I referred to this article in a query to Cancer Research UK and received the following reply today :
You asked about the use of the drug doxycycline to block the growth of cancer. This research has been mentioned in the media, because scientists at Stanford University in California have used doxycycline to "switch of the Myc gene". When it is "switched off" cancer cells do not develop and grow. Myc protein acts as a cellular conductor, sending messages that tell a cell to divide. Normal cells only make the protein when it's time to multiply. Cancer cells produce too much of this protein all the time, constantly prodding themselves to divide.
The Stanford research was conducted in the laboratory. Researchers found that the antibiotic doxycycline switched off the Myc gene in mice. But once they stopped taking the antibiotic, the Myc gene was switched back on and the cancer started to grow again.
Unfortunately, there is no evidence to suggest that doxycycline will switch off the Myc gene in humans. The treatment worked in the laboratory because the researchers were able to create a modified Myc gene. This responded to treatment with doxycycline. Scientists now need to find a drug that switches of the gene in humans. New developments like this may one day lead to treatments for cancer but this can take many years. Good research can be painstakingly slow. Unfortunately, this research is unlikely to benefit people who currently have cancer.