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Saturday, July 2, 2011

Elimination Of a Drug


Elimination of a Drug

Termination of drug effect > biotransformation > inactive > excreted

Organs of Excretion

Kidney, Liver.Lungs,Skin (sweat)

Kidney > Glom.Filtration,Tubular secretion Re-absorption by passive diffusion

o Glom. Filtration > passive process

o Removes mols. Size of small proteins

o Protein bound drug > less Glom.Filtration

o Free drug more filtration > rate approximates cratinine clearance

o Tubular secretion > actively secreted by proximal convoluted tubule

o Lipid membrane of nephron reabsorbs lipid soluble mol.

o Water soluble mol. excreted

Liver

o Actively secreted into bile after metabolism

o Enters G.I.T & may be reabsorbed into the blood > entero hepatic circulation

o E.g/ cardiac glycosides and some anti microbials

o Gastro Intestinal Tract > passive diffusion

o Drugs > blood>lipid membrane > lumen of G.I.T.

Clinical importance of excretion into lumen of gut

o Weakly basic drugs like morphine in high concentrations diffuses into acidic environment of stomach > poorly re absorbed as it is 100% ionized. Hence can be removed by gastric lavage.

o If not removed, it is re absorbed in the small intestine

Other organs of excretion

o LUNGS > Gaseous anaesthetics

o SWEAT

o SALIVARY GLANDS

o MILK

DRUG CLEARANCE

o Rate of elimination is relative to the conc. Of drug any biological fluid e.g. plasma, blood.

CL= Rate of elimination

Plasma conc.

o CL(renal) =

Rate of elimination(Kidney)

conc.in plasma (Cp)

o CL(liver) =

Rate of elimination(liver)

Cp

o Rate of elimination

o For an organ =QxC(in) – Qx C(o)

Q = rate of bl.flow

C(in) = amt of drug entering

C(o) = amt of drug leaving

o Bioavailability(F) = 1 – ER(extraction ratio)

o Dosing Rate = CL x Css (IV infusions)

o Half life proportional to VD & CL

o t1/2 =

0/693VD or 0.7VD

CL

Loading Dose = VD x Cpss

o Used for long t1/2 drugs > raise conc of drug to steady state value > amt. of drug in the body when steady state is reached

o E.g. If reqd. ss conc is 15microgms/ml and VD is 35L Then LD = 15 x 35 = 525mg

Dosage Regimen

o 1. Dose of drug

o 2. Route of administration

o 3. Dosing interval

o 4. Period of administration

o Clinical significance of dosing rate

o Eg. Desired Pl.conc of drug A is 15microgms/ml CL = 48ml/min/70Kg

o Dosing rate = 48 x 15 = 720mugm/min =0.72mg/min = 43.2mg/hour = rate of IV infusion


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