Drug cross various biological membranes 

Drug cross various biological membranes by following mechanism:

1. Simple transport:

I. Diffusion:

• It is most important means of drug passage across the biological membranes.
• In this process, drug present in aqueous medium partitions and dissolves in lipid phase of membrane and finally leaves it by dissolving again in aqueous medium at the other side of membrane.
• Both ionized and unionized drug penetrate through biological membranes. Unionized drugs are more lipid soluble than ionized drugs.
• Mainly drugs molecules move from a region of higher concentration to lower concentration until equilibrium is attained.
• Rate of diffusion is directly proportional to the concentration gradient across the membrane. Lipid-soluble drugs are transported across the membrane by passive diffusion. It does not require energy.
• For a drug to be readily diffusible, it must be largely lipid soluble and yet it must have some solubility in aqueous solution also.
• Water soluble drugs poorly penetrate the cell membrane. Extremely lipid-soluble drugs are also poorly diffusible because they are totally insoluble in aqueous body.
• Large surface area and thin membrane favors the process of diffusion.
• Passive diffusion of drug molecules across a biological membrane is best expressed by Fick’s first law of diffusion:

Rate of diffusion= (C₁-C₂)= A x  D x Km/w /T

where, C₁– higher concentration

            C₂– lower concentration

            C₁-C₂= difference in concentration of drug in two media across biological membrane called concentration gradient

          A= surface area of membrane

          D= diffusion coefficient of drug

          K_m/w = partition coefficient of drug in lipid membrane and aqueous fluid.

          T= thickness of the membranes

II. Filtration:

• It is the simple passage of drugs through pores or channels in the membranes.
• Filtration depends on the molecular size and weight of the drug.
• If drug molecules are smaller than pores, they are filtered easily though the membrane.

2. Specialized transport:

I. Active transport:

• Drug molecules move from a region of lower to higher concentration against the concentration gradient.
• It requires energy.
• Ex: absorption of levodopa from intestine.

II. Facilitated diffusion:

• It is a type of carrier-mediated transport and does not require energy.
• Drug attaches to a carrier in the membrane which facilitates its diffusion across the membrane.
• Transport of molecules is from region of higher to lower concentration.
• Ex: transport of glucose across the muscle cell membrane by a transporter GLUT₄.