Tuberculosis (TB) is more prevalent in the world today than at any other time in human history. Mycobacterium tuberculosis, the pathogen responsible for TB, uses diverse strategies to survive in a variety of host lesions and to evade immune surveillance. A key question is how robust are our approaches to discovering new TB drugs, and what measures could be taken to reduce the long and protracted clinical development of new drugs. The emergence of multi-drug-resistant strains of M. tuberculosis makes the discovery of new molecular scaffolds a priority, and the current situation even necessitates the re-engineering and repositioning of some old drug families to achieve effective control. Whatever the strategy used, success will depend largely on our proper understanding of the complex interactions between the pathogen and its human host. In this review, we discuss innovations in TB drug discovery and evolving strategies to bring newer agents more quickly to patients.

Respiratory syncytial virus (RSV) is a leading cause of pneumonia and bronchiolitis in young children and the elderly. Therapeutic small molecules have been developed that bind the RSV F glycoprotein and inhibit membrane fusion, yet their binding sites and molecular mechanisms of action remain largely unknown. Here we show that these inhibitors bind to a three-fold-symmetric pocket within the central cavity of the metastable prefusion conformation of RSV F. Inhibitor binding stabilizes this conformation by tethering two regions that must undergo a structural rearrangement to facilitate membrane fusion. Inhibitor-escape mutations occur in residues that directly contact the inhibitors or are involved in the conformational rearrangements required to accommodate inhibitor binding. Resistant viruses do not propagate as well as wild-type RSV in vitro, indicating a fitness cost for inhibitor escape. Collectively, these findings provide new insight into class I viral fusion proteins and should facilitate development of optimal RSV fusion inhibitors.

Respiratory syncytial virus is a major cause of acute lower respiratory tract infection in young children, immunocompromised adults, and the elderly. Intervention with small-molecule antivirals specific for respiratory syncytial virus presents an important therapeutic opportunity, but no such compounds are approved today. Here we report the structure of JNJ-53718678 bound to respiratory syncytial virus fusion (F) protein in its prefusion conformation, and we show that the potent nanomolar activity of JNJ-53718678, as well as the preliminary structure–activity relationship and the pharmaceutical optimization strategy of the series, are consistent with the binding mode of JNJ-53718678 and other respiratory syncytial virus fusion inhibitors. Oral treatment of neonatal lambs with JNJ-53718678, or with an equally active close analog, efficiently inhibits established acute lower respiratory tract infection in the animals, even when treatment is delayed until external signs of respiratory syncytial virus illness have become visible. Together, these data suggest that JNJ-53718678 is a promising candidate for further development as a potential therapeutic in patients at risk to develop respiratory syncytial virus acute lower respiratory tract infection.

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Six-helix bundle (6HB) formation is an essential step for many viruses that rely on a class I fusion protein to enter a target cell and initiate replication. Because the binding modes of small molecule inhibitors of 6HB formation are largely unknown, precisely how they disrupt 6HB formation remains unclear, and structure-based design of improved inhibitors is thus seriously hampered. Here we present the high resolution crystal structure of TMC353121, a potent inhibitor of respiratory syncytial virus (RSV), bound at a hydrophobic pocket of the 6HB formed by amino acid residues from both HR1 and HR2 heptad-repeats. Binding of TMC353121 stabilizes the interaction of HR1 and HR2 in an alternate conformation of the 6HB, in which direct binding interactions are formed between TMC353121 and both HR1 and HR2. Rather than completely preventing 6HB formation, our data indicate that TMC353121 inhibits fusion by causing a local disturbance of the natural 6HB conformation.cocrystal structure | respiratory syncytial virus | TMC353121 | viral fusion T o allow the deposition of their nucleic acid genome into a host cell, and to initiate their replication cycle, enveloped viruses have evolved complex membrane fusion machinery that includes a fusion protein (1, 2). Based on structural similarity, the viral fusion proteins from different viruses have been grouped into three distinct classes: I, II, and III (3, 4). Prototypic trimeric class I fusion proteins include HIV-1 gp41, influenza hemagglutinin and the fusion proteins from paramyxoviruses. The fusion protein (F) of respiratory syncytial virus (RSV), a paramyxovirus belonging to the pneumovirinae subfamily, assembles into a homotrimer that is cleaved at two proximal furin cleavage sites during biosynthesis, priming the protein for membrane fusion. Proteolytic cleavage of the fusion protein precursor (F 0 ) yields two polypeptides, F 1 and F 2 , joined by a disulfide bridge (Fig. 1). F 1 consists of an N-terminal hydrophobic fusion peptide, followed by a first heptad-repeat (HR1), an intervening globular domain, and a second heptadrepeat (HR2), which itself is N-terminal to the viral transmembrane and cytoplasmic regions (3). Once fusion is triggered, dramatic refolding of the prefusion conformation of the viral fusion protein occurs. Functional and structural studies have provided evidence that a folding intermediate is formed that contains a coiled-coil structure of three HR1 heptad repeats (5-8). This intermediate allows the fusion peptide to be inserted into the plasma membrane of a target cell. In the final stage of membrane fusion, the HR1-CTC structure irreversibly refolds into a 6HB complex with three HR2 heptad-repeats, resulting in membrane merger and stable fusion pore formation (5-14). In many viruses that rely on class I fusion proteins, the central HR1 trimeric coiled-coil (HR1-CTC) contains a hydrophobic pocket in each of its three grooves that has been proposed as a potential drug binding site (9, 10).The therapeutic value of inhibiting 6HB formation was establ...