Crucial detail to note right off the bat is one of size, a critical factor influencing tumor penetration by therapeutic molecules. The question references a 2001 paper (1) that discusses the tumor penetration capability of not a whole antibody but rather a piece of it, not a Monoclonal antibody – Wikipedia but Single-chain variable fragment – Wikipedia, which in this case was specific for HER2/neu – Wikipedia, an Oncogene – Wikipedia expressed by many cancers especially breast cancer.
A fraction of an antibody’s size, ScFv is an artificial construct consisting of just the heavy and light chains of an antibody joined together with a chemical linker, and is thus very different in structure and size from a mAb (see below right from 2).
Difficulty of the much smaller and structurally simpler ScFV to penetrate a tumor only implies it may be all the more difficult for much larger mAbs to do so, considering a wide variety of cells in the body express a wide variety of Fc receptor – Wikipedia that can non-specifically bind the Fc portion of an antibody (see below left from 3), which doesn’t apply to ScFv since they lack Fc receptors.
In order to be effective therapeutic agents once injected into the body, antibodies that target tumor antigens need to
- Localize, preferably selectively, to the tumor. Antibodies that don’t do so wouldn’t find or bind their targets and would instead be summarily flushed out of the body as waste.
- Translocate (extravasate) out of local blood vessels and capillaries into the solid tumor tissue.
- Penetrate the solid tumor tissue by simple diffusion to bind as many tumor cells as possible.
This answer explains
- How affinity influences tumor penetration ability and effectiveness of both high and low affinity antibodies.
- How penetration of tumor antigen-specific antibodies turns on not just their affinity but also dose and catabolism, tumor size, vascularization (blood vessel) degree and kind as well as antigen expression dynamics, that is not just its abundance but also how fast it’s internalized and recycled back up.
How Affinity Influences Tumor Penetration Ability & Effectiveness Of Both High & Low Affinity Antibodies
Antibodies bind antigens, which can be any type of molecule either expressed or secreted by a cell. Antibodies used to target tumors bind antigens expressed on their cell surface. Antibody affinity refers to how the paratope (part of antibody that binds antigen) and epitope (part of antigen that binds antibody) fit each other (their binding strength), and is mathematically expressed as an association constant that is calculated under equilibrium conditions.
An antibody’s affinity influences its tumor penetration ability as a result of the trade-off between rate at which antibody dissociates from the cell-surface antigen it bound versus how quickly the bound antibody gets internalized by the cell. Faster an antibody dissociates, higher its likelihood of remaining free to diffuse deeper into the tumor tissue. However, this trade-off is a double-edged sword in that it disadvantages both high and low affinity antibodies, though obviously in different ways.
High affinity Antibody: Association rates of high and low affinity antibodies are typically similar. In contrast, dissociation rates of high affinity antibodies are much slower, especially so with membrane (cell)-bound rather than soluble (free-floating) antigen, which is the situation with tumor antigens.
- High affinity antibodies could be further handicapped if their dissociation rate is slower than their internalization by cells, which is indeed often the case (4).
- In practical terms, high affinity antibodies tend to glom onto and get internalized by tumor cells located close to the capillaries out of which those antibodies extravasated while low affinity antibodies targeting the same tumor antigen(s) are more likely to dissociate before such tumor cells can internalize them, leaving them free to diffuse deeper into the tumor tissue.
Low affinity Antibody: Though low affinity antibodies may be more likely to penetrate deeper into tumor tissues, at least two factors work against them,
- Penetrating deeper may not be therapeutically helpful if antibody amounts don’t saturate the antigen since their effectiveness depends on how many of them need to bind antigens on each tumor cell to drive effector functions such as Antibody-dependent cell-mediated cytotoxicity – Wikipedia that will kill the cell.
- Their faster dissociation rates could also work against low affinity antibodies since routine fluid movement would simply flush them out of the tumor before they bound tumor cells in sufficient numbers to drive a meaningful therapeutic effect.
Antibody Dose & Catabolism, Tumor Size & Vascularization, & Antigen Expression Dynamics Influence Antibody Penetration Of Tumor-Specific Antibodies
Tumor outer surface and blood vessels offer entry points into solid tumors, which usually lack lymphatics thereby closing off a third entry point. As long as free antibodies remain in circulation, they will enter tumor tissue to bind their antigen.
- Entry from outside: Clearly higher the antibody dose, further the chance to penetrate deeper into tumor tissue (see below left from 5). However, higher antibody dose brings along increased risk for toxicity to healthy tissues. Arriving at optimal antibody dosing thus means walking a tightrope between optimal benefit and potential harm.
- Clearance within: As antibody diffuses into a tumor, the longer it takes for bound antibody to be internalized, deeper it can penetrate into tumor (see below right from 5) since newer antibodies entering from outside need to move deeper into tumor tissue in order to find free antigen available for binding.
Obviously tumors vary greatly in size and vascularization. Early in development or metastasis, tumors are small and lacking in blood vessels while developed tumors are larger as well as better vascularized. These differences in kind change requirements for effective antibody penetration into tumors (see below from 5).
- Outer surface area is a more important feature for an early tumor or micrometastasis since that will be the only site available for antibody uptake while blood vessel surface offers an additional entry point into better established, vascularized tumors.
- Antibody uptake by vascularized tumor turns on how well blood vessels perfuse a given tumor, and rates at which antibody crosses the capillary wall and tumor tissue. On the one hand, tumor blood vessels tend to be poorly constructed and hence ‘leaky’, which should favor efflux of antibodies out into tumor tissue. OTOH, since tumors tend to lack lymphatics, their fluid dynamics make them prone to higher interstitial fluid pressure compared to those in normal tissues, which tends to reduce efflux from vessels into tumor tissue. As a result, efflux rates into large, well-vascularized tumors can be slower than normal.
Tumor Antigen Features that influence Antibody Penetration: High expression, low turnover, and high recycling are antigen features favoring antibody effectiveness since higher the antigen expression, higher the likelihood of effective antibody binding and higher the possibility of anti-tumor effect. A tumor antigen expressed at high levels on the cell surface but internalized rapidly and recycled slowly makes for a more difficult target compared to a similarly expressed one that’s either internalized more slowly or recycled much faster (6). For example, Her2 is an antigen in breast and other solid tumors that makes for an attractive antibody target given its high expression of ~2*10^6 receptors per cell and high recycling (7).
Bibliography
1. Adams, Gregory P., et al. “High affinity restricts the localization and tumor penetration of single-chain fv antibody molecules.” Cancer research 61.12 (2001): 4750-4755. http://cancerres.aacrjournals.or…
2. Saeed, Abdullah FUH, et al. “Antibody engineering for pursuing a healthier future.” Frontiers in microbiology 8 (2017): 495. Antibody Engineering for Pursuing a Healthier Future
3. Shi, Jianguo, Richard S. Mcintosh, and Richard J. Pleass. “Antibody-and Fc-receptor-based therapeutics for malaria.” Clinical science 110.1 (2006): 11-19. https://www.researchgate.net/pro…
4. Kyriakos, Raymond J., et al. “The fate of antibodies bound to the surface of tumor cells in vitro.” Cancer research 52.4 (1992): 835-842. http://cancerres.aacrjournals.or…
5. Thurber, Greg M., Michael M. Schmidt, and K. Dane Wittrup. “Antibody tumor penetration: transport opposed by systemic and antigen-mediated clearance.” Advanced drug delivery reviews 60.12 (2008): 1421-1434. https://www.ncbi.nlm.nih.gov/pmc…
6. Vasalou, Christina, Gabriel Helmlinger, and Bruce Gomes. “A mechanistic tumor penetration model to guide antibody drug conjugate design.” PloS one 10.3 (2015): e0118977. http://journals.plos.org/plosone…
7. Hendriks, Bart S., H. Steven Wiley, and Douglas Lauffenburger. “HER2-mediated effects on EGFR endosomal sorting: analysis of biophysical mechanisms.” Biophysical journal 85.4 (2003): 2732-2745. http://www.cell.com/biophysj/pdf…