metabolism shown by 2-MeOE2bisMATE and its ability to inhibit tumour growth in vivo suggest that this compound should have considerable potential for development as a novel anticancer drug.

Steroid sulphatase inhibitors for breast cancer therapy

In contrast to aromatase inhibitors, which are now in clinical use, the development of steroid sulphatase (STS) inhibitors for breast cancer therapy is still at an early stage. STS regulates the formation of oestrone from oestrone sulphate (E1S) but also controls the hydrolysis of dehydroepiandrosterone sulphate (DHEA-S). DHEA can be reduced to 5-androstenediol (Adiol), a steroid with potent oestrogenic properties. The active pharmacophore for potent STS inhibitors has now been identified, i.e. a sulphamate ester group linked to an aryl ring. This has led to the development of a number of STS inhibitors, some of which are due to enter Phase I trials in the near future. Such first generation inhibitors include the tricyclic coumarin-based 667 COUMATE. Aryl sulphamates, such as 667 COUMATE, are taken up by red blood cells (rbc), binding to carbonic anhydrase II (CA II), and transit the liver without undergoing first-pass inactivation. 667 COUMATE is also a potent inhibitor of CA II activity with an IC50 of 17 nM. Second generation STS inhibitors, such as 2-methoxyoestradiol bis-sulphamate (2-MeOE2bisMATE), in addition to inhibiting STS activity, also inhibit the growth of oestrogen receptor negative (ER-) tumours in mice and are anti-angiogenic. As the active pharmacaphores for the inhibition of aromatase and STS are now known it may be possible to develop third generation inhibitors that are capable of inhibiting the activities of both enzymes. Whilst exploring the potential of such a strategy it was discovered that 667 COUMATE possessed weak aromatase inhibitory properties with an IC50 of 300 nM in JEG-3 cells. The identification of potent STS inhibitors will allow the therapeutic potential of this new class of drug to be explored in post-menopausal women with hormone-dependent breast cancer. Second generation inhibitors, such as 2-MeOE2bisMATE, which also inhibit the growth of ER- tumours should be active against a wide range of cancers.

The effects of 2-methoxy oestrogens and their sulphamoylated derivatives in conjunction with TNF-alpha on endothelial and fibroblast cell growth, morphology and apoptosis

2-methoxyoestradiol (2-MeOE2) is a potent anti-angiogenic agent. Its 3- and 17-sulphamoylated derivatives have been demonstrated to induce G2-M cell cycle arrest and apoptosis in breast cancer cells in vitro as well as tumour regression in rats in vivo with greater potency than the parent oestrogen. To determine whether the anti-cancer properties of these derivatives can be synergistically enhanced with low-dose TNF-alpha co-treatment, we investigated the effects of these treatments in adult human fibroblasts and human umbilical vein endothelial cells (HUVECs). Treatment of fibroblasts with 0.1 microM 2-methoxyoestradiol-3,17-bis sulphamate (2-MeOE2bisMATE) but not 2-MeOE2 caused a reversible morphology change and induced G2-M arrest (from 12 to 33%) but not subsequent apoptosis. In contrast, treatment of HUVECs did not induce morphology change or G2-M arrest. Using a nucleosomal ELISA assay, we showed that TNF-alpha (20 ng/ml) combination treatment synergistically increases 0.1 microM 2-MeOE2bisMATE-induced but not 0.1 microM 2-MeOE2-induced apoptosis in HUVECs. These results suggest that TNF-alpha co-treatment may be a beneficial method of increasing the potency of 2-substituted oestrogens as anti-angiogenic agents through synergistic induction of apoptosis in endothelial cells while maintaining low cytotoxicity to fibroblasts.

Inhibition of carbonic anhydrase II by steroidal and non-steroidal sulphamates

Carbonic anhydrases (CAs) are expressed by many solid tumours where they may act to confer a growth advantage on malignant tissues. In this study we have examined the ability of a series of steroidal and non-steroidal sulphamates (originally developed as steroid sulphatase inhibitors) and related compounds to inhibit human CAII (hCAII) activity in vitro. Using a 96-well plate assay, oestrone-3-O-sulphamate (EMATE) and two coumarin-based sulphamate drugs (667 COUMATE and STX 118) were found to have IC(50) values of 25-59 nM for the inhibition of hCAII activity. These compounds therefore have a similar CAII inhibitory potency to that of acetazolamide (IC(50)=25 nM), a known hCAII inhibitor. Docking studies have been performed with selected compounds to the crystal structure of hCAII and excellent correlation of scores with biological activity was observed. This agrees with our recent observations when we were the first to report the inhibition of hCAII by STS inhibitors. These studies and initial results with docking to the crystal structure of the extracellular domain of hCAXII indicate that the STS sulphamate ester inhibitors should also be interesting candidates to pursue as inhibitors of CA isozymes that are over-expressed in human tumours.

Growth inhibition of multi-drug-resistant breast cancer cells by 2-methoxyoestradiol-bis-sulphamate and 2-ethyloestradiol-bis-sulphamate

There is currently considerable interest in the use of the endogenous oestrogen metabolite, 2-methoxyoestradiol (2-MeOE2) for the treatment and prevention of breast cancer. We have previously shown that sulphamoylation of 2-MeOE2 and related derivatives greatly enhances their ability to inhibit the proliferation of ER+ and ER- breast cancer cells. In this study, we have compared the abilities of 2-methoxyoestradiol-bis-sulphamate (2-MeOE2bisMATE) and 2-ethyloestradiol-bis-sulphamate (2-EtE2bisMATE) with that of 2-MeOE2 to inhibit the proliferation of breast cancer cells when grown on three different substrata: plastic, collagen I and Matrigel. The human breast cell line MCF-7 was utilised for these studies together with its doxorubicin resistant variant, MCF-7 DOX40 and mitoxantrone resistant variant, MCF-7 MR, as a longitudinal model of in vitro drug resistance. On a plastic substratum all three cell lines were sensitive to the effects of 2-MeOE2bisMATE and 2-EtE2bisMATE whereas MCF-7 cells and the MCF-MR variant cells were resistant to the effects of 2-MeOE2 at 1 microM. The sensitivity of the cell lines to those compounds also remained significant when grown on more physiological substrata. All of the drugs tested arrested cells in the G2/M phase of the cell cycle. The finding that breast cancer cells that are resistant to conventional chemotherapeutic agents remain sensitive to 2-substituted oestrogen sulphamates offers considerable potential for the treatment of women with drug-resistant breast cancer.