Establishing efficacious combinations of targeted therapies for the treatment of glioma
While tumors of the brain are relatively uncommon they are one of the most lethal forms of cancer causing over 1000 deaths per year in Australia, and considerably more world-wide. The most common tumor of the brain is glioblastoma multiforme (GBM); accounting for approximately 25% of brain tumors. GBM is amongst the most lethal and difficult forms of cancer to treat with a median survival of 40-60 weeks from diagnosis, a dismally poor figure that has remained relatively unchanged for decades.
All cells contain a highly complex network of molecules, which in normal cells is tightly regulated and controlled. In cancer cells this regulation is lost through a plethora of mechanism leading to uncontrolled cell growth and increased cell survival. Targeted therapies are a new paradigm for the treatment of cancer because they specifically target key molecules in this complex network that contribute to the growth and survival of tumor cells. Therapies can target molecules such as proteins on the cell surface (i.e. receptors) that transmit growth and/or survival signals originating from outside cell to the inside of the cell. Alternatively, they may be directed protein molecules inside the cell (i.e. signalling molecules) that are responsible for converting the message from the different receptors on the cell surface into an appropriate response (e.g. move, divide etc). Targeted therapies are more specific for cancer cells and therefore have less side effects than current chemotherapy agents.
Currently there are 2 major classes of targeted therapeutics, the first of which is monoclonal antibodies (mAb's). Antibodies are a major component of the body's immune system that bind to foreign substances such as viruses. Once bound, antibodies can activate other parts of the immune system, which help destroy the foreign substance. Each antibody is unique, hence the term monoclonal, and can only bind a single target. Analogous to the situation, it is possible to generate and manufacture in culture, mAb's that bind specifically to receptors on the surface of cancer cells. Once bound the antibody can "switch-off the receptor causing a slowing of tumor growth, or activate the immune system leading to tumor damage or even destruction.
The second major class of targeted therapeutics are small chemical-based inhibitors (tyrosine kinase inhibitors or TKI's), which have the advantage of being able to target both receptors on the cell surface and signalling molecules found inside the cell. They can also be given orally while antibodies have to be delivered intravenously. Unlike antibodies they cannot activate the immune system and tend to have greater side-effects because they often bind additional targets; antibodies are exquisitely specific for their target. In some instances the promiscuous nature of TKI's is considered an advantage as one molecule may potentially block the function of several signalling molecules simultaneously. Like an antibody, once bound the TKI can "switch-off the signalling molecule causing an anti-tumor effect by reducing tumor cell growth or stimulating tumor cell death.
A small number of cancers are driven by the inappropriate activation of a single signalling pathway and therefore are susceptible to a single targeted therapy. However, most solid tumors such as GBM contain a range of inappropriately activated signalling molecules, all of which may contribute to its cancerous growth. Inappropriate activation can be caused mutation or the excessive presence of a particular signalling molecule. Thus, a single targeted therapy is unlikely be effective in the majority of GBM, especially long term, as other signalling molecules "step-in" to fill the gap created by blocking a single molecule.
The complex network of signalling molecules inside a cancer cell can be compared to the road map of a modern city. There are freeways which represent fundamental "pathways" that numerous signalling molecules feed into at different points. While different cancers, and even subtypes of the same cancer, will have their own set of freeways, I hypothesize that there will be some freeways essential to a particular cancer type such as GBM. Extending the analogy, these freeways are fed by a complex network of major roads which represent important signalling networks. These major roads will vary even between GBM patients, however some of them will occur in reasonable percentage of GBM's, and more importantly, some will be critical to the growth and/or survival of the cancer cells. Finally, there will be many small lane ways and one way streets that interact with the major roads. These smaller roads will vary dramatically between different GBM's and their function will be less critical.
I would suggest that there are currently 3 well defined freeways (i.e. fundamental signalling pathways) identified that are critical to the development of GBM. Activation of the phosphatidylinositol-3 kinase (PI3-K) pathway appears to be one of the most important signalling pathways in the development of GBM, therefore this freeway is a key one to target. Recent evidence suggests that activation of STAT3 pathway also appears to have a fundamental role in GBM progression and can be considered another freeway. Finally, inactivation of the p53 pathway represents another freeway important in GBM but is not been considered in this current proposal. All 3 freeways can be activated by a variety of mechanisms. A number of other freeways will probably become apparent in GBM over the next few years, but their number should be restricted.
This analogy of freeways and roads leads to a number of important questions. How do you efficiently block a freeway in a GBM cell? How many freeways do you have to block to prevent the growth/survival of GBM tumors? Do you have to concurrently block some of the major roads to efficiently inhibit tumor growth and survival? The overall aim of this grant is to address these key issues with respect to GBM.
Freeways could be blocked by targeting their entrances (i.e. receptors on the cell surface) or blocking key signalling molecules further along the freeway. The strategy my laboratory has chosen is to block freeways at their entrance by targeting receptors with antibodies. We have chosen the antibody approach for the reasons discussed above; exquisitely specific and low toxicity. For example, we have developed an antibody that blocks the function of a receptor known as the epidermal growth factor receptor (EGFR). Activation of this receptor in GBM leads to the subsequent activation of the PI3-K freeway. We believe that targeting signalling pathways at their beginning is the most efficient method of closing them down. Shutting down of freeways half way along allows some of the traffic to escape into major roads and is therefore a less efficient approach. The difficulty with our approach is that there are multiple entrances to each freeway. Therefore, you need to develop multiple blocking antibodies to the different receptors (i.e. entrances) involved in activating a particular freeway.
We already have two blocking antibodies to one receptor (i.e. EGFR) that should help close down the PI3-K freeway. We also have access to an antibody that blocks a molecule known as c-met; another receptor that activates the PI3-K freeway. We plan to determine if the strategy of targeting both these receptors is sufficient to inhibit the PI3-K freeway and if the inhibition of this one freeway is adequate to robustly prevent the growth of multiple GBM models. We are currently developing antibodies capable of blocking molecules that responsible for activating the STAT3 freeway. We will then combine antibodies to both these freeways to determine what effect blocking two signalling freeways has on GBM growth and survival.
Recent work suggests that a molecule known as src forms a major road in GBM signalling. Because src is inside the cell it cannot be targeted with antibodies. However, there is a TKI (Dasatinib) that inhibits the function of src, currently being tested in a variety of cancer patients. We will use Dasatinib to determine if blocking a major road in combination with freeways leads to enhanced anti-tumor response.
Targeted therapies are an important new class of evolving agents for the treatment of cancer including GBM. It is clear that a single target therapy will not be sufficient to cure GBM. The studies in this proposal are designed to understand how many targeted therapies are required to efficiently inhibit tumor growth, and more importantly, what type of combinations work the most efficiently. Long term it is hoped that these studies will lead to more effective treatment, and even cure, of GBM; an aggressive, almost universally fatal, form of cancer.