Background: PAZ is a multi-targeted tyrosine kinase inhibitor of VEGFR, PDGFR, and C-KIT, approved for metastatic RCC and refractory sarcoma based on phase III data showing prolonged PFS (JCO 2010;28:1061-8 and Lancet 2012;379:1879-86). PCI is a potent pan-HDAC inhibitor (pan-HDACi), observed in cell lines to change regulation of genes involved in cell signaling, apoptosis, proliferation, differentiation, and angiogenesis (Anticancer Res 2011;31:1115-23). Pre-clinical models suggest epigenetic modification with an HDACi potentiates PAZ’s efficacy by causing chromatin instability and gene expression changes involved in drug resistance (Can Res 2005;65:3815-22 and BJC 2009;100:758-63). We therefore designed a Phase Ia/b clinical trial combining PCI with PAZ in pts with advanced solid tumors, with an expansion cohort for preliminary efficacy in RCC and sarcoma. Methods: Primary objective of this phase Ia/b study is to evaluate the safety and tolerability of the combination of PAZ and PCI to determine the MTD and RP2D. In phase Ia, we utilized an accelerated phase I design. The phase Ib portion will include up to 20 pts per expansion cohort, for up to 32-70 pts enrolled. In phase 1a, pts receive run-in PCI alone on C1D-7 to D-4. Starting with C1D1, pts receive oral PCI on D1-5, 8-12, 15-19 BID 4 hrs apart and PAZ daily (D1-28) q28D cycle. CORRELATIVES: Pre- and post-treatment (Tx) H3 & H4 acetylation and HDAC activity in PBMCs. In phase Ib, these will also be studied in tumor biopsies. We will measure expression of VEGF, VEGFR, RAD51, HIF, Ki67; and analyze SNPs through genomic profiling. We will correlate response with pre- and post-Tx tumor thymidine uptake using 18F-fluorothymidine (FLT-PET) PET. Current Status: This is the 1^st trial exploring the combination of an HDACi with PAZ in RCC and sarcoma, where there is an unmet need for new tolerable therapies. It will study FLT-PET, an imaging correlate that captures tumor proliferation and may have a role as a predictive biomarker. We are currently in phase Ia. Enrollment in the 3^rd cohort exploring higher doses of PAZ will begin in Feb 2013. Clinical trial information: NCT01543763.