Mitotic inhibitor

A mitotic inhibitor, microtubule inhibitor, or tubulin inhibitor, is a drug that inhibits mitosis, or cell division, and is used in treating cancer, gout, and nail fungus. These drugs disrupt microtubules, which are structures that pull the chromosomes apart when a cell divides. Mitotic inhibitors are used in cancer treatment, because cancer cells are able to grow through continuous division that eventually spread through the body (metastasize). Thus, cancer cells are more sensitive to inhibition of mitosis than normal cells. Mitotic inhibitors are also used in cytogenetics (the study of chromosomes), where they stop cell division at a stage where chromosomes can be easily examined. Mitotic inhibitors are derived from natural substances such as plant alkaloids, and prevent cells from undergoing mitosis by disrupting microtubule polymerization, thus preventing cancerous growth. Microtubules are long, ropelike proteins, long polymers made of smaller units (monomers) of the protein tubulin, that extend through the cell and move cellular components around. Microtubules are created during normal cell functions by assembling (polymerizing) tubulin components, and are disassembled when they are no longer needed. One of the important functions of microtubules is to move and separate chromosomes and other components of the cell for cell division (mitosis). Mitotic inhibitors interfere with the assembly and disassembly of tubulin into microtubule polymers. This interrupts cell division, usually during the mitosis (M) phase of the cell cycle when two sets of fully formed chromosomes are supposed to separate into daughter cells. Tubulin binding molecules have generated significant interest after the introduction of the taxanes into clinical oncology and the general use of the vinca alkaloids. Examples of mitotic inhibitors frequently used in the treatment of cancer include paclitaxel, docetaxel, vinblastine, vincristine, and vinorelbine. Colchicine and griseofulvin are mitotic inhibitors used in the treatment of gout and nail fungus, respectively.

Microtubules

Microtubules are the key components of the cytoskeleton of eukaryotic cells and have an important role in various cellular functions such as intracellular migration and transport, cell shape maintenance, polarity, cell signaling and mitosis. They play a critical role in cell division by their involvement in the movement and attachment of chromosomes during various stages of mitosis. Therefore, microtubule dynamics are an important target for the developing anti-cancer drugs.

Structure Microtubules are composed of two globular protein subunits, α- and β-tubulin. These two subunits combine to form an α,β-heterodimer which then assembles in a filamentous tube-shaped structure. The tubulin hetero-dimers arrange themselves in a head to tail manner with the α-subunit of one dimer coming in contact with the β-subunit of the other. This arrangement results in the formation of long protein fibres called protofilaments. These protofilaments form the backbone of the hollow, cylindrical microtubule, which is about 25 nanometers in diameter and varies from 200 nanometers to 25 micrometers in length. About 12–13 protofilaments arrange themselves in parallel to form a C-shaped protein sheet, which then curls around to give a pipe-like structure called the microtubule. The head to tail arrangement of the hetero dimers gives polarity to the resulting microtubule, which has an α-subunit at one end and a β-subunit at the other end. The α-tubulin end has negative (–) charges while the β-tubulin end has positive (+) charges. The microtubule grows from discrete assembly sites in the cells called Microtubule organizing centers (MTOCs), which are networks of microtubule associated proteins (MAP). Two molecules of energy rich guanosine triphosphate (GTP) are also important components of the microtubule structure. One molecule of GTP is tightly bound to the α-tubulin and is non-exchangeable whereas the other GTP molecule is bound to β-tubulin and can be easily exchanged with guanosine diphosphate (GDP). The stability of the microtubule will depend on whether the β-end is occupied by GTP or GDP. A microtubule having a GTP molecule at the β-end will be stable and continue to grow whereas a microtubule having a GDP molecule at the β-end will be unstable and will depolymerise rapidly.

Microtubule dynamics Microtubules are not static but they are highly dynamic polymers and exhibit two kinds of dynamic behaviors : 'dynamic instability' and 'treadmilling'. Dynamic instability is a process in which the microtubule ends switches between periods of growth and shortening. The two ends are not equal; the α-tubulin ringed (-)end is less dynamic while the more dynamic β-tubulin ringed (+) end grows and shortens more rapidly. Microtubules undergo long periods of slow lengthening, brief periods of rapid shortening and also pauses in which there is neither growth nor shortening. Dynamic instability is characterized by four variables: the rate of microtubule growth; the rate of shortening; frequency of transition from the growth or paused state to shortening (called a 'catastrophe') and the frequency of transition from shortening to growth or pause (called a 'rescue'). The other dynamic behavior called treadmilling is the net growth of the microtubule at one end and the net shortening at the other end. It involves the intrinsic flow of tubulin sub-units from the plus end to the minus end. Both the dynamic behaviors are important and a particular microtubule may exhibit primarily dynamic instability, treadmilling or a mixture of both.

Mechanism of action

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