Nail Fold Microbiome Alterations in Patients Treated With Epidermal Growth Factor Receptor Inhibitors

Epidermal growth factor receptor inhibitors (EGFRI), targeted therapies for solid tumours, are associated with various cutaneous adverse effects, including acneiform eruptions, xerosis, mucositis, hair and nail changes, and photosensitivity [1]. Paronychia often arises during early EGFRI treatment due to inflammation, with later stages frequently complicated by Staphylococcus aureus infection [2]. Recent studies have also reported skin dysbiosis in EGFRI-treated patients [3]. However, most microbiome research has focused on facial skin, leaving limited data on the nail fold and other body sites. This study aimed to investigate microbiome alterations at the nail fold, the common site of paronychia, in patients receiving EGFRI. Eighteen patients with colorectal cancer or non-small cell lung cancer receiving EGFRI (erlotinib, gefitinib, afatinib, dacomitinib, or panitumumab) between 2017 and 2019 were recruited. Skin samples were collected from the lateral nail grooves of the right big and middle toes at baseline, months 1 and 6, following standardised protocol [4]. Fifteen patients completed sampling at all three time points, while three completed only two visits due to death (n = 2) or a change in systemic therapy (n = 1). Patient demographics are summarised in Table 1. No patients received systemic antibiotics or corticosteroids within 4 weeks, or topical antibiotics or corticosteroids within 1 week prior to sampling, and all sampled sites remained free of paronychia throughout the study. The 16S rRNA gene sequences in this study were processed following Mothur's standard operating procedures: were aligned against the Greengenes database for taxonomic classification, and deposited in the NCBI open access Sequence Read Archive database (accession number PRJNA1088904). Microbiome profiling of the lateral nail grooves of the right big and middle toes of 18 subjects revealed sufficient high-quality reads and percent sequence coverages (> 99%), and temporal alterations in microbial diversity and composition during EGFRI therapy. Analysis of alpha diversity Chao1 and Shannon demonstrated a general trend toward reduced richness and evenness over the 6-month treatment course, particularly evident between 0 and 6 months samples (Student's t test, p < 0.05) (Figure 1A). Specifically, the Chao1 and Shannon indices declined progressively from baseline to months 1 and 6, indicating a contraction in microbial community complexity. This decline may reflect selective pressures induced by EGFR inhibition, contributing to a less diverse and potentially dysbiotic local environment. At the phylum level, Firmicutes, Actinobacteria, and Proteobacteria predominated across all timepoints, although their relative abundances fluctuated over the treatment period (Figure 1B). Notably, Firmicutes increased largely, coinciding with elevations in Staphylococcus and an unclassified member in the order Bacillales. Meanwhile, the phyla Proteobacteria and Bacteroidetes were reduced, coinciding with reductions in Enhydrobacter and Acinetobacter, for example; and Actinobacteria showed a relatively stable trend (Figure 1B,C). For the species analyses, the baseline microbiota were characterised by commensals such as Cutibacterium acnes, Corynebacterium spp., and coagulase-negative Staphylococcus spp. However, with continued EGFRI treatments, a shift in microbial dominance was clearly observed in several bacterial taxa, that is, the aforementioned increases of Staphylococcus (Figure 1C), where some members in this genus are associated with skin conditions. Community-level comparisons using non-metric multidimensional scaling (NMDS) constructed from Morisita-Horn dissimilarity indices demonstrated high inter-individual variability in microbial profiles. This suggests that EGFRI-induced skin changes may not follow a uniform microbial pattern but rather heterogeneous ecological responses, possibly due to baseline individual skin microbiota, host immunity, and local mechanical or moisture conditions. Nevertheless, longitudinal samples along bacterial species representing analyses suggested certain taxa along some sample clusters. For instance, Staphylococcus epidermidis, unclassified species in Staphylococcus, and unclassified member in order Bacillales were associated with over 6-month EGFRI treatment of many subjects (Figure 1D,E). On the other hand, Prevotella, Ignatzschineria and Enhydrobacter might be associated with baseline samples. To investigate the potential emergence of pathogenic bacteria, the unclassified Staphylococcus species was manually BLASTN against non-redundant NCBI database and was identified as Staphylococcus aureus (BLASTN with E-value = 2e-118). Taken together, the findings indicated that EGFRI therapy affects the nail fold microbiome, reducing microbial diversity and promoting alterations in certain bacterial taxonomic structure over time. This study is limited by a small sample size and the absence of healthy or untreated control subjects, limiting causal inference regarding EGFRI-specific microbiome changes. Potential confounding factors, including systemic conditions and local hygiene practices, may also have influenced the observed microbiome profiles. EGFR inhibition disrupts epidermal barrier integrity and local immune regulation, including reduced antimicrobial peptide production [5, 6], creating a microenvironment prone to microbial imbalance. The resulting reduction in microbial diversity and enrichment of specific taxa may therefore contribute to inflammation and the development of EGFRI-associated paronychia. P.R. conceived and designed the study. T.P. recruited patients and collected clinical data and skin samples. N.S. and L.W. performed microbiome sequencing and bioinformatics processing. P.Ru., N.S. and L.W. analysed and interpreted the data. P.R. and P.Ru. drafted the manuscript. P.R. and N.S. critically revised the manuscript for important intellectual content. All authors reviewed and approved the final manuscript and agree to be accountable for all aspects of the work. This study has been supported by Center of Excellence for Skin and Allergy Research, Chulalongkorn University. This work was supported by the Ratchadapiseksomphot Endowment Fund, Faculty of Medicine, Chulalongkorn University (Grant No. RA61/079). Ethical approval for this prospective cohort study was obtained from the Institutional Review Board of the Faculty of Medicine, Chulalongkorn University (COA number 426/60). All patients included in the study provided informed consent. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.