Teratogenicity of the 13-cis and all-trans-isomers of the aromatic retinoid etretin: correlation to transplacental pharmacokinetics in mice during organogenesis after a single oral dose

Mechanisms regulating toxicant disposition to the embryo during early pregnancy: An interspecies comparison

The dose of toxicant reaching the embryo is a critical determinant of developmental toxicity, and is likely to be a key factor responsible for interspecies variability in response to many test agents. This review compares the mechanisms regulating disposition of toxicants from the maternal circulation to the embryo during organogenesis in humans and the two species used predominantly in regulatory developmental toxicity testing, rats and rabbits. These three species utilize fundamentally different strategies for maternal-embryonic exchange during early pregnancy. Early postimplantation rat embryos rely on the inverted visceral yolk sac placenta, which is in intimate contact with the uterine epithelium and is equipped with an extensive repertoire of transport mechanisms, such as pinocytosis, endocytosis, and specific transporter proteins. Also, the rat yolk sac completely surrounds the embryo, such that the fluid-filled exocoelom survives through most of the period of organogenesis, and can concentrate compounds such as certain weak acids due to pH differences between maternal blood and exocelomic fluid. The early postimplantation rabbit conceptus differs from the rat in that the yolk sac is not closely apposed to the uterus during early organogenesis and does not completely enclose the embryo until relatively later in development (approximately GD13). This suggests that the early rabbit yolk sac might be a relatively inefficient transporter, a conclusion supported by limited data with ethylene glycol and one of its predominant metabolites, glycolic acid, given to GD9 rabbits. In humans, maternal-embryo exchange is thought to occur via the chorioallantoic placenta, although it has recently been conjectured that a supplemental route of transfer could occur via absorption into the yolk sac. Knowledge of the mechanisms underlying species-specific embryonic disposition, factored together with other pharmacokinetic characteristics of the test compound and knowledge of critical periods of susceptibility, can be used on a case-by-case basis to make more accurate extrapolations of test animal data to the human.

Chromatographic and electrophoretic analysis of biomedically important retinoids

The determination of retinol (vitamin A) and its metabolites, as well as synthetic retinoids, in biological samples is a challenging task due to the sensitivity of these compounds to light, heat and oxygen, high protein binding, separation of geometric isomers and determination of low endogenous levels. Numerous procedures for sample preparation have been published for biological fluids and tissues, consisting of solvent extraction, solid-phase extraction (off-line) and HPLC with column switching (on-line solid-phase extraction). The last-mentioned technique has several advantages, including a high degree of automation, no evaporation of extraction solvents, protection from light and higher sensitivity. Due to the favourable UV characteristics of most retinoids, HPLC with UV detection is most often employed, and photodiode array detection is becoming more and more popular. Fluorescence and electrochemical detection have found only a limited field of application, but the use of LC-MS resulted in a few highly sensitive methods. Reconsideration of GC through the use of better deactivated columns and cold on-column injection and evaluation of new promising separation methods, such as supercritical fluid chromatography and capillary electrophoresis, have shown preliminary encouraging results, but appear to reach the required sensitivity only by coupling to MS. Therefore, HPLC with UV detection is still the method of choice for highly sensitive and selective retinoid determination, as well as for high sample throughput and robustness.

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)

Maternal toxicokinetics, metabolism, and embryo exposure following a teratogenic dosing regimen with 13-cis-retinoic acid (isotretinoin) in the cynomolgus monkey

The maternal pharmacokinetics, metabolism, and placental transfer of 13-cis-retinoic acid (isotretinoin) have been determined in the cynomolgus monkey using a dosing regimen which had been previously shown to result in retinoid-specific teratogenic effects [Hummler et al. (1990) Teratology 42:263-272]. The drug (2.5 mg/kg body weight) was administered by nasogastric intubation once a day between gestational days (GD) 16-26, and twice a day between GD 27-31. Maternal plasma kinetics were determined following dosing on GD 26 and GD 31, and placental transfer was studied following the last dose on GD 31. The plasma half life of 13-cis-retinoic acid in the monkey (13.2 h) was comparable to that in the human. The main plasma metabolite in the monkey was the 13-cis-4-oxo-retinoic acid which occurred at levels lower or comparable to those of the administered drug. During multiple dosing, this metabolite accumulated to the same degree as the parent drug. All-trans-retinoic acid was present in maternal plasma in very low concentrations (2% of 13-cis-retinoic acid). The beta-glucuronides of all-trans- and 13-cis-retinoic acid were further minor plasma metabolites. 13-cis-retinoic acid and its 4-oxo-metabolite reached the monkey embryo slowly but extensively during organogenesis and reached 24 h-AUC values of 956 and 590 ng.h/g embryo wet weight, resulting in embryo/maternal plasma concentration ratios of 0.41 and 0.33, respectively. The AUC value of all-trans-retinoic acid (316 ng.h/g) was only raised approximately 40% above the endogenous AUC level (225 ng.h/g); only at two time periods examined were the embryonic all-trans-retinoic acid concentrations above endogenous levels (at 4 h and 8 h; P < 0.01 and < 0.05, respectively; Student's t-test). The beta-glucuronides of all-trans- and 13-cis-retinoic acid were not detected in the embryo. Accumulation of 13-cis-retinoic acid and the 4-oxo-metabolite during the twice-per-day dosing regimen was apparent both in maternal plasma and embryo. An interspecies comparison suggests that the half life as well as the metabolic pattern of 13-cis-retinoic acid in plasma were similar in monkey and human: 13-cis-4-oxo-retinoic acid was the main metabolite in both species and the beta-glucoronides as well as all-trans-retinoic acid were minor metabolites. However, the plasma AUC values of the administered drug and particularly the 4-oxo-metabolite were found to be lower in the monkey as compared to the human.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Methylpyrazole partially ameliorated the teratogenicity of retinol and reduced the metabolic formation of all-trans-retinoic acid in the mouse

Oral administration of retinol (50 mg/kg) to NMRI mice on day 11 of gestation (vaginal plug = day 0) led to the metabolic formation of high quantities of all-trans retinoic acid and all-trans-4-oxoretinoic acid, both known as potent teratogenic agents in the mouse. A 96% reduction of the area under the concentration-versus-time-curve (AUC) of metabolically generated all-trans retinoic acid in maternal plasma, and an 84% decrease in the embryonic AUC were observed when mice had been pretreated with the alcohol dehydrogenase inhibitor 4-methylpyrazole. A similar reduction was observed for the major metabolite of all-trans retinoic acid in the mouse, all-trans-4-oxoretinoic acid. However, 4-methylpyrazole pretreatment decreased the AUC of retinol by 10% in maternal plasma and 15% in embryo. Treatment with retinol alone resulted in 55.6%, 43.9% and 56.0% skeletal anomalies of the forelimbs, hindlimbs and craniofacial structures, respectively. Pretreatment with 4-methylpyrazole lowered the retinol induced skeletal defects to 31.3%, 24.0% and 31.3%, respectively, in the forelimb, hindlimb and craniofacial region. Typical retinoid-induced malformations for gestational day 11, e.g. bent or reduced zeugopod or stylopod elements, or cleft palate, were significantly reduced by 4-methylpyrazole pretreatment but were still detected in significantly higher prevalence than in control mice. These data suggest that the teratogenic activity of a single high dose of vitamin A in mouse is partially but not exclusively dependent on the metabolic activation of retinol to all-trans retinoic acid. Thus it could be hypothesized that retinol is either a proximate teratogen or a coteratogen with all-trans retinoic acid.

Acitretin (Neotigason). A review of pharmacokinetics and teratogenicity and hypothesis on metabolic pathways

Acitretin was introduced as a replacement for etretinate, the ethyl ester of acitretin. Acitretin is eliminated at a much faster rate than etretinate. Although both drugs are teratogens, the replacement was important especially as it allowed for a much shorter post-medication period in which pregnancy should be precluded. Recent findings showed the presence of etretinate in the plasma of acitretin-treated patients. This article gives a review of known metabolic pathways of the retinoids and tries to elucidate the possible conversion of acitretin into etretinate after acitretin ingestion.

Acitretin. A review of its pharmacology and therapeutic use

Acitretin (etretin), a second generation monoaromatic retinoid for use in the treatment of severe psoriasis and other dermatoses, is the major active metabolite of etretinate and possesses a similar therapeutic index; i.e. a similar ratio of clinical efficacy to adverse effects. When used alone at a maintenance dosage of 30 to 50mg daily, acitretin is effective in the treatment of psoriasis, causing a reduction in the severity of scaling, erythema and induration. Efficacy appears to be further enhanced by combination with psoralen-ultraviolet A photochemotherapy (PUVA) or ultraviolet B irradiation (UVB). These combinations reduce the time to lesion clearance and reduce the total radiation dose, improving overall safety. Comparative studies have confirmed the equivalence of acitretin and etrtinate with regard to efficacy and toxicity. Adverse reactions are dose-related and generally typical of hypervitaminosis A. Alopecia and mucocutaneous symptoms such as cheilitis and drying of the mucous membranes are particularly prevalent. Hypertriglyceridaemia and elevation of cholesterol levels also occur. Examination of the pharmacokinetic profile of acitretin reveals its main advantage over etretinate. Acitretin is less lipophilic than etretinate, and its lack of sequestration into 'deep' fatty storage sites is reflected in a comparatively short terminal elimination half-life of 50 to 60 hours, compared with 120 days for etretinate. Due to its teratogenic potential, acitretin is strictly contraindicated in women of childbearing potential unless effective contraceptive measures are employed. Etretinate has been identified in plasma samples of some patients treated with acitretin. Thus, acetretin has an established place in the treatment of keratinising disorders, although its use in women of child-bearing potential must be accompanied by effective contraceptive measures, with a further 2-year contraceptive period after therapy completion.

Use of direct injection precolumn techniques for the high-performance liquid chromatographic determination of the retinoids acitretin and 13-cis-acitretin in plasma

A previously developed highly sensitive high-performance liquid chromatographic method for the determination of retinoids, using direct injection of large plasma volumes, on-line solid-phase extraction and ultraviolet detection, was improved and fully validated for the determination of acitretin and 13-cis-acitretin in plasma samples. The addition of acetonitrile to improve the recovery was performed on-line by a T-piece, avoiding any cis-trans isomerization which could occur when acetonitrile was added prior to storage in the autosampler. About 30 injections could be made onto one precolumn despite the large injection volume (1 ml of plasma containing the internal standard). Full automation was attained by the use of automated precolumn replacement. In addition, forward- and back-flush purging of the precolumn enhanced the longevity of the analytical column. This consisted of three coupled C18 columns of 125 mm length each. The quantification limit was 0.3 ng/ml, using ultraviolet detection at 360 nm, and the mean inter-assay precision was 3.8% for the two compounds.

Inhibition of isotretinoin teratogenicity by acetylsalicylic acid pretreatment in mice

Although isotretinoin (ITR) has been suggested to cause malformations via cytopathic effects on embryonic cells, the molecular mechanisms of ITR cytotoxicity in teratogenesis are not clear. Since ITR undergoes metabolism by prostaglandin synthase to a potentially cytotoxic peroxyl free radical, the possible role of prostaglandin synthase metabolism as a modulator of ITR teratogenicity was evaluated. Craniofacial and limb abnormalities were noted in fetuses on day 18.5 of gestation following administration of ITR to pregnant CD-1 mice in a three dose regimen of 100 mg/kg at 4 hr intervals on day 10.5 of gestation (plug day = day 0.5 of gestation). Mice were also treated with acetylsalicylic acid (ASA), an irreversible inhibitor of the cyclooxygenase component of prostaglandin synthase, at doses of 20 and 60 mg/kg body weight 2 hr prior to each ITR dose. ASA pretreatment of mice receiving ITR treatment showed a dose-dependent decrease in the overall incidence of malformations, number of defects per fetus, and the incidence of specific craniofacial and limb defects. Equivalent doses of ASA given to control mice did not cause malformations or alter the incidence of resorptions. These results demonstrate that ASA is able to ameliorate the teratogenic effects of ITR observed in fetal mice near term and indicate that prostaglandin metabolism could play a mechanistic role in ITR teratogenicity.