2732 Current Pharmaceutical Design, 2011, 17, 2732-2747 1381-6128/11 $58.00+.00 © 2011 Bentham Science Publishers Peptoids: Bio-Inspired Polymers as Potential Pharmaceuticals Michelle T. Dohm 1# , Rinki Kapoor 2# and Annelise E. Barron 1,2* 1 Department of Bioengineering and 2 Graduate Program in Biophysics, Stanford University, W300 James H. Clark Center, 318 Cam- pus Drive, Stanford, California 94305-5440, United States of America Abstract: Peptoids are a developing class of peptide-like oligomers originally invented for drug discovery in the early 1990s. While pep- tides hold great promise for therapeutic applications, current development of peptide-based pharmaceuticals is hindered by their potential for misfolding and aggregation, and particularly, for rapid in vivo degradation post-administration. Researchers have investigated alterna- tive peptide-like constructs that may be able to circumvent such complications. Peptoids comprise a peptide-based backbone and N- substituted glycines for side chain residues, resulting in complete protease-resistance. Synthesis of peptoid sequences up to 50 units in length allows for controlled sequence composition and incorporation of diverse side chain chemistries. Though the landscape of peptoid structure is not clearly defined, secondary, tertiary, loop, turn, and random structures have been identified. As protease-resistant isomers of peptides, peptoids are being developed as versatile molecular tools in biochemistry and biophysics, and are becoming attractive candi- dates for therapeutic and diagnostic applications. Peptoids have thus far demonstrated bioactivity as protein mimics and as replacements for small molecule drugs. In this review, we discuss the most recent advances in peptoid research on the therapeutic front in the last few years, including in vitro and in vivo studies in the fields of lung surfactant therapy, antimicrobial agents, diagnostics, and cancer. We par- ticularly focus on the biophysical activity of lipid-associated peptoids and their potential therapeutic applications. Keywords: Peptoids, peptide, mimic, peptidomimetic, amphipathic, lung surfactant, antimicrobial peptides, host defense peptides. INTRODUCTION Since oligomers of N-substituted glycines, or peptoids, were first reported by Bartlett, Zuckermann, and co-workers in 1992 [1- 2], researchers have explored the synthetic, conformational, and application-oriented aspects of the “peptoid landscape” with sig- nificant success. Peptoids are discrete oligomers with a peptidic backbone, where the side chains are appended to the backbone am- ide nitrogen rather than the -carbon. They are essentially peptide isomers, but the seemingly simple shifting of the side chains to the nitrogen significantly changes the properties of the isomer peptoid relative to the peptide. The most biologically relevant consequence of this shift is protease insusceptibility, which arguably increases their therapeutic applicability as pharmaceuticals relative to pep- tides. Furthermore, a straightforward solid-phase synthesis of pep- toids was developed using peptide synthesis equipment, making them amenable to library synthesis and a wide range of side chain functionalities. As a result, peptoids have since become attractive therapeutic candidates in biochemistry, molecular biophysics, diag- nostics, and medicine. Peptoids may be able to replace or supplement a protein or pep- tide required for a particular biological application. Peptides and proteins are very promising as therapeutics and account for 2% of marketed drugs and 50% of drugs in the pipeline for approval. However, these compounds have failed to deliver as initially ex- pected due to multiple complications. Firstly, peptides and proteins are costly to chemically synthesize and purify in sufficient quantity for systemic or localized treatments; and while recombinant protein expression is an efficient and inexpensive method for synthesizing proteins, it remains a poor alternative for complex protein se- quences or proteins with specific post-translationally ally modified functionalities. Extremely hydrophobic or structurally unstable peptides and proteins tend to easily aggregate or misfold, thus re- ducing the shelf life, bioavailability, or biostability to become manufactured drugs. The latter complication of biostability is par- ticularly cumbersome, as almost all peptides are rapidly degraded in *Address correspondence to the author at the Department of Bioengineering Stanford University, W300 James H. Clark Center, 318 Campus Drive, Stanford, California 94305-5440, United States of America; Tel: 650-796-4001; Fax: 650-723-9801; E-mail: aebarron@stanford.edu; # These authors contributed equally. vivo by proteases upon administration. This leads to manufacturing problems with delivery, dosage, efficacy, and formulation. The last major concern for peptide-based pharmaceuticals is potential for toxicity and immune response, as the body is likely to view admin- istered peptides and proteins as foreign invaders (this possibility exists for peptoids as well). Peptoids can match or exceed peptide capabilities on these fronts, but sequence design is a crucial factor for success. Peptoids are completely sequence-specific and, due to a lack of backbone hydrogen bonding, can easily be engineered for minimum aggrega- tion, appropriate solubility, yet high hydrophobicity. Peptoids are structurally stable even in the presence of denaturants [3], and as mentioned, are protease-invulnerable [4]. Even though peptoids should be viewed as invaders by the body, a recent study by Ko- dadek and co-workers showed that, when injected into the mouse, peptoids did not produce an anti-peptoid immune response [5]. In addition, the extent of immune response can be lowered through sequence design by including non-amino-acid-based side chains [6]. Finally, combination peptoid-peptide hybrids can also be syn- thesized to modulate the properties of peptide therapeutics. It would be unreasonable to say that peptoids do not have any disadvantages. Peptoids lack backbone hydrogen bonding due to the absence of backbone hydrogen bond donors, and hence, cannot form -sheets. However, a lack of -sheet formation is in a way advantageous because the resulting peptoids do not aggregate as easily as peptides. To develop peptoids as therapeutics, it is impor- tant to understand their fates in the body, such as: (1) how fast are the peptoids eliminated from the body? (2) where do they concen- trate or localize, if anywhere? (3) are peptoids xenobiotics? and (4) do peptoids produce hepatotoxicity? Unfortunately, not much has been published to address these concerns, and such studies are criti- cal for the development of peptoids as potential pharmaceuticals. In the realm of protein-protein interactions, peptoids could play a major role in replacing traditional small molecules that align with Lipinski’s rules but are too small to sufficiently cover the surface area of the targeted protein binding site. Protein-protein interactions are integral for normal biological functioning, and protein com- plexes have become therapeutic targets in virus inhibition, cancer, cellular signaling, gene expression activation, and other types of disease-related processes [7]. Investigators have created peptides