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Absorption in pharmacology
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- 2. Pharmacokinetics or DrugPharmacokineticsor Drug dispositiondisposition Processes of Pharmacokinetics The ADME scheme A - Absorption D - Distribution M- Metabolism or Biotransformation E- Excretion 2
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- 4. Passage of DrugsacrossPassage of Drugs across Biological membraneBiological membrane Drug transport is a foundation to all ADME processes Physicochemical determinants Membrane characteristics Drug properties Mechanism of drug transport Passive diffusion Carrier-mediated transport Bulk flow Filtration 4
- 5. Membrane characteristicsMembrane characteristics Bilayerof amphipathic lipids barriers Embedded proteins 5
- 6. Drug properties Molecular weight,shape and size Water solubility Lipid solubility Ionization 6
- 7. Lipid SolubilityLipid Solubility Movementdirectly through the lipid bilayer requires that the substance dissolve into the lipid bilayer Partition coefficient (Kp) = solubility in lipid / solubility in water Increase lipid solubility causes increased partition coefficient Increase in partition coefficient causes increased permeability. 7
- 8. IonizationIonization General rules A drugusually exists in 2 forms – unionized and ionized forms Unionized drug can passively diffuse across membrane The ratio of unionized drug will indicate direction of passive diffusion Factors affecting ionization are pH of the medium pKa (acid dissociation constant) of the drug Acidic drug tend to ionize in more basic medium pH – pKa = log (ionized / nonionzized) Basic drug tend to ionize in more acidic medium pH – pKa = log (nonionized / ionized) 8
- 9. Stomach fluid pH 2 Plasma pH7 More non-ionized Ionized Less non-ionized Ionized Example Acidic drug pKa 6 pH – pKa = log (ionized / non-ionized) 9
- 10. Some data forpractice calculation 10 Plasma pH 7.4 CSF 7.4 Stomach 1.4 Small Intestine 8.0 Acidic drugpKa Basic drug pKa Ampicillin 2.5 Strychnine 8.0 Sulfadiazine 6.5 Aminopyrine 5.0 Aspirin 3.4 Procaine 9.0
- 11. Mechanisms of DrugTransport Transcellular transport Passive diffusion Carrier-mediated transport Facilitated diffusion Active transport Ion-pair transport Endocytosis or Pinocytosis Paracellular transport Bulk flow Filtration 11
- 12. Passive diffusionPassive diffusion Drugsthat can passively diffuse through cell membrane must be Lipid soluble Unionized form Move according to concentration gradient 12
- 13. Carrier-mediatedCarrier-mediated transporttransport Facilitated diffusion Carrier needed Can be saturated No energy required Move along conc. gradient Active transport Carrier needed Can be saturated Energy required Move against conc. gradient 13
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- 17. Bulk flow andFiltrationBulk flow and Filtration Bulk flow is an important way of a drug to move out of blood vessel Filtration is an important way to excrete drug 17
- 18. Pharmacokinetics: DrugPharmacokinetics: Drug AbsorptionAbsorption Absorption describesthe rate and extent at which a drug leaves its site of administration. Bioavailability is the extent to which a drug reaches its site of action, or to a biological fluid (such as plasma) from which the drug has access to its site of action. 18
- 19. Factors Affecting DrugFactorsAffecting Drug AbsorptionAbsorption Dose, Concentration, and Rate of administration Dosage forms Physical and chemical properties of drugs Physiological Factors Routes of drug administration 19
- 20. Physicochemical Properties ofDrugPhysicochemical Properties of Drug Acid or Base Degree of ionization Polarity Molecular weight Lipid solubility or... Partition coefficient (Kp) 20
- 21. Physiological FactorsPhysiological Factors Gastricmotility Gastric emptying time pH at the absorption site Area of absorbing surface Blood flow Presystemic elimination Ingestion with or without food 21
- 22. Routes: Oral Ingestion(PO)Routes: Oral Ingestion (PO) Several factors affect absorption by this route - So absorption may change drastically based on these factors Dosage forms (physical state of drug) Drug concentration Surface area of absorption Blood flow to GI GI activity Food Bacteria in GI First past effect (hepatic metabolism) Entero-hepatic cycle 22
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- 24. Routes: Parenteral administrationRoutes:Parenteral administration Subcutaneous Slow absorption Intramuscular Fast absorption Intra-peritoneal Rapid absorption Large absorbing area Intravenous, Intra-arterial No absorption 24
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- 26. Routes: Pulmonary absorptionRoutes:Pulmonary absorption Gaseous and volatile drugs may be inhaled and absorbed by the pulmonary epithelium and mucous membranes of respiratory tract. -almost instantaneous absorption -avoids first-pass metabolism -local application 26
- 27. Routes: Topical (Transdermalroute)Routes: Topical (Transdermal route) Few drugs readily penetrate the skin - Absorption is proportional to surface area - More rapid through abraded, burned skin - Inflammation increases cutaneous blood flow and, therefore, absorption - Enhanced by suspension in oily vehicle and rubbing into skin 27
- 28. Bioavailability (F)Bioavailability (F) Thefraction of a dose that reaches the systemic circulation in a chemically unaltered form Fractional availability = F Quote as percentage or decimal e.g. 25% or 0.25 Has no units 28
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- 30. Incomplete oralIncomplete oral bioavailabilitybioavailability 30 2.Chemical, enzymatic or bacterial attack 3. Failure of absorption & pGP efflux 1. Failure of disintegration or 4. First pass metabolism in gut wall or liver Liver