A New Mouse Study Complicates a Decade of Polypodium leucotomos Optimism

A study published online September 10, 2026 in the Journal of Investigative Dermatology reports that dietary supplementation with Polypodium leucotomos extract, a fern-derived antioxidant widely marketed as an oral sunscreen adjunct, produced only minimal changes in acute UV-induced edema and erythema in SKH1 hairless mice, and failed to protect the animals from chronic UV-induced skin cancer over the course of long-term irradiation.

The finding, from a large multi-institutional team led by Mehdi Boostani and Gyorgy Paragh, sits awkwardly against a body of earlier animal work that had been read as encouraging. A 1999 pilot study in Photodermatology, Photoimmunology & Photomedicine by Alcaraz and colleagues reported that PL-treated hairless albino mice showed a significant reduction in skinfold thickness after UV exposure and fewer mice with visible tumors eight weeks after UVB dosing stopped. A decade later, a 2010 paper in the American Journal of Pathology, using Xpc+/− hairless mice fed PL extract for ten days before UV exposure, described decreased UV-induced Cox-2 expression and inflammation, enhanced DNA repair, and increased p53 expression and activity relative to vehicle-fed, UV-irradiated controls.

Those two papers, both conducted in hairless mouse models and both funded around commercial interest in PL as a photoprotective supplement, became touchstones cited in a 2015 review in the American Journal of Clinical Dermatology, which stated that oral P. leucotomos extract 'delays skin tumor development and increases epidermal p53 expression and the anti-oxidant status of UV-irradiated' skin. That review attributed the antioxidant activity chiefly to caffeic acid and ferulic acid content in the extract.

What separates the new SKH1 study from its predecessors is duration and endpoint. Where the 1999 and 2010 papers tracked short feeding windows, acute biochemical markers, or tumor counts at a single follow-up point, the 2026 paper tracked chronic UV-induced photocarcinogenesis across an extended timeline in the same hairless mouse strain long used in this literature. That longer window is precisely where the earlier positive results do not appear to hold: the acute inflammatory measures, edema and erythema, moved only slightly with supplementation, and the eventual tumor outcome showed no protective effect at all.

Human trial data referenced in an earlier ScienceDirect clinical study had already hedged its claims, concluding only that PL extract 'can potentially be used as an adjunctive agent to lessen the negative photobiologic effects of UVB,' language considerably more cautious than the tumor-prevention framing found in some rodent papers. A registered clinical trial, NCT02813902, is currently using actinic keratoses as its primary endpoint and skin cancer only as a secondary outcome, reflecting how thin the direct human evidence for cancer prevention still is even as PL supplements remain available over the counter and are frequently recommended as adjuncts to sunscreen.

The 2026 SKH1 findings do not overturn the earlier hairless mouse studies outright, since strain differences, extract sourcing, dosing protocols, and feeding duration all varied across the three decades of work. But the mismatch between short-term biochemical improvements and the absence of any long-term tumor benefit is the kind of gap that a supplement's marketing rarely mentions, and it is exactly the kind of gap that a chronic-exposure mouse model, run out to actual tumor formation, is built to expose.