PHAR 503 EXAM I Lecture Review (\(^.^)/)

(8/28) Jaki Lecture: Diuretics in Medicinal Chemistry Kidney Function: The functions of the Kidney are performed throughout the creation of urine

- Nephron: The functional unit of the Kidney, number about 1 million per kidney. Located in 2 tissue types o Cortex: Outer, Medulla: Inner - Each have their own electrolyte concentrations

- (1) Maintain Homeostatic balance of electrolytes and water ~essential for all metabolic reactions - (2) Excrete water-soluble end products of metabolism ~detoxify

Mechanisms of Urine Formation: Relies upon 2 important physical processes - Osmosis: Diffusion of water through a semi-permeable membrane from an area of high water potential (low

solute) to an area of low water potential (high solute concentration) - Active Transportation: Active transport creates the osmotic gradient, driving force = Na+/K+/ATPase - Goal: For efficient urine production, the goal of urine formation to create a filtrate iso-osmotic to the blood

Anatomical and Physiological components of Urine formation - Glomerular Filtration: Utilizing the hydraulic pressure from the pumping heart, small molecules are filtered

from the plasma by the glomerular capillaries forming the luminal fluid. As a result, there is a change in [osmotic] o 650ml of plasma flows through the Kidney each minute, only 125ml is filtered, and 1ml urine formed.

- Proximal Tubule Reabsorption: Water, Glucose, Vitamins, AA, Na+, Cl-, Bicarb, K+ are reabsorbed o Transcellular Re-absorption of Na+/HCO3- by Carbonic Anhydrase

§ Na+/H+ Antiport sends protons into the tubule lumen à Thus Na+ reabsorption § In the lumen, H+ + HCO3- à H2CO3, which is converted by CAIV to H2O and CO2 § CO2/H2O Symport reabsorbs the molecules, which is converted by CAII to H2CO3 § H2CO3 can spontaneously dissociate to H+ and HCO3-, thus restoring H+, and also supplying

HCO3- for symport at the basolateral membrane into the interstitial space with Na+ § Net Effect: Sodium and HCO3- are reabsorbed together from luminal fluid into tissue

o Transcellular Na+ Transport Coupled with Glucose, AA, Phosphate Reabsorption [Active+Passive] § Na+/K+/ATPase actively transports sodium out of luminal cells into lumen, thus creating an

osmotic gradient driving force § Na+/Glucose, AA, Phosphate Symport to reabsorb

o Paracellular Transport: Through cells without entering membranes (Na+/Cl-) Diffusion - [Passive] o OVERALL: 60% of filtered Na+ is removed in the proximal tubule

- Descending Loop of Henle: (Concentration) Alteration of Osmotic Concentration allows for Water reabsorption o As the nephron dips into the Medulla, it enters an area of high salt concentration. As a result of osmotic

pressure, water is drawn via osmosis into the surrounding tissue (removes 15% of the filtered water) o Surprisingly, Na+ is added to the luminal fluid from the surrounding tissues via [Active] transport

- Ascending Loop of Henle: (Dilution) Impermeable to water, This limb involves active transportation of salts o Transcellular Reabsorption of Na+ [Active+Passive]

§ Na+/K+-Antiport ATPase deposits K+ from blood to epithelium and returns Na+ to the blood • Creates an intracellular Na+ deficit

§ Na+/2Cl-/K+ Symporter withdraws from the lumen these electrolytes (with gradient) o Paracellular Transport [Passive] – Driving force is voltage difference

§ Na+, Ca2+, K+, Mg2+, (+) urine/lumen à (-) Interstitial/Blood § *Important to note: This compensates for Na+ content due to diuretics active in proximal tubule,

thus causing these diuretics to be relatively mild. The loop of Henle will compensate - Distal Tubule: Largely hormone-driven, Secretion of NH3, Uric Acid, PCN. Reabsorb Water

o Transcellular reabsorption of Na+ [Active+Passive] § Na+/K+-Antiport-ATPase causes an intracellular deficit of Na+, thus promoting Na+/Cl-

Symport from the urine. Therefore, the driving force, again, is the Na+/K+ ATPase - Collecting Duct: Further reabsorption of Water, and final adjustments of the electrolytes (Na+, K+, H+, HCO3-)

o Transcellular reabsorption of Na+ [Active+Passive] § Na+/K+-Antiport-ATPase causes an intracellular deficit of Na+, thus promoting Na+ import

coupled to K+ export into the urine. As a result, this is where a major loss of K+ occurs

**In reference to the Nephron, the membrane between the epithelial cell and blood will be referred to as the basolateral, anti-luminal, or epithelial membrane

Urine Formation Recap: The final goal of urine formation was producing iso-osmolarity between the Urine and ECF

Diuretic in General: Promote urination by increasing the rate of renal excretion of water and electrolytes

- By maintaining a high Na+ concentration in luminal fluid, you can prevent water reabsorption - Most diuretics work by inhibiting Na+ transport at one or more of the sites regulating Na+ reabsoption

o Decrease Na+ Reabsorption will increase Osmotic Gradient à Promotes water diffusion to lumen - Most diuretics enter luminal fluid by glomerular filtration and concentration by renal tubular secretion - Amount of diuretic entering the fluid depends on GFR, Plasma Drug concentration, and Protein binding - Diuretic concentration occurs: (1) OAT/OCT active secretion into proximal tubule, (2) Passive diffusion

o This is important because drugs with incur difficulties being cleared out through competition with Uric Acid for these OAT/OCT transporters. Diuretics may trigger gout flareups.

- All diuretics work to increase the osmotic concentration of the luminal fluid and resulting reduced water re-absorption

Osmotic Diuretics: Mannitol, Isosorbide, Sorbitol [Only diuretic completely filtered] - Structure: Low MW, freely filtered through Bowman’s capsule, highly water-soluble, minimally reabsorbed - MoA: Chemically, metabolically, and pharmacologically inert à Thus, little or no toxic side effects!

o These compounds artificially concentrate the urine, thereby increasing the osmotic gradient. As a result, there is decreased water reabsorption, thus increasing the volume of luminal fluid

- Special: They are weak diuretics, rarely used today. Only diuretic completely filtered Carbonic Anhydrase Inhibitors: Acetazolamide, Methazolamide, Sulfanilamide, Dichlorfenamide, Dorzolamide

- Indications: Glaucoma (Dorzolamide), certain types of epilepsy - Structure: 2 Classes of CAI, each require a free sulfonamide group, attached to an aromatic/heterocyclic ring

o Simple Heterocyclic Sulfonamides: Acetzolamide and Methazolamide o Meta-Disulfamoylbenzene derivatives: dichlorphenamide, dorzolamide

- General MoA: Work mainly at the proximal tubule. By inhibiting CA, there will be depleted H+ and HCO3- levels, therefore decreasing Na+/H+ antiport into the tissue. As a result of less Na+ reabsorption, less water will be reabsorbed. The net result is an increase volume of the luminal fluid

- Chemical MoA: CAI binds in enzyme active site in place of CO2 and H2O. Drug binds free –OH groups via hydrogen bonds at the Thr199, Thr200, His64. Also interact with Zn++ and heterocycle/aromatic at Phe131

Thiazide Diuretics: Aka Benzothiadiazines, includes: Chlorthiazide, Hydrochlorothiazide

- Indications: A Mild diuretic appropriate for the long-term use in - Structure: Many Thiazides have a free sulfonamide group with a nearby heterocyclic

structure, and therefore have CAI activity. The lead structure for this class is Chlorthiazide o S1/N-2, Carbonyl groups can replace the sulfonamide. (N-2 can only tolerate methyl) o C3: Lipophilic substituents increase duration of action o N4/C5/C8: Substitution of a methyl diminishes diuretic activity o C6: EWG is required for diuretic activity, deactivates the ring to protect from metabolic reactions. o C7: Sulfonamide groups is essential for diuretic activity

- MoA: Operating at the distal tubule, thiazide diuretics inhibit the luminal membrane Na+/Cl- symporter, responsible for removing Na+ and Cl- out of the urine.

- Efficacy: Mild, promotes a moderate increase in water excretion. - AE: Hypokalemia may occur due to excessive K+ excretion in response to increased levels of Na+ in entering the

downstream collecting duct (Na+/K+ exchange occurs, Na+ in, K+ out.) o Uremia: Thiazide/thiazide-like diuretics may induce hyperuricemia because it competes with uric acid for

OAT for secretion into the luminal fluid. Thus, buildup of uric acid may crystalize and precipitate gout. o Lithium toxicity: May need to adjust Lithium dose in long-term thiazide patients. o Majority of AE are due to possession of sulfamoyl moiety, associated with hypersensitivity reactions

(urticarial-hives), drug-induced fever, blood dyscrasias, interstitial nephritis [Thambi says this is untrue] Loop/High-Ceiling Diuretics: Furosemide (Lasix), Bumetanide (Bumex), Torsemide (Demedex)

- Indications: Edema - Structure: This class has diverse chemical structures, such that the class is best characterized

by pharmacologic similarities – Such a peak diuresis (“High ceiling”) o Sulfonamide moiety: Present on Furosemide and Bumetanide, this group exhibits weak CAI activity in

the proximal tubule, thereby increase phosphate and bicarbonate excretion - MoA: Operating in the thick ascending limb of the Loop of Henle, these drugs inhibit the luminal membrane

Na+/2Cl-/K+ Symporter - Phenoxyacetic Acid-type High Ceiling Diuretics: Operate on the basolateral membrane, inhibiting the Na+/K+-

ATPase pump, in addition to block the luminal membrane Na+/2Cl-/K+ symporter o Drugs: Etacrynic Acid, Indacrinone

Potassium-Sparing Diuretics: There are two separate classes of potassium sparing diuretics we will focus on - Selective Sodium Channel Blockers: Amiloride, Triamterene

o Indication: o Structure: These highly basic molecules are found in their cationic (BH+)

forms when active o MoA: Operating at the late distal tubule and collecting duct, these drugs

block sodium channels on the luminal membrane via a cationic exchange mechanism. By preventing sodium reabsorption, potassium is not lost (spared) and diuresis is promoted.

- Aldosterone Receptor Antagonists: Aldosterone, Spironolactone (Aldactone), Canrenone, Eplerenone o Indication: o Structure: Medications resemble the endogenous mineralcorticoid, Aldosterone, responsible for ADH

action o MoA: Operate in the distal tubule at MR (Mineralcorticoid receptors), acting as antagonists through

competitive inhibition, thereby preventing the transcription of collecting duct Na+ and Na/K-ATPase channels. Since these medications modulate protein expression at the genetic level, they take time to have an effect

Other Diuretics - The Methylxanthines (Theophylline, Caffiene, Theobromine) produce a diuretic effect due to their ability to

increase renal blood flow and the GFR. Of the 3, Theophylline is the most effective