Experimental and Computational Study on the Reaction Pathways of Diradical Intermediates Formed from Myers‐Saito Cyclization of Maleimide‐Based Enediynes

A carrier-free nanoparticle with dual NIR/acid responsiveness by co-assembly of enediyne and IR820 for combined PTT/chemotherapy

Combined photothermal therapy/chemotherapy by co-delivery of a photosensitizer (PS) and a chemotherapeutic drug has demonstrated great potential for cancer treatment. The intrinsic drawbacks of traditional drug delivery systems (DDSs), such as tedious synthetic procedures, side effects originated from the carrier materials, low loading efficiency, and uncontrolled drug release, however, have impaired their further advancement. On the other hand, enediyne antibiotics are highly cytotoxic toward cancer cells through the generation of lethal carbon radicals via thermal-induced cyclization, endowing them with great potential to achieve enhanced synergistic anticancer performance by incorporation with the photothermal effect of PS. To this end, a carrier-free and NIR/acid dual-responsive DDS was constructed for combined photothermal therapy/chemotherapy. The facile co-assembly of maleimide-based enediyne and PS IR820 was achieved in aqueous solution to give nanoparticles (EICN) with a hydrodynamic diameter of 90 nm and high stability. In vitro study confirmed the acid/NIR dual-responsive degradation and drug release, free radical generation and DNA-cleaving ability of EICN, which was accomplished by the corporation of enediyne and IR820 moieties. Further tests on HeLa cells verified the excellent synergistic anticancer performance of EICN including the improved cellular uptake, NIR-enhanced drug release, DNA damage and histone deacetylase inhibitor capacity. Overall, this carrier-free DDS with dual acid/NIR-responsivity would potentially provide new insights for the development of combined photothermal/chemotherapy.

Synthesis of maleimide-based enediynes with cyclopropane moieties for enhanced cytotoxicity under normoxic and hypoxic conditions

Myers-Saito cycloaromatization (MSC) is the working mechanism of many natural enediyne antibiotics with high antitumor potency. However, the presence of the equilibrium between diradical and zwitterionic intermediates in MSC severely hinders further improvement in cytotoxicity toward tumor cells. To this end, a series of maleimide-based enediynes with cyclopropane moieties were synthesized for enhanced cytotoxicity toward tumor cells. By taking advantage of radical clock reactions, the diradical intermediates generated from MSC would rearrange to new diradicals with much longer separation and weaker interactions between two radical centers. The computational study suggested a low energy barrier (4.4 kcal mol-1) for the radical rearrangement through the cyclopropane ring-opening process. Thermolysis experiments confirmed that this radical rearrangement results in the formation of a new diradical intermediate, followed by abstracting hydrogen atoms from 1,4-cyclohexadiene. Interestingly, the DNA cleavage ability and cytotoxicity of enediynes were significantly enhanced after the introduction of cyclopropane moieties. In addition, these maleimide-based enediynes exhibited a similar cytotoxicity under hypoxic conditions to that under normoxic conditions, which is beneficial for treating solid tumors where hypoxic environments frequently lead to deteriorated efficiency of many antitumor drugs. Docking studies indicated that the diradical intermediate was located between the minor groove of DNA with a binding energy of -7.40 kcal mol-1, which is in favor of intracellular DNA damage, and thereby inducing cell death via an apoptosis pathway as suggested by immunofluorescence analysis.