Stimulus-responsive viral vectors for controlled delivery of therapeutics

Abstract

Virus-based therapies have gained momentum as the next generation of treatments for a variety of serious diseases. In order to make these therapies more controllable, stimulus-responsive viral vectors capable of sensing and responding to specific environmental inputs are currently being developed. A number of viruses naturally respond to endogenous stimuli, such as pH, redox, and proteases, which are present at different concentrations in diseases and at different organ and organelle sites. Additionally, rather than relying on natural viral properties, efforts are underway to engineer viruses to respond to endogenous stimuli in new ways as well as to exogenous stimuli, such as temperature, magnetic field, and optical light. Viruses with stimulus-responsive capabilities, either nature-evolved or human-engineered, will be reviewed to capture the current state of the field. Stimulus-responsive viral vector design considerations as well as gaps in current research efforts will be identified.

Keywords: Bioactivatable; Bioresponsive; Gene delivery; Gene therapy; Viral vector.

Figure 1. Stimulus-responsive viruses can respond to endogenous (pH, redox, and proteases) and exogenous (temperature, magnetic field, and optical light) stimuli

These stimuli induce physiochemical changes in the viruses, generating responses such as increased delivery efficiency and specificity, as well as safety measures such as the halting of viral replication.

Figure 2. Proposed innate mechanisms of pH-dependent endosome membrane disruption by three different types of viruses

Some viruses are thought to hydrolyze pores in the membrane and slip through without rupturing the entire vesicle, while other viruses induce a physical disturbance in membrane shape to completely lyse the endosome. Adapted with permission from [38].

Figure 3. Examples of redox-responsive virus designs

(Left) Viruses have been engineered to internalize into cells only upon sensing reducing agents in the extracellular space. (Right) Some viruses have the natural ability to enter latency under oxidative conditions. Reducing agents can reactivate viruses from latency, resulting in viral replication.

Figure 4. Viral infection can be responsive to extracellular or intracellular proteases

The viral infection steps demonstrated thus far as being controllable with proteases (red pacmans) include receptor binding, cell entry, membrane fusion, endosomal escape, and viral replication.

Figure 5. Hyperthermia can serve as the stimulus to decrease viral replication or transgene expression

This strategy has been mainly used as a safety measure when using viruses in biomedical applications. An increase of 1–2 degrees Celsius can substantially decrease viral function.

Figure 6. Viruses can be engineered to be responsive to magnetic fields

(Left) Magnetic viruses can yield increased gene delivery efficiency and/or spatial patterning of gene delivery in vitro. (Right) Magnetic viruses can enable greater targeted delivery in vivo to desired tissues.

Figure 7. Light as a stimulus provides several potential benefits to gene delivery, including tunable intensity, temporal control, and spatial patterning

(Top) Gene expression levels can be tuned by modulating light intensity or using different wavelengths of light (e.g. far-red (FR) versus red (R) light depending on light-responsive system used). (Middle) Timing of gene expression may be controlled by adjusting when to switch on the stimulus and for how long. (Bottom) Spatial patterning of gene expression can be obtained by controlling what areas are exposed to light, potentially by using a photomask. Adapted with permission from Gomez et al [25].

Figure 8. Light-activatable drug delivery from virus capsids

The virus-like particle (VLP, blue icosahedral moiety) utilized two orthogonally performed bioconjugation reactions to improve solubility and to attach the photoactive payload. The first was a dibromomaleimide coupling (red moiety) to install a PEG chain and the second was a copper catalyzed alkyne-azide cycloaddition to install the payload. The payload contained a nitroveratryl alcohol (green moiety), which forms a radical at the carbamide under UVA irradiation. This results in heterolytic bond cleavage (squiggly line) releasing the doxorubicin (orange) cleanly from the VLP. (Figure and caption from J. Gassensmith.)