Aerosolized protein delivery in asthma: Gamma camera analysis of regional deposition and perfusion

Bioavailability of an aerosolized anti-inflammatory protein, soluble interleukin-4 receptor (IL,4R), was measured in patients with asthma using two different aerosol delivery systems, a prototype aerosol delivery system (AERx((R)) tethered model, Aradigm, Hayward, CA) and PARI LC STAR((R)) nebulizer (Pari, Richmond, VA). Regional distribution of the drug in the respiratory tract obtained by planar imaging using gamma camera scintigraphy was utilized to explain the differences in bioavailability. The drug, an experimental protein being developed for asthma, was mixed with radiolabel (99m)Technetium diethylene triaminepentaacetic acid (Tc-99m-DTPA). Aerosols were characterized in vitro using cascade impaction (mass median aerodynamic diameter [MMAD] and geometric standard deviation [GSD]); the AERx((R)) MMAD 2.0 mum (GSD 1.35), the PARI((R)) 3.5 mum (GSD 2.5). Four patients with asthma requiring maintenance aerosolized steroids were studied. First, regional volume was determined utilizing equilibrium Xe-133 scanning. Then, after a brief period of instruction, patients inhaled four breaths of protein using AERx((R)) (0.45 mg in total) followed 1 week later by inhalation via PARl((R)) (3.0 mg nebulized until dry). Each deposition image was followed by a measurement of regional perfusion using injected Tc-99m albumin macroaggregates. Deposition of Tc-99m-DTPA in the subjects was determined by mass balance. Regional analysis was performed using computerized regions of interest. The regional distribution of deposited drug was normalized for regional volume and perfusion. Following each single inhalation, serial blood samples were drawn over a 7-day period to determine area under the curve (AUC) of protein concentration in the blood. Median AUC(AERx)/AUC(PARI) was 7.66/1, based on the amount of drug placed in each device, indicating that AERx((R)) was 7.66 times more efficient than PARI((R)). When normalized for total lung deposition (AUC per mg deposited) the ratio decreased to 2.44, indicating that efficiencies of the drug delivery system and deposition were major factors. When normalized for sC/P and (pU/L)(Xe), ratios (central to peripheral and upper to lower ratios are parameters of regional distribution of deposited particles and regional perfusion ['p ']), AUC(AERx)/AUC(PARI) further decreased to 1.35, demonstrating that peripheral sites of deposition with the AERx((R)) affected the final blood concentration of the drug. We conclude that inhaled bioavailability of aerosolized protein, as expressed by AUC, is a quantifiable function of lung dose and regional deposition as defined by planar scintigraphy.