The relationship between dose and its effects on the exposed organism is of high significance in toxicology. Factors that influence chemical toxicity include the dosage, duration of exposure (whether it is acute or chronic), route of exposure, species, age, gender, and environment.

Toxicologists are experts on poisons and poisoning. There is a movement for evidence-based toxicology as part of the larger movement towards evidence-based practices. Toxicology currently contributes to cancer research, since some toxins can be used as drugs for killing tumor cells (e.g., ribosome-inactivating proteins in leukemia treatment).

Classification of Toxicology:

• Descriptive toxicology: Performs toxicity tests to obtain information used to evaluate the risk that exposure to a chemical poses to humans and the environment.
• Mechanistic toxicology: Attempts to determine how chemicals exert deleterious effects on living organisms.
• Regulatory toxicology: Judges whether a drug or other chemical has a low enough risk to justify making it available for its intended purpose.
• Forensic toxicology: Combines analytical chemistry and fundamental toxicology; concerned with the medicolegal aspects of chemicals. Assists in postmortem investigations to establish cause/circumstance of death.
• Clinical toxicology: Focuses on diseases caused by or uniquely associated with toxic substances. Clinical toxicologists treat poisoned patients and develop new diagnostic and treatment techniques.

The major sources of toxicities are:

1. Industrial chemicals / laboratory chemicals
2. Agrochemicals (pesticides, herbicides, germicides, rodenticides, insecticides)
3. Pharmaceutical products
4. Environmental contaminants / pollutants
5. Illegal use of drugs (overdose)
6. Food substances (coloring agents, flavoring agents, chemicals sprayed on food, toxic extraction from containers, etc.)
7. Accidental and occupational exposure to chemicals
8. Radiation (UV, IR, Microwave)
9. Drinking water polluted with Arsenic
10. Naturally occurring poisonous substances (e.g., aflatoxin)
11. Medical diagnosis (X-ray)

The toxicity of a drug depends upon the difference between the maximum safe dose (MSC) and the minimum effective dose (MEC).

• If drug concentration > MEC → therapeutic response.
• If drug concentration >> MEC → more therapeutic effects with longer duration.
• If drug concentration > MSC → toxic effects.

The dose-response relationship illustrates how the magnitude of a biological response increases with increasing dose of a toxin, up to a point of maximum effect or lethality.

Therapeutic Index (TI) is the ratio between the lethal dose and the effective dose, often expressed as:

TI = LD50 / ED50

Where:

• LD50 (Lethal Dose): The calculated dose at which a drug kills 50% of a group of test animals within a specified time.
• ED50 (Effective Dose): The calculated dose at which a desired non-lethal effect is obtained from 50% of a group of test animals within a specified time.

Properties of TI:

• If TI < 1 → drug is more risky/harmful.
• If TI > 1 and larger → drug is safer.
• Helps determine toxicity status. Example: Digoxin has TI < 1 (toxic); Paracetamol has TI > 1 (safer).

Limitations of LD50:

1. Measures mortality but not sublethal toxicity.
2. Varies widely between species; extrapolation to humans is unsafe.
3. Does not include delayed toxic responses leading to death.
4. Measures acute toxicity from a single dose only.
5. Provides only magnitude of toxicity, not mechanism.
6. Requires extrapolation; precise toxicant dose often not found.
7. Requires use of many animals.
8. Less performed nowadays; lacks biochemical/physiological insight.

Risk assessment evaluates the potential hazard of a chemical, considering both its toxicity and the likelihood of exposure. Paracelsus famously noted: "All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy."

It is not possible to simply categorize chemicals as safe or toxic. The real concern is the risk associated with use, which depends on dose, exposure conditions, and harmful effects (direct or via environment). A very toxic compound may pose less risk if exposure is minimal, while a relatively non-toxic compound may be harmful if used improperly or in large quantities.

Toxic effects of drugs can be classified as pharmacological, pathological, and genotoxic (DNA alteration). Incidence and seriousness relate to tissue concentration of the toxic chemical.

• Pharmacological toxicity: e.g., Excessive CNS depression by barbiturates.
• Pathological toxicity: e.g., Hepatic injury by acetaminophen.
• Genotoxic: e.g., Neoplasm produced by nitrogen mustard.

If tissue concentration does not exceed a critical level, effects are usually reversible. Pharmacological effects disappear when drug concentration decreases. Pathological and genotoxic effects may be repaired; if severe, death or long-term damage (e.g., cancer) may occur.

Specific toxic reactions include:

1. Allergic reaction
2. Chemical carcinogenesis
3. Idiosyncratic reactions
4. Interaction between chemicals

Toxicity testing in clinical settings involves hematological and biochemical tests to determine the magnitude and extent of toxicity.

A. Hematological Tests:

• Blood clotting: Prothrombin time and clotting parameters may be abnormal in poisoning with hepatotoxic agents, anticoagulant rodenticides, or certain snake bites.
• Hematocrit: Anemia may result from acute overdose (iron salts, NSAIDs, salicylates) or chronic exposure to heavy metals (arsenic, lead). G6PD deficiency with certain drugs (chloramphenicol, chloroquine) can also cause anemia.
• Leukocyte count: Leukocytosis may occur in acute metabolic acidosis (ethylene glycol, methanol) or secondary to hypostatic pneumonia after prolonged coma.

B. Biochemical Tests:

• Blood glucose: Hypoglycemia (ethanol, iron, paracetamol, salicylates); Hyperglycemia (salbutamol, theophylline overdose).
• Plasma enzymes: Non-specific increases in LDH, AST, ALT occur with shock, coma, convulsions. Specific patterns indicate organ damage (e.g., hepatic enzymes elevated in CCl4, copper, paracetamol poisoning). Depressed plasma cholinesterase indicates organophosphate/carbamate exposure.
• Electrolytes, blood gases, pH: Changed in various poisonings (e.g., acidosis in methanol/ethylene glycol).

Xenoestrogens are exogenous or synthetic substances that mimic estrogen. They are not produced by the body but bind to estrogen receptors (ERs) with varying affinity, disrupting endocrine function.

Examples: Bisphenol A (BPA), phthalates, PBDEs, DDT, diethylstilbestrol (DES).

Major Sources:

1. Oral contraceptives – can cause hepatotoxicity, cardiovascular toxicity, breast/endometrial/cervical cancer.
2. Synthetic hormones (e.g., DES) – used in cattle; causes reproductive issues in offspring.
3. Organochlorinated compounds (DDT, Lindane) – long half-life; accumulate in body; cause lactation failure, breast cancer.
4. Polychlorinated biphenyls (PCBs) – environmental persistence; linked to breast cancer, abortion, low birth weight.
5. Phytoestrogens in food – beans, cabbage, grains, spinach, peas.
6. Pharmaceuticals – post-coital preparations, Tamoxifen, Raloxifene metabolites.
7. Alkyl phenol polyethoxylates (APEs) – in cleaning products, paints, herbicides; enter via water/fish.
8. Industrial pollutants – Dioxins, Furans; antiestrogenic; affect reproduction.
9. Agrochemicals – Herbicides (atrazine), fungicides (vinclozolin) cause reproductive abnormalities.
10. Phthalates – plasticizers; leaching causes miscarriage, cancer.

Foods that may increase xenoestrogen exposure: Apples, berries, grapes, peaches, barley, oats, beer, coffee, olive oil, red wine, tea, almonds, flaxseeds, etc.

Environmental toxicants are chemicals in the environment that cause adverse health effects. Environmental estrogens (xenoestrogens) are a distinct class that mimic or interfere with endogenous estrogens.

Characteristics:

1. May not structurally resemble endogenous reproductive hormones.
2. May or may not resemble each other structurally.
3. Can modulate estrogenic pathways via ERs, AhR, other nuclear receptors.
4. Can induce, interfere, or inhibit estrogen/progesterone/testosterone action.
5. Can induce physiological/pathological responses without endogenous ligands.
6. Almost certainly damage reproductive organs/systems.

Main Classes:

1. Estrogen agonists
2. Antiestrogenic compounds
3. Antiandrogenic compounds

Human Reproductive System & Hormone Functions:

• Male: Testosterone >> Estrogen → spermatogenesis, male organ development, secondary sex characteristics.
• Female: Estrogen >> Progesterone >> Testosterone → ovulation, female organ development/maintenance, secondary sex characteristics, bone/behavior regulation.

Effects of Environmental Estrogens:

On males: Reduced sperm count/quality, cryptorchidism, hypospadias, testicular cancer, erectile dysfunction.

On females: Breast/uterine/ovarian/endometrial cancer, breast size reduction, unwanted hair growth, voice changes, miscarriage, reduced ova efficiency.

Mechanism similar to natural estrogen: Xenoestrogens bind to estrogen receptors (ERα in breast, ovary, brain, liver, bone, CVS, adrenal, testes, urogenital tract; ERβ in kidney, prostate, GIT).

Ligand-bound ER forms homodimers, translocates to nucleus, binds to estrogen-responsive elements on DNA, initiating transcription/translation of target genes.

Effects depend on class:

• Agonists: Stimulate tissue growth in breast/endometrium/ovary → cancer.
• Antagonists: Inhibit gene expression → interfere with menstrual cycle, reduce fertility, cause miscarriage/premature birth.
• Receptor destruction (in males): → infertility, prostate/testicular cancers.

Overall, xenoestrogens have estrogenic or anti-estrogenic properties leading to carcinogenesis, teratogenesis, and other toxic effects.

1. Use safer oral contraceptives.
2. Limit production and ensure safe disposal from industries.
3. Implement worldwide bans to prevent environmental contamination.
4. Government regulation for restricted use of xenoestrogens.