Since the discovery of the taxanes and recognition of tubulin polymerization as a clinically relevant target for anticancer drug development efforts, several other natural products derived from plant and marine sources that target tubulin dynamics have been identified. Figure 53-4 depicts the structures of the most relevant compounds at this time, epothilone B analogs, discodermolide, and eleutherobin. For the most part, these compounds share a similar pharmacophore and binding site with the taxanes.^{151,152,291–294} At this juncture, preclinical and clinical evaluations to discern whether these new chemotypes that work by a similar mechanism of action as the taxanes may portend higher therapeutic indices are ongoing.

Several natural products that are structurally dissimilar to the taxanes but share their mechanism of action and exhibit similar effects on tubulin and cytotoxicity have been identified. Like paclitaxel, rhazinilam is derived from tree bark, but it is the first nontaxane identified that induces cold-stable tubulin polymerization in vitro and microtubule bundling in cells.²⁹⁵ Although rhazinilam is capable of inducing tubulin polymerization at 0°C, its polymerized tubulin product is unstable. In contrast, discodermolide, which is derived from the marine sponge, Discodermia dissoluta, is much more potent at polymerizing tubulin than the taxanes, yielding polymerization products that are cold-stable, and also polymerizes tubulin almost as rapidly at 0°C.^{292–294,296–298} In contrast to rhazinilam, discodermolide-induced tubulin polymers resist dissolution following treatment with calcium and are composed of very short microtubules instead of tubulin spirals. Furthermore, discodermolide and paclitaxel have demonstrated synergistic cytotoxicity in vitro, indicating that their tubulin binding sites and microtubule effects may not be completely mutually exclusive.^298–300 In addition, discodermolide is a much poorer substrate for Pgp compared with the taxanes and epothilones. Early clinical evaluations with discodermolide have begun in patients with various advanced solid malignancies.

The epothilones, which are derived by microbial fermentation, appear to be more like the taxanes in that their polymerization products are relatively long, rigid, and resistant to destabilization by cold and calcium.^{292–294,301–307} These epothilones and their analogs are at least as potent as paclitaxel and cause mitotic arrest and microtubule bundling. Two epothilone B analogs are currently undergoing clinical evaluation.^298,299 Interestingly, these agents are metabolized differently, BMS247550 via cytochrome P450-related means and EPO906 via carboxylesterases, and they have distinctly different active chemical moieties. Furthermore, they produce different principal toxicologic effects, namely, diarrhea (EPO906) and both myelosuppression and neurotoxicity (BMS247550); however, the toxicities of these compounds overlap. In preliminary clinical trials, notable regressions of breast, lung, ovarian, and other types of advanced malignancies that are generally sensitive to the taxanes have been reported. Although several antitumor responses in patients who were previously treated with taxanes have been noted, it is not clear whether these agents possess appreciable clinical activity in patients with taxane-resistant neoplasms. However, several antitumor responses in patients with drug-refractory colorectal carcinoma, which is almost always inherently resistant to antimicrotubule agents, has been reported in preliminary studies. A derivative of epothilone D, which has been reported to be more potent than several epothilone A and B analogs against several cancer cell lines and xenografts, is also in early clinical evaluations.³⁰⁸

The marine soft coral-derived natural products sarcotidicytins A and B and eleutherobin also promote tubulin polymerization in a manner analogous to that of the taxanes.^292,309 Although all of the aforementioned compounds are likely substrates for Pgp to some extent, expressing varying degrees of cross-resistance against Pgp-expressing cells, it appears that the epothilones and discodermolide possess either low-level or no resistance conferred by Pgp overexpression, which may explain the clinical activity reported in colorectal malignancies that are derived from tissues that overexpress Pgp. Furthermore, other marine-derived, microtubule-stabilizing cytotoxins, particularly laulimalide and isolaulimalide, appear to be extremely poor substrates for Pgp drug efflux pump.³¹⁰ Because eleutherobin, epothilones A and B, and discodermolide competitively inhibit [³H]paclitaxel binding to microtubules, a common pharmacophore was sought and identified, which may enable the development of hybrid constructs with more desirable biologic characteristics.^151,152

Microtubule dynamics are also perturbed by other natural products and semisynthetic analogs that interact with tubulin in the Vinca alkaloid- or colchicine-binding domains. Among the most potent are the cryptophycins, which are a family of cyanobacterial macrolides that deplete microtubules in intact cells, including cells with the MDR phenotype.^311,312 The cryptophycins compete for the binding of [³H]VBL but for neither radiolabeled paclitaxel nor colchicine, and inhibit GTP hydrolysis by isolated tubulin. In addition, they possess cytotoxic activity against several types of tumor xenografts, including tumors resistant to the Vinca alkaloids.^311,312 The clinical development of cryptophycin-52, a semisynthetic cryptophycin analog, was recently discontinued because of the lack of a relevant level of therapeutic activity and a unacceptable degree of neurotoxicity and myelosuppression. The dolastatins constitute a series of oligopeptides isolated from the sea hare, Dolabela auricularia.³¹³ Two of the most potent dolastatins, dolastatin-10 and dolastatin-15, noncompetitively inhibit the binding of Vinca alkaloids to tubulin, inhibit tubulin polymerization and tubulin-dependent GTP hydrolysis, stabilize the colchicine binding activity of tubulin, and possess cytotoxic activity in the picomolar range.³¹⁴ Dolastatin-10 and semisynthetic dolastatin analogs are undergoing preclinical development and clinical evaluations. Phomopsin A, halichondrin B, homohalichondrin B, and spongistatin 1 that interact with tubulin in the Vinca alkaloid-binding domain and the natural products combretastatin and steganacin, and the synthetic compounds pyridine and pyridazine, and the pentaflurophenylsulfonamides, which interact with tubulin at the colchicine-binding domain, are currently being evaluated in preclinical and/or early clinical evaluations.³¹⁵

Although the majority of efforts are being directed toward antimicrotubule agents that interfere with tubulin, other potential strategic components of the microtubule, such as the kinesin motor proteins, are the focus of discovery and developmental efforts.^15,16 A potent inhibitor of KSP (GW57016), a kinesin found only in active proliferating cells, is currently undergoing early clinical evaluation.