A Trimer of Dimers Is the Basic Building Block for Human Immunodeficiency Virus-1 Capsid Assembly

Human immunodeficiency virus-1 (HIV-1) capsid protein (CA) has become a target of antiviral drug design in recent years. The recognition that binding of small molecules to the CA protein can result in the perturbation of capsid assembly or disassembly has led to mathematical modeling of the process. Although a number of capsid assembly models have been developed using biophysical parameters of the CA protein obtained experimentally, there is currently no model of CA polymerization that can be practically used to analyze in vitro CA polymerization data to facilitate drug discovery. Herein, we describe an equilibrium model of CA polymerization for the kinetic analysis of in vitro assembly of CA into polymer tubes. This new mathematical model has been used to assess whether a triangular trimer of dimers rather than a hexagonal hexamer can be the basic capsomere building block of CA polymer. The model allowed us to quantify for the first time the affinity for each of the four crucial interfaces involved in the polymerization process and indicated that the trimerization of CA dimers is a relatively slow step in CA polymerization in vitro. For wild-type CA, these four interfaces include the interface between two monomers of a CA dimer (<i>K</i>_D = 6.6 μM), the interface between any two dimers within a CA trimer of dimers (<i>K</i>_D = 32 nM), and two types of interfaces between neighboring trimers of dimers, either within the same ring around the perimeter of the polymer tube (<i>K</i>_D = 438 nM) or from two adjacent rings (<i>K</i>_D = 147 nM). A comparative analysis of the interface dissociation constants between wild-type and two mutant CA proteins, cross-linked hexamer (A14C/E45C/W184A/M185A) and A14C/E45C, yielded results that are consistent with the trimer of dimers with a triangular geometry being the capsomere building block involved in CA polymer growth. This work provides additional insights into the mechanism of HIV-1 CA assembly and may prove useful in elucidating how small molecule CA binding agents may disturb this essential step in the HIV-1 life cycle

Discovery of an Oral Respiratory Syncytial Virus (RSV) Fusion Inhibitor (GS-5806) and Clinical Proof of Concept in a Human RSV Challenge Study

GS-5806 is a novel, orally bioavailable RSV fusion inhibitor discovered following a lead optimization campaign on a screening hit. The oral absorption properties were optimized by converting to the pyrazolo­[1,5-<i>a</i>]-pyrimidine heterocycle, while potency, metabolic, and physicochemical properties were optimized by introducing the <i>para</i>-chloro and aminopyrrolidine groups. A mean EC₅₀ = 0.43 nM was found toward a panel of 75 RSV A and B clinical isolates and dose-dependent antiviral efficacy in the cotton rat model of RSV infection. Oral bioavailability in preclinical species ranged from 46 to 100%, with evidence of efficient penetration into lung tissue. In healthy human volunteers experimentally infected with RSV, a potent antiviral effect was observed with a mean 4.2 log₁₀ reduction in peak viral load and a significant reduction in disease severity compared to placebo. In conclusion, a potent, once daily, oral RSV fusion inhibitor with the potential to treat RSV infection in infants and adults is reported