STA-4783

· STA-4783 Mechanism of Action

· Summary of Publicly Presented Results (March 2007)

· STA-4783 is direct, potent activator of oxidative stress – pushing cancer cells beyond an oxidative stress

· breaking point, which triggers programmed cell death, while doing minimal damage to normal cells.

· 1. STA-4783 is a small molecule that enters cancer cells and activates a pathway that rapidly and potently

· induces reactive oxygen species (ROS). Examples of ROS include superoxide and hydrogen peroxide.

· 2. An elevated level of ROS inside cells is known as “oxidative stress”. There are several well-known,

· characteristic biological signatures of oxidative stress in cells. One of these is a dramatic increase in

· stress proteins such as heat-shock protein 70 (Hsp70). Low levels of Hsp70 increase can be protective;

· high levels signal a cell whose damage is beyond repair.

· 3. That cancer cells operate at an already much higher level of oxidative stress than normal cells has been

· known for several decades. The idea that an agent which further increases oxidative stress could be a

· novel anti-cancer therapeutic category, selectively pushing cancer cells beyond a breaking point, was

· first discussed in the 1960’s and has recently grown in interest (see, e.g., references on Synta website).

· 4. The evidence that the primary STA-4783 mechanism of action is through ROS:

· − Gene transcript profiles of cells before vs. after application of STA-4783 show the characteristic

· signatures of a strong, immediate oxidative stress response

· − Direct cellular measurements of ROS, e.g. hydrogen peroxide, show strong time-dependent and

· dose-dependent induction by STA-4783

· − Anti-cancer effects of STA-4783 are eliminated by applying an inhibitor of ROS-generation

· A summary of these results can be found in the STA-4783 presentation on the Synta website.

· 5. Once ROS levels in cancer cells exceed the breaking point, cell death occurs through two processes:

· − Triggering of apoptosis due to excessive ROS. This occurs through the release of cytochrome c

· from mitochondria, activating the caspase pathway.

· − Increase in signals targeting the cancer cell for elimination. One example is the increased

· expression of Hsp70 on the cell surface, which activates NK-cell mediated killing of cancer

· cells (activation of body’s immune system).

· In preclinical studies, we have seen evidence for both types of tumor-cell killing induced by STA-4783.

· 6. The killing of cancer cells by several chemotherapeutic agents, including paclitaxel, is known to be

· sensitive to the ROS level in cells. By increasing ROS, STA-4783 enhances the ability of paclitaxel and

· these other agents to induce apoptosis in cancer cells. There may in addition be other sources of

· synergy; these are currently under investigation.

· 7. While many agents affect ROS levels indirectly and to a weaker level, key to STA-4783 efficacy and

· specificity is the magnitude of ROS induction and that this is the primary, not a secondary, effect.

· 8. Opportunity of this mechanism category:

· − Broad additive potential to first-line therapies: the new way of differentiating cancer from

· normal cells, the attractive safety profile, and the potential synergy with numerous widely-used

· agents suggest broad potential for use in front-line combination therapy

· − Broad scope of tumor applicability: the mechanism is broadly applicable to tumor types with

· elevated oxidative stress, as opposed to highly dependent on a particular oncogene or target that

· selects one particular tumor or mutation type.

· 9. Additional mechanism of action and preclinical combination study results will be presented later in 2007