-Interactions of substances with living systems

-an applied science

- built on knowledge of all other basic sciences

binding to regulatory molecules, activating or inhibiting normal biochemical process of:

• the patient
• the invading organism
• abnormal cells (cancer

1. Pharmacokinetics
2. Pharmacodynamics
3. Pharmacotherapeutics
4. Pharmacocognosy
5. Toxicology

• How the body handles the drug
• absorption, distribution, metabolism and excretion 
• determines choice of route of administration as well as time course
• basic principles underlying drug interactions
• impact of systemic pathology on dosing (pts with liver of renal failure)

•  How drugs produce effects (desired, toxic)
• Mechanisms of action
• biological, biochemical interaction of drug and drug receptors
• Structure-Activity-Relationships (SAR)           (ex Key lock fit)

Clinical use of drugs in prevention, dx, and treatment of disease or modification of physiology (ie reproductive fxn)

• determination of proper drug and dosage for individual patients
• adverse effects & drug interactions

Study of poisons and poisonings, including treatment

• acute and chronic  (lead & mercury poisoning, ASA and Acetaminophen overdoses)
• Toxic doses of drugs (pharmacologic extension)

Influence of genetic variation on drug response by correlating gene expression or single-nucleotide polymorphisms with a drug's efficacy or toxicity

• Personalized medicine
• optimize drug therapy to patients genotype
• ensure max efficacy with min adverse effects

said "all things are poison and nothing is without poison, only the dose permits something not to be poisonous"

• the dose makes the poison

Selective action(s) of drugs

1. Drug acts via receptors
2. Small # of drugs are nod mediated via receptor interactions

Drug + Receptor ↔ Drug-Receptor Complex

                     ↓

Signal transduction

 ↓

Response

• Examples:
• Propranolol decreases heart rate
• Furosemide decreases plasma volume
• Imatinib- inhibits activity of Bcr-Abl tyrosine kinase
• Amoxicillin kills bacterial cells

• Prescribed
• Over-the-counter
• Herbal & Natural products
• Social
• Synthesized in the body
• Poisons (lead, arsenic)
• Toxins (poison of biologic origin)

Chemicals not synthesized in the body

• from Gr. xenos "stranger"

• Prevention
• Diagnosis
• Treatment (pharmacotherapy, chemotherapy, anesthesiology)

• Solid, liquid, gaseous (influences route of administration)
• Organic compounds (carbohydrates, proteins, lipids)
• Inorganic elements (lithium, iron, copper)
• Weak acids or bases (organic drugs: lidocaine, codeine)

• Small (lithium)
• Large (antiboidies: proteins, macromolecules; Alteplase: clot dissolving enzyme)
• range of 100 to 1000 molecular wt. 
• determine the drugs physical properties (diffusion, absorption, route of admin.) 

Drug receptor binding is the result of multiple chemical interactions between the drug and receptor (binding site)

Types of binding forces

• Covalent
• Electrostatic (H bonds, dipole interactions (Van der Waals forces))
• Hydorphobic

Sharing of electrons (Strong, not reversible)

• Very stable drug-receptor complex
• New receptor need to be synthesized

Weak bonds

• important in interactions of highly lipid-soluble drugs with lipids of cell membrane
• interactions of drugs with the internal walls of receptor "pockets"

Electrostatic attraction between atoms of opposite charge

• H bonds
• Dipole interactions (Van der Waals forces)
• Weaker than covalent bonds

• Interaction between polar molecules
• Weaker bonds than ionic bonds
• Drug and receptor site capable of forming multiple H bonds (overall bond strength is greater than a single ionic bond)

• Weak electrostatic interactions
• Weakest bonds
• Most important in drug-receptor binding
  • Numerous electrostatic interactions
  • Total binding force is much larger than other bonds
  • Allows precise "fit" between drug and receptor

Drugs interacting via weak bonds are generally more selective

• weak bonds require a very precise fit of the drug to a receptor

Drugs are 3 dimensional structures

• have certain shape, size and charge
• "keys"
• drugs shape is complementary to that of the receptor site
• Stereoisomerism

Phenomenon of chirality (left hand glove -- right hand)

• Enantiomers (S)(+) or (R)(-)
• Racemic mixture (racemate) contains both enantiomers
• one enantiomer can be much more 

epi has different optical isomers

• a racemic mixture of 2 stereoisomers
• Levorotory (l-): highly active (potent)

α-D-manose: sweet

β-D-manose:bitter

• S enantiomer is 4 x more potent than the R form
• S form has a stronger binding with its binding site on Vit K epoxide reductase

Stereochemistry effects of Thalidomide

• S enantiomer caused birth defects
• R form is employed in the treatment of leprosy and is an angiogenesis inhibitor

Non-selective beta blocker with alpha blocking activity

• different isomers have different activity
• (S)(-)isomer: potent beta blocker
• (R)(+)isomer: 100x weaker at the beta receptor
• (S)(-) and (R)(+) isomers: alpha blocking at equal potency

Both isomers are metabolized at different rates

- alpha blocker isomer accumulates in patients genetically deficient in CYP2D6

Pharmacokinetics of enantiomers can be vastly different, resulting in differences in:

half life of drug, duration of action, efficacy/toxicity

Metabolism: Different metabolic rate

•  
  • Enzymes are stereoselective
  • metabolic rate of one enantiomer can vastly differ from the other enantiomer

Drug Transporters: 

also stereoselective for drugs

How the drug acts/interacts in the biological system

• Mechanism of action
• How drug interacts with or perturbs biological system
• Determines the indications or contraindications of the drug (or that class of drugs)

1. Drug interacts with its receptor, resulting in a conformation change of the receptor complex
2. This brings about a complex set of events (intercellular signals)
3. Leads to Response(s)

Epinephrine activates receptors resulting in an increase in heart rate and force of contraction

• When we inject epi into a patient, we can expect and increase in heart rate and blood pressure

Propranolol blocks adrenergic receptors in the heart muscle

• Thus, endogenous epi is unable to activate its receptor
• Results in a decrease in heart rate and blood pressure

• Play a regulatory role in the biologic system
  • mediate actions of endogenous chemical signals such as neurotransmitters, autocoids, and hormones
• drug-receptor union mediates biochemical and physiologic changes within the organism

• Primary structure: aa sequence
• Secondary structure: interaction of nearby aa's in the polypeptide
• Tertiary structure: interaction of aa's that are relatively far apart in the polypeptide
• Quaternary structure: interaction of 2 or more polypeptides 

• macromolecules (most are proteins) that are twisted and folded in a specific configuration
• Structure of polypeptides give the necessary diversity and specificity (shape, charge)
• Has hydrophobic center and hydrophilic exterior
• Have unique 3D geometry... unique Binding sites, specific for drugs and endogenous compounds

Enzymes:

dihydrofolate reductase (methotrexate)

Angiotensin II converting enzyme (captopril)

Transport proteins

Na+/K+ ATPase (digitalis glycoside)

Calcium channels (Verapamil)

Structural proteins

tubulin (colchicine)

• receptor's affinity for binding a drug determines the concentration of drug required to form a significant # of drug-receptor complexes
• Total # of receptors may limit the max effect a drug may produce

• drug has a specific molecular size, shape, charge, which determines its specificity for a particular receptor
• changes in the chemical structure of a drug can dramatically increase/decrease a new drug's affinities for different classes of receptors
  • results in altered therapeutic and toxic effects

• actions mediated by receptors
• many drugs/endogenous chemical signals (ie. hormones) regulate the fxn of receptor macromolecules as agonists

•  actions regulated by receptors
• antagonists bind receptors without directly altering their fxn
• prevent antagonist from binding and block their biologic actions

Drug responses do not involve receptors

• antacids
• laxatives
• chelating agents (D- pencillamine)
• Osmotic diuretics (mannitol)

• selective for a specific glass of drugs
• a structural-activity relationship
• Different Classes:

1. Acetylcholine (ACh)
2. Adrenergic
3. Histamine
4. etc

• Nicotinic (N_N and N_M)
• Muscarinic (M₁, M₂, M₃, M₄, M₅)

 β receptors - via G_s, increases heart rate

• β₁, β₂, β₃

α receptors

• α₁, α₂, α₃
  
• α_1: via G_q causes vasodilation
• α_2: via G_i opens K⁺ channels

• Agonists
• Antagonists

• Drugs or endogenous molecules(ex hormones)
• Bind to receptor & initiate a cellular response
• Have both affinity and high intrinsic activity

• Bind to receptor without initiating a biological event (no stimulation of the receptor)
• Prevent agonist from binding to and activatind receptor
• Have affinity but NO intrinsic activity 

• High affinity = potent drug
  • requires less drug to produce and effect

• Low affinity
  • requires high drug concentration to have an effect

The ability of a drug to activate a receptor following binding

• High intrinsic activity
  • cause intense receptor activation
  • have a high max effect

a drug binding site adjacent to agonist binding site

• Drugs acting here can
  • facilitate action of the agonist: allosteric activator
  • inhibit the action of the agonist: allosteric inhibitor

Drug that binds to an allosteric binding site and inhibits the action of the agonist

• inhibition is not overcome by increasing the dose of the agonist

Drugs that mimic agonists:

Acetylcholine esterase inhibitors

• SARIN a nerve gas
• Inhibits ACh esterase at cholinergic nerve endings
• Increases synaptic concentrations of ACh
• ACh acts on its receptors

Competitive 

Irreversible 

Full 

Partial 

Inverse

Indirect

• Binds reversibly to same receptor as agonist
• In the presence of a fixed [ ] of agonist, increasing [ ] progressively inhibit agonist response
• High antagonist [ ] completely prevent response
• Inhibition can be overcome by increasing agonist [ ]
• shift dose effect curve to the right

Changes in the endogenous agonist (ie epinephrine) overcomes inhibition produced by propranolol, competitive β-adrenoceptor antagonist

Therapeutic implication: prescriber must always consider possible changes in endogenous agonist [ ] that could influence response of such a drug

Depends upon the [ ] of antagonist

• Binds to a different site on the receptor or irreversibly to the same site
• blockage is insurmountable (high affinity)
• Receptor is unavailable for binding agonist
  • usually via covalent bonds
• cause a downward shift of an agonists dose-response curve (only if spare receptors are present)
• Not displaced by High [ ] of agonist

Duration of action independent of rate of elimination; Dependent on rate of turnover of receptor molecules

• problem if overdose occurs

• Chemical antagonism
• Physiologic antagonism

type of non-receptor drug antagonism

• Protamine (+ charged protein at pH 7.2)
• Heparin, an anticoagulant, (- charged)
• Protamine binding w/ heparin ↓ availability of heparin for interactions with proteins involved in clot formation

type of non-receptor antagonism

• Exploit opposing regulatory pathways
• Glucocorticoid hormones lead to ↑ blood sugar, while insulin ↓ blood sugar
• Less specific and more difficult to control than using specific antagonists

Produce higher response at full receptor occupancy than a partial agonist

• saturation of receptor pool produces max response

produces a less than full effect, even when receptors are saturated

• coupling with receptors produces a lower response. It is not a difference in affinity
• competitively inhibit the responses produced by full agonist

a mix of full and partial agonist

• In the presence of a full agonist, a partial agonist acts as an inhibitor (competitive)
• Ex.: Pentazocine (partial agonist) vs Meperidine for pain relief

• bind to receptors similar to the agonist
• produces opposite effect as agonist
• Post synaptic cell membrane
  • GABA (n.t) acting of GABA_A receptors, results in chloride channel opening
  • chloride entering the post synaptic cell, results in hyperpolarization (sedation)
  • Benzodiazepine receptors: alprozolam, diazepam acting here facilitates action of GABA = sedation
  • R015-4513 acting here produces anxiety
  • Fulmazenil blocks benzodiazepine receptors 

endogenous molecules that bind a drug without initiating any of the drug's effects

• Plasma proteins: Albumin, orosomcoid 
• Corticosteroid-binding globulin (CBP)

• continuously recylced
• # of receptors present is normally constant
• # may change due to specific conditions
  • down-regulation receptors
  • up-regulation receptors

produced by agonist induced:

1. ↓ receptor biosynthesis 
2. ↑ in receptor internalization and degradation
3. hour to days
4. down regulation involving protein syn is less readily reversible

Caused by:

Decrease in neurotransmitter (ACh) quantal release

Denervation, nerve damage

• Caused by repeated or continuous administration of an agonist or antagonist 
• leads to changes in responsiveness of receptor
• Reversible:
  • Tolerances 
  • Tachyphylaxis

β-adrenergic receptor (model for most G coupled receptors)

• agonist binds receptor, activates βARK
• βARK phosphorylates carboxyl tail of receptor (serine or threonine)
• ↑ receptor affinity to β-arrestin, resulting in a decrease in receptor's ability to interact with G_S: ↓agonist response
• Removal of agonist, reverse events and return to normal

1. Lipid-soluble ligand (intracellular receptors)
2. Transmembrane receptors with enzymatic cytoplasmic domains
3. Transmembrane receptors that bind & stimulate protein tyrosine kinase
4. Ligand-gated transmembrane ion channel
5. Transmembrane coupled with G protiens

Lipid -soluble ligand

(Intracellular receptors)

• Drugs interacting with these receptors are lipophilic(can pass directly through membrane)
• Receptors include:

1. Transcription factors (cytoplasmic or nucleus) ex. Steroid hormones
2. Structural proteins (tubulin) ex. vincristine
3. Enzymes (ex. warfarin)
4. Signal  transduction molecules (ex. imatinib)

1. Nitric oxide (NO): stimulates guanylyl cyclase, which produces cGMP
2. Corticosteroids, sex steroids, mineralocorticoids, Vit. D, and thyroid hormone: Stimulate the transcription of genes by binding specific DNA sequences near the gene whose expression is to be regulated

• Delayed (lag) response: 30 min to several hours 
  • Corticosteroids will not immediately relieve the symptoms of acute bronchial asthma
• Effects can persist for hours to days:
  • Biological effects even after the drug is stopped or cleared

Transmembrane receptors transduce an extracellular ligand-binding interaction int an intercellular action via receptor-linked enzymatic domains

• involved in numerous phys. processes
  • cell metabolism, growth, differentiation

• Receptor Tyrosine Kinase
• Receptor Tyrosine Phosphatases
• Tyrosine Kinase-associated Receptors
• Receptor Serine/Threonine Kinases
• Receptor Guanylyl Cyclases

• Insulin
• epidermal growth factor (EGF)
• platelet derived growth factor (PDGF)
• atrial natriuretic factor (ANF)
• transforming growth factor-P (TGFP)

Protein tyrosine kinase

Serine kinase

Guanylyl cyclase

(auto)Cross-phosphorylation of intracellular domains occurs:

• results in downstream signaling (kinases)

Ligand binding results in endocytosis of receptor:

• New receptors form via de novo synthesis

• Growth hormone
• Erythropoietin
• several kinds of interferon
• other regulator of growth and differentiation

•  once drug is bound to extracellular domain of receptor, a separate protein tyrosine kinase, from the JAK family, binds noncovalently to the intracellular domain of the receptor
• JAK activates a STAT molecule (by phosphorylation) which then travels to the nucleus, where it regulates transcription of specific genes

1. Ligand gated: binding of ligand to channel → altered ion conductance
2. voltage-gated: Change in transmembrane voltage gradient → altered ion conductance

• Channels allow the passage of ions and other hydrophilic molecules across the plasma memb
• Highly selective for ions they conduct

Diverse functions

• Neurotransmission
• cardiac conduction
• muscle contraction

Synaptic transmtters

• Acetylcholine
• γ-aminobutyric acid(GABA)
• excitatory amino acids (EAA) (glycine, asparate, glutamate, etc.)

Receptor transmits its signal across the membrane by increasing transmembrane conductance of the relevant ion 

• alters electrical potential across the membrane

• Ligand domain can be extracellular, within the channel or intracellular
• time elapsed between the binding of the agonist and the cellular response is very quick (milliseconds) (contrasts different from other signaling mech, which can be sec-hours)

an example of a ligand gated ion channel

• has 5 subunits, including 2 α-subunits
• each α subunit has an extracellular binding site
• closed state: channel is occluded by a.a side chains
• binding of 2 ACh molecules causs a conformation change that opens the channel = open state

• Open 
• Closed
• Refractory or inactivated: channels permeability cannot be altered for a period of time

• most abundant class of receptors
• receptors have a extracellular domain, transverse the membrane and have an intracellular domain that interacts with G protiens

• large family of proteins
• each one mediates specific effects to a distinctive group of effector/sig pathway
• Different ones associated with different types of receptor
• different ligands associated with different 2nd messenger system. G protein-coupled signaling is involved in many different processes
• Vision, olfactory, neurotransmission, etc

• Increases cellular 2nd messengers (cAMP, phosphoinositides, Ca²⁺⁾
• 3 components:
• extracellular ligand couple with cell surface receptor
• activation of a G protein
• changes the activity of an effector element, usually an enzyme or ion channel

• Drugs may only bind to its receptor for a very short time. However, the downstream effect is longer
• G protein receptor activation allows the signal to be amplified

• cyclic adenosine monophosphate (cAMP)
• Calcium and Phosphoinositides
• Cyclic guanosine monophosphate (cGMP)
• Kinases

• antacids
• laxatives
• chelating agents (D-pencillamine)
• osmotic diuretics (mannitol)

• based on the concept of drug-receptor binding
• as ligands bind receptor and effect is seen
• max response when all receptors are occupied
• K_D is point where 50% of receptors are bound

• determined by bound vs free drug [ ] curve
• point where 50% of receptors are bound
• if low, binding affinity is high, and vice versa

• determined on a dose vs effect curve
• drug [ ], dose, where 50% of the max effect is seen
• measures a drugs potency
• the lower the EC₅₀ the more potent the drug

1. Graded dose-response
2. Quantal dose-response

When the response of a receptor-effector system is measured against increasing [ ] of drug (gradual)

• concepts in receptor occupancy
• determine E_max and EC₅₀ of a drug
• compare the potency of 2 drugs of the same class

• the graph of the drug [ ] or dose
• X-axis = increasing drug [ ]
• Y-axis = response (drug effect)
• as drug [ ] increases, there is a corresponding increase in the response until we reach a plateau 
• results in a hyperbolic curve
• can also be plotted in a log dose response curve

• formed when drug [ ] and drug effect data are plotted on semi-logarithmic axes
• Result in a sigmoid shape curve
• response of each dose is in terms of a % of maximal response = y axis
• drug [ ] is in terms of the log dose = x axis
• simplifies the mathematical manipulation of the dose response data

• results in a sigmoid curve
• x-axis = [ ] of drug (log dose)
• y-axis = % of max response
• E_max = max efficacy(effect) of the drug, reached when curve levels off at 100%

• max effect of the drug
• gives a measure of how effective the drug can be
• reached when the curve levels off at 100%
• the higher the curve reaches the greater the efficacy 
• can be produced by agonist if the dose is taken to very high levels
• determined mainly by the nature of the receptor and its associated effector system
• Measured with graded dose response curve (not quantal)

• population based
• where the response elicited with each dose of a drug is descried in terms of the cumulative % of subjects exhibiting a define all or none effect plotted against the log dose of the durg

quantal dose-effect curves

•  dose of drug required to produce a specified magnitude of effect in a large # of subjects
• Y-axis = cumulative frequency distribution of responders
• X-axis = Log dose

• relief of headache; effect on sleep etc
• specifying an increase in heart rate of 20 beats/min
• death of an exp animal

• find the min dose of the drug required to produce the response in a given individual
• plot the frequency of patients responding at each dose
• convert to a cumulative plot 
• results in a sigmoid curved curve

50% of the max effect and the dose causing this effect is called EC₅₀

• potency is determined by the receptor affinity

• 50% of the effect in the population studied
• ED₅₀ (median effective dose)
• TD₅₀ (toxic dose)
• LD₅₀ (lethal dose)

Ratio of TD₅₀ (or LD₅₀) to the ED₅₀ determined from quantal-dose response curves

toxic dose    =  TD₅₀

_{effective dose   ED50}

• Estimate of the safety of a drug (a poor safety estimate)
• safe drug will have a very large toxic dose and a small effective dose
• Therefore, the larger the TI the safer the durg

The dosage range between the min effective therapeutic [ ] (dose) (= the threshold), and the mim toxic [ ] 

• clinically relevant index of safety
• pt where effect is 1st seen until pt where toxicity is seen 

a more stringent index of safety

• calculated from quantal dose-response relationship for a therapeutic effect and a toxic effect
• Ratio between the dose that is lethal in 1% of subjects (LD₁) and the dose that produces an effect in 99% of the subjects (ED₉₉)

LD₁

ED₉₉

Depends on: 

1. its potency (ED₅₀)
2. its maximal efficacy 
3. ability to reach the relevant receptors (site of action)

•  
  •  
    • 3a) pharmacokinetics of the drug

• individuals vary in their responsiveness to a drug
• Compared to the general pop. each patient is hyporeactive or hyperreactive to a drug
• Drug tolerance
• Idiosyncratic drug response
• Hypersensitivity

Where the intensity of response to a given dose changes during the course of therapy

• responsiveness usually decreases as a consequence of continued drug administration

• an unusual drug response
• one that is infrequently observed in most patients
• usually caused by:
  • genetic differences in metabolism
  • immunologic mechanisms, including allergic reactions

1. Alteration in [ ] of drug that reaches receptor  
2. Variation in [ ] of endogenous receptor ligand
3.   Alterations in # of functioning receptors
4. Changes in components of response distal to receptor

propanolol, will ↓heart rate of pt with pheochromocytora, but will not affect

• changes in the post receptor processes
• dependent on correct diagnosis and understanding of the pathophysiologic mechanism responsible for the disease
• Physiological compensatory mechanisms

• no drug causes only a single, specific effect
• drugs are selective rather than specific in action
• Beneficial or therapeutic effects vs toxic effects (side effects) must be considered

1. Extension of pharmacological effect = same receptor effector mechanism
2. Beneficial and toxic effects mediated by identical receptors but in different tissues or different effector pathways
3. Beneficial and toxic effects mediated by different types of receptors

• insulin: decrease blood sugar - hypoglycemic coma
• anticoagulant therapy: bleeding
• Sedatives
• May be necessary for concomitant administration of another drug: hypertension

• methotrexate: cytotoxic to both normal and cancer cells
• Corticosteroids: anti-inflammatory but produces protein catabolism, psychosis etc
• Digitalis: increase contractility in myocardium but causes arrhythmia in conduction tissue

Beneficial and toxic effects mediated by different types of receptors

Tricyclic antidepressants:

• modulate several monoamines
• has anticholinergic properties