Toxicity of Retinoids in Humans and Animals

The limitations of some European healthcare databases for monitoring the effectiveness of pregnancy prevention programmes as risk minimisation measures

PURPOSE

Pregnancy prevention programmes (PPPs) exist for some medicines known to be highly teratogenic. It is increasingly recognised that the impact of these risk minimisation measures requires periodic evaluation. This study aimed to assess the extent to which some of the data needed to monitor the effectiveness of PPPs may be present in European healthcare databases.

METHODS

An inventory was completed for databases contributing to EUROmediCAT capturing pregnancy and prescription data in Denmark, Norway, the Netherlands, Italy (Tuscany/Emilia Romagna), Wales and the rest of the UK, to determine the extent of data collected that could be used to evaluate the impact of PPPs.

RESULTS

Data availability varied between databases. All databases could be used to identify the frequency and duration of prescriptions to women of childbearing age from primary care, but there were specific issues with availability of data from secondary care and private care. To estimate the frequency of exposed pregnancies, all databases could be linked to pregnancy data, but the accuracy of timing of the start of pregnancy was variable, and data on pregnancies ending in induced abortions were often not available. Data availability on contraception to estimate compliance with contraception requirements was variable and no data were available on pregnancy tests.

CONCLUSION

Current electronic healthcare databases do not contain all the data necessary to fully monitor the effectiveness of PPP implementation, and thus, special data collection measures need to be instituted.

Compliance with pregnancy prevention plan recommendations in 8672 French women of childbearing potential exposed to acitretin

PURPOSE

Acitretin is an oral synthetic aromatic analogue of retinoic acid available in most European countries since 1988. It is mainly used to treat severe psoriasis. Like all systemic retinoids, acitretin is teratogenic. Strict pregnancy prevention is required in women of childbearing potential who use acitretin. This study assessed compliance with Pregnancy Prevention Plan (PPP) recommendations, specifically looking at compliance with pregnancy testing (PT) and pregnancy occurrence.

METHODS

A cohort of 8672 women aged 15-49 years initiating acitretin treatment from 1 January 2007 through 31 December 2013 was identified using French SNIIRAM (administrative claims data) and PMSI (hospitalisations data) databases. Pregnancy tests (PTs) were identified from reimbursed serum βHCG and urine laboratory PTs. To satisfy PT criteria, patients who started treatment had to undergo a PT within 3 days before acitretin was dispensed. Pregnancies were identified by a pregnancy-related hospital stay or an outpatient medical abortion.

RESULTS

A PT was performed in only 12% of women starting treatment and was rarely performed during treatment or during the 24 months following discontinuation of treatment. Compliance with PPP recommendations although poor appeared to be better among private dermatologists, as a PT was performed for 16% of treatment initiations (vs. 4% for general practitioners, p < 0.001). Moreover, 470 pregnancies were reported corresponding to 27 pregnancies per 1000 person-years at risk of teratogenicity.

CONCLUSIONS

This study highlights poor compliance with acitretin PPP recommendations in France. Physicians and pharmacists must more rigorously apply the acitretin PPP recommendations, and patients must be better informed about acitretin's risk of teratogenicity.

Teratogenic effects of retinoic acid and related substances on the early development of the zebrafish (Brachydanio rerio) as assessed by a novel scoring system

The early development of the zebrafish Brachydanio rerio is shown to be suitable as a test system for prescreening drugs suspected to be hazardous to humans. The teratogenic effects of all-trans-retinoic acid (tretinoin) and eight chemically related substances, such as 3,4-didehydro-retinol (vitamin A(2)), 4-oxo-retinoic acid and several cis isomers, are analysed. Zebrafish development offers several advantages as a test system: large amounts of eggs can be collected throughout the year. Because the embryo is translucent, organ development can be monitored in vivo. After 1 day, eyes, heart and blood circulation is visible already, and the development is synchronous up to day 3 after fertilization. The effects of the retinoids can be scored easily, and the substances can be ranked and be compared by their effectiveness. Further, a new scoring method is presented which also allows comparison of the relative strength of the teratogenic effects of the substances on the various organs enabling investigation of the structure-activity relationship. Concentrations of 10(-19)-10(-6)M all-trans-retinoic acid generate malformations such as oedema, brain deformities (anophthalmy, microcephaly and acephaly), duplication of the otic placodes and otoliths, and a shortened and bent tail. The related substances display similar effects but they differ in their effectiveness. With respect to the chemical structure, the order of potency is found to be: acid > aldehyde > alcohol; all-trans 9-cis > 13-cis; tretinoin = 3,4-didehydro > 4-oxo. The same order of potency is reported for mammals. Because of this similarity early zebrafish development appears to be a proper model system for predicting teratogenic effects of drugs in mammals, especially of chemicals and their metabolites, which are able to cross the placental barrier and reach the mammalian embryo in utero.

Communication of medical product risk: how effective is effective enough?

Ever-increasing attention is being paid worldwide to the safety of medical products, and the risks associated with their use. The integral role of risk communication in overall risk management is demonstrated by several recent market withdrawals of drugs, in which a perceived incapability of healthcare systems to manage well-characterised, avoidable risks was a significant factor. With advances in clinical pharmacology, pharmacogenomics and pharmacoepidemiology expanding our knowledge of medical products, effective delivery of the latest safety-related information to health professionals and consumers becomes even more imperative. In this regard, it is important to evaluate whether current modes of risk communication lead to desired changes in relevant behaviours such as prescribing or drug monitoring, particularly in context with which achieved level of effectiveness is deemed acceptable. This is crucial, as there have been product-specific risk communication efforts that achieved a fair degree of success, yet were not seen as effective enough to prevent market withdrawal of the medical product in question. In the service of improving public health through enhanced risk communication, it is essential to critically assess current methods, both as to results achieved (or not), and whether each method is applicable to the various types of risks associated with medical product use. Furthermore, just as combining methods may well improve overall risk communication, there are societal and psychological factors that must be considered in attempting to maximise effectiveness. However, in assessing risk communication effectiveness, the particular benefit- risk relationship of any individual medical product must also be part of the evaluative process.

Histological observations of palatal malformations in rat embryos induced by retinoic acid treatment

Malformations of the palate were induced in white rat embryos following maternal exposure to retinoic acid (tretinoin). Five experimental groups and the controls were treated by the following protocol: Group 1: pregnant rats received 100 mg retinoic acid (RA)/kg b.w. suspended in corn oil on gestational day (GD) 11.5; Group 2: 20 mg RA/kg b.w. from GD 8-12; Group 3: 20 mg RA/kg b.w. from GD 7.5-11.5; Group 4: 100 mg RA/kg b.w. on GD 10-11; Group 5: 100 mg RA/kg b.w. on GD 10 and 12; Group 6 received corn oil vehicle from GD 7-14.5; and Group 6: served as non-injected controls. In all retinoic acid treated groups, varying degrees of clefts with occasional attempts of fusion were noted. The severity and frequency of the malformations were dependent on dosage or gestational day of drug treatment. Our results indicate that RA, even at the lowest dose tested (20 mg/kg b.w.) severely affects the various tissues constituting the embryonic palatal shelves by altering cell interaction and possibly programmed cell death. These events would then result in lack of or inadequate differentiation with subsequent formation of aberrant craniofacial architecture.

Analysis of cyclic and acyclic analogs of retinol, retinoic acid, and retinal by laser desorption ionization-, matrix-assisted laser desorption ionization-mass spectrometry, and UV/Vis spectroscopy

Laser desorption ionization (LDI)- and matrix-assisted laser desorption ionization (MALDI)-mass spectrometry (LDI-MS, MALDI-MS) at 337-nm laser wavelength were used to analyze retinol (vitamin A), retinoic acid, and retinal and their analogs 3-hydroxyretinol, 3-hydroxyretinoic acid, 3-hydroxyretinal, 4-oxoretinol, 4-oxoretinoic acid, 4-oxoretinal, 3,4-didehydroretinol (vitamin A(2)), 3,4-didehydroretinoic acid, 3,4-didehydroretinal, acycloretinol, acycloretinoic acid, and acycloretinal. The compounds exhibit sufficient ionizability which allows to obtain mass spectra by LDI which are similar in quality to those obtained by MALDI. Mass spectra were recorded with a linear time-of-flight (TOF) instrument or a reflectron-type (RETOF) instrument in positive-ion mode. Under the conditions of LDI-MS the compounds form abundant radical molecular ions (M+*), whereas in the MALDI mass spectra abundant protonated molecular ions ([M + H]+) are observed. Characteristic fragment ions provide additional structural information. High-performance liquid chromatography (HPLC) coupled with UV/Vis photodiode detection was used to assist in retinoid characterization. Synthesis of 3-hydroxyretinal, 4-oxoretinal, and acycloretinal was performed by oxidative cleavage of the all-trans-carotenoids of zeaxanthin, canthaxanthin, and lycopene.

Alcohol, vitamin A, and beta-carotene: adverse interactions, including hepatotoxicity and carcinogenicity

Isozymes of alcohol and other dehydrogenases convert ethanol and retinol to their corresponding aldehydes in vitro. In addition, new pathways of retinol metabolism have been described in hepatic microsomes that involve, in part, cytochrome P450s, which can also metabolize various drugs. In view of these overlapping metabolic pathways, it is not surprising that multiple interactions between retinol, ethanol, and other drugs occur. Accordingly, prolonged use of alcohol, drugs, or both, results not only in decreased dietary intake of retinoids and carotenoids, but also accelerates the breakdown of retinol through cross-induction of degradative enzymes. There is also competition between ethanol and retinoic acid precursors. Depletion ensues, with associated hepatic and extrahepatic pathology, including carcinogenesis and contribution to fetal defects. Correction of deficiency through vitamin A supplementation has been advocated. It is, however, complicated by the intrinsic hepatotoxicity of retinol, which is potentiated by concomitant alcohol consumption. By contrast, beta-carotene, a precursor of vitamin A, was considered innocuous until recently, when it was found to also interact with ethanol, which interferes with its conversion to retinol. Furthermore, the combination of beta-carotene with ethanol results in hepatotoxicity. Moreover, in smokers who also consume alcohol, beta-carotene supplementation promotes pulmonary cancer and, possibly, cardiovascular complications. Experimentally, beta-carotene toxicity was exacerbated when administered as part of beadlets. Thus ethanol, while promoting a deficiency of vitamin A also enhances its toxicity as well as that of beta-carotene. This narrowing of the therapeutic window for retinol and beta-carotene must be taken into account when formulating treatments aimed at correcting vitamin A deficiency, especially in drinking populations.

Pharmacokinetics and safety review of tazarotene

The safety profile of tazarotene is superior to that of orally administered retinoids. The limited percutaneous penetration of tazarotene limits its systemic absorption and this, combined with its rapid metabolism in the skin and the plasma to the more water-soluble active metabolite, tazarotenic acid, helps prevent accumulation of the drug in fat containing tissues. Urinary and fecal elimination are virtually complete within 2 to 3 days and 7 days after dosing, respectively. Tazarotene also exhibits no indication of mutagenicity, carcinogenicity, phototoxic potential, photoallergic potential, or contact sensitization.

Promoting effect of large amounts of vitamin A on cell proliferation of thyroid proliferative lesions induced by simultaneous treatment with thiourea

In order to examine modifying effects of simultaneous treatment with large amounts of vitamin A (VA) and thiourea (TU) on the thyroid tumorigenesis in rats, male F344 rats were initiated with N-bis(2-hydroxypropyl)nitrosamine (2800 mg/kg body weight, single s.c. injection), and starting 1 week later received diet containing 0.1% VA (VA group), drinking water containing 0.2% TU (TU group), 0.2% TU + 0.1% VA (TU + VA group) or tap water/basal diet (control group) for 19 weeks. Serum T3 and T4 in the TU and TU + VA groups were significantly decreased as compared to the control group, while serum TSH levels were remarkably increased. The ratios of T3 and T4 decrease and TSH increase in the TU + VA group were remarkably more pronounced than in the TU group. Thyroid neoplastic lesions were only induced in the TU and TU + VA groups. The multiplicity of intracapsular follicular cell proliferative foci in the TU + VA group was significantly increased as compared to the TU group value. Cell proliferation of hypertrophic and subcapsular follicular cells, as well as in hyperplasias, and neoplasias with adenomatous growth pattern was significantly higher in the combined treatment case than after TU alone. In the liver, centrilobular hypertrophy of hepatocytes was seen in the TU and TU + VA groups, this being especially marked in the latter group. In the combined group case the affected cells were strongly positive for GST-P antibody binding. The results of the present study suggest that cell proliferation of thyroid follicular cell proliferative lesions in rats is enhanced by strong TSH stimulation with simultaneous treatment of TU and large amounts of VA.

Maternal nutrition and obstetric outcome

In general, maternal nutritional status in civilized countries is not at risk. However, even a marginal malnutritional state for some (micro)nutrients for the pregnant women can adversely affect obstetrical outcome. From the data available so far, only folic acid supplementation is advised. However, the importance of an adequate iron and zinc status has to be stressed. In addition, women should be warned preconceptionally about excessive intake of vitamins, especially of those products containing large amounts of vitamin A.

Vulvo-vaginal candidiasis associated with acitretin

The aim of this study was to estimate the risk of vulvo-vaginal candidiasis among the users of acitretin. The incidence rate ratio of vulvo-vaginal candidiasis was estimated in a cohort of acitretin users by using prescription sequence analysis. Study subjects were 196 women between 15 and 45 years of age, exposed to acitretin in 1990. Prescriptions of drugs for treatment of vulvo-vaginal candidiasis were used as proxy for the occurrence of vulvo-vaginal candidiasis. Validation of the proxy drug was performed by record linkage of pharmacy and medical records. In addition to the risk estimation by incidence rate ratios a case-crossover study with patient-stratified logistic regression analysis was performed to control for distortion due to individual differences in contribution of exposure time. The positive predictive value of the proxy drug for vulvo-vaginal candidiasis ranged between 57 and 100%, the sensitivity was 87% and the specificity estimated 99%. The crude incidence rate ratio for vulvo-vaginal candidiasis following acitretin exposure was 2.8 (CI95%: 1.1-7.1). The pooled Mantel-Haenszel incidence rate ratio was 3.3 (CI95%: 1.1-9.6) after stratification for accumulated level of exposure. Patient-stratified analysis on the subgroup of cases (n = 15) revealed an odds ratio of 6.5 (CI95%: 2.3-18.2). This study strongly suggests that the higher occurrence of vulvo-vaginal candidiasis during acitretin exposure is attributable to the drug.

The role of pharmacists in the recruitment of a cohort for postmarketing surveillance. A case study of acitretin in The Netherlands

In October 1990, a recall procedure was issued regarding the drug acitretin. The recommended post-therapy contraception period after acitretin therapy was extended from 2 months to 2 years. For a postmarketing surveillance study, we recruited a cohort from the source population of women aged 15-45 years who were exposed to acitretin. Recruitment occurred through dermatologists, and pharmacists plus dispensing general practitioners. We describe the speed of and the response to the recruitment procedures, and the representativeness of the recruited cohort. We also studied whether the individuals who gave informed consent would have preferred to be recruited by either dermatologists or pharmacists, and whether the information obtained from pharmacists and dispensing general practitioners was valid. This study revealed that pharmacists and dispensing general practitioners (drug dispensers) recruited their subjects rapidly, with no or little selection; they attained a 42% response. Dermatologists recruited their subjects slowly and selectively; they attained a 24% response. The majority of women (60%) recruited by dermatologists would have given their informed consent if they would have been recruited by their pharmacists. Drug dispensers are essential contributors to the recruitment of a study population. We do advise that such recruitment for a postmarketing surveillance study occurs by means of a collaboration between pharmacists and physicians.

Teratogenicity of low doses of all-trans retinoic acid in presomite mouse embryos

This study was designed to examine the developmental dose response for all-trans retinoic acid (TRA) administered at presomite stages in mouse embryos. Previous studies using hamsters [Shenefelt (1972) Teratology 5:103-118] have shown that developmental stages corresponding to those present early on gestational day (GD) 7 in mice are most sensitive to retinoid-induced teratogenesis. Our preliminary studies showed that at this treatment time, gavage dosages of 7.5 mg/kg maternal body weight administered to C57B1/6N mice, an inbred strain, resulted in severe craniofacial malformations representing the holoprosencephaly, aprosencephaly spectrum. Additionally, in an outbred mouse strain, CD-1, exencephaly was induced by dosages of 2.5 mg/kg TRA and above. Readily detectable abnormalities of the eyes, including anophthalmia and severe microphthalmia and iridial colobomata, were induced by even lower doses cf TRA in the C57B1/6N strain. Incidences of micro/anophthalamia were 6.7%, 8.1%, 12.9%, and 32.4% at 0, 0.313, 0.625, and 1.25 mg/kg, respectively. The dosages required to induce significant incidences of exencephaly (2.5 mg/kg) and severe ocular abnormalities (1.25 mg/kg) on GD 7 in mice are approximately 50-100-fold less than those that are commonly used to examine the teratogenicity of this compound at later developmental stages in this species. The trend toward an increase in the incidence of severe ocular malformations at the lowest dose examined and the fact that subtle ocular malformations were not taken into account for this study suggest that even lower dosages may be effective.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of heterologously expressed recombinant retinoic acid receptors with natural or synthetic retinoids

The first step in retinoid action is binding to their nuclear receptors. Therefore, characterization of binding characteristics of retinoids is of major importance. Human retinoic acid receptors alpha (hRAR alpha), hRAR beta, and mouse RAR gamma (mRAR gamma) were expressed heterologously in Escherichia coli as a recombinant glutathione S-transferase (GST) fusion protein. The expressed fusion proteins were functional and bound specifically to the all-trans-retinoic acid (RA). The dissociation constants (Kd) for RA were 1.4 nM for GST-hRAR alpha, 1.4 nM for GST-hRAR beta, and 3.3 nM for GST-mRAR gamma, respectively. The fusion proteins were further used for competitive displacement assays to determine the displacement constant (DC50) for other selected retinoids. All-trans-RA and 4-oxo-all-trans-RA have high affinity with all three receptors (DC50 = 0.8-55 nM). The 13-cis RA binds to hRAR alpha with low affinity, but not to other RARs evaluated here. All-trans-N-ethylretinamide, all-trans-retinylacetate, and an ethyl ester of tetrahydronaphthalene derivative had no affinity to any RARs. The hRAR alpha and mRAR gamma receptors did not bind a naphthalene carboxylic acid derivative of RA, but hRAR beta binds this chemical with high affinity. Results indicated that the three recombinant proteins were functional in binding various RA congeners. The affinity and binding data of these retinoids were compared to their observed teratogenic activity.

Inability to detect plasma etretinate and acitretin is a poor predictor of the absence of these teratogens in tissue after stopping acitretin treatment

1. Concentrations of etretinate, acitretin and its main metabolite 13-cis-acitretin were measured in plasma and subcutaneous fat samples from 37 women of childbearing age exposed to acitretin before November 1990. Twenty of the women still used acitretin and 17 had stopped therapy for a period ranging from 1 to 29 months. 2. The prevalences of detectable etretinate concentrations were 45% and 83% in plasma and subcutaneous tissue, respectively, among current acitretin users and 18% and 86% among those who had stopped acitretin therapy. Thus, inability to detect plasma etretinate is a poor predictor of the absence of etretinate in fat. 3. Acitretin and/or etretinate were detectable in fat and in some cases in plasma from women who had ceased acitretin therapy for up to 29 months. 4. We suggest that (cis)-acitretin and etretinate should be monitored in subcutaneous tissue when plasma measurements are negative. The recommended contraception period of 2 years after cessation of acitretin therapy should be reconsidered to avoid the risk of teratogenicity.

Retinoic acid, local cell-cell interactions, and pattern formation in vertebrate limbs

Retinoic acid (RA), a derivative of vitamin A, has remarkable effects on developing and regenerating limbs. These effects include teratogenesis, arising from RA's ability to inhibit growth and pattern formation. They also include pattern duplication, arising as a result of the stimulation of additional growth and pattern formation. In this review we present evidence that the diverse effects of RA are consistent with a singular, underlying explanation. We propose that in all cases exogenously applied RA causes the positional information of pattern formation-competent cells to be reset to a value that is posterior-ventral-proximal with respect to the limb. The diversity of outcomes can be seen as a product of the mode of application of exogenous RA (global versus local) coupled with the unifying concept that growth and pattern formation in both limb development and limb regeneration are controlled by local cell-cell interactions, as formulated in the polar coordinate model. We explore the possibility that the major role of endogenous RA in limb development is in the establishment of the limb field rather than as a diffusible morphogen that specifies graded positional information across the limb as previously proposed. Finally, we interpret the results of the recent finding that RA can turn tail regenerates into limbs, as evidence that intercalary interactions may also be involved in the formation of the primary body axis.

Structure-affinity relationships of retinoids with embryonic cellular retinoic acid-binding protein

Separation and quantitation of cellular retinoic acid-binding protein (CRABP) in embryonic and fetal hamster tissues was accomplished with high-performance size-exclusion chromatography. Binding affinity of 26 retinoids was established by in vitro displacement of high specific activity all-trans-[3H2]retinoic acid from fetal CRABP. The CRABP concentration in presomite-to-early somite (Day 8) hamster embryos was 1.9 pmol/mg cytosolic protein and increased to 7.5 pmol/mg protein in Day 13 fetuses; CRABP concentrations subsequently declined as gestation progressed. CRABP was located primarily in fetal brain and skin (5.8 +/- 0.3 and 2.2 +/- 0.1 pmol/mg protein, respectively), whereas only trace concentrations were found in fetal liver, placenta, and maternal uterus. Retinoids that could displace all-trans-retinoic acid from CRABP had a free acid at the polar terminus (or were carboxylate esters that were readily hydrolyzed to the corresponding free acid) and had a hydrophobic ring at the distal position. The ligand specificity of the CRABP studied here suggests that this protein was analogous to the CRABP I isoform. The in vitro binding affinities of teratogenic retinoids that competed for embryonic CRABP failed to correlate directly with relative teratogenic potency. In some instances, the latter observation can be related to extensive in vivo biotransformation of retinoids to multiple teratogenic metabolites and to retinoid persistence in the embryo. Three analogs containing a free carboxy terminus, SRI 5898-21, SRI 7323-78, and SRI 6153-40, were identified with high teratogenic potency but failed to bind fetal hamster CRABP. The structure-activity and binding data of the analogs studied here indicate that many, if not most, teratogenic retinoids (or their acidic metabolites) bind with embryonic/fetal CRABP, but the present data question the role for CRABP in their teratogenic mechanism of action.

Teratogenicity of all-trans retinoic acid during early embryonic development in the cynomolgus monkey (Macaca fascicularis)

The embryotoxic and teratogenic potential of all-trans retinoic acid was assessed following exposure prior to and during early organogenesis in the cynomolgus monkey (Macaca fascicularis). Sixteen pregnant females were orally administered all-trans retinoic acid (Tretinoin, Hoffmann-La Roche) once daily from GD 10-20 and twice daily from GD 21-24 at three different dosages, 5 (n = 9), 10 (n = 6) and 20 mg/kg (n = 1). Adverse clinical signs resembling hypervitaminosis A were observed in one animal at 5 mg/kg, in three animals at 10 mg/kg, and in the animal treated with 20 mg/kg all-trans retinoic acid. Maternal weight loss was observed in the 10- and 20-mg/kg groups. A dose-dependent increase in embryolethality was observed, with 22% (2/9), 50% (3/6), and 100% (1/1) occurring at 5, 10, and 20 mg/kg, respectively. The majority of embryonic deaths occurred between GD 16 and 20; the incidence of these early losses was higher than in historical and concurrent controls. No malformations, but a single growth-retarded fetus, was observed in the 5-mg/kg group. Craniofacial malformations, consisting of external ear defects, mandibular hypoplasia, cleft palate, and temporal bone abnormalities, were seen in three viable fetuses in the 10-mg/kg group. Skeletal variations were common to the majority (70%, 7/10) of viable fetuses in both dose groups and were increased relative to historical controls (32%, 25/77). Unlike previous studies with 13-cis-retinoic acid during the pre- and early organogenic stages of development (Hummler et al., Teratology 42:263-272, 1990), no thymic hypo- or aplasia or heart anomalies were observed, which may be attributable to the slightly longer 13-cis retinoic acid treatment period, i.e., GD 10-27. However, external ear and temporal bone defects were common to both all-trans and 13-cis retinoic acid. The similarity observed in the malformation syndrome induced by both all-trans and 13-cis retinoic acid in the cynomolgus monkey and 13-cis retinoic acid embryopathy in humans supports this macaque species as a model for further developmental toxicity studies of vitamin A-related compounds.

Reactive oxygen species in the progression of CCl4-induced liver injury

Pretreatment of rats with large doses of vitamin A (retinol) dramatically increased the hepatotoxicity of carbon tetrachloride (CCl4). Experiments were performed to elucidate the mechanism of this potentiation. Hypervitaminosis A was produced by oral administration of retinol, 250,000 IU/kg for seven days. CCl4 was then administered at a dose of 0.15 ml/kg, ip. This large dose of vitamin A did not enhance the biotransformation of CCl4, but did produce a 4-fold increase in CCl4-induced lipid peroxidation, as assessed by ethane exhalation. Because vitamin A has been shown to activate macrophages, it was hypothesized that this increased lipid peroxidation and liver injury resulted from the release of reactive oxygen species from activated Kupffer cells. By using a chemiluminescence assay, an enhanced release of free radicals was detected in Kupffer cells isolated from vitamin A pretreated rats. In addition, Kupffer cells from vitamin A pretreated rats displayed enhanced phagocytic activity in vitro, towards sheep red blood cells. In vivo, vitamin A pretreated rats cleared carbon particles from the blood 2-3 times faster than non-pretreated rats. In vivo administration of superoxide dismutase (SOD) 2 hr after CCl4 exposure did not influence CCl4 toxicity in control rats but did block the enhanced ethane exhalation and also the potentiation of CCl4 liver injury in vitamin A treated rats. Administration of methyl palmitate, an inhibitor of Kupffer cell function, did not inhibit CCl4 toxicity in control rats, but did effectively block enhanced ethane exhalation and potentiation of CCl4 injury in vitamin A treated rats. We conclude that potentiation of CCl4 hepatotoxicity by hypervitaminosis A is mediated in part by reactive oxygen species released from activated Kupffer cells.

Percutaneous retinoid absorption and embryotoxicity

A single application of 17 micrograms/kg or 8.7 mg/kg all-trans-[10,11-3H2]-retinoic acid dissolved in acetone to shaved dorsal hamster skin resulted in rapid absorption and dose-dependent rates of elimination. An equation describing a two-compartment open model with a very brief lag time and first-order uptake and elimination was used to describe the central plasma compartment kinetics. Unchanged all-trans-retinoic acid represented less than or equal to 4% of the total circulating radio-activity. Peak circulating concentrations of parent all-trans-retinoic acid were less than those observed after an equivalent oral dose, but prolonged absorption from the skin appears to contribute to high total bioavailability of topical retinoid. Topical administration to intact skin of up to three consecutive doses of 10.5 mg/kg/d all-trans-retinoic acid or a single 5 mg/kg dose of etretinate (Ro 10-9359) during a critical stage of embryogenesis in hamsters caused erythema and/or dose-dependent epidermal hyperplasia at the site of application, but failed to induce a significant teratogenic response. Topical application of 0.01-1.0 mg/kg arotinoid Ro 13-6298 resulted in dose-dependent mucocutaneous toxicity and an increase in the numbers of dead embryos and malformed offspring. The marked skin toxicity and attenuated concentrations in maternal blood, compared to the oral route, limit the amounts of retinoid that can reach the hamster embryo. It is thus more important to compare the retinoid systemic values (absorbed dose) than it is to compare the oral or topical (applied) dose, when interpreting the results of conventional teratogenicity bioassays. The data suggest that in the human it is skin toxicity that limits the amounts of retinoid that can be applied and subsequently reach the embryo. In the rodent, overt skin toxicity under continued dosing could increase the amounts of retinoid penetrating the skin and reaching the embryo.

Induction of malformations in the cynomolgus monkey with 13-cis retinoic acid

The embryotoxic and teratogenic potential of 13-cis retinoic acid was assessed in the cynomolgus macaque (Macaca fascicularis). A total of 41 animals was orally administered 13-cis retinoic acid in four sequential experiments. In Exp. 1 three dose levels, 2, 10, and 25 mg/kg, were administered on gestational day (GD) 18-28; 5 mg/kg was administered as an equally divided dose twice daily in Exp. 2 and 3 on GD 21-24 and on GD 25-27, respectively; in Exp. 4 the drug was administered at 2.5 mg/kg once daily from GD 10 to 25 and twice daily (2 x 2.5 mg/kg) on GD 26 and 27. Maternal death and toxicity, manifested as reduction in maternal weight and food consumption, and diarrhea, was observed in Exp. 1 in all three dose groups. No significant maternal toxicity was observed in the treatment groups in Exp. 2, 3, and 4 or in the control group. The primary manifestation of developmental toxicity was embryolethality in Exp. 1 and 2. The incidence of embryonic deaths in Exp. 3 was comparable to the historical controls. No malformations in GD 100 fetuses were observed in Exp. 1, 2, and 3. In Exp. 4, five of seven fetuses (71%) had malformations of both external ears, four of seven fetuses (57%) exhibited hypo- or aplasia of the thymus, and two of seven (29%) had malformations (transposition of the great vessels, ventricular septal defect) of the heart. The teratogenic dose for the cynomolgus monkey in the present study was lower than that reported for all other experimental species. Although central nervous system and craniofacial defects were not observed, the incidence of ear and thymus defects was similar to that reported for the human. The cardiovascular defects resembled those reported clinically, but the incidence was lower in the cynomolgus monkey. The similarity in teratogenic sensitivity to humans supports the use of the monkey as a model for developmental toxicity studies of vitamin A-related compounds.

Aberrant differentiation of neuroepithelial cells in developing mouse brains subsequent to retinoic acid exposure in utero

All-trans-retinoic acid induced 2 types of disorganized neuroepithelium, localized and continuous, in the exencephaly of 9-day-old mouse embryos exposed to 60 or 40 mg/kg for 27 to 30 hr in utero. The localized effect appeared as a protuberance in the wall of the telencephalon and thick neural folds in the mesencephalon with the discontinuity of the apical terminal sheet. The continuous disorganization was seen from the olfactory placode to the myelencephalon with rosettes of cells and many dense bodies in the neuroepithelium. Ultrastructurally, cells in the localized disorganizations showed swelling of Golgi complexes, coated vesicles, and rough endoplasmic reticulum resulting in degeneration. The continuous disorganizations consisted of undifferentiated homogeneous cells in which the nuclei exhibited expansion of nucleolar granular portions and coagulated heterochromatin, and cytoplasm showed monosomal dispersion. In both types of disorganized neuroepithelium, junctional complexes were seen focally at the apical side or apical processes of the rosette, with few or no microfilament bundles. A layer of microfilaments at the base of the neuroepithelial cells in controls, just above the basal lamina, was not present in the monosome dispersed cytoplasm. In the neuroepithelium of controls, one phagosome was seen in the perinuclear region in 0.8% of the cells examined, whereas in the experimental neuroepithelium 2 or more phagosomes were seen in a cell, and phagocytosis occurred by pseudopods. These findings suggest that all-trans-retinoic acid induces not only cytotoxicity but also dedifferentiation in the neuroepithelial cells leading to more cell death, which activates the phagocytosis. These lesions in the neuroepithelium may be a cause of exencephaly.

Comparative distribution, pharmacokinetics and placental permeabilities of all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-4-oxo-retinoic acid, retinyl acetate and 9-cis-retinal in hamsters

Pregnant hamsters were given a single oral dose (35 mumol/kg) of all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-4-oxo-retinoic acid, 9-cis-retinal or all-trans-retinyl acetate during the early primitive streak stage of development. The radioactivity associated with the acidic retinoids was distributed to all tissues sampled (including placenta and fetus), with the largest accumulation in the liver and the least accumulation in fat. Radioactivity from 9-cis-retinal or retinyl acetate concentrated in the liver and lung. The all-trans-retinoic acid was oxidized in vivo to all-trans-4-oxo-retinoic acid and isomerized to 13-cis-retinoic acid: 13-cis-retinoic acid was oxidized to 13-cis-4-oxo-retinoic acid and isomerized to all-trans-retinoic acid. No parent 9-cis-retinal or retinyl acetate could be detected in maternal plasma. Plasma concentrations of the parent acidic retinoids reached their maxima within 60 min and then followed exponential decay. Of all the retinoids examined here, 13-cis-retinoic acid showed the largest area under the plasma curve, the slowest clearance and the longest elimination t1/2. Total plasma radioactivity, consisting of unidentified metabolites, remained elevated at 4 days after dosing. Maternal peak circulating concentrations of the parent retinoids, total radioactivity, plasma pharmacokinetic parameters or the total concentrations of residual radioactivity in fetal tissues could not be correlated with the differential teratogenic potencies of these retinoids.

Retinoid-induced hemorrhaging and bone toxicity in rats fed diets deficient in vitamin K

The recent increase in the clinical use of synthetic vitamin A compounds has led to concern of possible side effects. Some of these effects are known to be influenced by dietary levels of vitamin K. We therefore compared the toxic effects of 13-cis-retinoic acid (13cisRA), retinyl acetate (ROAc), and N-(4-hydroxyphenyl)retinamide (4HPR) in male Sprague-Dawley rats maintained on diets containing different levels of vitamin K. Animals were fed either an NIH-07 diet supplemented with menadione (3.1 ppm vitamin K3), an NIH-07 diet not supplemented with menadione, or an AIN-076 purified diet devoid of vitamin K. The retinoids had no effect on prothrombin times of animals fed the supplemented diet. When menadione was omitted from the diet, however, 4HPR-dosed animals had elevated prothrombin times. This effect was observed as early as Day 7 and was accompanied by one confirmed hemorrhagic death. 13cisRA-dosed animals showed no change in prothrombin times. In the high-dose ROAc group, there was a twofold increase in prothrombin times but only after prolonged dosing. In animals fed the NIH-07 diets, 13cisRA and ROAc induced multiple bone fractures at all dose levels. In contrast, 4HPR administered at the highest dose induced only one fracture in one animal. Animals fed the purified diet lost weight faster and diet sooner than those maintained on the other diets. Bone fractures were not observed in these animals because of early deaths resulting from hemorrhaging. For all retinoid-dosed groups maintained on the purified diet, changes in prothrombin times occured as early as 1 week. The order of effect was 4HPR greater than ROAc greater than 13cisRA, with increases in prothrombin times correlating with increases in hemorrhagic deaths. Hence, the degree of retinoid-induced hemorrhage, but not the incidence of bone fractures, was inversely related to vitamin K levels in the diet. 13cisRA and ROAc, but not 4HPR, caused a dose-dependent reduction in plasma osteocalcin, an effect that correlated with retinoid-induced bone effects. In contrast, serum alkaline phosphatase was elevated in animals dosed with 13cisRA or 4HPR but not in those dose with ROAc. For this enzyme, the electrophoretic pattern on agarose gel showed a decrease, compared to controls, in the major isozyme in serum of ROAc-dosed animals. Hence, plasma osteocalcin is a better predictor of retinoid-induced bone effects than serum alkaline phosphatase.

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)

Structure-toxicity relationships of the tetramethylated tetralin and indane analogs of retinoic acid

The teratogenicity of retinoids containing either tetramethylated tetralin (Ro 13-6307 or Ro 13-2389) or tetramethylated indane (Ro 13-4306) ring system substitutions was compared to the teratogenic potency of all-trans-retinoic acid. Single oral doses, administered to Syrian Golden hamsters at 10:00 A.M. on day 8 of gestation, induced a syndrome of malformations identical to that induced by treatment with all-trans-retinoic acid. These retinoids failed to induce signs of maternal hypervitaminosis A at doses associated with a significant teratogenic response. The tetramethylated tetralin retinoids and indane retinoid were 18 and 2.4 times as embryotoxic on a molar basis, respectively, as all-trans-retinoic acid. Introduction of a supplementary ring in the side-chain restricted polyene chain flexibility and maintained the hydrophobic plane of the chain. The present results are consistent with previous studies showing that the presence of or biotransformation to a free acid congener was necessary for retinoid teratogenic activity in hamsters and that increasing conformational restriction of acidic retinoids increased teratogenic potency.

Morphogenesis of isotretinoin-induced microcephaly and micrognathia studied by scanning electron microscopy

Isotretinoin ingestion during the first trimester of human pregnancy can induce malformations of the skull, ears, face, central nervous system, eyes, palate, lungs, circulatory system, limbs, and digits. A single oral dose of isotretinoin on day 8 of gestation in hamsters induces a similar syndrome of congenital malformation. The present study concerned scanning electron microscopic (SEM) observation of embryonic and fetal hamster craniofacial structures at 4, 8, 12, 24, 48, and 72 hr after administration of an oral dose of 50 mg/kg isotretinoin or an equivalent volume of the vehicle. The variability in development among control embryos recovered 4 hr after treatment precluded objective assessment of pathologic change by SEM at very early time points. Craniofacial damage was obvious within 8-12 hr of isotretinoin treatment, and it included hypoplasia of the maxillary and mandibular processes of the first branchial arch, a rudimentary second arch, and apparent collapse of the forebrain. Equivalent fusion between the lateral nasal process and the maxillary process and between the medial nasal process and the maxillary process in treated and control embryos accounts for the very low incidence of cleft lip observed in fetuses. The terminal microstomia was not associated with excessive merging or overgrowth of the first arch components. Hypoplasia of the first arch can account for retinoid-induced macrostomia and microstomia.