Summary Advanced basal cell carcinomas (BCCs) frequently acquire resistance to Smoothened (SMO) inhibitors through unknown mechanisms. Here, we identify SMO mutations in 50% (22/44) of resistant BCCs and show that these mutations maintain Hedgehog signaling in the presence of SMO inhibitors. Alterations include four ligand binding pocket mutations defining sites of inhibitor binding and four variants confering constitutive activity and inhibitor resistance, illuminating pivotal residues that ensure receptor autoinhibition. In the presence of a SMO inhibitor, tumor cells containing either class of SMO mutants effectively outcompete cells containing the wild type SMO. Finally, we show that both classes of SMO variants respond to aPKC-ι/λ or GLI2 inhibitors that operate downstream of SMO, setting the stage for the clinical use of GLI antagonists.
SUMMARY In a screen of drugs previously tested in humans we identified itraconazole, a systemic antifungal, as a potent antagonist of the Hedgehog (Hh) signaling pathway that acts by a mechanism distinct from its inhibitory effect on fungal sterol biosynthesis. Systemically administered itraconazole, like other Hh pathway antagonists, can suppress Hh pathway activity and the growth of medulloblastoma in a mouse allograft model and does so at serum levels comparable to those in patients undergoing antifungal therapy. Mechanistically, itraconazole appears to act on the essential Hh pathway component Smoothened (Smo) by a mechanism distinct from that of cyclopamine and other known Smo antagonists, and prevents the ciliary accumulation of Smo normally caused by Hh stimulation.
BACKGROUND Dysregulated hedgehog signaling is the pivotal molecular abnormality underlying basal-cell carcinomas. Vismodegib is a new orally administered hedgehog-pathway inhibitor that produces objective responses in locally advanced and metastatic basal-cell carcinomas. METHODS We tested the anti–basal-cell carcinoma efficacy of vismodegib in a randomized, double-blind, placebo-controlled trial in patients with the basal-cell nevus syndrome at three clinical centers from September 2009 through January 2011. The primary end point was reduction in the incidence of new basal-cell carcinomas that were eligible for surgical resection (surgically eligible) with vismodegib versus placebo after 3 months; secondary end points included reduction in the size of existing basal-cell carcinomas. RESULTS In 41 patients followed for a mean of 8 months (range, 1 to 15) after enrollment, the per-patient rate of new surgically eligible basal-cell carcinomas was lower with vismodegib than with placebo (2 vs. 29 cases per group per year, P<0.001), as was the size (percent change from baseline in the sum of the longest diameter) of existing clinically significant basal-cell carcinomas (−65% vs. −11%, P = 0.003). In some patients, all basal-cell carcinomas clinically regressed. No tumors progressed during treatment with vismodegib. Patients receiving vismodegib routinely had grade 1 or 2 adverse events of loss of taste, muscle cramps, hair loss, and weight loss. Overall, 54% of patients (14 of 26) receiving vismodegib discontinued drug treatment owing to adverse events. At 1 month, vismodegib use had reduced the hedgehog target-gene expression by basal-cell carcinoma by 90% (P<0.001) and diminished tumor-cell proliferation, but apoptosis was not affected. No residual basal-cell carcinoma was detectable in 83% of biopsy samples taken from sites of clinically regressed basal-cell carcinomas. CONCLUSIONS Vismodegib reduces the basal-cell carcinoma tumor burden and blocks growth of new basal-cell carcinomas in patients with the basal-cell nevus syndrome. The adverse events associated with treatment led to discontinuation in over half of treated patients. (Funded by Genentech and others; ClinicalTrials.gov number, NCT00957229.)
SUMMARY Smoothened (SMO) inhibitors are under clinical investigation for the treatment of several cancers. Vismodegib is approved for the treatment of locally advanced and metastatic basal cell carcinoma (BCC). Most BCC patients experience significant clinical benefit on vismodegib, however, some develop resistance. Genomic analysis of tumor biopsies revealed vismodegib resistance is associated with Hedgehog (Hh) pathway reactivation, predominantly through mutation of the drug target SMO and to a lesser extent through concurrent copy number changes in SUFU and GLI2. SMO mutations either directly impaired drug binding or activated SMO to varying levels. Furthermore, we found evidence for intra-tumor heterogeneity, suggesting that a combination of therapies targeting components at multiple levels of the Hh pathway is required to overcome resistance.
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