18 Vinca alkaloids as microtubule poisons: promoting microtubule depolymerisation

We are now in a position to understand how MT-targeting drugs work. We will start talking about vinca alkaloids, important drugs used in the clinic that target MT dynamics.

Vinva alkaloids such as Vincristine and Vinblastine are extracted from the periwinkle plant Vinca rosea (Figure 15.6). They are either natural products or are obtained by semi-synthesis. Vincristine and Vinblastine are quite complex natural products, which are structurally identical varying only in one position. In this position Vincristine has a formyl substituent, whereas Vinblastine contains a methyl group (Figure 15.7).

Madagascar rosy periwinkle from which vinca alkaloids can be isolated.

Figure 18.1: Madagascar rosy periwinkle from which vinca alkaloids can be isolated.

Although they have a wide range of cellular effects, the principle mechanism of cytotoxicity relates to the key protein target in cells, the MTs: vinca alkaloids depolymerise MTs.

Chemical structures of the natural products Vincristine and Vinblastine. These two differ only in a single position.

Figure 18.2: Chemical structures of the natural products Vincristine and Vinblastine. These two differ only in a single position.

What is the mechanism of action of vinca alkaloids? Since we understood how tubulin assembles into MTs we are now in the position to comprehend, how tubulin targeting drugs work. MTs are very dynamic, in particular during mitosis and there is polymerisation and depolymerisation taking place. Tubulin heterodimers incorporate into a MT or disassemble from the MT (Figure 15.8). Vinca alkaloids bind to free α,β-tubulin heterodimersand MT ends and disrupt the balance between polymerisation and depolymerisation. Tubulin subunits are eliminated from the equilibrium and the assembly of tubulin into MTs is inhibited. As a result, MTs are depolymerised. Since the mitotic spindle is build from MTs, it is destroyed. Without a proper mitotic spindle, cells arrest in mitosis. Mitotic arrest leads to apoptotic cell death. It should be mentioned that there are many types of tubulins, not only α, β, and γ, but also delta, epsilon tubulins and that these tubulins exist as different isoforms. Therefore, tubulin-targeting drugs can show tissue specific effects.

Mechanism of action of vinca alkaloids targeting microtubules.

Figure 18.3: Mechanism of action of vinca alkaloids targeting microtubules.

18.1 Administration, clinical uses and toxicities of vincristine

Vincristine and Vinblastine are administered intravenously. Vinca alkaloids are used either alone or in combination with other drugs to treat - Leukemia,

  • Hodgkins and non-Hodgkins lymphoma,

  • Small cell lung cancer,

  • In combination therapy it is also used against multiple myeloma and other tumours.

Vinca alkaloids induce a range of side effects including peripheral neuropathy and neutropenia. Neutropenia is the principal dose-limiting toxicity.

18.2 Taxanes as microtubule poisons: promoting microtubule polymerisation

We will now look at taxanes as inhibitors of MTs. Paclitaxel (Taxol) and Docetaxel (Taxotere) represent the taxane family of drugs. The different names may sometimes lead to some confusion.

Figure 15.9 shows the chemical structures of Taxol and Taxotere, which are quite complex. Taxol is a natural product and the molecule is similarly complex as the vinca alkaloids. Total synthesis has been achieved, but is difficult, because taxol contains 11 chiral centres. Several methods for total synthesis were developed requiring roughly 40 steps (add ref).

Chemical structures of the natural products Taxol and Taxotere.

Figure 18.4: Chemical structures of the natural products Taxol and Taxotere.

We would like to give you a brief overview on the discovery and clinical development of Taxol. In 1955, the National Cancer Institute in the US set up the Cancer Chemotherapy National Service Center to act as a public screening facility. From 1958, samples from about 1000 plants pecies were collected per year and tested. In 1962, the bark of the Pacific yew tree was collected and later shown to have anti-tumour activity. In 1966, the active ingredient, taxol, was isolated. From 1200 kg of bark, 28 kg crude extract was prepared that yielded 10 g of pure material. Unfortunately, the trees are killed during the process. In 1978, the efficacy of Taxol was shown in xenograft models. In 1988, the response rate of Taxol in a phase II clinical trial on ovarian cancer was 30%. In 1992, Taxol gained FDA approval. In summary, it took about 30 years from the discovery of a sample with cytotoxicity to the approval of the drug and gives you an idea about the complexity to develop a cancer drug!

The Pacific Yew Tree Taxus brevifolia.

Figure 18.5: The Pacific Yew Tree Taxus brevifolia.

In 1979, Prof. Susan Horwitz showed that Taxol had a previously unknown mechanism of action involving the stabilisation of MTs. Taxol acts by stabilising MTs, in contrast to vinca alkaloids, which destabilised MT, and thus interferes with MT dynamics. Taxol stabilises MTs and protects them from disassembly or depolymerisation. Taxol exhibitsc oncentration-dependent effects. In addition, differences in effects have been observed in cell culture and in tumours. For example in cell culture lagging chromosomes have been observed (Figure 15.11, whereas in tumours multipolar spindles have been observed (Figure 15.12). In any case, the effects of Taxol treatment lead to apoptotic cell death.

Mechanism of action of taxol in tumours. Treatment of tumours with taxanes lead to multipolar spindles with incomplete sets of chromosomes resulting in non-viable tumour cells, which eventually die.

Figure 18.6: Mechanism of action of taxol in tumours. Treatment of tumours with taxanes lead to multipolar spindles with incomplete sets of chromosomes resulting in non-viable tumour cells, which eventually die.

##Administration, clinical uses and toxicities of Taxol and Taxotere

Taxol is given intravenously. Taxol is very lipophilic and therefore has poor aqueous solubility, which requires the need for a particular formulation. Taxol is formulated with a castor oil and ethanol. Part of the clinical toxicity of Taxol is associated with the solvent Cremophor EL. Albumine-bound taxol (trade name Abraxane) is an alternative formulation where Taxol is bound to albumine. The advantage of albumine-bound taxol over taxol administered with castor oil and ethanol is, that it may have fewer side effects. There seems to be no anaphylactic reaction and shorter infusion times can be achieved.

Taxol is used to treat patients with - Lung, - Ovarian, - Breast, - Head and neck, - Bladder, - Prostate cancer, - as well as advanced forms of Kaposi‘s sarcoma.

Abraxane is currently being used for

  • Advanced breast cancer,

  • Lung cancer.

    The treatment of advanced pancreatic cancer is not recommended anymore, due to its limited benefits, compared to current treatments.

The principle toxicity is neutropenia, but there are other toxicities as well including neurotoxocity, nausea and vomiting, alopecia and several others. One of the side effects is anaphylactic shock.

18.3 Bibliography

Beth A. Weaver. (2014). How taxol/paclitaxel kills cancer cells. Molecular Biology of the Cell* 25, 2677-2681.

Total synthesis of taxol.

More references to be a added