The primary focus of this application is to develop more potent and less toxic, isoform-selective histone deacetylase (HDAC) inhibitors (HDACi) for use as anti-myeloma and anti-cancer drug candidates, biological tools for investigation of specific functions of individual HDACs and extension of this chemical class beyond antineoplastic indications. Specifically, we plan to optimize the structures of macrocyclic peptide analogues of largazole, and related naturally occurring HDAC inhibitors for HDAC class- selectivity by altering the amino acid sequence and zinc-binding functionality in accord with parameters derived from crystal structures of the target enzymes deploying additional computational and empiric insights. Analogues will be assayed for inhibitory activity against HDACs 1-11 in the laboratories of co- investigator Dr. James E. Bradner at the Broad Institute of Harvard-MIT and the Dana-Farber Cancer Institute and Dr. James R. Berenson at the Institute for Myeloma and Bone Cancer Research. Assessment of the safety and pharmacodynamic effect will be performed by Prof. Douglas Thamm and co- workers at the CSU College of Veterinary Medicine and Biomedical Sciences. Insight from the results of these assays will guide further alterations of the candidate structures. A computational co-investigator, Prof. Olaf Wiest, at the University of Notre Dame, will deploy homology model-based docking studies to further guide the design and optimization of new synthetic HDACi's. We have developed scaleable solution-phase syntheses of the known, highly potent natural HDACi's largazole and FK228 as well as the corresponding peptide isosteres and numerous synthetic analogs of these potent and naturally occurring HDACi's. Structural diversity within the analogs will be explored by varying three parameters: (a) amino acid sequence and constitution; (b) macrocycle size; and (c) zinc-binding arms. Particularly potent and/or isoform-selective macrocyclic peptide inhibitors that are produced by this approach will be targeted for re-synthesis as the corresponding depsipeptide congeners and profiled for potency and specificity in biochemical and cellular assays. Select isoform-specific analogs will be evaluated for in vivo tolerability and antineoplastic activity.