teratogenic drugs and teratogenicity tests

Teratogenicity of Drugs &

Teratogenicity Tests

Teratogenicity of

Drugs

• Any substance that can induce or increase the incidence of a congenital malformation

• Recognition of human teratogens offers the opportunity to prevent exposure

• Frequency of congenital malformations in women exposed is greater

• This data is sometimes not available for humans & is not unbiased

• There are clearly species differences between teratogenic effects

• Drugs are classified as to their teratogenic potential, based on anecdotal information or animal studies

• Less than 2% of congenital malformations are caused by drugs or chemicals

• Teratogenic drugs should be avoided either during or prior to conception

• Women to avoid all medications in the first 8 weeks after conception

• Effects of teratogens, during the

developmental period, results in an “all or

none effect.”

Drugs can affect the foetus at 3 stages:

• Fertilization & implantation – conception to 17 days

• Organogenesis- 18 to 55 days of gestation

• Growth & development-56 days onwards

Risk Category of Drugs during Pregnancy:

• Cat A- Adequate studies in pregnant women have failed

to demonstrate a risk to the foetus

• Cat B- Adequate human studies are lacking, but animal

studies demonstrate a risk to foetus

• Cat C- No adequate studies in pregnant women &

animal studies are lacking or have shown an adverse

effect on foetus, but potential benefit may require use

in pregnant women.

• Cat D- There is evidence of human foetal risk,

but the potential benefits from use of the drug

may be acceptable despite the potential risk.

• Cat X- Studies in animals or humans have

demonstrated foetal abnormalities, and

potential risk clearly outweighs possible

benefits.

TERATOGENIC MECHANISMS:

• Folate antagonism

•Neural crest cell disruption

• Endocrine disruption

•Oxidative stress

• Vascular disruption and specific receptor

• Enzyme-mediated teratogenesis

Proven Human Teratogens:Drug AbnormalityThalidomide Phocomelia, multiple defects

Anti-neoplastic drugs Multiple defects, foetal death

Androgens Virilization, esophageal, cardiac defects

Progestins Virilization of female foetus

Stilboestrol Vaginal carcinoma

Tetracyclines Discoloured teeth, bone defects

Warfarin Nose, Eye, Hand defects, Growth retardation

Phenytoin Cleft lip/palate, microcephaly, hypoplastic phalanges

Alcohol Low IQ, Growth Retardation, Foetal alcohol syndrome

ACE inhibitors Hypoplasia of organs, growth retardation, foetal loss

Lithium Foetal goiter, Cardiac abnormality

Antithyroid Drugs

Foetal goiter, hypothyroidism

Indomethacin Premature closure of ductus arterious

Isotretinoin Craniofacial, Heart & CNS defects

Thalidomide: (Thalidomide disaster 1958-61)• Hypnotic agent widely used in Europe in 1959• An estimated 7000 infants born with

thalidomide syndrome or focomelia

• Characteristic features include limb abnormalities

• Other malformations - Absence of the internal

and external ears, hemangiomas, congenital

heart disease & urinary tract malformations

• Critical period of exposure 24 to 36 days

after fertilization

Retinoic acid or Vitamin A derivatives:

• Even at very low doses isotretinoin is potent teratogen• Malformations include

Craniofacial dysmorphisms, Cleft palate, Thymic aplasia Neural tube defects

• Critical period of exposure 2nd to 5th week of gestation.

Anti-neoplastic/chemotherapeutic agents:

• Highly teratogenic inhibit rapidly dividing

cells

• Malformations include cranial defects,

leukopenia and malformed extremities

• Occasionally used in the 3rd trimester when they

are urgently needed to treat the mother.

Anticonvulsants:

• Use of anticonvulsants leads to double the risk

for malformations

• Malformations like cleft lip, cleft palate,

congenital heart disease, neural tube defects,

microcephaly

Anticoagulants :

• Warfarin (Coumarin) has been associated with

Chondrodysplasia punctata

• Other malformations like nasal hypoplasia,

bone stippling seen on radiologic examination,

bilateral optic atrophy and mental retardation

Thyroid and Antithyroid Drugs

• Propylthiouracil (PTU) and methimazole both cross the placenta and may cause some degree of fetal goiter

• Goal of such therapy during pregnancy is to keep the mother slightly hyperthyroid to minimize fetal drug exposure.

Tetracycline:

• Protein synthesis inhibitor

• Readily cross the placenta.

• Brown discoloration of the deciduous teeth,

hypoplasia of the enamel, and inhibition of

bone growth

• Critical period of exposure- 2nd & 3rd trimester

Streptomycin and Kanamycin:

• Associated with congenital deafness

• Ototoxicity was reported with doses as low 1 g streptomycin

• Critical period of exposure- 1st trimester

Androgenic Steroids:

• Androgens may masculinize a developing

female fetus

• Danazol to produce mild clitoral enlargement

and labial fusion

• Critical period of exposure 10 to 12 weeks

after conception

ACE Inhibitor:

• It can cause- fetal hypotensionrenal failureoligohydromniosdeath

• Critical Period of exposure 2nd – 3rd trimester

Lithium:

• Malformations caused are–Hypotonia–Cyanosis– Lethargy

• Critical period of exposure 1st Trimester

Nicotine:

• Constrict blood vessels• This decreases oxygen delivery to the fetus

Alcohol:

• Malformation caused are– Low IQ–Growth Retardation– Foetal alcohol syndrome

Teratogenicity Test:

• Only Mammalian Species are to be used

• Studies are carried in two animal species (rats

& rabbits)

• Drug is given after mating, during the period of

organogenesis

• Foetus is then examined for visceral or skeletal

abnormalities

Testing Protocols:

• Under the guidelines of FDA • Under the guidelines of ICH

(International Conference on Harmonisation)

Test under FDA:

• Multigenerational studies

• Single generational studiesa) Segment I:Evaluation of Fertility and

Reproductive Performanceb) Segment II: Assessment of Developmental

Toxicityc) Segment III: Postnatal Evaluation

Multigenerational Study:

The animals are mated.

Continuous exposure of a rodent species (usually mice)

F1

Exposed shortly after weaning (30–40 days of age)

The effects of the test is monitored through each generation.

The measured parameters : Fertility Litter size Neonatal viability

F2F3

Evaluation of Fertility and Reproductive Performance:

Male rodents are treated for 70 days and nonpregnantfemales for 14 days .Treatment is continued in the females during Mating Pregnancy Lactation

50% of the females are sacrificed and the foetus are examined for presence of malformations. The other 50% areallowed to give birth. After weaning, these offspring are sacrificed and examined

Assessment of Developmental Toxicity:

Treatment of pregnant females only during implantation through organogenesis

One day prior to birth, the animals are sacrificed

Fetuses examined for 1. Viability 2. Bodyweight 3. Presence of malformation

Postnatal Evaluation:

Pregnant animals are treated from the last trimester of pregnancy until weaning.

Evaluation: • Parturition process • Late fetal development • Neonatal survival• Growth • Presence of any malformations

Test under ICH:

• Fertility Assessment

• Postnatal Evaluation and Pregnancy State Susceptibility

• Assessment of Developmental Toxicity

Fertility Assessment:

• 1.• 3.

Males Females

Exposed for four weeks before mating

Two weeks before mating

Evaluation Evaluation

Reproductive organs weight Fertility

Histological analysis Litter size

Sperm count & mobility Viability of conceptus

Postnatal Evaluation and Pregnancy State Susceptibility:

Comparing the degree of toxicity of the non pregnant female to that of the pregnant female

Evaluation : Maternal toxicity Growth Functional development(off springs)

Assessment of Developmental Toxicity:

Pregnant animals are exposed from implantation through organogenesis

The study is conducted using atleast two species, one rodent and one non rodent.

One day prior to birth , the animals are killed & foetus are examined.

Evaluation : Viability Body weight Presence of malformation

Alternative Test Methods:

• Micro mass test ( cells from the limb buds & brains of rat embroys)

• Whole embryo culture test(whole embryos of rats)

• Embryonic stem cell test

Conclusions: Understanding the mechanisms of the induction

of birth defects is key to determine how to prevent these effects

Further increasing the accuracy of experimental animal extrapolation will aid in the interpretation of experimental data in order to more accurately determine the risk of a given compound to elicit birth defects in humans

Thank you