Resistance to SMO Inhibitors in Advanced Basal Cell Carcinoma: A Case Highlighting the Role of Molecular Tumor Profiling

Basal cell carcinoma (BCC) is the most common skin cancer, predominantly affecting sun-exposed areas. It typically grows slowly and rarely metastasizes, though untreated cases can cause significant tissue destruction and morbidity. Its pathogenesis primarily involves dysregulation of the Hedgehog (HH) signaling pathway, mainly through mutations in <i>PTCH1</i> or <i>SMO</i> genes, leading to chronic activation of downstream GLI transcription factors. Accordingly, current targeted therapies for locally advanced, unresectable, or metastatic BCC focus on SMO inhibition, using orally administered drugs such as vismodegib and sonidegib. Although these therapies have shown success, many patients develop resistance, with about 50% harboring mutated SMO. In numerous cases, genetic determinants (sometimes pre-existing) of resistance remain unidentified, complicating patient management. Here, we report a case of a 58-year-old female with advanced BCC who initially exhibited a favorable response to sonidegib but developed resistance after approximately one year. This resistance was not attributable to the acquired mutations in <i>SMO</i> but rather to intra-tumor heterogeneity and additional mutations in critical driver genes, including <i>TP53</i>, <i>APC</i>, <i>FGFR1</i> and <i>NOTCH1</i>, which likely enable HH pathway inhibition. To our knowledge, this is the first report documenting a sonidegib resistance mechanism in BCC that is independent of HH pathway mutations. This case highlights the complexity of resistance mechanisms to HH inhibitors and underscores the critical need for comprehensive molecular tumor profiling prior to initiating targeted therapy.