Aerosol Drug Delivery
Systems
There are two main types of hand held devices for delivering
doses of aerosol medicament to a patient. Both are a
propellant-driven metered dose inhaler (MDI) and a dry powder
inhaler (DPI).

In a metered dose inhaler, the manufacturers either suspend or
dissolve the medicament in a propellant. They then "contain" the
propellant in a pressurized canister with a metered valve. Upon
activation, the propellant produces a single dose of the
medicament in the form of a gas stream. The device may include
a tapered discharge nozzle baffle or a venturi to accelerate
particles through a discharge nozzle, and to remove oversized
particles. The propellants usually are
low-boiling-point-halocarbons such as hydrofluorocarbons,
hydrofluorochlorocarbons and fluorochlorocarbons. Are you
familiar with the name- "Freon"?

However, only about 8% percentage of the medicament is
delivered to a patient's lung, because of incomplete evaporation
of the propellant and sticking of medications to the back of the
throat. This sticking of medications may cause localized problems
in the impact area, other unwanted systemic side effects if
patients are swallowed. Furthermore, metered dose inhalers
require coordination between activation and inhalation, which may
exclude Infants, small children and the elderly from the use of the
medications.

To overcome this problem, manufacturer often adds a space in
metered dose inhalers to provide an additional volume for the
propellant to evaporate. However, fine particle fractions deposit
within the spacer, and lower the absorption of the medications.
However, this may avoid the deposition of the drug to the throat
and relieve some systemic side effects.

In a dry powder inhaler, manufacturers use no propellant but the
device relies upon a burst of inspired air drawn through the unit by
the patient. However, the force of inspiration varies considerably
from person to person. Some patients with lung problems are
unable to generate sufficient air in-flow to activate the device. Dry
powder inhalers may have incomplete particle dispersion and
also the impact of particle at the back of the throat.

In an attempt to overcome the problem of particle aggregation, the
manufacturers usually formulate the medicament with a carrier in a
particular way to aid de-agglomeration. Thus dry powder inhalers
are unsuitable for high dose medications. Other common
problems may lead to low absorption include high adhesiveness
between the drug and the carrier and high moisture affinity of
predominantly amorphous structure, e.g  blend of
dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol
(PG) (DPPC : PG 7:3).

                            
Aerosol
                  metered dose inhaler
            Testing of Metered Dose Inhaler

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