Inhaled aerosolization of all-trans-retinoic acid for targeted pulmonary delivery

Retinoids have shown promising activity for both cancer chemoprevention and as a treatment for emphysema. However, chronic oral administration of these drugs is limited by systemic side effects, including hepatic dysfunction, skeletal malformations, hyperlipidemia, hypercalcemia, and other reactions. In order to improve the pulmonary targeting of this potentially useful therapy, we developed a system for aerosolization of retinoids that substantially increased their local bioavailability. We compared the biodistribution and pharmacokinetics of an inhaled formulation of all-trans-retinoic acid (all-trans-RA), which was packaged in a metered dose inhaler, following both intratracheal (IT) and intravenous (IV) administration in male Sprague-Dawley rats. After drug administration, anesthetized animals were killed at 5 min, and at 1, 2, 4, 6 and 24 h. Plasma and emulsified samples of liver and lung tissues were dissected, extracted, and frozen prior to measurement of all-trans-RA concentration by high-performance liquid chromatography (HPLC). Aerosolization and IT injection of all-trans-RA resulted in a significantly longer pulmonary half-life of the drug (both 5-17 h), lower peak serum concentrations (aerosol 71 +/- 31 ng/ml, IT 68 +/- 50 ng/ml), and lower liver levels (aerosol 111 +/- 28 ng/g, IT 753 +/- 350 ng/g) than the same dose administered IV (2 h, 838 +/- 56 ng/ml, 4258 +/- 1006 ng/g, respectively; P < 0.05 for each comparison). Histologic examination of lungs and trachea showed no focal irritation attributable to the drug after single-dose administration. These results suggest that aerosolization of retinoids may offer a practical alternative to systemic oral administration for chemoprevention trials or treatment of lung diseases. This method may substantially increase the therapeutic index of these compounds by reducing systemic complications associated with long-term dosing.