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Factors affecting bioavailability.

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A brief presentation on the factors affecting bioavailability of drugs along with a quick overview on what is bioequivalence, clinical equivalence, therapeutic equivalence, chemical equivalence and pharmaceutical equivalence.

Publié dans : Santé & Médecine
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Factors affecting bioavailability.

  1. 1. Factors Affecting Bioavailability Dr. Salman H. Rizvi JR1 Dept. of Pharmacology Dr. V. M. Govt. Medical College Solapur (08/08/2018)
  2. 2. Objectives: – Bioavailability – Bioequivalence – Chemical Equivalence – Pharmaceutical Equivalence – Therapeutic Equivalence – Clinical Equivalence – Factors Affecting Bioavailability • Pharmaceutical factors • Pharmacological factors
  3. 3. What is BIOAVAILABILITY? • The rate and extend to which the active concentration of a drug is available at the desired site of action (or bloodstream).
  4. 4. How does it differs from BIOEQUIVALENCE? • If two or more, similar dosage forms of the same drug reach the blood circulation at the same relative rate and extend, those are BIOEQUIVALENT preparations of that generic drug. • Difference in bioavailability is usually seen with ORAL dosage forms, bioavailability of I.V is 100%, I.M and S.C are assumed to be close to 100%. • Differences of less than 25% in bioavailability among several formulations of one drug will usually have no significant effect on clinical outcome, hence such formulations can be called BIOEQUIALENT.
  5. 5. • Chemical Equivalence – If two or more dosage forms of the same drug contains the same labeled quantities of the drug as specified in Pharmacopoeia, these are Chemical Equivalent drugs. • Pharmaceutical Equivalence – If the two or more Chemical Equivalent drug’s dosage forms also contains other similar ingredients, e.g. excipients (binders), those dosage forms are considered to be Pharmaceutical Equivalent.
  6. 6. • Therapeutic Equivalence – If they provide an identical in vivo pharmacological response, as measured by control of symptoms or disease and safety profile. • Clinical Equivalence – If one structurally different drug can provide the same clinical response as another mechanically related drug, they are considered to exhibit clinical equivalence.
  7. 7. What are the factors influencing BIOAVAILABILITY? • Pharmaceutical factors • Pharmacological factors
  8. 8. • Pharmaceutical Factors: – It is expected that, bioavailability of drugs to be in this decreasing order- • Solutions > Suspension > Capsule > Tablet > Coated Tablet
  9. 9. The Pharmaceutical factors include: – Particle Size • The rate at which a drug is dissolved can be increased by increasing its surface area by decreasing its PARTICLE SIZE. – Salt Form • The rate at which a particular salt dissolves differs from its parent compound. • Salts of weakly acidic drugs are highly water soluble, free acidic drugs is precipitated from these salts is micro crystalline form, which has a faster dissolution rate and increases bioavailability. – Crystal Forms • The rate of absorption and bioavailability of a drug also depends on its crystalline form. • This is because amorphous forms dissolve faster compared to crystalline forms, because no energy is needed to break up the crystalline lattice, thus increasing their bioavailability.
  10. 10. – Water of Hydration • Many drugs incorporate water to produce a crystalline form called hydrates. • If water molecules are already present in a crystal structure, the tendency of the crystal to attract additional water to initiate dissolution process is reduced, compared to anhydrous forms. – Nature of Excipients and Adjuvants • These pharmacologically inert substances , (e.g starch, lactose, calcium sulfate, gum) which are added as filling material or as binding agents or to obtain proper granular size, have tremendous effects on bioavailability of drugs. • Some of these excipients are wetting agents, which enhance solvent penetration and ensures faster dissolution and in turn absorption. • We should be particularly careful in drugs which follows zero order or mixed order kinetics or have a low margin of safety. – Degree of Ionisation • Non-ionised, lipid soluble drugs are better absorbed, increasing their bioavailability, compared to strongly acidic or strongly basic drugs or highly ionised drugs.
  11. 11. • Pharmacological Factors:
  12. 12. The Pharmacological factors include: – Gastric Emptying and Gastrointestinal Motility • Factors that accelerate gastric emptying increases the bioavailability. • This is because the drug is exposed to the larger surface area of the small intestine. – Gastrointestinal Diseases • There are many gastrointestinal diseases which have an effect on drug absorption, the outcome of Coeliac disease is complex, it increases the absorption of cephalexin, whereas reduces of amoxycillin. • In case of Crohn’s disease, there is disproportionate absorption of individual components of cotrimoxazole, increases absorption of sulfamethoxazole, decreases of trimethoprim – Food and Other Substances • In general, GI absorption rate is reduced after ingestion of food, although it has no effect on extend of absorption. • Both rate and extend of absorption of certain antibiotics like rifampicin is reduced after meals.
  13. 13. • Absorption of tetracyclines is reduced if taken with milk because it forms poorly absorbed complexes with calcium. • Vit. C increases absorption of iron because it keeps it in it’s ferrous form. – First Pass Metabolism • It means that drug degradation occurs, reducing its bioavailability, when it passed through GIT wall and then through portal system, before it reaches systemic circulation. – Drug-Drug Interactions • Drug-drug interactions can also cause difference in bioavailability. • Liquid paraffin decreases the bioavailability of fat soluble vitamins as it emulsifies fat. • Antacids reduces bioavailability of tetracyclines because it forms chelated complex. – Pharmacogenetic Factors • Large difference in bioavailability often occurs among humans due to pharmacogenetic reasons. • Slow and fast acetylators show increased and decreased bioavailability of isoniazid respectively.
  14. 14. – Miscellaneous Factors • Area of Absorptive Surface • State of Circulation at the Site of Absorption (shock, where tissue perfusion decreases) • Route of Administration
  15. 15. References: – Goodman L. S, Goodman & Gilman’s Pharmacological Basis of Therapeutics, 13th Edition, New York; New Delhi, TataMcGraw-Hill Education, 2017. – Katzung B. G, Basic and Clinical Pharmacology, 14th Edition, New York; New Delhi, TataMcGraw-Hill Education, 2017. – Tripathi K.D, Essentials of Medical Pharmacology, 7th Edition, New Delhi, Jaypee Brothers Publications, 2013. – Sharma H. L, Principles of Pharmacology, 3rd Edition, New Delhi, Paras Medical Publisher, 2018
  16. 16. Thank You! 

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