Small molecule distribution in rat lung: a comparison of various cryoprotectants as inflation media and their applicability to MSI

Given the recent explosion of mass spectrometric imaging (MSI), it has become easier to assess drug tissue localisation without the use of radiolabeling and other more complex methods (such as PET and MRI). For MSI tissue preparation is of utmost importance, however, the lung in particular does pose some difficulties with imaging since it is made up of a number of air-filled alveoli. These organs are known to collapse when the thoracic cavity is pierced, losing its structural integrity and giving poor histological representation for drug distribution analysis. The use of cryoprotectants as a tissue inflation media will aid in the preservation of the lung's structural integrity during MSI experiments involving small molecule distribution. Various established cryoprotectants (DMSO, PvP, ethylene glycol, sucrose, DMEM, control serum, OCT) were selected as lung inflation media for MSI analysis of gatifloxacin (GAT). Female Sprague-Dawley rats were treated with GAT (10 mg/kg b.w) via i.p. injection. After 15 min the animals were terminated by halothane overdose, and each set of tissue inflated with a specific agent. Cryosections were made and MSI conducted to determine drug tissue distribution. During the early stages of the experimental procedure some crypreservatives were eliminated due to difficulties with sample preparation. While others displayed excellent preservation of the tissue structure and integrity. Following MSI analysis, some agents showed homogenous drug distribution while some displayed heterogeneous distribution favoring the basal periphery. Taking into account the physiology of the lung and previous MRI investigations of its perfusion, it is expected that a systemically administered drug would localize in the basal areas. DMSO and DMEM proved to display this distribution pattern while keeping structural integrity intact. However, the later was ruled out since it showed complete suppression of GAT in solution. From the cryoprotectants selected for this study, DMSO is the most promising lung inflation media focusing on small molecule distribution via MSI.