Synthesis, Biological Profiling and Determination of the Tubulin-Bound Conformation of 12-Aza-Epothilones (Azathilones)

Abstract

12-Aza-epothilones (azathilones) incorporating quinoline side chains and bearing different N12-substituents have been synthesized via highly efficient RCM-based macrocyclizations. Quinoline-based azathilones with the side chain N-atom in the meta-position to the C15 atom in the macrocycle are highly potent inhibitors of cancer cell growth in vitro. In contrast, shifting the quinoline nitrogen to the position para to C15 leads to a ca. 1000-fold loss in potency. Likewise, the desaturation of the C9-C10 bond in the macrocycle to an E double bond produces a substantial reduction in antiproliferative activity. This is in stark contrast to the effect exerted by the same modification in the natural epothilone macrocycle. The conformation of a representative azathilone bound to α/β-tubulin heterodimers was determined based on TR-NOE measurements and a model for the posture of the compound in its binding site on β-tubulin was deduced through a combination of STD measurements and CORCEMA-ST calculations. The tubulin-bound, bioactive conformation of azathilones was found to be overall similar to that of epothilones A and B.

Synthesis, Microtubule-Binding Affinity, and Antiproliferative Activity of New Epothilone Analogs and of an EGFR-Targeted Epothilone-Peptide Conjugate

“…Not unexpectedly, the microtubule-binding affinity of the simplified epothilone analog 1 even exceeds that of natural Epo A (K b of 7.85 × 10 7 M −1 vs. 3.17 × 10 7 M −1 for Epo A; Table 1 ); this observation is in line with existing SAR data for other epothilone analogs that are derived from trans -Epo A and/or incorporate a benzimidazole side chain [ 24 , 25 , 44 ], although analog 1 has not been investigated previously. Compared to 1 , the K b for thioacetate 14 is considerably lower (>10-fold), but its microtubule-binding affinity is still comparable to that of other epothilone analogs that have shown nanomolar antiproliferative activity against different cancer cell lines [ 45 ]. Intriguingly, analog 16 and conjugate 4 exhibit similar microtubule-binding affinity as thioacetate 14 , in spite of the significant enlargement in size of the substituent group on the benzimidazole side chain.…”

Epothilones as Natural Compounds for Novel Anticancer Drugs Development

“…It is known that the C12-13 epoxide is not essential for binding to microtubules because deoxyepothylone B (Epothilone D, UTD1 or KOS-862) lacking the epoxide, is more active in microtubule stabilizing in vitro when compared to epothilone A or B. Furthermore, derivatives containing nitrogen at C12 (azathilones) show higher pharmacological activity in vitro [ 10 , 49 ].…”

Breakthroughs in Medicinal Chemistry: New Targets and Mechanisms, New Drugs, New Hopes

“…One approach to natural product-based medicinal chemistry is the development of synthetic technologies to facilitate the synthesis of new analogues for structure-activity relationship (SAR) studies. Karl-Heinz Altmann and his team [ 13 ] used ring-closing olefin metathesis as a key synthetic technology to prepare 12-aza-epothilones (also known as azathilones). Highly potent inhibitors of cancer cell growth in vitro were obtained, and meaningful SARs were delineated .…”