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Pharmacokinetics

  

Goal – Max beneficial effect + Minimal Adverse effect

(Target effect/conc.)       (Toxic conc.)

  1. Absorption
  2. Distribution
  3. Metabolism
  4. Excretion

 

 

       

Volume of distribution

  • Vd – Amount of drug in body (Determined by dose given)
  • Concentration of drug in plasma/ blood/ water

                Cp          Cb           CU

 

*High Vd - high extravascular distribution

  • Vd can not be less than 2.8 L/70kg (as whole, drug will be in plasma)
  • Redistribution of drug: - Highly lipophilic drug enter adipose tissue hence the conc. fall rapidly in plasma
  • High Vd – Then adipose act as source of drug back to plasma (e.g. Barbiturates)

                           Rate of elimination (Ek)


Clearance Cl    =   

                              Drug conc. (c)

  • E – From Kidney, lungs, liver, skin etc
  • Yield respective clearance of organs
  • Total systemic Clearance– C-kidney + C-liver + C-lung + C-skin
  • C-kidney – unchanged Clearance
  • Cliver/lung – Biotranformation (changed Cl)
  • Elimination = Cl X Conc of drug                                                                     

Clearance:

 

First order (Still unsaturated)

Zero order (mech. saturated)

  • As, E = CL*C                                         
  • E∞conc.                                                
  • E∞Cl
  • ∆ = Area Under Curve OF TIME – conc. Graph
  • Cl= dose (mg)/ AUC (mg)
  • Cl is constant
  • Elimination is not dependent and on conc. of drug
  • No meaning of Cl – Not determined by AUC
  • E.g. – alcohol, Warfarin, Phenytoin, Aspirin, Zidovudine

 

Elimination:

 

Capacity Limited elimination           

Flow limited elimination

Saturable                                                               - Dose/conc. Dependant
Michelis-menten type

- Flow of blood to organ
- More of high extractable drug
- Confounded- Drug protein binding  
                         - Drug partition coefficient.

                               

Elimination –        Vmax × coc.
                                                


                             Km + conc.


Km – drug conc. at which elimination is 50% of conc. ≥ Km

  • Elimination is independent of conc. (pseudo-zero order kinetics)

Drug conc.: ≤ Km – 1st order

                   ≥Km ≤Vm – Pseudo o order

                   ≥ Vm – zero order

Eg – Ethanol, phenytoin

 





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