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In vitro suppression of Pseudomonas cepacia after limited exposure to subinhibitory concentrations of amiloride and 5‐(N,N‐hexamethylene) amiloride

Amiloride (A) for aerosolized therapy in cystic fibrosis (CF) is under investigation. Antipseudomonal properties of A and A analogs in vitro have recently been described. This study was performed to determine if there was suppression of P. cepacia growth after a limited 2‐hour exposure to (subinhibitory) concentrations of A ± tobramycin (T) or to the analog 5‐(N,N‐hexamethylene) amiloride (HMA) in vitro. The MIC of each drug against five different P. cepacia strains was determined. Cells were prepared in Mueller Hinton broth to [106] colony forming units (CFU)/mL and incubated at 35°C for 2 hours in the presence and absence of subinhibitory A, HMA, T, or A + T. The CFU were measured before and at 1 ‐hour intervals after dilutional removal of drug. The post‐antibiotic effect (PAE) was defined as the time (in minutes) required for the test culture counts to increase 10‐fold minus the time required for control counts to increase 10‐fold. At 400 μ/mL or 200 μ/mL, the PAE for A against five different strains was 139 ± 23 and 83 f 24 (mean 5 SD) minutes, respectively. For 100 μ/mL and 50 μ/mL HMA, the PAE was 122 ± 38 and 65 f 25 minutes. For T and A + T (200 μ/mL + 32 μ/mL) the PAE was 168 ± 30 and 258 ± 30 minutes. We conclude that A and the analog HMA in (subinhibitory) concentrations have a suppressive effect on P. cepacia after drug removal and potentiate the effect of T in vitro. Pediatr Pulmonol. 1994; 17:149–154. © 1994 Wiley‐Liss, Inc.