Common Antibiotic Blunts the Power of a Brain Cancer Drug in Cell Studies

A routine antibiotic given to protect vulnerable cancer patients from life-threatening infections may quietly undermine one of the drugs used to fight their tumors. That is the unsettling conclusion of a new laboratory study published in the journal Medical Oncology, in which researchers at Ondokuz Mayıs University in Samsun, Turkey, found that cefazolin—a first-generation cephalosporin administered countless times a day in hospitals worldwide—reduced the killing power of the chemotherapy agent irinotecan in two aggressive cancer cell lines: LN-18 glioblastoma cells and SH-SY5Y neuroblastoma cells. The finding, while strictly preliminary and confined to culture dishes, adds a fresh and clinically consequential dimension to a growing body of evidence that the drugs oncologists prescribe for supportive care can interfere, sometimes invisibly, with the drugs they prescribe to cure.

The stakes could hardly be higher. Brain tumors remain among the most lethal malignancies in medicine, characterized by dismal prognoses and stubborn resistance to conventional therapies, while neuroblastoma—a cancer of the sympathetic nervous system that strikes mainly in early childhood—poses its own formidable treatment challenges. Patients undergoing intensive chemotherapy for these diseases are profoundly immunosuppressed, and bacterial infections during treatment are common, feared, and frequently fatal. Antibiotic prophylaxis and empiric treatment are therefore standard practice, not optional extras. Clinical studies have documented significant rates of bacterial and fungal infection during induction chemotherapy for high-risk neuroblastoma, and febrile neutropenia in solid tumor patients is routinely managed with broad-spectrum cephalosporins like cefazolin’s relatives. If those protective drugs simultaneously erode the efficacy of the anticancer regimen, the trade-off at the bedside becomes far more complicated than anyone assumed.

The Turkish team, led by İlayda Şişli together with Melek Yüce and Esra Albayrak, set out to probe precisely this dual role. Their question was deceptively simple: what happens to cancer cells when a widely used antibiotic and a chemotherapy drug share the same culture medium? Cefazolin, marketed in some countries as Cefamezin, is a beta-lactam antibiotic that works by disrupting bacterial cell wall synthesis—utterly irrelevant machinery for human cells, which have no cell walls. Yet a accumulating literature suggests cephalosporins are not pharmacologically inert toward tumor cells. Some studies have reported that certain cephalosporins can suppress cancer growth, for example by triggering ferroptosis, an iron-dependent form of cell death, in nasopharyngeal carcinoma cells, or by targeting Aurora B kinase in lung cancer models. Others have found the opposite: ceftazidime and cefepime were shown to antagonize the effect of 5-fluorouracil in colon cancer cells. The picture is inconsistent, drug-specific, and cell-type-specific—which is exactly why direct experimental testing matters.

To interrogate the interaction, the researchers used two complementary experimental readouts. The first was the MTT assay, a colorimetric test in which metabolically active cells convert a yellow tetrazolium compound into a purple formazan product; the amount of purple dye serves as a proxy for the number of viable, metabolically competent cells. The second was flow cytometry, a technique that fires cells one by one through a laser and quantifies their DNA content and fluorescent markers, allowing researchers to determine what fraction of the cell population sits in each phase of the cell cycle—G0/G1, S, G2/M—and what fraction has fallen into the sub-G0/G1 debris characteristic of dying cells. Together, these methods capture both the net effect on cell survival and the mechanistic fingerprint of how the drugs act on the cell division machinery.

The results were clear on both fronts. Irinotecan, a topoisomerase I inhibitor that works by trapping the enzyme responsible for relieving torsional stress in DNA during replication, thereby converting an essential enzyme into a DNA-damaging poison, induced significant cytotoxicity in both LN-18 glioblastoma and SH-SY5Y neuroblastoma cells. Flow cytometry showed the expected signature: an increase in the sub-G0/G1 fraction, indicating cells with fragmented DNA heading toward death, alongside arrest in the S phase, where DNA replication stalls, and the G2/M phase, where cells with damaged DNA halt before mitosis. This is textbook topoisomerase poison behavior—the drug prevents cells from copying and dividing their genome cleanly, and the damaged cells accumulate and die.

Then came the twist. When cefazolin was added alongside irinotecan, the chemotherapy’s grip on the cells loosened. In the SH-SY5Y neuroblastoma cells, co-treatment with the antibiotic attenuated both the cytotoxicity and the cell cycle disruption that irinotecan alone had produced; the drug’s ability to push cells into the lethal sub-G0/G1 compartment and to arrest them in S and G2/M was measurably blunted. In the LN-18 glioblastoma cells, the pattern was similar but not identical: cefazolin partially weakened irinotecan’s inhibition of cell viability, while necrosis rates—death by a messier, non-programmed route—were not significantly altered. In both cell lines, the direction of the interaction was the same, and it was the wrong direction for a cancer patient: the antibiotic softened the chemotherapy’s blow.

What might explain this antagonism at the molecular level? The study does not pin down a mechanism, but the authors and the surrounding literature point to several plausible candidates. Antibiotics and chemotherapeutics can compete for cellular transport systems, altering how much drug actually reaches its intracellular target—a phenomenon first documented more than fifty years ago, when researchers showed that certain antibiotics interfered with the cellular transport and antitumor activity of methotrexate in leukemia cells. Antibiotics can also modulate drug efflux pumps such as BCRP/ABCG2, the molecular bouncers that expel chemotherapy agents from cells; indeed, one antibiotic, novobiocin, is famous precisely because it reverses such resistance. More speculatively, cefazolin could be altering mitochondrial function, reactive oxygen species generation, or the metabolic state of the tumor cells in ways that buffer them against topoisomerase-induced stress. Disentangling these possibilities will require the kind of mechanistic follow-up this in vitro study deliberately leaves open.

The broader context makes the finding more than a laboratory curiosity. A substantial clinical literature has already linked antibiotic exposure to worse cancer outcomes, most prominently with immune checkpoint inhibitors, where antibiotic use around the time of treatment has been associated with reduced clinical activity in advanced renal cell and non-small-cell lung cancer—largely attributed to disruption of the gut microbiome. A retrospective cohort study in glioblastoma patients has examined whether antibiotic drug use affects outcome and therapy-related toxicity, and reviews have framed antibiotics as a double-edged sword in oncology: indispensable against infection, potentially harmful against the tumor. Drug–drug interactions in cancer patients treated with oral anticancer agents are common and often overlooked, and fatal adverse drug events remain a sobering reminder that polypharmacy in oncology is a minefield. The new study extends this concern from the microbiome and the immune system down to the level of direct pharmacological interference between a cephalosporin and a topoisomerase inhibitor inside the tumor cell itself.

The researchers are careful about what their data can and cannot say. This was an in vitro study: two immortalized cell lines, culture dishes, and defined drug concentrations. Real glioblastomas and neuroblastomas live behind the blood–brain barrier, embedded in a microenvironment that no monolayer culture reproduces, and the concentrations and timing of cefazolin exposure in a treated patient differ from those in a flask. The authors explicitly note that their findings suggest cefazolin may antagonize the cytotoxic effects of irinotecan in vitro and underscore the need for further validation in in vivo and clinically relevant models. Animal studies and, ultimately, carefully designed clinical pharmacology studies would be needed before anyone could say whether a patient receiving cefazolin during irinotecan-based therapy actually experiences a meaningful reduction in tumor control. It is also worth remembering that irinotecan is not a mainstay of standard glioblastoma care, so the immediate clinical implications for current treatment protocols are limited; the finding speaks more to the principle than to a specific prescription today.

Even so, the message lands with force. Antibiotics are among the most frequently co-administered drugs in oncology, handed out for prophylaxis, for fever during neutropenia, and for intercurrent infections across every tumor type and every age group, from children with neuroblastoma to adults with glioblastoma. If a drug as ordinary as cefazolin can measurably weaken a chemotherapy agent in the dish, then the possibility deserves systematic scrutiny rather than assumption of safety. The study, funded by the Ondokuz Mayıs University Scientific Research Project Office and conducted with cell lines provided by Yeditepe University in Istanbul, is a small piece of evidence—but it points at a large blind spot. As cancer therapy grows ever more complex, the drugs given to protect patients from infection must be evaluated as potential players in the treatment itself, not as pharmacological bystanders. The next experiments, in animal models and eventually in patients, will determine whether this laboratory warning becomes a clinical rule.

Subject of Research: Interaction between the antibiotic cefazolin and the chemotherapeutic drug irinotecan in glioblastoma and neuroblastoma cells

Article Title: Antibiotic-chemotherapy interaction in brain tumors: cefazolin reduces chemotherapeutic efficacy of irinotecan in glioblastoma and neuroblastoma cells

Article References: Şişli, İ., Yüce, M., & Albayrak, E. (2026). Antibiotic-chemotherapy interaction in brain tumors: cefazolin reduces chemotherapeutic efficacy of irinotecan in glioblastoma and neuroblastoma cells. Medical Oncology, 43(11), Article 302. https://doi.org/10.1007/s12032-026-03403-6

Image Credits: AI Generated

DOI: 10.1007/s12032-026-03403-6

Keywords: cefazolin, irinotecan, glioblastoma, neuroblastoma, antibiotics, chemotherapy, drug interactions, cell cycle, flow cytometry, MTT assay, topoisomerase inhibitor, cancer pharmacology