Routes of Administration

• Oral Administration

  • Most convenient, most economical

  •   Disadvantages:

    • Emesis (drug irritation of the gastrointestinal mucosa)

    • Digestive enzymes/gastric acidity destroys the drug

    • Unreliable or inconsistent absorption due to food or other drug effects

    • Metabolism of the drug by gastrointestinal flora

  • Factors determining rate of drug effect onset

    • Primary factor:

      • Rate and absorption extent by GI tract

    • Absorption Site:

      • Mainly small intestine because of large surface area

    • Drug ionization state:

      • Nonionized (lipid-soluble) forms favor absorption

        • Weak acids may be highly ionized in the alkaline intestinal pH (not favoring absorption) but this effect is counterbalanced by the large surface-area effect.

        • Drugs which are weak acids are readily absorbed in the stomach

  • First-Pass Effect

    • Drugs absorbed from the GI tract passes through the  portal venous system then through the liver and finally into the systemic circulation when drugs interact with receptors in target tissues.

    • Extensive hepatic metabolism/extraction result in minimal drug delivery to the systemic circulation for certain agents.

    • Drugs with large first pass effect exhibit significant differences in pharmacological effects comparing oral vs. IV administration

      • Examples:

        • Propranolol

        • Ldocaine

Transdermal Administration

• Advantages:

  • Sustained, therapeutic plasma levels (reduced peaks/valleys associated with intermittent drug administrations)

  • Avoids continuous infusion technique difficulties

  • Low side effect incidence (smaller doses)

  • Generally good patient compliance

• Factors contributing to reliable transdermal drug absorption:

  • Molecular weight < 1000

  • pH range 5-9 in aqueous medium

  • No histamine-releasing action

  • Daily drug requirement <10 mg

• Example of drugs available for transdermal delivery:

  • Scopolamine:-tolerance may eventually occur; resulting in loss of therapeutic action

  • Fentanyl (Sublimaze)

  • Clonidine (Catapres)

  • Nitroglycerin-tolerance may eventually occur; resulting in loss of therapeutic action.

Rectal Administration

• Proximal rectum administration:

  • Absorption into superior hemorrhoidal veins then enters the portal venous system  then to the liver (possible first pass hepatic effect) and finally into the systemic circulation

• Low rectal administration of drug may allow the drug to enter the systemic circulation without passing through the liver

• Generally unpredictable pharmacological responses for the above reasons

• Rectal mucosal irritation possible

Parenteral Administration

• Ensures active drug absorption

• Subcutaneously intramuscular injection: is often more rapid and predictable compared to the oral administration route

• May be the only route of administration acceptable for:

  • Uncooperative patients

  • Unconscious patients

• Factors the determine rate of systemic absorption:

  • Absorbing capillary membrane surface area

  • Drug solubility in interstitial fluid

  • Aqueous channels promote high diffusion rates of drugs, independent of their lipid solubility

• Advantages of IV administration

  • Rapid/precise blood drug levels obtained since the drug does not first pass through the liver where the drug may be metabolized before reaching the systemic circulation. 

    • If the drug is metabolized by the liver first, such as after oral administration, this occurrence is described as the "first-pass effect."

  • Irritant drug are more comfortably administered by IV since blood vessels are relatively insensitive.

  • Drug rapidly diluted in the blood volume, particularly if the drug is administered into large forearm vein.