L1 - Ligand Receptor Interactions

Definition of Pharmacology

• Pharmacology is defined as the interaction of chemicals with biological systems to yield therapeutic or other beneficial effects.

Selected Characteristics of Pharmacological Agents

• Intended Purposes: Pharmacological agents are specifically designed to fulfill certain therapeutic goals.

• Prevent or Treat: These agents are typically used for preventing illnesses or treating existing medical conditions.

• Physical Form: Medications must be administered in an appropriate physical form that can be utilized by the body.

• Dosing Regimens: Specific dosing regimens must be followed for effective treatment.

Drug Action

Differentiating Pharmacology and Toxicology

• Pharmacology is the study of how chemicals interact with biological systems to provide therapeutic or beneficial effects.

• Toxicology, by contrast, studies chemical interactions that yield adverse effects.

Definitions

• Adverse: This term refers to harmful, unfavorable, or unintended effects of drugs.

• Unintended effects may lead to drug repurposing, exemplified by sildenafil citrate.

History of Pharmacology

16th Century Developments

• Specificity: The idea that diseases have identifiable causes which can be targeted by specific therapies.

• Active Ingredients: Recognition that natural remedies contain active compounds that provide therapeutic effects.

• Dose-Response: Establishing that the response magnitude to treatment depends on the administered dose.

Paracelsus

• Notable Quote: "All substances are poisons: there is none which is not a poison. The right dose differentiates poison from remedy."

• Key Contribution: Emphasized dose-response relationships and proposed a continuum of toxicity and therapy; at the right dose, therapeutic effects occur, while higher doses lead to toxicity.

18th Century Contributions

William Withering

• Studied Digitalis purpurea, or the foxglove plant, for treating congestive heart failure (‘dropsy’).

• Reported outcomes from a study involving 158 patients using foxglove extracts, noting common toxicities such as vomiting, leading to the identification of active ingredients.

19th Century Advancements

• Friedrich Serturner: Isolated morphine from poppy plants, thus marking the birth of medicinal chemistry.

  • Named morphine after Morpheus, the Greek god of dreams, due to its sedative effects.

  • This era saw increased advances in chemistry facilitating the isolation of pure compounds, beginning alkaloid pharmacology.

• Rudolf Buchheim: Established the first pharmacology department at the University of Dorpat in 1846, redefining drug classification systems to focus more on physiological effects rather than traditional uses.

  • Championed systematic experimental methods and statistical analysis in drug studies.

Contributions of Oswald Schmiedeberg

• A disciple of Buchheim, he was critical in Pharmacology as a practical discipline.

• Focused on sedative hypnotic drugs, how digitalis affected heart function, liver detoxification methods, and identified effects of nicotine.

20th Century U.S. Pharmacology

• John Jacob Abel: Recognized as the “Father of American Pharmacology.” He founded the Pharmacology department at the University of Michigan and made significant contributions including the purification of epinephrine and insulin.

Rationale for Drug Design

• Josef von Mering and Emil Fischer: Developed structure-activity relationships in drug design.

  • Achieved their first chemically designed drug, Barbital (diethyl barbituric acid), which was marketed as a sleep aid (1903-1950s).

Pharmacology's Core Components

Pharmacodynamics and Pharmacokinetics

• Pharmacodynamics (PD): The study of the biochemical and physiological effects of drugs and their mechanisms of action.

• Pharmacokinetics (PK): Encompasses absorption, distribution, metabolism, and excretion of drugs, often summarized as ADME.

Mechanisms of Drug Action

Drug-Receptor Interaction

• Drug Receptor: A macromolecule that interacts with a drug to elicit a cellular response; drugs typically enhance or modify intrinsic cellular responses rather than creating novel ones.

Occupancy Theory

• The magnitude of drug effect increases with the occupancy of receptors by the drug/ligand.

• Binding site is where the drug attaches, with greater occupancy leading to a graded and cumulative response.

Agonists

• Agonists are drugs that mimic the action of endogenous compounds at receptors, binding at various sites:

  • Allosteric Agonists: Bind at sites other than the active site but still mimic the effects of primary agonists.

  • Primary Agonists: Bind to the active site of the receptor.

Antagonists

• Antagonists reduce or block agonist actions. Types include:

  • Competitive: Compete with agonists at the overlapping receptor site.

  • Noncompetitive: Interact with different sites.

  • Functional: Indirectly inhibit effects of agonists.

Molecular Interaction Models

Lock & Key Model

• This traditional model does not account for changes in receptor conformation upon drug binding.

Induced Fit Model

• Suggests that drug binding causes the receptor to change shape, enhancing the drug's affinity for it.

Properties Influencing Drug-Receptor Binding

Key Factors

• Hydrophobicity/Lipophilicity: Affects permeability through membranes and access to binding sites.

• Ionization (pKa/local pH): Determines drug charge and membrane penetration capabilities associated with ionic interactions and distribution.

• Stereochemistry/Chirality: Different enantiomers can have significantly varied interactions and effects on receptors, impacting pharmacodynamics.

Patient Case Example: Local Anesthetics

• Ionization Example: Local anesthetic drugs function distinctly in charged and uncharged states, affecting penetration rates and effectiveness within neural tissues.

  • Uncharged forms penetrate membranes, while charged forms block sodium channels to prevent depolarization.

Chiral Drugs and Receptor Affinity Differences

Warfarin and Sotalol Examples

• Warfarin: The S enantiomer holds greater potency than the R enantiomer due to more effective binding characteristics.

• Sotalol: The l enantiomer functions better as a beta-adrenergic antagonist, while d and l enantiomers share efficacy in K+ channel blocking but with differing safety profiles.

Receptor Structure and Drug Interactions

Levels of Protein Structure

• Primary: Amino acid sequence.

• Secondary: Patterns from interactions between amino acids in the same chain.

• Tertiary: Three-dimensional folding stabilized by ionic bonds and disulfide links.

• Quaternary: Assembly of multiple protein units.

Drug-Receptor Bond Types

Reversibility of Binding

• Most drug-receptor interactions are reversible due to noncovalent forces, allowing for safety and adaptability in therapeutic effects.

• Some drugs form covalent bonds with receptors, fundamentally altering their function.

Case Study: Imatinib as a Targeted Tyrosine Kinase Inhibitor

Mechanism of Action

• Imatinib targets the BCR-Abl Kinase, preventing the activation loop from being phosphorylated and thereby inhibiting downstream signaling crucial for cell proliferation in certain cancers.

Intermolecular Interactions

• Imatinib interacts with key amino acids in the BCR-Abl protein through both hydrogen bonds and van der Waals forces, demonstrating its design to target specific biochemical pathways effectively.

Affinity, Selectivity, and Intrinsic Activity

• These factors dictate a drug's efficacy, with selectivity describing how effectively a drug targets its intended receptor relative to others in the body, which is crucial for minimizing side effects.

Adrenergic Receptor Selectivity

• Understanding the different specific agonists and antagonists acting on alpha and beta adrenergic receptors, categorizing their effectiveness and selectivity, impacts therapeutic strategies for cardiovascular diseases and broader pharmacotherapeutics.

Example Question Analysis

• Determine the selective antagonist for beta-1 receptors from a provided chart, reinforcing the analytical approach needed to integrate pharmacological knowledge into pharmacotherapy.