Maytansinoids continue to excite interest almost after a decade after their discovery as potent antimitotic tubulin binders.[1–3] Their highly efficient mechanism of capping MT-dynamics[4] has led to their successful application in cancer treatment as antibody-drug conjugates (ADCs).[5–7] This success has further prompted their development as targeted cancer therapeutics, for example, in the form of nanoparticles[8,9] or recently reported immune checkpoint-targeting maytansinoid conjugates.[10] However, the high-affinity of maytansine towards β-tubulin (KD=6.8–14 nM) not only makes it well suited as a cytotoxin, but also derivatives with a similar affinity could be used as molecular probes.[11,12] As of today, effortless generation of maytansine-based molecular probes is hampered by major drawbacks such as the complexity of the natural product scaffold and lack of SAR studies, which could suggest suitable points for attachment of fluorophore tags or radionuclides. Here, we report on both of these aspects: the chemistry of maytansinol (Figure 1, 1) for the creation of maytansinoid conjugates and the suitability of the C3-position to tolerate bulky substituents without affecting the binding mode. Therefore,
we provide a solid basis for in-depth exploration of maytansinoids as molecular probes to study microtubule dynamics. Previously, it has been shown that the ester at the C-3 position of ansamitocins, maytansine, and maytansinoids plays an important role in biological activity and cell permeability.[3,13,14] In our recent study, we extensively investigated a series of C-3 derivatized maytansinoids obtained by acylation of maytansinol.[12] By X-ray crystallography experiments, we observed that the studied maytansinoids retained a fundamental spatial arrangement with respect to β-tubulin. The binding mode of the core structure of maytansinol remained unaltered independently of the introduced substituents. Furthermore, the binding affinities of the C3-substituted maytansinoids obtained are very similar to the one of maytansine. Based on these observations, we decided to further investigate the potential of C-3 functionalized maytansinoids as molecular probes by creating novel tubulin binders. The design of novel maytansinoids was led by the high-resolution X-ray
crystallography structures of recently obtained maytansinoidtubulin complexes (PDB IDs: 5SB9, 5SBA and 5SBB). The maytansine binding site is located in proximity of the GDP nucleotide bound to the E-site and a GDP-coordinated Mg2+. The first interesting factor that could be exploited to exert a
novel effect on MTs is the Mg2+ ion coordinated by the nucleotide in close proximity to the maytansine binding site. The design of bivalent compounds containing maytansinol and nucleotide mimetics would a) increase the understanding of the ability of maytansinoids to accommodate in the binding site
independently of the size of the substituents, and b) investigate the ability of these molecules to interact with Mg2+ and to displace the nucleotide from the binding site. Tubulin inhibition by nucleotide analogues is very challenging because of GTP concentrations around 300 μM inside the cells.[15] However, we sought to investigate whether the presence of the maytansinol moiety in a bivalent compound could favor binding of the nucleotide portion through an
entropic effect. The maytansinol moiety would act as an anchor point holding the modified nucleotide portion in close proximity of the E-site through a flexible linker, thereby favoring the nucleotide exchange. Accordingly, we decided to prepare long-chain maytansinol derivatives and maytansinoid conjugates to target either the Mg2+ ion or the nucleotide binding site. Introducing different ubstituents at the 3-O position should allow the protein-ligand
interaction to be optimized and new molecules capable of producing different effects on tubulin structure and dynamics to be found