Supramolecular Photosensitizers Based on HMeQ[6] and Their Photodynamic Effects on Triple-Negative Breast Cancer Cells

The principal challenge in the development of efficient porphyrin-based photosensitizers is the intrinsic aggregation-induced quenching effect, which significantly impairs the generation efficiency of singlet oxygen (¹O₂) in photodynamic therapy (PDT). This study addresses this limitation through a supramolecular approach grounded in host-guest chemistry. Partially methyl-substituted cucurbit[6]uril (HMeQ[6]) was selected as the macrocyclic host owing to its smaller portal size and larger outer diameter, features that facilitate both strong binding affinity and effective spatial isolation. A porphyrin derivative functionalized with two cationic arms (DPPY) was designed and synthesized as the guest molecule. The results derived from ¹H NMR titration and UV spectroscopy analyses demonstrate that, in aqueous solution, these components self-assemble via host-guest interactions to form a 2:1 stoichiometric supramolecular complex (DPPY@HMeQ[6]) with a binding constant of 2.11 × 10⁵ M^-1. TEM, AFM, and DLS analyses indicate that this complex further assembles into nanosheet structures with dimensions of approximately 100 nm. Spectroscopic analyses reveal that encapsulation by HMeQ[6] effectively inhibits π-π stacking aggregation of DPPY molecules, resulting in an approximate threefold increase in fluorescence intensity and an extension of fluorescence lifetime from 3.2 ns to 6.2 ns. Relative to free DPPY, the complex demonstrates a sixfold enhancement in ¹O₂ generation efficiency. Subsequently, 4T1 cells, derived from mouse triple-negative breast tumors, were selected as the experimental model. These cells exhibit high invasiveness and metastatic potential, thereby effectively recapitulating the pathological progression of human triple-negative breast cancer. In vitro cellular assays indicate efficient internalization of the complex by 4T1 cells, inducing a concentration-dependent increase in reactive oxygen species (ROS) and oxidative stress following light irradiation. The in vitro cytotoxicity of the supramolecular photosensitizer was assessed employing the CCK-8 assay and flow cytometry techniques. The half-maximal inhibitory concentration (IC₅₀) against cancer cells is 1.8 μM, with apoptosis rates reaching up to 65.3%, while exhibiting minimal dark toxicity. This study expands the potential applications of methyl-substituted cucurbiturils within functional supramolecular assemblies and proposes a viable approach for the development of efficient and activatable supramolecular photosensitizers.