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Pharmacokinetics

Pharmacokinetics. Based on the hypothesis that the action of a drug requires presence of a certain concentration in the fluid bathing the target tissue. In other words, the magnitude of response (good or bad) depends on concentration of the drug at the site of action. Pharmacokinetics.

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Pharmacokinetics

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  1. Pharmacokinetics • Based on the hypothesis that the action of a drug requires presence of a certain concentration in the fluid bathing the target tissue. • In other words, the magnitude of response (good or bad) depends on concentration of the drug at the site of action

  2. Pharmacokinetics • Absorption • Distribution • Metabolism • Elimination

  3. Study of [drug] over time

  4. How are [drug] measured? • Invasive: blood, spinal fluid, biopsy • Noninvasive: urine, feces, breath, saliva • Most analytical methods designed for plasma analysis • C-14, H-3

  5. Therapeutic Window • Useful range of concentration over which a drug is therapeutically beneficial. Therapeutic window may vary from patient to patient • Drugs w/ narrow therapeutic windows require smaller & more frequent doses or a different method of administration • Drugs w/ slow elimination rates may rapidly accumulate to toxic levels….can choose to give one large initial dose, following only with small doses

  6. Shape different for IV injection

  7. Distribution • Rate & Extent depend upon • Chemical structure of drug • Rate of blood flow • Ease of transport through membrane • Binding of drug to proteins in blood • Elimination processes

  8. Partition Coefficients: ratio of solubility of a drug in water or in an aqueous buffer to its solubility in a lipophilic, non-polar solvent • pH and ionization: Ion Trapping

  9. The Compartment Model • WE can generally think of the body as a series of interconnected well-stirred compartments within which the [drug] remains fairly constant. BUT movement BETWEEN compartments important in determining when and for how long a drug will be present in body.

  10. Partitioning into body fat and other tissues • ·A large, nonpolar compartment. Fat has low blood supply—less than 2% of cardiac output, so drugs are delivered to fat relatively slowly • For practical purposes: partition into body fat important following acute dosing only for a few highly lipid-soluble drugs and environmental contaminants which are poorly metabolized and remain in body for long period of time

  11. IMPORTANT EFFECTS OF PH PARTITIONING: • ·        urinary acidification will accelerate the excretion of weak bases and retard that of weak acids; alkalination has the opposite effects • ·        increasing plasma pH (by addition of NaHCO3) will cause weakly acidic drugs to be extracted from the CNS into the plasma; reducing plasma pH (by administering a carbonic anhydrase inhibitor) will cause weakly acidic drugs to be concentrated in the CNS, increasing their toxicity

  12. Renal Elimination • Glomerular filtration: molecules below 20 kDa pass into filtrate. Drug must be free, not protein bound. • Tubular secretion/reabsorption: Active transport. Followed by passive & active. DP=D + P. As D transported, shift in equilibrium to release more free D. Drugs with high lipid solubility are reabsorbed passively & therefore slowly excreted. Idea of ion trapping can be used to increase excretion rate---traps drug in filtrate.

  13. Plasma Proteins that Bind Drugs • albumin: binds many acidic drugs and a few basic drugs • b-globulin and an a1acid glycoprotein have also been found to bind certain basic drugs

  14. A bound drug has no effect! • Amount bound depends on: • 1)     free drug concentration • 2)     the protein concentration • 3) affinity for binding sites % bound: __[bound drug]__________ x 100 [bound drug] + [free drug]

  15. % Bound • Renal failure, inflammation, fasting, malnutrition can have effect on plasma protein binding. • Competition from other drugs can also affect % bound.

  16. An Example • warfarin (anticoagulant) protein bound ~98% • Therefore, for a 5 mg dose, only 0.1 mg of drug is free in the body to work! • If pt takes normal dose of aspirin at same time (normally occupies 50% of binding sites), the aspirin displaces warfarin so that 96% of the warfarin dose is protein-bound; thus, 0.2 mg warfarin free; thus, doubles the injested dose

  17. Volume of Distribution • C = D/Vd • Vd is the apparent volume of distribution • C= Conc of drug in plasma at some time • D= Total conc of drug in system\ Vd gives one as estimate of how well the drug is distributed. Value < 0.071 L/kg indicate the drug is mainly in the circulatory system. Values > 0.071 L/kg indicate the drug has gotten into specific tissues.

  18. Conc. Vs. time plots C = Co - ktln C = ln Co - kt

  19. Zero Order Kinetics Rate = k C = Co - kt C vs. t graph is LINEAR First Order Kinetics Rate = k C C = Co e-kt C vs. t graph is NOT linear, decaying exponential. Log C vs. t graph is linear. Types of Kinetics Commonly Seen

  20. First-Order Kinetics

  21. First Order Elimination [drug] decreases exponentially w/ time Rate of elimination is proportional to [drug] Plot of log [drug] or ln[drug] vs. time are linear t 1/2 is constant regardless of [drug] Zero Order Elimination [drug] decreases linearly with time Rate of elimination is constant Rate of elimination is independent of [drug] No true t 1/2 Comparison

  22. Half-Life • C = Co e - kt • C/Co = 0.50 for half of the original amount • 0.50 = e – k t • ln 0.50 = -k t ½ • -0.693 = -k t ½ • t 1/2 = 0.693 / k

  23. Use of t 1/2 & kel data • If drug has short duration of action, design drug with larger t 1/2 & smaller kel • If drug too toxic, design drug with smaller t 1/2 & larger kel

  24. Clearance • Volume of blood in a defined region of the body that is cleared of a drug in a unit time. • Clearance is a more useful concept in reality than t 1/2 or kel since it takes into account blood flow rate • Clearance varies with body weight • Also varies with degree of protein binding

  25. Clearance • Rate of elimination = kel D, • Remembering that C = D/Vd • And therefore D= C Vd • Rate of elimination = kel C Vd • Rate of elimination for whole body = CLT C Combining the two, CLT C = kel C Vd and simplifying gives: CLT = kel Vd

  26. Problem 5.5

  27. Calculate kel Calculate Vd Calculate CL C = Co e-kt ln C – ln Co = - k t C = D/Vd CLT = kel Vd Problem 5.5

  28. AUC: IV Administration

  29. AUC • For IV bolus, the AUC represents the total amount of drug that reaches the circulatory system in a given time. • Dose = CLT AUC

  30. AUC: Oral Administration

  31. Bioavailability • The fraction of the dose of a drug (F) that enters the general circulatory system, F= amt. Of drug that enters systemic circul. Dose administered F = AUC/Dose

  32. Bioavailability • A concept for oral administration • Useful to compare two different drugs or different dosage forms of same drug • Rate of absorption depends, in part, on rate of dissolution (which in turn is dependent on chemical structure, pH, partition coefficient, surface area of absorbing region, etc.) Also first-pass metabolism is a determining factor

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