Ch 3 Organ-Selective Toxicity

Why does organ-selective toxicity occur?

Different organs differ in:

• Exposure

• Metabolism

• Molecular targets

• Transporters

• Susceptibility

• Ability to adapt or repair.

Local Toxicity

Occurs at the site of first contact with the toxicant

Local Toxicity eg

Irritation of skin or gastrointestinal tract

Organotropic Toxicity

Occurs in a specific target organ after systemic uptake

Name one drug which induces renal toxicity

Cephaloridine

Cephaloridine

β-lactam antibiotic

Which transporter does Cephaloridine have a high affinity for?

OAT1

Which transporter does Cephaloridine have a low affinity for?

OAT4

Where does uptake occur for Cephaloridine?

Proximal tubule cells

What is the consequence of Cephaloridine?

Mitochondrial toxicity

What drug inhibits renal toxicity of Cephaloridine?

Probenecid

Why does paracetamol preferentially cause liver toxicity?

High expression of CYP2E1 in the liver

Phase II metabolism of paracetamol

• Glucuronidation

→ glucuronide conjugate

• Sulfation

→ sulfate conjugate

These metabolites can then be excreted

Normal NAPQI and detoxification

Paracetamol → CYP2E1 → NAPQI → detoxified by GSH → non-toxic conjugates → excretion

Paracetamol overdose

More paracetamol → more NAPQI formation → GSH becomes depleted → NAPQI no longer adequately detoxified → NAPQI interacts with cellular macromolecules/proteins → oxidative stress + mitochondrial dysfunction + cellular injury → hepatocyte death → acute liver injury/failure

Why specifically the liver?

1. High metabolic capacity

2. High CYP2E1 expression

3. High exposure

4. Metabolic function

5. Cellular susceptibility

High metabolic capacity

Liver contains large amount of drug-metabolising enzymes

High exposure

The liver receives a large amount of blood and is exposed to absorbed xenobiotics

Metabolic function

The liver is the major organ responsible for xenobiotic metabolism

Cellular susceptibility

Hepatocytes can be particularly vulnerable when reactive metabolites cause mitochondrial and oxidative injury. The liver receives a large blood supply and contains many drug-metabolising enzymes

Which part of the liver has relatively high CYP2E1 activity?

Centrilobular/zone 3 hepatocytes

What can diethylstilbestrol (DES) bind to?

Estrogen receptor (ER)

DES mechanism

DES → ER binding → altered expression of ER-responsive genes → abnormal developmental signalling → reproductive tractabnormalities

What can phthalates activate?

PPARα

Where is PPARα particularly abundant in?

The liver

What can PPARα activation cause?

• Peroxisome proliferation

• Liver effects

• In rodents, non-genotoxic hepatocarcinogenesis

Egs of mechanisms protecting against stress

• Antioxidant responses

• Stress responses

• Detoxification pathways

• DNA repair

• Protein repair/removal

• Autophagy

• Regeneration

Why are the liver and kidney common target organs?

• Receive a large amount of blood

• Have high levels of metabolic enzymes

• Have highly polarised cells

• Express many transporters

• Can concentrate xenobiotics/metabolites

• Can have high local concentrations of toxic substances

Why is specifically the liver a common target organ?

• It is the first major organ exposed to many absorbed xenobiotics.

• Toxic compounds can become concentrated in bile.

• It contains many resident immune cells, allowing immune-mediated toxicity

Why is specifically the kidney a common target organ?

• Proximal tubules have extensive transporter-mediated uptake.

• Renal excretion can concentrate xenobiotics/metabolites in tubular fluid.

• Tubular cells therefore may experience high local exposure

Cholestasis

Impaired formation, secretion or flow of bile

What does bile contain?

• Bile acids

• Bilirubin

• Cholesterol

• Phospholipids

• Xenobiotics/metabolites

What happens when bile transport is disrupted?

Bile components can accumulate in hepatocytes and/or the biliary system. This can cause hepatocellular injury and cholestatic liver injury.

Basolateral/sinusoidal membrane

Faces the blood

Apical/canalicular membrane

Faces the bile canaliculus

Uptake transporters

• OATP

• OAT

• Other organic ion transporters

Efflux transporters

• BSEP

• MRP2

• MDR1/P-glycoprotein

• MDR3

Where are many of the transporters located in the liver?

Apical/canalicular membrane (t)

How can a xenobitioic inhibit tranpsorters?

Drug inhibits BSEP (Bile Salt Export Pump) → reduced bile acid export → bile acid accumulation → hepatocyte injury

How can a xenobitioic reduce tranpsorter expression?

↓ transporter expression → ↓ transport → accumulation of substrate

→ toxicity

How can a xenobitioic reduce tranpsorter expression?

The transporter may be present in the cell but not correctly positioned in the membrane

How can a xenobitioic cause tranpsorter internalisation?

Transporter proteins can be removed from the plasma membrane. This reduces functional transport

Cholestasis and accumulation mechanism

Toxicant/drug→ Transporter inhibition or dysfunction → Reduced bile export → Accumulation of bile acids/drugs/metabolites → Cellular stress and injury → Cholestasis

ABC function

ATP-binding cassette transporters

• Use ATP energy to transport substrates. Therefore they can transport substances against their concentration gradient

• Export pumps

Where are ABCs often located?

Apical membrane

ABC egs

• P-glycoprotein/MDR1

• MRP2

• BSEP

How do ABC transporters normally protect?

Toxic compound → ABC efflux → removal from cell

→ protection

What can excess efflux from ABC transporters produce?

Reactive metabolite → export into bile

→ high concentration in biliary compartment → local damage

Give an example of an extracellular compartment where ABC transport activity can lead to up-concentration of reactive metabolites

Bile canaliculi

What happens if an efflux transporter stops working?

↓ efflux → Intracellular accumulation of toxic compounds → Cellular toxicity