A postulated mechanism of beta-sympathomimetic induction of rib and limb anomalies in rat fetuses

Mechanism of Developmental Effects in Rats Caused by an N-Phenylimide Herbicide: Transient Fetal Anemia and Sequelae during Mid-to-Late Gestation

BACKGROUND

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects [VSDs] and wavy ribs) was produced by an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. Major characteristics of the developmental toxicity included species difference between rats and rabbits, compound-specific difference among structurally similar herbicides, and sensitive period. Protoporphyrin accumulation in treated fetuses closely correlated with the major characteristics. Iron deposits in erythroblastic mitochondria and degeneration of erythroblasts were observed in treated rat fetuses. In this study we investigated fetal anemia and subsequent developmental effects in rats, and inhibition of PPO in rats, rabbits, and humans by the herbicides in vitro.

METHODS

Fetuses were treated on gestational day (GD) 12 and removed on GDs 13 through 20. All litters were examined externally. One half of litters were examined for blood and skeletal development, and the other half for interventricular foramen closure. Effects on PPO were determined in mitochondria from embryos and adult livers.

RESULTS

Fetal anemia in rats was evident on GDs 13 through 16. Subsequently, enlarged heart, delayed closure of the foramen, reduced serum protein, and retarded rib ossification were observed. In vitro PPO inhibition exhibited species- and compound-specific differences corresponding to the developmental toxicity.

CONCLUSION

We propose that developmental toxicity results from PPO inhibition in primitive erythroblasts, causing transient fetal anemia followed by death. Compensatory enlargement of the fetal heart results in failure of interventricular foramen closure and VSD. Reduced serum protein leads to delayed ossification and wavy ribs.

Relationship between bent long bones, bent scapulae, and wavy ribs: malformations or variations?

BACKGROUND

Shortened and bent long bones and bent scapulae are sometimes reported in fetuses with wavy ribs (Carney and Kimmel, ). Wavy ribs are typically seen in the presence of maternal and developmental toxicity, are transient and reversible postnatally, and are considered to be variations rather than malformations.

METHODS

We further assessed the literature cited in Kimmel and Carney () as well as papers published since then to determine under what conditions bent long bones in the absence of gross limb defects and bent scapulae were reported and whether information was available on the transient or permanent nature of these effects.

RESULTS

Long bone and/or scapular changes almost always occurred at a lower incidence than wavy ribs. In every case, maternal and fetal toxicity occurred at the same dose levels. In a few studies, pups were followed sequentially after birth and bent long bones and scapulae were transient in nature and appeared normal by the time of weaning. Rabbits were much less likely to show wavy ribs or long bone and scapular changes at birth, even in the presence of severe maternal and fetal toxicity. This species difference may be due in part to the great increase in bone mass and remodeling that occurs during the first few postnatal weeks in rodents, but which takes place during the longer fetal period in rabbits.

CONCLUSION

Our conclusion from this review is that bent long bones and scapulae, like wavy ribs, appear to be secondary to maternal and developmental toxicity, are transient, and like wavy ribs should be considered variations rather than malformations.

Oral and Dermal Developmental Toxicity Studies of Phenylethyl Alcohol in Rats

Phenylethyl alcohol (PEA) was tested for developmental toxicity. Pregnant rats were fed 0, 83, 266, or 799 mg/kg/d PEA on gestation days (GDs) 6 to 15; only minimal, nonsignificant effects were observed. In dermal studies, PEA (neat) was applied to the skin on GDs 6 to 15 at dosages of 0, 140, 430, or 1400 mg/kg/d and at 0, 70, 140, 280, 430, or 700 mg/kg/d in a corroborative study. Observations included maternal and embryo-fetal toxicity/abnormalities at 1400 mg/kg/d, increased incidences of rudimentary cervical ribs at ≥430 mg/kg/d, and reduced fetal body weights at ≥140 mg/kg/d. Dermal maternal and developmental no-observed-adverse-effect levels are 70 mg/kg/d, based on dermal irritation and reductions (nonsignificant) in fetal body weights. Human exposure from fragrances is 0.02 mg/kg/d, resulting in a margin of safety >2600, when marked differences in dermal absorption between rats and humans are considered. Under normal fragrance use conditions, PEA is not a developmental toxicity hazard for humans.

The preclinical toxicology of salmeterol hydroxynaphthoate

An extensive toxicology programme on salmeterol hydroxynaphthoate (SereventTM), a marketed long-acting β2-adrenoceptor agonist, has been carried out. The studies evaluated both the local (respiratory tract) and systemic tolerance to single and repeated dosing, effects on all stages of reproduction, as well as the genotoxic and oncogenic potential. High acute doses were well tolerated and caused no specific target organ toxicity. In repeat dose studies, animals tolerated salmeterol very well both locally and systemically. No significant effects on the respiratory tract of dogs were seen and only minor laryngeal changes, typical of those occurring with many inhaled medicines, were noted in rats. The high systemic concentrations achieved resulted in a number of changes that are considered to be the result of excessive and prolonged β 2-adrenoceptor stimulation. These included tachycardia, skeletal muscle hypertrophy and minor haematological and blood biochemical changes in general toxicity studies, foetal effects in rabbit organogenesis studies and increased incidences of smooth muscle tumours of the mesovarium in the rat and of the uterus in the mouse oncogenicity studies. Salmeterol showed no evidence of any genotoxic potential. Results of the extensive toxicology programme provide good assurance of the safety for the inhaled use of salmeterol in patients; this has ben confirmed by many years of clinical experience during its development and marketing.

Developmental toxicity study of Aquacoat ECD ethylcellulose aqueous dispersion administered orally to rats

A developmental rat toxicity study of Aquacoat((R)) ECD was performed as part of a program to evaluate the safety of the product. Groups of 25 presumed-pregnant Charles River Sprague-Dawley CD rats received doses of 0, 903, 2709 and 4515mg/kg/day (dry weight basis) of Aquacoat ECD administered undiluted once daily via oral gavage on days 6-15 of gestation. All surviving dams underwent caesarean sectioning on day 20 of gestation. Foetuses were weighed, sacrificed and subject to external, visceral and skeletal evaluations. No test material-related maternal deaths occurred; one high-dose female died on day 14 due to gavage error. The only treatment-related clinical sign noted among dams receiving 2709mg/kg/day and greater was pale faeces which was attributed to the presence of the test material in the faeces. No statistically significant differences were noted among the measured maternal parameters. Foetal sex ratios and body weights were similar in all groups. The results of external and visceral foetal evaluations revealed no treatment-related alterations. The only statistically significant findings noted during the skeletal evaluation were increased litter incidences of incompletely ossified or wavy ribs noted among foetuses receiving 4515mg/kg/day, and a significant increase in the litter incidence of thickened ribs at doses of 2709 and 4515mg/kg/day. Given the nature of these findings and the lack of effects on any other parameter measured in this study, they were not considered adverse effects of treatment. Under the conditions of this study, the maternal and foetal no-observed-adverse-effect level (NOAEL) is in excess of 4515mg/mg/day.

"Wavy ribs". A reversible pathologic finding in rat fetuses

A literature of 74 scientific papers addressing the occurrence of "wavy ribs" in fetuses of small rodents has been collected with the support of a computer-assisted search on MEDLINE, TOXLINE, BIOSIS and EXCERPTA MEDICA systems (1982-1993) and has been critically examined. Numerous compounds of a large variation in chemical structure and biological activity, if given to pregnant rats during the later period of organogenesis, are known to cause "wavy ribs" in their fetuses, a reversible pathologic finding which is rarely observed in untreated controls. According to literature, causative factors, like delayed development of alkaline phosphatase, decreased fetal protein, dietary contents, maternal toxicity, renal loop diuretics, myometrial constriction, endocrine disturbances and beta-adrenoceptor stimulation or blocking have been observed and discussed, but, a general explanation for the lesions is still missing. Therefore, the following hypothesis may be added: the high doses used in reproductive toxicology may induce maternal and placental circulatory disturbances leading to reductions in fetal serum proteins, with delayed development of osteoblast progenitors and low fetal serum alkaline phosphatase in the day 17-20 fetus. Normalization of the neonate's homeostasis (serum protein) leads to enhanced ossification and concomitant bone repair until weaning.

Effects of sodium bicarbonate and ammonium chloride on the incidence of furosemide-induced fetal skeletal anomaly, wavy rib, in rats

Furosemide produces fetal wavy ribs when administered to pregnant rats during late gestation. The compound is also known to produce metabolic alkalosis in laboratory animals and man. In order to evaluate the effect of furosemide on maternal blood pH, Crj:CD(SD) female rats received an oral administration of 150 or 200 mg/kg of furosemide by gavage on day 16 of gestation and were bled at 4 hr post-dose. Compared to an average pH of 7.39 in control females, there was a significant elevation in blood pH in these furosemide-treated females (average pH of 7.44 to 7.48). When 2% sodium bicarbonate was provided in the drinking water for females treated with 150 mg/kg of furosemide, there was a further rise in maternal blood pH (7.52) compared to females treated with furosemide alone. Associated with this elevation in maternal blood pH was a marked increase in the incidence of fetal wavy ribs (87.6% compared to 27.6%). When females treated with 200 mg/kg of furosemide were provided with 0.5% ammonium chloride, furosemide-induced maternal alkalosis was corrected (pH decreased to 7.35) and there was a reduction in the incidence of fetal wavy ribs (7.0% compared to 37.2%). In addition, maternal blood pH among individual females was positively correlated with the incidence of fetal wavy ribs (r = 0.714). These results suggest that maternal metabolic alkalosis is involved in the pathogenesis of furosemide-induced wavy ribs.

Review of drug-induced limb defects in mammals

The objective of this paper was to illustrate the spectrum of possible limb malformations in mammals resulting from drug exposure. A bibliography of 171 papers from 20 journals was generated from which pertinent data (drug used, limb defects reported, predominant defect location) were tabulated. These data should provide a basis for predictions about types of defects that might be expected in further studies and for judging postulated drug-induced human limb defects. However, direct extrapolation to humans is inappropriate. The following trends were observed: 1) Distal limb defects (autopod) are almost twice as common as proximal limb defects (stylopod and zygopod). 2) Ectrodactyly is the single most common type of limb defect, accounting for over half of the autopod defects. 3) Ectrodactyly is almost twice as common in the hindlimb as in the forelimb. 4) Postaxial ectrodactyly is over twice as common as preaxial ectrodactyly in the forelimb, but preaxial ectrodactyly is four times more common in the hindlimbs. 5) Polydactyly occurs with approximately equal frequency in forelimbs and hindlimbs, and preaxial polydactyly is most common in both fore and hindlimbs. 6) Polymelia (supernumerary limbs) occurred in one case, and may have been a spurious result. 7) Either transverse hemimelia is greatly underreported in teratology studies or it essentially does not occur. We have concluded that, at least in some cases, acetazolamide, adenine, 1,7-dimethylxanthine, and xanthine derivative aminophylline, retinoic acid, acetoxy-methyl-methylnitrosamine, aspirin, and cadmium can all cause unilateral defects.

Role of myometrial constriction in the induction of wavy ribs in rat fetuses

Furosemide, a loop diuretic, was orally administered to pregnant Crj: CD (SD) rats at a dose of 300 mg/kg once on day 16 of gestation. Cartilage bone double staining of day 17 fetuses revealed a delay in ossification. Characteristic rib deformities were not seen until day 18 of gestation. Concomitant with this observation were reductions in the amount of amniotic fluid, compared to controls, on day 18 of gestation and thereafter. When isoxsuprine HCl (2 mg/kg, b.i.d.), a uterine muscle relaxant, was additionally injected ip to pregnant rats on days 17-19 of gestation, the incidence of wavy ribs was significantly (P less than or equal to .05) less than that seen after treatment with furosemide alone. These observations indicate that the myometrial constriction plays a significant role in the induction of furosemide-induced wavy ribs.