Abstract: Cyclic peptides are widely produced in nature and possess a broad range of biological activities. Their enhanced proteolytic stability in vivo and improved receptor binding affinity/specificity makes them excellent drug candidates, molecular probes and targeting agents. In fact, many cyclic peptides are clinically used therapeutic agents. Combinatorial library approaches provide powerful tools for the rapid identification of compounds with desired properties from large pools in biological and biomedical studies. However, the synthesis and screening of cyclic peptide libraries in a combinatorial format has been challenging. To overcome the issue, we have successfully developed one-bead-two-compound (OBTC) libraries with a cyclic peptide displayed on the bead surface accessible for protein targets screening, while the bead interior contains the corresponding linear peptide served as an encoding tag for hit identification. The primary goal of my research is to identify novel biologically active cyclic peptides, beyond what nature has provided us. By applying cyclic peptide library approach, we have successfully identified high affinity ligands against various biological targets, including: extracellular protein receptors (human prolactin receptor), intracellular protein domains (the capsid domain of HIV-1 Gag polyprotein and calcineurin catalytic domain) and enzymes (Pin1 catalytic domains). In the meantime, we have continued to improve the methodologies associated with combinatorial chemistry. To facilitate the process and improve the screening results, such as avoiding false positives, we have developed many cyclic library approaches including libraries on different solid supports, reduced surface density libraries, high diversity libraries with different ring sizes and library compatible with rapid solution phase validation. These new approaches greatly facilitate the ligands discovery process. My final work focused on the intracellular delivery of cyclic peptides. Little is known about how cyclization would affect peptides membrane permeability and the results from existing studies are controversial. With a combination of biophysical approaches and cell based studies, we have found that cyclization has a dramatic effect on the cell permeation of peptides with certain residues. By applying the rules, we were able to design cell permeable cyclic peptide inhibitors against various intracellular protein targets. Our studies provide guiding principles for designing membrane penetrating cyclic peptidyl drugs.

A great number of cellular and biological processes depend, at some level, on protein-protein interactions (PPI). Their manipulation with chemical compounds has provided a great potential for the discovery of new drugs. Despite the increasing demand for molecules able to interrupt specific PPI, the development of small PPI inhibitors is beset by a number of challenges such as the large size of the interaction interface. Based on the interface's nature, the ability to mimic protein secondary structures is very important to bind a protein and inhibit PPI. With their interesting peptidomimetic abilities and pharmacological properties, cyclic peptides are very promising templates to discover protein ligands and development new PPI inhibitors. To fully exploit the great diversity accessible with cyclic peptides, the one-bead-one-compound (OBOC) combinatorial method is certainly the most accessible and powerful approach. Unfortunately, the use of cyclic peptides in OBOC libraries is limited by difficulties in sequencing hit compounds after the screening. Lacking a free N-terminal amine, Edman degradation cannot be used on cyclic peptides and complicated fragmentation patterns are obtained by tandem mass spectrometry (MS/MS). In this regard we have designed and developed new convenient ring-opening approaches to prepare and decode OBOC cyclic peptide libraries. Our strategy was to introduce a cleavable residue in the macrocycle and as a linker to allow linearization of peptides and their release from the beads for sequencing by MS/MS. First, amino acid residues sensible to nucleophiles, ultraviolet irradiation or cyanogens bromide were introduced in a model cyclic peptide. Afterward, the most promising residues were used to design and develop tandem ring-opening/cleavage approaches to decode OBOC cyclic peptide libraries. In the first approach a methionine residue was introduced in the macrocycle and as a linker to allow a simultaneous ring-opening and cleavage from the beads upon treatment with cyanogens bromide. In the second approach, a photosensitive residue was used in the macrocycle and as a linker for a dual ring-opening/cleavage upon UV irradiation. The linear peptide generated by these approaches can be efficiently sequenced by tandem mass spectrometry. Finally, an OBOC library has been prepared and screened against the HIV-1 Nef protein to identify selective ligands. The development of these methodologies will prompt the use of macrocyclic compounds in OBOC libraries and be an important contribution in medicinal chemistry for the discovery of protein ligands and the development of PPI inhibitors.

During the course of evolution, an imbalance was created between the rate of vertebrate genetic adaptation and that of the lower forms of living organisms, such as bacteria and viruses. This imbalance has given the latter the advantage of generating, relatively quickly, molecules with unexpected structures and features that carry a threat to vertebrates. To compensate for their weakness, vertebrates have accelerated their own evolutionary processes, not at the level of whole organism, but in specialized cells containing the genes that code for antibody molecules or for T-cell receptors. That is, when an immediate requirement for molecules capable of specific interactions arose, nature has preferred to speed up the mode of Darwinian evolution in pref- ence to any other approach (such as the use of X-ray diffraction studies and computergraphic analysis). Recently, Darwinian rules have been adapted for test tube research, and the concept of selecting molecules having particular characteristics from r- dom pools has been realized in the form of various chemical and biological combinatorial libraries. While working with these libraries, we noticed the interesting fact that when combinatorial libraries of oligopeptides were allowed to interact with different selector proteins, only the actual binding sites of these proteins showed binding properties, whereas the rest of the p- tein surface seemed "inert. " This seemingly common feature of protein- having no extra potential binding sites--was probably selected during evolution in order to minimize nonspecific interactions with the surrounding milieu.

The continued successes of large- and small-scale genome sequencing projects are increasing the number of genomic targets available for drug d- covery at an exponential rate. In addition, a better understanding of molecular mechanisms—such as apoptosis, signal transduction, telomere control of ch- mosomes, cytoskeletal development, modulation of stress-related proteins, and cell surface display of antigens by the major histocompatibility complex m- ecules—has improved the probability of identifying the most promising genomic targets to counteract disease. As a result, developing and optimizing lead candidates for these targets and rapidly moving them into clinical trials is now a critical juncture in pharmaceutical research. Recent advances in com- natorial library synthesis, purification, and analysis techniques are not only increasing the numbers of compounds that can be tested against each specific genomic target, but are also speeding and improving the overall processes of lead discovery and optimization. There are two main approaches to combinatorial library production: p- allel chemical synthesis and split-and-mix chemical synthesis. These approaches can utilize solid- or solution-based synthetic methods, alone or in combination, although the majority of combinatorial library synthesis is still done on solid support. In a parallel synthesis, all the products are assembled separately in their own reaction vessels or microtiter plates. The array of rows and columns enables researchers to organize the building blocks to be c- bined, and provides an easy way to identify compounds in a particular well.

Including case studies of macrocyclic marketed drugs and macrocycles in drug development, this book helps medicinal chemists deal with the synthetic and conceptual challenges of macrocycles in drug discovery efforts. Provides needed background to build a program in macrocycle drug discovery –design criteria, macrocycle profiles, applications, and limitations Features chapters contributed from leading international figures involved in macrocyclic drug discovery efforts Covers design criteria, typical profile of current macrocycles, applications, and limitations

A peptidomimetic is a small protein-like chain designed to mimic a peptide with adjusted molecular properties such as enhanced stability or biological activity. It is a very powerful approach for the generation of small-molecule-based drugs as enzyme inhibitors or receptor ligands. Peptidomimetics in Organic and Medicinal Chemistry outlines the concepts and synthetic strategies underlying the building of bioactive compounds of a peptidomimetic nature. Topics covered include the chemistry of unnatural amino acids, peptide- and scaffold-based peptidomimetics, amino acid-side chain isosteres, backbone isosteres, dipeptide isosteres, beta-turn peptidomimetics, proline-mimetics as turn inducers, cyclic scaffolds, amino acid surrogates, and scaffolds for combinatorial chemistry of peptidomimetics. Case studies in the hit-to-lead process, such as the development of integrin ligands and thrombin inhibitors, illustrate the successful application of peptidomimetics in drug discovery.

Traditionally, the search for new compounds from natural products has been a time- and resource-intensive process. The recent application of combinatorial methods and high-throughput synthesis has allowed scientists to generate a range of new molecular structures from natural products and observe how they interact with biological targets. Combinato