Background: The BLASST-1 study is multi-center phase II trial evaluating the combination of nivolumab (N) with gemcitabine-cisplatin (GC) as a neoadjuvant therapy for patients (pts) with MIBC undergoing radical cystectomy (RC). The primary endpoint was pathologic down staging (PaR; ≤pT1N0). Safety, Relapse-free survival (RFS), Progression-free survival (PFS) and biomarker analyses were secondary endpoints. We previously reported a PaR rate of 65.8% and pCR rate of 49%. There were no safety concerns or delays to surgery. (ASCO GU 2020) Here, we correlate PaR with biomarkers (Tumor mutational burden (TMB), PD-L1 and molecular subtypes) and provide updated clinical follow-up (FU) data. Methods: Forty-one pts with MIBC (cT2-T4a, N≤1, M0) and candidates for RC were enrolled between Feb 2018 and June 2019; (cT2N0 90%, cT3N0 7%, cT4N1 3%). Pts received C (70mg/m2) IV on D1, G (1000mg/m2) on D1, D8 and N (360 mg) IV on D8 every 21 days for 4 cycles followed by RC within 8 weeks. For RNA-based analysis, GeneChip Human Exon 1.0 ST Array (Affymetrix) was used; baseline tumors from 37 patients passed quality control and had available transcriptome data. A cohort (n=223) of patients treated with NAC+RC was used as a comparator for molecular subtyping analysis. DNA was extracted from baseline pre-treatment tumor samples and sequenced to an average depth of 150X and the DNA extracted from matched normal tissue (peripheral blood) to a mean depth of 50X. PD-L1 expression was assessed using IHC 28-8 antibody on baseline tumors. Results: At a median FU of 15.8 months,12-month RFS rate was 85.4% and PFS including death from any cause was 83%. There were no long-term safety concerns. Molecular subtyping found patients with a basal-type tumor (Basal or Claudin-low) had a more favorable overall PaR in 13/18 = 73% with PaR in 9/13 in basal (69%) and 4/5 in claudin-low (80%) compared to overall PaR of 58% for the luminal-type tumors (Luminal or Infiltrated luminal) with a breakdown of PaR in 5/8 (63%) in luminal and 6/11 (54%) in infiltrated luminal. In contrast, in the comparator NAC cohort, the PaR rates were similar for basal-type and luminal-type tumors, with 44% and 48% respectively. There was no correlation of PaR with TMB or PD-L1 expression from bassline pre-treatment tumors. Biomarker analyses from residual tumors in RC tissues are ongoing. Conclusions: The combination of N+GC was safe and efficacious in MIBC with encouraging outcomes of pathologic down staging and relapse-free survival at a median FU of 15.8 months. Molecular subtyping results suggest basal-type tumors may respond more favorably to this chemo-immunotherapy treatment regimen. Clinical trial information: NCT03294304.