Pharmacology 3 Flashcards
List some altered physiology in pregnancy that can affect drug kinetics
- Increased plasma volume
- Chronic metabolic alkalosis
- Increased volume of distribution
- Altered protein binding (increased free fraction)
- Increased gastric pH
- Decreased GI motility
- Altered cytochrome function (increased metabolism due to increased energy requirements)
Give some special pharmacokinetic considerations in pregnancy directly relating to the foetus
- Foetal trapping can occur
- Foetus more acidic than maternal side, so will trap weak bases due to becoming ionised
Give some potential detrimental effects of drugs on a foetus
- Teratogenic effects (e.g. nitrous oxide)
- Prevention of implantation
- Early termination
- Mutagenesis
Give some practical considerations of therapy during pregnancy
- Difficult to predict pharmacokinetics
- Avoid drugs where possible, use those with marketing authorisation
- Many are “off-label” in pregnancy
List some pharmacokinetic effects of neonates
- Increased surface area to volume ratio in neonate (allometric scaling)
- Increased metabolic rate but decreased metabolic function
- GI absorption variable
- More rapid topical absorption
- Increased Vd for on-lipophilic drugs
- BBB not complete first few days postpartum
- Lower adipose content
- Reduced hepatic function (species specific)
- GFR takes time to become normal
- Tubular secretion takes longer
Explain the GI absorption in neonates
- Variable
- Altered gastric emptying
- Irregular peristalsis
- Increased permeability of mucosa so absorption easier
Explain why neonates have an increased metabolic rate but decreased metabolic function
Lower amount of CYP enzymes but higher general metabolic rate
Why do neonates have more rapid topical absorption?
Immature percutaneous barrier
Why do neonates have an increased volume of distribution for non-lipophilic drugs?
- Greater water content
- Decreased plasma protein binding
Explain how the lower adipose content of neonates affects pharmacokinetics
- Less fat uptake and sequestration of drug
- May affect maintenance of plasma level
- Dosing, dosing frequency needs to be considered
Explain why neonates have slower tubular secretion
Immature expression or formation of transporters in tubules in kidney
What is the effect of neonatal status on drug dose?
Usually decrease dose as increased risk for toxicity
- Sometimes complete contraindication
Outline some pharmacokinetic effects of old age
- Gastric pH increased
- Less microvilli
- Reduced gastric motility and emptying
- Lower expression of enzymes
- Decreased body mass
- Less water content
- Increased adipose tissue
- Increased Vd for fat soluble drugs
- Less Vd for water soluble drugs
- Decreased plasma albumin
- Minimal effects on metabolism!
- Decreased renal elimination
- Concurrent disease likely
How does altered gastric function in elderly patients have an effect on pharmacokinetics?
- Delayed disintegration of tablets
- Altered ionisation
- Less mixing and dissolution
- Slower absorption
How does plasma albumin in elderly patients affect pharmacokinetics?
- Lower plasma albumin, more free drug
- Increased potential for toxicity
- Only a concern in IV administration
What factor has the greatest effect on pharmacokinetics in elderly patients?
- Reduced renal elimination
- Decreased renal mass, GFR and tubular secretion
- Similar to animal with chronic disease
Give examples of concurrent disease in elderly patients and how this may affect pharmacokinetics
- Chronic cardiovascular disease: decreased mentation, increased effects of sedatives when used, decreased cardiac output
- Respiratory disease: altered serum pH and protein binding
Outline how hepatic failure affects pharmacokinetics
- Content and activity of Phase I and II reactions decreased
- Little effect on drug metabolism until 80% functional loss
- Most antimicrobials well tolerated, but not licosamide, macrolides, sulphonamides and chloramphenicol
Outline how renal failure affects pharmacokinetics
- Most profound changes in PK
- Gradual loss of urine concentrating ability and ability to acidify
- Altered drug distribution patterns
- Loss ofability to acidify iincreases retention of basic drugs and excretion of acidiic drugs
How does uraemia affect pharmacokinetics?
- Chronic acidosis, reduced albumin binding of drug as less albumin present
- Leads to less hepatic metabolism
Define the term therapeutic index
A comparison of the amount of therapeutic agent that causes the therapeutic effect to the amount that causes toxic effects
Give examples of drugs with a high therapeutic index
- NSAIDs
- Sedative/hypnotics (e.g. benzodiazepines)
- Most antibiotics
- Beta-blockers
Give examples of drugs with low therapeutic indices
- Lithium
- Anaesthetics
- Neuroleptics (e.g. phenytoin, phenobarbital)
- Some antibiotics (e.g. gentamycin, vancomycin, amikacin)
- Digoxin
- Immunosuppressives
- Alcohol
Give examples of factors affecting the margin of drug safety
- Receptor saturation (e.g. medetomidine, alpha-2 agonsit at presynaptic cleft)
- Ability to bind to multiple targets
- Volume of distribution
Explain the effect of volume of distribution on drug safety margins
- More dangerous with high Vd
- Lots of distribution into tissues
- Tend to be lipophilic so binds to many targets in body, poor selectivity
- Small changes in plasma concentration reflect large changes in tissue
What is the importance of drug monitoring?
- Avert toxicity
- Optimise dose/therapeutic response
- Detect changes in pharmacokinetics
- Monitor compliance
When is drug monitoring particularly important?
- Low therapeutic index
- Chronic therapy
- Combination of both
- Polypharmacy
What is the importance of sampling at steady state when monitoring drug therapy?
- Provides best correlation between serum drug concentrations and clinical status
- Amount of drug eliminated is equal to amount administered
How is drug toxicity assessed for monitoring drug therapy?
- Sample at peak concentration
- Difficult to determine as may be individual variability
How is drug efficacy assessed for monitoring drug therapy?
Sample at concentration trough, just before next dose is administered, thus easier to time correctly
In drug clearance, what factors determine the time for the concentration in the plasma to fall to a set level?
- Rate of elimination
- Dose given
- Volume of distribution
- Method of elimination
- Blood flow
- Properties of the drug
Which of theese factors determine the peak plasma concentration after a bolus: dosage, volume of distribution, clearance?
- Dosage
- Volume of distribution
What would be the effect on drug distribution in an emaciated/starved animal with poor body condition?
Reduced adipose and muscle tissue for drug to distribute into, plasma volume also reduced. Phase I rate would be reduced (rate constant reduced), leading to faster phase II concentration drop
What is the clinical significance of K12 and K21 rate in race horses or dairy cows?
High K12 or low K21 would extend the withdrawal period of a drug
What is the importance of Veterinary medicines regulation in the UK?
- Control risks to human health, animal health and environment
- Provide assurance on efficacy
- Provide reliable information to users
When did the Veterinary Medicines Regulations come into force and what is covered?
2013
- Medicines as well as medicated feeds and some feed additives
List the processes by which regulation is achieved for a veterinary licensed medicinal product
- Data assessment and authorisation
- Certification and qualification
- Prescribing, dispensing and supply
- Testing, inspection and investigation
- Post authorisation
List the categories of veterinary medicines in the EU
- POM-V
- POM-VPS
- NFA-VPS
- AVM-GSL
- Schedule 6 exemption products
Who can prescribe POM-V medications?
Vets only
Who can prescribe POM-VPS medications?
Vets, pharmacists and suitably qualified persons
Who can prescribe NFA-VPS?
Vets, pharmacists and suitably qualified persons
Who can prescribe AVM-GSL medications?
General sales list (i.e. anyone)
When can POM-V drugs be prescribed?
Only by vet after clinical assessment of animal or group of animals under their care
Who can supply a POM-V medication?
- Vet giving prescription
- Another vet or pharmacist in accordance with written prescription
- Client may request written prescription if do not want prescribing vet to supply medication
Where can POM_V medications be supplied from?
Only from registered premises
When is a drug classified as POM-V?
- Requires strict limitation on use for specific safety reasons
- requires specialised knowledge of vet surgeon for its use/application
- Has narrow safety margin requiring above average care in use
- Government policy to demand professional control at high level e.g. antimicrobials and controlled drugs