Drug Repurposing for Treatment of Recessive Dystrophic Epidermolysis Bullosa Cutaneous Squamous Cell Carcinoma

Abstract Background Recessive dystrophic epidermolysis bullosa (RDEB) is an inherited skin disorder that leads to severe wounding and blistering of the skin. Patients with RDEB develop aggressive cutaneous squamous cell carcinoma (cSCC) with a mortality rate of approximately 90% by age 55 years. The current primary treatment is wide excision of tumours, often leading to amputations. Identifying new therapeutic avenues for RDEB cSCC remains a priority. Here, we approach this in two ways. Firstly, we identified a SMAD4-dependent sphingosine pathway response to endogenous transforming growth factor (TGF)-β, essential for RDEB cSCC proliferation which could be targeted as a potential therapeutic strategy. Secondly, we utilized a US Food and Drug Administration (FDA) screen of > 3000 drugs to identify potential druggable pathways that could be used to treat RDEB cSCC. Methods RDEB cSCC cell lines were used to test Siponimod, a sphingosine-1 receptor modulator, on proliferation, clonogenicity and invasion in vitro. Siponimod was used in vivo to test tumour growth effect of targeting the sphingosine pathway. We carried out an FDA-approved drug screen on four RDEB cSCC cell lines using the Opera Phenix high-throughput microscope to identify targets. Results Treatment of RDEB cSCC cell lines with Siponimod showed compromised proliferation compared with untreated cells in addition to significant reduction in colony formation. There was a significant decrease in the invasive capability of RDEB SCC cells. Treatment of normal human keratinocytes showed no effect on proliferation. Our FDA-approved screen identified multiple drugs that target MEK, mechanistic target of rapamycin complex, histone deacetylase, signal transducer and activator of transcription, and topoisomerase I/II in the top 50 hits. The most common target was DNA damage/repair pathways, but cytoskeletal signalling, transmembrane transporters and proteases were identified as other potential avenues for treatment of RDEB cSCC. Conclusions There is an unmet need for therapeutic options in RDEB cSCC. Here, we have demonstrated the potential of repurposing previously licensed drugs to identify treatment avenues for RDEB cSCC.