Although oral fluoroquinolones (FQs) and trimethoprim-sulfamethoxazole (TMP-SMX) have been routinely recommended as step-down therapy for treatment of Enterobacterales bacteremia owing to their high bioavailability, increasing evidence suggests that beta-lactam (BL) antibiotics (particularly those with high bioavailability, such as cephalexin) are as effective without the attendant adverse risks associated with FQs—with increasing FDA warnings—and TMP-SMX.^1,2

In the largest study to date involving a retrospective review of over 4,000 cases of Enterobacterales UTI-associated bacteremia (eg, E. coli, Proteus spp., Klebsiella spp) in Veterans Affairs hospitals, no significant difference in the main outcome (composite of 30-day all cause mortality or 30-day recurrent bacteremia) was found between the oral beta-lactam and FQ/TMP-SMX groups (4.4% vs 3.0%, respectively); additionally, when examined separately, no significant difference in mortality (3.0% vs 2.6%) or recurrent bacteremia (1.5% vs 0.4%) was found. ¹

A meta-analysis of 8 retrospective studies (2019) also failed to find a significant difference in mortality or recurrent bacteremia between BLs and FQs or TMP-SMX groups; it did find a higher odds of any recurrent infection, however (5.5% vs 2.0% (O.R. 2.06, 1.18-3.61). ²

Before selecting an antibiotic, however, it’s important to recall that not all oral BLs are  created equal, with some having better bioavailability than others.   More specifically, it may not be common knowledge that cephalexin (“Keflex”), a commonly prescribed and inexpensive cephalosporin with great safety profile, has 90-100% bioavailability, rivaling those of FQs or TMP-SMX.

 Of interest, in a subset of patients who received cephalexin as step-down therapy (n=245) in the VA study above, the outcomes were nearly identical to those who received FQ or TMP-SMX, with a 30-d recurrent bacteremia of 0% and a 30-day mortality of 2% (vs 0.4% and 2.5% for ciprofloxacin and 1.0% and 2.4% for TMP-STX, respectively). Of note nearly one-half of the cephalexin group received a higher dose of 500 mg 4x/day, with the rest receiving less frequent dosing. 

These findings makes one wonder whether suboptimal oral BL dosing may not have contributed to the discrepant results from earlier studies suggesting the superiority of FQs or TMP-SMX over oral BLs as step-down therapy. ^1,2

 

Bonus Pearl: Did you know that cephalexin may be given up to 4 gm/day in 4 divided doses with 90% of the drug excreted unchanged in the urine? ³

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References

1. Sutton JD, Stevens VW, Chang NCN, Khader K, et al. Oral beta-lactam antibiotics vs fluoroquinolones or trimethoprim-sulfamethoxazole for definite treatment of Enterobacterales bacteremia from a urine source. JAMA Network Open 2020;3 (10):e20220166. Oral β-Lactam Antibiotics vs Fluoroquinolones or Trimethoprim-Sulfamethoxazole for Definitive Treatment of Enterobacterales Bacteremia From a Urine Source – PubMed (nih.gov)
2. Punjabi C, Tien V, Meng L, et al. Oral fluoroquinolone or trimethoprim-sulfamethoxazole vs beta-lactams as step-down therapy for Enterobacteriaceae bacteremia: systematic review and meta-analysis. Open Forum Infect Dis 2019;6:ofz364 doi:10.1.1093/ofid/ofz364   https://pubmed.ncbi.nlm.nih.gov/31412127/
3. Herman TF, Hasmi MF. Cephalexin. StatPearls (internet). https://www.ncbi.nlm.nih.gov/books/NBK549780/ Accessed July 10, 2022.

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