Human respiratory syncytial virus (RSV) is a major cause of respiratory tract infections worldwide. Several novel small-molecule inhibitors of RSV have been identified, but they are still in preclinical or early clinical evaluation. One such inhibitor is a recently discovered triphenol-based molecule, VP-14637 (ViroPharma). Initial experiments suggested that VP-14637 acted early and might be an RSV fusion inhibitor. Here we present studies demonstrating that VP-14637 does not block RSV adsorption but inhibits RSV-induced cell-cell fusion and binds specifically to RSV-infected cells with an affinity corresponding to its inhibitory potency. VP-14637 is capable of specifically interacting with the RSV fusion protein expressed by a T7 vaccinia virus system. RSV variants resistant to VP-14637 were selected; they had mutations localized to two distinct regions of the RSV F protein, heptad repeat 2 (HR2) and the intervening domain between heptad repeat 1 (HR1) and HR2. No mutations arose in HR1, suggesting a mechanism other than direct disruption of the heptad repeat interaction. The F proteins containing the resistance mutations exhibited greatly reduced binding of VP-14637. Despite segregating with the membrane fraction following incubation with intact RSV-infected cells, the compound did not bind to membranes isolated from RSV-infected cells. In addition, binding of VP-14637 was substantially compromised at temperatures of <22°C. Therefore, we propose that VP-14637 inhibits RSV through a novel mechanism involving an interaction between the compound and a transient conformation of the RSV F protein.

Here we present data on the mechanism of action of VP-14637 and JNJ-2408068 (formerly R-170591), two small-molecule inhibitors of respiratory syncytial virus (RSV). Both inhibitors exhibited potent antiviral activity with 50% effective concentrations (EC 50 s) of 1.4 and 2.1 nM, respectively. A similar inhibitory effect was observed in a RSV-mediated cell fusion assay (EC 50 ‫؍‬ 5.4 and 0.9 nM, respectively). Several drug-resistant RSV variants were selected in vitro in the presence of each compound. All selected viruses exhibited significant cross-resistance to both inhibitors and contained various single amino acid substitutions in two distinct regions of the viral F protein, the heptad repeat 2 (HR2; mutations D486N, E487D, and F488Y), and the intervening domain between HR1 and HR2 (mutation K399I and T400A). Studies using [ 3 H]VP-14637 revealed a specific binding of the compound to RSV-infected cells that was efficiently inhibited by JNJ-2408068 (50% inhibitory concentration ‫؍‬ 2.9 nM) but not by the HR2-derived peptide T-118. Further analysis using a transient T7 vaccinia expression system indicated that RSV F protein is sufficient for this interaction. F proteins containing either the VP-14637 or JNJ-2408068 resistance mutations exhibited greatly reduced binding of [ 3 H]VP-14637. Molecular modeling analysis suggests that both molecules may bind into a small hydrophobic cavity in the inner core of F protein, interacting simultaneously with both the HR1 and HR2 domains. Altogether, these data indicate that VP-14637 and JNJ-2408068 interfere with RSV fusion through a mechanism involving a similar interaction with the F protein.

The methods available to efficiently transduce human CD34(+) hematopoietic stem cells (HSCs) derived from mobilized peripheral blood, such that they fully retain their engraftment potential and maintain high levels of transgene expression in vivo, have been unsatisfactory. The current murine retrovirus-based gene transfer systems require dividing cells for efficient transduction, and therefore the target HSCs must be activated ex vivo by cytokines to cycle, which may limit their engrafting ability. Lentivirus-based gene transfer systems do not require cell division and, thus, may allow for efficient gene transfer to human HSCs in the absence of any ex vivo cytokine stimulation. We constructed human immunodeficiency virus (HIV)-based vectors and compared them in vitro and in vivo with MuLV-based vectors in their ability to transduce unstimulated human CD34(+) HSCs isolated from mobilized peripheral blood. Both sets of vectors contained the marker gene that expresses the enhanced green fluorescent protein (EGFP) for evaluating transduction efficiency and were pseudotyped with either vesicular stomatitis virus glycoprotein (VSV-G) or the amphotropic murine leukemia virus envelope (A-MULV Env). The VSV-G-pseudotyped HIV-based vectors containing an internal mouse phosphoglycerate kinase promoter (PGK) were able to transduce up to 48% of the unstimulated CD34(+) cells as measured by EGFP expression. When these cells were injected into the human fetal thymus implants of irradiated SCID-hu Thy/Liv mice, up to 18% expressed EGFP after 8 weeks in vivo. In contrast, the MULV-based vectors were effective at transducing HSCs only in the presence of cytokines. Our results demonstrate that the improved HIV-based gene transfer system can effectively transduce unstimulated human CD34(+) HSCs, which can then differentiate into thymocytes and provide long-term transgene expression in vivo.