Antimicrobial resistance (AMR) in urinary tract infections (UTI) is one of the leading causes of death associated with resistance worldwide.¹ In 2019, more than 26,000 deaths directly attributable to AMR were caused by Escherichia coli – the most common uropathogen limiting empirical therapies for severe UTI, owing to its multidrug-resistant (MDR) strains ST131 and ST1193.^2–4

Oral ciprofloxacin is the mainstay treatment for UTIs owing to its excellent bioavailability, tissue penetration and ability to achieve high concentrations in the urine versus plasma.⁵ A 12 hourly dose of 250 mg is typically prescribed for uncomplicated UTIs⁶, while ‘high dose’ ciprofloxacin is defined as 750 mg administered 12 hourly.⁷ In 2022, both the Clinical & Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) established breakpoints (concentrations at which bacteria are susceptible to antibiotic treatment) for ciprofloxacin based on the standard dosing of 500 mg 12 hourly.^7,8

Although there are currently no urine-specific breakpoints established for ciprofloxacin, it is hypothesized that urinary isolates with low-level resistance to ciprofloxacin could be effectively treated with high-dose
ciprofloxacin, given that high concentrations of the drug can be retained in the urine after oral dosing.⁵

Abbott and colleagues aimed to test this hypothesis by performing pharmacodynamic profiling of different oral ciprofloxacin dosing schedules using clinical isolates of ceftriaxone-resistant E. coli. The isolates’ ciprofloxacin MICs were assessed in triplicate using the broth microdilution method (BMD), and whole genome sequencing (WGS) was performed to determine the sequence type (ST), phylogenetic relatedness, quinolone-resistance determinants and β-lactamase genes of the isolates. The researchers then established a dynamic bladder infection in vitro model consisting of fresh media reservoirs containing modified synthetic human urine (mSHU) that flowed continuously into an ‘intestinal’ compartment with 2000 mg/L ciprofloxacin and into a ‘circulatory’ compartment. From the ‘circulatory compartment’, the medium was pumped into 16 ‘bladder’ compartments that were individually inoculated with 10 mL of the E. coli isolates (10⁶ cfu/mL, equivalent to the total bacterial count typically seen in human UTIs i.e., ≥10⁵ cfu/mL in 200 mL void).

Ciprofloxacin dosing regimens (250 mg daily; 500 mg daily; 250 mg 12 hourly; 500 mg 12 hourly; and 750 mg 12 hourly) were administered as a 3-day treatment course. The primary endpoint of the study was end-of-treatment (72 h) change in bacterial density, measured via serial dilution and agar plating. Secondary endpoints were the change in bacterial density at follow-up (96 h), and total bacterial response measured by the area under the bacterial kill curve (AUBKC_0–96). The researchers also assessed the relationship between ciprofloxacin exposure (AUC_0–24/MIC and C_max/MIC) and bacterial response, and used Monte Carlo simulations (MCS) to determine the probability of pharmacodynamic target attainment (PTA) for each ciprofloxacin dosing regimen.

Key study findings include⁵:

• A total of 93 ceftriaxone-resistant coli urinary isolates were collected for ciprofloxacin MIC testing; 15 were selected for further testing in the bladder infection in vitro model alongside E. coli ATCC 25922.
  • The 15 clinical isolates selected for additional testing had MICs ranging from 0.25 to 512 mg/L. They possessed a diverse range of STs and did not belong to any transmission cluster (i.e., >45 single nucleotide polymorphisms [SNP] between all five coli ST131).
  • parC S80I mutation was observed in isolates with MIC ≥4 mg/L.
• In all dosing regimens, coli ATCC 25922 (MIC 0.008 mg/L) was eradicated.
• Among the 15 clinical isolates, six (MIC ≥16 mg/L) had near maximal regrowth at 72 h (>1.9 Δlog₁₀ cfu/mL) in all dosing regimens.
• Regrowth at 72 h for the remaining nine clinical isolates (MIC 0.25–8 mg/L) was observed in:
  • Three isolates (MICs 4–8 mg/L) after 250 mg and 500 mg daily
  • One isolate (MIC 8 mg/L) after 250 mg and 500 mg 12 hourly
  • No isolate regrew after 750 mg 12 hourly ciprofloxacin
• At follow-up (96 h), regrowth among the remaining nine isolates was observed in:
  • Seven isolates (MICs 0.5–8 mg/L) after 250 mg daily
  • Five isolates (MICs 0.5–8 mg/L) after 500 mg daily
  • Three isolates after 250 mg and 500 mg 12 hourly
  • One isolate (MIC 4 mg/L) after 750 mg 12 hourly
• Appreciable increase in MIC was observed in only two isolates: Isolate 019 (MIC 0.5 to 2 mg/L after 250 mg 12 hourly) and isolate 127 (MIC 32 to 128 mg/L after 750 mg 12 hourly).
• No emergence of resistance was detected on the Mueller-Hinton agar (MHA) plates supplemented with 2 and 128 mg/L ciprofloxacin.
  • Isolates with MIC <2 mg/L were suppressed in MHA containing 2 mg/L ciprofloxacin.
  • MHA containing 128 mg/L ciprofloxacin suppressed the growth of isolates with MIC 2–64 mg/L, except selected isolates.
• The results of the PK/PD analyses and MCS demonstrated that:
  • 750 mg 12 hourly ciprofloxacin achieved a 3 log₁₀ kill effect at 72 h in isolates with MIC ≤1 mg/L, with 90% maximal activity (AUBKC_0–96) and a 1 log₁₀ kill at follow-up (96 h).
  • Standard-dose ciprofloxacin (500 mg 12 hourly) promoted 3 log₁₀ kill at 72 h in isolates with MIC ≤ 1 mg/L; however, a lower MIC of ≤0.5 mg/L was needed to achieve 1 log₁₀ kill at 96 h and 90% maximal activity.
  • Low-dose ciprofloxacin (250 mg 12 hourly, or 500 mg daily) achieved 3 log₁₀ kill at 72 h in isolates with MIC ≤ 0.5 mg/L, 1 log₁₀ kill at 96 h and 90% maximal activity in isolates with MIC ≤ 0.25 mg/L.

Overall, the results of the study support the efficacy of high-dose oral ciprofloxacin (750 mg 12 hourly) against ceftriaxone-resistant E. coli isolates with ciprofloxacin MIC ≤ 1 mg/L, highlighting the potential of expanding antimicrobial activity against urinary isolates with low-level ciprofloxacin resistance. Notably, an overall lack of emergence of resistance was observed during follow-up MIC testing and MHA plating with 2 and 128 mg/L ciprofloxacin. The authors speculate these results to be either owing to the downregulation of genes prior to MIC testing, or that regrowth reflects tolerance, persistence and quiescence.^9,10

Although the study had a few key limitations (i.e., lack of host response and bladder tissue architecture, and uncertain applicability of study results to other uropathogens and cases of complicated UTI or renal dysfunction), the authors nonetheless encourage meticulous application of urinary-specific ciprofloxacin breakpoints in clinical scenarios and strong antimicrobial stewardship practices.

References

1. Murray CJ, et al. The Lancet. 2022;399(10325):629-655.
2. Pitout JDD, et al. Antimicrob Agents Chemother. 2022;66(7).
3. Centers for Disease Control and Prevention. Antibiotic resistance threats in the United States, 2019. https://www.cdc.gov/drugresistance/pdf/threats-report/2019-ar-threats-report-508.pdf. Accessed 2 October 2023.
4. Li X, et al. J Clin Med. 2022;11(10):2817.
5. Abbott IJ, et al. J Antimicrob Chemother. 2023;78(2):397-410.
6. Abbott IJ, et al. Clin Microbiol Infect. 2023;29(10).
7. European Committee on Antimicrobial Susceptibility Testing. Clinical breakpoints and dosing of antibiotics. https://www.eucast.org/clinical_breakpoints. Accessed 2 October 2023.
8. Clinical and Laboratory Standards Institute. Performance standards for antimicrobial susceptibility testing, 33rd edition. https://clsi.org/standards/products/microbiology/documents/m100/. Accessed 2 October 2023.
9. Ortiz-Padilla M, et al. J Antimicrob Chemother. 2020;75(8):2124-2132.
10. Leatham-Jensen MP, et al. mSphere. 2016;1(1).