Secondary lymphedema typically develops after cancer surgery, particularly when lymph nodes are removed or irradiated as part of treatment for breast cancer, melanoma, or gynecological malignancies. Yet clinicians have long observed something puzzling: the condition often appears months or even years after the operation, long after the surgical wound has healed. This delayed onset has led researchers to suspect that the surgery alone cannot explain the full picture, and that cancer treatments themselves, particularly chemotherapy, may quietly undermine the lymphatic vessels that drain fluid from tissues. When those vessels fail, protein-rich fluid accumulates in the limbs, causing swelling, thickening of the skin, recurrent infections, and a lasting reduction in quality of life.

The Toulouse team, led by Barbara Garmy-Susini, focused on 5-fluorouracil, or 5-FU, a first-line chemotherapeutic used against colorectal, breast, and many other cancers. In laboratory experiments, they found that 5-FU inflicts measurable damage on lymphatic endothelial cells, the specialized cells that line the inside of lymphatic vessels and coordinate their growth, maintenance, and pumping function. The drug disrupted the expression of master transcription factors that keep lymphatic vessels healthy, including GATA2, FOXC2, and SOX18, a trio of genetic regulators that act as the operational blueprint of the lymphatic system. When these genes are silenced or diminished, lymphatic vessels lose their structural integrity and their capacity to transport fluid, setting the stage for lymphedema.

The mechanism behind this damage turned out to be mitochondrial. 5-FU impaired the power plants of lymphatic endothelial cells, reducing their maximal respiration, depleting their ATP production, and eroding their spare respiratory capacity, which is the reserve a cell can call upon under stress. At the same time, mitochondria in the treated cells accumulated superoxide, a highly reactive and destructive oxygen radical. The result was a double blow: less energy to power the vessel’s contractile and barrier functions, and more oxidative stress to corrode cellular components. The researchers also observed an increase in mitochondrial mass, a compensatory response that in this context appears maladaptive, adding more dysfunctional machinery rather than repairing the existing network.

Enter metformin. Although best known as a first-line treatment for type 2 diabetes, metformin has attracted intense scientific interest in recent years for its effects on metabolism, aging, and cancer. At the molecular level, it acts as an inhibitor of complex I of the mitochondrial electron transport chain, subtly modulating how cells generate energy. That same property, the researchers discovered, is precisely what makes it protective for lymphatic vessels under chemotherapy attack. When lymphatic endothelial cells were treated with metformin alongside 5-FU, the mitochondrial damage was largely prevented: maximal respiration, ATP production, and spare respiratory capacity were restored, superoxide accumulation was blocked, and the pathological expansion of mitochondrial mass was curtailed.

The protective effect extended beyond cellular energetics to the genetic identity of the vessels themselves. Metformin restored the expression of GATA2, FOXC2, and SOX18 in cells exposed to 5-FU, effectively preserving the molecular program that defines functional lymphatic endothelium. Notably, the team found that the antioxidant effect of metformin depends on the GATA2 pathway, linking the drug’s metabolic action to the transcriptional machinery that safeguards lymphatic vessel integrity. This mechanistic connection is significant because it suggests that metformin is not merely mopping up free radicals generically, but is reinforcing a master regulatory circuit that lymphatic cells rely on to survive metabolic stress.

To test whether these cellular findings translated into living organisms, the researchers turned to mouse models. When animals received 5-FU, their lymphatic vessels collapsed, a structural failure that would be expected to impair drainage. In mice that also received metformin, this vessel collapse was not observed. The team then used a murine model of lymphedema to ask the more clinically relevant question: could metformin treat established chemotherapy-associated lymphatic damage? The answer was encouraging. Metformin reduced the limb swelling caused by 5-FU treatment, markedly decreased the reflux of lymph back into tissues, lessened the abnormal thickening of the skin, and corrected the disorganized proliferation of lymphatic vessels that characterizes the disease.

These results carry considerable weight because metformin brings an unusually favorable profile to the table. It has been used in clinical practice for decades, has a well-characterized safety record, is inexpensive, and is already taken by a large share of cancer patients who have coexisting diabetes or insulin resistance. A preventive or therapeutic strategy for lymphedema that could be delivered with an existing, orally available drug would represent a major departure from current care, which relies mainly on compression garments, manual lymphatic drainage, and, in select cases, surgical procedures such as lymphovenous anastomosis or vascularized lymph node transfer. None of these approaches reverse the underlying cellular dysfunction; they manage symptoms or reroute fluid around damaged vessels.

The study also reframes how the field thinks about lymphedema risk. If chemotherapy agents such as 5-FU can compromise lymphatic mitochondria and transcriptional programs, then the timing of interventions could shift. Rather than waiting for swelling to appear, oncologists might one day identify patients at risk and protect their lymphatic system during treatment itself. The work was supported by the European Union’s Horizon 2020 programme through the Theralymph project and by the French National Cancer Institute, reflecting a coordinated effort to bring lymphatic protection into mainstream cancer care. The researchers caution, as with any preclinical study, that mouse models do not perfectly recapitulate human disease, and clinical trials in cancer patients will be needed to establish the right dosing, timing, and patient selection before metformin can be recommended for lymphedema prevention.

Even so, the convergence of evidence is striking. A drug that inhibits complex I, restores mitochondrial respiration, suppresses superoxide, preserves GATA2-dependent gene expression, prevents vessel collapse, and reduces limb volume in vivo offers a coherent mechanistic story from molecule to organ. For the millions of cancer survivors who live with or fear lymphedema, the prospect that a decades-old diabetes pill could protect the vessels that surgery and chemotherapy put at risk is a compelling one. The Toulouse findings now provide the biological rationale to test that prospect in the clinic, and they underscore a broader lesson of modern pharmacology: some of the most valuable new applications may come from old drugs whose full range of actions we are only beginning to understand.

Subject of Research: Metformin's reversal of chemotherapy-induced lymphatic mitochondrial dysfunction and secondary lymphedema

Article Title: Metformin reverses lymphatic mitochondrial dysfunction induced by 5-FU chemotherapy

Article References: Loi, H., Draia-Nicolau, T., Balzan, E., Morfoisse, F., Lacazette, E., Prats, A.-C., & Garmy-Susini, B. (2026). Metformin reverses lymphatic mitochondrial dysfunction induced by 5-FU chemotherapy. Cell Death Discovery. https://doi.org/10.1038/s41420-026-03403-5

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03403-5

Keywords: metformin, lymphedema, 5-fluorouracil, chemotherapy, lymphatic endothelial cells, mitochondria, GATA2, superoxide, lymphatic vessels, drug repurposing, cancer survivors, Cell Death Discovery