Tumor microenvironment-responsive charge reversal zinc phthalocyanines based on amino acids for photodynamic therapy
Graphical abstract
Amino acid substituted zinc phthalocyanines which are capable of reversing their charge from negative to positive in response to tumor microenvironment have been prepared for photodynamic therapy.
Introduction
Cancer is a major public health problem in the world. Every year, over eight million casualties worldwide are due to cancer [1], [2]. Despite enormous efforts, there are still few existing anticancer drugs can meet the multiple requirements of effective transmission, high selectivity and excellent anticancer activity at the same time. Charge which is closely relate to blood transmission, uptake mechanisms, intracellular fate and activity of drugs has attracted people's considerable attention [3], [4], [5], [6]. Generally, neutral particles are prone to aggregate or precipitate in water [7]; positively charged particles have enhanced water solubility, cellular uptake rates and anticancer activity [3], [4], [5], [6], [8], [9], but can cause severe serum proteins inhibition and then induce hemolysis or get cleared from the body [3], [10], [11]; negatively charged particles have good blood transmission performance, while show poor cellular uptake [5], [9], [12]. Hence, ideally drugs should be negatively charged under normal physiological conditions to ensure effective blood transmission, while be positively charged in tumor tissues to obtain better cellular uptake and anticancer activity. Thus, the preparation of anticancer drugs which can switch their charge from negative to positive in response to tumor microenvironment will be very meaningful.
It is well known that the pH value in tumor tissue (∼6.5) is more acidic than in blood and normal tissue (∼7.4) [13], [14], [15]. On this basis, Shen, Wang and Lee have reported several tumor acidity activated charge reversal macromolecules containing β-carboxylic acid amide groups [11], [16], [17]. Na has reported charge-switchable nanoparticles containing imidazole and carboxyl groups [18]. However, most of the above charge reversible systems were based on macromolecules or nanoparticles, charge-conversion drug molecules have been reported rarely [19]. Furthermore, the development of alternative pH sensitive drugs that can accurately and efficiently respond to the tumor microenvironment is still great challenges because of the small pH difference between normal and tumor tissues. Photodynamic therapy (PDT) is a well consolidated, promising new treatment method for cancer [20], [21], [22]. The final therapeutic outcome of PDT is largely determined by photosensitizer (PS) [23], [24]. Due to the unique properties which are easy modification, strong absorption in phototherapeutic window (650–800 nm), low dark toxicity and high phototoxicity, phthalocyanines (Pcs) have been considered as promising PSs for PDT [25], [26].
In this study, we have prepared tumor microenvironment responsive zinc phthalocyanines (ZnPcs) which are capable of reversing their charge from negative to positive for PDT. We chose amino acids (AAs) as the pH sensitive moieties to conjugate with ZnPcs because they are typical of pH activated charge reversal compounds that are negatively charged when environmental pH (pHe) is above isoelectric pH (pI) and positively charged when pHe is under pI. The AAs based zinc phthalocyanines (AAZnPcs) are also expected to have good biocompatibility and high water solubility due to the presence of AAs which contain carboxyl and amino groups [27], [28]. To ensure the prepared AAZnPcs are capable of reversing their charge from negative to positive in response to the tumor microenvironment accurately, the pIs of AAZnPcs were adjusted to fall within the range of pH 6.5–7.4 by regulating the spaces between amino and carboxyl groups (Scheme 1a). After that, the charge reversal sensitivity of the candidate AAZnPcs were adjusted by increasing the number of suitable AA groups (Scheme 1b).
Section snippets
Materials and instrumentation
All necessary solvents were used after purification according to the reported literature [29]. All chemical reagents were obtained from commercial suppliers and used as received unless otherwise stated. 1, 8-diazabicyclo-[5.4.0] undec-7-ene (DBU), 9, 10-anthracenedipropionic acid (ADPA) and [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide] (MTT) were obtained from Sigma–Aldrich. Dulbecco's modified Eagle's medium (DMEM) was purchased from Gibco. Chromatographic purifications
Synthesis
The synthesis of the AAZnPcs (TyrZnPc, AspZnPc, GluZnPc, LysZnPc and OGluZnPc) and their precursors are shown in Scheme 2. The AAZnPcs were prepared through annelation reactions using DBU as the catalyst. It is worth mentioning that if the protected LysZnPc was first treated with KOH/NaOH at reflux, the LysZnPc could not be successfully obtained. This may be due to the easy hydrolysis of α-carboxylic acid amide.
pIs and zeta potentials of AAZnPcs
All of the AAZnPcs show good water solubility due to the presence of hydrophilic
Conclusion
In summary, we have prepared and characterized several pH sensitive AAZnPcs. The pIs of them were adjusted by regulating the spaces between amino and carboxyl groups to screen out charge reversible AAZnPcs in response to tumor microenvironment. The charge reversal sensitivity of candidate AAZnPcs was further enhanced by increasing the numbers of suitable AA groups. Finally, GluZnPc and OGluZnPc come to the fore because of their tumor microenvironment responsive charge reversal abilities. They
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21201102), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 13KJA150003), 2014 PhD thesis of excellent selection scheme (1812000002131), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials.
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The two authors contributed equally to this work.