Inhaled Vs Systemic Corticosteroid Sensitivity Of A Combined Ovalbumin And LPS Model Of Asthma Exacerbation In Guinea-pigs

APP Lowe, WR Ford, EJ Kidd, KJ Broadley. Cardiff University, Cardiff, UK

Asthmatics that experience exacerbations show increased symptom severity and decreased responsiveness to inhaled corticosteroids (Fahy et al, 1995; in\'t Veen et al, 1999). Complete insensitivity to oral corticosteroids is rare and most patients lie on a continuum of steroid responsiveness (reviewed in Adcock & Barnes, 2008). We have developed a model of asthma exacerbation, in which lipopolysaccharide (LPS) exposure is superimposed on ovalbumin (Ova) sensitization and inhalation (Lowe et al, 2012). The current study determined this models sensitivity to both inhaled and systemic corticosteroid treatment.

Guinea-pigs (200-250g, male, Charles River, N=5-6) were sensitised with an intra-peritoneal injection of 150µg Ova and 100mg Al(OH)3 (day 1, 4 and 7). Airway responses to inhaled (1h) Ova (0.03%), LPS (2x 30μg/m, 48h apart) or LPS and Ova co-administered to conscious guinea-pigs were determined whole body plethysmography (Buxco Systems, USA) on day 21. Changes in specific airways conductance (sGaw) were determined over 12hrs and at 24hrs. Fluticasone propionate (Fp, 0.5 mg/ml daily) or dexamethasone (20mg/kg daily) was administered by inhalation or intra-peritoneal injection, respectively, for 6 days before challenge. Airways hyper-responsiveness (AHR) was investigated from the bronchoconstriction by inhaled histamine (0.3mM) 7 days pre- and 24hr post-Ova challenge. Inflammatory cell influx and total protein were determined by bronchoalveolar lavage (BAL) at 24hr post-challenge. T-tests and ANOVA were performed where relevant.

Animals challenged with both Ova and LPS demonstrated an elongated early asthmatic response (EAR), late asthmatic responses (LAR), increased AHR to histamine, BAL protein and mixed granulocytic airway inflammation, compared to Ova alone. Inhaled Fp failed to significantly attenuate EAR (-61.1±6.8%) or LAR (-27.1±4.8%) compared to vehicle (-65.5±5.5% and -34.6±6.7%, respectively). Contrastingly, dexamethasone significantly reduced the duration of the EAR (time to 50% of peak value, 2.1±0.5h compared to 4.8±0.5h, P>0.01) but not the peak LAR compared to vehicle (-18.2±5.7%, -28.5±5.7% respectively). AHR to histamine following Ova and LPS challenge was not reduced by Fp (-34.3±12.8% compared to vehicle, -41.1±7.0%) or dexamethasone (-36.0±12.3% compared to vehicle, -43.2±5.8%). Airway inflammation (total cells) was decreased by dexamethasone (11.2±1.9x106/ml compared to vehicle, 25.8±1.6x106/ml, P>0.001), but was Fp-insensitive (22.3±1.6x106/ml compared to vehicle, 22.2±1.3x106/ml). Total BAL protein, an indicator of airway oedema was attenuated by both Fp (1.3±0.1mg/ml, P>0.01) and dexamethasone (0.7±0.2mg/ml, P>0.01) compared to vehicle (3.1±0.4mg/ml and 2.4±0.4mg/ml, respectively), demonstrating that steroid resistance is not related to delivery issues. Both steroids at the doses used were found effective at reducing LAR, AHR, airway inflammation and BAL protein in animals challenged with Ova alone.

Functional and inflammatory responses to allergen and LPS co-challenge were resistant to inhaled corticosteroid but airway inflammation and the prolonged EAR were sensitive to systemic corticosteroid. This data has clinical relevance as individuals poorly treated by inhaled steroids often require acute oral corticosteroid treatment to improve symptoms (Chan et al, 1998). This model may be useful as a preclinical model for developing novel drugs for steroid-resistant asthma.

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in\'t Veen et al (1999)Am.J.Respir.Crit.Care.Med160:93–99

Lowe et al (2012)Am.J.Respir.Crit.Care.Med185:A5641.

This work was supported by an MRC/GlaxoSmithKline Studentship to A.P.P.L

All animal studies were carried out in accordance with Animals (Scientific Procedures) Act 1986