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Tubulin and Antimitotics

The cellular scaffolding protein tubulin is the monomeric unit that forms the complex, highly dynamic network of cellular microtubules - gigantic, noncovalent polymeric assemblies that act as roads for cellular trafficking, parking lots for resting proteins, and as girders for applying or sensing forces within (and outside of) cells. The microtubules are continuously being built up, torn down, modified with chemical flags (PTMs), and coordinated by a host of proteins into rapidly changing shapes. These dynamics are crucial to the correct funcitioning of a massive range of anisotropic cellular processes, most famously, to the separation of genetic material and daughter cells during mitosis/meieosis. Many of the most powerful anticancer drugs (taxanes, vinca alkaloids, discodermolides) work by altering these microtubule dynamics so that cells can no longer divide correctly and instead go into apoptosis.

We achieved much success with a series of azobenzene-based microtubule inhibitors called PSTs that act as light-switchable versions of the known microtubule inhibitor CA4 (a) (2015 paper in Cell, highlighted in Nature, Cancer Discovery, etc). Precisely-targeted blue or green illumination generates either the bioactive cis isomer or the innocuous trans, so PSTs can be used in cellular microscopy to stop or restart mitosis at will in genetically unmodified animals - and so, study and alter the course of normal embryonic development (b). This level of cell-by-cell, reversible control over fundamental biological processes had never been possible without genetic engineering, prior to our work. Because blocking mitosis eventually results in cell death, we could also use PSTs to kill cells by pulsing them with brief blue-light illuminations, while cells kept in the dark suffer no ill effects (c). Because of the excellent photoswitching properties of the azobenzene core, we can also rapidly and reversibly switch cells between phases of microtubule blockage and normal dynamics (d), to study the influence of short-term temporal patterning of drug activity. PSTs have since been used by >100 research groups for cytoskeleton studies that are appearing in Science, Nature Cell Bio, etc (see Publications).

These exciting and previously impossible results were made possible by merging the right photoswitch unit with the appropriate drug core, and taking this through early biological testing. We are now stretching the chemical space of antimitotics (and developing a boutique photoswitch unit) to enable new generations of bioactive photoswitch drugs with drastically improved potency, in vivo compatibility, and other layers of functionality including tissue-retention, subcellular localisation and extreme long-wavelength response. All these qualities can be installed by appropriate chemical designs. Our second generation synthetic programs to specialise molecules for these diverse purposes include alternative chemical photoswitch scaffolds such as SBTs (Li) and HTIs (Alex) as well as alternative drug cores (Yvonne, Li and Adrian) (see also Publications); challenges and rewards await for chemically and/or biologically intrepid students eager to start work on the third generation compounds.

The principle of photoswitchable toxins also points the way towards light-localised antimitotic therapy. Applying photoswitchable compounds systemically, then locally activating them in tumours (by fibre optic, LED implant, etc), could reduce the systemic side-effects that hinder the success of current antimitotic chemotherapeutics. In a separate project, we are now performing preclinical trials of our molecules PST-1P and PST-2S in mouse tumour models for exactly this purpose (see Project CytoSwitch).

Lastly: a particular advantage of tubulin: it's easy to image so it's straightforward to understand when your reagents affect it. These pictures were taken with a cell phone (cell-phone) through our ordinary walk-on microscope (note: human eye has far better optical resolution than iPhone 6; objects in the picture are fescher than they appear. HeLa cells treated with low dose microtubule stabilising compound).

Click for higher resolution image.