The differential effects of dupilumab and Janus kinase 1-selective inhibitors on the skin microbiome in adult patients with moderate-to-severe atopic dermatitis using metagenomic sequencing: results from the BioDay registry

The skin is more than just a protective covering; it is a bustling ecosystem teeming with microscopic life. In a healthy state, this community of bacteria and fungi thrives in a balanced diversity, acting as a natural defense system that keeps harmful invaders at bay. However, in people with atopic dermatitis, a chronic and often itchy inflammatory skin condition, this balance is disrupted. The skin becomes a less diverse environment where a specific type of bacteria, Staphylococcus aureus, often takes over, crowding out the beneficial microbes and worsening the inflammation. For decades, doctors have treated this condition with medications that calm the immune system, but a new generation of drugs has arrived that works in different ways. One type, known as dupilumab, acts like a precise switch, blocking specific signals that drive the inflammation. Another type, called Janus kinase 1-selective inhibitors, works by turning down a broader range of immune signals. While both treatments are effective at clearing the visible rash and reducing itching, scientists have long wondered if they also restore the skin's microscopic ecosystem to a healthy state, or if the underlying imbalance remains hidden beneath the surface.

To answer this question, researchers at the University Medical Center Utrecht in the Netherlands set out to compare how these two distinct treatments reshape the skin's microbiome. They followed two groups of adults with moderate-to-severe atopic dermatitis. One group of ten patients received dupilumab, while a second group of fifteen patients received a Janus kinase inhibitor. For comparison, they also collected samples from seventeen healthy individuals who did not have the skin condition. The team took swabs from the patients' active, red patches of skin and from their facial skin, which served as a sample of non-affected areas. They collected these samples at the start of treatment and again after several weeks, using a powerful technique called metagenomic sequencing. This method allowed them to read the genetic material of every microbe present, providing a complete census of the bacterial and fungal communities rather than just looking for a few known species.

The results revealed a clear difference in how the two drugs affected the skin's inner world. In the patients treated with dupilumab, the skin microbiome underwent a significant transformation. After sixteen weeks of treatment, the abundance of the harmful Staphylococcus aureus bacteria dropped dramatically, nearly disappearing from the affected skin. At the same time, the overall diversity of the microbial community increased, shifting the ecosystem to look much more like the diverse, balanced communities found on the skin of healthy people. This change happened in both the inflamed patches and the facial skin, suggesting that the drug helped restore the skin's natural balance alongside its ability to clear the rash.

In contrast, the patients treated with the Janus kinase inhibitor showed a different story. While their skin cleared up just as well as the dupilumab group, with similar improvements in redness and severity scores, their skin microbiome did not change in the same way. Although there was a slight reduction in the harmful bacteria, it was not as complete as with the other treatment, and the overall diversity of the microbial community did not shift toward the healthy state. Even after twenty-eight weeks of treatment, the skin of these patients still looked different from that of healthy individuals, retaining a microbial profile that was distinct from the healthy baseline. This suggests that while the drug successfully calmed the immune response enough to heal the visible skin, it did not fully reset the underlying microbial ecosystem.

The researchers also looked closely at fungi, specifically a type called Malassezia, which has been linked to a side effect known as head and neck dermatitis that some patients experience with dupilumab. They found that while the abundance of this fungus increased slightly during dupilumab treatment, it remained far lower than in healthy people, and none of the patients in the study developed the specific side effect. Similarly, another fungus often associated with acne did not show a significant increase in the patients taking the Janus kinase inhibitor, even though acne-like eruptions are a known side effect of that drug. This indicates that the specific mechanisms behind these side effects are complex and may not be solely driven by the simple overgrowth of these microbes.

Ultimately, the study highlights that two drugs can produce the same visible result—clear skin—while working through different biological pathways. Dupilumab appears to do more than just suppress inflammation; it actively helps the skin's microbial community recover its natural diversity and push out harmful bacteria. The Janus kinase inhibitor, while effective at treating the symptoms, seems to leave the skin's microbial landscape largely unchanged. This distinction is important because it suggests that the health of the skin is not just about the absence of a rash, but also about the restoration of the invisible community of life that lives on it. The findings do not declare one treatment superior to the other for clearing the skin, but they do offer a deeper understanding of how these powerful medicines interact with the human body's most immediate environment.