Drug development for pulmonary use necessitates inhalation toxicology studies. Most of these studies require the use of relatively large amounts of drug due to losses before and during exposure. Ben-Jebria et al. (1999) developed unique particles for inhalation use. These particles are porous with a large geometric diameter (~10 um) and low mass density (<0.1g/cm3). They hypothesized that these particles would be too large to be phagocytosed by macrophages and would, therefore, remain in the lungs longer than conventional particles. An experimental model was developed using rats and guinea pigs to measure the efficacy of drug delivery using these particles.

Both polymeric and nonpolymeric excepients were used to prepare these large particles with low mass density. Solvent evaporation and spray drying was used in both cases as described in detail in the reference (below). The polymeric microsphere preparation was made of biodegradable poly(lactic acid-co-glycolic acid) (PLGA) polymer particles. The nonpolymeric large porous particles consisted of human serum albumin, lactose, dipalmitoyl phosphatidylcholine (DPPC) and albuterol sulfate. Particles were also made of DPPC, albumin, lactose, and estradiol. The DPPC allows these nonpolymeric particles to be relatively water insoluble permitting a sustained release of hydrophilic or lipophilic drugs such as albuterol or estradiol over one to two days. The DPPC particles all had geometric diameters of ~10um and bulk (tapped) densities of approximately 0.08 g/cm3. Control non-porous particles made of lactose and albuterol had a smaller geometric diameter and larger bulk density (diameter=3u and bulk density of 0.45 g/cm3).

These particles were aerosolized with a Spinhaler dry powder inhaler and characterized in vitro with a cascade impactor and Aerosizer as detailed in the reference. About 50% of the large porous particles had a MMAD of <4.7 um while 16% of the small nonporous particles were <4.7 um or "fine particles". In addition, Spinhaler results showed that large porous particles were less aggregated than nonporous particles. The MMAD of large porous particles was ~2.15 um while the MMAD of nonporous particles was ~ 4.53 as measured with the Aerosizer. The smaller MMAD results in greater respirability in vitro.

Figure 1:  Click on Figure for larger version.  This is the inhalation system for particle delivery to the respiratory tract of rats (from Ben-Jebria et.al., 2000)

The final test of the utility of large porous particles for both delivery and sustained release into the lungs required the development of a novel rodent exposure system. Rats were used to test the release over time of insulin, testosterone, and estradiol. Guinea pigs were used to assess the bronchodilation effect of albuterol. The animal is anaesthetized and placed supine on a surgical board. The proximal trachea is exposed and a blunt cannula inserted and connected to a ventilator as shown in Figure 1. The animal is still able to breathe through proximal and upper airways because the outer diameter of the cannula is is 1 mm whereas the inner diameter of the trachea is 3-4 mm. (The 1mm diameter cannula has greater airway resistance than the 3-4 mm airway). A pipette with a rubber bulb is attached to the inhalation side of the ventilator tubing for the purpose of introducing the particles. The bulb is filled with a set amount of particles and a portion is introduced as the operator squeezes it while the respirator is set to inhale. The portion is delivered in one 30 to 60 sec insufflation by forced ventilation at 3 ml tidal volume and 100 strokes/min frequency. Several repetitions of the procedure deliver 3 to 5 mg of particles over a period of 10-15 min.

Figure 2:  Total particle mass (percent of mass delivered) recovered from rat respiratory tract after bronchoalveolar lavage.  "Respirable" is the fraction of the delivered particle mass that is recovered in rat lungs beyond the carina, after bronchoalveolar lavage; and "Nonrespirable" is the fraction of teh delivered particle mass that is recovered in the tubing and trachea.  (from Ben-Jebria et al, 2000)

The bronchoalveolar lavage fluid (BALF) was collected 10 min after the inhalation procedure and the number of particles in each lobe of the lobe was counted as described in the reference. Figure 2 shows that most of the large porous particle mass was deposited in the lungs distal to the carina. Less than 45% was left in the tubing and trachea. However only 20% of the mass of small nonporous particles was deposited in the respiratory tract. There were no significant changes in cell population or protein recovered from the BALF in guinea pigs treated with large porous particles with and without albuterol.

The bioavailability of the drugs, insulin, testosterone and estradiol administered to the lungs with large porous particles was compared to the bioavailability when administered subcutaneously. Radioimmunoassay was used to quantitate the amount of insulin, estradiol and testosterone. 87.5% more estradiol, 177% more insulin, and 86% more testosterone were available from inhaled large porous particles compared to subcutaneous administration of these drugs. In contrast the small nonporous particles yielded less bioavailable insulin (12%) and estradiol (18%). Blood concentrations of these drugs remained elevated significantly longer when they were administered on large porous particles compared to administration on nonporous particles. There was also a sustained inhibition of carbachol-induced bronchoconstriction when guinea pigs inhaled large porous particles with albuterol for 15 min.

The authors conclude that large porous particles reach the deep lung more efficiently than do more conventional small nonporous particles in this experimental rodent model. In addition large porous particles have a longer residence time in the lungs than do small nonporous particles. The large porous particles have relatively low mass density. This causes better penetration of the large porous particle because the aerodynamic diameter of these particles is similar to that of nonporous particles. Greater aerosolization efficiency allows less chance of loss in large airways and tubing. The insufflation technique for aerosol delivery described is concluded to be useful for dry powders because it allows a small amount of aerosol to be delivered in a short period of time.

Reference: Inhalation System for Pulmonary Aerosol Drug Delivery in Rodents Using Large Porous Particles. Abdellaziz Ben-Jebria, Mary Lou Eskew, and David A. Edwards. Aerosol Sci. and Technol. 21:421-433 (2000).

Also: Ben-Jebria, A., Chen, D., Eskew, M.L., Van-bever, R., Langer, R., and Edwards, D.A. (1999). Large Porous Particles for Sustained Protection from Carbachol-induced Bronchoconstriction in Guinea Pigs, Pharm. Res. 16:555-561.