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Pulmonary absorption studies utilizing in situ rat lung model: designing dosage regimen for bronchial delivery of new drug entities.

The absorption of 7-methylsulfinylxanthone-2-carboxylic acid, 7-(methylthio)xanthone-2-carboxylic acid, and their sodium salts from the respiratory tract of anesthetized rats was studied after intratracheal administration of 0.1 ml of a solution or suspension containing the drug. At various times after administration, the lungs and trachea were removed and assayed radiochemically for unabsorbed drug. Sodium 7-(methylthio)xanthone-2-carboxylate from solution was absorbed approximately 20 times faster than sodium 7-methylsulfinylxanthone-2-carboxylate from a solution. The absorption from solutions was three to four times faster than the absorption from suspensions. For inhalation aerosol dosage forms intended for prophylactic use, the drug entity with slower systemic absorption probably would be more desirable than the drug entity with rapid absorption. Rapid systemic absorption following inhalation of powder or liquid aerosols would lead to more frequent dosing if the biological activity is related to the drug concentration in the tracheobronchial tissues. Therefore, the powder or liquid inhalation aerosols of organic acids rather than the corresponding sodium salts may be preferable for designing a dosage regimen. However, if the dosage form is intended for utilizing the bronchodilator activity of the compound, the drug entity with rapid absorption is more desirable. Therefore, in the treatment of asthmatic attacks, liquid or powder inhalation aerosols of the sodium salt of the rapidly absorbing drug entity are preferable. The absorption rates were directly proportional to concentration when the initial concentration of sodium 7-methylsulfinylxanthone-2-carboxylate was varied over a 333-fold range. The effect of the pH of the drug solution administered intratracheally to the rat lung indicated a sharp increase in the pulmonary absorption at a pH near the pKa. The results suggested that structurally related xanthones are absorbed possibly by passive diffusion across the lipoidal region of the pulmonary membranes and that the absorption of organic acids and organic electrolytes is mainly controlled by the lipid solubility of the unionized species.