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Bacterial load may predict response to inhaled antibiotics in bronchiectasis

Among patients with bronchiectasis, inhaled antibiotics improved quality of life only in those with high bacterial load, according to data published in the American Journal of Respiratory and Critical Care Medicine.

“The principle underpinning inhaled antibiotic treatment in bronchiectasis is that decreases in bacterial load will in turn reduce symptoms and the frequency of exacerbations. However, most trials have not reached their primary endpoints in bronchiectasis,” the researchers wrote.

Recent studies, however, have not assessed outcomes with inhaled antibiotic treatment in relation to baseline bacterial load, they noted. To fill gaps in knowledge, the researchers performed three studies. For all studies, they used sputum quantitative analysis to categorize patients as having low (< 10⁵ colony-forming units [cfu]/g), moderate (10⁵-10⁶ cfu/g) and high bacterial load ( 10⁷ cfu/g).

In the first study, when compared with patients with moderate and low bacterial load, those with high bacterial load had worse quality of life, as measured by the Quality of Life-Bronchiectasis-Respiratory Symptoms Score (QoL-B-RSS; 50 vs. 59 vs. 61; P = .01), and more severe disease, as measured by the Bronchiectasis Severity Index (9.3 vs. 7.7 vs. 5.2; P < .001).

The second study included an exacerbation and a stability study. In the exacerbation study, bacterial load decreased with antibiotic treatment in patients with low, moderate and high bacterial load (P < .001), but it increased again in all groups during follow-up (P < .001). In the stability study, only two of 10 patients experienced significant changes in bacterial load during follow-up, whereas the other eight maintained the similar levels.

The third study was a secondary analysis of two randomized, double-blind, placebo-controlled phase 3 trials — AIR-BX1 and AIR-BX2 — of inhaled aztreonam in bronchiectasis. As in the second study, bacterial load decreased with treatment at days 14 and 28 on treatment in the high, moderate and low bacterial load groups. After aztreonam discontinuation, however, patients’ bacterial loads returned to levels that were equivalent to that of their respective groups at baseline.

Also in the third study, QoL-B-RSS at week 4 did not improve with inhaled aztreonam vs. placebo in patients with low bacterial load (mean difference, 1.6; 95% CI, –4.9 to 8.3) and moderate bacterial load (mean difference, 0.9; 95% CI, –4.8 to 6.8). However, there was significant improvement with inhaled aztreonam in patients with high bacterial load at baseline (mean difference, 9.7; 95% CI, 3.4-16).

Furthermore, the proportion of patients who achieved an increase in QoL-B-RSS above the minimum clinically important difference of 8 points or more was higher in the aztreonam group vs. placebo at week 4 (63% vs. 37%; P = .01) and week 12 (62% vs. 38%; P = .01) only in patients with high bacterial load at baseline.

“In conclusion, we have shown that bacterial load is potentially a key bronchiectasis treatable trait that predicts response to inhaled antibiotics,” the researchers wrote. – by Melissa Foster

Disclosures: Healio Pulmonology could not confirm the authors’ relevant financial disclosures at the time of publication.