HLTH 340 Section E: Toxicokinetics of Elimination PDF

HLTH 340 Section E: Toxicokinetics of Elimination Elimination via the Urinary System ▪ The urinary system is composed of primarily four parts: 1. The kidneys 2. The ureter 3. The bladder 4. The urethra ▪ However, for the purposes of excretion of xenobiotics,...

HLTH 340 Section E: Toxicokinetics of Elimination Elimination via the Urinary System ▪ The urinary system is composed of primarily four parts: 1. The kidneys 2. The ureter 3. The bladder 4. The urethra ▪ However, for the purposes of excretion of xenobiotics, we will be focusing on the role of the kidneys. Elimination via the Urinary System The Kidney ▪ The kidney has two main functions: ▪ Removes metabolic waste from the body (excretion) ▪ Regulates the water and ion content in the blood. ▪ Functions achieved through excretion of a dilute solution (urine) which contains urea, mineral ions, water, and xenobiotics from the blood. The two kidneys have a very extensive blood supply and the whole blood supply passes through the kidneys every five minutes ensuring that the waste materials don’t build up. Elimination via the Urinary System Besides urine formation, the kidney has the following functions: ▪ Plays a major role in regulating blood volume because it controls the amount of water to be excreted and the amount of water to be reabsorbed. ▪ Renal damage can impact blood pressure ▪ Regulates electrolytes in the blood by controlling the secretion and reabsorption of sodium and potassium ions. ▪ Regulates the pH of the blood by controlling the secretion and reabsorption of hydrogen ions. ▪ More hydrogen ions are excreted from the blood, it renders the blood less acidic (more alkaline). ▪ Hydrogen ions are retained in the blood, this renders the blood more acidic (less alkaline). ▪ Strict homeostatic control of blood pH is maintained by varying pH of urine Elimination via the Urinary System ▪ Regulates blood pressure by regulating to amount of water excreted and the amount of water reabsorbed back into the blood. ▪ Plays a role in the regulation of red blood cell production. When the number of red blood cells decreases, the level of oxygen in the blood will also decrease. This causes the kidney to secrete a substance called erythropoietin. ▪ Erythropoietin travels to the bone marrow and causes it to produce more red blood cells. When enough red blood cells have been produced, this process is shut down via a negative feedback mechanism. Elimination via the Urinary System The renal artery carries blood to the kidneys and the renal vein carries blood away from the kidneys. Blood flows to the kidneys of the adult human at a rate of roughly 1 L/min. The most important part of the kidneys is a tubule called a nephron. The adult human kidney contains approximately 1 million nephrons. The Nephron Source: http://www.tutorvista.com/content/biology/biology-iv/excretion/nephron-structure.php Kidney Nephron System Glomerular ultrafiltration about 1/3 of blood plasma is filtered by the glomerulus to form dilute urine (ultrafiltrate) Tubular secretion PCT selectively secretes certain xenobiotics into urine Tubular reabsorption DCT and CD may reabsorb Source: many xenobiotics into blood http://www.tutorvista.com/content/biology/biology- by passive diffusion iv/excretion/nephron-structure.php Elimination via the Urinary System There are four primary mechanisms involved in urinary excretion: 1. Filtration -- from the blood to the glomerulus. Blood is filtered at the beginning of the renal tubule. The filtration membrane contains large pores so small molecules readily pass into the tubule. About 99% of the filtrate is reabsorbed into blood, the remaining 1% is excreted as urine. The primary factors that influence filtration are size of the molecule and glomerular filtration, as determined by blood flow and pressure. Proteins and blood cells are too large to pass through the pores. Elimination via the Urinary System Depending upon the toxic agent and/or disease state, glomerular filtration may increase or decrease. Therefore, glomerular filtration rate may be used as an indicator for clearance of a compound that is excreted by renal filtration. Elimination via the Urinary System 2. Passive diffusion -- from the blood to the renal tubules. This is an important route for lipid soluble compounds. If the compound is non-ionised passive diffusion is high. However, if it remains non-ionised in the tubule, reabsorption may take place just as easily. Thus retention in the tubule (and passive diffusion as a means of renal excretion) requires ionisation of the compound in the tubule, which may be altered by pH of the tubular fluid. The primary determinant of diffusion is the concentration gradient of the compound. Elimination via the Urinary System 3. Active transport -- of chemicals from the blood to the tubular lumen by energy-dependent carrier proteins. ▪ These protein transport systems are specific for weak acids or bases and may become saturated, setting a limit on the amount of material excreted in a given time period. 4. Facilitated diffusion -- a process that essentially resembles active transport, except that it does not require energy. Elimination via the Urinary System Other factors affecting urinary excretion include: ▪ Plasma protein binding: ▪ given a compound that is highly bound, its ability to undergo glomerular filtration will be decreased, note that its ability to be actively transported will NOT be affected. ▪ pH ▪ if the toxicant is a weak acid, alkalinisation of the urine will increase its excretion by maintaining it in the ionised state. The converse is true for weak bases. Renal ultrafiltration and selective tubular reabsorption Log Kow versus Clearance in the Kidney and Liver non-renal clearance (liver) renal clearance (kidney) hydrophilic lipophilic log Kow Failures of Homeostasis ▪ Kidney Disease - caused by long term diabetes, infections, and chemical poisoning. ▪ Bladder & Kidney Infections caused by enteric bacteria entering urethra. All of these impair the function of the kidneys, ▪ Kidney Stones - crystallization of mineral salts and uric acid directly impacting on the that block passage of urine. body’s ability to successfully excrete xenobiotics before toxicity can occur. RECAP: Urinary Excretion RECAP: Glomerular Filtration Class Question What would it mean if one measured albumin in urine? Kidneys receive about 25% of first pass cardiac output, and ~20% is filtered through the glomeruli (25 g/day of urea is filtered and excreted). The glomerular capillaries have very large pores (70 nm) and allow compounds up to a molecular weight of 60 kDa (< albumin) to be filtered. Passive transport Tubular Secretion and Reabsorption Tubular Secretion Active transport for acids, bases, neutrals into renal tubules OCT: organic cation transporter OAT: organic anion transporter MDR/MRP: multidrug resistant transporters Tubular Reabsorption Passive: depends on ionization of xenobiotic; lipophilic substances will be reabsorbed from the tubules more than hydrophilic substances: under high urinary pH, excretion of acids is increased under low urinary pH, excretion of bases is increased Active: OCT’s, peptide transporters (PEP), MRPs Xenobiotic Efflux Pumps (e.g., P-gp/MDR, MRP) ▪ Mediate active transport of hydrophobic endobiotics and xenobiotics ▪ All belong to ABC transporter superfamily ▪ P-gp or P-glycoprotein (MDR1) ▪ No endogenous substrates yet identified ▪ Wide range of xenobiotics ▪ “Hydrophobic vacuum” ▪ Pore forming protein with ABC ▪ MRP1-7 ▪ Overlap in substrates with MDR/P-gp ▪ Also effective on organic anions Renal Excretion and Reabsorption through Transporters Disposition of Xenobiotics By Efflux Pumps and Metabolic Enzymes Efflux Pumps E.g. P-gp Metabolic Enzymes E.g. CYP3A4 Active Transport and Facilitated Diffusion Systems Across Membrane Barriers Kidney cells Active Transport Efflux Pumps (MDR, MRP, P-gp) and Facilitated Diffusion Channels (OAT, OCT) in the Kidney OCT OAT Quaternary Carboxylic acids Ammonium Ions R-COO– Nicotine Metabolites Cyperquat conjugated with: Dopamine Glucuronide MPP+ Glutathione Sulfate E.g. E2-SO42- Elimination of Xenobiotics Urine via Feces via GI Tract and Renal System Liver Filtration Metabolism Excretion Excretion Expired air via OTHERS Lungs Secretions in Breast milk Exhalation Sweat, Saliva etc. Fecal Excretion ▪ Fecal excretion is the second major pathway for the elimination of xenobiotics. ▪ It is a complex process. ▪ Excretion of toxicants via the feces can result from: ▪ Direct elimination of non-absorbed compounds in the GI tract ▪ Delivery to the GI tract via the bile ▪ Secretion into the intestinal luminal contents from enterocytes Biliary Elimination ▪ Biliary elimination is a significant source contributing to the fecal excretion of xenobiotics and is even more important for the excretion of metabolites. ▪ The liver plays an important role in removing xenobiotics from the blood after gastrointestinal absorption because blood from the GI tract passes via the portal circulation before reaching the general circulation. ▪ First Pass Effect ▪ The liver is also the main site for biotransformation of xenobiotics, and the resulting metabolites (typically now more hydrophilic) may be excreted directly into the bile. First-pass effect by the liver Xenobiotics in food and water filtered blood to systemic circulation carried from gut to liver via hepatic portal vein Xenobiotics in systemic circulation are carried to liver via hepatic artery Liver filters out a fraction of lipophiles into hepatocytes lipophiles Remaining lipophiles enter systemic blood via hepatic vein systemic blood to liver Hepatocytes secrete their intracellular lipophiles to bile Biliary excretion carries lipophiles to gut --> feces bile to GI tract blood from GI tract  Structure of the Liver Excretory Unit: the Hepatobiliary Sinusoid Active Transport and Facilitated Diffusion Systems Across Membrane Barriers Kidney cells “Phase 3 Metabolism” in the Liver Facilitated Diffusion Channels Active Transport Pumps Biliary Elimination ▪ Xenobiotics and/or their metabolites excreted into the bile enter the intestine and may be excreted with feces. ▪ However, if the physicochemical properties favour reabsorption, an enterohepatic circulation may result. ▪ The factors determining biliary excretion are not well understood, but in general the following is true: ▪ Xenobiotics bound to plasma proteins are fully available for active biliary excretion. ▪ Low molecular-weight compounds are poorly excreted into bile, while larger compounds or their conjugates (with MWs < 325) can be excreted in large quantities. ▪ Glutathione and glucuronide conjugates have a high probability of being excreted in bile. Biliary Elimination ▪ Substances excreted into the bile typically are divided into three classes based on the ratio of their concentration in bile compared to that in plasma. ▪ Class A: Ratio of nearly 1. Include sodium, potassium, glucose, mercury, thallium, cesium, and cobalt. ▪ Class B: Ratio of bile to plasma usually between 10 and 1000. Include bile acids, bilirubin, lead, arsenic, manganese, and many other xenobiotics. ▪ Class C: Ratio below 1. Include inulin, albumin, zinc, iron, gold, and chromium. ▪ Class B substances are typically rapidly excreted into bile.  Biliary Excretion ▪ Biliary excretion is regulated primarily by xenobiotic transporters present on the canalicular membrane, including: MRP2: Transport of organic anions including glucuronide and glutathione conjugates of many xenobiotics BCRP: Affinity for sulfated conjugates of xenobiotics MDR1: Primarily transports a variety of substrates into bile MATE1: Transport of organic cations BSEP: Secretion of bile salts and the regulation Source: Casarett & Doull’s, 8th edition of bile flow Amanitin ▪ Oligopeptide toxin produced by Amanita phalloides (toadstool) ▪ Lethal human poisoning after ingestion (8-24 h post exposure) ▪ High oral bioavailability Inhibits RNA polymerase II → blocks mRNA synthesis Follows bile salt uptake transporter (facilitated diffusion) OATP1B3 – Humans (Liver, hepatocyte basolateral membrane) NCTP – Rats So treatment for this toxin is to provide an OAT1PB3 transporter substrate (competitive inhibitor), such as rifampicin or cyclosporin. Hepatotoxicity: Cholestasis ▪ Cholestasis is reduction or stoppage of bile flow. ▪ It can be causes by disorders of the liver, bile duct, or pancreas. ▪ The skin and whites of the eyes look yellow, the skin itches, urine is dark, and stools may become light-coloured and smell foul. ▪ It can be noted by the presence of bile plugs in canaliculi; accumulated bile in hepatocytes. ▪ Results in hepatocyte retention of compounds usually eliminated in bile. ▪ It can be caused by an impairment of transporters, such as their expression being down-regulated. ▪ The resulting build up of endobiotics and xenobiotics in hepatocytes can lead to liver toxicity. Enterohepatic Circulation ▪ An important concept relating to biliary excretion is the phenomenon of enterohepatic circulation. ▪ After an xenobiotic is excreted into bile, it enters the intestine where it can be reabsorbed or eliminated with feces. ▪ Many organic compounds are conjugated with UDP-glucuroinic acid, sulfate, or glutathione before excretion into bile, and these polar metabolites are not sufficiently lipid soluble to be reabsorbed. ▪ However, enzymes found in the intestinal microflora may hydrolyze them, making them more lipophilic and increasing the likelihood of reabsorption. Enterohepatic Cycle: Lipophilic Xenobiotics Excreted into Bile May be Reabsorbed in the Gut Liver (conjugated) portal blood bile small intestine (deconjugation by bacteria) (beta-glucuronidase) Enterohepatic circulation causes increased retention of xenobiotics conjugated by glucuronic acid because they are deconjugated in the intestine and reabsorbed. Enterohepatic Cycle: Lipophilic Xenobiotics Excreted into Bile May be Reabsorbed in the Gut Liver excretes both lipophiles and conjugated lipophiles into the bile via efflux pumps (biliary excretion). Lipophiles travel in bile and empty into small intestine at the duodenum. Some fraction of the lipophile is excreted in feces. Remaining fraction is reabsorbed from the gut into blood via passive diffusion. Enterohepatic Cycle: Lipophilic Xenobiotics Excreted into Bile May be Reabsorbed in the Gut Hepatic portal vein carries reabsorbed lipophile back to the liver Enterohepatic cycle slows the clearance of many highly lipophilic xenobiotics Liver metabolism can often biotransform a lipophile to a conjugated lipophile to prevent enterohepatic cycling --> faster clearance Lipophile conjugation groups may be hydrolyzed in the gut by microbial enzymes (beta-glucuronidases)  Example: Diethylstilbestrol (DES) Classified as endocrine disruptor — a synthetic nonsteroidal estrogen OH Exposure to DES occurs through dietary ingestion from supplemented cattle feed. From 1940s-1970s, was used in pregnant women to reduce complications, but caused vaginal cancer in daughters of women who used DES during HO Diethylstilbestrol pregnancy. Undergoes enterohepatic circulation and is retained in the body by sequential conjugation and deconjugation. Biliary Excretion Depends on Phase 1, 2, and 3 Metabolism lipophile (L) degradation hydrophile (Hfree) L Hfree lipophile (L) Hconj conjugation conjugated hydrophile (Hcong) Elimination of Lipophilic Xenobiotics Lipophiles are often difficult to excrete (kidney and liver) ▪ Tend to be persistent in biota ▪ Degree of persistence is measured by the elimination half-life Elimination half-life (t1/2) - measures persistence in body ▪ Rate of elimination of a xenobiotic from body by passive excretion is proportional to blood concentration (first-order kinetics) ▪ t1/2 = time taken to eliminate one-half of remaining xenobiotic ▪ Amount of chemical remaining at each half-life interval ▪ 1-->1/2 --> 1/4 --> 1/8 -->etc. Excretion of Lipophilic Xenobiotics Lipophiles often have a prolonged elimination half-life ▪ Poorly excreted through the kidney due to tubular reabsorption ▪ Enterohepatic circulation limits biliary excretion ▪ This results in bioaccumulation in to create high body burden Body burden ▪ Concentration of a chemical (or chemicals) in body tissues ▪ Most tissues cannot be directly sampled ▪ Biomonitoring studies rely on biomarkers in urine, blood, saliva, etc. ▪ Can “back-extrapolate” from blood concentrations → estimated tissue concentrations → health effects Excretion of Lipophilic Xenobiotics Lipophile elimination speeded by biliary excretion via the liver ▪ Active transport systems -- not dependent on passive diffusion ▪ Conjugation reactions metabolize lipophiles ▪ Several active transport systems secrete conjugated lipophiles into the bile for excretion Persistent Organic Pollutants (POPs) ▪ Highly lipophilic organochlorine (OC) compounds ▪ ‘Dirty Dozen’ - persistent, bioaccumulative, toxic (PBT criteria) ▪ Very slow or non-existent conjugation in liver --> hard to excrete in bile  Thank You for Taking HLTH 340 I hope you enjoyed learning more about environmental toxicology and how it is integral to many facets of public health. If you found environmental toxicology and/or risk assessment interesting, I encourage you to get in touch with Dr. Brian Laird ([email protected]), Dr. Phil Bigelow ([email protected]) or myself ([email protected]) A number of other universities also have strong toxicology/public health graduate programs. I am always happy to discuss my academic career in toxicology and provide guidance where I can!