Synthesis and Superpotent Anticancer Activity of Tubulysins Carrying Non‐hydrolysable N‐Substituents on Tubuvaline

Abstract

Synthetic tubulysins 24 a-m, containing non-hydrolysable N-substituents on tubuvaline (Tuv), were obtained in high purity and good overall yields using a multistep synthesis. A key step was the formation of differently N-substituted Ile-Tuv fragments 10 by using an aza-Michael reaction of azido-Ile derivatives 8 with the α,β-unsaturated oxo-thiazole 5. A structure-activity relationship study using a panel of human tumour cell lines showed strong anti-proliferative activity for all compounds 24 a-m, with IC values in the sub-nanomolar range, which were distinctly lower than those of tubulysin A, vinorelbine and paclitaxel. Furthermore, 24 a-m were able to overcome cross-resistance to paclitaxel and vinorelbine in two tumour cell lines with acquired resistance to doxorubicin. Compounds 24 e and 24 g were selected as leads to evaluate their mechanism of action. In vitro assays showed that both 24 e and 24 g interfere with tubulin polymerization in a vinca alkaloid-like manner and prevent paclitaxel-induced assembly of tubulin polymers. Both compounds exerted antimitotic activity and induced apoptosis in cancer cells at very low concentrations. Compound 24 e also exhibited potent antitumor activity at well tolerated doses on in vivo models of diffuse malignant peritoneal mesothelioma, such as MESOII peritoneal mesothelioma xenografts, the growth of which was not significantly affected by vinorelbine. These results indicate that synthetic tubulysins 24 could be used as standalone chemotherapeutic agents in difficult-to-treat cancers.

Thiazole Moiety: An Interesting Scaffold for Developing New Antitumoral Compounds

“…It blocks cells in G2/M phase of the cell cycle and stimulates apoptosis. In line with these results, attachment of alkyl groups at mentioned Tuv N 14 as benzyl, 4fluorobenzyl, and cyclopropylmethyl in tubulysins also led to superpotent cytotoxic activity [75]. More Tuv modifications have been reported, like the incorporation of tetrahydropyranyl ring by Diels-Alder reaction for conformational restriction of tubulysin [76], but rigidification seemed to affect negatively to polymerization inhibition.…”

Improved Total Synthesis of Tubulysins and Design, Synthesis, and Biological Evaluation of New Tubulysins with Highly Potent Cytotoxicities against Cancer Cells as Potential Payloads for Antibody–Drug Conjugates

“…The tubulysins are among the most potent cytotoxic compounds ever discovered from Nature. − Their mechanism of action involves depolymerization of microtubules with disintegration of the cytoskeleton as a consequence. − Isolated from the myxobacteria Archangium gephyra and Angiococcus disciformis , , these natural products elicited intense research efforts directed toward their total synthesis, analogue design and synthesis, and biological investigations as part of anticancer drug discovery and development programs. − Thus, total syntheses of the naturally occurring tubulysins A, B, C, D, − G, I, U ( Tb46 , Figure ), ,− and V ( Tb45 , Figure ) ,− and pretubulysin D ( PTb-D43 , Figure ), , as well as numerous analogues, have been accomplished. − From the latter, N 14 -desacetoxy­tubulysin H ( Tb1 , Figure ) is distinguished for its methyl, instead of the acyl methyl, substituent on N14 of tubulysins A–I ,− , and its high potency. , We have recently published a total synthesis of N 14 -desacetoxy­tubulysin H ( Tb1 , Figure ) and several of its analogues (e.g., Tb32 ,…”

Design, Synthesis, and Cytotoxic Activity of New Tubulysin Analogues

“…Structure-activity relationship studies (SAR) disclosed that the N,O-acetal group is labile to acidic and basic conditions and is unnecessary for tubulysin cytotoxicity. 11,12 The replacement of this group with N-alkyl substituents (methyl, benzyl, cyclopropylmethyl) resulted in analogues that remained anticancer activity in nanomolar range [13][14][15][16] and are at least double more active than those replaced with a hydrogen atom. 16,17 In addition, the chemical modifications at the Mep and Tup positions were well tolerated.…”

“…11,12 The replacement of this group with N-alkyl substituents (methyl, benzyl, cyclopropylmethyl) resulted in analogues that remained anticancer activity in nanomolar range [13][14][15][16] and are at least double more active than those replaced with a hydrogen atom. 16,17 In addition, the chemical modifications at the Mep and Tup positions were well tolerated. Recently, syntheses of simplified tubulysin analogues have focused mainly on the chemical modification of the N-and C-terminal amino acids and the N,O-acetal moiety.…”