toxicology TDM Flashcards
Main functions of a clinical toxicology lab
- therapeutic drug monitoring ( TDM)
- ID of drugs in acute intoxication
- Urine testing for drugs of abuse or performance enhancers
- testing for environmental or occupational exposure
Therapeutic drug monitoring
analysis, assessment, and evaluation of circulating concentrations of drugs in Serum,
Plasma, or Whole blood
Main purposes of TDM
ensure drug dosage is within the Therapeutic Range - The range that produces the
maximum therapeutic benefit
identify when the drug is outside of the therapeutic range
•Too low - drug inefficiency
•Too high - toxicity
Not used for all drugs
Indications for Therapeutic Drug Monitoring (TDM)
When is it necessary?
- Drugs with low therapeutic index (ie. Narrow range between therapeutic and toxic levels)
- Start or change in therapy
- Neonates/ infants due to maturing renal and hepatic functions
- During illness
- When a drug is unexpectedly ineffective or toxic
- When a new drug is added due to possible drug interactions
Factors to consider in TDM
- Route of administration ( ex. orally, muscular injection)
- Rate of absorption
- Distribution of the drug within in the body
- Metabolism ( liver)
- Rate of elimination (kidney)
protein bound vs free drug
Only free drug can give a biologic response
Not protien bound drug
Pharmacodynamics
study of the biochemical and physiological effects of drugs
and the mechanisms of their actions
site of action
site at which a drug acts to initiate a specific biological effect
Drug action is usually mediated through receptors such as cellular enzymes and
structural or transport proteins
Pharmacokinetics definition & how its used
activity or fate of drugs in the body over a period of time.
•Takes into account all of the factors that determine concentration of a serum drug
level (absorption, distribution, metabolism and excretion).
•Assists the health care provider in establishing or modifying a dosage regimen for
each patient.
•Uses mathematical modelling to determine the drug concentration in circulation
Drug concentrations will swing back and forth between a maximum (peak) and minimum
(trough) concentration level.
Goal:
•To get the peak and trough levels within the therapeutic range.
•To make sure the peak is not in the toxic range
steady state
intake and elimination of a drug are in equilibrium
5 to 7 doses of a drug are usually required to achieve a steady state
factors affecting blood levels of a drug physiologial & non
Physiological Factors: •Absorption into blood (IV, IM, orally) •Distribution in body tissues •Metabolism by liver •Excretion by the kidney •Maturation or aging •Organ disease (liver & kidney esp.) •Diurnal variation (daily fluctuations) •Drug interactions
Non-physiological Factors:
•Drug formulation
•Medication Errors
•Patient compliance
during drug therapy what is affected by a patients history
A- Absorption
D- Distribution
M-Metabolism
E- Excretion
routes of administration
intravenous **
-most direct & effective delivery to sites of action
intramuscular (IM) injection
Subcutaneous (SC) injection
Transcutaneous
- absorbed through skin
- ex. transdermal patch
rectal delivery
- ex. suppository
- used in infants or when oral delivery is not possible
Oral administration **
- Most common method
- Least invasive
Drug Absorption for oral drugs; efficiency of absorption from GI tract depends on:
efficiency of absorption from GI tract depends on:
•Dissociation from its administered form
•Solubility in gastrointestinal fluids
•Diffusion across gastrointestinal membranes
drug absorption ( Dissolution)
- Tablets and capsules must dissolve before being absorbed
* Liquid solutions absorbed more rapidly
drug absorption ( solubility )
- Weak acids are absorbed well in stomach (due to gastric acidity)
- Weak bases best absorbed in intestine (pH is more alkaline)
drug absorption (Diffusion)
Absorption occurs in predictable manner in healthy people
•Absorption rates can be altered by changes in:
•Intestinal mobility
•pH
•Inflammation
•Presence of food or other drugs
Absorption rates can also be affected by:
•Age
•Pregnancy
•Pathologic conditions
drug bioavailability
The bioavailability is the fraction of a drug dose that is absorbed into the
systemic circulation
**In order to be useful, a drug’s bioavailability needs to be greater than 70%
First pass effect
Drugs administered orally are absorbed in the small intestine and are carried by the
portal vein directly to the liver.
•Some drugs have a high hepatic excretion rate whereby low amounts reach the systemic circulation (low bioavailability).
first-pass effect - extensive metabolism of a drug with a high
hepatic extraction rate by the liver before it reaches systemic circulation
drug distribution ( protein bound )
- Drug distribution occurs after a drug enters the vascular department.
- Many drugs bind to protein in the blood
- Acidic drugs are primarily bound to albumin
- Basic drugs are primarily bound to globulins
drug distribution ( free drugs)
Only the free (unbound) fraction of drug can interact with the site of action & cause
biologic response.
•Free drug is also termed active
Percentage of free drug depends on physiologic & biochemical parameters:
•Inflammation, malignancies, pregnancy, hepatic disease, nephrotic syndrome, malnutrition, acid–
base disturbances
•Concentration of other substances competing for binding sites (eg. other drugs, urea, bilirubin,
hormones)
Example: Phenytoin
•In a healthy individual: 90% bound; 10% free
•In uremic patients: 20-30% free
•Drug dose would need to be adjusted in uremic patients
drug metabolism by liver
All substances absorbed from intestine enter hepatic portal system
Metabolism involves the biotransformation and excretion of the drug
Rate of metabolism can vary between different patients as well as with the same patient
at different times and in different circumstances
Biotransformation
•Chemical alteration of a xenobiotic within the body that generally enhances the xenobiotic’s aqueous solubility and excretion.( makes drugs water soluble & easier to excrete in urine )
**•The main biochemical pathway for drug metabolism is the hepatic mixed-function
oxidase (MFO) system
Converts lipophilic non-polar molecules to more polar water-soluble forms
patients with liver disorders may require lower dosages
Pharmacogenomics
study of genetic variations in patient drug metabolism pathways and
development of drug therapies to compensate for genetic differences impacting therapy
regimens
- Responders
- Patients benefitting from therapeutic and desired effects of the drug
- Nonresponders
- Patients not benefitting from therapeutic and desired effects of the drug