Early observations and pilot data that first suggested a new direction
The global spread of carbapenem-resistant Enterobacterales (CRE), multidrug-resistant Pseudomonas aeruginosa, and carbapenem-resistant Acinetobacter baumannii created a crisis in infectious disease. Colistin (polymyxin E), a toxic antibiotic abandoned in the 1970s, was revived as the last-resort agent for these pathogens, despite nephrotoxicity rates of 20-60% and suboptimal pharmacokinetics. The emergence of plasmid-mediated colistin resistance (mcr-1) in 2015 raised the specter of truly untreatable infections. Mortality rates for CRE bloodstream infections exceeded 40-50% with colistin-based regimens, and the pipeline for new antibiotics was alarmingly thin after decades of pharmaceutical underinvestment in antibacterial development.
Landmark RCTs and pivotal trials that established the evidence base
Ceftazidime-avibactam (Avycaz) was the first novel beta-lactam/beta-lactamase inhibitor combination active against KPC-producing CRE, approved in 2015. Retrospective cohort studies demonstrated dramatically improved outcomes compared to colistin-based regimens, with mortality reductions from 32% to 9% for CRE bloodstream infections. The TANGO I and TANGO II trials established ceftazidime-avibactam's efficacy for complicated UTI and CRE infections respectively. Meropenem-vaborbactam (Vabomere) followed, with the TANGO II trial showing superiority over best available therapy for CRE infections (clinical cure 65.6% vs 33.3%, mortality 15.6% vs 33.3%). These agents transformed CRE from a near-untreatable condition to one with effective targeted therapy.
Follow-up studies, subgroup analyses, and real-world validation
Cefiderocol (Fetroja), a novel siderophore cephalosporin that exploits bacterial iron transport systems to penetrate the outer membrane, was developed specifically for difficult-to-treat resistant gram-negative organisms including CRE, MDR Pseudomonas, and Acinetobacter. The CREDIBLE-CR trial demonstrated efficacy against carbapenem-resistant infections, though with a complex safety signal including higher mortality in Acinetobacter infections. Imipenem-cilastatin-relebactam (Recarbrio) added another option for MDR Pseudomonas. Newer combinations like ceftolozane-tazobactam (Zerbaxa) proved particularly effective for MDR Pseudomonas infections. The pipeline expanded with aztreonam-avibactam targeting metallo-beta-lactamase producers, the last major gap in the armamentarium.
Integration into clinical practice guidelines and recommendations
IDSA published landmark guidance on the treatment of antimicrobial-resistant gram-negative infections in 2023, providing organism-specific and resistance-mechanism-specific treatment algorithms. For KPC-producing CRE: meropenem-vaborbactam or ceftazidime-avibactam preferred over colistin. For MBL-producing CRE: ceftazidime-avibactam plus aztreonam or cefiderocol. For MDR Pseudomonas: ceftolozane-tazobactam, ceftazidime-avibactam, or imipenem-relebactam based on susceptibility. These guidelines fundamentally shifted treatment away from colistin toward targeted beta-lactam combinations, marking the end of the colistin era for most resistant gram-negative infections.
IDSA Guidance on Treatment of Antimicrobial-Resistant Gram-Negative Infections
Resistance-mechanism-specific approach: meropenem-vaborbactam or ceftazidime-avibactam for KPC-CRE; cefiderocol or ceftazidime-avibactam + aztreonam for MBL-CRE; colistin no longer preferred for most indications
ESCMID Guidelines for Treatment of Infections Caused by MDR Gram-Negative Bacilli
New beta-lactam/beta-lactamase inhibitor combinations preferred over polymyxins for carbapenem-resistant infections when susceptibility confirmed
Now
Current standard of care and ongoing research directions
The treatment landscape for MDR gram-negative infections has been transformed from near-hopelessness to having multiple targeted options. The remaining major gap is metallo-beta-lactamase (MBL) producers, for which aztreonam-avibactam received FDA approval and addresses a critical unmet need. Rapid molecular diagnostics that identify resistance mechanisms within hours are enabling faster targeted therapy, replacing days of empiric broad-spectrum coverage. The major challenges now are global access to new antibiotics (most are unavailable in low-income countries where resistance burden is highest), rising resistance to the new agents themselves, and the economic unsustainability of antibiotic development. Novel approaches including bacteriophage therapy, anti-virulence strategies, and microbiome restoration are being explored as adjuncts to conventional antibiotics.
Why is colistin no longer the preferred treatment for CRE?+
New beta-lactam/beta-lactamase inhibitor combinations (ceftazidime-avibactam, meropenem-vaborbactam) have demonstrated dramatically better outcomes than colistin for CRE infections, with mortality reductions from 30-40% to 10-15%. Colistin also has significant nephrotoxicity (20-60%), poor pharmacokinetics, and the emergence of plasmid-mediated colistin resistance (mcr-1) further undermines its utility.
How do I choose between the new agents for CRE?+
The choice depends on the resistance mechanism. For KPC-producing CRE (most common in the US): meropenem-vaborbactam or ceftazidime-avibactam. For MBL-producing CRE (NDM, VIM, IMP): cefiderocol or ceftazidime-avibactam combined with aztreonam (or aztreonam-avibactam). For OXA-48-like producers: ceftazidime-avibactam. Rapid molecular diagnostics identifying the carbapenemase gene are essential for optimal selection.
What is cefiderocol and when should it be used?+
Cefiderocol is a siderophore cephalosporin that uses bacterial iron transport systems to bypass outer membrane barriers. It has the broadest spectrum of any current beta-lactam, covering CRE (including MBL producers), MDR Pseudomonas, and carbapenem-resistant Acinetobacter. It is best reserved for extensively drug-resistant organisms when other targeted options are unavailable, given its broad spectrum and the need for antibiotic stewardship.
