In vitro and in vivo metabolism of all-trans- and 13-cis-retinoic acid in hamsters. Identification of 13-cis-4-oxoretinoic acid

One-Step, Low-Cost, Operator-Friendly, and Scalable Procedure to Synthetize Highly Pure N-(4-ethoxyphenyl)-retinamide in Quantitative Yield without Purification Work-Up

It is widely reported that N-(4-hydroxyphenyl)-retinamide or fenretinide (4-HPR), which is a synthetic amide of all-trans-retinoic acid (ATRA), inhibits in vitro several types of tumors, including cancer cell lines resistant to ATRA, at 1–10 µM concentrations. Additionally, studies in rats and mice have confirmed the potent anticancer effects of 4-HPR, without evidencing hemolytic toxicity, thus demonstrating its suitability for the development of a new chemo-preventive agent. To this end, the accurate determination of 4-HPR levels in tissues is essential for its pre-clinical training, and for the correct determination of 4-HPR and its metabolites by chromatography, N-(4-ethoxyphenyl)-retinamide (4-EPR) has been suggested as an indispensable internal standard. Unfortunately, only a consultable old patent reports the synthesis of 4-EPR, starting from dangerous and high-cost reagents and using long and tedious purification procedures. To the best of our knowledge, no article existed so far describing the specific synthesis of 4-EPR. Only two vendors worldwide supply 4-ERP, and its characterization was incomplete. Here, a scalable, operator-friendly, and one-step procedure to synthetize highly pure 4-EPR without purification work-up and in quantitative yield is reported. Additionally, a complete characterization of 4-EPR using all possible analytical techniques has been provided.

Chemical modulation of chaperone-mediated autophagy by retinoic acid derivatives

Chaperone-mediated autophagy (CMA) contributes to cellular quality control and the cellular response to stress through the selective degradation of cytosolic proteins in lysosomes. Decrease in CMA activity occurs in aging and in age-related disorders (for example, neurodegenerative diseases and diabetes). Although prevention of this age-dependent decline through genetic manipulation in mouse has proven beneficial, chemical modulation of CMA is not currently possible, due in part to the lack of information on the signaling mechanisms that modulate this pathway. In this work, we report that signaling through the retinoic acid receptor alpha (RARα) inhibits CMA and apply structure-based chemical design to develop synthetic derivatives of all-trans-retinoic acid (ATRA) to specifically neutralize this inhibitory effect. We demonstrate that chemical enhancement of CMA protects cells from oxidative stress and from proteotoxicity, supporting a potential therapeutic opportunity when reduced CMA contributes to cellular dysfunction and disease.

Physiological insights into all-trans-retinoic acid biosynthesis

All-trans-retinoic acid (atRA) provides essential support to diverse biological systems and physiological processes. Epithelial differentiation and its relationship to cancer, and embryogenesis have typified intense areas of interest into atRA function. Recently, however, interest in atRA action in the nervous system, the immune system, energy balance and obesity has increased considerably, especially concerning postnatal function. atRA action depends on atRA biosynthesis: defects in retinoid-dependent processes increasingly relate to defects in atRA biogenesis. Considerable evidence indicates that physiological atRA biosynthesis occurs via a regulated process, consisting of a complex interaction of retinoid binding-proteins and retinoid recognizing enzymes. An accrual of biochemical, physiological and genetic data have identified specific functional outcomes for the retinol dehydrogenases, RDH1, RDH10, and DHRS9, as physiological catalysts of the first step in atRA biosynthesis, and for the retinal dehydrogenases RALDH1, RALDH2, and RALDH3, as catalysts of the second and irreversible step. Each of these enzymes associates with explicit biological processes mediated by atRA. Redundancy occurs, but seems limited. Cumulative data support a model of interactions among these enzymes with retinoid binding-proteins, with feedback regulation and/or control by atRA via modulating gene expression of multiple participants. The ratio apo-CRBP1/holo-CRBP1 participates by influencing retinol flux into and out of storage as retinyl esters, thereby modulating substrate to support atRA biosynthesis. atRA biosynthesis requires the presence of both an RDH and an RALDH: conversely, absence of one isozyme of either step does not indicate lack of atRA biosynthesis at the site. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

A novel retinoic acid analogue, 7-hydroxy retinoic acid, isolated from cyanobacteria

BACKGROUND

All-trans retinoic acid (RA) is a low-molecular compound derived from vitamin A. It induces events in various ways by binding with the retinoic acid receptor (RAR), a nuclear receptor, in animal cells. RA and its metabolites have been found in animal tissues. In this paper, we report a novel RA analogue found in cyanobacterial cells, describe the method for its isolation, and compare its photo-stability with that of all-trans RA.

METHODS

The new A analogue was extracted from cells of Microcystis aeruginosa and Spirulina sp. and fractionated by high-performance liquid chromatography. The analogue was analysed using a yeast two-hybrid assay method to measure in vitro RAR-agonistic activity. Liquid chromatography-mass spectrometry/mass spectrometry analyses was performed to elucidate the chemical structure of this RA analogue.

RESULTS

The results of the analysis of the fragments revealed that the novel RA analogue was 7-hydroxy RA. The yields from 3.5 μg (4.5% of the total RAR-agonistic activity of Spirulina sp. cells) of 7-hydroxy RA was a mixture of 4 isomers due to cis-trans isomerisation coupled with keto-enol tautomerism; its relative RAR agonistic activity was 0.49 ± 0.01 (n=3) when the activity of all trans RA was set up to 1.00. Under fluorescent light, the mixture of 7-hydroxy RA isomers was more stable than all- trans RA.

CONCLUSIONS

We isolated a novel RAR-activating compound, 7-hydroxy RA, from cyanobacteria.

GENERAL SIGNIFICANCE

7-hydroxy RA is more stable than all-trans RA under UV-A.

Assessment of dermatopharmacokinetic approach in the bioequivalence determination of topical tretinoin gel products

BACKGROUND

A new dermatopharmacokinetic (DPK) approach has been proposed for bioequivalence determination of topical drug products by comparing the drug content kinetics in stratum corneum.

OBJECTIVE

We sought to establish any correlation between clinical safety/efficacy and DPK approach in bioequivalence determination of tretinoin gel 0.025%.

METHODS

Tretinoin and isotretinoin were quantified in human volar forearm stratum corneum as a function of time with 3 tretinoin gel 0.025% products in 49 patients. Stratum corneum layers were harvested using multiple adhesive disks, which were subsequently extracted and quantified for both isomers by high-performance liquid chromatography.

RESULTS

Products with similar composition and therapeutic equivalence were found bioequivalent, and products with different composition and clinical profiles were found bioinequivalent by DPK methodology.

CONCLUSIONS

There is a direct correlation between DPK parameters in healthy patients and clinical safety/efficacy of tretinoin gel products in patients with acne. Data support the use of DPK parameters and methodology in the bioequivalence assessment of topical tretinoin gel products.

Liarozole markedly increases all trans-retinoic acid toxicity in mouse limb bud cell cultures: a model to explain the potency of the aromatic retinoid (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid

The remarkable toxicity of (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid (TTNPB) compared to all trans-retinoic acid (tRA) is due to multiple factors, including reduced affinities for cytosolic binding proteins (CRABPs), resistance to metabolism, and prolonged nuclear receptor activation. To further investigate the role of half-life in retinoid toxicity, experiments were performed to determine whether, and to what extent, inhibition of tRA metabolism by liarozole increased its toxicity comparable to that of TTNPB in the mouse limb bud system. Liarozole is a known inhibitor of tRA 4-hydroxylation (CYP26). In the absence of liarozole, the IC50 for inhibition of chondrogenesis by tRA was 140 nM compared to 0.3 nM for TTNPB, a 467-fold difference. Following the addition of liarozole (10(-6) M) to limb bud cultures, the potency of tRA to inhibit chondrogenesis was increased approximately 14-fold (IC50 of 9.8 nM). Although liarozole markedly increased toxicity of tRA in mouse limb bud micromass cultures, tRA metabolism was inhibited only about 10%. These results indicate that a relatively minor decrease in the metabolism of tRA in the mouse limb bud system is associated with a marked enhancement of toxicity that is likely related to the prolongation of tRA half-life during a critical period of development. Thus, the prolonged half-life of TTNPB is the most significant factor contributing to the remarkable teratogenicity of this synthetic aromatic retinoid.

Identification of human cytochrome P450s involved in the formation of all-trans-retinoic acid principal metabolites

Cytochrome P450 (P450)-dependent metabolism of all-trans-retinoic acid (atRA) is important for the expression of its biological activity. Because the human P450s involved in the formation of the principal atRA metabolites have been only partially identified, the purpose of this study was to identify the human P450s involved in atRA metabolism. The use of phenotyped human liver microsomes (n = 16) allowed the identification of the following P450s: 2B6, 2C8, 3A4/5, and 2A6 were involved in the formation of 4-OH-RA and 4-oxo-RA; 2B6, 2C8, and 2A6 correlated with the formation of 18-OH-RA; and 2A6, 2B6, and 3A4/5 activities correlated with 5, 6-epoxy-RA formation (30-min incubation, 10 microM atRA, HPLC separation, UV detection 340 nm). The use of 15 cDNA-expressed human P450s from lymphoblast microsomes, showed the formation of 4-OH-RA by CYP3A7 > CYP3A5 > CYP2C18 > CYP2C8 > CYP3A4 > CYP2C9, whereas the 18-OH-RA formation involved CYPs 4A11 > 3A7 > 1A1 > 2C9 > 2C8 > 3A5 > 3A4 >2C18. Kinetic studies identified 3A7 as the most active P450 in the formation of three of the metabolites: for 4-OH-retinoic acid, 3A7 showed a V(max)/K(m) of 127.7, followed by 3A5 (V(max)/K(m) = 25.6), 2C8 (V(max)/K(m) = 24.5), 2C18 (V(max)/K(m) = 15.8), 3A4 (V(max)/K(m) = 5.7), 1A1 (V(max)/K(m) = 5.0), and 4A11 (V(max)/K(m) = 1.9); for 4-oxo-RA, 3A7 showed a V(max)/K(m) of 13.4, followed by a 10-fold lower activity for both 2C18 and 4A11 (V(max)/K(m) = 1.2); and for 18-OH-RA, 3A7 showed a V(max)/K(m) of 10.5 compared with a V(max)/K(m) of 2.1 for 4A11 and 2.0 for 2C8. 5,6-Epoxy-RA was only detected at high substrate concentrations in this system (>10 microM), and P450s 2C8, 2C9, and 1A1 were the most active in its formation. The use of embryonic kidney cells (293) stably transfected with human P450 cDNA confirmed the major involvement of P450s 3A7, 1A1, and 2C8 in the oxidation of atRA, and to a lesser extent, 1A2, 2C9, and 3A4. In conclusion, several human P450s involved in atRA metabolism have been identified, the expression of which was shown to direct atRA metabolism toward the formation of specific metabolites. The role of these human P450s in the biological and anticancer effects of atRA remains to be elucidated.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

A novel assay for retinoic acid catabolic enzymes shows high expression in the developing hindbrain

We have employed a novel technique that determines the relative capacity of tissues to catabolize all-trans retinoic acid (RA) to a metabolite incapable of activating a RA reporter cell line. This assay uses the microsomal fraction of tissues from the developing mouse and detects a pathway which requires NADPH and is inhibitable by ketoconazole, suggesting that a cytochrome P450-dependent enzyme may be required. High catabolic activity was detected transiently in the developing cerebellum which peaked at postnatal day 2. The medulla oblongata was the only other CNS region with high catabolic capacity, its earlier expression peak, between embryonic days 16 and 17, likely reflecting its earlier maturation. In the CNS, the hindbrain is exceptional in its high expression of RA catabolic enzymes, suggesting a unique function for regulated RA levels in this region.

Liarozole inhibits human epidermal retinoic acid 4-hydroxylase activity and differentially augments human skin responses to retinoic acid and retinol in vivo

Metabolic inactivation of all-trans retinoic acid to 4-hydroxy retinoic acid occurs via a cytochrome P-450 enzyme. We investigated the effects of liarozole on the retinoic acid 4-hydroxylase activity of human epidermis and its ability to modify in vivo human skin responses to retinoic acid and all-trans retinol. Retinoic acid 4-hydroxylase activity induced in vivo by 4 d treatment with retinoic acid (0.1%) was inhibited in vitro by liarozole in a concentration-dependent manner. Comparable micromolar concentrations of liarozole were extracted from stratum corneum-free epidermis treated with 3% liarozole. Retinoic acid levels in liarozole-treated skin increased to 19 +/- 5 ng/g wet wt (mean +/- SEM, p < 0.002, n = 17) at 18 h and to 6 +/- 2 ng/g wet wt (p = 0.38, n = 17) at 48 h as compared to vehicle (not detectable). At 48 h, retinoic acid 4-hydroxylase activity was induced 9-fold over vehicle (p < 0.03, n = 8). At 96 h, no significant erythema or increased epidermal thickness was found when either retinoic acid (0.001%), all-trans retinol (0.0250%), or liarozole (3%) was applied individually, but when 0.001% retinoic acid and 3% liarozole were applied together, both erythema and increased epidermal thickness occurred. In contrast, 0.025% all-trans retinol and 3% liarozole together caused increased epidermal thickness but no erythema. These data demonstrate that, at doses used here, liarozole, although an effective inhibitor of retinoic acid 4-hydroxylase, cannot function alone like a retinoid in vivo, probably because of retinoic acid 4-hydroxylase induction. In the presence of a low dose retinoic acid or all-trans retinol, however, liarozole can amplify human skin responses to each retinoid in a manner characteristic of the retinoid at a higher dose (erythema and hyperplasia with retinoic acid; no erythema but hyperplasia with all-trans retinol).

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.

Enzymes and binding proteins affecting retinoic acid concentrations

Free retinoids suffer promiscuous metabolism in vitro. Diverse enzymes are expressed in several subcellular fractions that are capable of converting free retinol (retinol not sequestered with specific binding proteins) into retinal or retinoic acid. If this were to occur in vivo, regulating the temporal-spatial concentrations of functionally-active retinoids, such as RA (retinoic acid), would be enigmatic. In vivo, however, retinoids occur bound to high-affinity, high-specificity binding proteins, including cellular retinol-binding protein, type I (CRBP) and cellular retinoic acid-binding protein, type I (CRABP). These binding proteins, members of the superfamily of lipid binding proteins, are expressed in concentrations that exceed those of their ligands. Considerable data favor a model pathway of RA biosynthesis and metabolism consisting of enzymes that recognize CRBP (apo and holo) and holo-CRABP as substrates and/or affecters of activity. This would restrict retinoid access to enzymes that recognize the appropriate binding protein, imparting specificity to RA homeostasis; preventing, e.g. opportunistic RA synthesis by alcohol dehydrogenases with broad substrate tolerances. An NADP-dependent microsomal retinol dehydrogenase (RDH) catalyzes the first reaction in this pathway. RDH recognizes CRBP as substrate by the dual criteria of enzyme kinetics and chemical crosslinking. A cDNA of RDH has been cloned, expressed and characterized as a short-chain alcohol dehydrogenase. Retinal generated in microsomes from holo-CRBP by RDH supports cytosolic RA synthesis by an NAD-dependent retinal dehydrogenase (RalDH). RalDH has been purified, characterized with respect to substrate specificity, and its cDNA has been cloned. CRABP is also important to modulating the steady-state concentrations of RA, through sequestering RA and facilitating its metabolism, because the complex CRABP/RA acts as a low Km substrate.

The retinoid ligand 4-oxo-retinoic acid is a highly active modulator of positional specification

Retinoids (vitamin A and its metabolites) are suspected of regulating diverse aspects of growth, differentiation, and patterning during embryogenesis, but many questions remain about the identities and functions of the endogenous active retinoids involved. The pleiotropic effects of retinoids may be explained by the existence of complex signal transduction pathways involving diverse nuclear receptors of the retinoic acid receptor (RAR) and retinoid X receptor (RXR) families, and at least two types of cellular retinoic acid binding proteins (CRABP-I and -II). The different RARs, RXRs, and CRABPs have different expression patterns during vertebrate embryogenesis, suggesting that they each have particular functions. Another level at which fine tuning of retinoid action could occur is the metabolism of vitamin A to active metabolites, which may include all-trans-retinoic acid, all-trans-3,4-didehydroretinoic acid, 9-cis-retinoic acid, and 14-hydroxy-4,14-retroretinol. Formation of the metabolite all-trans-4-oxo-retinoic acid from retinoic acid was considered to be an inactivation pathway during growth and differentiation. We report here that, in contrast, 4-oxo-retinoic acid is a highly active metabolite which can modulate positional specification in early embryos. We also show that this retinoid binds avidly to and activates RAR beta, and that it is available in early embryos. The different activities of 4-oxo-retinoic acid and retinoic acid in modulating positional specification on the one hand, and growth and differentiation on the other, interest us in the possibility that specific retinoid ligands regulate different physiological processes in vivo.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Chromatography of retinoids

This article reviews the determination of retinoic acids and their metabolites (first-generation retinoids), aromatic retinoids (second generation) and arotinoids (third generation) in biological samples. Because of the sensitivity of the retinoids to isomerization and oxidation, special care has to be taken from sample collection and storage, throughout extraction, till the final chromatographic separation. High and strong protein binding, and insolubility in aqueous solutions hamper the extraction from biological samples. Various extraction procedures are discussed, mainly involving liquid-liquid extraction of biological fluids or lyophilized tissue samples. The new technique involving direct injection of biological fluids or tissue homogenates, using high-performance liquid chromatography (HPLC) with automated column switching, provides full protection from light and simplifies sample work-up. HPLC with ultraviolet detection is the method of choice for the determination of retinoids, because it is rapid, sensitive and allows separation of geometric isomers and metabolites within a wide polarity range. Gas chromatography-mass spectrometry is not appropriate for first- and second-generation retinoids because of isomerization, but allows very sensitive determination of third-generation retinoids, although very extensive sample clean-up and derivatization are necessary. However, direct injection of large volumes of biological fluids into HPLC systems, using on-line solid-phase extraction and automated column-switching, results in very sensitive methods even with simple ultraviolet detection and may become the method of choice for routine analyses.

Human biliary metabolites of isotretinoin: identification, quantification, synthesis, and biological activity

1. The metabolites of isotretinoin (13-cis-retinoic acid, Accutane) were investigated in the bile of two patients with biliary T-tube drainage after administration of a single, oral, 80-mg dose of 14C-isotretinoin. Radioactivity measurements showed that the two patients excreted 22.7 and 17.1% of the dose in their bile in 4 days. 2. The two major drug-related components in the bile were identified as the glucuronide conjugates of 4-oxo-isotretinoin and 16-hydroxy-isotretinoin. Two minor components were identified as the glucuronide conjugates of isotretinoin and 18-hydroxy-isotretinoin. 3. H.p.l.c. analyses of Glusulase-treated bile samples indicated that the glucuronides of isotretinoin and the two major metabolites accounted for about 48% and 44% of the total radioactivity in the bile of the two patients. 4. Racemic 16-hydroxy-isotretinoin was synthesized and evaluated for its effect on human sebocytes in vitro. This metabolite and the other major metabolites of isotretinoin were less active than isotretinoin in inhibiting the proliferation of the sebocytes.

Evidence for percutaneous absorption of isotretinoin from the photo-isomerization of topical tretinoin

Tretinoin (0.1% Retin-A cream) was topically applied to human cadaver skin in vitro using Franz diffusion chambers. The photo-isomerization of tretinoin and retinoic acid percutaneous absorption in the absence of metabolic activity were assessed with and without ambient light exposure to the skin. Using HPLC, UV, and GC/MSD, a retinoid exhibiting identical chromatographic and spectral characteristics of isotretinoin was observed in the samples from the skin exposed to light, but was virtually absent in the skin samples maintained in the dark. From a single topical application of tretinoin, isotretinoin was as abundant as tretinoin in the chamber receiver solution, dermis, epidermis, and on the skin surface at 24 h after topical application. The data suggest the possibility that isotretinoin may have an important role in the pharmacology of topically applied tretinoin.

Synthesis and evaluation of some new retinoids for cancer chemoprevention

Many derivatives and analogs of the natural vitamin A compounds have been synthesized, and some of these retinoids are very active in various bioassays that may be predictive for cancer chemopreventive activity. Analogs have been synthesized in which parts of one, or more, of the three traditional subdivisions of the retinoid structure--the terminal polar group, the conjugated side chain, and the cyclohexenyl region--have been altered or replaced. In addition, structural alterations have produced compounds (arotinoids) in which two, or more, aromatic rings replace much of the classical retinoid structure. Further structural modifications that effected replacement of the entire retinoid structure by aromatic groups and certain linking groups have been made. Some of the arotinoids and further structural variants, which appear superficially to represent immense alteration of the classical retinoid structure, have potent activity in various retinoid bioassays. Four types of new classical retinoids (retinoylamino acids, bifunctional analogs, retinyl ethers, and 4-oxoretinoid derivatives) have been synthesized and evaluated in our laboratories. All-trans- and 13-cis-retinoyl alpha-amino acids are active in a hamster trachea organ culture assay and show much promise in potentiating immune responses. Retinoic acid analogs that have a bifunctional terminus bind to CRABP even though they have a bulky alpha-substituent, induce differentiation of F9 embryonal carcinoma cells, and have good activity in an immune-potentiation assay. A new retinyl either, retinyl propynyl ether, has chemopreventive activity against mammary carcinogenesis in vivo. Oral administration of this retinoid produces high and sustained concentrations in the rat mammary gland. New congeners of 4-oxoretinoic acid demonstrate high activity in a spectrum of bioassays.

Structure-activity relationships of retinoids in developmental toxicology. III. Contribution of the vitamin A beta-cyclogeranylidene ring

The teratogenic potency of congeners of all-trans-retinoic acid (all-trans-RA) containing modifications or substitution of the naturally occurring beta-cyclogeranylidene ring was determined in Golden hamsters and compared to that of all-trans-RA. The following ring-modified retinoids were screened: phenyl (Ro 8-8717), furyl (Ro 8-9750), 4-methoxy-2,3,6-trimethylphenyl (Ro 21-6667), which also has a thiomethylene group in place of the trans-8,9 double bond of the etretin side chain, 4-hydroxy-2,3,6-trimethylphenyl (Ro 11-4768), 2-chloro-3,6-dimethyl-4-methoxyphenyl (Ro 12-0995), 2-(1-methoxyethyl)-5,5-dimethyl-1-cyclopentenyl (Ro 10-1770), 2-acetyl-5,5-dimethyl-1-cyclopentenyl (Ro 8-7699), and 10,11-epoxy-11,11-dimethyl (juvenile hormone III), which also has the bonds corresponding to the 7,8- and 11,12-double bond of the retinoid skeleton saturated. The retinoids Ro 12-4824, Ro 12-4825, and SRI2712-24 had C4-keto, C18-hydroxyl, and C18-methyl substituents, respectively. Motretinid (Ro 11-1430) had both 4-methoxy-2,3,6-trimethylphenyl ring and ethyl amide polar group modifications. Single oral retinoid doses administered to pregnant dams at 10:00 AM on Day 8 neither induced signs of hypervitaminosis A nor induced weight loss in any of the treated groups. Teratogenically active retinoids induced a malformation syndrome identical to that induced by all-trans-RA. At retinoid doses that were associated with malformations in all of the fetuses, embryolethality remained near that of vehicle-treated controls. The phenyl retinoid Ro 8-8717 was embryolethal but was not teratogenic. The ethyl amide derivative of the human and animal teratogen etretinate, motretinid, was teratogenic only at the highest dose administered, 350 mg/kg. The retinoids Ro 12-4824, Ro 12-4825, Ro 8-7699, and SRI 2712-24 were as potent as all-trans-RA. The chlorine substituted retinoid, Ro 12-0995, was sixfold more teratogenic than all-trans-RA, and the cyclopentene retinoid, Ro 10-1770, was 19 times more potent than all-trans-RA. The retinoids with furyl or epoxy group substitution for the cyclohexenyl ring were devoid of teratogenic activity up to equimolar doses of 75 mg/kg of all-trans-RA, and Ro 21-6667 was teratogenically inactive at a dose equivalent to 150 mg/kg of all-trans-RA. Major modifications of the beta-cyclogeranylidene ring can be made without altering teratogenic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Lack of biological activity of physiological metabolites of all-trans-retinoic acid on vaginal epithelial differentiation

It has been of interest to determine whether the metabolites of physiological doses of retinoic acid represent active forms of vitamin A. Previous work (Biochem. J. 206, 33-41, 1982) studied the metabolites produced from 2-micrograms doses of all-trans-retinoic acid in the vitamin A-deficient rat. Four major metabolites common to all of the tissues studied were discovered. In the present work, three of these metabolites are isolated from vitamin A-deficient rats given physiological doses (5 micrograms) of all-trans-retinoic acid and from vitamin A-sufficient rats given high doses (1 mg) of all-trans-retinoic acid. Cochromatography on anion-exchange and reverse-phase high-performance liquid chromatography showed that metabolites resulting from high doses of retinoic acid contained the metabolites generated from physiological doses of retinoic acid. Quantities of these metabolites were isolated, purified, and tested for their epithelial-differentiating activity in the vitamin A-deficient rat vagina. The metabolites were inactive at all dose levels tested. These metabolites have less than 10% the biological activity of all-trans-retinoic acid. Therefore, these metabolites appear to be products of the inactivation of all-trans-retinoic acid. Based upon these and previous data, it seems likely that all-trans-retinoic acid or its beta-glucuronide derivative is the most likely active form of vitamin A in the maintenance of normal epithelial differentiation.

The in vitro metabolism of 13-cis-retinoic acid in a model sebaceous structure, the rat preputial gland

In order to study the metabolism of 13-cis-retinoic acid (13-cis-RA) in animal sebaceous glands and analogues, preputial glands from normal and vitamin A-deficient male rats were incubated with [3H]13-cis-RA for up to 24 h; vitamin A-normal hamster costovertebral glands (flank organs) were incubated for 24 h as well. High-performance liquid chromatography was used to identify the metabolites. [3H]13-cis-RA was rapidly converted to a less polar compound, [3H]all-trans-retinoic acid, by the preputial glands from both normal and deficient rats. In normal preputial glands, the level of [3H]all-trans-RA decreases and two more polar compounds, metabolite I and [3H]4-keto-13-cis-RA appear. In contrast, [3H]all-trans-RA is not metabolized further by the preputial glands from deficient rats, while [3H]13-cis-RA in the hamster costovertebral glands remains intact for up to 24 h. The major metabolite of [3H]13-cis-RA in rat preputial glands is [3H]4-keto-13-cis-RA. Initially, [3H]13-cis-RA is converted to [3H]all-trans-RA. In vitamin A-deficient rats the preputial glands fail to further metabolize [3H]13-cis-RA to the more polar [3H]13-cis-RA derivatives. This may be due to the reduced level of P-450 enzyme in vitamin A-deficient rat preputial glands.

The biological activity of cyclopropyl analogs of all-trans- and 13-cis-retinoic acid in the rat vaginal smear assay

The biological activity of a series of cyclopropyl analogs of all-trans- and 13-cis-retinoic acid has been evaluated in the vaginal smear assay carried out in vitamin A-deficient rats. These analogs were designed to probe the role of the 13-cis isomer in the actions of the parent all-trans-retinoic acid by blocking the interconversion of these two compounds. Although relatively less active, the potency of some of the cyclopropyl analogs suggests that 13-cis-retinoic acid is a fully active metabolite of all-trans-retinoic acid. Since 13-cis-retinoic acid represents a small percentage of the retinoic acid metabolites, the physiological significance of this activity is still unclear. Possible reasons for the reduced activity of the cyclopropyl analogs, as well as an aromatic analog of retinoic acid, are discussed.

Demonstration of retinoic acid isomers in human urine under physiological conditions

Untransformed retinoic acid has never been demonstrated in human excreta under normal physiological conditions. We have developed a two-step liquid chromatographic system for the demonstration of subnanogram amounts of this compound in human urine without administration of any precursor.

Comparative pharmacokinetics of vitamin A (retinol) in rats and hamsters

The absorption and elimination of all-trans-retinol in the plasma of rats and hamsters were studied after an oral dose of 45 mg/kg body weight. The hamsters exhibited a higher pretreatment mean circulating retinol concentration than rats maintained on an identical diet. The increase in plasma retinol after a single oral dose was much greater in hamsters than rats. The area under the plasma concentration v. time curve was approximately 60% greater for rats than for hamsters. The elimination half-times for rats were much longer than for hamsters. Plasma retinol uptake and disappearance were consistent with a two-compartment open model with first-order absorption.

Comparative teratogenic activities of two retinoids: effects on palate and limb development

Two closely related retinoids, all-trans and 13-cis retinoic acids, were assessed for their relative activities as teratogens in ICR mice by monitoring the frequency with which either isomer produced discrete dysmorphogenesis of the embryonic limb and the secondary palate. A single oral dose of all-trans retinoic acid at 100 mg/kg on either day 11.5 or 12.0 of gestation (plug day = day one) was maximally effective; more than 90% of the treated embryos developed reduction defects of the limb bones and an equally high percentage also had cleft palate. The limb development was most sensitive on day 11.5 of gestation while the peak susceptibility for palatal clefts began on day 12.0. Under identical experimental conditions, treatment with 100 mg/kg 13-cis retinoic acid produced no apparent teratogenic effects. By assessing the relative incidence of readily identifiable malformations of the limb and palate associated with various doses of the two isomers, we found that 13-cis retinoic acid was four to eight times less embryopathic than all-trans retinoic acid. Since the mechanism of teratogenic action of retinoids is still far from clear, it is suggested that further studies on causative factors will be greatly assisted by the use of these two closely related retinoids, which substantially differ from each other in their teratogenic potency.

The biological activity of rat intestinal retinoic acid metabolites in the vaginal smear assay

Vitamin A-deficient rats were given a single intrajugular injection of 1 mg all-trans-[11-3H]retinoic acid and 3 h later the rats were killed. The small intestines were extracted and chromatographed by high-performance liquid chromatography to yield distinct metabolites. These were quantitated using the assumption that the specific activity of the metabolite is equal to that of the parent [3H]retinoic acid. The biological activity of all discernible metabolites was determined in the vitamin A-deficient female rat by vaginal smear assay. Retinoic acid and retinoyl-beta-glucuronide from the preparation had equal activity while no activity was found for any of the other metabolite fractions. Thus, no evidence for an unknown metabolite having potent epithelial differentiating activity could be found in this target tissue of vitamin A action.

Biliary metabolites of all-trans-retinoic acid in the rat: isolation and identification of a novel polar metabolite

The biliary metabolites from normal rats dosed with either pharmacological or physiological doses of all-trans-[11,12-3H2]retinoic acid were investigated. Biliary metabolites excreted during the first 24 h account for approximately 60-65% of the radiolabeled dose. A major polar metabolite was purified to homogeneity by using Sephadex LH-20 chromatography and several high-performance liquid chromatographic procedures. This metabolite was negatively charged as revealed by high-performance liquid chromatography on ion-exchange columns and accounts for 10% of the total biliary radioactivity (6% of the dose). The polar compound was positively identified by using Fourier transform proton nuclear magnetic resonance spectroscopy, high- and low-resolution mass spectrometry, fast atom bombardment mass spectrometry, ultraviolet absorption spectrophotometry, Fourier transform infrared spectroscopy, amino acid analysis, and chemical derivatization as 2-[8-[6-(hydroxymethyl)-2,6-dimethyl-3-oxo-1-cyclohexen-1-yl]-2,6- dimethyl-5,7-octadienamido]ethanesulfonic acid. The metabolic transformations required for the generation of this metabolite from all-trans-retinoic acid are the following: (1) allylic oxidation at carbon 4 of the cyclohexene ring to produce a 4-keto group, (2) hydroxylation of one of the methyl groups at carbon 1 of the cyclohexene ring, (3) saturation of the two terminal double bonds in the side chain, (4) loss of the terminal carboxyl group of the side chain via decarboxylation, and (5) conjugation of the resulting retinoid with taurine. To our knowledge, this metabolite represents the first taurine conjugate of a fat-soluble vitamin to be identified.

Metabolism of all-trans-retinyl acetate to retinoic acid in hamster tracheal organ culture

All-trans-[3H]retinyl acetate has been shown to be metabolized to all-trans-[3H]retinoic acid in a target tissue of vitamin A action, the hamster trachea in organ culture. That the compound produced is indeed all-trans-retinoic acid is demonstrated by chromatography of the biosynthetically produced retinoic acid with synthetic all-trans-retinoic acid in two different high-pressure liquid chromatographic systems, either as the free acids in a reverse-phase system or as the methyl esters in a normal-phase system. The all-trans-[3H]retinoic acid was also found in the tracheal epithelium and cartilage as well as in the medium. In addition the tracheal tissue also contained retinyl palmitate and other esters. Finally, further in vitro metabolism of [3H]retinyl acetate paralleled the metabolism of [14C]retinoic acid suggesting that these two compounds are being metabolized through similar pathways.