e15543 Background: Up to 40% of patients with non-invasive bladder cancer (BC) will develop invasive disease progression despite locally-directed therapy. Overcoming the urothelial barrier is a challenge for intravesical drug delivery. We designed a biodegradable poly(lactide-co-glycolide) (PLGA) NP coated with a novel cell penetrating polymer, poly (guanidinium oxanorbornene) (PGON) for testing against BC in vitro and in vivo. We chose to deliver the HDAC inhibitor belinostat (bel) for its BC cell cytotoxicity and inhibition of invasion & migration pathways; key mechanisms that enable progression of BC. Methods: Fluorophore (C6) or bel was encapsulated in PLGA using an oil/water nanoemulsion method, and surface coated with PGON, and then characterized for morphology, size, and loading. BC cell lines UM-UC-3 and T24 were treated with belinostat or NP-bel-PGON vs controls for assessment of cytotoxicity and acetyl-H4 histone expression over time. In vivo murine bladder and ex vivo human ureter were treated with NP-C6-PGON, and compared to NPs coated with chitosan or PEG for urothelial penetration using FACS analysis, tissue extraction, and fluorescence microscopy. UM-UC-3 murine flank xenografts were treated locally biweekly with NP-bel-PGON or controls and assessed for tumor size and acetyl-H4 expression. Results: C6 extraction of intravesically treated mouse bladder and ex-vivo human ureter showed uptake improved ten-fold in NP-C6-PGON compared to other NPs and corroborated by fluorescent microscopy. In vitro, NP-bel-PGON and bel had similar IC50 of ~2.0 μM in UM-UC-3 & T24 lines, and no effect from PGON. Significantly, the NP-bel-PGON treated groups retained 30% of max H4 hyperacetylation whereas bel groups declined to basal at 12hr post wash, which supports a mechanism of intracellular NP release and activity. In vivo, xenografts treated with NP-bel-PGON showed tumor volume reduced 70% and had 2.5 fold higher intratumoral acetyl-H4 expression compared to vehicle three days post final treatment. Conclusions: NP-bel-PGON penetrates the urothelium, is taken up by BC cells, sustains HDAC inhibition, and causes tumor regression. These data demonstrate the potential for NP-bel-PGON as an intravesical nanotherapy of BC.