Lecture 9 - Biopharmaceutics

Our goal when we treat patients is to cure the condition or relieve the symptoms without causing any unnecessary side effects. To do that we must deliver the correct amount of drug (not too little or too much) to the site in the body where it must do its work. The most efficient way to do this is to inject the drug directly into the site of action. This is often impractical. We, therefore, must understand the factors that affect the uptake of the drug from wherever we have placed it.

Injection is a viable route of administration but is has some drawbacks. Among them are: it is a painful method and self administration using injection is often resisted by the patient. We also can apply drug to the skin for local effect and for systemic effect. This has limited usefulness because of the limited surface area available and the nature of our skin as a barrier to passage of material into the body. There are two other routes of administration which can be used. We can ask the patient to inhale the drug. This exposes the drug to a large surface area (about 70 m²) which has a thin layer of epitheliae cells separating the drug from the blood supply. The last is the GI tract which is a complex organ system designed by the body to absorb food and fluid. The total surface area of the GI tract is 120 to 200 m². It also contains enzymes which actively transport drugs from the lumen on the GUT to the blood supply.

What are the advantages of an IV dosage form?

What are the advantages of an IM dosage form?

What are the advantages of an inhaled dosage form?

What are the advantages of a dermal dosage form for systemic drugs?

What are the advantages of a solid oral dosage form?

What are the advantages of a liquid oral dosage form?

Types of Absorption and the Factors that Affect Them

Name	Driving Force	Needs Energy	Other Factors
Passive Diffusion	Concentration gradient	no	Surface area of membrane, solubility of compound
	q= -D(A/h)K(C_o - C_i)		Membrane thickness, pH of medium
Convective Transport	Concentration Gradient	no	Pore Diameter, hydrostatic pressure
	q= {nr4(pi)/8(Viscosity)}(C_o -C_i)		Size of membrane, numbers of pores.
Active Transport	Enzyme System	YES	Structure of Cpd., quantity of Enzyme
	-dc/dt ={dc/dt_MAX [S]}/K_c + [S]		Unidirectional and saturable
Facilitated Transport	Concentration Gradient, & Carrier	Yes	Example Vit B₁₂
Endocytosis - Pinocytosis	Concentration Gradient	Yes	Example Oil soluble Vitamins A, D, E, & K

Which of these types of absorption occurs in which region of the GI tract??

What is the impact of pH of the GI contents on absorption of drugs?

What is the impact of the contents of the GI tract on stability of the drug?

What can we do to improve or control the outcome??