Association of localized high-risk prostate cancer (PC) and an androgen receptor low subpopulation susceptible to HER2 inhibition.

401 Background: Patients diagnosed with localized high-risk PC have higher rates of recurrence, and neoadjuvant intensive hormonal therapies seek to treat occult micrometastatic disease by addition to definitive treatment. We completed a phase 2 trial of neoadjuvant androgen deprivation therapy (ADT) plus the androgen receptor (AR) inhibitor enzalutamide (enza) for 6 months prior to radical prostatectomy (RP), in which 22 out of 37 patients exhibited poor pathologic responses (NCT02430480). In the current study, we aimed to identify targetable mechanisms of poor response evident in baseline biopsies that correlated with pathologic response. Methods: RNA-seq was performed on 147 laser microdissected foci from 99 biopsy blocks acquired prior to the initiation of neoadjuvant therapy. Pathologic response was measured directly from posttreatment RP H&E slides. Differential expression was determined using linear mixed-effects models with histologic features and patients as random effects and residual cancer volume as a fixed effect. Differentially enriched pathways were identified using Ingenuity Pathway Analysis. Protein and phosphoprotein levels were measured in tissues using immunohistochemistry and analyzed using machine learning algorithms. Sensitivity and differential responses in cell lines and patient-derived organoids were measured using dose responsive curves, RNA-seq, single-cell sequencing and flow cytometry. Results: Gene signatures of higher AR activity were associated with exceptional responses (defined as less than 0.05 cm 3 of residual tumor in the RP), which was inversely associated with HER2 activity. High HER2 activity was associated with poorer outcomes. The inverse relationship between AR and HER2 activity was validated in three additional cohorts by RNA-seq and multiplex immunofluorescence. Tumors demonstrating resistance to ADT+enza expressed higher levels of HER2 and pHER2 protein at baseline. PC cell lines (including LNCaP and LAPC-4) treated with low doses of HER2 inhibitors (afatinib and neratinib) selected for populations with low AR activity as measured by RNA-seq, immunoblotting and flow cytometry. Combining enza+neratinib increased cell death, but patient-derived PC organoids exposed to 3-5 μM neratinib alone resulted in 90% cell death after 48h. Using single-cell sequencing and AR/HER2/PSA multiplex immunofluorescence, we found that most high-grade hormone-sensitive prostate tumors harbor a HER2-high subpopulation of cells with lower levels of AR activity that are resistant to AR inhibitors but sensitive to HER2 inhibition. Conclusions: Nascent human prostate tumors adopt an AR activity-low state prior to antiandrogen exposure that can be exploited by treatment with HER2 inhibitors, including novel combinations in the neoadjuvant or adjuvant setting. Future NCI clinical trials will evaluate such combinations. Clinical trial information: NCT02430480 .