Response to mTOR and PI3K inhibitors in enzalutamide-resistant luminal androgen receptor triple-negative breast cancer patient-derived xenografts

Abstract

Luminal androgen receptor (LAR) breast cancer accounts for 10% of all triple-negative breast cancers (TNBC). Anti-androgen therapy for this subtype is in development, but yields only partial clinical benefits. In this study, we aimed to characterize the genomic alterations of LAR TNBC, to analyze activation of the PI3K signaling pathway and to compare the response to PI3K pathway inhibitors with that to anti-androgen therapy in patient-derived xenografts (PDX) of LAR TNBC. <b>Methods</b>: Four LAR PDX models were identified, on the basis of their transcriptomic profiles, in a cohort of 57 PDX models of TNBC. The expression of <i>AR</i>-related genes, basal and luminal cytokeratins and EMT genes was analyzed by RT-PCR and IHC. <i>AKT1</i> and <i>PIK3CA</i> mutations were identified by targeted NGS, and activation of the PI3K pathway was analyzed with a reverse-phase protein array. Three LAR PDXs with a <i>PIK3CA</i> or <i>AKT1</i> mutation were treated with the AR inhibitor enzalutamide, a PI3K inhibitor, a <i>dual</i> PI3K-mTOR inhibitor and a mTORC1-mTORC2 inhibitor. Finally, we screened a clinical cohort of 329 TNBC for <i>PIK3CA</i> and <i>AKT1</i> hotspot mutations. <b>Results</b>: LAR TNBC PDXs were significantly enriched in <i>PIK3CA</i> and <i>AKT1</i> mutations, and had higher levels of luminal-androgen-like gene expression and a higher PI3K pathway protein activation score than other TNBC subtypes. Immunohistochemistry analysis revealed strong expression of the luminal cytokeratin CK18 and AR in three LAR PDX models. We found that mTOR and PI3K inhibitors had marked antitumor activity <i>in vivo</i> in PDX harboring genomic alterations of <i>PIK3CA</i> and <i>AKT1</i> genes that did not respond to the AR antagonist enzalutamide. <i>PIK3CA</i> mutations were detected in more than one third of AR+ TNBC from patients (38%), and only 10% of AR-negative TNBC. <b>Conclusion</b>: Our results for PDX models of LAR TNBC resistant to enzalutamide indicate that <i>PIK3CA</i> and <i>AKT1</i> are potential therapeutic targets.