Breast cancer is the most common cancer among women, excluding skin cancers, and the second leading cause of cancer death in women, second only to lung cancer. The American Cancer Society estimates that in 1999 about 175,000 new cases of invasive breast cancer will be diagnosed among women in the United States, and this will result in about 43,000 deaths. Although early detection methods and new therapies have become available, there is still significant mortality from metastatic breast cancer. Therefore, there remains a tremendous need for more effective chemotherapeutic agents for the treatment of metastatic breast cancer.

The development and progression of breast cancer is linked to genetic changes, including the loss of functional tumor suppressor p53. The one class of chemotherapeutic agents that are equally effective in cells with normal or mutant p53 status is the microtubule-targeting compounds. The effectiveness of this single class of anticancer agents is significant and leads us to predict that new antimicrotubule agents will be found with superior properties that may be useful in the treatment of breast cancer.

Through a collaborative research program involving Utah State University and the Cancer Research Center of Hawaii, we have discovered that the sponge-derived macrolides laulimalide and isolaulimalide are potent cytotoxins with paclitaxel-like
antimicrotubule-stabilizing activity. Laulimalide is a potent inhibitor of cellular proliferation with an IC50 in the low nanomolar range, and it maintains activity against a drug-resistant P-glycoprotein overexpressing ovarian cancer cell line. In the National Cancer Institute's (NCI's) 60-cell line assay, laulimalide showed especially interesting profiles of activity in the prostate and breast tumor panels, resulting in a request from the NCI for additional laulimalide with which to begin in vivo evaluation. Laulimalide represents a lead compound for a new class of microtubule-stabilizing agents with activities that may prove therapeutically useful for the treatment of breast cancer.

The aim of this project is to utilize a combinatorial solid-phase synthetic approach for the construction of a library of laulimalide analogs for structure activity relationship (SAR) studies. A solid-phase synthesis will have the advantages of (a) simplified reaction work-ups requiring only simple washing of the resin; (b) being able to use excess reagents to ensure that the reaction proceeds to completion with respect to the resin-bound intermediates; and (c) excellent mass conservation of resin and correspondingly of the attached compound.

The strategy for library construction will be guided by the desire to (1) improve the chemical stability of the biologically active compound, i.e., minimize the possible rearrangement of laulimalide to isolaulimalide; (2) simplify preparation of the active compound; and (3) establish a working understanding of the structural constraints for microtubule stabilizing activity, cytotoxicity toward drug resistant breast tumor cell lines, and P-glycoprotein-dependent drug efflux.

In an effort to discover a new chemotherapeutic agent for the treatment of breast cancer, we propose to do the following: (1) transfer our current solution-phase synthetic approach to solid phase; (2) prepare 260 laulimalide analogs using a split and pool strategy; (3) test laulimalide analogs for microtubule-stabilizing activity, cytotoxicity against both drug-sensitive and drug-resistant breast cell lines; and (4) submit active analogs to the NCI for screening in the 60-cell line.