Principles of Toxicology & Chronopharmacology
This concluding unit covers two important areas: (1) Toxicology — the study of adverse effects of chemicals on living organisms. It covers types of toxicity (acute, subacute, chronic), special toxicities (genotoxicity, carcinogenicity, teratogenicity, mutagenicity), general principles of poisoning management, and specific treatment of barbiturate, morphine, organophosphorus, and heavy metal (lead, mercury, arsenic) poisoning. (2) Chronopharmacology — the study of how biological rhythms influence drug effects, leading to the concept of chronotherapy (timing drug administration for optimal efficacy and minimal toxicity).
Syllabus & Topics
- 1Toxicology – Definitions & Scope: Toxicology = study of adverse effects of chemical, physical, or biological agents on living organisms. Toxicant/Toxin: any substance that can cause harm. Poison: substance that causes harm at relatively low doses. ‘The dose makes the poison’ (Paracelsus). LD₅₀ (Lethal Dose 50%): dose that kills 50% of test animals — used to measure acute toxicity. TD₅₀ (Toxic Dose 50%): dose that causes toxicity in 50%. Therapeutic Index (TI) = LD₅₀/ED₅₀ — higher TI = safer drug.
- 2Types of Toxicity – Acute: Acute toxicity: adverse effects occurring within 24 hours (usually minutes to hours) of a single exposure or multiple exposures within 24 hours. Examples: paracetamol overdose (hepatotoxicity within 24-72 h), cyanide poisoning (minutes). LD₅₀ determined by acute toxicity studies. Signs: rapid onset, often affects CNS, CVS, respiratory system first. In drug development: acute toxicity studies done in 2 species (rodent + non-rodent) using single doses via intended route.
- 3Types of Toxicity – Subacute & Chronic: Subacute (repeated-dose) toxicity: adverse effects from repeated exposure over 14-90 days. Identifies target organs, dose-response for organ toxicity, NOAEL (No Observed Adverse Effect Level). Studies: 28-day or 90-day in 2 species. Chronic toxicity: adverse effects from continuous exposure over >90 days (often 6-12 months to 2 years). Detects cumulative damage. Important for drugs taken long-term (antihypertensives, antidiabetics). Carcinogenicity studies are part of chronic toxicity (2-year rodent studies).
- 4Special Toxicities: Genotoxicity: damage to DNA (mutations, chromosome breaks, DNA strand breaks). Tests: Ames test (bacterial reverse mutation — Salmonella with histidine deficiency), Micronucleus test (chromosome breaks in bone marrow), Chromosomal aberration test. Carcinogenicity: ability to cause cancer. 2-year rodent bioassay. Teratogenicity: ability to cause birth defects (structural malformations in embryo/fetus). Critical period: organogenesis (1st trimester). Example: Thalidomide (phocomelia). Mutagenicity: ability to cause heritable gene mutations.
- 5General Principles of Poisoning Management: (1) Stabilize: ABC (Airway, Breathing, Circulation). (2) Identify the poison: history, clinical signs, toxicology screen. (3) Decontamination: Skin (remove clothes, wash with water), Eyes (irrigate), GI (gastric lavage if within 1-2 hours, activated charcoal — adsorbs most poisons, whole bowel irrigation for sustained-release drugs). (4) Enhance elimination: Forced diuresis (alkaline diuresis for aspirin, acid drugs), Hemodialysis (for dialyzable poisons — methanol, ethylene glycol, lithium, salicylates). (5) Specific antidote (if available). (6) Supportive care.
- 6Activated Charcoal: ‘Universal antidote’ — adsorbs most drugs/poisons in GI tract → prevents absorption. Given as suspension (50 g in adults). Most effective within 1 hour of ingestion. Repeated doses (multiple-dose activated charcoal — MDAC) for: Theophylline, Carbamazepine, Dapsone, Phenobarbital (enhances elimination by ‘GI dialysis’). NOT effective for: Alcohols (methanol, ethanol, ethylene glycol), Iron, Lithium, Cyanide, Corrosives (acids/alkalis), Hydrocarbons.
- 7Barbiturate Poisoning: Barbiturates: CNS depressants (Phenobarbital — long-acting, Secobarbital — short-acting). Poisoning: CNS depression → drowsiness → coma → respiratory failure → death. Pupils: constricted initially, dilated in severe overdose. Hypothermia, hypotension, bullous skin lesions. Management: (1) ABC — airway management may need intubation. (2) Activated charcoal (if within 1 h). (3) Alkaline diuresis: IV NaHCO₃ → ↑urine pH → ↑ionization of phenobarbital (weak acid, pKa 7.4) → ↑renal excretion (‘ion trapping’). (4) Hemodialysis for severe cases. (5) Supportive: IV fluids, vasopressors for hypotension. NO specific antidote (Flumazenil is for benzodiazepines, NOT barbiturates).
- 8Morphine/Opioid Poisoning: Classic triad: (1) Pinpoint pupils (miosis — due to parasympathetic stimulation of Edinger-Westphal nucleus). (2) Respiratory depression (depressed medullary respiratory center — THE cause of death). (3) Coma/unconsciousness. Additional: hypotension, bradycardia, hypothermia, pulmonary edema. Management: (1) ABC — bag-valve-mask ventilation or intubation. (2) SPECIFIC ANTIDOTE: Naloxone (pure μ-opioid receptor antagonist). IV 0.4-2 mg, repeat every 2-3 min until respiration improves. Short t½ (~1 h) — may need continuous infusion or repeated doses (morphine t½ longer). (3) Activated charcoal if oral ingestion within 1 h. (4) Monitor for re-sedation.
- 9Organophosphorus (OP) Poisoning: OPs: organophosphate insecticides (Malathion, Parathion, Chlorpyrifos) and nerve agents (Sarin). MOA: irreversibly inhibit acetylcholinesterase (AChE) → accumulation of acetylcholine at ALL cholinergic synapses. Clinical features (cholinergic crisis): DUMBELS (Diarrhea, Urination, Miosis, Bronchospasm/Bradycardia, Emesis, Lacrimation, Salivation) + Sweating + Skeletal muscle fasciculations → paralysis → respiratory failure → death. Garlic/kerosene-like odor.
- 10OP Poisoning – Treatment: (1) Decontamination: remove clothes, wash skin thoroughly (wear gloves!). (2) Atropine: competitive muscarinic receptor antagonist. Blocks muscarinic effects (bronchoconstriction, bradycardia, secretions). Large IV doses (2-5 mg initially, double every 5-10 min until atropinization — dry mouth, tachycardia, pupil dilation). Does NOT reverse nicotinic effects (muscle fasciculations/paralysis). (3) Pralidoxime (2-PAM): AChE reactivator — removes OP from the enzyme active site (cleaves the OP-enzyme bond). Must be given within 24-48 hours BEFORE ‘aging’ (permanent bond). (4) Diazepam: for OP-induced seizures. (5) Supportive: ventilation, atropine infusion.
- 11Lead Poisoning: Sources: old paint (pre-1978), contaminated water (lead pipes), industrial exposure, Ayurvedic medicines (sometimes contain lead). Acute: abdominal pain (lead colic), constipation, vomiting. Chronic: CNS effects (encephalopathy in children — developmental delay, IQ reduction; adults — peripheral neuropathy with ‘wrist drop’), Hematological (basophilic stippling of RBCs, sideroblastic anemia — lead inhibits δ-ALA dehydratase and ferrochelatase in heme synthesis), Renal (lead nephropathy, gout), Lead line (Burton’s line — blue-black line on gums). Treatment: Chelation therapy — CaNa₂EDTA (IV, forms soluble chelate excreted in urine), Succimer/DMSA (oral, for children), D-penicillamine (oral, milder cases).
- 12Mercury Poisoning: Forms: Elemental (inhaled vapor — thermometers, dental amalgam), Inorganic (mercuric chloride — corrosive), Organic (methylmercury — fish contamination → Minamata disease). Acute inorganic: corrosive GI injury, renal failure. Chronic: CNS effects (erethism — tremor, insomnia, memory loss, personality changes → ‘Mad Hatter’s disease’), Oral (stomatitis, gingivitis, metallic taste), Renal (nephrotic syndrome). Management: Chelation — Dimercaprol (BAL — British Anti-Lewisite, IM for acute/severe), Succimer (DMSA, oral), Penicillamine. For methylmercury: Succimer preferred.
- 13Arsenic Poisoning: Sources: contaminated groundwater (West Bengal, Bangladesh — chronic), pesticides, industrial exposure. Acute: severe GI (rice-water diarrhea/cholera-like, garlic breath), cardiovascular collapse, muscle cramps. Chronic: Rain-drop pigmentation (diffuse darkening with hypopigmented spots), keratosis (palms/soles), peripheral neuropathy, Mees’ lines (white transverse bands on nails), hepatomegaly, ↑risk of skin/lung/bladder cancer. Management: Chelation — Dimercaprol (BAL, IM — drug of choice for acute), Succimer (DMSA, oral — for chronic), D-penicillamine. Whole bowel irrigation (if ingested). Supportive: IV fluids, electrolyte correction.
- 14Chronopharmacology – Concepts: Biological rhythms: Circadian (24 h — most important: sleep-wake, BP, cortisol, body temperature), Ultradian (<24 h — hormonal pulsatile release, sleep stages), Infradian (>24 h — menstrual cycle). Biological clock: Suprachiasmatic nucleus (SCN) of hypothalamus — ‘master clock,’ synchronized by light-dark cycle (Zeitgeber). Clock genes: CLOCK, BMAL1, Per, Cry — form transcription-translation feedback loops. Periphery has local oscillators in liver, heart, kidney → influence drug metabolism and response.
- 15Chronotherapy: Timing drug administration according to biological rhythms to maximize efficacy and minimize toxicity. Examples: (1) Asthma: worst at 4 AM (↑bronchoconstriction, ↑inflammation) → sustained-release Theophylline at bedtime. (2) Hypertension: BP surges in early morning (‘morning surge’) → time antihypertensives to cover this period. (3) Corticosteroids: cortisol peaks at 8 AM → give morning dose to mimic physiological pattern → ↓HPA suppression. (4) Cholesterol: HMG-CoA reductase activity peaks at night → short-acting statins (Simvastatin) given at bedtime. (5) Pain: certain pains follow circadian patterns (RA morning stiffness → pre-dose NSAIDs). (6) Cancer: Chronochemotherapy — timing 5-FU infusion to reduce bone marrow toxicity.
Learning Objectives
Exam Prep Questions
Q1. Why Must Pralidoxime Be Given Early in OP Poisoning?
Pralidoxime is used in poisoning caused by Organophosphate compounds. These compounds inhibit Acetylcholinesterase by forming a covalent bond with the enzyme. Initially, this bond is reversible and pralidoxime can cleave the bond, restoring enzyme activity.
However, after a period of time the inhibited enzyme undergoes a process called Enzyme aging in which the organophosphate–enzyme complex loses an alkyl group. This strengthens the covalent bond and makes the inhibition irreversible. Once aging occurs, pralidoxime can no longer reactivate the enzyme. Therefore, pralidoxime must be administered as early as possible—preferably within hours and ideally within 24 hours of exposure. Another antidote, Atropine, can be given at any time because it works by blocking muscarinic receptors rather than restoring the enzyme.
Q2. What Is the Difference Between Naloxone and Naltrexone?
Naloxone and Naltrexone are both competitive antagonists at opioid receptors such as the Mu opioid receptor.
Naloxone has a very rapid onset and a short half-life of about one hour. It is usually administered intravenously, intramuscularly, or as a nasal spray and is used for emergency reversal of acute opioid overdose, especially to treat respiratory depression caused by drugs like Morphine or Heroin. Because its duration of action is short, repeated dosing may be required.
Naltrexone has a longer duration of action and is typically administered orally. It is used for long-term management of opioid or alcohol dependence because it blocks the euphoric effects if opioids are taken. It is not used for acute overdose because oral absorption is slower and it may precipitate withdrawal symptoms in dependent individuals.
Q3. Why Is Alkaline Diuresis Used for Barbiturate Poisoning?
Phenobarbital is a weak acid with a pKa around 7.4. In cases of poisoning, urinary alkalinization using intravenous Sodium bicarbonate raises the urine pH to about 7.5–8.0. According to the Henderson Hasselbalch equation, increasing the pH shifts the drug into its ionized form.
Ionized molecules cannot easily cross the renal tubular membrane and therefore cannot be reabsorbed back into the bloodstream. This phenomenon, known as Ion trapping, increases renal excretion of the drug and accelerates detoxification. The same principle is also used in poisoning caused by weak acids such as Aspirin.
Q4. What Is Chronotherapy?
Chronotherapy refers to the practice of administering medications at specific times of the day to align with the body’s natural biological rhythms. Many physiological processes—including blood pressure, hormone secretion, enzyme activity, and immune function—follow a predictable 24-hour circadian pattern.
By synchronizing drug administration with these rhythms, chronotherapy can maximize therapeutic effectiveness while minimizing adverse effects. For example, Simvastatin is often taken at bedtime because the activity of HMG CoA reductase peaks during the night and early morning. Taking the drug at this time ensures that its peak action coincides with peak cholesterol synthesis, improving lipid-lowering efficacy.