Structural, biophysical, and virological mechanistic characterization of HIV-1 capsid-targeting antivirals

This study demonstrates that specific chemical modifications at the R1 and R3 positions of the HIV-1 capsid inhibitor PF74 yield novel antivirals with enhanced potency, stability, and binding affinity, offering valuable insights for designing future clinical candidates.

The Gist

Imagine HIV as a master thief trying to break into a bank (your body's cells). To succeed, the thief needs a special, armored getaway car called the Capsid. This car protects the thief's stolen goods (the viral DNA) while they drive through the city (your bloodstream) to reach the bank vault (the cell nucleus).

For a long time, scientists have tried to build a "trap" that either breaks the car's wheels or jams the engine so the thief can't get to the vault. One famous trap, called PF74, was good at this, but it had two big problems: it didn't stick very well (it fell off the car easily), and it broke down too quickly in the body.

Recently, a new, super-strong trap called Lenacapavir was approved. It's amazing, but it's also huge and complex, making it hard for scientists to tweak and improve.

The Goal of This Study
The scientists in this paper asked a simple question: "Can we take the old, smaller trap (PF74) and fix its weak spots to make it work almost as well as the new super-trap, but easier to build?"

They took the PF74 design and started swapping out tiny parts of its chemical structure, like changing the tires or the engine bolts. They tested over 300 different versions and found 10 "champion" candidates. The best one, named ZW-1261, is the star of the show.

How They Tested the New Traps

1. The "Sticky" Test (Binding):
   Imagine trying to stick a magnet to a fridge. The old PF74 magnet was a bit weak; it would slide off easily. The new ZW-1261 magnet? It's like a super-magnet. The scientists found that by adding a tiny "hook" (a chemical group called a 5-hydroxy group) to the trap, it latched onto the HIV car much tighter and stayed there longer.

2. The "Car Ride" Test (Stability):
   Normally, the HIV car is supposed to fall apart at the right moment to let the thief out.

   • The Problem: Sometimes the car falls apart too early (the thief gets caught). Sometimes it stays together too long (the thief gets stuck in the car and can't enter the vault).
   • The Solution: The new traps are smart. At low doses, they make the car wobble a bit (which stops the thief). But at higher doses, they actually harden the car's armor so much that it becomes a solid, unbreakable block. The thief is trapped inside the car, unable to get out to steal the DNA. It's like turning the getaway car into a steel safe.

3. The "Bank Vault" Test (Nuclear Import):
   The thief needs to get the car to the bank vault to deposit the stolen goods. The scientists watched what happened when the virus tried to enter the nucleus.

   • With the old trap, the car got stuck at the door.
   • With the new ZW-1261 trap, something interesting happened: at high doses, the car actually got into the vault, but because the armor was so hard, the thief couldn't get out to do the job. It's like the thief drove the car into the vault, but the doors were welded shut from the inside.

4. The "Crowd Control" Test (CPSF6):
   Inside the bank, there are security guards (host proteins) that usually help the thief. The virus tricks these guards into forming a crowd to help it. The new traps are like a master of ceremonies who not only stops the crowd from forming but also breaks up a crowd that has already gathered. This is a feature the new super-trap (Lenacapavir) doesn't have, making the new PF74 version unique.

The Secret Ingredient: The "FG" Pocket
The scientists discovered why the new traps work so well. The HIV car has a specific keyhole called the "FG pocket."

• The old trap (PF74) just sat in the keyhole.
• The new trap (ZW-1261) reached out with a tiny arm and grabbed onto a handle on the other side of the keyhole. This double-grip makes it incredibly hard to pull the trap off.

Why This Matters
This study is like finding a way to build a better, cheaper, and more customizable version of a high-tech security system.

• It works on different types of HIV: The new traps work on many different "flavors" of the virus, not just the most common one.
• It's a blueprint: By understanding exactly how these tiny chemical changes make the trap stick better, scientists can now design even better drugs in the future.

In Summary
The scientists took an old, slightly flawed drug design, added a few tiny "super-glue" features, and created a new generation of HIV traps. These new traps stick tighter, break the virus's plans in clever new ways, and offer a promising path toward even better treatments for people living with HIV.

Technical Summary

1. Problem Statement

The HIV-1 capsid is a critical viral component essential for replication, acting as a protective shell that facilitates nuclear entry and integration. While the capsid is a validated drug target (exemplified by the recent approval of the long-acting drug lenacapavir), earlier candidates like PF74 were discontinued due to poor metabolic stability and insufficient potency. Lenacapavir, while highly potent, is a large, complex molecule that makes structure-guided optimization difficult. There is a need for smaller, chemically tractable capsid inhibitors with improved potency, stability, and bioavailability to serve as next-generation therapeutics or prophylactics.

2. Methodology

The authors employed a multi-disciplinary approach combining virology, biophysics, and structural biology to characterize a series of PF74 analogs (specifically focusing on 10 lead compounds derived from modifications at the R1 and R3 positions of the PF74 scaffold).

• Virological Assays:

  • Antiviral Potency: Tested EC50 values against various HIV-1 subtypes (B, C, AE, AG) and HIV-2 using TZM-GFP reporter cells.
  • Stage-Specific Inhibition: Differentiated between early (entry to integration) and late (assembly/maturation) replication stages using pseudotyped viruses and single-cycle infection assays.
  • Capsid Stability (CypA-DsRed): Used time-resolved confocal imaging to monitor the disassembly of native HIV-1 cores in permeabilized virions.
  • Fate-of-the-Capsid: A fluorescent Western blot assay to distinguish between soluble capsid monomers (destabilized) and pelletable cores (stabilized).
  • Nuclear Import: Assessed the translocation of viral capsids into the nucleus via fractionation and Western blotting.
  • CPSF6 Aggregation: Immunofluorescence microscopy to observe the translocation and disassembly of CPSF6 aggregates in nuclear speckles.

• Biophysical Characterization:

  • Biolayer Interferometry (BLI): Measured binding kinetics ($K_D$, $k_{on}$, $k_{off}$) of lead compounds to crosslinked wild-type (WT) HIV-1 CA hexamers.

• Structural Biology:

  • X-ray Crystallography: Determined high-resolution structures of full-length WT HIV-1 CA in complex with three lead compounds (ZW-1261, ZW-1514, and ZW-1527) to visualize interactions at the "FG" binding pocket.

3. Key Contributions

• Identification of Potent PF74 Analogs: The study validates that specific chemical modifications at the R1 and R3 positions of PF74 can yield compounds with significantly improved antiviral potency and metabolic stability compared to the parent compound.
• Discovery of a Dual-Phase Mechanism: The lead compound ZW-1261 exhibits a concentration-dependent dual mechanism: it destabilizes the capsid at low concentrations (similar to PF74) but stabilizes it at higher concentrations (similar to lenacapavir).
• Structural Insights into Binding: The study provides the first crystal structures of these specific analogs, revealing novel interactions with the C-terminal domain (CTD) of neighboring CA monomers that were not possible with PF74.
• Broad-Spectrum Activity: Demonstrated that the optimized compounds retain high potency against diverse HIV-1 subtypes and HIV-2, addressing a limitation of some existing capsid inhibitors.

4. Key Results

• Antiviral Potency:

  • ZW-1261 emerged as the most potent lead, with an EC50 of 0.022 µM against HIV-1 subtype B.
  • It showed 10–28-fold greater potency than PF74 across HIV-1 subtypes (B, C, AE, AG) and ~7-fold greater potency against HIV-2.
  • The compounds effectively inhibited both early and late stages of the viral life cycle.

• Capsid Stability & Nuclear Import:

  • Stabilization: In CypA-DsRed assays, compounds like ZW-1261 maintained capsid integrity for up to 1 hour, whereas the control (DMSO) showed rapid disassembly.
  • Dual-Phase Effect (ZW-1261):
    • Low Concentration (0.5–1 µM): Induced capsid destabilization (soluble CA).
    • High Concentration (2.5–5 µM): Induced capsid hyper-stabilization (pelletable cores).
  • Nuclear Import Correlation: Consistent with the stability data, low concentrations of ZW-1261 reduced nuclear entry, while high concentrations increased nuclear entry (likely due to hyper-stabilization preventing premature uncoating but allowing transport to the nucleus).

• CPSF6 Dynamics:

  • ZW-1261 blocked the formation of CPSF6 aggregates in nuclear speckles during infection.
  • Crucially, when added after infection (post-aggregation), ZW-1261 actively disassembled pre-formed CPSF6 complexes, mimicking PF74 but differing from lenacapavir precursor GS-CA1 (which cannot disassemble pre-formed complexes).

• Binding Kinetics (BLI):

  • ZW-1261 bound to CA hexamers with a $K_D$ of 105 nM, significantly tighter than PF74 ($365$ nM).
  • This improvement was driven by a slower dissociation rate ($k_{off}$), indicating a more stable complex.

• Structural Mechanisms (X-ray Crystallography):

  • ZW-1261: The R3 5-hydroxy group forms a hydrogen bond with K182 and a water-mediated bridge to Q179 on the neighboring CA monomer's CTD. This interaction with the CTD is absent in PF74 and explains the increased stability and potency.
  • ZW-1514: An N1-ethyl group creates a hydrophobic interaction with the main chain of Q67, shifting the indole ring slightly.
  • ZW-1527: A bulky N1-isopropyl group causes a steric clash with Q63, forcing the entire R3 indole ring to flip ~90°. This reorientation points the isopropyl group toward the CTD, suggesting a new vector for future drug design.

5. Significance

This research provides a critical roadmap for the rational design of next-generation HIV-1 capsid inhibitors. By optimizing the PF74 scaffold, the authors have created compounds that:

1. Overcome PF74's limitations: They offer superior potency and stability.
2. Reveal new binding modes: The structural data highlights the importance of interacting with the CTD of neighboring monomers (specifically residues K182 and Q179) to achieve high-affinity binding.
3. Offer mechanistic flexibility: The discovery of a concentration-dependent dual mechanism (destabilization vs. stabilization) suggests these compounds could be tuned for different therapeutic windows.
4. Broaden applicability: The efficacy against non-B subtypes and HIV-2 makes these candidates viable for global treatment and pre-exposure prophylaxis (PrEP) strategies.

These findings bridge the gap between the small-molecule tractability of PF74 and the high potency of lenacapavir, offering promising candidates for clinical development.