350 Background: Small cell neuroendocrine carcinoma of the bladder (SCCB) is rare but aggressive form of genitourinary cancer that can arise de novo or in conjunction with urothelial carcinoma (UCB). Methods: DNA was extracted from 40 microns of FFPE specimen from 29 cases of relapsed, refractory and metastatic SCCB and 1,113 UCB. Comprehensive genomic profiling (CGP) was performed using a hybrid-capture, adaptor ligation based next generation sequencing assay to a mean coverage depth of > 503X. Tumor mutational burden (TMB) was calculated from a minimum of 1.11 Mb of sequenced DNA as previously described and reported as mutations/Mb. The results were analyzed for all classes of genomic alterations (GA), including base substitutions, insertions and deletions (short variants; SV), fusions, and copy number changes including amplifications (amp) and homozygous deletions. Results: 29 SCCB cases were confirmed on routine histology and featured positive IHC staining for chromogranin, synaptophysin or both. Patients had a mean age of 68.1 years (range 49-90 years) and 25 (86%) were male. At the time of CGP, 3 (10%) SCCB were Stage III and 26 (90%) were stage IV. The primary SCCB was used for sequencing in 14 (48%) of cases and a metastasis sample in 15 (52%). The 29 SCCB featured 2.86 GA/case.The genomics of SCCB differed significantly from UCB (Table). The most frequent clinically relevant GA in SCCB were RICTOR amp (21%) and PIK3CA (10%), BRCA1, HGF, FBXW7 and CCND2 SV (7% each). The relatively high TMB in SCCB (7% TMB > 20 mut/Mb and 28% TMB > 10 mut/Mb) is similar to that seen in UCB. No SCCB cases were MSI-high. ERBB2 and FGFR1 GA frequencies (both 3%) in SCCB were lower than in similarly studied UCB. Conclusions: SCCB differs in genomic landscape from UCB in having higher frequencies of TP53 and RB1 GA and lower frequencies of FGFR3 and ERBB2 GA. However, like UCB, SCCB shares the presence of multiple GA associated with potential responses to targeted therapies and high TMB associated with response to immune checkpoint inhibitor therapy. [Table: see text]
710P Comprehensive genomic profiling (CGP) of small cell neuroendocrine carcinoma of the bladder (NEBC)
