Scientific Report 2005

Molecular Biology

Antibody Catalysis and Organic Synthesis

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

Our main research interests are antibody catalysis and the applications of antibody catalysts in organic synthesis, prodrug activation, and the development of cell-targeting antibody constructs. In addition, we also focus on synthetic and medicinal chemistry, including the total synthesis of biologically important natural products and synthetic compounds and their analogs and new methods of synthesis.

Antibody Catalysis and Its Applications

Aldolase antibodies 38C2, 84G3, and 93F3 produced by the reactive immunization technique are highly useful in synthetic organic chemistry, as indicated by their application in the syntheses of a number of natural products, including epothilones. These antibodies catalyze both aldol and retro-aldol reactions and yield products with high enantioselectivities. The high catalytic rate of the retro-aldol reaction makes the antibodies useful in prodrug therapy.

In prodrug therapy, an enzyme or a catalytic antibody is used to activate a nontoxic prodrug at a targeted site, thereby producing a cytotoxic drug. We are developing prodrugs of cytotoxic molecules, including paclitaxel, doxorubicin analogs, enediynes, CBI analogs, and epothilones, that can be activated efficiently by aldolase antibodies. In particular, we prepared and evaluated several prodrugs of the analogs of dynemicin B and doxorubicin. The prodrugs of dynemicin analogs are activated by using antibody 38C2; those of doxorubicin analogs, by 93F3 (Fig. 1). On the basis of these studies, we are developing new linkers for the prodrugs so that activation of the prodrugs can be selectively achieved at high catalytic rate.

Fig.1. Structure of the prodrugs of doxorubicin (DOX) analogs.

Using antibody 38C2, we also developed antagonist-38C2 conjugates. The conjugates bound efficiently to cells expressing the integrins α_vβ₃ and α_vβ₅. The conjugates have several advantages, including prolongation of half-life of the antagonist and in vivo assembly of the conjugate. On the basis of our initial studies, in collaboration with C.F. Barbas, Department of Molecular Biology, we synthesized a series of antagonist-38C2 conjugates and evaluated them by using breast cancer cell lines that express the integrins α_vβ₃ and α_vβ₅. Several conjugates (Fig. 2) bound to these cell lines with high affinity. Our findings, which were supported by molecular docking studies, provided preliminary information on how the compounds should be derivatized.

Fig. 2. Structure of the compounds that target the integrins αvβ3 and αvβ5 for conjugation with 38C2.

Synthesis of Natural Products and Their Analogs

In the past year, we focused on the synthesis of naturally occurring macrocyclic molecules, sorangiolides A and B, and the library of bis-tetrahydrofuran annonaceous acetogenins. Sorangiolides (Fig. 3) are weakly active against gram-positive bacteria. Our goal is to synthesize analogs of sorangiolides that are highly active. The bis-tetrahydrofuran acetogenins are among the most active cancer agents and are toxic to a number of human cancer cell lines at much lower concentrations than doxorubicin is. In collaboration with E. Keinan, Department of Molecular Biology, we synthesized an analog of asimicin, an annonaceous acetogenin, for photoaffinity labeling of the corresponding receptor. In other studies, we developed a bidirectional approach for the synthesis of all 64 diastereomers of the adjacent bis-tetrahydrofuran acetogenins (Fig. 3).

Fig. 3. Structure of sorangiolides A and B (top) and a general structure of bis-tetrahydrofuran annonaceous acetogenins (bottom).

Starting with 8 diene lactones, we synthesized 36 bifunctional adjacent bis-tetrahydrofuran lactones by using 5 key reactions: (1) monooxidative or bis-oxidative cyclization mediated by rhenium(VII) oxides, (2) Shi monoasymmetric or bis-asymmetric epoxidation, (3) Sharpless asymmetric dihydroxylation, (4) Williamson-type etherification, and (5) Mitsunobu inversion. Further studies are in progress.

Publications

Li, L.-S., Rader, C., Matsushita, M., Das, S., Barbas, C.F. III, Lerner, R.A., Sinha, S.C. Chemical-adaptor immunotherapy: design, synthesis and evaluation of novel integrin-targeting devices. J. Med. Chem. 47:5630, 2004.

Saphier, S., Hu, Y., Sinha, S.C., Houk, K.N., Keinan, E. Origin of selectivity in the antibody 20F10-catalyzed Yang cyclization. J. Am. Chem. Soc. 127:132, 2005.

Sinha, S.C., Li, L.-S., Watanabe, S.-I., Kaltgrad, E., Tanaka, F., Rader, C., Lerner, R.A., Barbas, C.F. III. Aldolase antibody activation of prodrugs of potent aldehyde-containing cytotoxics for selective chemotherapy. Chemistry 10:5467, 2004.