Carl B. Goodman, Ph.D.

Professor Pharmacology

College of Pharmacy & Pharmaceutical Sciences

Florida A&M University

308E FSH-SRC

carl.goodman@famu.edu

599-3128

1. Most drugs are chemically synthesized (or at least modified, e.g. the penicillins) – the larger the molecules, the more difficult the synthesis, and the lower the yield will be.

2. Drugs need to reach their targets in the body, which means they need to be able to cross membrane barriers by diffusion. Diffusion becomes increasingly difficult with size.

Affinity Constant = K(D)

a low KD = a high affinity

a K(D) of 10E-6 or more is considered too weak

Cell Surface Receptors

Receptors vs. Binding Sites7 transmembrane loopG protein-coupledSignaling Transduction PathwayEndogenous Ligand

Ion channels

Carrier pumps Some diurectics act on the Na+/Cl-

cotransporter in the distal tubules Some diurectics act on the Na+/K+

antiporter in the collecting ducts Some antiulcer/reflux drugs work on

the H+/K+ antiporter

Enzymes NSAIDs inhibit cyclo-oxygenase which

blocks prostaglandin production

ACE inhibitors inhibit angiotensin converting enzyme to decrease blood pressure

HMGCoA reductase inhibitors inhibit this to reduce lipid concentration

Nuclear Receptors/RNA/DNA

Steroid Hormones- Estrogen and Androgens

Intracellular Structural Proteins

Inhibition Constant = K(I)

Measures degree of inhibition of an enzyme

Treatment of a disease can be by different targets:

Ex. to treat Gastric Hyperacidity: Neutralizing drug (NaHCO3) Proton pump inhibitor (Omeprazole) Histamine 2 receptor blocker

(Cimetidine)

At the same receptor, but distributed throughout the body in different places

More than one target

Ex. Caffeine - Nonselective adenosine receptor antagonist

- Competitive nonselective phosphodiesterase inhibitor

Propranolol

Molecular: is a competitive inhibitor by binding to beta 1 receptors

Cellular: prevents increase in cAMP, protein phosphorylation

Physiological: reduces cardiac heart rate and contractile force

Therapeutic: used to treat angina

4 Basic Mechanisms for Transmembrane Signaling

Ligand gated channels

G protein-coupled receptors/second messengers

Intracellular receptors

Ligand-regulated transmembrane enzymes

Nicotinic Acetylcholine Receptor

Where: found on skeletal muscle Function: when activated causes contraction Structure: pentamer crossing lipid bilayer Mechanism of action: acetycholine binds to

alpha subunit --> conformational change --> trransient opening of central aqueous channel --> Na+ flows down concentration gradient --> depolarisation --> contraction

Signaling system involves:

Extracellular drug binds to cell surface receptor

Receptor triggers activation of a G protein on cytoplasmic face of plasma membrane

Activated G protein alters activity of an effector element (e.g. enzyme or ion channel)

Effector element changes concentration of intracellular second messenger

Contains alpha, beta and gamma subunits

The beta-gamma anchors the G protein to the membrane

The G protein is not normally bound to the receptor but is free floating in the cell membrane (cytoplasmic side

G protein Structure

1. When the receptor is not occupied by a drug: the G protein is in a resting state, where beta-gamma anchors G protein to membrane, and GDP occupies site on alpha subunit

2. The ligand binds to the receptor, which alters the conformation of the receptor, exposing the binding site for G protein

3. The G protein binds to the receptor, greatly weakening the affinity of the G protein for GDP

4. GDP dissociates, allowing GTP to bind to the alpha subunit

5. This causes the alpha subunit to change conformation, it now dissociates from the receptor and the beta and gamma subunits

6. The free alpha subunit now changes conformation so that it can bind with its target enzyme

7. This target enzyme acts, e.g. converts ATP-->cAMP, ion channel etc.

8. Hydrolysis of the GTP by the alpha subunit returns the subunit to its original conformation, causing it to dissociate from the target enzyme and reassociate with the beta and gamma subunits

Gs is a G protein that acts on adenyl cyclase, hence converts ATP-->cAMP

Gi is a G protein that inhibits adenyl cyclase, hence prevents ATP-->cAMP

1. cAMP many drugs/hormones act by increasing or

decreasing the adenyl cyclase which in turn increases or decreases cAMP

2. cGMP

3. Calcium-phosphoinositide pathway (IP3/DAG)

Mechanisms of this pathway:

Gs stimulates phospholipase C (a membrane enzyme)

Hydrolysis of IP2 into DAG (diacyclglycerol) and IP3 (inositol-1,4,5-triphosphate) DAG activates protein kinase C --> phosphorylates

intracellular proteins --> altered cellular function (e.g. contraction of smooth muscle)

IP3 triggers release of Ca++ from storage vesicles (in smooth muscle this will increase contraction)

These signals are terminated by the following: dephosphorylation of IP3 phosphorylating DAG to arachidonic acid actively removing the Ca++

Ligands for intracellular receptors include: steroids, vitamin D, thyroid hormone –regulate gene expression

Mechanism: ligand is lipid soluble, hence can cross plasma membrane binds with intracellular receptor intracellular receptors increases gene transcription by directly

or indirectly acting on promoter region

Drugs that act on intracellular receptors are slow acting because gene transcription takes time, but endure hours after the drug has been eliminated from the body

Protein Tyrosine Kinase

Mechanism: Ligand (includes insulin, growth factors) binds to

receptor

Enzymes (tyrosine kinases) located in the cytoplasmic domains (receptors have large extracellular and intracellular domains) are activated and cause phosphorylation of certain targets

These targets are activated or inactivated by this