Background: MM pts with LMD have a dismal prognosis, with median overall survival (OS) < 3 months, no approved therapies and extremely limited clinical trial options. We previously reported initial safety findings from an open label, single arm, single center phase I/IB trial (NCT03025256), in which IT and IV N were well tolerated, without any CNS-specific or unexpected toxicity. Here we report an update on safety and maximum tolerated dose (MTD) for all patients enrolled, and efficacy for the completed dose cohorts. Methods: MM patients aged >18 with evidence of LMD by MRI and/or CSF cytology, ECOG PS ≤2 were treated with IT and IV N. Dexamethasone ≤4mg/daily and concurrent BRAF/MEK inhibitor(i) treatment was allowed. For cycle 1, IT N was administered via intraventricular reservoir on day (D)1. For subsequent cycles (every 14 days), pts received IT N on D1, followed by IV N 240 mg on D2. IT N doses evaluated were 5, 10, 20 mg and 50 mg. Blood and CSF were collected at multiple time points for translational research. The primary objectives of this first-in-human study were to determine the safety and MTD of IT N given with IV N in MM pts with LMD. Bayesian mTPI methodology was used to define the MTD. Results: To date, 23 pts have been treated: two at 5, three at 10, fourteen at 20 mg and four at 50 mg IT N. Median age at LMD diagnosis was 42 (28-73); 12 pts are male. All pts had radiographic evidence of LMD and neurological symptoms; 14 pts had positive CSF cytology at baseline. 21 pts received prior therapies for their metastatic melanoma: anti-PD1 (n = 19), BRAFi/MEKi (n = 14), chemo (n = 2), IT IL2 (n = 4) other (n = 2). 19 pts had prior XRT, including whole brain RT (n = 7). Two pts were treatment-naïve. The median number of IT N doses was five (1- 66). The combination regimen was well tolerated by all evaluable pts (n=23), with only five pts (22%) experiencing gr 3 AEs, and no reported gr 4 or 5 toxicities. Nausea (30%), diarrhea (26%), and rash (22%) were the most common AEs. Eight pts (23%) experienced AEs after IT N administration, all gr 1. Initial efficacy analysis included only pts (n=19) treated with first three dose levels (5-20mg). Median follow-up for these pts is 4.5 months (mos) (1.1, 31.5 mos) and median OS is 63 % at 3 mos, 42 % at 6 mos and 30% at 12 mos. Conclusions: The trial demonstrates the feasibility and safety of IT administration of modern immunotherapy for MM pts with LMD. No unexpected systemic or neurological toxicity was observed with 20mg IT N. 2 additional patients are required to complete the 50mg IT N cohort. OS rates at 6 and 12 mos are encouraging and support further evaluation of IT administration of immunotherapy agents for pts with MM and LMD. Final presentation will include results of LMD for all dose cohorts, composite response assessment and comparative analysis of longitudinal CSF samples to assess immunologic effects. Clinical trial information: NCT03025256