Pharmacology Lecture: Toxicology and Post-Marketing Surveillance

Introduction to Toxicology and PCL 511 Course Outline

Toxicology is a multidisciplinary science focused on the study of harmful actions of chemical substances on biological material. It specifically examines the toxic or harmful effects observed with chemical agents, including drugs, when administered at excessive doses. As a field, it involves the study of the adverse effects of xenobiotics on living systems, assimilating knowledge and techniques from biochemistry, biology, chemistry, genetics, mathematics, medicine, pharmacology, physiology, and physics. Expert practitioners, known as toxicologists, apply safety evaluations and risk assessments to explore the mechanisms by which chemicals produce adverse effects in biological systems. Modern toxicology has evolved to incorporate safety evaluation and risk assessment into its core methodology.

The course PCL 511, taught by Prof. (Mrs) O.J. Owolabi at Delta State University, Abraka, covers several critical areas. These include the mechanism of substance toxicity, general principles of toxicity management, management of acute poisoning, toxicological evaluation of new drugs, and post-marketing surveillance of drugs. This comprehensive framework aims to give 500-level Pharmacy students an exhaustive understanding of how chemical agents interact with life and how those interactions are managed and monitored.

Historical Foundations of Toxicology

Throughout history, toxicological science has shaped society through its applications in the healing arts, war, agriculture, and policy making. In ancient history, a Greek physician serving the Roman emperor Nero made the first documented attempt to classify plants according to their toxic and therapeutic effects. Significant further developments occurred in the 9th or 10th century when Ibn Wahshiyya authored the Book on Poisons, followed in $1360$ by the Indian text Khagendra Mani Darpana.

Modern toxicology entered a more formal scientific era in $1813$ with Mathieu Orfila, who is considered the father of modern toxicology. He published the first formal treatment of the subject, Traité des poisons, or Toxicologie générale. In $1850$, Jean Stas achieved the first successful isolation of plant poisons from human tissue, which led to the first forensic toxicology conviction. He identified nicotine as the poison used in the Bocarme murder case, leading to the conviction of Belgian Count Hippolyte Visart de Bocarme for the murder of his brother-in-law.

Theophrastus Phillipus Auroleus Bombastus von Hohenheim ($1493$–$1541$), widely known as Paracelsus, is also credited as a founding father of the discipline. He argued his studies were superior to those of the Roman physician Celsus, hence his name. Paracelsus is most famous for the maxim: "Alle Dinge sind Gift und nichts ist ohne Gift; allein die Dosis macht, dass ein Ding kein Gift ist," which translates to: "All things are poisonous and nothing is without poison; only the dose makes a thing not poisonous." This is frequently condensed to the scientific fundamental: "The dose makes the poison."

Modern Scope and Branches of Toxicology

The scope of toxicology is categorized into three principal areas: environmental toxicology (covering pollution, residues, and industrial hygiene), economic toxicology (covering medicines, food, food additives, pesticides, dyestuffs, and industrial chemicals), and forensic toxicology (focusing on intoxication, diagnosis, and therapy). The ultimate goal of the discipline is to contribute knowledge regarding the harmful actions of chemicals, study their mechanisms of action, and estimate risks to humans based on biological testing. Modern toxicology has shifted toward "green chemistry" to minimize negative impacts on humans and the environment, and it increasingly emphasizes alternative, harm-free routes of experimentation to replace traditional animal testing.

Toxicology is divided into numerous specialized branches. Analytical toxicology involves the detection and assay of poisonous chemicals and metabolites. Applied toxicology applies modern technologies for early detection in field settings. Clinical toxicology focuses on the diagnosis and treatment of human poisoning. Veterinary toxicology deals with animal poisoning and the transmission of toxins to humans via animal products like milk, meat, and fish. Environmental toxicology studies the presence and degradation of toxicants in the environment. Forensic toxicology utilizes analytical chemistry, pharmacology, and clinical chemistry to aid medical and legal investigations into death and drug use. Other branches include Medical, Agro, and Computational toxicology, the latter of which uses mathematical and computer-based models to predict adverse effects.

Fundamental Divisions and Acute Toxicity Mechanisms

Toxicology is broadly divided into two domains: Toxicokinetics and Toxicodynamics. Toxicokinetics denotes the absorption, distribution, metabolism, and excretion (ADME) of toxins. Toxicodynamics refers to the study of the toxic doses of therapeutic agents and their metabolites and their effects on the body.

Mechanisms of acute toxicity involve high-dose effects that can lead to death or severe incapacitation. Many effects are reversible if intervention is rapid. Simple asphyxiants, such as inert gases, cause toxicity through anoxia (lack of oxygen). Deprivation of oxygen to the brain leads to unconsciousness in seconds and death if not removed. Delayed rescue can lead to irreversible brain damage due to the death of non-regenerative neurons. Chemical asphyxiants like Carbon Monoxide ($CO$) compete with oxygen for binding sites on hemoglobin in red blood cells, depriving tissues of oxygen for energy metabolism. Survival depends on removal from the source and treatment with oxygen.

Cyanide is another potent chemical asphyxiant that interferes with cellular metabolism and the utilization of oxygen. Treatment involves administering sodium nitrite, which converts hemoglobin to methemoglobin. Because methemoglobin has a higher binding affinity for cyanide than the cellular target does, it sequestered the toxin away from critical sites. Central Nervous System (CNS) depressants, such as various solvents, cause sedation or unconsciousness by interacting with cell membranes in the CNS to impair electrical and chemical signaling. This toxicity is usually reversible upon removal from the exposure source.

Skin and Eye Toxicity Mechanisms

Adverse skin effects range from irritation to corrosion. Strong acids and alkaline solutions are corrosive, causing chemical burns and permanent scarring due to the death of deep dermal cells. Skin sensitization is a specialized mechanism where chemicals like $2,4$-dinitrochlorobenzene bind with skin proteins, causing the immune system to recognize the complex as foreign. This trigger involves the release of cytokines. This reaction mirrors the response to poison ivy and typically requires two exposures: one for sensitization and subsequent ones to trigger the reaction. Treatment involves symptomatic therapy with steroid-containing anti-inflammatory creams or systemic immunosuppressants like prednisone in severe cases.

Eye toxicity ranges from outer layer reddening to cataract formation or iris damage. Mechanisms of skin corrosion often apply to the eye. Materials with a $pH < 2$ (strong acids) or a $pH > 11.5$ (alkalis) are highly corrosive and can cause blindness. Surface-active agents like detergents and surfactants can also cause injury. Cationic (positively charged) surfactants are particularly dangerous, potentially causing burns, permanent corneal opacity, and vascularization. Dinitrophenol specifically causes cataract formation, an example of pharmacokinetic effects where the chemical concentrates within the eye tissue.

Subchronic, Chronic, and Special Toxicity Mechanisms

Subchronic and chronic toxicity occur through repeated exposure to lower doses over time, often producing different mechanisms than single high doses. Alcohol is a primary example: high acute doses affect the CNS, while repeated lower doses cause liver damage. Anticholinesterase inhibition is the primary mechanism for organophosphate pesticides. These are typically activated in the liver and then inhibit acetylcholinesterase ($AChE$), the enzyme responsible for terminating acetylcholine stimulation. Overstimulation of the cholinergic system leads to respiratory arrest and death. Treatment involves Atropine (which blocks acetylcholine effects) and Pralidoxime chloride (which reactivates inhibited $AChE$).

Cancer development is a multi-stage process involving alterations in DNA (somatic mutations) in critical genes. Contributions include natural and synthetic chemicals (e.g., benzidine dyes, or chemicals in cooked beef and fish) and physical agents (UV light, radon, gamma radiation). Genetics play a major role; for example, individuals with xeroderma pigmentosum lack DNA repair mechanisms and are highly susceptible to skin cancer. Reproductive and developmental toxicity can be caused by viruses (Rubella), bacterial infections, and drugs (Thalidomide, Vitamin A). Studies show ethylene glycol creates abnormal developmental effects in animals due to maternal metabolic acidic metabolites, specifically glycolic and oxalic acid, which affect the placenta and fetus.

General Principles of Toxicity Management

A poison is a chemical substance that produces harmful effects when entering the body in small or excessive doses, resulting in tissue damage, functional distortion, or death. Harm is dependent on two variables: the dosage and the length of exposure. These variables are influenced by the total amount of chemical, route of administration, rate of absorption, membrane traversal ability, receptor site location, and rate of detoxification or elimination. Management begins with supportive care, including resuscitation and maintenance of respiratory and cardiovascular functions.

Antidotes function by preventing absorption, neutralizing the toxin, antagonizing end-organ effects, or inhibiting toxic metabolite conversion. Examples include naloxone for opioid narcotics and epinephrine for histamine-induced anaphylaxis. There are three mechanisms in antidotal therapy: limiting drug-receptor interaction by altering absorption/distribution, protecting the organism by elevating the toxicity threshold, and hastening biotransformation or elimination.

Symptomatic treatment includes emesis and gastric lavage. Emesis via Syrup of Ipecac (a gastric irritant and chemoreceptor trigger zone stimulant) is effective if performed within $2$ to $3$ hours of ingestion but is contraindicated in patients with altered consciousness, corrosive ingestion, or petroleum distillate ingestion. Gastric lavage involves repeated stomach washings. Once a chemical enters the small intestine, it cannot be recalled. Activated charcoal is used to adsorb chemicals; the standard dose is two tablespoons ($50\,mg$) in a glass of water ($400\,ml$). Neutralization of acids with bases is unsatisfactory due to heat and corrosion; dilution with milk or water is preferred. Specific antidotes include calcium for fluoride, sodium chloride for silver nitrate, EDTA for heavy metal chelation, and ethanol for methanol poisoning.

Management of Acute Poisoning and Emergency Stabilization

Acute poisoning results from suicidal, accidental, or homicidal exposure. Evaluation involves identifying the toxic agent via symptoms or history, assessing vital signs and consciousness, and conducting laboratory tests. Emergency stabilization prioritizes establishing a clear airway, ensuring adequate ventilation/breathing, and addressing CNS or respiratory depression. For topical exposure, areas should be flushed with large volumes of fluid (water). For inhalational exposure, the patient must be moved to fresh air and given oxygen.

In hospital settings, gastric lavage is performed with saline. Activated charcoal is administered as an adsorbent, and cathartics may be used to increase rectal elimination of unabsorbed drugs. If a victim is in a coma, naloxone hydrochloride is administered for narcotic overdose and $50\%$ glucose for insulin shock. The fundamental principle is to "treat the patient, not the poison," meaning respiration and circulation maintenance take precedence over specific antidotal therapy unless both can occur simultaneously.

Toxicological Evaluation of New Drugs

Safety assessment or toxicity testing determines the degree of damage a substance can cause. This preclinical development stage includes in vitro and in vivo research. Rats are recommended for sub-chronic, chronic, carcinogenicity, and reproduction studies. Mice are used for carcinogenicity, dogs for subchronic/chronic studies, rabbits for eye and skin irritation, and guinea pigs for dermal sensitization. Dose levels for subchronic studies include four groups: a control, a low-dose (no toxicity), a mid-dose (minimal toxicity), and a high-dose (toxicity without excessive fatalities). For non-rodents, there should be no fatalities in the high-dose group.

Acute toxicity studies involve single exposures on one day to identify potential hazards and parameters for protective measures. Subchronic studies monitor effects from repeated exposure over roughly $30\%$ of a rodent's lifetime (dosing starts shortly after weaning, at $6$ to $8$ weeks for rats, and $4$ to $9$ months for dogs). Parameters monitored include body weight, food consumption, ophthalmological exams, hematology (blood cell formation, clotting), clinical chemistry (liver, kidney, heart function), and urinalysis. After sacrifice, weights of the brain, gonads, liver, and kidneys are recorded during necropsy.

Chronic toxicity studies assess hazards from long-term exposure ($12$ to $24$ months). Carcinogenicity focus aims to detect neoplastic lesions (tumors) at the Minimal Toxic Dose ($MTD$). These studies last $24$ months in rats and $18$ months in mice. Specialized studies include Developmental Toxicity (assessing structural abnormalities and growth in offspring), Reproduction Studies (multigenerational effects on neonatal morbidity), Mutagenicity (affecting genetic material), and Neurotoxicity (assessing structural or functional nervous system damage).

Post-Marketing Surveillance and Pharmacovigilance

Post-marketing surveillance (PMS) is the practice of monitoring drug safety after market release. This is a critical component of pharmacovigilance (PV), which is the science of detecting, assessing, and preventing adverse drug reactions (ADRs). PMS is necessary because clinical trials involve small numbers of selected participants who rarely have the co-morbidities found in the general population. PMS methods include spontaneous reporting databases, prescription event monitoring, electronic health records, patient registries, and data mining.

PMS helps identify rare events and validate causality. FDA may require post-marketing study commitments or Phase IV trials to gather data on long-term effectiveness, drug-drug interactions, and effects on specific populations (e.g., pregnant women). Phase IV trials usually include thousands of participants and can include pharmacoeconomic studies or drug utilization research. Vigilance by physicians remains the best defense against adverse actions, as clinical trials cannot provide a full safety profile before marketing.