Threshold size for optimal passive pulmonary targeting and retention of rigid microparticles in rats

Abstract

The relationship between microparticle (MP) size and lung targeting efficiency, intra-lung distribution and retention time was systematically studied after intravenous administration of rigid fluorescent polystyrene MPs of various sizes (2, 3, 6 and 10 microm) to Sprague Dawley rats. Total fluorescence was assessed and it was found that 2 microm and 3 microm MPs readily passed through the lung to the liver and spleen while 10 microm MPs were completely entrapped in the lung for the one-week duration of the study. Approximately 84% of 6 microm MPs that were initially entrapped in the lung were cleared over the next 2 days and 15% were cleared over the remaining 5 days. A Caliper IVIS 100 small animal imaging system confirmed that 3 microm MPs were not retained in the lung but that 6 microm and 10 microm MPs were widely distributed throughout the lung. Moreover, histologic examination showed MP entrapment in capillaries but not arterioles. These studies suggest that for rigid MPs the optimal size range required to achieve transient but highly efficiently targeting to pulmonary capillaries after IV injection is >6 microm but <10 microm in rats and that systemic administration of optimally sized MPs may be an efficient alternative to currently used inhalation-based delivery to the lung.

Fig. 1

Numeric (solid) and Volumetric (dashed) distribution of polystyrene MPs: 3 μm (red); 6 μm (green), 10 μm (blue). The size of the 3 μm, 6 μm and 10 μm polystyrene MPs as reported by the manufacturer were: 3.005 ± 0.112 μm, 6.098 ± 0.573 μm, 10.94 ± 0.396 μm, respectively; the volumetrically measured size of polystyrene MPs were: 3.066 ± 0.236 μm, 5.915 ± 0.205 μm, and 11.06 ± 0.309 μm, respectively.

Fig. 2

SEM of 6 μm polystyrene MPs. MPs appear to be discrete with a smooth surface and have low polydispersity.

Fig. 3

Biodistribution studies of MPs in the filter organs and hearts of healthy rats was determined using a fluorescent plate reader (n = 3) at different time points after IV injection: (A) 2 μm; (B) 3 μm; (C) 6 μm; and (D) 10 μm.

Fig. 4

A representative picture of the spatial distribution of 3, 6 and 10 μm fluorescently labeled carboxylate polystyrene MPs in the lung of healthy rats, imaged using the IVIS® 100 at 1 hour, 12 hours, 24 hours and 168 hours post injection. The MPs were highly distributed throughout the lung tissue.

Fig. 5

H & E stained healthy rat lung in GMA polymer matrix: control rat lung (Panel A & B); rat lung 48 hours after administration of 10 μm MPs (Panel C & D). MPs are indicated by arrows; showing they were entrapped in the pulmonary septal microvessels. Magnification of Panels A & C: 20x; Panels B & D: 60x.