433 Background: After cisplatin-based neoadjuvant chemotherapy (NAC) almost two thirds of patients have residual muscle-invasive bladder cancer (MIBC) present at radical cystectomy (RC). The alterations induced by NAC in these cisplatin-resistant tumors remain largely unstudied. Here, we aim to investigate the characteristics of cisplatin-resistant tumors. Methods: RC samples were available for gene expression analysis from 133 patients with residual invasive disease after cisplatin-based NAC, of whom 116 had matched pre-NAC samples. In addition, the tumor bed (scar tissue) of 21 post-NAC RC specimens with no residual tumor was profiled. Unsupervised consensus clustering (CC) was performed and the CC were investigated for their biological and clinical characteristics. H&E and immunohistochemistry (KRT5/6, GATA3, KI67 and CD8) were used to confirm tissue sampling and gene expression analysis. Results: Unsupervised consensus clustering yielded four distinct consensus clusters (CC). Consistent basal-(CC1) and luminal-like (CC2) phenotype similar to pre-NAC subtyping was observed in 42% of cases. One third of cases became immune-infiltrated (CC3) in the post-NAC setting but lacked basal and luminal markers. These tumors expressed a strong T-cell signature, chemokine signaling and checkpoint molecules. Conversely, CC4 was associated with healing/scarring. This ‘scar-like’ character of CC4 was consistent with the scar samples. Despite being pathological non-responders, the relative risk of death for CC4 was 2.8 and 3 times less than CC2-Luminal (p = 0.038) and CC3-Infiltrated (p = 0.018), respectively. Luminal-like pre-NAC samples were more likely to adopt a scar-like character (CC4) in the post-NAC setting, while the basal-like tumors were more likely to develop luminal features (CC2). Conclusions: This study expands our knowledge of cisplatin-resistant MIBC by suggesting molecular subtypes to understand the biology of these tumors. Clinical trials are necessary to test the impact of these molecular subtypes with respect to selection of adjuvant and salvage treatments. Post-NAC immune infiltration could have implications for subsequent immunotherapy.
MP47-04 REFINING SELECTION CRITERIA FOR NEOADJUVANT CHEMOTHERAPY IN PATIENTS WITH MUSCLE INVASIVE BLADDER CANCER: COMBINING CLINICAL RISK FEATURES AND INTRINSIC MOLECULAR SUBTYPES