Research Article: Mechanistic Studies and Modeling Reveal the Origin of Differential Inhibition of Gag Polymorphic Viruses by HIV-1 Maturation Inhibitors

Date Published: November 28, 2016

Publisher: Public Library of Science

Author(s): Zeyu Lin, Joseph Cantone, Hao Lu, Beata Nowicka-Sans, Tricia Protack, Tian Yuan, Hong Yang, Zheng Liu, Dieter Drexler, Alicia Regueiro-Ren, Nicholas A. Meanwell, Mark Cockett, Mark Krystal, Max Lataillade, Ira B. Dicker, Ronald Swanstrom.


HIV-1 maturation inhibitors (MIs) disrupt the final step in the HIV-1 protease-mediated cleavage of the Gag polyprotein between capsid p24 capsid (CA) and spacer peptide 1 (SP1), leading to the production of infectious virus. BMS-955176 is a second generation MI with improved antiviral activity toward polymorphic Gag variants compared to a first generation MI bevirimat (BVM). The underlying mechanistic reasons for the differences in polymorphic coverage were studied using antiviral assays, an LC/MS assay that quantitatively characterizes CA/SP1 cleavage kinetics of virus like particles (VLPs) and a radiolabel binding assay to determine VLP/MI affinities and dissociation kinetics. Antiviral assay data indicates that BVM does not achieve 100% inhibition of certain polymorphs, even at saturating concentrations. This results in the breakthrough of infectious virus (partial antagonism) regardless of BVM concentration. Reduced maximal percent inhibition (MPI) values for BVM correlated with elevated EC50 values, while rates of HIV-1 protease cleavage at CA/SP1 correlated inversely with the ability of BVM to inhibit HIV-1 Gag polymorphic viruses: genotypes with more rapid CA/SP1 cleavage kinetics were less sensitive to BVM. In vitro inhibition of wild type VLP CA/SP1 cleavage by BVM was not maintained at longer cleavage times. BMS-955176 exhibited greatly improved MPI against polymorphic Gag viruses, binds to Gag polymorphs with higher affinity/longer dissociation half-lives and exhibits greater time-independent inhibition of CA/SP1 cleavage compared to BVM. Virological (MPI) and biochemical (CA/SP1 cleavage rates, MI-specific Gag affinities) data were used to create an integrated semi-quantitative model that quantifies CA/SP1 cleavage rates as a function of both MI and Gag polymorph. The model outputs are in accord with in vitro antiviral observations and correlate with observed in vivo MI efficacies. Overall, these findings may be useful to further understand antiviral profiles and clinical responses of MIs at a basic level, potentially facilitating further improvements to MI potency and coverage.

Partial Text

Currently there are more than 1.2 million individuals (age 13 years older) in the United States (CDC data)[1] and more than 35 million worldwide infected with HIV, with 39 million people already having died from the disease and 2.3 million new cases reported in 2013.[2] There are presently >35 FDA-approved HIV therapies or combinations of agents which can be categorized into different classes: NRTIs, NNTRIs, PIs, integrase and entry inhibitors, (the latter includes attachment and fusion inhibitors, along with CCR5 antagonists).[3, 4] However, co-morbidities associated with long-term use of antiretrovirals (ARVs)[4–6] and the continued development of resistance remains a problem. [7, 8] Thus, there is a continuing need for new HIV-1 drugs which lack cross-resistance to existing classes and have excellent long term safety profiles.

An early MI failed in the clinic due to inability to inhibit ~50% of viruses containing polymorphic variation in Gag near the site of MI action. The 2nd generation MI, BMS-955176, is active toward these viruses. In this study we sought to understand the mechanistic origin for the improved antiviral activity of BMS-955176, and to model this behavior as a function of Gag polymorph cleavage rates, MI affinity and MI concentration, with consideration as to how this information relates structurally to MI binding. Such an approach may have utility in interpreting pre-clinical antiviral results and clinical data on MI action, and may also be helpful in the discovery of MIs with further improvements to potency and spectrum.




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