Background Adult dCBT recipients are at significant risk for aGVHD especially involving the GI tract. Investigation of augmented prophylaxis is indicated. Methods Based on adult donor allograft data (Kennedy et al, Lancet 2015; Drobyski et al Haematologica 2018), we are investigating the addition of a single dose of tocilizumab 8 mg/kg (cap 800mg) on day -1 to standard cyclosporine/ MMF starting day -3 in adult patients (pts) with hematologic malignancies undergoing intermediate intensity Cy 50/ Flu 150/ Thio 10/ TBI 400 dCBT. We report an interim analysis of the first 16 pts of a 27 pt phase II trial (NCT03434730). All pts received standard bacterial/ fungal prophylaxis & Letermovir if CMV seropositive. Results 16 pts [median 50 years (range 27-59), median 85 kg (range 59-125), 6 AML, 5 ALL, 2 MPAL, 2 MDS, 1 CML, median aaHCT-CI of 3 (range 0-6)] received double-unit CB grafts with a median CD34+ cell dose of 1.33 × 105/kg/unit (range 0.24-3.83) & median unit-recipient 8-allele HLA-match of 5/8 (range 3-6). Pt outcomes are shown (Table). All (100%) pts engrafted with a median neutrophil recovery of 26 days (range 19-40). 94% of pts engrafted platelets (median 44 days, range 32-59). All pts had single unit dominance (median day 28 blood winning unit chimerism 100% (range 58-100). Prior to engraftment, 11/16 (69%) had no neutropenic fever, 3/16 (19%) pts had fever with bacteremia, 1 pt (6%) had febrile neutropenia (duration 2 days) responding to antibiotics, & 1 pt (6%) had initial fever due to bacteremia followed by fever/ rash attributed to pre-engraftment syndrome (PES). An additional 2 pts developed rash without fever prior to engraftment attributed to PES. Thus, a total of 3/16 (19%) pts had PES (onset ranging 12-14 days). Ten pts developed grade II aGVHD (median onset 45 days, range 26-81) for a cumulative incidence of 63% (95%CI: 33-82). This was limited primarily to skin &/or the upper gut with one pt having stage I lower GI. No pt has developed grade III-IV aGVHD & no pt has had stage II-IV lower GI aGVHD to date (Figure). With a median follow-up of 147 days (range 74-214), 15 pts are alive & disease-free (1 death due to HHV-6 pneumonitis). Conclusions Preliminary data suggests tocilizumab is safe with no adverse effects upon engraftment or unit dominance. Tocilizumab mitigates PES (expected incidence 65% in dCBT controls, Bhatt et al, ASBMT 2019). Expected days of febrile neutropenia were also reduced especially given the prolonged neutrophil recovery. Notably, lower gut aGVHD appears markedly reduced with no cases of grade III-IV aGVHD or aGVHD mortality to date. Our findings appear consistent with the published adult donor data. Overall, one tocilizumab dose has improved the early post-dCBT morbidity & investigation of this promising & potentially cost-effective approach is ongoing.