Total Synthesis and Biological Evaluation of Tubulysin Analogues

Improved Access to Potent Anticancer Tubulysins and Linker-Functionalized Payloads Via an All-On-Resin Strategy

Tubulysins are among the most recent antimitotic compounds to enter into antibody/peptide-drug conjugate (ADC/PDC) development. Thus far, the design of the most promising tubulysin payloads relied on simplifying their structures, e. g., by using small tertiary amide N-substituents (Me, Et, Pr) on the tubuvaline residue. Cumbersome solution-phase approaches are typically used for both syntheses and functionalization with cleavable linkers. p-Aminobenzyl quaternary ammonium (PABQ) linkers were a remarkable advancement for targeted delivery, but the procedures to incorporate them into tubulysins are only of moderate efficiency. Here we describe a novel all-on-resin strategy permitting a loss-free resin linkage and an improved access to super potent tubulysin analogs showing close resemblance to the natural compounds. For the first time, a protocol enables the integration of on-resin tubulysin derivatization with, e. g., a maleimido-Val-Cit-PABQ linker, which is a notable progress for the payload-PABQ-linker technology. The strategy also allows tubulysin diversification of the internal amide N-substituent, thus enabling to screen a tubulysin library for the discovery of new potent analogs. This work provides ADC/PDC developers with new tools for both rapid access to new derivatives and easier linker-attachment and functionalization.

Tunable Cysteine-Targeting Electrophilic Heteroaromatic Warheads Induce Ferroptosis

Once considered potential liabilities, the modern era witnesses a renaissance of interest in covalent inhibitors in drug discovery. The available toolbox of electrophilic warheads is limited by constraints on tuning reactivity and selectivity. Following our work on a class of ferroptotic agents termed CETZOLEs, we discovered new tunable heterocyclic electrophiles which are capable of inducing ferroptosis. The biological evaluation demonstrated that thiazoles with an alkyne electrophile at the 2-position selectively induce ferroptosis with high potency. Density functional theory calculations and NMR kinetic studies demonstrated the ability of our heterocycles to undergo thiol addition, an apparent prerequisite for cytotoxicity. Chemoproteomic analysis indicated several potential targets, the most prominent among them being GPX4 protein. These results were further validated by western blot analysis and the cellular thermal shift assay. Incorporation of these heterocycles into appropriate pharmacophores generated highly cytotoxic agents such as the analogue BCP-T.A, with low nM IC50 values in ferroptosis-sensitive cell lines.

Structural convergence for tubulin binding of CPAP and vinca domain microtubule inhibitors

Microtubules are dynamic assemblies of αβ-tubulin that are involved in key cellular functions, including cell division and intracellular transport. Microtubule dynamics is inhibited by several families of small molecules, some of which are used in oncology. The extent to which these compounds target the binding sites of cellular partners of tubulin remains poorly characterized. We show here that a region of the CPAP protein binds to the so-called vinca domain of β-tubulin in a way very similar to that of peptide-like inhibitors produced by bacteria and fungi. Therefore, our work identifies a structural convergence for tubulin binding between inhibitors and a regulator of microtubule dynamics.

Microtubule dynamics is regulated by various cellular proteins and perturbed by small-molecule compounds. To what extent the mechanism of the former resembles that of the latter is an open question. We report here structures of tubulin bound to the PN2-3 domain of CPAP, a protein controlling the length of the centrioles. We show that an α-helix of the PN2-3 N-terminal region binds and caps the longitudinal surface of the tubulin β subunit. Moreover, a PN2-3 N-terminal stretch lies in a β-tubulin site also targeted by fungal and bacterial peptide-like inhibitors of the vinca domain, sharing a very similar binding mode with these compounds. Therefore, our results identify several characteristic features of cellular partners that bind to this site and highlight a structural convergence of CPAP with small-molecule inhibitors of microtubule assembly.

Synthetic Approaches Towards the Total synthesis of tubulysin and its fragments: A review

BACKGROUND

Tubulysins, linear tetrapeptides show extraordinary cytotoxicity against various cancer cells, with IC50 values in nano or picomolar range. Due to their extremely vigorous anti-proliferative and antiangiogenic characteristics, tubulysins exhibit captivating prospects in the development of anticancer drugs. This review focuses on diverse routes for the total synthesis of natural and synthetic tubulysins as well as their fragments.

OBJECTIVE

The purpose of this review is to present the synthetic strategies for the development of antitumor agents, tubulysins.

CONCLUSION

A range of synthetic pathways adopted for the total synthesis of tubulysins and their fragments have been described in this review. Synthesis of fragments, Tuv, Tup, and Tut can be accomplished by adopting appropriate strategies such as Manganese-mediated synthesis, Ireland-Claisen rearrangement, Mukaiyama aldol reaction, and Mannich process etc. Tubulysin B, D, U, V, and N14-desacetoxytubulysin H have been prepared through Mitsunobu reaction, tert-butanesulfinamide method, Tandem reaction, aza-Barbier reaction, Evans aldol reaction, and C-H activation strategies etc. The remarkable anticancer potential of tubulysins toward a substantiate target make them prominent leads for developing novel drugs against multidrug-resistant cancers.

Diastereocontrol in Radical Addition to β-Benzyloxy Hydrazones: Revised Approach to Tubuvaline and Synthesis of O-Benzyltubulysin V Benzyl Ester

Radical addition to chiral N-acylhydrazones has generated unusual amino acids tubuphenylalanine (Tup) and tubuvaline (Tuv) that are structural components of the tubulysin family of picomolar antimitotic agents and previously led to a tubulysin tetrapeptide analog with a C-terminal alcohol. To improve efficiency in this synthetic route to tubulysins, and to address difficulties in oxidation of the C-terminal alcohol, here we present two alternative routes to Tuv that (a) improve step economy, (b) provide modified conditions for Mn-mediated radical addition in the presence of aromatic heterocycles, and (c) expose an example of double diastereocontrol in radical addition to a β-benzyloxyhydrazone with broader implications for asymmetric amine synthesis via radical addition. An efficient coupling sequence affords 11-O-benzyltubulysin V benzyl ester.

Design, Synthesis, and Biological Evaluation of Tubulysin Analogues, Linker-Drugs, and Antibody-Drug Conjugates, Insights into Structure-Activity Relationships, and Tubulysin-Tubulin Binding Derived from X-ray Crystallographic Analysis

Molecular design, synthesis, and biological evaluation of tubulysin analogues, linker-drugs, and antibody-drug conjugates are described. Among the new discoveries reported is the identification of new potent analogues within the tubulysin family that carry a C11 alkyl ether substituent, rather than the usual ester structural motif at that position, a fact that endows the former with higher plasma stability than that of the latter. Also described herein are X-ray crystallographic analysis studies of two tubulin-tubulysin complexes formed within the α/β interface between two tubulin heterodimers and two highly potent tubulysin analogues, one of which exhibited a different binding mode to the one previously reported for tubulysin M. The X-ray crystallographic analysis-derived new insights into the binding modes of these tubulysin analogues explain their potencies and provide inspiration for further design, synthesis, and biological investigations within this class of antitumor agents. A number of these analogues were conjugated as payloads with appropriate linkers at different sites allowing their attachment onto targeting antibodies for cancer therapies. A number of such antibody-drug conjugates were constructed and tested, both in vivo and in vitro, leading to the identification of at least one promising ADC (Herceptin-LD3), warranting further investigations.

Tubulysin Synthesis Featuring Stereoselective Catalysis and Highly Convergent Multicomponent Assembly

A concise and modular total synthesis of the highly potent N14-desacetoxytubulysin H (1) has been accomplished in 18 steps in an overall yield of up to 30%. Our work highlights the complexity-augmenting and route-shortening power of diastereoselective multicomponent reaction (MCR) as well as the role of bulky ligands to perfectly control both the regioselective and diastereoselective synthesis of tubuphenylalanine in just two steps. The total synthesis not only provides an operationally simple and step economy but will also stimulate major advances in the development of new tubulysin analogues.

Design and validation of fibroblast activation protein alpha targeted imaging and therapeutic agents

Background: Cancer-associated fibroblasts (CAFs) comprise a major cell type in the tumor microenvironment where they support tumor growth and survival by producing extracellular matrix, secreting immunosuppressive cytokines, releasing growth factors, and facilitating metastases. Because tumors with elevated CAFs are characterized by poorer prognosis, considerable effort is focused on developing methods to quantitate, suppress and/or eliminate CAFs. We exploit the elevated expression of fibroblast activation protein (FAP) on CAFs to target imaging and therapeutic agents selectively to these fibroblasts in solid tumors. Methods: FAP-targeted optical imaging, radioimaging, and chemotherapeutic agents were synthesized by conjugating FAP ligand (FL) to either a fluorescent dye, technetium-99m, or tubulysin B hydrazide. In vitro and in vivo studies were performed to determine the specificity and selectivity of each conjugate for FAP in vitro and in vivo. Results: FAP-targeted imaging and therapeutic conjugates showed high binding specificity and affinity in the low nanomolar range. Injection of FAP-targeted 99mTc into tumor-bearing mice enabled facile detection of tumor xenografts with little off-target uptake. Optical imaging of malignant lesions was also readily achieved following intravenous injection of FAP-targeted near-infrared fluorescent dye. Finally, systemic administration of a tubulysin B conjugate of FL promoted complete eradication of solid tumors with no evidence of gross toxicity to the animals. Conclusion: In view of the near absence of FAP on healthy cells, we conclude that targeting of FAP on cancer-associated fibroblasts can enable highly specific imaging and therapy of solid tumors.

Direct Catalytic Asymmetric Synthesis of Trifluoromethylated γ-Amino Esters/Lactones via Umpolung Strategy

Enabled by the discovery of new cinchonium salts and coadditives, a direct and efficient asymmetric access to trifluoromethylated γ-amino esters/lactones has been realized through the enantioselective and diastereoselective umpolung reaction of trifluoromethyl imines with acrylates or α,β-unsaturated lactones as carbon electrophiles. At 0.5-5.0 mol % catalyst loadings, the newly developed catalytic system activates a variety of imine substrates as unconventional nucleophiles to mediate highly chemo-, regio-, diastereo-, and enantioselective C-C bond forming reactions. The developed synthetic protocol represents an excellent strategy to target a series of versatile and enantiomerically enriched γ-amino esters/lactones in good to excellent yields from the readily available starting materials. Additionally, we found that the epi-vinyl catalysts based on cinchonidine and quinine promote a similarly high enantioselective reaction generating the opposite configuration of chiral products in a highly efficient manner, which allows convenient access to either the R- or S-enantiomer of the chiral amine products in high yields and excellent enantioselectivities.

Catalytic Asymmetric Synthesis of Trifluoromethylated γ-Amino Acids through the Umpolung Addition of Trifluoromethyl Imines to Carboxylic Acid Derivatives

Novel cinchona alkaloid derived chiral phase-transfer catalysts enabled the highly chemo-, regio-, diastereo-, and enantioselective umpolung addition of trifluoromethyl imines to α,β-unsaturated N-acyl pyrroles. With a catalyst loading ranging from 0.2 to 5.0 mol %, this new catalytic asymmetric transformation provides facile and high-yielding access to highly enantiomerically enriched chiral trifluoromethylated γ-amino acids and γ-lactams.

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

Improved, streamlined total syntheses of natural tubulysins such as V (Tb45) and U (Tb46) and pretubulysin D (PTb-D43), and their application to the synthesis of designed tubulysin analogues (Tb44, PTb-D42, PTb-D47-PTb-D49, and Tb50-Tb120), are described. Cytotoxicity evaluation of the synthesized compounds against certain cancer cell lines revealed a number of novel analogues with exceptional potencies [e.g., Tb111: IC₅₀ = 40 pM against MES SA (uterine sarcoma) cell line; IC₅₀ = 6 pM against HEK 293T (human embryonic kidney cancer) cell line; and IC₅₀ = 1.54 nM against MES SA DX (MES SA with marked multidrug resistance) cell line]. These studies led to a set of valuable structure-activity relationships that provide guidance to further molecular design, synthesis, and biological evaluation studies. The extremely potent cytotoxic compounds discovered in these investigations are highly desirable as potential payloads for antibody-drug conjugates and other drug delivery systems for personalized targeted cancer chemotherapies.

Current Advances of Tubulin Inhibitors in Nanoparticle Drug Delivery and Vascular Disruption/Angiogenesis

Extensive research over the last decade has resulted in a number of highly potent tubulin polymerization inhibitors acting either as microtubule stabilizing agents (MSAs) or microtubule destabilizing agents (MDAs). These inhibitors have potent cytotoxicity against a broad spectrum of human tumor cell lines. In addition to cytotoxicity, a number of these tubulin inhibitors have exhibited abilities to inhibit formation of new blood vessels as well as disrupt existing blood vessels. Tubulin inhibitors as a vascular disrupting agents (VDAs), mainly from the MDA family, induce rapid tumor vessel occlusion and massive tumor necrosis. Thus, tubulin inhibitors have become increasingly popular in the field of tumor vasculature. However, their pharmaceutical application is halted by a number of limitations including poor solubility and toxicity. Thus, recently, there has been considerable interests in the nanoparticle drug delivery of tubulin inhibitors to circumvent those limitations. This article reviews recent advances in nanoparticle based drug delivery for tubulin inhibitors as well as their tumor vasculature disruption properties.