Dubbed Vacquinol-1, the compound kills GBM cancer cells via a new mechanism that the researchers describe as “catastrophic vacuolization”. The study was published in Cell in April.
I was quite impressed with the extent of the study and the amount of work that went into it. I also found its step-by-step account of the process of early drug discovery interesting. In addition, the study was built on a somewhat new approach: tumor cells have genetic mutations that make them cancerous (e.g. by enhancing their survival, proliferation or migration capacities), and current therapies target the biological pathways affected by such mutations ; the researchers reflected that tumor cells may also contain genetic mutations affecting other biological pathways that would not be directly involved in making the cells cancerous, but that would still make them different from normal cells and would therefore possibly render them vulnerable to a drug when a normal cell would not be.
Working on this hypothesis, the researchers screened a library of chemical compounds to identify which of them, if any, would be toxic to GBM cells but not to normal cells. From that initial screen and additional tests emerged Vacquinol-1. The researchers then investigated the compound’s mechanism of action, its pre-clinical drug profile, its chemical structure and its efficacy in vitro and in vivo.
1. Identification of a compound of interest: Vacquinol-1
The researchers used two cell lines derived from the tumor cells of GBM patients to screen a library of 1,364 compounds. Practically, that means adding the compounds to cultures of tumor cells and assessing whether the viability of the cells is affected in any way. This screen identified 234 molecules of interest. Since the aim was to find compounds that would be toxic to cancer cells but not to normal cells, the 234 compounds were added to cultures of mouse embryonic stem cells and human fibroblasts. This second round of screening left the researchers with a list of 63 molecules. The list was then refined by performing a number of other tests further quantifying the toxicity of the compounds, their selectivity against GBM cells versus normal cells, and their in vivo toxicity and efficacy (in zebra fish). From all those tests, the researchers selected one compound for further study ; they named it Vacquinol-1, because of its quinoline-alcohol chemical structure. After yet another battery of tests to confirm and characterize the selective pharmacological activity of Vacquinol-1 against GBM cells, the researchers went on to try and understand by which mechanism Vacquinol-1 was toxic to tumor cells.
2. How Vacquinol-1 induces tumor cell death
A series of experiments revealed that Vacquinol-1 did not induce death in GBM cells by a conventional apoptotic mechanism (apoptosis is a kind of cell death), but by the formation of vacuoles inside the cells that increased in number and size over time, eventually leading to cell rupture and death. More experiments showed that Vacquinol-1 prompted the cells to ingest large amounts of the surrounding extracellular fluid (in a process called macropinocytosis), which led to the observed formation of vacuoles inside the cells and eventually death.
3. Pre-clinical profiling of Vacquinol-1
It’s not enough for a compound to be toxic to tumor cells to be a potential drug. It has to have the right chemical properties to make it possible to administer it to patients (for example, if it’s only soluble in a solvent that is itself toxic, it’s not going to be usable) ; it should remain active once inside a living organism ; it should reach the site of the tumor (in the case of GBM, the brain) ; it should be toxic to tumor cells in concentrations that can be reached in the patient without inducing severe toxic side effects. The researchers therefore performed experiments to evaluate the potential of Vacquinol-1 as a drug: they looked at its chemical properties and behavior, at how it was metabolized by liver cells in vitro, and at its bioavailability when given orally, intravenously, or intraperitoneally to mice (i.e., looking at what happened to Vacquinol-1 in a living organism: how it was metabolized, distributed to the different tissues, and eliminated from the body).
4. Preliminary efficacy in animal models
After determining that Vacquinol-1 had suitable properties to at least be further considered for drug development, the researchers assessed its efficacy in vivo, using a glioblastoma tumor model in zebra fish and in mice. They found that Vacquinol-1 induced tumor cell death in vivo in the same way as it had done in vitro (vacuolization of the cells), and that its toxic effect was selective for GBM cells (no vacuolization was observed in parts of the brain other than the tumor sites). Preliminary experiments showed that Vacquinol-1 given orally to mice once daily for 5 days after the tumor was established in the brain decreased tumor progression and prolonged survival.
5. Advantage of Vacquinol-1 over current therapies
The main treatment currently used in GBM is chemotherapy with temozolomide and radiotherapy (in addition to surgery). Because of their mode of action, these therapies act mainly on cells that are rapidly dividing. While most tumor cells fit that description, there is a small population of GBM cells with stem/progenitor cell characteristics that are responsible for growth of new tumors and metastasis, and that are believed to be relative quiescent (non-dividing). Such cells would therefore not be affected by current therapies and would drive disease recurrence after the treatment ends. Since Vacquinol-1 kills tumor cells in a way that does not require the cells to be actively dividing, the researchers suggest that it could also kill the more quiescent stem-like tumor cells and therefore decrease the risk of tumor recurrence. In addition, since Vacquinol-1 targets a pathway different from those already targeted by current therapies, it could be efficacious against tumors resistant to conventional therapy.
6. Will Vacquinol-1 make it as a drug for the treatment of glioblastoma multiforme?
Although the data presented in this study make Vacquinol-1 look like a promising compound for the treatment of GBM, it is still a while before it may become a drug that can be used in the clinic, and many things could go wrong on the way. This study is just the first step in the multi-stage process that is drug development. For example, the researchers mention that the therapeutic window of Vacquinol-1 may not be wide enough (the therapeutic window is the range of concentrations at which the compound is toxic to cancer cells without being toxic to normal cells). The team is therefore trying to find chemical analogs of Vacquinol-1 that would be more potent and hope to be able to move to phase I clinical studies soon. In the meantime, the researchers (from the Karolinska Institute, Stockholm) have filed a patent application in the US and in Sweden.
7. Phenotypic screening vs. target-based drug discovery
As another scientist note in an accompanying comment in the same issue of Cell, the approach used in this study (an unbiased phenotypic screen) is not really a new one in drug development, but rather one that is being revisited. Once upon a time, drug discovery was mainly based on phenotypic screens (in short, throwing chemicals at cells in culture and finding those that have the desired effect on the cells). Then came the era of genetics/genomics, and drug discovery became more target-based (in short, identifying key genetic mutations leading to tumor development, and blocking the corresponding mutated proteins). However, in the case of GBM, the target-based approach has been relatively ineffective and the old method is being revisited: the idea is that it may be more effective to try and find a drug that affects the GBM cancer cell as a whole than to try and hit individual mutant proteins in tumor cells that turn out to have multiple mutations in diverse biological pathways, making the blocking of any given pathway ineffective.
Vulnerability of glioblastoma cells to catastrophic vacuolization and death induced by a small molecule. Kitambi SS, Toledo EM, Usoskin D, Wee S, Harisankar A, Svensson R, Sigmundsson K, Kalderén C, Niklasson M, Kundu S6, Aranda S, Westermark B, Uhrbom L, Andäng M, Damberg P, Nelander S, Arenas E, Artursson P, Walfridsson J, Forsberg Nilsson K, Hammarström LG, Ernfors P. Cell. 2014 Apr 10;157(2):313-28. doi: 10.1016/j.cell.2014.02.021
Kitambi SS, Toledo EM, Usoskin D, Wee S, Harisankar A, Svensson R, Sigmundsson K, Kalderén C, Niklasson M, Kundu S, Aranda S, Westermark B, Uhrbom L, Andäng M, Damberg P, Nelander S, Arenas E, Artursson P, Walfridsson J, Forsberg Nilsson K, Hammarström LG, & Ernfors P (2014). Vulnerability of glioblastoma cells to catastrophic vacuolization and death induced by a small molecule. Cell, 157 (2), 313-28 PMID: 24656405