MECHANISM OF CELLULAR DRUG ABSORPTION

MECHANISM OF DRUG ABSORPTION

Link for video demonstration of this topic 👇

 https://youtu.be/v-nVH7y1Cc8

The three broad categories of drug transport mechanisms involved in absorption are – 

A. Transcellular/intracellular transport

B. Paracellular/intercellular transport 

C. Vesicular transport

TRANSCELLULAR/INTRACELLULAR TRANSPORT

It is defined as the passage of drugs across the GI epithelium. It is the most common pathway for drug transport. The 3 steps involved in transcellular transport of drugs are – 

(i) Permeation of GI epithelial cell membrane, a lipoidal barrier –this is the major obstacle to drug absorption. 

(ii) Movement across the intracellular space (cytosol). 

(iii) Permeation of the lateral or basolateral membrane- this is of secondary importance. 

The various transcellular transport processes involved in drug absorption are –

1. Passive Transport Processes – These transport processes do not require energy other than that of molecular motion (Brownian motion) to pass through the lipid bilayer. Passive transport processes can be further classified into following types – 

a. Passive diffusion. 

b. Pore transport. 

c. Ion-pair transport. 

d. Facilitated- or mediated-diffusion.

a. Passive transport

b. Pore transport

c. Ion-pair transport

d. Carrier mediated transport

2. Active Transport Processes – This transport process requires energy from ATP to move drug molecules from extracellular to intracellular area. These are of two types –

a. Primary active transport(use of ATP). 

b. Secondary active transport(use of any other form of energy) – this process is further subdivided into two –

i. Symport (co-transport). 

ii. Antiport (counter-transport).

PARACELLULAR/INTERCELLULAR TRANSPORT

It is defined as the transport of drugs through the junctions between the GI epithelial cells. This pathway is of minor importance in drug absorption. The two paracellular transport mechanisms involved in drug absorption are – 

1. Permeation through tight junctions of epithelial cells – this process basically occurs through openings which are little bigger than the aqueous pores. Compounds such as insulin and cardiac glycosides are taken up this mechanism. 

2. Persorption – is permeation of drug through temporary openings formed by shedding of two neighbouring epithelial cells into the lumen.

VESICULAR/CORPUSCULAR TRANSPORT

Like active transport, these are also energy dependent processes but involve transport of substances within vesicles into a cell. Since the mechanism involves transport across the cell membrane, the process can also be classified as transcellular. Vesicular transport of drugs can be classed into two categories –

1. Pinocytosis. 

2. Phagocytosis.

Illustrative comparison of transcellular, paracellular and vesicular transport.

Figure 1: comparison of transcellular, paracellular and vesicular transport mechanisms.