Targeting tumours: putting our best foot forward

Artwork combining green and blue colours with lines depicting circles and protein structure images

Thu, 02/01/2024 - 12:00

In an unremarkable African shrub is a hidden weapon in the fight against cancer: maytansine. This natural molecule, originally extracted from the Maytenus ovatus plant, is powerfully toxic to our cells, disrupting microtubule formation and impacting cell growth and regeneration. However, by targeting this molecule to attack only specific cells, it has the potential to be a weapon against cancer.

Disrupting tumour cell division

Let’s first start with microtubules - these are transport networks of the cell, transporting vital materials and orchestrating cellular activities. During cell division, they transform into a delicate spindle, ensuring each dividing cell receives its fair share of genetic material. Maytansine throws a wrench into this vital process. It binds to a key protein called beta-tubulin, the building block of microtubules, preventing the formation of these long fibres that are so vital for cell division. Without these microtubules, the cancer cells that are so dependent on rapid division are disrupted, effectively halting their growth.

But maytansine comes with a caveat. While potent against cancer cells, it's also incredibly toxic to our other cells, making clinically effective doses too dangerous for direct use in humans. However, there is hope with the development of modified maytansine molecules, known as maytansinoids. Examples that show particular promise are antibody-linked derivatives, known as antibody-maytansine conjugates (AMCs).

Targeting the molecular cargo

AMCs are designed to act like stealth bombers, with the antibody travelling to the site of attack (the tumour cells) where the payload (maytansinoid) is then released. The maytansinoid is linked to the antibody via a disulphide bond, which can be broken in the presence of glutathione. As the glutathione concentration is significantly higher in the cell than in the blood, the active maytansinoid molecule is released only once it reaches its desired destination.

Once inside the cell, the maytansinoid can disrupt microtubule formation, slow the cancer cell division and effectively halt its growth. AMCs have been found to be at least as potent as unmodified maytansine, with remarkable specificity to target cells.

Schematic image displaying cell in blue with the process of maytansinoids and antibodies entering the cell — Schematic image displaying the mechanism of AMCs. The antibody-bound maytanisoids reach the target cell by antibody-target recognition. Inside the cell, the maytansinoid is cleaving from the antibody, activating it to bind to beta-tubulin and disrupt microtubule formation.
*Image from Zafar, S. et al. (2023). New insights into the anticancer therapeutic potential of maytansine and its derivatives. Biomedicine & Pharmacotherapy, 165, 115039.* *https://doi.org/10.1016/j.biopha.2023.115039* *via Creative Commons CC BY 4.0:* *https://creativecommons.org/licenses/by/4.0/*.

Knowing your left and right

However, it has been noted that the potency of various AMCs can differ significantly, with some hundreds of times more effective in preventing microtubule formation than others. A recent study by Li et al. looked at two such maytansinoid molecules, each composed of maytansine with an additional C3 ester side chain that contains the thiol group required for antibody linking.

These two molecules differ only by the chirality around one specific methyl group on this side chain. A difference in chirality doesn’t involve any difference in the atoms that make up the molecule but only in their handedness. For example your left and right foot are essentially the same composition but are mirror images of each other. In this case, the two maytansinoid molecules are also mirror images, with one in the L-configuration (L-DM1-SMe) and the other in the D-configuration (D-DM1-SMe).

image displaying molecule chemical diagrams in black and white schematics — Image displaying the 2D chemical structure of D-DM1-SMe (CCD ID HZ0 in PDB entry 7e4r) and L-DM1-SMe (CCD ID BKX in PDB entry 7e4q). The shared, equivalent maytansine structure of these compounds is shown on the right, while on the right, the C3 ester side chain structure is displayed separately for each of D-DM1-SMe and L-DM1-SMe.

You may think at first that this is a minor change to the molecule, however this can have a significant impact on the binding of these molecules - think of putting your left foot in your right shoe! For these maytasinoids, the L-configuration has between 100 and 400 fold higher potency than the D-configuration.

Understanding conformations

Why is this difference in potency so significant? Li et al. set out to understand why, using X-ray crystallography to determine structures that could compare the binding to beta-tubulin of each of these maytansinoids in addition to an unmodified maytansine compound (maytansinol). The interactions between beta tubulin and the core maytansine region for each form of the C3 ester maytansinoids (and maytansinol) were broadly similar. However, the C3 ester side chain for each of the L- and D- forms were in different positions, due to twisting of the molecular structure around the chiral centre.

Superposition of L-DM1-SMe and D-DM1-SMe from PDB IDs 7e4q and 7e4r

This change in the C3 side chain conformation appears to be driven by changes in the interactions formed within the molecule itself, known as intramolecular interactions. These intramolecular interactions are present in the L-configuration structure but absent in the D-configuration, allowing the L-configuration maytansinoid to fold in on itself. This restricts the conformation of the structure, by fixing the side chain against the main maytansine structure. This restriction on the molecular flexibility reduces the ‘conformational entropy’ and in doing so increases the binding affinity.

But what does conformational entropy mean and why does this have an impact on binding affinity?

Let’s go back to our feet analogy. Imagine you are putting shoes on someone else’s feet. What if they were constantly flexing their foot and wiggling their toes? It might be difficult to get the shoe on. If you ask them to keep the foot still and in one place, they reduce the conformational entropy and it makes it easier to get the shoe on. If they kept their foot too rigid however, it might struggle to fit into their shoe. So there is a sweet spot for conformational entropy - flexible enough to find the best fit, but not so flexible that it makes it difficult to fit it in there in the first place!

What next?

What does all this information mean for the future of cancer treatment? Well, it highlights that AMCs are a great tool for targeted attack of tumour cells. They are already being used for treatment of breast cancer, with a number of clinical studies in recent years conducted on AMCs in various types of cancer. By understanding more about the specific conformations these different molecules adopt we can find the optimum tools for the job, and ensure that we put our best foot forward.

David Armstrong

About the artwork

Bella Frewin, from Thomas Gainsborough school, chose the cancer-fighting molecule maytansine as the subject of their layered artwork. Looking at PDB structures containing maytansine, Bella was captivated by the colours reflecting off the alpha helical coils of the protein's structure. She used paints and dyes to construct her artwork, layering two complementary pieces to create the final design. Through this creative exploration, she not only honed her artistic skills in layering, but also discovered a unique perspective on a potentially life-saving molecule.

View the artwork in the virtual 2023 PDB Art exhibition.