Background: Histone deacetylase inhibitors (HDIs) are epigenetic therapies in development. To exploit the unique activity in impairing DNA repair, HDIs have been combined with chemotherapy. Belinostat is a potent HDI combined with Cis and Etop based on enhanced DNA damage and apoptosis in small cell lung cancer (SCLC) cells. Methods: Patients with relapsed/refractory cancer or previously untreated advanced stage SCLC were eligible. Belinostat was administered by continuous infusion (CIV) over 48h, from 400 mg/m²/24h, in cohorts of 3. Cis was administered on day 1 and Etop daily X3. Belinostat pharmacokinetics (PK) and several pharmacodynamic (PD) measures were assessed, including lysine acetylation in peripheral blood mononuclear cells (PBMCs) and γH2Ax staining in PBMCs and in hair follicles. Results: Five dose levels were explored in 20 patients with solid tumors, including 5 patients with SCLC, two who had no prior therapy. At the first dose level, dose-limiting toxicities (DLT) of gr 4 ANC in 1, and gr 3 HTN in 1 were observed. Cis and Etop were reduced to 60 mg/m² and 80 mg/m², respectively, and the dose level repeated without DLT. At the next dose level, 800 mg/m²/24h belinostat, grade 3 HTN and grade 4 pneumonitis were observed. At the MTD of 600 mg/m²/24h belinostat, DLT was seen in 1 of 6 pts; however, all 6 pts required later dose reductions. We thus considered 500 mg/m²/24h in combination with Cis and Etop to be the recommended Phase II dose; confirmation ongoing. PKs show belinostat levels at 1 uM over the 48h infusion, decreasing rapidly to the 60h timepoint. In total 11 pts, 3 with SCLC, completed 6 cycles. PR was seen in 6 pts (3 with SCLC). PD studies confirmed γH2AX staining in PBMCs and hair follicles, peaking at 36h and 60h, respectively. Tubulin and lysine acetylation (Ac-K) in PBMCs peaked at 36h; Ac-K recovered more rapidly than tubulin, mirroring γH2AX. Conclusions: The MTD of belinostat over 48h by CIV was 600 mg/m²/24h, in combination with Cis 60 mg/m² on day 1 and Etop 80 mg/m² on days 1 - 3. PD endpoints indicate that belinostat is active in promoting both acetylation and DNA damage. The HDI combined with chemotherapy requires dose reduction and likely represents an on-target increase in DNA damage. Clinical trial information: NCT00926640.