Discodermolide: just another microtubule-stabilizing agent? No! A lesson in synergy.

In our increasingly sophisticated world, it is remarkable how, even today, most decisions on combination chemotherapy regimens are still made on purely empirical grounds. Whereas several strategies have been pursued, the most common approach is to combine agents based on their single-agent activity and toxicity profile. Thus, if drug A has 20% activity in disease X as a single agent, and drug B has 20% activity, drugs A and B are combined. An alternate strategy that is increasingly being used is to combine a new agent (gemcitabine, for example) with as many existing agents as possible. Other strategies exist. Using these empirical approaches, insightful oncologists have crafted successful combinations in a variety of diseases, including malignant lymphomas, testicular cancer, and others (1, 2). To be sure, the road has been bumpy, and success was not always forthcoming. For example, a detour was taken in the treatment of lymphoma, for which combination regimens containing eight drugs were used before returning to simpler regimens (3). Meanwhile, two drugs with highly similar mechanisms of action (cisplatin and cytoxan) were combined in the treatment of ovarian cancer, with arguable improvement in efficacy (4). The virtue of this latter combination may have been to allow one to administer more of this class of active agent with less toxicity. However, it failed to exploit the potential for synergy from combining agents from different drug classes (see below). An increasing number of attempts are being made to aid this process with preclinical studies, but this is often wanting because: (a) clinical development cannot wait for the data; (b) there is a reluctance to obtain such data because in vitro observations are not always predictive of clinical efficacy; and (c) in vivo combination studies in animals are difficult to perform, time consuming, and expensive. All too often, the data emerge after clinical trials have been completed. For example, the order of administration of paclitaxel in combination regimens was empirical before preclinical studies demonstrated that additive responses were seen most often when paclitaxel was administered first, with antagonism found in several combinations when paclitaxel was administered second (5). An “unwritten tenet” has guided most choices: combination regimens should include only one drug from each of the different “classes” of agents (alkylating agents, antimetabolites, antimitotics, anthracyclines, topoisomerase poisons, and so forth). Although this tenet appears reasonable, one can envision some exceptions. For example, the combination of a microtubule depolymerizing agent (an antimitotic) with a microtubule polymerizing agent (another antimitotic) can be synergistic. Although these agents share the same target, they act in different ways. But would one combine two antimitotic agents with a similar mechanism of action? Probably not. However, the study by Martello et al. (6) in this issue suggests otherwise. Martello et al. (6) report that Taxol and discodermolide represent a synergistic drug combination. This was a surprise to us and, we suspect, to them . This was a surprise because in may ways, their mechanism of action appears indistinguishable: (a) both agents induce polymerization of purified tubulin with and without microtubule-associated proteins or GTP; (b) both agents form polymers that are stable to cold and calcium; (c) discodermolide competitively inhibits the binding of [H]paclitaxel to tubulin polymers; and (d) both agents cause cell cycle perturbations and induction of a hypodiploid cell population (7, 8). But are Taxol and discodermolide truly indistinguishable? The answer to this is a clear no! To Martello et al. (6), the first clue came from the characterization of a Taxol-resistant cell line, A549-T12. Previous studies have shown that A549-T12 cells are not only resistant to Taxol, they are also dependent on Taxol for growth (9). When they examined the cross-resistance profile of this human lung carcinoma subline to various microtubule active agents, they found much less cross-resistance to discodermolide than to Taxol. Furthermore, when they examined the ability of three distinct classes of tubulin-stabilizing agents to substitute for Taxol in sustaining the growth of A549-T12 cells, the differences between discodermolide and the other microtubule-stabilizing agents became even more apparent. Whereas the epothilones and the eleutherobins could substitute for Taxol, discodermolide could not. The observation that followed was possibly expected: in A549-T12 cells, discodermolide was significantly more potent in the presence of Taxol; whereas Taxol did not affect the potency of the epothilones or the eleutherobins. But was this unique to this drug-resistant cell line? Surprisingly, no. Using the combination index method of Chou and Talalay, Martello et al. (6) show unequivocally, in four different human carcinoma cell lines, a schedule-independent synergistic interaction between Taxol and discodermolide. This effect was not observed between Taxol and epothilone B. The challenge now is to understand how this synergy occurs. Martello et al. (6) demonstrate that the combination of Taxol and discodermolide at doses that “do not induce mitotic arrest” causes an increase in the hypodiploid population. They conclude this indicates that “a possible mechanism for the observed synergy is the potentiation of apoptosis.” However, apoptosis is the end result of something more basic and “upstream” that we do not yet understand. For students of microtubules, this underscores how much more is yet to be learned. Received 2/24/00; accepted 3/3/00. 1 To whom requests for reprints should be addressed, at Medicine Branch, Division of Clinical Sciences, National Cancer Institute, NIH, Building 10, Room 12N226, 9000 Rockville Pike, Bethesda, MD 20892. Phone: (310) 496-4916; Fax: (310) 402-0172; E-mail: tfojo@helix. nih.gov. 1613 Vol. 6, 1613–1615, May 2000 Clinical Cancer Research