Protein and Peptide Delivery through Respiratory Pathway - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 434

by sedimentation. The particles that deposit in the upper and central airways are
rarely absorbed systemically, as these are efficiently and rapidly removed by muco-
ciliary clearance.
9.3.6.2 Methods for Assessing Peptide Absorption from the Lung
Peptide drug absorption from the lung may be assessed by a variety of techniques,
viz. in vitro or in situ methods.
9.3.6.2.1 In Vitro and In Situ Techniques
In vitro techniques generally utilize fundamental biophysical principles along with
the aerodynamic principles of the particle. In vitro methods are useful in estimat-
ing the fraction of therapeutic compound that might be deposited at the absorption
site. Impaction and light-scattering methods are performed for particle size mea-
surements in aerosols. Frequently used commercial equipment utilizing impaction
technology includes the Twin Stage Impinger and the Andersen Cascade Impactor;
and laser diffraction includes the HIAC/ROYCO, Coulter, and Malvern instru-
ments. Isolated organ (lung) studies serve as a valuable tool for rapid screening of
pulmonary absorption of therapeutic compounds [22,23] . Rabbit, dog, and rat lungs
have been investigated in isolated perfusion studies for a variety of compounds,
including isoproterenol, methadone, nicotine, parathion, and pentobarbital [24,25] .
In vitro permeability studies using cultivated epithelial cell culture monolayers from
different anatomical regions of the respiratory tract [26,27] provide valuable insights
into the transport properties of peptidic drugs across the lung tissues and transepithe-
lial flux.
9.3.6.2.2 In Vivo Techniques
Because in vitro and in situ techniques serve as important tools for screening absorption
characteristics and understanding their underlying mechanisms, they do not generally
account for the bioavailability and pharmacokinetics of the inhaled drug in the living
system. Thus, in vivo studies are fundamentally important. Tracheostomized animal
and intratracheal instillation are the two most commonly used techniques for inhalation
delivery in animals [28] . Techniques for clinical studies include gamma scintigraphy to
assess regional deposition and distribution of aerosolized drug in the airways; plasma
or urine pharmacokinetic studies to assess the rate and the extent of pulmonary drug
absorption; and pharmacodynamic studies in patients to assess the impact and safety
of formulation(s) as a function of drug dose. Pharmacokinetics relates drug dosime-
try with plasma profiles, whereas pharmacodynamics relates drug concentrations with
pharmacological endpoints indicative of efficacy. The pulmonary deposition and phar-
macokinetics of human growth hormone (hGH), administered by aerosol and instillate,
have been studied in five male New Zealand white rabbits. Gamma scintigraphy indi-
cated that the peripheral-central deposition ratio was greater for aerosol (1.54) than for
instillate (0.8), and the bioavailable fraction for aerosolized hGH (45%) was greater
than for instilled hGH (16%) [29] .