A non-aggregated zinc(II) phthalocyanine with hexadeca cations for antitumor and antibacterial photodynamic therapies

Highlights

• A super cationic phthalocyanine with hexadeca quaternized ammoniums was synthesized.

• It is highly hydrophilic and completely non-aggregated in aqueous media.

• It shows highly efficient in vitro and in vivo photodynamic antitumor activities.

• It can also be used for photodynamic inactivation of bacteria efficiently.

Abstract

With a view to developing highly efficient photosensitizers for both antitumor and antimicrobial photodynamic therapies, herein, we reported a super cationic zinc(II) phthalocyanine (Pc4), which was prepared through the quaternization of the N, N-dimethyl-3-aminophenoxyl-hexadeca-substituted precursor Pc3. Meanwhile, two disubstituted analogues (Pc1 and Pc2) were also prepared as controls. The cationic Pc2 and Pc4 had higher photoactivities including fluorescence and singlet oxygen than the neutral counterparts Pc1 and Pc3, probably because of the inhibition of intramolecular charge transfer (ICT) effect of the amino groups. With the bulky steric effect and high hydrophilicity, Pc4 presented non-aggregated behavior in aqueous solutions. Therefore, it exhibited the highest in vitro photodynamic activity toward HepG2 cancer cells with an IC₅₀ value as low as 0.04 μM. Furthermore, Pc4 showed a highly efficient in vivo PDT effect on H22 tumor-bearing mice with 98.7% tumor growth inhibition. In addition, Pc4 also exhibited an excellent in vitro and in vivo photodynamic inactivation against S. aureus. The results indicate that the non-aggregated hexadeca-cationic Pc4 could serve as a promising photosensitizer for both antitumor and antimicrobial photodynamic therapies.

Introduction

As a non-invasive therapeutic modality, photodynamic therapy (PDT) has been wildly used for cancer treatment, which uses light to activate a photosensitizer (PS) and then generate cytotoxic reactive oxygen species (ROS) to eliminate tumor tissues [[1], [2], [3], [4]]. Besides cancer therapy, PDT has also been utilized for the treatment of pathogenic microorganism infection, namely antimicrobial PDT (APDT) [[5], [6], [7]]. With the ROS-induced mechanism, APDT can efficiently overcome the issue of drug-resistance, which is one of the most outstanding defect of antibiotic-based treatment [[8], [9], [10]]. Owing to some unique advantages, PDT and APDT have been caused considerable attention, and a large amount of effort has been devoted to develop ideal photosensitizers to improve the therapeutic efficacy [5,11].

Zinc(II) phthalocyanines (ZnPcs) are one of the most common and efficient PSs used in PDT owing to their strong absorption in red light region, highly efficient ROS generation, and low dark toxicity [2,12,13,41]. However, most phthalocyanines tend to aggregate in biological solutions, due to the high hydrophobicity of the conjugated macrocyclic skeleton, leading to the quenching of photodynamic activities [[14], [15], [16]]. Therefore, in order to improve the photodynamic efficacy, it is imperative to improve hydrophilicity and reduce aggregation of phthalocyanines, especially to obtain the phthalocyanines completely non-aggregated in aqueous media.

Introduction of cationic groups on the skeleton of phthalocyanines is an efficient strategy for improving their hydrophilicity [17,18]. In addition, it is believed that cationic PSs would be more conductive to cellular uptake of bacteria and enhancing photodynamic antibacterial efficacy, because of the negatively charged cell wall of bacteria [3,[19], [20], [21]]. In recent years, some cationic ZnPcs have been synthesized for either anticancer or antimicrobial PDT. For example, the cationic ZnPc substituted with 4-dimethylaminopyridine reported by Miretti and co-workers showed the higher inactivation toward Escherichia coli [22]. Fujishiro and co-workers also prepared the N-methyl-pyridinium-substituted cationic ZnPc, which exhibited a high photocytotoxicity toward HEp2 cells with IC₅₀ = 5.3 μM [18]. Recently, we also have reported a series of cationic phthalocyanines, such as N-methylated-morpholinyl-substituted ZnPcs [23], dodeca-cationic ZnPc containing trimethylaminomethyl moieties [24], and N-methylated-piperidine axially substituted silicon(VI) phthalocyanines (SiPcs) [25]. However, in these works, only the photodynamic antimicrobial activities were investigated.

As an ongoing interest in the development of cationic phthalocyanines, in this work, we reported a super cationic ZnPc (Pc4) with hexadeca quaternized ammoniums, which was prepared by N-methylation of its neutral counterpart Pc3 substituted with N, N-dimethyl-3-aminophenoxyl groups. Meanwhile, the disubstituted analogues (Pc1 and Pc2) were also prepared as controls. Both the photodynamic anticancer and antibacterial activities were evaluated. With the hexadeca quaternized ammoniums, Pc4 was completely in the form of monomer in aqueous solutions. Moreover, it showed significantly enhanced singlet oxygen generation efficiency compared with its precursor Pc3, because the intramolecular charge transfer (ICT) effect of Pc3 was eliminated after quaternization [26,27]. Therefore, Pc4 exhibited the highly efficient photodynamic anticancer and antibacterial activities.