Drug handling in liver impairment Flashcards
In relation to drug handling, what parameters are affected by LD?
Drug clearance, especially if a drug has higher hepatic clearance
Biotransformation is the process by which substances that enter the body are changed from hydrophobic to hydrophilic molecules to facilitate elimination from the body
Pharmacokinetics
What are some of the factors that are responsible for the alteration in parameters? What is the overall effect?
Intestinal absorption
Plasma protein binding due to reduce synthesis of the proteins. If a drug is highly plasma bound, this means in LD there will be an increase in the ‘free’ drug
Hepatic extraction ratio
Liver blood flow
Portal systemic shunting due to portal hypertension, meaning that the liver can be bypassed
Biliary excretion
Enterohepatic circulation
Reduced renal clearance due to reduced renal blood flow
Essential all of these factors can contribute to an increased bioavailability, meaning that usual doses can become toxic
Are the effects on drug handling predictable in a patient with liver disease?
Unlike dose adjustments in renal function, both the severity and type of liver disease does not always correlate to elevations in liver function tests and increase in bioavailability.
There are no specific dose reductions that are required.
What is phase I metabolism?
Phase I biotransformation reactions introduce or expose functional groups on the drug with the goal of increasing the polarity of the compound. As most small molecule drugs are lipophilic in nature, drug metabolism converts these hydrophobic compounds into more water soluble compounds that can be excreted.
What are some examples of processes that occur in phase I metabolism?
Oxidation (e.g. Azathioprine)
Reduction (e.g. Halothane)
Hydrolysis (e.g. Atropine)
What is phase II metabolism?
Phase 2 metabolism involves conjugation - that is, the attachment of an ionised group to the drug. The attachment of an ionised group makes the metabolite more more water soluble ready for excretion out of the body.
What are some examples of processes that occur in phase II metabolism?
Glucuronidation (e.g. Paracetamol)
Sulphonation (e.g. Steroids)
Acetylation (e.g. Hydralazine)
Methylation (e.g. Nicotine)
What is the rate of hepatic clearance dependent on?
Both the metabolism capacity but also the blood flow through the liver
What does having a high extraction ratio mean?
That the clearance of the drugs in this categories is highly dependent on hepatic blood flow.
What does having a low extraction ratio mean?
That the clearance of the drugs in this category is not dependent on hepatic blood flow but instead relies more heavily on metabolism.
What are examples of drugs within the high extraction ratio?
Chlormethiazole,
Lignocaine,
Morphine,
Propranolol,
Verapamil,
Metoprolol,
Pethidine
What are examples of drugs within the low extraction ratio?
Chlorpropamide,
Phenytoin,
Diazepam,
Warfarin
Atenolol,
Frusemide,
Prednisolone,
Lorazepam
What do drugs with a high extraction ratio undergo?
Extensive first pass metabolism by the liver
How do the extraction ratios differ between low and high?
Drugs with a high extraction ratio will have this value as close to 1 and those with a low extraction ratio this will be close to 0.
Which type of drug require a greater reduction in dose?
Drugs with a high extraction ratio although unlike renal impairment there are no set requirements.