Ansamitocin P3, a structural analogue of maytansine, was isolated from Nocardia. Structurally maytansine and ansamitocin P3 are polyketide macrolactams differing in the acyl groups at the C3 position. Ansamitocin P-3 inhibited the polymerization of tubulin and depolymerized the once polymerized tubulin. Ansamitocin P3 exerts antitumour properties.

SB-743921 is a synthetic small molecule with potential antineoplastic properties. SB-743921 selectively inhibits the ATP-binding domain of the kinesin spindle protein (KSP), an important protein involved in the early stages of mitosis that is expressed in proliferating cells. Inhibition of KSP results in inhibition of mitotic spindle assembly and interrupts cell division, thereby causing cell cycle arrest and induction of apoptosis. SB-743921 has greater than 40,000 fold selectivity for KSP over other kinesins. SB-743921 has demonstrated promising anti-cancer activity in a variety of in vivo and in vitro human cancer models in preclinical studies. Furthermore, anti-cancer effect has been demonstrated in taxane-refractory malignancies with SB-743921. Toxicity studies demonstrated predictable neutropenias and gastrointestinal toxicities without clear evidence of neurotoxicity. The recommended phase II dose for SB-743921 as a 1-h infusion every 21 days is 4 mg/m2.

Baccatin III is an isolate of the Pacific yew tree (Taxus brevifolia) and related species. Baccatin III is the precursor to paclitaxel/ taxol. Antileukaemic and antitumor agent, Baccatin III, binds to tubules when they are assembled in the cell and stabilizes the polymerized form of tubules so that they remain assembled even under conditions in which microtubules dissociate into tubulin subunits. Baccatin III is also an inducer of apoptosis and shows antitumor properties in vitro. Baccatin III exerts anti-tumor immunomodulatory activity in very low doses (0.05-0.5mg/kg), although it is regarded as an inactive derivative of paclitaxel. Oral administration of Baccatin III significantly reduced the growth of tumors induced by engrafting BALB/c mice with either 4 T1 mammary carcinoma or CT26 colon cancer cells. Baccatin III reduced tumor progression by inhibiting the accumulation and suppressive function of MDSCs. Baccatin III inhibited cell proliferation of a number of cancer cell lines. The cytotoxic activity exhibited by fungal taxol and Baccatin III involves the same mechanism, dependent on caspase-10 and membrane potential loss of mitochondria, with taxol having far greater cytotoxic potential.