The Skaggs Institute
for Chemical Biology

Scientific Report 2006

Synthesis of Natural and Nonnatural Products and Prodrug Therapy

S.C. Sinha, R.A. Lerner, Z. Chen, S. De, S. Das, S. Abraham, F. Guo

Our main research interests are synthesis of biologically important natural and nonnatural molecules, synthetic methods, and antibody catalysis in organic synthesis and selective drug delivery. During the past year, we focused on the synthesis of new adjacent bis-tetrahydrofuran annonaceous acetogenins, the antibacterial macrocyclic lactones sorangiolides, nonnatural small-molecule compounds that target G protein–coupled receptors, and protease inhibitors. In our work on antibody catalysis, we developed a proadapter approach for production of the chemically programmed aldolase antibody 38C2 and new doxorubicin prodrugs that are catalyzed by antibody 38C2 faster than the previously reported doxorubicin prodrugs are.

Selective Chemotherapy with Catalytic Aldolase Antibodies

For the selective chemotherapy, we are developing drug conjugates and prodrugs that will target cell-surface receptors, including the glycoprotein integrins αβ₃ and αβ₅, and prostate-specific membrane antigens. The integrins αβ₃ and αβ₅ are directly implicated in tumor angiogenesis; they are overexpressed in the vasculature of angiogenic tumors and in numerous cancer cells but are less expressed on quiescent blood vessels. Using antibody 38C2 and small-molecule antagonists of αβ₃ and αβ₅ (or a targeting agent), we developed antagonist-38C2 conjugates, also known as chemically programmed 38C2 (Fig. 1).

Fig. 1. Schematic drawings of the diketone and the proadapter strategies used to produce chemically programmed antibody constructs that target cells expressing the integrins αvβ3 and αvβ5. Abbreviations: Ab, antibody; TA, targeting agent.

The conjugation between the targeting agents and 38C2 takes place in the binding sites though the diketone or vinyl ketone linkers. Because the vinyl ketones are highly reactive, we used the corresponding acetone adduct as the prolinker, which undergoes a 38C2-catalyzed reaction to produce the active linker before the active linker reacts. This strategy has been termed the proadapter approach. The conjugates prepared via both approaches bound efficiently to cells expressing αβ₃ and αβ₅, including human breast cancer cell line MDA-MB-231, and inhibited the growth of both the primary tumors and secondary metastasis in distant organs.

Development of these strategies for the formation of antibody constructs can have a marked effect on the treatment of various diseases, including cancers. We anticipate that our strategy offers an opportunity to break the 1 antibody–1 target axiom. Thus, different low molecular weight targeting agents (programming agents or adapters) can be used to selectively target the same antibody to different sites for different uses.

In the alternative approach, we are developing prodrugs that can be efficiently activated by the aldolase antibodies 38C2 and 93F3. These antibodies will be targeted to tumor cells or the tumor vasculature by using antagonists of αβ₃ and αβ₅. In the past year, we developed new doxorubicin prodrugs that are not only more stable than the previously reported analogous prodrugs but also activated faster by using antibody 38C2. We also produced 38C2-antagonist conjugates. The conjugates bound efficiently to MDA-MB-231 cells expressing αβ₃ and αβ₅. We also found that the modified antibody can activate new doxorubicin prodrugs.

Therefore, we have all the tools to investigate prodrug therapy in animal models. The selective chemotherapy studies are carried out in collaboration with C.F. Barbas, Skaggs Institute, and B. Mueller, La Jolla Institute for Molecular Medicine, San Diego, California.

Synthesis of Adjacent Bis-Tetrahydrofuran Acetogenins

Adjacent bis-tetrahydrofuran annonaceous acetogenins are among the most cytotoxic compounds. Each acetogenin may have as many as 64 stereoisomers because of the bis-tetrahydrofuran fragment alone flanked by a hydroxy group on each side. In order to understand the structure-activity relationship of these compounds and carry out comprehensive biological studies, total synthesis of all 64 stereoisomers is required. This synthesis has been the focus of research for several years, not only by us but also by many others. However, despite continued effort, synthesis of all 64 stereoisomers of asimicin or bullatacin, which are among the most cytotoxic adjacent bis-tetrahydrofuran acetogenins, has not yet been achieved. We are using a bidirectional and nondiasteromeric approach to construct these molecules so that more than 2 compounds can be achieved in a single chemical transformation. We have synthesized a complete library of the bifunctional bis-tetrahydrofuran intermediates that can be used in the synthesis of asimicin and bullatacin and their libraries (Fig. 2).

Fig. 2. Structures of the adjacent bis-tetrahydrofuran annonaceous acetogenins asimicin, bullatacin, and 27-hydroxybullatacin.

Publications

Guo, F., Das, S., Mueller, B.M., Barbas, C.F. III, Lerner, R.A., Sinha, S.C. Breaking the one antibody-one target axiom. Proc. Natl. Acad. Sci. U. S. A. 103:11009, 2006.

Popkov, M., Rader, C., Gonzalez, B., Sinha, S.C., Barbas, C.F. III. Small molecule drug activity in melanoma models may be dramatically enhanced with an antibody effector. Int. J. Cancer 119:1194, 2006.