Target Organ Toxicity: Liver, Kidney, and Heart

Follow-up on Biomarkers of Alcohol Consumption

• Open-Ended Question Context: From a previous lecture, students were asked to identify biomarkers for specific scenarios.

• Multiple Options: Any of the following four biomarkers are acceptable for Part A of the question:

  • Ethyl glucuronide.

  • Ethyl sulfate.

  • Fatty acid ethyl esters.

  • Phosphatidylethanol.

• Grading Criteria: For Part B, the identified enzyme must match the biomarker selected in Part A. If the enzyme identified in Part B does not correspond to the biomarker from Part A, no mark is awarded for Part B.

Introduction to Target Organ Toxicity

• Scope of the Lecture: The lecture focuses on specific toxicity occurring in particular tissues or organs.

• Learning Objectives: The primary focus is to "explain" mechanisms rather than merely describe occurrences. This involves detailing the processes and underlying physiological or chemical triggers of toxicity.

• Exposure Types:

  • Intentional: Self-harm or suicide attempts (e.g., deliberate paracetamol overdose).

  • Unintentional: Inadvertent consumption, such as taking a cough syrup unaware that it already contains paracetamol, leading to unintended overdose.

• Reversibility of Damage:

  • Reversible: Occurs when a substance causes a functional change in enzymes or receptors. Toxicity disappears once the offending drug is removed.

  • Irreversible: Causes structural damage, such as cell death, which cannot be undone.

Factors Influencing Target Organ Toxicity

• Organ-Specific Factors:

  • Perfusion: Highly perfused organs receive more blood and, consequently, more exposure to the toxicant.

  • Accumulation: Certain organs may naturally accumulate specific toxicants.

  • Metabolic Profile: Organs like the liver have high levels of drug-metabolizing enzymes (responsible for detoxifying or bioactivating substances).

  • Transporters: The expression of specific transporters (e.g., on the basolateral membrane of hepatocytes) can actively move toxicants into cells.

  • Target Expression: A drug target might be expressed across multiple organs. Toxicity occurs when a drug binds to these targets in organs unrelated to the primary therapeutic effect.

• Patient-Related Factors:

  • Genetics: Genetic variations influence response and metabolism, leading to abnormal individual reactions.

  • Polypharmacy: Concurrent use of multiple medications increases risks of drug-drug interactions.

  • Dietary Interactions: Drug-food interactions can alter a patient’s ability to eliminate drugs, causing accumulation (e.g., in the kidneys).

  • Lifestyle: Habits such as smoking or drinking alcohol affect organ function and the capacity to handle toxicants.

Drug-Induced Liver Injury (DILI)

• Susceptibility of the Liver:

  • High perfusion.

  • Abundant expression of drug-metabolizing enzymes and transporters.

  • Site of both detoxification and bioactivation (the process where relatively inert substances are converted into toxic/reactive metabolites).

• Classification of DILI:

  • Intrinsic DILI: Predictable, dose-dependent, and reproducible in animal models. Example: Paracetamol (Acetaminophen).

  • Idiosyncratic DILI: Rare, unpredictable, and specific to the individual. It is not dose-dependent and often not reproducible in animal models. Example: NSAIDs like Diclofenac.

• General Mechanisms of Liver Injury:

  • Uptake: Transporters on the basolateral side move the drug/metabolite into hepatocytes.

  • Adduct Formation: Reactive metabolites form adducts with cellular proteins, reducing protein function and causing cellular stress (specifically in the mitochondria).

  • Mitochondrial Damage: Interruption of the electron transport chain leads to reduced $ATP$ synthesis and subsequent cell death.

  • Immune Trigger: When cells lyse, drug-protein adducts are released. These act as "neoantigens," triggering hypersensitivity reactions.

  • Bile Accumulation: Metabolites can inhibit efflux transporters, such as the Bile Salt Export Pump ($BSEP$). This leads to the accumulation of bile acids in hepatocytes, causing mitochondrial stress and apoptosis.

Paracetamol (Acetaminophen) Induced Hepatotoxicity

• Epidemiology: While many cases are intentional overdose, some are due to misuse. Data from 2007 to 2017 shows an increase in poisoning cases related to 4-aminophenol derivatives.

• Metabolism Pathways:

  • Phase II Reactions: Paracetamol undergoes glucuronidation and sulfation.

  • Adults: Glucuronidation is the predominant pathway.

  • Fetus/Pediatric: Sulfation is dominant because $UGT$ (UDP-glucuronosyltransferase) expression is very low at birth and increases with age.

  • Bioactivation: Paracetamol is converted into a toxic metabolite, $NAPQI$ (N-acetyl-p-benzoquinone imine), by cytochrome $P450\,2E1$ ($CYP2E1$).

  • Detoxification: In normal doses, $NAPQI$ is quickly detoxified through glutathione conjugation.

• Mechanism of Toxicity:

  • Glutathione Depletion: In overdose, the abundance of $NAPQI$ exceeds the body's glutathione supply.

  • Cellular Stress: Loss of glutathione breaks the balance between oxidation and reduction.

  • JNK Activation: Dephosphorylation and translocation of c-Jun N-terminal kinase ($JNK$) to the mitochondria occur.

  • ROS Production: This interrupts the electron transport chain and generates reactive oxygen species ($ROS$), leading to hepatocyte death.

• CNS Metabolites (FYI): In the central nervous system, paracetamol is converted to p-aminophenol and then to $AM404$, which is a pharmacologically active metabolite potentially responsible for its therapeutic analgesic effects.

Organ-on-a-Chip Research

• Concept: Since animal and static in vitro models have limitations (e.g., interspecies differences or lack of fluid exchange), "Liver-on-a-Chip" technology is used.

• Findings in Paracetamol Study:

  • The setup mimics blood flow and hepatocyte channels.

  • Data shows a concentration-dependent reduction in glutathione ($GSH$) and $ATP$ synthesis.

  • Visible formation of $ROS$ is indicated by purple coloring in imaging.

  • Presence of a $GSH$-depleting agent significantly increases $ROS$ production even at lower paracetamol concentrations (e.g., $3\,mM$).

Idiosyncratic DILI and NSAIDs

• NSAIDs Class: Most drugs in the non-steroidal anti-inflammatory class cause idiosyncratic liver injury, with the exception of Aspirin (which is dose-dependent/intrinsic).

• Diclofenac: The leading cause within the NSAID class, responsible for over one-third of idiosyncratic DILI cases.

• Mechanism: Metabolism produces reactive and toxic metabolites (some ending in quinone imine structures similar to $NAPQI$). These cause mitochondrial injury and oxidative stress. Adducts released after cell death trigger immune responses.

• Enzymatic Note: $UGT2B7$ (also involved in morphine metabolism) can produce reactive metabolites from diclofenac.

Drug-Induced Nephrotoxicity (Kidney)

• Factors: Susceptibility depends on existing kidney function, patient age/comorbidities, and specific drug properties.

• Major Nephrotoxic Classes:

  • NSAIDs.

  • Aminoglycosides (to be covered in future lectures on antibacterials).

• NSAID Mechanism in the Kidney:

  • Normal Physiology: Cyclooxygenase ($COX$) converts arachidonic acid into Prostaglandin $G2$, then $H2$, and finally Prostaglandin $E2$ ($PGE2$).

  • Role of PGE2: In the kidney, $PGE2$ causes vasodilation (relaxation of renal vessels), maintaining renal blood flow, especially in compromised patients.

  • Toxicity: NSAIDs inhibit $COX-1$ and $COX-2$, reducing $PGE2$ levels. This leads to vasoconstriction, reduced renal blood flow, and acute renal dysfunction.

  • Acute Interstitial Nephritis: A separate mechanism involving a Type 4 delayed hypersensitivity reaction where drug-protein adducts trigger inflammation in the renal tubules.

Cardiotoxicity and Trastuzumab

• Drug Profile: Trastuzumab is a monoclonal antibody (mab), a large biologic molecule administered via injection (not oral).

• Primary Indication: Treats $HER2$-positive breast cancers by binding to the extracellular domain of the Human Epidermal Growth Factor Receptor 2 ($HER2$).

• Anti-Cancer Mechanisms:

  • Prevents cleavage of the extracellular domain (limiting constitutive signaling).

  • Prevents receptor dimerization.

  • Induces endocytosis and degradation of the receptor complex.

  • Antibody-dependent cellular cytotoxicity ($ADCC$) mediated by T cells.

• Toxicity Mechanism:

  • $HER2$ is also expressed on cardiomyocytes in the heart, where it normally mediates protective survival signaling.

  • Trastuzumab disrupts this protective pathway in the heart, leading to $ROS$ production and apoptosis of cardiomyocytes.

• Note on Selectivity: This is an issue of tissue selectivity (breast cancer cells vs. cardiomyocytes) rather than drug selectivity (which refers to discriminating between different targets).

• Reversibility: Trastuzumab-induced cardiotoxicity is generally reversible upon cessation of the drug because it is based on target expression rather than immediate structural destruction.

Questions & Discussion

• Question: What immunocomponent mediates Type 1 drug hypersensitivity reactions?

• Answer: Immunoglobulin E ($IgE$). This is an immediate reaction.

• Question: What mediates Type 4 drug hypersensitivity reactions?

• Answer: T cells. This is a delayed hypersensitivity reaction.

• Transcription Note on Nomenclature: The lecturer clarified that acronyms like $NAPQI$ must be exact; written errors such as "NAPQ1" (using the number one instead of the letter I) are marked incorrect in exams.