6)Drug excretion. Drugs with salivary excretion. Drugs affecting the salivary secretion Flashcards
Drug excretion
Removal of drug from body via a number of routes:
- Most important route ⇢ via kidney into urine
- Other routes ⇢ tears, perspiration, intestine, lung, saliva
Renal elimination
Rate of R.E can vary based on drug, eg penicillin is almost completely cleared from drug in a single transit to kidney
3 aspects to R.E
- Glomerular filtration
- Active tubular secretion
- Passive diffusion across tubular epithelium
Glomerular Filtration
- Drug enters kidneys via renal arteries (divide forming glomerular capillary plexus)
- The free/unbound to albumin drug flows into bowman’s capsule as part of glomerular filtrate
Active tubular secretion
- There are 2 non-selective carrier systems ⇢ one for acids + one for bases
- This is the main mechanism for drug elimination in kidneys
- It works well for drugs bound to proteins ⇢ can be properly excreted
Passive diffusion across tubular epithelium
- 1% of fluid is filtered from glomerular filtration ⇢ excreted as urine
- If all drugs passed via passive diffusion ⇢ rate of excretion would be very slow
- Lipid-soluble drugs are poorly excreted as they pass through w/ease
- Water-soluble polar drugs remain in lumen + more likely to be excreted
Renal Clearance (ml/min)
Renal clearance (ml/min) = Urinary conc x rate of urine flow ÷ plasma conc
Half life of drug
½ life = time required for [drug] in blood plasma to be reduced by ½
Steady state
when a drug has built up therapeutically effective levels in blood stream
Extraction ratio
- The decline of [drug] in plasma from arterial to venous side of kidney
- Drugs enter kidneys at [C1] and leave at [C2]
- extraction ratio = C2/C1
Excretion rate
Excretion rate = [clerance] x [plasma]
Drugs with salivary excretion
Clindamycin
Rifampicin
Caffeine
Metroidazole
Theophyline
Drugs affecting salivary excretion
Atropine + atropine-like antispasmodics
Tricyclic antidepressants ⇢ amitriptyline
Tetracycline antidepressants ⇢ maprotiline
Clonidine
Anti muscarinic drugs ⇢ pirenzepine