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Pulmonary Drug delivery

The respiratory tract is divided into two main parts:

•The upper respiratory tract, consisting of the nose, nasal cavity and the pharynx; and
•The lower respiratory tract consisting of the larynx, trachea, bronchi and the lungs.


The trachea which begins at the edge of the larynx, divides into two bronchi and continues into the lungs. The trachea allows air to pass from the larynx to the bronchi and then to the lungs. 
The bronchi divide into smaller bronchioles which branch in the lungs forming passageways for air. 
The terminal parts of the bronchi are the alveoli. The alveoli are the functional units of the lungs and they form the site of gaseous exchange.


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Lungs provide :

- Large absorptive surface area (up to 100 m2 ) ;
- Extremely thin (0.1 μm – 0.2 μm) absorptive mucosal membrane,
- Good blood supply;
- Low metabolic effect.

Therefore,
Growing attention has been given to the potential of a pulmonary route as an non-invasive administration for systemic delivery of therapeutic agents (mainly peptides and proteins)

Advantages of Pulmonary Drug Delivery

1.Used for local delivery of drugs to the lungs for asthma treatment.
2.Non-invasive route of systemic drug delivery.
3.Rapid absorption and fast onset of action (comparable to IV drug administration)
4.Avoid harsh conditions of the GIT
5.Drug efflux transporters and metabolizing enzymes are present in the lung at much lower levels than the gastrointestinal tract

Limitations to Pulmonary Drug Delivery

1.Inhaled drug particles must possess specific particle diameter in order to pass from nose and mouth filters and at the same time dispose inside the lung
2.Excipients used in drug formulation can cause irritation or permanent injury to the lung.
3.High disposition of drug in the oro-pharynx region.

Factors affecting Pulmonary Drug Delivery

1.Physiological factors
2.Drug properties

1.Physiological factors
- Blood-alveolar permeable barrier allows drug rapid diffusion into the systemic circulation.
- Low metabolic activity enhances drug bioavailability.
- Protein binding receptors favors the passage of peptides and proteins through the alveolar wall to the blood stream.

2.Drug-related factors 1-Particle diameter:
•Drug particle size affects the mechanism of drug disposition into the lungs:
•Drug particles in the range of 0.5-5μm: sedimentation
•Drug particles> 5μm: gravity
•Drug particles <0.5μm: diffusion (Brownian motion)

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