Maytansine is a potent microtubule-targeted compound that induces mitotic arrest and kills tumor cells at subnanomolar concentrations. However, its side effects and lack of tumor specificity have prevented successful clinical use. Recently, antibody-conjugated maytansine derivatives have been developed to overcome these drawbacks. Several conjugates show promising early clinical results. We evaluated the effects on microtubule polymerization and dynamic instability of maytansine and two cellular metabolites (S-methyl-DM1 and S-methyl-DM4) of antibody-maytansinoid conjugates that are potent in cells at picomolar levels and that are active in tumor-bearing mice. Although S-methyl-DM1 and S-methyl-DM4 inhibited polymerization more weakly than maytansine, at 100 nmol/L they suppressed dynamic instability more strongly than maytansine (by 84% and 73%, respectively, compared with 45% for maytansine). However, unlike maytansine, S-methyl-DM1 and S-methyl-DM4 induced tubulin aggregates detectable by electron microscopy at concentrations ≥2 μmol/L, with S-methyl-DM4 showing more extensive aggregate formation than S-methyl-DM1. Both maytansine and S-methyl-DM1 bound to tubulin with similar K_D values (0.86 ± 0.2 and 0.93 ± 0.2 μmol/L, respectively). Tritiated S-methyl-DM1 bound to 37 high-affinity sites per microtubule (K_D, 0.1 ± 0.05 μmol/L). Thus, S-methyl-DM1 binds to high-affinity sites on microtubules 20-fold more strongly than vinblastine. The high-affinity binding is likely at microtubule ends and is responsible for suppression of microtubule dynamic instability. Also, at higher concentrations, S-methyl-DM1 showed low-affinity binding either to a larger number of sites on microtubules or to sedimentable tubulin aggregates. Overall, the maytansine derivatives that result from cellular metabolism of the antibody conjugates are themselves potent microtubule poisons, interacting with microtubules as effectively as or more effectively than the parent molecule. Mol Cancer Ther; 9(10); 2689–99. ©2010 AACR.

Maytansine and its analogues (maytansinoids) are potent microtubule-targeted compounds that inhibit proliferation of cells at mitosis. Antibody-maytansinoid conjugates consisting of maytansinoids (DM1 and DM4) attached to tumor-specific antibodies have shown promising clinical results. To determine the mechanism by which the antibody-DM1 conjugates inhibit cell proliferation, we examined the effects of the cleavable anti-EpCAM-SPP-DM1 and uncleavable anti-EpCAM-SMCC-DM1 conjugates on MCF7 human breast tumor cells. We also examined the effects of the free maytansinoids, maytansine and S-methyl DM1 (a version of DM1 that is stable in cell culture medium), for comparison. Both the conjugates and free maytansinoids potently inhibited MCF7 cell proliferation at nanomolar and subnanomolar concentrations, respectively, by arresting the cells in mitotic prometaphase/metaphase. Arrest occurred in concert with the internalization and intracellular processing of both conjugates under conditions that induced abnormal spindle organization and suppressed microtubule dynamic instability. Microtubule depolymerization occurred only at significantly higher drug concentrations. The results indicate that free maytansinoids, antibody-maytansinoid conjugates, and their metabolites exert their potent antimitotic effects through a common mechanism involving suppression of microtubule dynamic instability. Mol Cancer Ther; 9(10); 2700–13. ©2010 AACR.

• Spontaneous remission (SR) of cancer is a rare but well-documented phenomenon. • Mechanisms of SR are described in detail. • Knowing the intricacies of SR would help in devising novel treatment strategies.

We have shown previously that an antitussive plant alkaloid, noscapine, binds tubulin, displays anticancer activity, and has a safe pharmacological profile in humans. Structure-function analyses pointed to a proton at position-9 of the isoquinoline ring that can be modified without compromising tubulin binding activity. Thus, many noscapine analogs with different functional moieties at position-9 were synthesized. Those analogs that kill human cancer cells resistant to other antimicrotubule agents, vincas and taxanes, were screened. Here, we present one such analog, 9-nitro-noscapine (9-nitro-nos), which binds tubulin and induces apoptosis selectively in tumor cells (ovarian and T-cell lymphoma) resistant to paclitaxel, vinblastine, and teniposide. 9-Nitro-nos treatment at doses as high as 100 μM did not affect the cell cycle profile of normal human fibroblasts. This selectivity of 9-nitro-nos for cancer cells represents a unique edge over the other available antimitotics. 9-Nitro-nos perturbs the progression of cell cycle by mitotic arrest, followed by apoptotic cell death associated with increased caspase-3 activation and appearance of terminal deoxynucleotidyl transferase dUTP nick-end labeling-positive cells. Thus, we conclude that 9-nitro-nos has great potential to be a novel therapeutic agent for ovarian and T-cell lymphoma cancers, even those that have become drug-resistant to currently available chemotherapeutic drugs.