Defining dosimetry and markers of inhaled exposure in drug development

Development of pharmaceuticals as dry powder formulations for treatment of respiratory diseases has significant challenges including: (i) distinction between adverse toxicopathology and histopathological changes attributable to particle clearance mechanisms; (ii) low ‘safety margins’ arising from scaling factors applied for interspecies differences in lung deposition and for non-monitorable lesions, i.e. tissue changes seen in toxicology species that cannot be monitored clinically using markers in readily available specimens e.g. peripheral blood. Furthermore, development programs for inhaled products consume high masses of drug relative to other exposure routes. This limits the extent to which the inhaled route is utilised in early studies designed to identify issues and thus discharge liabilities before committing significant resources and finances to clinical development. This is especially true of conventional inhalation exposure systems used for dry powder aerosol delivery to animals. Strategies proposed to use less material in early respiratory drug development include: (i) progression to Phase I clinical trials using nebulised (liquid) formulations to identify unforeseen toxicity or undesirable PK/PD (liabilities such as tolerability or altered PK/PD are addressed later when the final particulate formulation is developed); (ii) design of aerosol delivery systems to reduce drug requirements for inhalation studies in early development. A clear understanding of differences in dosimetry, drug efficacy and toxicology of aerosols presented as dusts or droplets is pivotal to these strategies. The objectives of this project are to assess the extent to which changing the presentation of an inhaled drug alters its rate and extent of systemic exposure, lung dose and the drug’s efficacy and toxicology. Laboratory animals are not dosed by inhalation per se but are exposed, i.e. breathe passively from an atmosphere containing the test material. The ‘inhaled dose’ is estimated according to the following equation: Estimated inhaled dose = (C x RMV x T x IF) / BW Where: C is aerosol concentration T is duration of inhalation exposure IF is the inhaled (respiratory) fraction BW is body weight RMV is the Respired Minute Volume (measured or calculated as 0.608 x BW 0.852 ).  Respiratory tract deposition may influence dose and depends upon the aerodynamic properties of a particle, respiratory tract anatomy and nature/pattern of breathing. Work was undertaken to measure drug concentrations in rat plasma and lung tissue and to consider limitations in established dosimetry methods. Male rats were administered micronized GSK-CMPD, by snout-only inhalation exposure, at nominal doses of 1 or 45 mg/kg for up to 28 days. Aerosols were sampled for analysis of chamber concentration and particle size distribution using HPLC/UV quantitation. Plasma samples were taken post exposure on Days 1 and 28 for toxicokinetic analysis. Lungs were taken after a single exposure (immediately and 23 hours post dose) and 23 hours after the 28 th dose. Lungs were homogenised and GSK-CMPD concentrations in plasma and lung tissue analysed by HPLC/MS/MS. All animal studies were ethically reviewed and performed in accordance with Animals (Scientific Procedures) Act 1986 and the GSK Policy on the Care, Welfare and Treatment of Animals. Inhalation exposure of rats to micronized GSK-CMPD for 28 days produced drug accumulation with systemic exposure increasing ≥4-fold and lung concentrations increasing ≥7-fold from Days 1 to 28 of exposure. Sub-proportional increases in systemic exposure (3 to 6-fold) and lung concentrations (16 to 22-fold) were evident for a 40 to 57-fold increase in the estimated inhaled dose. Lung concentrations were very variable, particularly after a single dose. High and low lung concentrations were inconsistent (i.e. not proportional) with corresponding estimated RMV values, which were also less variable. The relative accumulation in lung tissue was higher than that observed in plasma, indicating saturable mechanisms of pulmonary absorption or clearance. Further experiments will evaluate systemic and lung exposure after inhaled administration of alternate aerosol forms of GSK-CMPD, i.e. spray dried powder and nebulised formulations. High intra-group variation in lung concentration, relative to ‘body weight derived RMV’, suggests potential for established methods of estimating ‘inhaled dose’ to oversimplify dosimetry and yield misleading data. Direct measurement of RMV will permit investigation of this phenomenon.