Lecture 9 - Nephrotoxicity Flashcards
(28 cards)
Kidneys: what are their key functions and their functional units?
- Excretion of waste products and unwanted substances - urea, creatinine, uric acid
Xenobiotics and their metabolites - Regulation of extracellular fluid
- Retention of vital substances (proteins, glucose)
- Conservation of water, electrolytes, amino acids, and other solutes
- Maintenance of acid-base balance
- Endocrine activity - renin, calcitriol, erythropoietin
Nephron - 1-2 million per kidney
Nephron: what are its components and their function?
Glomerulus and Bowman’s capsule:
* Filtration and collection of fluid
Proximal convoluted tubule (PCT):
* Re-absorption of nutrients and most ions
* Tubular secretion
* Movement of waste products
* Xenobiotics and metabolites move from blood into lumen
* Recapture of filtered protein via endocytosis
Loop of Henle:
* Water and NaCl re-absorption
Distal convoluted tubule:
* Hormone controlled re-absorption of Na⁺ and Ca²⁺ and secretion of K⁺
Collecting duct:
* Water re-absorption
* Urea recycling
* Ion homeostasis
Kidney: is it susceptible to toxic injury, and why?
Yes:
* High blood flow (high delivery of toxicants)
~2000 L/day ; 180-200 L filtered every day: GFR ~125 mL/min
* Bioactivation of toxicants - high expression of metabolising enzymes (e.g. CYP450s): toxic metabolite formation
* Powerful concentrating mechanisms (re-absorption & secretion)
* Only 1-2 L of urine produced per day: ~99% of filtrate is re-absorbed
* Unwanted substances (including xenobiotics) secreted into tubules from blood
* High expression of multiple membrane transporter proteins
* High local concentrations of potential toxicants: many readily enter kidney cells. May cause significant damage.
- Proximal convoluted tubule particularly susceptible: major site of reabsorption and tubular secretion, high levels of transporter expression.
Renal injury: how does it manifest in response to toxicants?
- ~25% of serious adverse drug reactions affect the kidneys
- Typically causes ‘acute kidney injury’
- Manifests as decreased Glomerular Filtration Rate (GFR), decreased urinary output, increased Blood Urea Nitrogen (BUN), increased serum creatinine, and altered ion concentrations in blood
- May be reversible, but can sometimes progress to CKD
AKI: what is it, in what percentage of cases is drug nephrotoxicity implicated, and is it reversible?
Acute kidney injury - consensus term for acute renal failure: rapid loss of excretory function (occurring within hours/days – potentially fatal)
Drug nephrotoxicity is implicated in ~25% of all AKI cases
AKI is potentially reversible but associated with long-term changes in kidney function. Can lead to chronic kidney disease (CKD): progressive and irreversible kidney damage.
Detecting renal injury: how frequently is nephrotoxicity noticed during drug development, at what point is it typically noticed within people, and what biomarkers are there?
Most drugs that display nephrotoxicity in clinical trials did not display any in pre-clinical testing: we need better tests
Excretion is typically unaffected until kidney damage is advanced - difficult to assess damage directly – typically need a biopsy
Traditional biomarkers (e.g. BUN, serum creatinine) not very reliable - only increased after significant damage, many factors may affect them including nutrition, biological sex
Renal toxicity testing - in vitro approaches: why are they difficult?
Renal cells (ie PCT) are highly polarised and are affected by various conditions that are difficult to replicate in vitro (mechanical stress, liquid flow, etc)
Renal toxicity testing - traditional biomarkers: what are they and why are they not ideal?
- Blood urea nitrogen (BUN)
- Serum creatinine
Only present following increased after significant damage, not very reliable (may get false positives), and may be affected by individual differences (nutrition, sex, race, etc)
Renal toxicity testing - novel biomarkers: why are they ideal, what would they need to be, and what is a possible example of one?
Indicate renal injury at a much earlier point - allow faster and better treatment
Ideally organ specific, proportional to damage, detectable before appreciable damage occurs, urinary (less invasive than blood)
Kidney injury molecule-1 (KIM-1)
KIM-1: what is it, why can it be a novel biomarker, and how has it been used in animal testing and human testing?
Kidney injury molecule-1 - a PCT transmembrane protein, induced following kidney injury
Presence in urine due to damage to PCT cells - indicates renal damage
- Qualified by regulatory agencies FDA & EMEA as a urinary biomarker in rats
- Use of KIM-1 in first-in-human studies on a case-by-case basis, and used together with 5 other urinary biomarkers and traditional serum biomarkers in Phase I clinical trials where there is concern that tubular toxicity may occur
Renal toxicity testing - a new in vivo approach
Miox a kidney-specific protein, abundant in PCT, that may indicate renal injury
Miox: what is it, how can it indicate renal injury, and how much better is it than traditional biomarkers?
Myo-inositol oxygenase - kidney specific protein, abundant in PCT
Miox levels in serum elevated in patients and animal models of AKI
Plasma Miox levels elevated ~2d before detectable increase in serum creatinine
Renal toxicity: what are the types of damage that may be caused?
- Glomerular injury
- Acute tubular injury
- Acute interstitial nephritis
- Injury sustained by the insolubility of drugs in urine
Renal toxicity: what is the mechanism of glomerular injury?
Damage to glomerulus
Can occur through gentamycin - causes proliferation and contraction of mesangial cells in glomerulus, reducing glomerular filtration
Renal toxicity: what is the mechanism of acute tubular injury?
Toxicant enters kidney epithelial cells (especially PCT) and causes damage e.g. cisplatin, cadmium
Renal toxicity: what is the mechanism of acute interstitial nephritis?
T-cell mediated immune response; inflammation, including infiltration of kidney by immune cells (e.g. immune checkpoint inhibitor drugs, beta lactam antibiotics (e.g. penicillin))
Renal toxicity: what is the mechanism of injury sustained by the insolubility of drugs in urine?
Drugs may crystallise (‘crystal nephropathy’) or form non-crystalline aggregates which can block tubules and/or cause physical tubular damage and inflammation (e.g. methotrexate (crystallises), vancomycin (forms non-crystalline aggregates (casts) in combination with the kidney protein uromodulin))
Often worse in people with pre-existing renal insufficiency; urine pH and flow rate can have significant effects
Transport in the PCT: why is it significant, what are the types, and what are examples of transporters within these types?
PCT cells express a large variety of transporters: secretion of unwanted substances (including xenobiotics) from blood into lumen; recapture of filtered substances from lumen
Apical transport (mostly extrusion into lumen):
- MATEs
- MDR1; MRPs
- OCTNs
- BCRP
- Megalin
Basolateral transport (uptake from blood):
* OCTs
* OATs
* OATPs
Apical PCT transporters: where do they transport substances from and what are some examples?
Mostly extrusion into lumen
- Multidrug and toxin extrusion proteins (MATEs)
- Multidrug resistance (proteins) (MDR1; MRPs)
- Organic cation/carnitine transporters (OCTNs)
- Breast cancer resistance protein (BCRP)
- Megalin: 600kDa membrane protein in apical membrane of epithelial cells: involved in recapture (by endocytosis) of filtered proteins and vitamins
Aminoglycosides: what are they, what is their known nephrotoxicity, how do they enter the kidneys, how concentrated may they be in the kidneys, and how may they interact with cells?
Antimicrobial antibiotics used for serious Gram-negative infections
Notoriously nephrotoxic - neomycin has the greatest nephrotoxicity
Enters PCT cells from the glomerular filtrate (urine) via endocytosis via Megalin (apical membrane of PCT cells)
~5% of the administered dose of the drug may accumulate in PCT cells
- Accumulation in lysosomes - binds phospholipids and eventually leads to lysosome leakage
- Enter cytoplasm and damage mitochondria - ROS production and apoptosis
Basolateral PCT transporters: where do they transport substances from and what are some examples?
Uptake from blood
- Organic Cation Transporters (OCTs)
- Organic Anion Transporters (OATs)
- Organic Anion Transporting Polypeptides (OATPs)
Cadmium: what is it, where is it found, how does it enter the body, what occurs as it enters the body, and what occurs with its administration in mice?
Cadmium is a non-essential metal anti-corrosive agent
A component of some batteries, and a contaminant in food and tobacco
Handling of cadmium by the body:
* Taken up by liver; induces metallothionein (MT) production
* Complexes of cadmium (C) with MT released into the blood
* C/MT complexes are endocytosed into PCT cells via megalin from the filtrate
* Degradation of the complex within PCT cells releases cadmium
* Can induce MT expression in the kidney, but if levels are exceeded, free cadmium accumulates
* Free ions also possibly enter cells via some metal transporters and ion channels
* If free in the cytosol, cadmium induces ROS generation and oxidative stress
Cadmium administration in mice - GSH depletion, increased lipid peroxidation
Initial changes in PCT cell adhesion, altered signalling and autophagy, and then apoptosis
Cadmium and oxidative stress: what does cadmium do at a cellular level?
Increases ROS production and reduces protection:
* Inhibits complex III of mitochondria
* Increases expression of NOX in the kidney
* Cadmium ions can displace copper and iron ions from carrier proteins - promotes Fenton reaction (HO* production)
* GSH and Cysteine bind Cd via –SH groups; can lead to GSH depletion
* Cadmium forms complexes with Selenium in body; excreted via bile, leads to Selenium deficiency
* If both GSH and Se depleted - Peroxiredoxin, glutathione peroxidase, glutaredoxin and thioredoxin are unable to function, then only SOD and catalase are available for enzymatic ROS detoxification and loss of protection from ROS probably the main reason for oxidative stress due to exposure to cadmium
PCT Damage from Basolateral Uptake
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