Inhibition of both HIV-1 reverse transcription and gene expression by a cyclic peptide that binds the Tat-transactivating response element (TAR) RNA.

The RNA response element TAR plays a critical role in HIV replication by providing a binding site for the recruitment of the viral transactivator protein Tat. Using a structure-guided approach, we have developed a series of conformationally-constrained cyclic peptides that act as structural mimics of the Tat RNA binding region and block Tat-TAR interactions at nanomolar concentrations in vitro. Here we show that these compounds block Tat-dependent transcription in cell-free systems and in cell-based reporter assays. The compounds are also cell permeable, have low toxicity, and inhibit replication of diverse HIV-1 strains, including both CXCR4-tropic and CCR5-tropic primary HIV-1 isolates of the divergent subtypes A, B, C, D and CRF01_AE. In human peripheral blood mononuclear cells, the cyclic peptidomimetic L50 exhibited an IC(50) ∼250 nM. Surprisingly, inhibition of LTR-driven HIV-1 transcription could not account for the full antiviral activity. Timed drug-addition experiments revealed that L-50 has a bi-phasic inhibition curve with the first phase occurring after HIV-1 entry into the host cell and during the initiation of HIV-1 reverse transcription. The second phase coincides with inhibition of HIV-1 transcription. Reconstituted reverse transcription assays confirm that HIV-1 (-) strand strong stop DNA synthesis is blocked by L50-TAR RNA interactions in-vitro. These findings are consistent with genetic evidence that TAR plays critical roles both during reverse transcription and during HIV gene expression. Our results suggest that antiviral drugs targeting TAR RNA might be highly effective due to a dual inhibitory mechanism.

Simultaneous recognition of HIV-1 TAR RNA bulge and loop sequences by cyclic peptide mimics of Tat protein.

The interaction of the HIV-1 transactivator protein Tat with its transactivation response (TAR) RNA is an essential step in viral replication and therefore an attractive target for developing antivirals with new mechanisms of action. Numerous compounds that bind to the 3-nt bulge responsible for binding Tat have been identified in the past, but none of these molecules had sufficient potency to warrant pharmaceutical development. We have discovered conformationally-constrained cyclic peptide mimetics of Tat that are specific nM inhibitors of the Tat-TAR interaction by using a structure-based approach. The lead peptides are nearly as active as the antiviral drug nevirapine against a variety of clinical isolates in human lymphocytes. The NMR structure of a peptide-RNA complex reveals that these molecules interfere with the recruitment to TAR of both Tat and the essential cellular cofactor transcription elongation factor-b (P-TEFb) by binding simultaneously at the RNA bulge and apical loop, forming an unusually deep pocket. This structure illustrates additional principles in RNA recognition: RNA-binding molecules can achieve specificity by interacting simultaneously with multiple secondary structure elements and by inducing the formation of deep binding pockets in their targets. It also provides insight into the P-TEFb binding site and a rational basis for optimizing the promising antiviral activity observed for these cyclic peptides.

Structure-guided peptidomimetic design leads to nanomolar beta-hairpin inhibitors of the Tat-TAR interaction of bovine immunodeficiency virus.

The Tat protein of immunodeficiency viruses is the main activator of viral gene expression. By binding specifically to its cognate site, the transactivator response element (TAR), Tat mediates a strong induction of the production of all viral transcripts. In seeking a new chemical solution to inhibiting viral protein-RNA interactions, we recently identified inhibitors of the viral Tat protein from the bovine immunodeficiency virus (BIV) using conformationally constrained beta-hairpin peptidomimetics. We identified a micromolar ligand, called BIV2, and the structure of its complex with BIV TAR was determined by NMR. In this work, we demonstrate that this chemistry can rapidly yield highly potent and selective ligands. On the basis of the structure, we synthesized and assayed libraries of mutant peptidomimetics. Remarkably, we were able in just a few rounds of design and synthesis to discover nanomolar inhibitors of the Tat-TAR interaction in BIV that selectively bind the BIV TAR RNA compared to RNA structures as closely related as the HIV-1 TAR or RRE elements. The molecular recognition principles developed in this study have been exploited in discovering related peptidomimetic inhibitors of the Tat-TAR interaction in HIV-1.

TAR RNA recognition by a cyclic peptidomimetic of Tat protein.

The search for new antiviral drugs that repress HIV viral replication by blocking transactivation of viral RNA transcription has long been advocated as an approach to novel antiviral therapy. However, research in this area has so far failed to yield attractive lead compounds because of the insufficient development of RNA-based medicinal chemistry. One difficulty in efforts to inhibit protein-RNA interactions using small druglike molecules is the large surface areas typically found at these interfaces. To overcome this problem, we sought to identify constrained peptidomimetic inhibitors that would provide potential new drug leads. We previously reported the discovery of a cyclic peptide mimic of the RNA-binding domain of BIV Tat protein based on a designed beta-hairpin scaffold. We demonstrated that the cyclic peptide bound BIV TAR RNA with an affinity comparable to that of the RNA-binding domain of the Tat protein and inhibited protein binding to the RNA. In this study, we report the structure of the complex of the cyclic peptide bound to BIV TAR RNA determined using heteronuclear NMR methods. The structure reveals a beta-hairpin conformation in the bound peptide, which adopts an unexpected orientation in the major groove of the RNA opposite those observed for peptides derived from the Tat protein. This structure suggests many ways in which to optimize the compound and enhance its activity and pharmacological potential and represents a further step in the rational design of a new class of HIV-1 virus replication inhibitors based on peptidomimetic chemistry.

Structural mimicry of retroviral tat proteins by constrained beta-hairpin peptidomimetics: ligands with high affinity and selectivity for viral TAR RNA regulatory elements.

An approach is described to the design of beta-hairpin peptidomimetic ligands for bovine immunodeficiency virus (BIV) Tat protein, which inhibit binding to its transactivator response element (TAR) RNA. A library of peptidomimetics was derived by grafting onto a hairpin-inducing d-Pro-l-Pro template sequences related to the RNA recognition element in Tat. One hairpin mimetic was identified that binds tightly (K(d) approximately 150 nM) to BIV TAR, and another that binds also to HIV-1 TAR RNA (K(d) approximately 1-2 microM). (In the same assay, the wild-type BIV Tat(65-81) peptide binds to BIV TAR with K(d) approximately 50 nM.) The high-affinity BIV-Tat mimetic was shown to adopt a stable beta-hairpin conformation in free solution by NMR methods. Amino acid substitutions in this mimetic were shown to impact on the hairpin structure and to disrupt binding to the RNA. This family of conformationally constrained peptidomimetics affords insights into the structural requirements for binding to TAR RNA and provides a basis for the design of new ligands with increased inhibitory activity and specificity to both BIV and HIV TAR RNAs.

Macrocyclic hairpin mimetics of the cationic antimicrobial peptide protegrin I: a new family of broad-spectrum antibiotics.

The problems associated with increasing antibiotic resistance have stimulated great interest in newly discovered families of naturally occurring cationic antimicrobial peptides. These include protegrin, tachyplesin, and RTD-1, which adopt beta-hairpin-like structures. We report here an approach to novel peptidomimetics based on these natural products. The mimetics were designed by transplanting the cationic and hydrophobic residues onto a beta-hairpin-inducing template, either a D-Pro-L-Pro dipeptide or a xanthene derivative. The mimetics have good antimicrobial activity against Gram-positive and Gram-negative bacteria (minimal inhibitory concentration approximately 6-25 microgram mL(-1)). Analogues with improved selectivity for microbial rather than red blood cells (1 % hemolysis at 100 microgram mL(-1)) were identified from a small library prepared by parallel synthesis. Thus, it is possible to separate the antimicrobial and hemolytic activities in this class of mimetics. NMR studies on one mimetic revealed a largely unordered structure in water, but a transition to a regular beta-hairpin backbone conformation in the presence of dodecylphosphocholine micelles. This family of mimetics may provide a starting point for the optimization of antimicrobial agents of potential clinical value in the fight against multiple-drug-resistant microorganisms.