Development of Cytotoxic Targeted Therapies
In experimental cancer models, anti-angiogenic agents were shown to “starve” the cancer by inhibiting the development of blood vessels essential for nourishing tumor growth and maintenance and thus act cytostatically. Anti-angiogenic therapies differed from traditional cytotoxic strategies in two fundamental ways.
- First, anti-angiogenic agents target normal tissue required by the tumor for growth. In other words, anti-angiogenic agents retard the growth of new blood vessels feeding the tumor, not the cancer itself.
- Second, the anti-angiogenic strategy does not seek to eradicate the tumor, but simply to keep it from growing. This strategy reasons that tumor growth kills by eroding the anatomic and physiologic integrity of the body leading ultimately to death.
The logic of cytostatic chemotherapy is simple: stop tumor growth, stop its erosive power and a person with cancer will not succumb to it. Moreover, the collateral damage to normal tissues from conventional cytotoxic drugs is minimized or eliminated.
If cytostatic therapy can be realized, cancer treatment will no
longer be crisis management and instead will become analogous to
the treatment for other chronic diseases. However, anti-angiogenic
therapy (Genetech’s Feb. 2004, launch of Avastin), while promising,
has serious limitations. It must be administered by intravenous injection
requiring a person with cancer to regularly visit a doctor's office
or medical clinic, and it is extremely expensive. Most importantly,
the target is secondary; anti-angiogenic therapies do nothing to
the cancer itself but instead target a single component of the cancer's "infrastructure."
Recent clinical observations indicate that this focus on a single component of the cancer's infrastructure can be unproductive because cancer cells can "find" or evolve a way around this single point blockade.
The ability of cancer to “find” its way around a single point blockade has doctors now prescribing combinations and even “cocktails” of drugs to close off multiple paths or points. The problem is there are simply too many points for the cancer to evolve and continue to grow.