In a recent article posted by the Fred Hutchinson Cancer Research Center, a novel therapy is shown to improve the length of survival of patients with high-risk forms of glioblastoma. After looking at the primary research article, I noticed the researchers used many of the principals associated with personalized and targeted therpies we've recently discussed in class. But first, a little background information on glioblastoma.
According to Maciej Mrugala, the study's lead neuro oncologist, "glioblastoma remains one of the most devastating cancers with a median survival of only 12-15 months" (1). The current treatment strategy is to surgically dissect the tumor if possible, and then to treat the patient with a combination of radiation and chemotherapy. While the treatments are effective at killing the tumor, the side effects are often so severe that dosages must be reduced or treatments completely suspended. This results in less effective treatment and the faster death of the patient. However, it looks as if Adair and colleagues have developed a treatment that circumvents the common problem of toxicity and suppression of blood stem cells (HSCs).
In the study, researchers selected three patients with a particularly lethal case of glioblastoma. Patients' tumors were biopsied and the genomes were sequenced in order to discover the methylation status of the MGMT gene using the expression profile technique we talked about in class on Monday. The MGMT gene of all three patients was turned on due to a lack of methylation. The MGMT gene produces a DNA repair protein that counter-acts the effects of many chemotherapeutic agents, most commonly temozolomide. This makes treatment much less effective. In order to block the chemoprotective effects of the MGMT protein, another drug is added to the treatment plan: benzylguanine. Benzylguanine blocks the MGMT gene, thus making the cancer cells responsive to temozolomide. In the process, though, the toxic effects of the drugs causes the hematopoetic stem cells in the bone marrow to stop proliferating. This results in decreased blood cells, increased infection rates, and a host of associated symptoms.
Adair and colleagues genetically modified each patients' natural hematopoetic stem cells by removing them and letting them bind to P140K. P140K is a modified version of the MGMT protein that does prevent the HSCs from differentiating into blood cells when the body needs them, and still has the DNA repairing properties as MGMT (1). And because it binds to the same place that MGMT protein binds, the HSCs will be protected from benzylquanine. Once the patients' HSCs were bound to P140K, they were grafted back into the bone marrow of each patient. Researchers then administered the clinically accepted doses of telozomide and benzylguanine. They then stimulated the production of new blood cells by injecting the patients with CD34+ colony-stimulating factor and then took blood samples to run tests and measure results.
The graphs below represent each patients' blood cell count up to 600 days after the genetically-modified HSCs were transplanted back into their bone marrow. The data points represented by the black squares are the number of P140K-modified HSCs. (The circles and diamonds are normal RBSc and WBCs). The graphs on the right were measured using the virus probe technique we discussed in class. The viral probe bound to all blood cells that expressed the P140K gene sequence (2).
Researchers were pleasantly surprised to find that, even after multiple rounds of chemotherapy, the P140K-modified HSCs survived. Furthermore, when the HSCs prolifereated, they were able to maintain the P140K modification, meaning that patients would not have to have fresh P140K-modified HSC transplants before every single chemotherapy treatment. This is promising because it makes the treatment even more accessible to those in need of it.
The diagram to the right demonstrates that the repopulating HSCs do in fact contain the P140K chemoprotective modification. The top figure represent colonies that contain the 140K modification, and were taken throughout the experiment. The bar graph shows the percentage of colonies positive for the viral probe that binds the P140K gene domain. The numbers above the bars indicate the number of telozolomide and benyzlguanine chemotherapy cycles each patient received before the colony-forming assay was taken. It is clear that even after as many as 8 rounds of chemotherapy, HSCs present in the patients' bloodstream still contained the chemoprotective P140K modification (2).
In terms of tumor progression, researchers found that two of the three patients showed disease stabilization. (See brain MRI images below). MRI images A, B, and C are of one patient. The arrow in image A shows the initial glioblastoma at the time of diagnosis. (A biopsy was done to determine the tumor is, in fact, a glioblastoma). Image B shows the progression of the tumor 6 months after diagnosis and right before the tumor was surgically removed. Radiation and chemotherapy was not effective for this individual. Image C was taken 6 months after the tumor was ressected. In that time, 4 cycles of chemotherapy were administered after P140K-modifications were made to the individual. Although some tumor cells are re-forming, they are growing at a slower rate, presumably because, with the P140K modification, the patient was able to tolerate consistent and effective doses of radiation and chemotherapy.
Images D, E, F, and G are MRI scans of another patient. Figure D shows the size of the tumor at diganosis, while image E shows tumor progression after the first round of radiation. Image F was taken 6 months after diagnosis and after the individual had received two rounds of radiation and chemotherapy after receiving P140K-modification therapy. Eleven months after diagnosis and 4 rounds of chemotherapy with chemoprotected HSCs, the patient's tumor has not continued to grow (2).
With the P140K gene modification therapy, the average survival time of the three patients was 22 months, nearly twice the national average, which remains as low as 12 months (1). Although two of the patients have since died, one man is still alive today, suriving 34 months after the treatment.
While this method of protecting hematopoetic stem cells from chemotherapeutic drugs extends the life of individuals diagnosed with glioblastoma, more needs to be done. Two of the three patients still died from the disease. P140K-modified HSCs is a great example of a targeted drug therapy in that it protects HSCs from being suppressed while still allowing the chemotherapeutic drugs to damage cancer cell DNA. But because two of the individuals died, even after surgical removal of the tumor and many rounds of radiation and chemotherapy, scientists still have yet to find a truly effective course of treatment for this disease. As we discussed in class, it seems probable that researchers are lacking imperative information about the signal transduction pathway associated with the cellular oncogenes of this disease. Hopefully this new therapy will allow glioblastoma patients and researchers more time to understand the mechanisms behind this disease.
