Are topical retinoids teratogenic?

Retinoic acid is a physiological compound of human blood. Blood levels range from 1000 to 7000 pg/mL (usually 1500-5000 pg/mL). Results of studies on absorption of topical retinoic acid in laboratory animals, although rather conflicting, demonstrate that it induces plasma concentrations which are well below concentrations caused by non-teratogenic oral doses. In humans, minimal percutaneous absorption of tretinoin was observed after topical applications. Neither single dose nor long-term treatment with topical tretinoin affect the endogenous levels of retinoic acid or its metabolites. Topical application of tretinoin at doses used in acne unlikely induces systemic effects. Although some clinical cases of suspected tretinoin-related embryotoxicity have been described, three prospective cohort studies clearly demonstrated the safety of topical tretinoin as an embryotoxic agent.

Delivery of retinoid-based therapies to target tissues

Through its various metabolites, vitamin A controls essential physiological functions. Both naturally occurring metabolites and novel retinoid analogues have shown effectiveness in many clinical settings that include skin diseases and cancer, and in animal models of human conditions affecting vision. In this review, we analyze several potential retinoid-based therapies from the point of view of drug metabolism and transport to target tissues. We focus on the endogenous factors that affect the absorption, transport, and metabolism of retinoids by taking into account data obtained from the analysis of animal models that lack the enzymes or proteins involved in the storage and absorption of retinoids. We also discuss findings of toxicity associated with retinoids in an effort to improve the outcome of retinoid-based therapies. In this context, we review evidence that esterification of retinol and retinol-based drugs within target tissues provides one of the most efficient means to improve the absorption and to reduce the toxicity associated with pharmacological doses of retinoids. Future retinoid-based therapeutic strategies could involve targeted delivery mechanisms leading to lower toxicity and improved effectiveness of retinoids.

Vitamins as hormones

Vitamins A and D are the first group of substances that have been reported to exhibit properties of skin hormones, such as organized metabolism, activation, inactivation, and elimination in specialized cells of the tissue, exertion of biological activity, and release in the circulation. Vitamin A and its two important metabolites, retinaldehyde and retinoic acids, are fat-soluble unsaturated isoprenoids necessary for growth, differentiation and maintenance of epithelial tissues, and also for reproduction. In a reversible process, vitamin A is oxidized IN VIVO to give retinaldehyde, which is important for vision. The dramatic effects of vitamin A analogues on embryogenesis have been studied by animal experiments; the clinical malformation pattern in humans is known. Retinoic acids are major oxidative metabolites of vitamin A and can substitute for it in vitamin A-deficient animals in growth promotion and epithelial differentiation. Natural vitamin A metabolites are vitamins, because vitamin A is not synthesized in the body and must be derived from carotenoids in the diet. On the other hand, retinoids are also hormones - with intracrine activity - because retinol is transformed in the cells into molecules that bind to and activate specific nuclear receptors, exhibit their function, and are subsequently inactivated. The mechanisms of action of natural vitamin A metabolites on human skin are based on the time- and dose-dependent influence of morphogenesis, epithelial cell proliferation and differentiation, epithelial and mesenchymal synthetic performance, immune modulation, stimulation of angiogenesis and inhibition of carcinogenesis. As drugs, vitamin A and its natural metabolites have been approved for the topical and systemic treatment of mild to moderate and severe, recalcitrant acne, photoaging and biologic skin aging, acute promyelocytic leukaemia and Kaposi's sarcoma. On the other hand, the critical importance of the skin for the human body's vitamin D endocrine system is documented by the fact that the skin is both the site of vitamin D (3)- and 1,25-dihydroxyvitamin D (3) [1, 25(OH) (2)D (3)]-synthesis and a target organ for 1,25(OH) (2)D (3). 1,25(OH) (2)D (3) is not only essential for mineral homeostasis and bone integrity, but also for numerous further physiologic functions including regulation of growth and differentiation in a broad variety of normal and malignant tissues, including cells derived from prostate, breast and bone. In keratinocytes and other cell types, 1,25(OH) (2)D (3) regulates growth and differentiation. Consequently, vitamin D analogues have been introduced for the treatment of the hyperproliferative skin disease psoriasis. Other newly detected functions of vitamin D analogues include profound effects on the immune system as well as protection against cancer and other diseases, including autoimmune and infectious diseases, in various tissues. Current investigation of the biological effects of vitamin D analogues are likely to lead to new therapeutic applications that, besides cancer prevention, may include the prevention and treatment of infectious as well as of inflammatory skin diseases. This review summarizes existing knowledge on vitamins A and D, the major vitamin-hormones of the skin.

High performance liquid chromatographic analysis and preclinical pharmacokinetics of the heteroarotinoid antitumor agent, SHetA2

BACKGROUND

SHetA2 {[(4-nitrophenyl)amino][2,2,4,4-tetramethylthiochroman-6-yl)amino]methane-thione], NSC 726189} is a sulfur-containing heteroarotinoid, which selectively inhibits cancer cell growth and induces apoptosis without activation of nuclear retinoic acid receptors (RARs). The objective was to develop and validate a HPLC/UV method for the determination of SHetA2, and study the pharmacokinetics of SHetA2 in the mouse.

METHODS

SHetA2 and the internal standard, methylated XK469 (MeXK469) were isolated from 0.2 ml of mouse plasma by solid phase extraction. The analytes were separated on a narrow-bore C18 column, with the mobile phase consisting of 60% acetonitrile in water at a flow rate of 0.2 ml/min. UV detection was set at 341 nm. Pharmacokinetic studies of SHetA2 were carried out in mice following i.v. bolus dose at 20 mg/kg and oral administrations at 20 and 60 mg/kg.

RESULTS

The standard curves were linear between 25 and 2,500 nM and the lower limit of quantification (LLOQ) was 25 nM. The within-run coefficients of variation (CVs) were 11.1% at 10, 9.4% at 100, and 5.2% at 2,500 nM and the respective between-run CVs were 10.9, 3.1, and 1.5% (all n=5). The recovery was 85.8% for SHetA2 and 80.6% for MeXK469. Following i.v. bolus dose, plasma concentrations of approximately 10 microM were achieved at 5 min in mice and declined biexponentially with detectable levels at 60 h. The data were fitted with a two-compartment model, which gave a mean initial t1/2 of 40 min and terminal t1/2 of 11.4 h (n=6). The total body clearance was approximately 1.81 l/h/kg. The volume of distribution at steady state (Vdss) was 20.8 l/kg. Plasma protein binding was found to be 99.3-99.5% at low micromolar concentrations. Plasma concentration data for the i.v. and p.o. doses were also fitted interactively to a two-compartment deconvolution model. From this result, oral bioavailability values of 15% at 20 mg/kg and 19% at 60 mg/kg were obtained.

CONCLUSIONS

A highly sensitive HPLC/UV method for the quantification of SHetA2 in plasma has been developed to support pharmacokinetics of SHetA2 in the mouse. Pharmacokinetic behaviors of this drug appear to be favorable for future development.

An update on the safety and efficacy of topical retinoids

Adapalene, tazarotene, and tretinoin are topical retinoids that are approved by the US Food and Drug Administration (FDA) and have overall favorable safety profiles. These agents are effective in a variety of skin disorders. Local adverse events, seen especially during the early course of treatment, include peeling, erythema, dryness, burning, and itching. The degree of local skin intolerance varies among patients and may relate to the vehicle formulation used. These local effects can be minimized by decreasing exposure to sunlight, avoiding extreme temperatures, and using moisturizers. Use of these agents has been found to be advantageous over current systemic retinoids from a toxicologic perspective. This article reviews the safety, efficacy, and photostability of the available topical retinoids.

A review of clinical experience and recommendations for improving patient care

This article reviews the history of retinoids from the 1960s to the present, clinical experiences, and recommendations for improving care for patients with acne. How a retinoid is chosen by the clinician based on irritation potential, treatment outcome, patient profile, type of acne, and the patient's clinical experience is discussed, as well as the use of retinoids in the treatment of photoaging and other approved and off-label uses.

Effect of organochlorine contaminants and individual biological traits on blubber retinoid concentrations in bottlenose dolphins (Tursiops truncatus)

Here we assessed retinoids as biomarkers of contaminant exposure by studying whether the sex, age, lipid content and organochlorine concentrations of bottlenose dolphins induced variation in retinoid status and its deposition in blubber. Blubber samples were collected from 47 individuals of known age and gender from Sarasota Bay in June 2000 and 2001. The sample included a representative cross-section of the resident dolphin community, with ages ranging from 2 to 50 years. Organochlorine levels showed the age- and sex-related variation commonly observed in other species, with concentrations increasing in youngsters of both sexes and in adult males, and decreasing in adult females after the onset of maturity. Blubber lipid content was low in the overall population and significantly decreased with age in adult males. Retinoid blubber concentrations were comparable to other odontocete species previously studied, and were strongly determined by lipid content. As a consequence of the latter, retinoid concentration was observed to decrease with age in adult males. This effect could not be statistically dissociated from the negative correlation observed between levels of organochlorines and retinoid blubber concentration. Consequently, we could not clarify whether high organochlorine loads in this population lowered retinoid concentrations or, conversely, whether depleted lipid reserves were indeed responsible for the high organochlorine concentrations and the low retinoid levels detected in blubber. With the current knowledge, both options should be considered and investigated, with initial focus on male dolphins.

Penetration and Metabolism of Topical Retinoids in ex vivo Organ-Cultured Full-Thickness Human Skin Explants

The human epidermis contains endogenous retinoids [retinol (vitamin A) and retinyl esters] and carotenoids (mostly beta-carotene). Previous studies in the mouse have shown that the enzymes involved in retinoid metabolism are present in the epidermis. In this study, we wanted to assess the skin penetration and metabolism of topical retinoids in the human. To do this, fresh surgically excised human abdominal skin was mounted on Franz perfusion cells. Topical retinoic acid, retinal, retinol and retinyl palmitate were applied at 2.5 mg/cm(2) in oil-in-water creams containing 0.05% retinoids on the donor compartment, while the receptor compartment was filled with culture medium. The skin was incubated for 24 h at 37 degrees C, then epidermal retinoid concentrations were determined by HPLC. The same experiment was performed with mouse back skin mounted on Franz cells. Finally, topical retinoids were applied on the back of hairless mice for 24 h; then the mice were sacrificed and retinoid concentrations were assayed in the epidermis. In all three models, retinol and its esters were found to be endogenous, as was the case in previous studies in the mouse in vivo. The four applied retinoids penetrated well into the epidermis. Topical retinoic acid did not increase endogenous retinoids, whereas the latter were greatly increased following topical retinal in the mouse. Retinal was also metabolized into retinoic acid, unlike topical retinol and retinyl palmitate, which only increased endogenous retinoids. Topical retinal and retinol did undergo a higher metabolism in both mouse models than in human skin. In summary, the penetration and metabolism patterns of topical retinoids were quite similar in the two mouse models used, indicating that the Franz cells appear to be a good model to predict in vivo metabolism of topical retinoids. When applying this concept to our results obtained in Franz cells with human skin, we conclude that topical retinol and retinal load human skin with both storage and functional vitamin A.

A phase 1 study of tazarotene in adults with advanced cancer

Tazarotene is an acetylenic retinoid which is metabolised to tazarotenic acid and which binds selectively to the retinoid receptors RARbeta and RARgamma. The safety, toxicity and pharmacokinetics of oral tazarotene were determined over 12 weeks of treatment in 34 patients with advanced cancer. Commonly seen toxicities were mucocutaneous symptoms, musculoskeletal pain and headache. Dose-limiting toxicities were hypercalcaemia, hypertriglyceridaemia and musculoskeletal pain. The maximum tolerated dose of tazarotene in this schedule is 25.2 mg day(-1). Plasma concentrations of tazarotenic acid were found to peak rapidly within 1-3 h of dosing and thereafter declined quickly. The C(max) and AUC values on day 0, and weeks 2 and 4 were similar indicating no drug accumulation. The dose-normalised C(max) and AUC values at different dose levels and different study days appeared to be similar indicating linear pharmacokinetics. No objective responses were seen, although stable disease was seen in six out of eight evaluable patients receiving the three highest dose levels of tazarotene (16.8, 25.2 or 33.4 mg day(-1)). We conclude that oral tazarotene is well tolerated when administered daily for 12 weeks, has a favourable toxicity profile compared with other retinoids and merits further investigation as an anticancer therapy.

Retinoids as a potential treatment for experimental puromycin-induced nephrosis

1 Puromycin aminonucleoside (PAN)-induced nephrosis is a model of human minimal change disease. In rats, PAN induces nephrotic-range proteinuria, renal epithelial cell (podocyte) damage, infiltration of mononuclear leukocytes, and apoptosis of several renal cell types. 2 Retinoic acid (RA) modulates a wide range of biological processes, such as inflammation and apoptosis. Since renal damage by PAN is characterized by inflammatory infiltration and epithelial cell death, the effect of treatment with all-trans RA (tRA) was examined in the PAN nephrosis model and in the cultured differentiated podocyte. 3 Treatment with tRA 4 days after PAN injection did not inhibit the proteinuria peak but reversed it significantly. However, treatment with tRA both before and 2 days after the injection of PAN protected the glomerular epithelial cells, diminishing the cellular edema and diffuseness of the foot process effacement. Preservation of the podocyte architecture correlated with the inhibition of proteinuria. The anti-inflammatory effect of tRA was evidenced by the inhibition of PAN-induced interstitial mononuclear cell infiltration and the decreased renal expression of two molecules involved in monocyte infiltration: fibronectin and monocyte chemoattractant protein-1. TUNEL assays showed that tRA inhibited the PAN-induced apoptosis of cultured differentiated mouse podocytes. 4 We conclude that tRA treatment may prevent proteinuria by protecting the podocytes from injury and diminishing the interstitial mononuclear infiltrate in the model of PAN nephrosis. Retinoids are a potential new treatment for kidney diseases characterized by proteinuria and mononuclear cell infiltration.

Retinol-binding protein-deficient mice: biochemical basis for impaired vision

We reported previously that mice lacking plasma retinol-binding protein (RBP) are phenotypically normal except that they display impaired vision at the time of weaning. This visual defect is associated with greatly diminished eyecup levels of retinaldehyde and is reversible if the mutants are maintained for several months on a vitamin A-sufficient diet. Here we provide a biochemical basis for the visual phenotype of RBP-deficient mice. This phenotype does not result from inadequate milk total retinol levels since these are not different for RBP-deficient and wild-type mice. The eye, unlike all other tissues that have been examined, takes up dietary retinol very poorly. Moreover, compared to other tissues, the eye displays a strong preference for retinol uptake when retinol is delivered bound to RBP. The poor uptake of dietary retinol by the eye coupled with its marked ability to take up retinol from RBP, we propose, provides a basis for the impaired vision observed in weanling RBP-deficient mice. Further study of the mutants suggests that the impaired vision is reversible because the eyes of mutant mice slowly acquire sufficient retinol from the low levels of retinol present in their circulation either bound to albumin or present in lipoprotein fractions. Thus, the eye is unlike other tissues in the body in that it shows a very marked preference for acquiring retinol needed to support vision from the retinol-RBP complex and is unable to meet adequately its retinol need through uptake of recently absorbed dietary retinol. This provides an explanation for the impaired vision phenotype of RBP-deficient mice.

Induction of apoptosis of human B-CLL and ALL cells by a novel retinoid and its nonretinoidal analog

We have recently described a novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (CD437/AHPN) that induces apoptosis in a number of malignant cell types. We now describe our studies examining the effects of CD437 and a nonretinoidal analog (MM002) on the in vitro proliferation of the ALL-REH cell line, the in vitro and in vivo growth of a novel Epstein-Barr virus-negative (EBV(-)) B-cell chronic lymphocytic leukemia (B-CLL) cell line (WSU-CLL), and primary cultures of human B-CLL and acute lymphoblastic leukemia (ALL) cells. CD437 and MM002 induce apoptosis in both cell lines, as indicated by the activation of caspase-2 and caspase-3, cleavage of poly(adenosine diphosphate-ribose) (poly(ADP-ribose)) polymerase, increase in annexin V binding, and subsequent nuclear fragmentation. CD437-mediated apoptosis was not associated with the modulation of Bcl-2, Bax, or Mcl-1 levels, but was associated with the cleavage of the antiapoptotic protein Bcl-X(L) to a proapoptotic 18-kD form. This cleavage of Bcl-X(L) was dependent on caspase-3 activation since Bcl-X(L) cleavage and apoptosis were inhibited by the caspase-3 inhibitor Z-DVED-fmk. CD437 markedly inhibited the growth of WSU-CLL cells in severe combined immunodeficiency (SCID) mice. Tumor growth inhibition, growth delay, and log cell kill were 85.7%, 21 days, and 2.1, respectively, in the treated mice. Moreover, 1 of the 5 treated mice was tumor-free longer than 150 days and thus was considered cured. Exposure of primary cultures of both B-CLL and ALL cells obtained from patients to CD437 and MM002 resulted in their apoptosis. These results suggest that CD437 and MM002 analogs may have a potential role in the treatment of B-CLL and ALL.

Tazarotene cream for the treatment of facial photodamage: a multicenter, investigator-masked, randomized, vehicle-controlled, parallel comparison of 0.01%, 0.025%, 0.05%, and 0.1% tazarotene creams with 0.05% tretinoin emollient cream applied once daily for 24 weeks

OBJECTIVE

To assess the safety and efficacy of 4 concentrations of tazarotene cream in the treatment of facial photodamage.

DESIGN

Prospective weekly multicenter, investigator-masked, randomized, parallel-group study.

SETTING

University hospitals and clinical research centers.

PATIENTS

Three hundred forty-nine subjects with facial photodamage.

INTERVENTION

Daily topical application of tazarotene cream (0.01%, 0.025%, 0.05%, and 0.1%) compared with its vehicle and with 0.05% tretinoin emollient cream.

RESULTS

Tazarotene cream and tretinoin cream significantly improved mottled hyperpigmentation and fine wrinkles. At week 24, treatment success rates based on global responses were 67% (39 of 58 subjects) with 0.1% tazarotene, 52% (30 of 58 subjects) with 0.05% tazarotene, 36% (21 of 58 subjects) with 0.025% tazarotene, 41% (24 of 59 subjects) with 0.01% tazarotene, 55% (32 of 58 subjects) with 0.05% tretinoin, and 22% (13 of 58 subjects) with vehicle. Local adverse events, although more frequent with tazarotene at higher concentrations, were generally mild to moderate.

CONCLUSIONS

Tazarotene in a cream formulation is safe and is associated with positive changes in the treatment of photodamaged facial skin.

The role of metabolism and toxicokinetics in retinoid teratogenesis

Retinoids (vitamin A and its analogs) exert profound effects on a wide variety of life processes, including morphogenesis and embryonic development. Several retinoids are also effective drugs for therapy of skin diseases and some types of cancer. However, the applicability of this class of compounds is limited by their teratogenic activity. A major question in retinoid toxicology has been the marked interspecies differences in the lowest teratogenic doses of 13-cis-retinoic acid and retinol. In addition, great attention has been drawn to the risk assessment of embryotoxicity resulting from excessive intake of vitamin A by pregnant women. The present review first gives an overview of the biochemistry, metabolism and mode of action of retinoids as well as their role in embryonic development. It then summarizes the results of recent studies on retinoid metabolism, toxicokinetics, and embryonic exposure and discusses how the available information provides explanation of the aforementioned interspecies variations. Finally, it presents some approaches for risk assessment of high vitamin A intake by humans based on various animal models and epidemiological studies.

Retinoids for the treatment of psoriasis: outlook for the future

Despite the demonstrated clinical success of retinoid therapy in psoriasis, its mechanism of action has not been fully elucidated, and investigators are confronted with two paradoxes. Firstly, the binding of retinoids to nuclear retinoic acid receptors (RARs) does not match their therapeutic efficacy. Secondly, formation of retinoic acid is probably increased in the psoriatic lesions. Answering these questions should result in: (i) the better use of acitretin, an oral synthetic retinoid, and tazarotene, the first compound for topical use; (ii) the development of new retinoids with specific pharmacological profile such as subtype-selective retinoids including molecules with an 'antiretinoid' activity and dissociating antiproliferative retinoids; and (iii) the better characterization of non-genomic effects of retinoids.

The absorption of retinoic acids from the gastrointestinal tract is dependent upon chemical structure

PURPOSE

The gastrointestinal permeability of a number of retinoic acids was determined in order to evaluate whether the gastrointestinal membrane was able to distinguish between retinoids in which the polyene chain was present in several different isomeric forms. In addition, the structure of the six-membered ring was varied in order to determine which portion of the molecule was most important for its recognition by the membrane. The role of bile salt micelle composition in the intestinal absorption of retinoids was also evaluated.

METHODS

In situ perfused rat intestinal segment preparations (= 78) were used, and the retinoids were each perfused at a concentration of approximately 1 microg/ml in either simple micelles of sodium taurocholate (10 mM) or mixed micelles of sodium taurocholate/egg phosphatidylcholine (10 mM/10 mM). The flow rate of the perfusate was either 0.1 or 0.35 ml/min.

RESULTS

For each retinoid, the mixed micelles were associated with a higher degree of retinoid uptake into the jejunal cells than were the simple micelles. In addition, the permeability was higher when the perfusate flow was greater, indicating that the aqueous boundary layer of the intestine contributes to the resistance to the disappearance of the retinoid from the intestinal lumen. Retinoid structure was also found to have a significant effect on the permeability in the mixed micelle systems at both low and high flow rates, but not with simple micelles. The structure of the six-membered ring was not a major determinant of the permeability. However, the permeability of the retinoids with the polyene chain in the 13-cis position was significantly greater than when the chain was all-trans or in the 9-cis position.

CONCLUSIONS

The isomeric position of the polyene chain and the presence of phospholipid in the micellar vehicle have a significant influence on the membrane transport of the retinoic acids.

Synergistic teratogenic effects induced by retinoids in mice by coadministration of a RARalpha- or RARgamma-selective agonist with a RXR-selective agonist

To study the interaction of retinoid-induced limb defects and cleft palate on day 11 of gestation, a RXR-selective agonist (AGN191701, an arylpropenyl-thiophene-carboxylic acid derivative, 20 mg/kg orally) was coadministered with a RARalpha-agonist (Am580, an arylcarboxamidobenzoic acid derivative, 5 mg/kg orally) to NMRI mice. AGN191701 was neither fetotoxic nor teratogenic at the dose used but potentiated Am580-induced limb defects and cleft palate and prevented Am580-induced fetal weight retardation. These results suggest that Am580-induced limb defects and probably cleft palate on day 11 of gestation may be mediated via RARalpha-RXR heterodimerization, particularly in the absence of toxicokinetic interactions. AGN191701 was also coadministered with a RARgamma-agonist (CD437, an adamantyl-hydroxyphenyl naphthoic acid derivative, 15 mg/kg orally) on days 8 and 11 of gestation to investigate which CD437-induced defects are mediated via RARgamma-RXR heterodimerization. On day 8 of gestation, AGN191701 potentiated CD437-induced embryolethality, exencephaly, spina bifida aperta, cleft palate, and tail defects, as well as visceral and skeletal defects, but not micrognathia. On day 11 of gestation, the incidence of CD437-induced cleft palate and limb defects was also potentiated when coadministered with the RXR agonist. These results suggest that synergistic teratogenic effects can be induced by coadministration of two receptor-selective retinoids, indicating the importance of RARalpha-RXR and RARgamma-RXR heterodimers in producing structural defects during organogenesis.

Oral retinoids in the treatment of seborrhoea and acne

Isotretinoin is an extremely effective drug if given systemically in severe forms of seborrhoea and acne, being the only retinoid with potent sebostatic properties. Its unique activity on the sebaceous gland still remains unclear since isotretinoin barely binds to cellular retinoic-acid-binding proteins and to retinoic acid receptors. Its bioavailability is approximately 25% and can be increased by food 1.5-2 times; after 30 min, the drug is detectable in the blood and maximal concentrations are reached 2-4 h after oral intake. The major metabolites of isotretinoin in blood are 4-hydroxy- and 4-oxo-isotretinoin, while several glucuronides are detectable in the bile. 4-Oxo-isotretinoin is present in plasma in a 2- to 4-fold higher concentration 6 h after a single dose. Steady-state concentrations appear after 1 week. The half-life elimination rate of the parent compound ranges from 7 to 37 h while that of some metabolites does so from 11 to 50 h. Isotretinoin crosses the placenta and is recognized as a strong teratogenic compound. About 10-30% of the drug is metabolized via its isomer tretinoin. Excretion of isotretinoin occurs after conjugation with the faeces or after metabolization with the urine. The epidermal levels of isotretinoin are rather low and no progressive accumulation, either in serum or in the skin, is found. After discontinuation of therapy, isotretinoin disappears from serum and skin within 2-4 weeks. Isotretinoin is the most effective drug in reducing sebaceous gland size (up to 90%) by decreasing proliferation of basal sebocytes, suppressing sebum production and inhibiting sebocyte differentiation in vivo. The molecular basis for its antisebotrophic activity has not been fully elucidated. Isotretinoin also exhibits anti-inflammatory activities. Systemic isotretinoin is today the regimen of choice in severe seborrhoea, since it reduces sebocyte lipid synthesis by 75% with daily doses as low as 0.1 mg/kg after 4 weeks. Patients who have received oral isotretinoin therapy for seborrhoea do not usually experience a relapse for months or years. In severe acne, a 6- to 12-month treatment with isotretinoin 1 mg/kg/day reduced to 0.5 or 0.2 mg/kg/day according to the response is recommended (cumulative dose of > 120 mg/kg). Contraception is essential during isotretinoin treatment in women of childbearing age 1 month before, during and for 3 months after discontinuation of treatment.

Primary cultures of brain microvessel endothelial cells: a valid and flexible model to study drug transport through the blood-brain barrier in vitro

Studies on drug entry into the brain and permeation of the blood-brain barrier start to gain more and more importance in neuropharmaceutical research in order to develop new drugs for the therapy of central nervous system diseases. Procedures that provide quick access to permeation properties of those drugs with high throughput are difficult to achieve with animal models. Although various useful cell culture models approaching this issue have been described, results are often not comparable among each other unless determined with an equal experimental setup. Reproducibility of cell culture methods as well as corresponding findings gathered with these tools are often impeded due to the lack of details in experimental manuals. Here we present a precise manual for preparation and maintenance of porcine brain microvessel endothelial cells, serving as a culture model of the blood-brain barrier. Furthermore experimental details for blood-brain barrier transport investigations are presented. Validation of this model was carried out by determination of bioelectric properties and permeation experiments using various marker molecules reflecting paracellular and transcellular blood-brain barrier penetration. Results obtained with our model are closely resembling the in vivo-situation although astrocytes are not included. This simplification of the system is one of the major advantages towards robot derived cell cultures necessary for high throughput screening.

Distribution, teratogenicity, and embryonic delivered dose of retinoid Ro 23-9223

Ro 23-9223 is a highly lipophilic aromatic retinoid with antiproliferative and sebum supressive effects in preclinical disease models of acne. To investigate the relation between Ro 23-9223 developmental toxicity, drug distribution, and transplacental transfer, groups of pregnant hamsters were given oral doses of 50-500 mg/kg Ro 23-9223 on days 8 and 9 of gestation. The teratogenic phenotype induced at doses greater than 125 mg/kg per day was similar to that found after exposure to doses of 13-cis-retinoic acid (isotretinoin, Accutane) greater than 37.5 mg/kg per day. Oral bioavailability of Ro 23-9223 was very low compared to 13-cis-retinoic acid. The highest concentrations of Ro 23-9223 were found in maternal liver, lung, adipose tissue, cardiac muscle, and placenta, whereas only little of the compound crossed the blood-brain barrier. Based on embryo AUC, Ro 23-9223 had a 30- to 50-fold greater embryo:maternal concentration ratio than 13-cis-retinoic acid plus its bioactive metabolites following similar doses of the two retinoids. In preclinical pharmacology studies, oral doses of Ro 23-9223 (5 mg/kg per day) and 13-cis-retinoic acid (10 mg/kg per day) produced comparable gland size reductions in the hamster ear sebaceous gland reduction assay. Under these conditions, Ro 23-9223 plasma AUC was 40 times smaller than that of 13-cis-retinoic acid plus its bioactive metabolites. Assuming that the near linear dose-exposure relationship of Ro 23-9223 extends beyond the dose range of this study, embryo AUCs of Ro 23-9223 and 13-cis-retinoic acid (plus metabolites) would be near identical following pharmacologically equivalent doses. A comparison of embryo retinoid AUCs suggests a 4-fold lower teratogenic potency of Ro 23-9223 compared to with 13-cis-retinoic acid. Despite high embryo levels in hamsters, the data suggest an improved therapeutic index for Ro 23-9223 compared with 13-cis-retinoic acid in a preclinical acne disease model.

Selective agonists of retinoic acid receptors: comparative toxicokinetics and embryonic exposure

Three biologically active synthetic retinoids were investigated that bind selectively to retinoic acid receptors RARs (alpha, beta and gamma). The retinoids were previously demonstrated to have different teratogenic effects in the mouse in terms of potency and regioselectivity. The teratogenic potency rank order (alpha >beta >gamma) was found to be more or less compatible with the receptor binding affinities and transactivation potencies of the retinoid ligands to their respective receptors. The RARalpha agonist (Am580; CD336) induced a wide spectrum of malformations; CD2019 (RARbeta agonist) and especially CD437 (RARgamma agonist) produced more restricted defects. In the current study we tried to address whether the differences in teratogenic effects are solely related to binding affinity and transactivation differences or also due to differences in embryonic exposure. Therefore, transplacental kinetics of the ligands were assessed following administration of a single oral dose of 15 mg/kg of either retinoid given to NMRI mice on day 11 of gestation. Am580 was rapidly transferred to the embryo resulting in the highest embryonic exposure [embryo to maternal plasma area under the time vs concentration curve (AUC)(0-24 h )ratio (E/M) was 1.7], in accordance with its highest teratogenic potency. The low placental transfer of CD2019 (E/M of 0.3) was compatible with its lower teratogenic potential. Of major interest was the finding that the CD437, though being least teratogenic, exhibited considerable embryonic exposure (E/M of 0.6). These findings suggest that both the embryonic exposure and receptor binding transactivation selectivity are crucial determinants of the teratogenicity of these retinoid ligands.

Retinoid metabolism in the prostate: effects of administration of the synthetic retinoid N-(4-hydroxyphenyl)retinamide

We have carried out a series of complementary in vivo and in vitro studies to better understand the metabolism of vitamin A by the prostate gland. Male Sprague-Dawley rats were fed either a control diet sufficient in vitamin A [CON group; 0.8 microg retinol equivalents (RE)/g diet] or a CON diet supplemented with the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR; CON+/-4HPR group; 1,173 )microg of 4-HPR/g diet). After an i.v. injection of a physiological radiolabeled dose of retinol, the vitamin A content and radioactivity of plasma and a number of tissues, including the prostate glands, were monitored for time periods ranging between 30 min and 41 days. On the basis of the results of these vitamin A turnover studies, we developed tissue subsystem models to describe vitamin A dynamics in the prostates of both the CON and CON+4HPR groups. There was a gradual decrease in the vitamin A content of the prostates of the 4-HPR-treated group as compared with the control, such that by the end of the study period, the CON+4HPR group averaged 0.166 +/- 0.0827 (mean +/- SD) REs, whereas the CON group was 0.732 +/- 0.190 REs. The fraction of vitamin A exiting the prostate each day was not significantly different in the CON as compared with the CON+4HPR group [0.149 +/- 0.103 versus 0.155 +/- 0.191 h(-1) (mean +/- FSD), respectively]; however, the average amount of vitamin A turning over from the CON+4HPR group prostates (0.0885 /microg/day) was nearly three times less than that of the CON group (0.243 microg/day). To obtain more detailed information on the mechanisms that might be involved in the changes in vitamin A kinetics observed in our in vivo studies, we used both a normal human prostate cell line (PrEC) and a human prostate adenocarcinoma cell line (LNCaP) to monitor in vitro retinol and 4-HPR dynamics. Cells were treated with 4-HPR for different time periods up to 48 h (PrEC) or 96 h (LNCaP). Retinol in the media was taken up readily by both PrEC and LNCaP cells, and there was conversion of retinol to the major storage esters of vitamin A, retinyl palmitate and retinyl stearate, as well as several minor retinyl esters, in a pattern indicative of normal retinoid esterification activity. Although 4-HPR was taken up readily and over time accumulated in both cell lines, conversion of 4-HPR to its major metabolite, N-[4-methoxyphenyl]retinamide, as well as several other metabolites of 4-HPR was apparent only in the LNCaP cells. Our findings would suggest that a study design that includes appropriately designed complementary in vivo and in vitro experimental systems represents a useful approach to better understanding possible mechanisms involved in basic retinoid functioning and interactions in the prostate as well as in other organs and related tissue culture systems.

Teratology of retinoids

Either an excess or a deficiency of vitamin A and related compounds (retinoids) causes abnormal morphological development (teratogenesis). Potential retinoid sources come from dietary intake, nutritional supplements, and some therapeutic drugs. Therefore, understanding the mechanisms of retinoid teratogenesis is important. This review first gives an overview of the principles of teratology as they apply to retinoid-induced malformations. It then describes relevant aspects of the biochemical pathway and signal transduction of retinoids. The teratogenic activity of various retinoid compounds, the role of the retinoid receptors, and important toxicokinetic parameters in teratogenesis are reviewed.

Phase I and pharmacologic study of the arotinoid Ro 40-8757 in combination with cisplatin and etoposide in patients with non-small cell lung cancer

This phase I study was performed to assess the feasibility of combining cisplatin/etoposide (VP-16) with the arotinoid Ro 40-8757 and to determine the dose-limiting toxicity (DLT) of Ro 40-8757 in this combination. Patients with non-small cell lung cancer were eligible. Treatment consisted of Ro 40-8757 p.o. day 1-21, cisplatin 100 mg/m2 i.v. on day 2 and VP-16 100 mg/m2 i.v. on day 2-4, repeated every 3 weeks. Eighteen patients were evaluable for toxicity and response. The doses of Ro 40-8757 ranged from 84 mg/m2 once daily to 42 mg/m2 thrice daily (tid). DLT consisting of delayed nausea/vomiting was reached at 42 mg/m2 tid. Consequently, the maximum tolerated dose was set at one dose level below the DLT, i.e. 28 mg/m2 tid. Skin toxicity occurred but was well manageable. Pharmacological analyses showed a small increase in the volume of distribution of cisplatin and VP-16 between the first and third course. However, no relationship with side effects was found. A response was achieved in 50% of patients. The combination of cisplatin/VP-16 with Ro 40-8757 appears to be feasible at a dose schedule of 28 mg/m2 tid. The response rate was at the upper rate of what can be expected with cisplatin and VP-16.

An improved low-permeability in vitro-model of the blood-brain barrier: transport studies on retinoids, sucrose, haloperidol, caffeine and mannitol

Primary cultures of porcine brain capillary endothelial cells grown on collagen coated polycarbonate membranes were used to build up an in vitro-model for the blood-brain barrier. Improved cultivation techniques allowed cell-storage and experiments under serum-free conditions. We employed this model to perform permeability studies in vitro with the radioactively labelled marker substances sucrose, retinoic acid, retinol, haloperidol, caffeine, and mannitol. Permeability values obtained with this blood-brain barrier model (1. 0x10-6 cm/s for sucrose, 6.2x10-6 cm/s for retinoic acid, 4.8x10-6 cm/s for retinol, 49.5x10-6 cm/s for haloperidol, 62.4x10-6 cm/s for caffeine, and 1.8x10-6 cm/s for mannitol) show a good correlation to data which are already known from in vivo-experiments. As judged by the sucrose permeability our blood-brain barrier model is less permeable than numerous other models published so far. Therefore it represents a powerful tool for in vitro-prediction of blood-brain barrier permeability of drugs and offers the possibility to scan a large quantity of drugs for their potential to enter the brain.

Metabolism to a response pathway selective retinoid ligand during axial pattern formation

We report identification of 9-cis-4-oxo-retinoic acid (9-cis-4-oxo-RA) as an in vivo retinoid metabolite in Xenopus embryos. 9-Cis-4-oxo-RA bound receptors (RARs) alpha, beta, and gamma as well as retinoid X receptors (RXRs) alpha, beta, and gamma in vitro. However, this retinoid displayed differential RXR activation depending on the response pathway used. Although it failed to activate RXRs in RXR homodimers, it activated RXRs and RARs synergistically in RAR-RXR heterodimers. 9-Cis-4-oxo-RA thus acted as a dimer-specific agonist. Considering that RAR-RXR heterodimers are major functional units involved in transducing retinoid signals during embryogenesis and that 9-cis-4-oxo-RA displayed high potency for modulating axial pattern formation in Xenopus, metabolism to 9-cis-4-oxo-RA may provide a mechanism to target retinoid action to this and other RAR-RXR heterodimer-mediated processes.

[Synthetic peptide as retinoid vector and antiproliferative agent]

First part: Structure, conformational behaviour and vectorization properties of a peptide (PFNLS) designed by association of a fusion peptide and a nuclear localization sequence is described. Tryptophan fluorescence quenching measurements show that ten peptide molecules bind one all trans retinol or all trans retinoic acid molecule with a strong affinity (Kd' = 40 nM). And is able to help the internalization of all-trans retinol in human fibroblasts. Stoichiometry, structure and affinity of the binding can be compared with those of cellular retinoid binding proteins (CRBP), the structure of which is an antiparallel beta barrel. Second part: Cytotoxic properties of the amphiphilic synthetic peptide are presented. Comparative analysis of proliferating, differentiated and confluent H9C2 adherent cells shows a correlation between toxicity and cell cycle stage (proliferating cells). Electrophysiological measurements on Xenopus laevi oocytes bathed in the peptide also demonstrate the induction of cationic currents, which are voltage dependent. These results allow us to hypothesize that the observed toxicity is related to membrane hyperpolarization of proliferating cells at the G1/S cell cycle phase transition. An important point is that in the case of the "peptide-retinoid" complex, no cytotoxicity is observed.

Future trends: a new generation of retinoids

The existence of several different types of retinoid receptors, response elements, and cofactors means that retinoid physiology is mediated by multiple discrete pathways and is highly complex. As a result, non-selective retinoids have a multitude of physiologic effects and are usually associated with toxicity problems that limit their therapeutic usefulness. In contrast, because receptor-selective retinoids have a more focused and targeted action, they are likely to have a better therapeutic index. Tazarotene is the first of a new generation of receptor-selective retinoids. Its actions are targeted on 2 retinoic acid receptors (RARs), RAR-beta and RAR-gamma. Current retinoid research is leading to the development not only of many more novel receptor-selective retinoids but also of novel types of function-selective retinoids such as RAR inverse agonists and RAR antagonists. These retinoids are expected to be of clinical benefit not only in dermatology but also in oncology, diabetes, and diseases associated with the human papilloma virus.

Selective activation of an apoptotic retinoid precursor in macrophage cell lines

Advances in the understanding of the retinoid signaling mechanism has allowed the discovery of highly selective retinoids that activate only one specific receptor class, subtype, or signaling pathway. These novel compounds lack certain of the common retinoid toxicities and therefore suggest promising new approaches for therapeutic applications. We describe here a new compound, 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid methyl ester (MX84), that is selectively activated in macrophages, leading to killing of only macrophage monocyte type cells in vitro. We provide evidence that MX84 is an inactive precursor that is converted into an active apoptosis-inducing retinoid in macrophages. The macrophage activity is also secreted, and our data suggest that the secreted activity is a phospholipase D type activity. Our observation may lead to the development of molecules that are highly macrophage-selective apoptosis inducers in vivo and that could represent important novel therapeutics against diseases caused by excessive macrophage activity.

Clinical safety of tazarotene in the treatment of plaque psoriasis

Oral retinoids are effective in the treatment of psoriasis, but their use is limited by concerns for teratogenic potential and systemic side effects. Tazarotene is a novel acetylenic retinoid undergoing clinical trials for the topical treatment of mild-to-moderate plaque psoriasis. The safety and tolerability of tazarotene 0.1% and 0.05% gels were examined in a series of preclinical and clinical trials. In preclinical studies topically applied tazarotene gel was nonmutagenic, noncarcinogenic, and nonteratogenic. Tazarotene gel was not sensitizing, phototoxic, or photosensitizing in a series of studies in human volunteers. Treatment-related systemic adverse effects were not observed in clinical trials involving approximately 2000 patients treated with tazarotene 0.1% or 0.05% gel for periods of up to 1 year. Adverse effects appear limited to manageable, mainly mild-to-moderate local skin irritation.

Studies on the metabolism and disposition of the new retinoid 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl] benzoic acid. 4th communication: absorption, metabolism, excretion and plasma protein binding in various animals and man

4-[(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl]benzoic acid (CAS 94497-51-5, Am-80) is a new synthetic retinoid which has been shown to have a potent topical antipsoriatic activity. Pharmaco-kinetic profiles of Am-80 were studied in dogs, mice and rabbits after percutaneous or subcutaneous administration of 14C-Am-80. Plasma protein binding of 14C-Am-80 was also studied in rats, dogs and humans. After topical application of 14C-labeled Am-80 by occlusive dressing technique at a dose of 1 mg 14C-Am-80/1,000 mg ointment/kg, the blood and plasma levels of radioactivity were below the detection limit in normal-skin dogs. In normal skin mice and rabbits, the plasma radioactivity peaked at 8 h (40.8 ng eq./ml) and at 12 h (34.0 ng eq./ml) after application, respectively. Percutaneous absorption of 14C-Am-80 was less than 2% of the dose for dogs, 34% for mice and 23% for rabbits. After subcutaneous administration at a dose of 1 mg/kg to mice, dogs and rabbits, plasma levels of radioactivity peaked at 1, 4 and 4 h after dosing with a concentration of 614.0, 902.9 and 757.7 ng eq./ml and then it declined with half-lives of 2.4, 7.2 and 4.1 h, respectively. Urinary and fecal excretion of radioactivity after subcutaneous administration at a dose of 1 mg/kg was 3.5 and 94.7% of the dose in dogs, 27.0 and 73.2% in mice and 43.5 and 45.6% in rabbits. A possible gastrointestinal secretion, which might lead to excretion into feces, was suggested from the results with bile-duct-cannulated dogs. Unchanged Am-80 was present in high amounts in the plasma and bile or feces of all animal species tested except in rat bile, in which Am-80 was predominantly detected in the form of its taurine conjugate (M-6). Hydroxylation of Am-80 to yield 7-hydroxy-Am-80 (M-4) and 6-hydroxy-Am-80 (M-3), which lead to the formation of 6-oxo-Am-80 (M-5), were commonly observed in all animal species. Taurine conjugation reaction of unchanged Am-80 and hydroxy-Am-80 (to form M-6 and both M-1 and M-2, respectively) was distinct in rats and dogs, but, hardly detected in mice and rabbits. The presence of tetrahydro-tetramethyl-naphtylamine (TTNA) was most marked in mice, followed by rabbits and rats, but it was almost absent in dogs. HPLC-RIA analysis of human samples obtained from the phase II and phase III clinical trials of Am-80 ointment suggested that fecal excretion was the major elimination route, and that hydroxylation and taurine conjugation reaction of unchanged and hydroxy-Am-80 also occurred. Unchanged Am-80 was predominant in human plasma as compared with metabolites M-1 to M-6. In vitro binding of 14C-Am-80 to the plasma protein was found to be more than 99% in rats, dogs and humans. In vivo plasma protein binding of 14C-Am-80 and/or its radioactive metabolites was also found to be more than 98% in rats and dogs after subcutaneous administration of 14C-Am-80. In both dogs and humans, in vitro. 14C-Am-80 appeared to be bound predominantly to serum albumin. The binding of 14C-Am-80 to human serum albumin was scarcely affected in the presence of diazepam, digitoxin or warfarin, indicating that there are no specific binding sites for Am-80 on serum albumin.

Current use and future potential role of retinoids in dermatology

Since their introduction 15 years ago, retinoids have been increasingly used for topical and systemic treatment of psoriasis and other hyperkeratotic and parakeratotic skin disorders, keratotic genodermatoses, severe acne and acne-related dermatoses, and also for therapy and/or chemoprevention of skin cancer and other neoplasia. Oxidative metabolites of vitamin A (retinol) are natural retinoids present at low levels in the peripheral blood. Synthetic retinoids are classified into 3 generations including nonaromatic, monoaromatic and polyaromatic compounds. They are detectable in plasma 30-60 minutes after systemic administration, and reach maximum concentrations 2 to 4 hours later. Elimination half-life is 10 to 20 hours for isotretinoin, 80 to 175 days for etretinate and 2 to 4 days for, trans-acitretin; the latter, however, partially converts into etretinate. Retinoid concentrations in skin are rather low in contrast to subcutaneous fat tissue. Intracellularly, retinoids interact with cytosolic proteins and specific nuclear receptors. Two classes of nuclear receptors have been suggested to mediate retinoid activity at the molecular level, RARs and RXRs. The expression of retinoid receptors is tissue specific; skin mainly espresses RAR gamma and RXR alpha. Retinoids affect epidermal cell growth and differentiation as well as sebaceous gland activity and exhibit immunomodulatory and anti-inflammatory properties. Current retinoid research targets the development of receptor-selective retinoids for tailoring and/or improving their therapeutic profile. Currently, tretinoin is used systemically for acute promyelocytic leukaemia, etretinate and acitretin for psoriasis and related disorders, as well as other disorders of keratinisation and isotretinoin for seborrhoea, severe acne, rosacea and acneiform dermatoses. Systemic retinoids are also applied for chemoprevention of epithelial skin cancer and cutaneous T cell lymphoma. The major adverse effect of retinoids is teratogenicity; all other adverse effects are dose-dependent and controllable. Contraception is, therefore, essential during retinoid treatment in women of child-bearing age. Clinical monitoring requires physical examination for adverse effects every 3 to 4 weeks and proper laboratory investigations, also including analysis of retinoid bioavailability in selected cases. Topical retinoids are rapidly developing at present and seem promising for the future; their clinical application includes acne, aging, photodamage, precanceroses, skin cancer and disorders of skin pigmentation. The development of receptor-specific retinoids for topical treatment of psoriasis and/or acne may lead to interesting new compounds based on our current concepts of retinoid function.

Studies on the metabolism and disposition of the new retinoid 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl]benzoic acid. 5th communication: factors affecting percutaneous absorption in rats

4-[(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl] benzoic acid (CAS 94497-51-5, Am-80) is a new synthetic retinoid which has been shown to have a potent topical antipsoriatic activity. Factors affecting the percutaneous absorption of Am-80 were studied with the intention of obtaining information for toxicity and clinical investigations. The percutaneous absorption of radioactivity was compared after topical application of 0.1% 14C-Am-80 ointment to female rats by simple application (SA), occlusive dressing technique (ODT), and application using lint (AUL). After single topical application to normal skin female rats, the percutaneous absorption of radioactivity was very low with no significant differences in the extent of absorption among the three application methods. In the stripped skin female rats, a distinct increase of the percutaneous absorption was observed indicating that it was markedly affected by the lack of the stratum corneum. Compared with the single dosing, a considerable increase of percutaneous absorption was observed following repetitive topical application once daily for 4 or 7 days to the normal skin female rats. The extent of increase was highest in ODT rats followed by SA rats, but was relatively low in AUL rats. The effects of concentration, dose and application area of 14C-Am-80 ointment on the percutaneous absorption of radioactivity were studied following topical application of 0.002%-0.008% 14C-Am-80 ointment to normal skin male rats by ODT to areas of 72 cm2/kg-360 cm2/kg (5%-25% of the body surface area) at ointment doses of 2g/kg-8 g/kg. When the application area and the amount of ointment applied were fixed at 144 cm2/kg (10% of the body surface area) and 2 g/kg, respectively, the amount of radioactivity absorbed increased in proportion to the 14C-Am-80 concentration in the ointment, whereas the rate of percutaneous absorption, expressed as the percent of dose, was nearly constant. When the 14C-Am-80 concentration in the ointment and the amount of ointment applied were fixed at 0.08% and 2 g/kg, respectively, both the amount of radioactivity absorbed and the rate of percutaneous absorption markedly increased with an increase in the application area. When the concentration of 14C-Am-80 in the ointment was set at 0.008% and the application area at 72 cm2/kg, 144 cm2/kg or 288 cm2/kg (5%, 10% or 20% of the body surface area), the amount of radioactivity absorbed increased as the amount of ointment applied increased for areas of the same, though the rate of percutaneous absorption remained almost constant. When the 14C-Am-80 concentration in the ointment was fixed at 0.008%, the amount of radioactivity absorbed increased markedly about 20-fold with 4-fold simultaneous increases in both the application area (from 72 cm2/kg to 288 cm2/kg) and the amount of ointment applied (from 2 g/kg to 8 g/kg).

Molecular epidemiology and retinoid chemoprevention of head and neck cancer

Head and neck cancer is a major worldwide health problem; it has been estimated that approximately 900,000 people were diagnosed with this disease in 1995. Patients are generally treated with surgery and/or radiation therapy. Treatment, especially of patients with early stage (I or II) head and neck squamous cell carcinoma, is often successful. A serious concern, however, is the fact that these patients subsequently develop second primary tumors at an annual rate of 4%-7%. Molecular analyses of premalignant and malignant tissues have produced strong evidence that clonal genetic alterations occur during the early stage of aerodigestive tract carcinogenesis. Although the roles of tobacco and diet in head and neck carcinogenesis have been the subjects of epidemiologic investigations for many years, it has only recently become possible to integrate information regarding genetic susceptibility factors into the development of comprehensive risk models for these cancers. The molecular and epidemiologic studies provide the foundation on which clinical trials can be designed to evaluate the role of retinoids and other compounds in the reversal of premalignancy and the prevention of second primary tumors (i.e., in chemoprevention). This translational approach has led to studies of the utility of intermediate end point markers, such as the nuclear retinoic acid receptors, in chemoprevention strategies. Given the rapid advances occurring in this area of research, it may soon be possible to use these biomarkers to identify patients who are most at risk for developing head and neck cancer and who are most likely to benefit from chemopreventive interventions.

Studies on the metabolism and disposition of the new retinoid 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl] benzoic acid. 2nd communication: absorption, distribution and excretion after single and consecutive subcutaneous administration in rats

4-[(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl]benzoic acid (CAS 94497-51-5, Am-80) is a new synthetic retinoid which has been shown to have a potent topical antisporiatic activity. The accumulation characteristics of Am-80 were examined in rats after a single and consecutive subcutaneous administration of 14C-labeled Am-80 once a day for 24 days, at a daily dose of 0.2 mg/kg. As compared with the single administration, Tmax (1-2 h) and Cmax (about 50 ng eq./ml) of the blood radioactivity were not altered markedly after the consecutive administration. During the daily subcutaneous dosing, the blood level of radioactivity at 24 h after each dosing was also very low. These findings suggested that accumulation in the blood was low after long term consecutive administration of Am-80. The plasma levels of total radioactivity and the proportion of unchanged Am-80 to the total plasma radioactivity, being about 80% at 2 h after administration, and plasma elimination half-life of Am-80, being approximately 3 h, appeared to be hardly affected by the consecutive administration. The cumulative excretion of radioactivity at 168 h after the final dosing was 6.7% and 89.1% in the urine and feces, respectively. The radioactivity remaining in the carcass at this time was about 3% of the total dose. The excretion profile was not altered by the consecutive administration. In most tissues, the concentration of radioactivity at 24 h after each dose reached a steady-state within 24 doses. At 2 h after the consecutive administration for 24 days, the highest concentration of radioactivity was found in the liver followed by the adrenal gland. Accumulation and delayed elimination of radioactivity in most tissues, especially in the adrenal gland, fat, skin and epididymis, were evidently observed as predicted from the previous study where 14C-Am-80 was administered at a dose of 1 mg/kg. The profile of accumulation and retention of radioactivity after the consecutive administration may be considered as a common characteristic of retinoids, such as etretinate.

Multiple factors contribute to the toxicity of the aromatic retinoid, TTNPB (Ro 13-7410): binding affinities and disposition

The aromatic retinoid (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1 -propenyl] benzoic acid (TTNPB) is 1000-fold more potent as a teratogen than all trans-retinoic acid (tRA) in several species and in the inhibition of chondrogenesis in the mouse limb bud cell culture. Factors responsible for the potency of TTNPB were investigated including binding to nuclear retinoic acid receptors (RARs and RXRs), cytosolic binding proteins (CRABPs), and metabolic disposition of TTNPB. For competitive binding assays and saturation kinetics, nucleosol or cytosol fractions were obtained from COS-1 cells transfected with cDNAs encoding the appropriate nuclear receptor or binding protein. TTNPB binds to RAR alpha, beta, and gamma with Kds in the nanomolar range; however, these binding affinities are 10-fold less than those of tRA. Although the affinities are high for TTNPB, it is unlikely that the binding affinities to nuclear receptors alone account for the potency of TTNPB. The binding affinities of TTNPB for the CRABPs are significantly lower than those of tRA. TTNPB did not compete with [3H]9-cis RA for binding to RXR alpha, beta, or gamma. Mouse limb bud cell cultures, a well characterized model for retinoid teratogenesis, were used to compare the metabolic disposition of TTNPB and tRA. In the media of limb bud cell cultures treated with either retinoid, the disappearance of TTNPB was significantly slower than that of tRA over 72 hr. Both retinoids reached approximately equal concentrations in cell uptake experiments; however, TTNPB disappeared from the limb bud cell at a significantly slower rate than did tRA. Collectively, these results indicate that high affinity binding to RARs, lower affinity to CRABPs, and resistance to metabolism contribute to the potency of TTNPB.

Studies on the metabolism and disposition of the new retinoid 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl]benzoic acid. 3rd communication: placental transfer and excretion into milk in rats

4-[5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)carbamoyl]benzoic acid (CAS 94497-51-5, Am-80) is a new synthetic retinoid which has been shown to have a potent topical antipsoriatic activity. Placental transfer and excretion into milk after administration of 14C-Am-80 to pregnant or nursing rats were investigated in view of reproductive and developmental toxicity studies. When 14C-Am-80 was administered topically at a dose of 10 mg/kg to normal-skin pregnant rats on the 12th day of pregnancy, plasma radioactivity in the dam and fetus was detected only at low levels. However, at a dose of 1 mg/kg to the stripped-skin pregnant rats, radioactivity levels peaked at 6 h in the maternal plasma (188.7 ng eq./g) and fetus (64.6 ng eq./g) and at a dose of 10 mg/kg, the peak maternal plasma level of radioactivity and the concentration of radioactivity in the fetus up to 24 h after dosing rose about 10-fold in proportion to the increased dose. At both doses, the radioactivity level in the fetus at the peak corresponded to approximately one-third of the maternal plasma level. When 14C-Am-80 was administered subcutaneously at a dose of 1 mg/kg to pregnant rats on the 12th day of pregnancy, radioactivity in the fetus peaked at 4 h after dosing, being about one-fourth of the maternal plasma level at the same time point. Radioactivity in the fetus after subcutaneous administration of 14C-Am-80 at a dose of 1 mg/kg to pregnant rats on the 19th day of pregnancy peaked (156.4 ng eq./g) at 4 h after dosing, corresponding to approximately one-half the maternal plasma level at the same time point, and then decreased gradually. Among the fetal tissues, relatively high radioactivity was found in the liver. Whole-body autoradiography showed that in most tissues in the dam, the distribution pattern of radioactivity was similar to that in the non-pregnant rat. The concentration of radioactivity in the milk after subcutaneous administration of 14C-Am-80 at a dose of 1 mg/kg to lactating rats on the 9th day after delivery peaked at 8 h after dosing, being 94 times greater than that in the plasma. Unchanged Am-80 in the milk was largely recovered after hydrolysis of hexane extracts of the intact milk with lipase, suggesting extensive incorporation of Am-80 into the triglyceride in the milk because of its benzoic acid structure and high lipophilicity. As for radioactive metabolites which have hitherto been identified in rats, only M-6 (taurine conjugate of Am-80) and tetrahydro-tetra-methyl-naphthylamine (TTNA) were detectable in small amounts in the milk.

Biochemical pathways of retinoid transport, metabolism, and signal transduction

Retinoids control the processes of differentiation, proliferation, and apoptosis from the conception of vertebrates through their deaths. It may seem improbable that a few small, simple lipids could contain and communicate such extensive and disparate information as the retinoids, but the permutations possible make this notion plausible. Retinoid actions result from coordinated interplay among more than one hormone and multiple binding proteins, biosynthetic and catabolic enzymes, receptors (acting alone or in combination not only with retinoid but also with other lipophilic hormone receptors), and receptor-response elements. This symphony is conducted in all vertebrates in deliberate spatially and temporally specific patterns. This article reviews the absorption and formation of retinol, the parent and major circulating naturally occurring retinoid; the role of retinol as substrate for biosynthesis of two hormones, i.e., all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA); the integrated operations of retinoid metabolism and the intracellular retinoid-binding proteins, cellular retinol-binding protein (CRBP), and cellular RA-binding protein (CRABP), which protect (retinoids), serve (as substrate), and control (metabolic reactions); and the mechanism of RA and 9cRA action as activators of the transcription factors RA-receptor (RAR) and the retinoid X receptor (RXR). The discussion offers a mechanistic rationale for the pleiotropic actions of retinoids.

Pharmacokinetics of retinoids in women after meal consumption or vitamin A supplementation

These studies were conducted to evaluate the pharmacokinetics of several retinoids after meal consumption or vitamin A supplementation to establish a reference for future assessment of teratogenic risks of retinoid therapeutic agents. In the first study, 36 healthy young female volunteers consumed single meals containing vitamin A amounts ranging from 1,305 to 169,474 IU. In the second study, 24 other female volunteers took vitamin A supplements at a dose level of 5,000, 10,000, or 25,000 IU/day for 60 days. Plasma concentrations of tretinoin, isotretinoin, 4-oxo-tretinoin, and 4-oxo-isotretinoin in samples collected during the studies were analyzed using a high-performance liquid chromatography method with ultraviolet detection. Pharmacokinetic parameters for the retinoids were calculated using model-independent methods. Plasma concentrations of tretinoin were not altered by meal consumption or vitamin A supplementation. Plasma levels of 4-oxo-tretinoin were below the assay detection limit (0.3 ng/mL) in the majority of samples collected throughout the studies. Linear relationships between dose and maximum concentration (Cmax) and dose and area under the concentration-time curve (AUC) for isotretinoin and 4-oxo-isotretinoin were derived from data from the meal study. For the most bioavailable formulation used in the supplement study, daily ingestion of 5,000 IU of vitamin A caused increases of 141 +/- 53% and 171 +/- 77% from baseline in the 24-hour AUCs of isotretinoin and 4-oxo-isotretinoin, respectively. Dose-related increases in systemic exposure to retinoids were observed after ingestion of vitamin A by means of a meal or a supplement. Findings from these studies can be used as a basis for future safety evaluations of retinoid compounds.

Fully automated determination of selective retinoic acid receptor ligands in mouse plasma and tissue by reversed-phase liquid chromatography coupled on-line with solid-phase extraction

A fully automated reversed-phase HPLC method was developed for the quantitative assay of three retinoids (Am-580, CD-2019 and CD-437) which selectively activate the retinoic acid receptors RAR alpha, RAR beta and RAR gamma, respectively. Mouse plasma, embryo and maternal tissues were prepared for injection by on-line solid-phase extraction (SPE) and valve-switching techniques. Following automatic injection, the sample was loaded on preconditioned disposable cartridges, cleaned-up and then transferred onto the analytical column to be eluted in the backflush mode, separated by gradient elution and detected by UV, while a new cartridge was concomitantly conditioned. The overall recovery was quantitative allowing for external standardization. The calibration curves were linear in all biological samples tested so far, with a correlation coefficient (r) >0.99. The intra-day precision was < or = 7.8% (n = 5-6) and the inter-day variability was < or = 9.4% (n = 3). The lower limit of detection was 2.5 ng/ml or ng/g for CD-2019 and CD-437, and 5 ng/ml for Am-580 with a S/N ratio of 5 using a sample weight of 25 microliters or mg. The method is now in routine use in our laboratory for the assessment of the pharmacokinetic profiles of these retinoids. The small sample size required, the simple sample preparation and the rapid analysis with high degree of automation make this method convenient for microanalysis of biological samples both in animal and human studies.

Determination of the arotinoid mofarotene in human, rat and dog plasma by high-performance liquid chromatography with automated column switching and ultraviolet detection

A sensitive and specific high-performance liquid chromatographic method was developed and validated for the determination of the third-generation retinoid (arotinoid) mofarotene (Ro 40-8757) in human, rat and dog plasma, using direct injection of deproteinated plasma samples, automated column switching (on-line solid-phase extraction) and ultraviolet detection. Plasma (0.5 ml) was deproteinated by adding ethanol (1 ml) containing the internal standard Ro 42-8659 (200 ng/ml). After centrifugation, 0.9 ml of the supernatant were directly injected onto a precolumn packed with C18 Corasil 37-50 microns. Polar plasma components were washed out from the precolumn using 1% ammonium acetate-acetic acid-acetonitrile (900:9:100, v/v/v). After valve switching, the pre-concentrated compounds were transferred to the analytical column (C18) in the backflush mode, separated by gradient elution and detected at 300 nm. The retention times (total run times) were approximately 15 and 20 min for the internal standard and mofarotene, respectively. The method was linear in the range 10-1000 ng/ml with a limit of quantification of 10 ng/ml. The mean recoveries were 80.4%, 81.7% and 77.8% (range 10-1000 ng/ml) and the inter-assay precision was 2.7% (range 20-1000 ng/ml), 1.5% and 2.0% (both range 100-1000 ng/ml) for human, rat and dog plasma, respectively. Mofarotene was found to be stable in human, rat and dog plasma stored at -20 degrees C for 3 months and 22 degrees C for 24 h. The method was successfully applied to clinical, pharmacokinetic and toxicokinetic studies.

Chemical structure--teratogenicity relationships, toxicokinetics and metabolism in risk assessment of retinoids

Retinoic acid, an oxidative metabolite of vitamin A, is involved in the control of many biological processes including embryonic development and excess as well as deficiency of retinoids has been found to be teratogenic. The effects of retinoids in normal as well as abnormal development may be mediated by two members of retinoid receptors, the RARs and RXRs, which exhibit specific temporal and spatial expression during development. Evidence accumulates that any alteration of this complex retinoid system may be related to teratogenic effects. Here we investigate the influence of toxicokinetic parameters, including aspects of metabolism and placental transfer, on the teratogenic potency of retinoids. It is demonstrated that activation (oxidation of retinoic acids; hydrolysis of glycoconjugates) and deactivation reactions (isomerization from trans- into cis-configuration; beta-glucuronidation) relate to teratogenesis. The beta-glucuronides of retinoic acids show poor placental transfer and prolonged presence in the maternal organism. Non-retinoid compounds such as antiepileptic agents may exert some of their teratogenicity via alteration of endogenous retinoid levels.

Retinoid absorption from simple and mixed micelles in the rat intestine

The absorption of three retinoid analogs etretinate (ET), acitretin (ETA), and motretinid (MOE) from two distinct micellar systems was studied in the rat intestine. Each of the three drugs was loaded into simple micelles consisting of 10 mM sodium taurocholate (NaTC) and mixed micelles consisting of 10 mM egg phosphatidylcholine (PC) and 10 mM NaTC. Following perfusion through the jejunum segments, both the fraction of drug disappearing from the segment and the permeability of the drug from the lumen into the gut wall (Peff) was greater with the mixed micelles as compared to the simple micelles. Perfusion flow rate had an influence on the Peff for ET and ETA. Similar trends as for the jejunum were seen in the ileum perfusions. The simultaneous uptake of PC and NaTC during the retinoid perfusions was monitored. There appeared to be a correlation between the Peff values for PC and that of the retinoids. The viability of the in-situ perfusion system was confirmed histologically. There is evidence to indicate that the permeability of the intestine is sensitive to subtle differences in the chemical structure of the retinoids.

Metabolism of mofarotene in hepatocytes and liver microsomes from different species. Comparison with in vivo data and evaluation of the cytochrome P450 isoenzymes involved in human biotransformation

The arotinoid mofarotene is a novel potent anticancer compound. The metabolic profiles obtained from rat, dog, and human plasma showed a good correlation with the corresponding in vitro profiles observed with liver microsomes and hepatocytes. Interspecies differences in its metabolism were investigated using microsomes prepared from the livers of the mouse, rat, dog, cynomolgus monkey, and humans. These in vitro experiments showed that, both qualitatively and quantitatively, the metabolic profiles obtained with cynomolgus monkey liver samples were similar to those observed with human liver material. However, rat and dog were also confirmed to be suitable species for assessing the safety of mofarotene, and were used in toxicology. The involvement of cytochrome P450 (CYP) in the metabolism of mofarotene was examined with human liver microsomes. CYP3A4 plays a major role in the metabolism, and CYP1A2 might be responsible for a minor pathway. Finally, the potential induction by mofarotene of four major CYP isoenzymes was investigated in rats. These experiments showed that CYP1A1 was clearly induced, whereas a slight induction of CYP3A and CYP2B was observed. Repeated administration of mofarotene had no effect on CYP2E1. These studies with liver microsomes and hepatocytes aided the selection of appropriate species for toxicology, and have provided information that will help to predict potential drug-drug interactions in clinical trials.

Comparative disposition, receptor affinity, and teratogenic activity of sulfon arotinoids

To investigate the relationship between sulfon arotinoid biotransformation and teratogenic activity, the potency of the ethyl (Ro 15-1570) and methyl (Ro 14-9706) arotinoid sulfones and their in vivo disposition in pregnant hamsters were studied. Administration of Ro 15-1570 was teratogenic, but Ro 14-9706 showed no such activity. Total absorbed doses of the ethyl and methyl sulfones (measured as maternal plasma AUC) were very similar. Total delivered dose of Ro 14-9706 to liver and lung was 120-160% that of Ro 15-157, and Ro 14-9706 was transferred in greater amounts to the embryo as well. Placenta AUC for parent sulfon arotinoids was 160-250% that in the embryo. Plasma analyses by HPLC suggested that the ethyl sulfone was oxidized and appeared in maternal plasma as the corresponding sulfinic (Ro 14-9572) and sulfonic (Ro 14-3899) acids, amounting to 10% and 16%, respectively, of the mean maternal ethyl sulfone Cmax value. The concentrations of sulfinic and sulfonic metabolites were always less than the analytical limit of detection in placenta and embryo after maternal ethyl sulfone intubation. Neither the sulfinic nor the sulfonic acid were ever detected in maternal circulation, placenta, or embryo after methyl sulfone intubation. Comparisons of their binding affinities found that neither the ethyl nor the methyl arotinoid sulfone could act as a ligand for cellular retinoic acid-binding protein (CRABP), nor could these compounds bind retinoid nuclear receptors (RAR). Transcriptional activation of RARs was weak and similar for both compounds. The sulfinic and sulfonic acid arotinoids bind and transactivate RARs, and bind CRABP with efficiencies similar to all-trans-retinoic acid. Furthermore, they are active in cultured limb bud chondrocytes. The results suggest that the methyl sulfone (in accord with its lack of activity in cultured limb bud chondrocytes) is of no toxicologic significance in hamster embryo--even after relatively high delivered dose. Teratogenicity of the ethyl sulfone (which shows marked inhibition of chondrogenesis in cultured limb bud) does not appear to depend on measurable concentrations of these sulfinic/sulfonic acid metabolites in the hamster embryo.

Diminished teratogenicity of retinoid X receptor-selective synthetic retinoids

One feature that contraindicates the wide therapeutic use of retinoids is their teratogenicity. Synthetic retinoids are distinguishable from each other on the basis of their partial or exclusive preference in binding and activation of all-trans retinoic acid receptors (RARs) or retinoid X receptors (RXRs). Using mouse embryo limb bud cells in micromass cultures as a bioassay, we examined the inhibitory activities of a number of standard and novel retinoids on chondrogenic cell differentiation. Transient cotransfection of HeLa cells was used to measure the ability of each retinoid to induce transcription of a reporter gene by activating RAR alpha, RAR beta, RAR gamma, or RXR alpha chimeric constructs. All retinoids in this study that activated RARs to any degree in the cotransfection assay also inhibited chondrogenesis in vitro, whereas retinoids that were either specific for RXR or inactive in the cotransfection assay did not. The activity of RAR-selective agonists and the inactivity of RXR-specific agonists in the cotransfection assay correlated well with the relative teratogenicity of six of the representative retinoids studied when orally administered at day 11 to pregnant ICR mice.

Retinoids as teratogens

Vitamin A is a necessary nutrient in the diet. However, excessive doses of retinoids by pregnant women result in teratogenesis. In this chapter, we initially discuss the occurrence and characteristics of fetal malformations associated with maternal ingestion of natural and synthetic retinoids in both experimental animals and humans. We then turn to an examination of the pharmacology of teratogenic retinoids, focusing on structure-function relationships and pharmacokinetics. Finally, we review the current literature on the molecular mechanism of action of teratogenic doses of retinoids and the role of the retinoic acid receptors and other target genes in this process.

A new extrapolation method from animals to man: application to a metabolized compound, mofarotene

Allometric scaling (a technique which uses data obtained in laboratory animals to predict human pharmacokinetics) works well for drugs that are cleared intact, but is less successful with extensively metabolised compounds. This paper describes a new method to improve the accuracy of such projections, by integrating metabolic data obtained in vitro (e.g. with liver microsomes or hepatocytes) into these calculations. The approach was used prospectively, to predict the clearance of mofarotene (Ro 40-8757) in humans from in vivo kinetic data obtained in mouse, rat and dog. This compound was selected to illustrate this approach because it is exclusively eliminated through metabolism. Without the metabolic correction or using empirical correcting factors, the values predicted for man were 2.7 and 0.6 ml/min/kg. This fell outside the range subsequently obtained in healthy volunteers dosed orally with 300 mg of mofarotene (7.5 +/- 4.0 ml/min/kg, n = 12). However, inclusion of the microsomal or hepatocyte data gave values of 5.1 and 4.2 ml/min/kg, respectively, illustrating that the integration of in vitro metabolic data improves the accuracy of kinetic extrapolations. In contrast to the existing empirical techniques, this approach offers a rational basis to predict clearance of metabolized compounds in human.