What are Antimicrotubule Drugs?
Antimicrotubule drugs are used in the treatment of a wide variety of cancers. They are plant-derived medicines that disrupt microtubule function; microtubules being polymers of tubulin that form part of the cytoskeleton of cells.
In the treatment of cancer, there are two main classes of antimicrotubule drugs:
- Vinca alkaloids – vincristine, vinblastine, vinorelbine
- Taxanes – paclitaxel, docetaxel
Of course, these two classes are not the only medicines that disrupt microtubule function.
Other drugs that disrupt microtubule function include:
- Colchicine – a drug derived from the Autumn crocus [Colchicum autumnale], a medicine used to treat gout
- Griseofulvin – an antifungal drug derived from a Penicillium species
- Podophyllotoxin – a drug derived from the mayapple / wild mandrake plant; a drug used to treat genital warts and water warts
For the anticancer medicines, the original plant sources include:
- Madagascar periwinkle [Catharanthus roseus] – source of vinca alkaloids
- Pacific yew tree [Taxus brevifolia] – source of taxanes
Of course, synthetic variations are now created through other, more convenient means.
Antimicrotubule drugs are used to treat a wide variety of cancers.
- Vincristine – Hodgkin’s disease, non-Hodgkin’s lymphoma, leukemia, neuroblastoma
- Vinblastine – Hodgkin’s disease, lymphoma, breast cancer, testicular cancer
- Vinrelbine – non-small cell lung cancer
- Paclitaxel – ovarian cancer, breast cancer, Kaposi’s sarcoma, non-small cell lung cancer
- Docetaxel – breast cancer, head and neck cancer, prostate cancer, non-small cell lung cancer
Here, we’ve listed many of their most common uses – however, any of the above drugs may be used as part of a wider regimen to treat other cancers not listed in this guide.
With these indications in mind, let’s take a closer look at how both anticancer drug classes work for, though they are both antimicrotubule agents, they work by entirely different means.
Mechanism of action
Both drug classes – taxanes and vinca alkaloids – act on microtubules.
Microtubules form an essential, structural role; a fundamental part of the cytoskeleton. Microtubules are composed of two proteins:
Microtubules are needed for cell division, among other functions. Microtubules are not static structures. They are in a near-constant cycle of assembly and disassembly. That is where the two medicine classes come in.
One class impacts “assembly”, whereas the other class impacts “disassembly”.
Taxanes work by preventing microtubule disassembly – meaning cell division, or mitosis, cannot take place. In contrast, vinca alkaloids prevent microtubule formation – preventing assembly of tubulin into microtubulin.
Because cell division cannot take place, the cell is stunned into arrest before undergoing apoptosis, or programmed cell death.
Unlike other anticancer drug classes, such as the alkylating agents, antimicrotubule drugs are cell cycle specific – that is to say, they act at specific points along the cell cycle.
In contrast, alkylating agents are non-cell cycle specific – acting irrespective of the cell cycle phase.
- Vinca alkaloids bind to tubulin molecules in the S phase, preventing the necessary microtubule formation needed in M phase.
- Taxanes prevent cell cycle at different points. Paclitaxel prevents the cell cycle at the boundary of G2-M phases, whereas docetaxel prevents the cycle in the S phase.
Let’s now review the potential side effect profiles associated with both drug classes.
Side effects associated with taxanes include:
- Hair loss – because taxanes are cytotoxic to all rapidly dividing cells
- Low blood cell counts – particularly neutropenia
- Shortness of breath
- Nausea and vomiting
- Muscle pains and aches
- Nail color changes
Side effects associated with vinca alkaloids include:
- Hair loss
- GI effects – particularly constipation
- Peripheral neuropathy
- Loss of appetite
Of course, because these are anticancer drugs, they are linked to a wide variety of other potential side effects. Above, we’ve listed many of the most common side effects.
As well as this, both drug classes are linked to drug interactions.
Because anticancer drugs are given in regimens, the oncologist will determine the risk-ratio of the regimen and the potential for side effects, sometimes serious if not potentially fatal side effects. Ultimately, this is how treatment is determined.
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