The role of the microbiome in topical treatment efficacy for non-melanoma skin cancer in a UV-induced non-melanoma skin cancer mouse model

Summary: Non-melanoma skin cancer (NMSC) lesions are effectively treated with surgical therapies; however, patients with multiple lesions benefit from topical therapies that provide a field treatment effect. Topical therapies, such as 5-fluorouracil (5-FU) and imiquimod (IMQ), have significant inflammatory side effects that result in poor treatment adherence and suboptimal clinical outcomes. We showed topical application of N-phosphonacetyl-L-aspartate (PALA) reduced tumor numbers and area of tumor tissue, without inducing inflammation in a clinically relevant, murine UVB-induced NMSC model. Head-to-head comparisons of PALA to 5-FU and IMQ in this model demonstrate that local skin inflammation is not required for an effective anti-tumor response. Skin microbiome profiling showed distinct treatment-dependent shifts in microbiome diversity and composition. Of note was a substantial increase in the <i>Cutibacterium: Staphylococcus</i> genus ratio and large decreases in the prevalence of pro-inflammatory genera uniquely on PALA treated mice indicating a less inflammatory local microbiome. Reduction of the pro-inflammatory microbiome of IMQ treated mice by topical antiseptic treatment improved therapeutic outcomes, while perturbation of the anti-inflammatory microbiome of PALA treated mice resulted in larger tumors. <i>In vitro</i> antimicrobial testing demonstrated that 5-FU ointment was toxic to many skin commensals, while IMQ and PALA topicals had minimal direct antimicrobial activity. These results indicate that topical NMSC therapies modify the skin microbiome by different mechanisms and microbial modulation impacts therapeutic efficacy. This study provides a foundation for future research into the cross-talk of immune responses and the skin microbiome to improve treatment of skin cancers. Kala Mahen¹, Malia Valder², Isabel Johnston³, Alec Minikowski³, Naseer Sangwan⁴, Edward Maytin⁵, George Stark¹, Christine McDonald³ 1. Cancer Biology, Cleveland Clinic, Cleveland, OH, United States. 2. CCLCM, Case Western Reserve University, Cleveland, OH, United States. 3. Inflammation & Immunity, Cleveland Clinic, Cleveland, OH, United States. 4. CVMS, Cleveland Clinic, Cleveland, OH, United States. 5. Biomedical Engineering, Cleveland Clinic, Cleveland, OH, United States. UV Biology/Injury and Non-melanoma Cancers