Inhibitors of Myc: Novel Drugs for Breast Cancer

Institution: Scripps Research Institute
Investigator(s): Peter Vogt, Ph.D. -
Award Cycle: 2003 (Cycle IX) Grant #: 9WB-0022 Award: $277,887
Award Type: STEP Award
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure

Initial Award Abstract (2003)
Breast cancer cells are characterized by changes in the levels and activities of growth regulatory proteins. One of these proteins is Myc which shows enhanced function in about 50 percent of breast cancers. Increased Myc activity is correlated with the more aggressive forms of breast cancer that invade neighboring tissues and grow very rapidly. In model animal systems, elevated Myc induces various forms of cancer by deregulating specific genes. In human breast cancer, Myc has a similar deregulating effect, affecting the cellular response to hormones and altering the activity of proteins that control the cycle of cellular replication and division. The accumulated evidence on Myc activity in breast cancer indicates that Myc plays a major role both in the causation of the tumor and in the progression to an aggressive cancer. Myc is therefore an important target for therapy. Drugs that control the activity of Myc could be of great value in breast cancer therapy and in prevention.

Myc is an important genetic regulator; it changes the activity of numerous subordinate genes. However, Myc can perform this function only with the aid of the second protein, termed Max. Myc must bind to Max, and it is this pairing of these two proteins that can activate other genes. The central hypothesis of this project is that small molecules can be identified that interfere with Myc-Max binding and thus control Myc activity. Conventional wisdom says that such protein-protein interactions are too powerful to be controlled by small molecules. All major pharmaceutical companies have therefore stayed away from this problem, considering it as too risky. However, we have produced and purified modified versions of the Myc and Max proteins. In our system, when these proteins bind to each other, they generate a light signal that can be easily detected. If Myc-Max binding is inhibited, the light signal disappears. This simple test allows us to screen large numbers of newly synthesized chemical compounds containing tens of thousands of individual drug-like compounds for inhibitors of Myc-Max binding. For these screens, we have available a robot that can perform several hundred of these tests simultaneously. A large collection of novel chemical compounds for these tests will be produced at The Scripps Research Institute by the laboratories of Drs. Dale Boger and Kim Janda. Both researchers are leaders in the new technology that permits simultaneous synthesis of large numbers of novel compounds.

Our eventual aim is to identify small molecules that inhibit Myc-Max interactions, and could be developed into drugs that have a major impact on breast cancer. We believe that these novel drugs, aimed at a neglected but important target, could be of significant benefit to breast cancer patients.

Final Report (2006)
The Myc protein is a transcriptional (i.e. gene activity) regulator that shows gain of function in many human cancers. It is an important oncoprotein in breast cancer, responsible for enhanced cell growth and for resistance to anti-cancer drugs. To become functionally active, Myc must dimerize with a partner protein, Max. Only the Myc-Max heterodimer interacts specifically with DNA to regulate gene transcription. Small molecules that inhibit the interaction of Myc and Max, or that stabilize the Max homodimer, are candidates for cancer drugs. Using CBCRP funding along with NIH funding, we have isolated two novel classes of Myc inhibitors and characterized their interaction with the Myc-Max complex and studied their effects on the oncogenic functions of Myc using whole cells.

Our first aim was to isolate small molecule inhibitors of Myc. In collaboration with the laboratory of Dr. Kim Janda at the Scripps Research Institute, we have tested a novel combinatorial “library” and characterized lead compounds. The library contains molecules that have a flat scaffold structure. The idea is that molecules that have a flat structure would have the best chance of getting inserted between the smooth protein interacting surfaces. As some locks can be picked with a credit card, so can protein-protein complexes be pried apart with flat molecules. Thus, we refer to these inhibitors as a "credit card library". Using this approach we have identified three inhibitors of Myc-Max dimerization. Since the library is relatively small (<500 compounds), this remarkable “hit rate” suggests that our approach may be generally successful.

In parallel to the “credit card” approach, we are looking at ways to target Myc by enhancing the stability of the Max homodimer. This should make Max less available for dimerization with other proteins and could therefore “downregulate” the entire transcriptional network to which Myc belongs. We have, therefore, embarked on an identification of small molecules that stabilize the Max homodimer using used an “in silico” docking program, called Autodoc, developed by Dr. Art Olson at the Scripps Research Institute. Thus far, we have identified eight compounds that specifically stabilized the Max homodimer as measured by fluorescence resonance energy transfer (FRET). The stabilizers of the Max homodimers represent a new class of Myc inhibitors.

Our final aims was to test the Myc inhibitors for therapeutically significant effects on breast cancer cells. We have used the breast cancer cell line MCF7-35im. This is a cell line that carries an estrogen-responsive endogenous Myc and a transgene Myc that can be controlled with doxycycline. Treatment of these cells with our lead compounds from the credit card library interfered with the activity of Myc, controlled by the endogenous gene as well as the transgene, preventing the activation of Myc by estrogen and by doxycycline. The effect of the credit card inhibitors is also still being studied in collaboration with the laboratory of Dr. Steven Martin, a recipient of CBCRP support at the University of California, Berkeley. We have also studied the effects of the Max stabilizers with the breast cancer cell MCF7im. We find that the Max stabilizers specifically inhibits the growth of these cells. We are now testing several other breast cancer cell lines for their response to the Max stabilizers. We expect that growth inhibition will be preferentially effective in cell lines that show a gain of Myc function. We have also set up tests to measure the expression of known transcriptional targets of Myc to determine whether our drugs have the desired effects. An important concern in the development of small molecule inhibitors is toxicity, possibly caused by “off-target” effects. Thus far, the lead molecules we have studied do not affect the growth of normal cells even during exposure times that exceed four weeks. More detailed toxicological studies in animals are planned.

The support of the CBCRP has enabled us to isolate and characterize promising inhibitors of Myc and to specifically test them in breast cancer cell lines. We plan to expand these results to the next level by: (1) identifying of inhibitors with substantially improved effectiveness, and (2) characterizing the inhibitory effects on cellular Myc using breast cancer cells.

A credit-card library approach for disrupting protein-protein interactions.
Periodical:Bioorganic Medical Chemistry
Index Medicus: Bio Med Chem
Authors: Xu Y, Shi J, Yamamoto N, Moss JA, Vogt PK, Janda KD
Yr: 2006 Vol: 14 Nbr: Abs: Pg:2660-2673