Combination therapy may be effective against S. maltophilia

Meropenem plus levofloxacin demonstrated Etest synergy in 60% of Stenotrophonomas maltophilia isolates, according to study results presented at the 2010 Annual Conference on Antimicrobial Resistance.

From May 2007 to June 2009, researchers, led by George A. Pankey, MD, director of infectious disease research at Ochsner Clinic Foundation in New Orleans and member of the Infectious Disease News editorial board, collected 20 genetically-unique clinical S. maltophilia isolates from patients to evaluate potential synergistic activity of meropenem plus levofloxacin against S. maltophilia. The researchers used Etest MICs and synergy method, which was performed in triplicate with the summation fractional inhibitory concentration calculated for each set of MICs, and the mean used for comparison to the time-kill assay (TKA). Isolates had the following susceptibility:

• 35% for ceftazidime.
• 10% for chloramphenicol.
• 95% for minocycline.
• 35% for ticarcillin/clavulanate.
• 100% for trimethoprim/sulfamethoxazole.

Results revealed synergy with levofloxacin plus meropenem in 12 of 20 of S. maltophilia isolates while eight of 20 showed indifference. TKA data indicated synergy in nine of 20 isolates, with the remaining 11 showing indifference. Researchers also found that two of three non-susceptible levofloxacin isolates also demonstrated synergy by both Etest and TKA.

Presently, the mechanism of in vitro synergy between levofloxacin and meropenem remains unclear, and the researchers concluded that evaluation with more non-susceptible levofloxacin S. maltophilia isolates would be required to maintain the 67% synergy shown with levofloxacin plus meropenem in the three non-susceptible levofloxacin isolates.

In an interview, Pankey said that limited options for resistant isolates necessitates looking at combination therapies, and said that this combination may hold promise as a therapeutic option, but in vivo studies are needed.

For more information:

• Pankey GA. Etest synergy testing of meropenem plus levofloxacin against S. maltophilia. Presented at: 2010 Annual Conference on Antimicrobial Resistance; Feb. 1-3, 2010; Bethesda, Md.