How an old drug could make a comeback against antibiotic resistance

Five years ago at Goethe University Hospital in Frankfurt, a patient in the intensive care unit was battling with a stubborn urinary tract infection. Despite multiple rounds of antibiotics, nothing seemed to make a difference. Then, as a last resort, doctors turned to an old drug from the 1960s—nitroxoline, typically reserved for uncomplicated urinary tract infections (UTIs). To their surprise, it worked.

Doctor and researcher Stephan Göttig was investigating this drug. “I realised that the potential of nitroxoline is huge towards multidrug-resistant Gram-negative bacteria, so we started studying its spectrum and mechanism”, he explains.

Targeting hospital pathogens

Nitroxoline is only used in a handful of countries today, after the rise of newer antibiotics and lack of clinical research. In Germany, where Göttig and the first author of the paper, Elisabetta Cacace were based (Cacace has now moved to ETH Zurich), it is an approved drug and it is still commonly prescribed for uncomplicated UTIs. However, it is not included in the WHO’s essential medicines list nor is it FDA approved in the US. “New pharmaceutical compounds are difficult to market, so it is useful to repurpose existing antibiotics”, explains Cacace.

Göttig and Cacace investigated more than a thousand isolates of different Gram-negative bacteria that nitroxoline could potentially fight against. Gram-negative bacteria have an outer membrane that can block many antibiotics and even some immune system components. They focused on Enterobacteriaceae, a group of bacteria that includes notorious hospital pathogens such as Klebsiella pneumoniae and Escherichia coli.

Metal imbalance

The team joined forces with the team at ESRF’s ID16A to study nitroxoline’s mode of action using X-ray nanofluorescence. The experiment was challenging due to the fragility of the samples. Peter Cloetens, ESRF scientist in charge of the beamline explains: On ID16A we are specialised in tracking metals at the nano level in cells, but individual bacteria remained challenging due to their size, fragility and the low concentrations involved. The higher brightness and coherence with the Extremely Brilliant Source has proven to be very beneficial for this kind of biomedical studies."

They discovered that nitroxoline disrupts metal balance leading to zinc and copper overload, impairing bacterial enzymes and damaging their outer membrane, which is particularly important in Gram-negative species. The results also showed that nitroxoline can resensitize Klebsiella pneumoniae and Escherichia coli cultures resistant to colistin, an antibiotic with many side effects used only as last-resort after all other drugs fail. This synergy opens the door to combination therapies that could restore the effectiveness of existing antibiotics.

It is important to note that nitroxoline resistance is rare in bacteria. Whilst the team identified resistance mutations, these took a toll on the health of the bacteria, which suggests that widespread resistance to nitroxoline might be harder to evolve than with many modern antibiotics.

“The more we know about the mechanism of these drugs, the better we will understand antibiotic resistance”, explains Göttig. “It is important to communicate on this finding, so that medical organisations all over the world may consider the use of nitroxoline in future cases of antibiotic resistance”, he concludes.

Reference:

Cacace, E., et al. Uncovering nitroxoline activity spectrum, mode of action and resistance across Gram-negative bacteria. Nat Commun 16, 3783 (2025).  https://www.nature.com/articles/s41467-025-58730-5

Text by Montserrat Capellas Espuny

Top image: Nitroxoline action on Acinetobacter baumannii, one of the most critical bacteria in the AMR crisis. Credits: Cacace.