Paclitaxel bundles

Taxanes (paclitaxel, docetaxel) are derivatives of yew tree bark (Taxus brevifolia). They stabilize microtubules in the polymerized state leading to nonfunctional microtubular bundles in the cell. Inhibition occurs during G2- and M-phases. Taxanes are also radiosensitizers. Unwanted effects include bone marrow suppression and cumulative neurotoxicity. 

The answer is c. (Hardman, pp 1260—1262.) Paclitaxel is a large structural molecule that contains a 15 membered taxane ring system. This anti cancer agent is an alkaloid derived from the bark of the Pacific yew tree. Its chemotherapeutic action is related to the microtubules in the cell. Paclitaxel promotes microtubule assembly from dimers and causes microtubule stabilization by preventing depolymerization. As a consequence of these actions, the microtubules form disorganized bundles, which decreases... 

Vinblastine (6.73) is an antimitotic drug that prevents polymerization of tubulin (Figure 6.26). When incubated with tubulin, vinblastine complexes in a 1 1 ratio with tubulin proteins. By blocking polymerization, vinblastine prevents microtubule formation and therefore mitosis. In contrast, paclitaxel (Taxol, 6.74) and epothilone B (6.75) stabilize aggregated tubulin. As a result, in the presence of paclitaxel and epothilone B, cells form static bundles of microtubules that are nonfunctional. Vinblastine and paclitaxel are both approved for clinical use against cancer. Ixabepilone (6.76), an analogue of epothilone B (6.75), has been approved by the FDA for treatment of certain forms of breast cancer. The European Medicines Agency (EMEA) did not approve ixabepilone out of concern over severe side effects.27... 

Fig. 7). Therefore, the investigators attributed the antimigratory activity of paclitaxel to its effect on the actin bundles. 

Paclitaxel acts by enhancing microtubule assembly and stabilizing microtubules (1,2). Microtubules consist of polymers of tubulin in dynamic equilibrium with tubulin heterodimers. Their principal function is the formation of the mitotic spindle during cell division, but they are also active in many interphase functions, such as cellular motility, intracellular transport, and signal transmission. Paclitaxel inhibits the depolymerization of tubulin, and the microtubules formed in the presence of paclitaxel are extremely stable and dysfunctional. This stabilization impairs the essential assembly and disassembly required for dynamic cellular processes, and death of the cell results through disruption of the normal microtubular dynamics required for interphase processes and cell division. In tumor cells, cytotoxicity is represented by the appearance of abnormal microtubular bundles, which accumulate during G2 and mitosis, blocking the cell cycle (3). 

Docetaxel is the second representative of the new entity of drugs that have a unique taxane ring in common, such as the one seen in pachtaxel. Being more efficacious than paclitaxel, docetaxel is used in ovarian cancer. Paclitaxel and docetaxel inhibit microtubule depolymerization, thereby reducing the formation of stable microtubule bundles (see also Figure 15). 

Interest in paclitaxel was stimulated by the finding that the drug possessed the unique ability to promote microtubule formation at cold temperatures and in the absence of guanosine triphosphate (GTP). It binds specifically to the P-tubulin subunit of microtubules and antagonizes the disassembly of this key cytoskeletal protein, with the result that bundles of microtubules and aberrant structures derived from microtubules appear in the mitotic phase of the cell cycle. Arrest in mitosis follows. Cell killing (CK) is dependent on both drug concentration and duration of cell exposure. 

Cardiovascular Heart failure (New York Heart Association classes II-IV) has been observed in patients receiving trastuzumab, alone or in combination with paclitaxel or docetaxel, particularly after chemotherapy containing an anthracycline (doxorubicin or epirubicin) [303, 304, 305, 306. It can be moderate or severe and can be fatal. The results of many randomized trials have shown that the degree of cardiotoxicity is generally acceptable the incidence of cardiac damage caused by trastuzumab was 0.4-4.1% [307 ]. Older age, lower left ventricular ejection fraction, and antihypertensive medications are associated with an increased risk of cardiac dysfunction in patients receiving trastuzumab [308 "]. The cardiac dysfunction associated with trastuzumab is usually reversible on withdrawal and standard medical therapy [309 ]. In one case, trastuzumab-associated cardiomyopathy presented with complete left bundle-branch block mimicking acute coronary syndrome [310" ]. 

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