What are the intramuscular (IM) options for treating urinary tract infections (UTIs) caused by Escherichia coli (E. coli)?

Intramuscular Options for E. coli UTIs

For E. coli UTIs requiring parenteral therapy, ceftriaxone 1g IM or gentamicin (dosed at 5-7 mg/kg) IM are the primary intramuscular options, with ceftriaxone preferred for most outpatient scenarios due to once-daily dosing and broader safety profile. 1, 2

Primary IM Antibiotic Options

Ceftriaxone (Third-Generation Cephalosporin)

• FDA-approved for both complicated and uncomplicated UTIs caused by E. coli via IM administration 1
• Dosing: 1g IM as a single dose for uncomplicated UTIs, or 1-2g IM once daily for complicated UTIs/pyelonephritis 1
• Particularly useful as an initial IV/IM dose when transitioning to oral β-lactams for pyelonephritis, as recommended by the American College of Physicians 3
• Provides targeted coverage while preserving carbapenems for more resistant infections 4
• Major advantage: Once-daily dosing makes it ideal for outpatient parenteral antibiotic therapy (OPAT) 1

Gentamicin (Aminoglycoside)

• FDA-approved for serious UTIs caused by E. coli, including complicated infections 2
• Dosing: 5-7 mg/kg IM once daily (single-dose aminoglycoside regimen) 5, 2
• Single-dose aminoglycoside achieves urinary concentrations 25- to 100-fold higher than plasma levels, with therapeutic levels persisting for days 5
• Microbiologic cure rates of 87-100% for lower UTIs in meta-analysis of 13,804 patients 5
• Particularly effective for simple cystitis due to carbapenem-resistant E. coli (CRE), though evidence is limited for complicated UTIs 5
• Critical caveat: Monitor renal function and avoid in patients with baseline renal impairment due to nephrotoxicity risk 2

Amikacin (Alternative Aminoglycoside)

• Maintains better activity against ESBL-producing E. coli compared to gentamicin 5, 6
• Dosing: 15 mg/kg IM once daily 6
• Demonstrated feasibility as OPAT for non-bacteremic UTIs caused by ESBL-producing E. coli without significant nephrotoxicity or ototoxicity in small case series 6
• Consider when gentamicin resistance is suspected or documented 5

Clinical Decision Algorithm

For Uncomplicated Cystitis Requiring Parenteral Therapy:

• First choice: Gentamicin 5-7 mg/kg IM as single dose 5, 2
• Alternative: Ceftriaxone 1g IM single dose 1

For Pyelonephritis or Complicated UTI (Outpatient Setting):

• First choice: Ceftriaxone 1-2g IM once daily for 7-14 days 3, 4, 1
• Alternative: Gentamicin 5-7 mg/kg IM once daily (if renal function normal and no hearing impairment) 2
• Transition to oral therapy once clinical improvement observed (typically 48-72 hours) 4

For ESBL-Producing E. coli:

• Amikacin 15 mg/kg IM once daily 6
• Ceftriaxone may have reduced efficacy; verify susceptibility 4

For Carbapenem-Resistant E. coli (CRE):

• Single-dose gentamicin or amikacin for simple cystitis only 5
• For complicated UTI/pyelonephritis: IM options insufficient; requires IV therapy with ceftazidime-avibactam, meropenem-vaborbactam, or imipenem-cilastatin-relebactam 5, 3

Critical Pitfalls to Avoid

• Do not use aminoglycosides for complicated UTIs or pyelonephritis as monotherapy without strong evidence of susceptibility - insufficient data supports their use beyond simple cystitis in CRE infections 5
• Avoid gentamicin in elderly patients or those with baseline renal dysfunction - nephrotoxicity risk significantly increases 2
• Do not assume ceftriaxone efficacy for ESBL-producing strains without susceptibility testing - resistance patterns vary geographically 4
• Ensure adequate treatment duration for pyelonephritis (7-14 days total) - premature discontinuation increases recurrence risk 3, 4
• Monitor for clinical response within 48-72 hours; if no improvement, obtain repeat culture and consider resistance 4