Microtubule Poisons

VincaAlkaloid and Taxanes

What are microtubules and what is there cellular function?

Components (filament type) of the cytoskeleton. The cytoskeleton provides cellular shape and structure.

What are the three types of filaments in the cytoskeleton?

• Actin Filaments

  • Tracks for myosin motors

  • Generate contractile forces, both in muscle and non-muscle cells.

• Intermediate Filaments

  • Organise the 3-D structure of the cell for example, by anchoring organelles

• Microtubules

  • Tracks for kinesins and dyneins

Microtubule Cellular Functions

• Maintain the structure and shape of cells by providing structural support (cytoskeleton)

• Provide a platform for Intracellular transport

  • Members of the two motor protein families, kinesins and dyneins move along microtubules in either the plus (cell periphery) and minus (cell center) ends respectively to unidirectionally transport a variety of cargoes such as vesicles, organelles and even chromosomes.

• Involvement is meiosis and mitosis

  • Help in chromosome alignment and separation

  • Formation of spindle apparatus.

What are microtubules formed of?

Microtubules are formed of highly dynamic structures composed of tubulin protein dimers, which consist of alpha and beta tubulin subunits that polymerize to create hollow tubes.

They form stable heterodimers that assemble into linear protofilaments, ultimately forming the cylindrical structure of microtubules.

In each protofilament, the heterodimers are oriented with the beta tubulin monomer pointing towards the faster-growing plus end and the alpha tubulin monomer pointing towards the slower growing minus end.

What is microtubule growth inittiated by

• y-tubulin

• Capping proteins

usually on the - end

How does microbial growth occur?

• by the addition of tubulins or dimers, a process driven by GTP hydrolysis called polymerisation

• Microtubules can shrink or depolymerase.

This polymerisation and depolymerisation is responsible for the highly dynamic nature of microtubules in the cell.

Microtubules are particularly dynamic during mitosis where their turnover is about 20 minutes.

Cell cycle steps

G1: cell grows, duplication of organelles

S : DNA replication, chromosomes are duplicates

G2: cell grow, preparation for mitosis

Mitosis : cell divides

G1,S,G2: Interphase

What are the cell cycle checkpoints?

The asses DNA damage and if found, the cell stalls the cycle to repair the damage or if a repair can’t be made, targets the cell for destruction via the apoptotic pathway.

G1/S: everything is ready for DNA replication

G2/S: Everything is ready to enter mitosis

Mitotic Checkpoint: make sure chromosomes are properly aligned in the metaphase plate

Microtubules Function in Mitosis

1. Prophase: duplicated centrosomes start to separate

2. Prometaphase: bipolar spindle starts to form. Chromosomes are captured by kinetochore MTs

3. Metaphase: Chromosomes are aligned at metaphase plate

4. Anaphase: sister chromatids start to separate, by moving to opposite spindle poles

5. Telophase & cytokinesis: genetic information is equally distributed between two daughter cells, which will physically separate at the end of process

What are two medicines known as Microtubule Poisons ?

• Plant Vinca Alkaloid: Vincristine/Vinblastine

• Taxanes: Paclitaxel

How do Vinca Alkaloid inhibitors work?

They depolymerase microtubules, and hence act as disruptors of MT assembly

Where are Vinca Alkaloids derived from

They are derived from the periwinkle plant Vinca rosea.

Natural products and semisynthetic derivatives

What is the difference between Vinblastine and Vincristine?

They are highly very structurally similar, they only vary in one position.

In this position, Vincristine has a formyl group substitute (R= CHO), while Vinblastine has a methyl group (R= CH3)

Mechanism of Action

They reversibly bind to:

• Free alpha-beta-tubulin heterodimers

• The ends of microtubules, where growth normally occurs (+ve end)

Therefore, Vinca Alkaloids disrupt the balance of the constant MT polyemrisation and depolymerisation by:

• Blocking microtubule assembly (they stop the tubulin from building into microtubules)

• Creating something called a “kinetic cap”, which stops any further growth

This therefore causes:

• Dissolution of MTs: Microtubules break down

• The mitotic spindle (needed to divide chromosomes during cell division) is destroyed

• Mitotic arrest: Cells get stuck in mitosis (the division phase) and can’t continue

• This leads to subsequent cell death

Uses of Vincristine

1. Leukaemia

2. Hodgkins and Non-Hodgkins lymphoma

3. Small cell lung cancer

4. Combination therapy: Multiple myeloma

Vinca Alkaloid Toxicity

Main Toxicity- Peripheral Neuropathy

• Damage or disease affecting nerves

• Pain and loss of deep tendon reflexes

• Motor dysfunction, ataxia, and paralysis

• Back, bone, and limb pains.

Neutropenia (DLT)

Where is Taxols dervied from?

Bark of Pacific Yew Tree

Taxol Mechanism of action

• Stabilises microtubules (MTs) and protects them for disassembly or depolymerisation

• This disrupts the normal dynamic balance of MT assembly/disassembly needed for mitosis.

• MTs become abnormally bundled, and cells can’t divide properly.

Does taxol exhibits concentration-dependent effects?

• Low doses: slow cell division

• High doses: major structural abnormalities, mitotic arrest

What are the differences observed between effects of Taxals in cell culture and tumour?

o Tumours: can lead to multipolar spindles, abnormal mitoses, and eventually apoptosis (cell death)

In cell culture: mitotic arrest (cells freeze during division) and lagging chromsomes

Uses of Taxels

Lung, ovarian, breast, head and neck,

bladder, prostate and advanced forms of Kaposi’s sarcoma

Toxicity of Taxol

• Neutropenia (Principal)

• neurotoxicity

• nausea & vomiting,

• alopecia

• myalgia (muscle pain),

• hypersensitivity reactions

• asthenia (weakness)

• Anaphylactic Shock

Taxol Structure

Taxol (Paclitaxel) has a complex structure with 11 chiral centres, which contributes to its lipophilic (fat-soluble)nature.

However, due to this lipophilicity, Taxol is not naturally soluble in water, and thus requires a formulation to be administered intravenously.

How is Taxol formulated to be soluble for IV administration?

1. Cremophor EL

• A polyethoxylated castor oil

  • Non-ionic surfactant, used to solubilise the drug and make it water miscible and bioavailable

• Ethanol is also included in the formulation to help dissolve Taxol.

What Side effects is Cremophor EL resposnible for?

• Anaphylactic shock (allergic reaction): This is a rare but serious side effect.

  • Phase I reactions: Up to 30% of patients may experience this, which includes hypersensitivity reactions, such as skin rash or difficulty breathing.

Why do the S/E happen?

• Cremophor EL can induce histamine release and other allergic responses, leading to the risk of anaphylactic reactions.

• The ethanol in the formulation may also contribute to some side effects, like flushing or discomfort at the injection site.

What is Abraxane

It is an Albumin Bound Taxol

Uses of Abraxane

It is used to treat Advanced breast cancer, non-small cell lung cancer, and advanced pancreatic cancer ((not justified anymore due to its limited benefits compared to current treatments).

Positives of Abraxne

1- Albumin Nanoparticle: In Abraxane, paclitaxel is bound to albumin (a naturally occurring protein in the body), forming nanoparticles. This binding helps paclitaxel be delivered more efficiently to tumor cells.

2- No Solvent Needed: Unlike Taxol, which requires Cremophor EL (a solvent that can cause side effects like anaphylaxis), Abraxane does not need a solvent for paclitaxel to be delivered. This reduces the risk of severe allergic reactions like anaphylactic shock that can occur with Taxol.

3- Abraxane allows for a much shorter infusion time—around 30 minutes—compared to 3 hours required for the traditional Taxol infusion.

• This makes it more convenient for patients and reduces the time spent in treatment.

Why is a shorter infusion time in Abraxane?

Abraxane is already in a nanoparticle form, which allows it to be more easily transported through the bloodstream and delivered directly to the tumor site. The albumin molecules help paclitaxel to cross cell membranes more efficiently, reducing the need for a slow infusion to prevent adverse effects.