Drug Repurposing for Antimicrobial Resistance: Turning Old Drugs Into New Weapons
The numbers hit hard. In 2019, bacterial antimicrobial resistance directly killed 1.27 million people. That’s more than HIV or malaria. And it contributed to nearly 5 million deaths overall. By 2050, if we keep going the way we are, we could hit 10 million deaths a year from resistant infections. The WHO has been shouting from the rooftops for years, but the pipeline for new antibiotics is still dangerously thin.
There’s a reason for that, and it’s not just about the science. Developing a new antibiotic costs somewhere north of $1.6 billion and takes 10 to 17 years. And here’s the kicker: antibiotics are prescribed in short courses and held back as last-line treatments, so the return on investment is terrible compared to drugs for chronic conditions. Big pharma walked away from antibiotic R&D decades ago. We’ve been running on fumes.
But here’s what I find interesting: we already have thousands of drugs approved for other conditions. Their safety profiles are known. Their manufacturing processes exist. Their pharmacology is documented. What if some of them accidentally kill bacteria too? That’s the idea behind drug repurposing, and honestly, it might be one of the most practical strategies we have while waiting for the antibiotic pipeline to catch up.
What drug repurposing actually means
Drug repurposing — also called drug repositioning — means taking existing, approved drugs and using them for something completely different. This isn’t some wild new idea. Sildenafil was supposed to be an angina drug before it found its, uh, more famous niche. Thalidomide had a tragic history before it was repurposed for multiple myeloma. But applying this approach to antimicrobial resistance is relatively new, and it’s picking up real momentum.
The reason is obvious if you think about the numbers. A repurposed drug has already cleared Phase I safety trials. Doctors know its pharmacokinetics, toxicity, and dosing. The manufacturing line already exists. Instead of 10 to 17 years, you might get there in 3 to 12. Instead of 1.6billion,maybe300 million. Not pocket change, but a fraction of the standard cost, and the risk of late-stage failure drops way down.
Three ways repurposed drugs fight resistance
Repurposed drugs don’t all work the same way. Broadly, they fall into three buckets.
Some work as straight-up antimicrobials. They kill bacteria directly, even though that’s not what they were designed for. Some work as adjuvants — they don’t kill bacteria themselves, but they make existing antibiotics effective again by blocking things like efflux pumps or biofilms. And some work through host-directed therapy, which means they strengthen the patient’s immune system rather than going after the bacteria directly.
All three have shown real promise. And we’re starting to see actual data, not just lab bench optimism.
Cancer drugs vs. bacteria
This one surprised me. Cancer drugs and bacteria share more traits than you’d expect. Both involve rapidly dividing cells. Both develop resistance through efflux pumps, drug modifications, and metabolic workarounds. Both lean on similar survival pathways. So it turns out some anticancer agents also work against bacteria.
Cisplatin, the chemotherapy drug that crosslinks DNA, shows activity against Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. Etoposide kills Pseudomonas by inducing oxidative stress. 5-Fluorouracil messes with virulence factors and biofilm production in Staphylococcus. Gemcitabine disrupts bacterial cell walls in MRSA. Gallium nitrate interferes with iron transport in Mycobacterium tuberculosis.
The list goes on. But here’s the catch. Cancer drugs are designed to kill fast-dividing cells, which includes immune cells. Using them as antimicrobials means walking a tightrope — you don’t want to knock out the patient’s defenses while fighting the infection. So far, it’s a balancing act, not a done deal.
Antidepressants and antipsychotics as resistance breakers
I’ll be honest, this one blindsided me. Antidepressants like paroxetine, sertraline, and fluoxetine — the SSRIs that millions of people take daily — have been shown to block bacterial efflux pumps. Those are the little molecular machines bacteria use to spit antibiotics out before they can work.
Paroxetine blocks the NorA efflux pump in Staphylococcus aureus. Sertraline hits E. coli and Pseudomonas. Fluoxetine breaks down biofilm formation in Proteus mirabilis. Thioridazine, an antipsychotic, reduces efflux pump activity in Staphylococcus.
The problem? The concentrations needed for antimicrobial effects are way higher than what people take for depression or psychosis. That raises some serious toxicity questions. And long-term use of these drugs can mess with the gut microbiome in ways we’re still figuring out. So the concept is exciting, but the clinical path is complicated.
Antibiotic adjuvants: making old antibiotics work again
This might be the most immediately useful approach. An adjuvant doesn’t kill bacteria by itself, but it makes existing antibiotics more effective. Think of it as unlocking a door that bacteria have bolted from the inside.
The textbook example is the beta-lactamase inhibitor. Bacteria make beta-lactamase enzymes that chop up penicillin-like antibiotics. Pair amoxicillin with clavulanic acid — a beta-lactamase inhibitor — and suddenly the antibiotic works again. That combination, Augmentin, has been around for decades. Newer inhibitors like avibactam and vaborbactam are pushing the same strategy against even more resistant strains.
Efflux pump inhibitors work differently. Instead of protecting the antibiotic, they disable the bacteria’s ejection seat. PAßN (phenylalanine-arginine-beta-naphthylamide) is one example. Pair it with levofloxacin, and it works against Pseudomonas aeruginosa. CCCP, another inhibitor, goes with erythromycin.
Biofilm disrupters tackle a whole different headache. Bacteria in biofilms wrap themselves in a protective goo that antibiotics can’t easily penetrate. Agents like N-acetyl cysteine, DNase I, and chitosan break that goo down, letting the antibiotics through.
Host-directed therapies: a different way of thinking
What if, instead of attacking the pathogen, you made the host better at fighting it off? That’s the idea behind host-directed therapy. Antimicrobial peptides, vitamin D, phenylbutyrate, and similar immune modulators can boost the body’s natural defenses. This approach matters especially for tuberculosis, where the pathogen hides inside immune cells and antibiotics struggle to reach it.
The big advantage: pathogens are less likely to evolve resistance to something that targets the host rather than the microbe. The downside: these therapies tend to be less immediately potent than direct antimicrobials, and clinical trials are harder to design.
The science behind the strategy
Modern drug repurposing isn’t guesswork. It’s driven by data. High-throughput screening lets researchers test thousands of compounds against bacterial targets in a matter of days. Computational methods — molecular docking, QSAR models, machine learning — have sped things up even more.
Structure-based drug design starts with a 3D model of a bacterial protein. Researchers dock virtual compounds into its active site, rank them by binding energy, and only synthesize the best candidates. Machine learning models can predict drug-target interactions, match drugs to completely different diseases, and flag candidates that humans would probably miss.
During COVID, these tools were put through the wringer at record speed. AI models screened millions of compounds in silico before any wet lab work began. Now the same tools are pointed at AMR, and they’ve already delivered. A deep learning model trained on 39,000 compounds identified halicin — a drug nobody had ever thought of as antibacterial — as a potent antimicrobial against Acinetobacter baumannii and Clostridioides difficile.
The elephant in the room: money and patents
If repurposing is so promising, why aren’t we doing more? The answer, as always, is economics.
Most repurposable drugs are off-patent. That’s great for patients but terrible for investors. A company that drops $100 million on clinical trials for a repurposed indication can’t recoup that through high prices because generics will undercut them. “New use” patents are much weaker than “new molecule” patents.
This is a structural problem. The market incentives that power innovation in chronic disease drugs don’t apply to antimicrobials. Some governments have noticed and started funding repurposing research, offering milestone grants, and exploring subscription-style payment models — the “Netflix model” for antibiotics. But these programs are early stage.
Where we go from here
Drug repurposing isn’t the single answer to AMR. We still need novel antibiotics, better diagnostics, stricter infection control, and more responsible use of what we have. But repurposing can fill critical gaps, especially for the nastiest pathogens where treatment options have basically disappeared.
What makes it worth paying attention to is the speed. For patients with infections that shrug off every last-resort antibiotic, a repurposed drug that’s already been tested in humans can reach the bedside in years, not decades. This is already happening — colistin, fosfomycin, and minocycline are older drugs being redeployed against multidrug-resistant bugs right now.
The road ahead needs coordination. Researchers need access to drug libraries and clinical data. Regulators need streamlined pathways for repurposed indications. And governments need to build incentive structures that make repurposing financially viable, not just scientifically interesting.
We’re not going to discover our way out of the AMR crisis. We’re going to have to work smarter, move faster, and get more creative with what we already have. Drug repurposing is one of the better ideas on the table. It won’t save us by itself, but it might buy us the time we need.
