Table 2.
Targeting systems
| Approach | Principle | Advantages | Disadvantages | Examples |
|---|---|---|---|---|
| Pseudotyping | ||||
| Approach overview | Use of a viral attachment protein from a different virus strain or family | Technically easy when the biology is supportive or compatible | Limited availability of pseudotypes that fit the desired target cell; possible reduction of transfection efficiency (retrovirus) | Ad (in vitro)18 (in vivo)120; AAV (in vivo)138; Lentivirus (in vivo)30 |
| Adaptor systems | ||||
| Approach overview | Use of a molecule that binds both the vector and target-cell receptor to facilitate transduction | Limited knowledge of capsid structure is sufficient; flexibility; no/minimal change in vector structure; easy preclinical testing of different targeting ligands | Two-component system; stoichiometry of adaptor to vector might vary between batches; two molecules must be produced separately; issues with regulatory agencies; adaptor might dissociate in vivo; clinical applicability can be limited | |
| Receptor–ligand | A native viral receptor is fused to the targeting ligand | Easy preclinical testing | Correct folding of each new receptor–ligand pair must be determined | Ad (in vivo)22; retrovirus (in vitro)24 |
| Bispecific antibody | Two antibodies are coupled, with the resulting molecule having specificity for the vector and the target | Using existing reagents, the antibody is easy to make; screening for different targets is readily possible | Binding affinity of the targeting complex to the vector can vary | Ad (in vivo)139; AAV (in vitro)145; coronavirus (in vitro)148 |
| Chemical linkage | Targeting moiety is bound to the vector by chemical means | A covalent bond is formed with the targeting complex, thus no adaptor dissociation from the vector | Technically more demanding than other adaptor systems (but nevertheless scaleable for clinical applications) | Ad (in vitro)29,31 |
| Avidin–biotin | Biotin is coupled to the vector and then bound to the avidin–ligand complex | High-affinity binding of the targeting complex to the vector; allows easy vector purification | Some risk for toxicity in clinical applications (biotin from the circulation could be complexed) | Ad (in vitro)38; AAV (in vitro)39; retrovirus (in vitro)34,35 |
| Antibody | Antibody binds to a genetically incorporated Ig-binding domain of the vector | Vast pool of available antibodies for targeting; easy coupling | Antibodies from the circulation could interfere with targeting | Ad (in vitro)44; AAV (in vitro)43; retrovirus (in vitro)45 |
| Genetic systems | ||||
| Approach overview | A polypeptide is incorporated into the vector by genetic means to facilitate transduction | Single-component system; favoured for clinical application; ease of high-titre vector production | Technically more challenging than adaptor approaches; can be detrimental to vector or ligand structure | |
| Serotype switching | Use of a different serotype from within the same virus family | Biological compatibility makes it feasible | Limited availability of serotypes; the precise cellular receptor is frequently unknown | AAV (in vivo)140; Ad (in vivo)149 |
| Small targeting motifs | Small peptides are inserted into the capsid or viral attachment protein | Minimal disturbance of vector structure | Broadens tropism without ablating native tropism; limited number of available motifs, thus not applicable for all cell types | Ad (in vivo)76; AAV (in vitro)70; retrovirus (in vitro)73; phage–AAV (in vivo)20 |
| Single-chain antibody | A single-chain antibody is incorporated into the viral attachment protein | Vast pool of tested antibodies available for targeting | Antibody might need adaptation to a biosynthetic pathway of virus protein production (Ad) | Ad (in vivo)19; AAV (in vitro)47; retrovirus (in vivo)48 |
| Mosaic viral attachment proteins | Two viral attachment proteins with different properties are combined, allowing targeting, production or imaging in parallel | True multifunctionality in a virion can be achieved | Desired stoichiometry can be difficult to achieve | Ad (in vitro)93; AAV (in vitro)70 |
| Ablation of native tropism | Mutation of the amino acids responsible for native tropism | Can be combined with other techniques | Can confound production in packaging cell line | Ad (in vivo)88; AAV (in vivo)87; Lentivirus (in vivo)13 |
| AAV, adeno-associated virus; Ad, adenovirus. | ||||