Growth suppression of human breast carcinoma cells in culture by N-(4-hydroxyphenyl)retinamide and its glucuronide and through synergism with glucarate

Improved Synthesis of the C-Glucuronide/Glycoside of 4-Hydroxybenzylretinone (4-HBR)

Improvements in the synthesis of carbon-linked glucuronide/glucoside conjugates of cancer chemopreventive retinoids have been achieved starting with 2,3,4,6-tetra-O-benzyl-D-glucopyranose. The revised approach demonstrates better yields, eliminates the use of an expensive, carcinogenic protecting group reagent, and avoids much painstaking chromatography. The new approach should allow synthesis of larger quantities of the agents for detailed animal and mechanistic studies.

Terpenoids and breast cancer chemoprevention

Cancer chemoprevention is defined as the use of natural or synthetic agents that reverse, suppress or arrest carcinogenic and/or malignant phenotype progression towards invasive cancer. Phytochemicals obtained from vegetables, fruits, spices, herbs and medicinal plants, such as terpenoids, carotenoids, flavanoids, phenolic compounds, and other groups of compounds have shown promise in suppressing experimental carcinogenesis in various organs. Recent studies have indicated that mechanisms underlying chemopreventive action may include combinations of anti-oxidant, anti-inflammatory, immune-enhancing, and anti-hormone effects. Further, modification of drug-metabolizing enzymes, and influences on cell cycling and differentiation, induction of apoptosis, and suppression of proliferation and angiogenesis that play a role in the initiation and secondary modification of neoplastic development, have also been under investigation as possible mechanisms. This review will highlight the biological effects of terpenoids as chemopreventive agents on breast epithelial carcinogenesis, and the utility of intermediate biomarkers as indicators of premalignancy. Selected breast chemoprevention trials are discussed with a focus on strategies for trial design, and clinical outcomes. Future directions in the field of chemoprevention are proposed based on recently acquired mechanistic insights into breast carcinogenesis.

N-(4-Hydroxyphenyl)retinamide induced differentiation with repression of telomerase and cell cycle to increase interferon-gamma sensitivity for apoptosis in human glioblastoma cells

Glioblastoma is the most malignant and prevalent brain tumor in humans. It is composed of heterogenic abnormal astroglial cells that avoid differentiation, maintain proliferation, and hardly commit apoptosis. N-(4-Hydroxyphenyl)retinamide (4-HPR) induced astrocytic differentiation and increased sensitivity to interferon-gamma (IFN-gamma) for apoptosis in human glioblastoma A172, LN18, and SNB19 cells. Combination of 4-HPR and IFN-gamma significantly inhibited human telomerase reverse transcriptase (hTERT), cyclin dependent kinase 2 (CDK2), and survivin to up-regulate caspase-8, caspase-9, and caspase-3 for increasing apoptosis in all glioblastoma cell lines. Hence, combination of 4-HPR and IFN-gamma should be considered for controlling growth of different human glioblastoma cells.

The unhydrolyzable fenretinide analogue 4-hydroxybenzylretinone induces the proapoptotic genes GADD153 (CHOP) and Bcl-2-binding component 3 (PUMA) and apoptosis that is caspase- dependent and independent of the retinoic acid receptor

The synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) induces apoptosis in a variety of cell lines and has shown promise as an anticancer agent both in vitro and in vivo. The clinical dose of 4-HPR, however, is limited by residual-associated toxicities, indicating a need for a less toxic drug. In this study, we show that 4-hydroxybenzylretinone (4-HBR), the unhydrolyzable analogue of 4-HPR, is effective in producing apoptosis in a variety of 4-HPR-sensitive cell lines, including breast cancer, neuroblastoma, and leukemia cells. We also show through the use of a pan-caspase inhibitor that this 4-HBR-induced apoptosis is dependent, at least in part, on caspase activity. 4-HBR is shown to exhibit binding to the retinoic acid receptors (RAR) at concentrations necessary to induce cell death and induces expression of all-trans-retinoic acid-responsive genes that can be blocked by a RAR pan-antagonist. However, through the use of this RAR pan-antagonist, 4-HBR-induced apoptosis and cell death is shown to be independent of the RAR signaling pathway. To further characterize the mechanism of action of 4-HBR, expression of the endoplasmic reticulum stress-induced genes GADD153 and Bcl-2-binding component 3 was examined. These mRNAs are shown to be rapidly induced in 4-HBR-treated and 4-HPR-treated breast cancer cells, and this up-regulation is also shown to be independent of the RARs. These results suggest that a stress-mediated apoptotic cascade is involved in the mechanism of action of these retinoids.

Antiangiogenic strategies in neuroblastoma

Promising new antiangiogenic strategies are emerging for the treatment of cancer and the inhibition of angiogenesis could represent a powerful adjunct to traditional therapy of malignant tumors. Over the last ten years several reports have been published concerning the relationship between tumor progression and angiogenesis in neuroblastoma in experimental models in vitro and in vivo. Moreover, a high vascular index in neuroblastoma correlates with poor prognosis, suggesting dependence of aggressive tumor growth on active angiogenesis. Here, we present an overview of recent advances in antiangiogenesis in neuroblastoma and describe the most important active substances, preclinical and clinical data, as well as future perspectives.

Protection of Ewing's sarcoma family tumor (ESFT) cell line SK-N-MC from betulinic acid induced apoptosis by alpha-DL-tocopherol

Betulinic acid (BA) is known to induce apoptosis in melanoma neuroectodermal and malignant brain cancer cell lines. Present report describes the role of antioxidants on the BA-induced toxicity to human cell line SK-N-MC. Hydrophilic antioxidants viz., L-ascorbic acid (VitC) and N-acetyl-L-cysteine (l-NAC) had no protective effect on BA-induced apoptosis at the maximal concentrations tested. The lipophilic antioxidant, alpha-DL-tocopherol (VitE) showed a concentration and a time dependent effect on the protection of SK-N-MC cells from BA-induced apoptosis. The apoptotic parameters were analyzed using FACS analysis of propidium iodide (PI) stained nuclei, PS externalization using Annexin-V assay and changes in mitochondrial membrane potential. Generation of superoxide radical was monitored by the fluorescent dye hydroethidium (HE). Cells showed Annexin-V positivity and an increase in the propidium iodide (PI) uptake in the early hours of treatment with BA, which was concomitant with the loss of mitochondrial membrane potential. Addition of alpha-DL-tocopherol to the cell cultures 1-h prior to the treatment with BA abolished all the effects of BA-induced apoptosis. These observations suggest that BA initiates events at membrane level leading to induction of apoptosis. The observed ineffectiveness of hydrophilic antioxidants and substantial protection by lipophilic antioxidants indicate involvement of membrane-associated damages that form the basis of BA-induced cytotoxicity.

Hydrolysis of 4-HPR to atRA occurs in vivo but is not required for retinamide-induced apoptosis

The retinamide, N-(4-hydroxyphenyl)retinamide (4-HPR), has shown promising anti-tumor activity, but it is unclear whether this compound is hydrolyzed to all-trans retinoic acid (atRA) and if so, whether this plays any role in its chemotherapeutic activity. To address this issue, the ability of 4-hydroxybenzylretinone (4-HBR), a carbon-linked analog of 4-HPR, to support growth in vitamin A-deficient (VAD) animals and to activate an atRA-responsive gene in vivo was compared to 4-HPR and atRA. Further, the non-hydrolyzable 4-HBR analog was used to determine whether the presence of the labile amide linkage in 4-HPR is essential for the induction of apoptosis in cultured MCF-7 breast cancer cells. Studies in VAD rats showed that 4-HPR, like atRA, supports animal growth and induces CYP26B1 mRNA expression in lung whereas 4-HBR does not. Analysis of plasma from 4-HPR- and atRA-treated VAD animals revealed the presence of atRA whereas it was not detected in plasma from animals given 4-HBR. To determine whether hydrolysis to atRA is necessary for apoptosis induced by 4-HPR in MCF-7 breast cancer cells, morphological and biochemical assays for apoptosis were performed. 4-HBR, like 4-HPR, induced apoptosis in MCF-7 cells. Apoptosis was not induced even at high concentrations of atRA, showing that 4-HPR and 4-HBR act in cells via a distinct signaling pathway. These results show that although limited hydrolysis of 4-HPR occurs in vivo, the ability to liberate atRA is not required for these 4-hydroxyphenyl retinoids to induce apoptosis in MCF-7 breast cancer cells. Thus the non-hydrolyzable analog, 4-HBR, may have significant therapeutic advantage over 4-HPR because it does not liberate atRA that can contribute to the adverse side effects of drug administration in vivo.

Inhibition of neuroblastoma-induced angiogenesis by fenretinide

Retinoids are a class of natural or synthetic compounds that participate in the control of cell proliferation, differentiation and fetal development. The synthetic retinoid fenretinide (HPR) inhibits carcinogenesis in various animal models. Retinoids have also been suggested to be effective inhibitors of angiogenesis. The effects of HPR on certain endothelial cell functions were investigated in vitro, and its effects on angiogenesis was studied in vivo, by using the chorioallantoic membrane (CAM) assay. HPR inhibited vascular endothelial growth factor- (VEGF-) and fibroblast growth factor-2- (FGF-2)-induced endothelial cell proliferation without affecting endothelial motility; moreover, HPR inhibited growth factor-induced angiogenesis in the CAM assay. Furthermore, a significant antiangiogenic potential of HPR has also been observed in neuroblastoma (NB) biopsy-induced angiogenesis in vivo. We previously demonstrated that supernatants derived from NB cell lines stimulated endothelial cell proliferation. In the present study, we found that this effect was abolished when NB cells were incubated in the presence of HPR. VEGF- and FGF-2-specific ELISA assays, performed on both NB cells derived from conditioned medium and cellular extracts, indicated no consistent effect of HPR on the level of these angiogenic cytokines. Moreover, RT-PCR analysis of VEGF and FGF-2 gene expression confirmed the above lack of effect. HPR was also able to significantly repress the spontaneous growth of endothelial cells, requiring at least 48-72 hr of treatment with HPR, followed by a progressive accumulation of cells in G(1) at subsequent time points. Finally, immunohistochemistry experiments performed in the CAM assay demonstrated that endothelial staining of both VEGF receptor 2 and FGF-2 receptor-2 was reduced after implantation of HPR-loaded sponges, as compared to control CAMs. These data suggest that HPR exerts its antiangiogenic activity through both a direct effect on endothelial cell proliferative activity and an inhibitory effect on the responsivity of the endothelial cells to the proliferative stimuli mediated by angiogenic growth factors.

Synthesis and biological activity of glycosyl conjugates of N-(4-hydroxyphenyl)retinamide

Three glycosyl (glucosyl, galactosyl, mannosyl) conjugates of 4-hydroxyphenylretinamide have been synthesized and tested on a broad variety of tumor cells. All three compounds are active on promyelocytic leukemia cell lines HL60 but less than the parent compound 4-HPR. Among them, the mannosyl analog stands out by its very low toxicity.

pRb and Cdk regulation by N-(4-hydroxyphenyl)retinamide

The cancer chemopreventive synthetic retinoid N-(4-hydroxyphenyl)retinamide (HPR) can inhibit the growth and induce apoptosis of tumor cells. In this study we analysed the growth suppressive effect of HPR on human breast cancer cell lines in vitro and the role of the retinoblastoma protein (pRb) in this response. Treatment of MCF7, T47D and SKBR3 for 24 - 48 h with 3 microM HPR, a concentration attainable in vivo, resulted in growth inhibition and marked dephosphorylation of pRb involving Ser612, Thr821, Ser795 and Ser780, target residues for cyclin-dependent kinase 2 (Cdk2) the former two, and Cdk4 the latter two. Interestingly, this dephosphorylation of pRb occurred in S-G2-M phase cells, as revealed by experiments on cells fractionated by FACS according to the cell cycle phase, hence suggesting that the retinoid interferes with the regulation of pRb phosphorylation. The in vitro phosphorylation of a GST-pRb recombinant substrate by Cdk2 immunocomplexes from MCF7, T47D and SKBR3 was markedly suppressed after HPR treatment, whereas that by Cdk4 complexes was suppressed in T47D and SKBR3 but not in MCF7. The steady-state levels of Cdk2, Cdk4 and Cyclin A proteins were unaffected by HPR, while those of Cyclin D1 were significantly reduced in all three cell lines. Interestingly, Cyclin D1 downregulation by HPR correlated with transcriptional repression, but not with enhanced proteolysis of Cyclin D1 typically elicited by other retinoids. Collectively, our data suggest that the antiproliferative activity of HPR arises from its capacity to maintain pRb in a de-phosphorylated growth-suppressive status in S-G2/M, possibly through Cyclin D1 downregulation and inhibition of pRb-targeting Cdks. Oncogene (2000) 19, 4035 - 41.

4HPR triggers apoptosis but not differentiation in retinoid sensitive and resistant human embryonal carcinoma cells through an RARgamma independent pathway

Retinoids signal biological effects through retinoic acid receptors (RAR) and retinoid X receptors (RXR) and their co-regulators. We previously reported that all-trans retinoic acid (RA) triggers terminal differentiation in the human embryonal carcinoma cell line NTERA-2 clone D1 (NT2/D1), through an RARgamma dependent pathway. RARgamma repression in NT2/D1-R1 cells accounts for RA resistance in this line. This report finds RARgamma repression is due to selective repression of RARgamma but not RARbeta transcription in NT2/D1-R1 cells. The repression is neither due to mutations in RARgamma nor its promoter containing the RA response element. Prior work was confirmed and extended by demonstrating that an RARgamma selective agonist preferentially signals differentiation of NT2/D1 cells, while RARalpha/beta, RARbeta, RXR agonists and an RAR pan-antagonist do not even when NT2/D1 cells are treated with these retinoids at 10 microM dosages. None of these examined retinoids induced differentiation of the RA resistant NT2/D1-R1 cells. In contrast, N-(4-hydroxyphenyl)retinamide (4HPR), a reported transcriptional activator of RARgamma was shown to potently induce growth inhibition and apoptosis in both NT2/D1 and NT2/D1-R1 cells. 4HPR-induced apoptosis was unaffected by co-treatment of both cell lines with equimolar RAR antagonist. Semi-quantitative reverse transcription-polymerase chain reaction (RT - PCR) assays of total RNA from 4HPR-treated NT2/D1 and NT2/D1-R1 cells did not reveal RARgamma induction. Since 4HPR signals in RA-resistant NT2/D1-R1 cells having an RARgamma transcriptional block, these results indicate that 4HPR triggers apoptosis but not differentiation through an RARgamma independent pathway. Taken together, these findings implicate a therapeutic role for 4HPR mediated apoptosis in germ cell tumors even when a maturation block is present.

Fenretinide-induced apoptosis of human head and neck squamous carcinoma cell lines

BACKGROUND

Squamous cell carcinoma of the head and neck (HNSCC) has a high incidence of recurrence and associated second primary malignancy. The retinoid 13-cis-retinoic acid has been shown to be effective as both a chemopreventive and chemotherapeutic agent for HNSCC, but often with treatment-limiting toxicity. The synthetic retinoid fenretinide (N-(4-hydroxyphenyl)retinamide) (HPR) has significant antiproliferative activity against a number of animal and human malignancies and has been used in clinical trials as a chemopreventive agent in patients with breast and prostate cancer and oral leukoplakia. HPR has been shown to have a toxicity profile lower than that for other retinoids used in clinical trials.

PURPOSE

The aim of this study was to investigate the effect of HPR on the growth of HNSCC cell lines in vitro.

METHODS

Four HNSCC cell lines (JHU-011-SCC, JHU-020-SCC, JHU-022-SCC, and FaDu) were treated with a range of concentrations of HPR for various times. After HPR exposure, cell viability was determined by tetrazolium dye (MTT) colorimetric assay, comparing cell survival with that of untreated control cells. HPR-induced apoptosis was determined by flow-cytometric deoxyribonucleic acid cell-cycle analysis, ultrastructural analysis with electron microscopy, and deoxyribonucleic acid fragmentation detected by gel electrophoresis.

RESULTS

HPR caused significant growth inhibition in three of the four HNSCC cell lines in a dose- and time-dependent fashion. In two cell lines (JHU-011-SCC, JHU-020-SCC) a significant antiproliferative effect was achieved between 1 and 2.5 micromol/L HPR after 72 hours of treatment. By deoxyribonucleic acid cell-cycle analysis, electron microscopy, and gel electrophoresis, HPR was shown to induce apoptosis in the JHU-011-SCC and JHU-020-SCC cell lines, but not in the FaDu cell line, which was insensitive to the growth inhibitory effect of HPR.

CONCLUSIONS

This study has demonstrated that HPR reduces cell viability in HNSCC cells in vitro at clinically relevant doses, with the growth inhibition occurring through the induction of apoptosis.

4-Hydroxyphenyl retinamide is a highly selective activator of retinoid receptors

Retinoids have shown promise as anti-cancer and cancer preventative agents. All-trans-N-(4-hydroxyphenyl)retinamide (4HPR) belongs to a new group of retinoids that not only inhibit the proliferation of cancer cells but also can induce apoptosis in certain cancer cells. Because of its increased efficacy against cancer cells and its low toxicity it has been entered into a number of clinical trials. However, its mechanism of action is not known, and it had been assumed that it is not a true retinoid. Here we analyze its ability to function as an activator of nuclear retinoid receptors (RARs and RXRs). We observe that, in transactivation assays, 4HPR is a potent transactivator with RARgamma and a moderate activator with RARbeta but is not an activator with RARalpha and RXRalpha. Furthermore, RARgamma-selective transactivation by 4HPR is enhanced on some response elements and reduced on others when compared to natural retinoids. In contrast to transactivation, 4HPR in transrepression assays functions mostly with RARalpha, RARbeta, and RXRalpha. Optimal receptor activation is seen at 4HPR concentrations at which it is a potent growth inhibitor and inducer of apoptosis. We conclude that 4HPR is a highly selective activator of retinoid receptors. We propose that this selective activation of the nuclear receptors is likely to be the basis for its specific biological activities and its favorable pharmaceutical properties.

All-trans-retinoyl-beta-glucuronide is a potent teratogen in the mouse because of extensive metabolism to all-trans-retinoic acid

All-trans-retinoyl-beta-D-glucuronide (all-trans-RAG) is a water-soluble derivative of all-trans-retinoic acid (all-trans-RA) and has been characterized as an endogenous metabolite of vitamin A in rat bile and kidney. All-trans-RAG was previously demonstrated to be a major metabolite after application of all-trans-RA in several species (mouse, rat, rabbit, monkey); all-trans-RAG was described in these experiments to exhibit a very low placental transfer to the embryo. Because retinoid-like activity has been found after application of all-trans-RAG in vivo as well as in several in vitro systems, and because of its low placental transfer, this glycoconjugate appeared to be an interesting retinoid with possible therapeutic activity, but reduced teratogenicity. Here we investigated the teratogenic activity of all-trans-RAG in comparison to all-trans-RA in mice, and performed accompanying pharmacokinetic studies. Surprisingly, all-trans-RAG was more teratogenic than equimolar doses of all-trans-RA following subcutaneous application on day 11 of gestation in the mouse (20 mumol/kg body weight). Pharmacokinetic studies revealed that all-trans-RAG was extensively hydrolyzed to all-trans-RA and that the plasma area under the concentration-time curve (AUC) of all-trans-RA following all-trans-RAG application exceeded the plasma AUC value of all-trans-RA following application of all-trans-RA. Extensive hydrolysis of all-trans-RAG was also observed after intravenous application of this glycoconjugate. Transfer of all-trans-RAG to the embryo was low, but transfer was high to maternal organs such as the liver and kidney. These in vivo studies suggest that all-trans-RAG serves as a precursor of all-trans-RA by the intravenous and subcutaneous routes, and application of all-trans-RAG results in high and teratogenic in vivo exposure to all-trans-RA.

Biological activity of N-(4-hydroxyphenyl) retinamide-O-glucuronide in corneal and conjunctival cells of rabbits and humans

Previous studies of topical retinoic acid for treatment of ocular surface disease met with limited success due to instability and irritancy of the retinoid and lack of efficacy in keratoconjunctivitis sicca. There has, however, been continued interest in the treatment of mucin deficiency and cicatrizing conjunctival diseases, such as ocular cicatricial pemphigoid (OCP), topically with retinoids. In this study the biological activity of stable, water-soluble, synthetic retinoid, N-(4-hydroxyphenyl) retinamide-O-glucuronide (4-HPROG) was investigated in vivo and in vitro using conjunctival and corneal epithelium and fibroblasts. Vitamin A-deficient rabbits with stage 3-4 corneal xerosis and squamous metaplasia confirmed by conjunctival impression cytology were treated with topical 0.1% 4-HPROG in an artificial tear vehicle for 3 weeks. Impression cytology was repeated at 2 and 3 weeks and at 3 weeks conjunctival biopsies were fixed for histology. Growth curves were generated using conjunctival fibroblasts of rabbits and humans (normals and patients with cicatrizing conjunctival disease including OCP and Stevens-Johnson syndrome) cultured in the 10(-8)-10(-6) M 4-HPROG. In vivo, corneal xerosis cleared in three days. A normal conjunctival epithelium was restored by 2 weeks and goblet cells were present by 3 wk, with no change in vehicle-treated controls. No ocular irritation occurred. In vitro, 10(-6) M 4-HPROG inhibits growth of rabbit conjunctival fibroblasts. The retinoid had no effect on proliferation of conjunctival fibroblasts from normal humans but the doubling time of cells from patients with OCP increased significantly, from 50.9 +/- 10.01 h (control) to 61.5 +/- 8.95 h (retinoid). Proliferation of conjunctival fibroblasts from a patient with Stevens-Johnson syndrome was also inhibited. N-(4-hydroxyphenyl) retinamide-O-glucuronide is biologically active and merits further study to determine its efficacy in controlling conjunctival fibrosis and treating ocular surface squamous metaplasia.

N-(4-hydroxyphenyl) retinamide induces cell cycle specific growth inhibition in PC3 cells

The synthetic retinoid N-(4-hydroxyphenyl) retinamide (4-HPR) has been demonstrated to inhibit the development of primary and metastatic neoplasms in several animal models. In order to investigate the effect of 4-HPR on human prostate adenocarcinoma, we designed a series of in vitro experiments with the PC3 cell line to evaluate effects on proliferation, cell cycle kinetics, and c-myc mRNA expression. 4-HPR demonstrated cytotoxicity only at the supraphysiologic concentration of 10.0 microM. However, asynchronously growing cells exposed to 1 microM 4-HPR demonstrated a 51% reduction in proliferation rate, associated with an accumulation of cells in the G0/G1 phase of the cell cycle. PC3 cells synchronized with serum deprivation or aphidicolin exhibited significant decreases in DNA synthesis when treated with 1 microM 4-HPR. Additionally, these cells were found to accumulate in G0/G1 and S phase. Northern blots indicated a significant decrease in c-myc mRNA expression in asynchronously growing cells with continuous administration of 1 microM 4-HPR for 6 days. These data suggest that 4-HPR can inhibit growth of PC3 cells as a consequence of a block in cell cycle transition from G1 to S phase at a concentration of 1 microM, and that this inhibition is associated with a suppression of c-myc gene expression.

Pharmacokinetics relevant to the anti-carcinogenic and anti-tumor activities of glucarate and the synergistic combination of glucarate:retinoid in the rat

Alone and in synergistic combination with retinoids, dietary glucarate inhibits both the chemical induction and growth of rat mammary tumors. To investigate the pharmacokinetics of glucarate, [14C]glucarate was synthesized, converted to the calcium salt, and administered to rats bearing primary mammary tumors. When given by gavage, [14C]glucarate, as the calcium salt, showed a biphasic response in the blood. After peaking within 1 hr of administration at a level of 0.4 mumol/mL (normal endogenous level is approximately 0.04 mumol/mL), its plasma concentration dropped to 0.1 mumol/mL at 3 hr. In the second phase, there was a semilog increase to 0.6 mumol/mL at 15 hr, followed by a slow rise to 0.75 mumol/mL at 24 hr. Of the 38% of the administered glucarate that was recovered, 38% was excreted in the urine, and 30% remained in the gastrointestinal tract at 24 hr. Glucarate was concentrated 3- to 4-fold in the liver and intestinal mucosa, compared to the level in serum. With minor exception, the pharmacokinetics of [14C]13-cis-retinoic acid administered by gavage to rats was similar or not the semipurified diets were supplemented with 64 mmol/kg of calcium glucarate. During the interval between 5 and 10 hr post-administration of [14C]13-cis-retinoid, there was a transient 35-50% rise in the plasma level in rats on the glucarate-supplemented diet. This rise had no observable effect on the level of retinoid in major organs or in the tumor. A glucarate-binding protein was detected in the tumor cytosol. This potential receptor had a Ka of 1.49 x 10(7) M-1.

Activity of D-glucarate analogues: synergistic antiproliferative effects with retinoid in cultured human mammary tumor cells appear to specifically require the D-glucarate structure

D-Glucarate has shown modest chemopreventive and synergistic chemopreventive effects with retinoids in a number of tumor models as well as a similar antiproliferative effect in MCF-7 human tumor cells in culture. It has been postulated that D-glucarate exerts some of its effects by equilibrium conversion to D-glucarolactone, a potent beta-glucuronidase inhibitor. In the present study, D-glucarate and a number of its analogues, including D-glucarolactone, were evaluated as antiproliferatives in the MCF-7 model with and without added retinoid. Results suggest that the effects of glucarate are reasonably specific for its structure and may not require conversion to glucarolactone.