Tubulin-inhibitors

Tubulin inhibitors are chemotherapeutic agents that directly interfere with the tubulin system (see microtubules below) and are in contrast to chemotherapeutic agents that act on DNA.

Microtubules play an important role in the function and division of eukaryotic cells. The structural proteins alpha- and beta-tubulin are the main components of microtubules. They have attracted considerable interest due to their function and biophysical properties and have become the subject of intense study.

Tubulin-binding molecules (tubulin -ligands): The addition of tubulin ligands can affect microtubule stability and function, including mitosis, cell movement, and intracellular organelle transport. Tubulin-binding molecules have attracted considerable medical interest, following the general use of vinca alkaloids, and the introduction of taxanes into clinical oncology. Tubulin-binding molecules inhibit cell mitosis by binding to the protein "tubulin" in the mitotic spindle, thereby preventing polymerization or depolymerization into microtubules. Tubulin inhibitors thus have a fundamentally different starting point than other cytostatic drugs that attack DNA. Colchicine was the first known compound to bind to tubulin. Colchicine, an alkaloid isolated from the autumn crocus (Colchicum autumnale), did not find its way into tumour therapy, but was successfully used in the treatment of acute gout attacks. The first oncological drugs from this drug family were the vinca alkaloids vinblastine and vincristine, isolated from extracts of the plant Catharanthus roseus (Vinca rosea). Later, other tubulin ligands were the taxanes (paclitaxel, docetaxel, cabazitaxel) extracted from the bark of the yew tree (Taxus brevifolia). However, their tubulin-inhibiting effect only became known in 1979. In the meantime, this substance group is produced synthetically. Paclitaxel was approved for chemotherapy in 1992.

Tubulin polymerization inhibitors

Colchicine binding site (colchicine-domain-binder):

• Colchicine
• Combrestatins (fosbreabulin, verubulin, crinobulin, ombrabulin)
• Methoxybenzenesulfonamides (E7010)
• Tirbanibulin (non-ATP-competitive inhibitor of proto-oncogenic non-receptor tyrosine kinase (SRC) + potent tubulin polymerization inhibitors). Tirbanibulin was shown to induce tubulin depolymerization and cell cycle arrest in G2/M phase at low nanomolar concentrations. The compound acts similarly to colchicine (Niu L et al. 2019).

Vinca alkaloid binding site:

• Vinblastine
• Vincristine
• Vinorelbine
• Vinflunine
• Dolastatin (brentuximab)
• Halichondrins (Eribulin)
• Maytansinoids (Mertansinde ADCs)
• Hemiasterol
• Cryptophysin 52

Estramustine binding site (Microtubule-associated protein)

• Estramustine: weakly inhibits the polymerization of tubulin to microtubules by binding to the "microtubule-associated protein". This binding site is different from the colchicine or vinblastine binding sites.

Tubulin depolymerization inhibitors

Paclitaxel binding site:

• Paclitaxel
• Epothilones (Ixaepilone, Patupilone, Sagopilone)
• Docetaxel
• Discodermolide

Agents that act as tubulin inhibitors also act as inhibitors of cell division. A microtubule is in a continuous dynamic state of growth and shortening through reversible association and dissociation of α/β-tubulin heterodimers at both ends. This dynamic behavior and the resulting control over the length of the microtubule is critical for the proper functioning of the mitotic spindle during mitosis, or cell division. Tubulin inhibitors act by interfering with microtubule dynamics, i.e., their growth (polymerization) and shortening (depolymerization).

Polymerization inhibitors: A class of inhibitors acts by inhibiting the polymerization of tubulin to form microtubules. These substances are called polymerization inhibitors. These include colchicine and various colchicine analogues. Colchicine analogues as well as the Vinca alkaloids. Groups of substances have different binding sites on the microtubili. At high concentrations, they reduce the mass of microtubule polymers in cells and act as microtubule destabilizers.

Binding site of tubulin inhibitors (colchicine and vinca alkaloids): Colchicine analogues block cell division by disrupting microtubules. Colchicine binding Involves the β-subunit of tubulin. Colchicine binds to the soluble tubulin. An and colchicine-tubulin complex is formed. This complex is then polymerized together with the normal tubulins to form the microtubule (T-C complex). However, the presence of this T-C complex causes a conformational change that prevents the tubulin dimers from further attachment. Further growth of the microtubule is prevented. As the T-C complex slows the addition of new dimers, the microtubule disintegrates due to structural imbalance or instability during the metaphase of mitosis.

Vinca alkaloids bind with high affinity to the β-subunit of tubulin d imers in the so-called vinca-binding domain. Unlike colchicine, the vinca alkaloids bind directly to the microtubule. However, they do not complex with soluble tubulin. However, this high-affinity binding leads to a conformational change of tubulin and a strong kinetic suppression of tubulin exchange. This is true even at low drug concentrations. There exists a second tubulin binding site of the vinca alkaloids but this is with low affinity. Even the low affinity binding leads to depolymerization of microtubules at relatively high drug concentration.

Depolymerization inhibitors (taxane group): The other class of inhibitors inhibits the depolymerization of polymerized tubulin, thereby increasing the mass of microtubule polymers in cells. In contrast to colchicine and vinca alkaloids, paclitaxel and versch. Paclitaxel analogues enhance microtubule polymerization by promoting both the nucleation and elongation phases of the polymerization reaction and by decreasing the critical tubulin subunit concentration (i.e., the concentration of soluble tubulin at steady state). Microtubules polymerized in the presence of paclitaxel are extremely stable. Thus, suppression of microtubule dynamics is thought to be the major cause of inhibition of cell division and tumor cell death in cells treated with paclitaxel.

1. Haider K et al (2019) Tubulin inhibitors as novel anticancer agents: an overview on patents (2013-2018). Expert Opin Ther Pat 29:623-641
2. Knaur R et al (2014) Recent developments in tubulin polymerization inhibitors: An overview European Journal of Medicinal Chemistry 87: 89-124.
3. Niu L et al (2019) Reversible binding of the anticancer drug KXO1 (tirbanibulin) to the colchicine-binding site of β-tubulin explains KXO1's low clinical toxicity. J Biol Chem 294:18099-18108.