Phthalocyanines are excellent photosensitizers for photodynamic therapy as they have strong absorbance in the near infra-red region which is most relevant for in vivo activation in deeper tissular regions. However, most phthalocyanines present two major challenges, ie, a strong tendency to aggregate and low water-solubility, limiting their effective usage clinically. In the present study, we evaluated the potential enhancement capability of glycerol substitution on the photodynamic properties of zinc (II) phthalocyanines (ZnPc). Three glycerol substituted ZnPc, 1-3, (tetra peripherally, tetra non-peripherally and mono iodinated tri non-peripherally respectively) were evaluated in terms of their spectroscopic properties, rate of singlet oxygen generation, partition coefficient (log P), intracellular uptake, photo-induced cytotoxicity and vascular occlusion efficiency. Tetrasulfonated ZnPc (ZnPcS4) was included as a reference compound. Here, we showed that 1-3 exhibited 10-100 nm red-shifted absorption peaks with higher molar absorptivity, and at least two-fold greater singlet oxygen generation rates compared to ZnPcS4. Meanwhile, phthalocyanines 1 and 2 showed more hydrophilic log P values than 3 consistent with the number of glycerol attachments but 3 was most readily taken up by cells compared to the rest. Both phthalocyanines 2 and 3 exhibited potent phototoxicity against MCF-7, HCT-116 and HSC-2 cancer cell-lines with IC50 ranging 2.8-3.2 µM and 0.04-0.06 µM respectively, while 1 and ZnPcS4 (up to 100 µM) failed to yield determinable IC50 values. In terms of vascular occlusion efficiency, phthalocyanine 3 showed better effects than 2 by causing total occlusion of vessels with diameter <70 µm of the chorioallantoic membrane. Meanwhile, no detectable vascular occlusion was observed for ZnPcS4 with treatment under similar experimental conditions. These findings provide evidence that glycerol substitution, in particular in structures 2 and 3, is able to improve the photodynamic properties of ZnPc.
We previously developed a new zinc(II) phthalocyanine (ZnPc) derivative (Pc 1) conjugated to poly-L-glutamic acid (PGA) (1-PG) to address the limitations of ZnPc as part of an antitumor photodynamic therapy approach, which include hydrophobicity, phototoxicity, and nonselectivity in biodistribution and tumor targeting. During this study, we discovered that 1-PG possessed high near-infrared (NIR) light absorptivity (λmax = 675 nm), good singlet oxygen generation efficiency in an aqueous environment, and enhanced photocytotoxic efficacy and cancer cell uptake in vitro. In the current study, we discovered that 1-PG accumulated in 4T1 mouse mammary tumors, with a retention time of up to 48 h. Furthermore, as part of an antitumor PDT, low dose 1-PG (2 mg of Pc 1 equivalent/kg) induced a greater tumor volume reduction (-74 ± 5%) when compared to high dose ZnPc (8 mg/kg, -50 ± 12%). At higher treatment doses (8 mg of Pc 1 equivalent/kg), 1-PG reduced tumor volume maximally (-91 ± 6%) and suppressed tumor size to a minimal level for up to 15 days. The kidney, liver, and lungs of the mice treated with 1-PG (both low and high doses) were free from 4T1 tumor metastasis at the end of the study. Telemetry-spectral-echocardiography studies also revealed that PGA (65 mg/kg) produced insignificant changes to the cardiovascular physiology of Wistar-Kyoto rats when administered in vivo. Results indicate that PGA displays an excellent cardiovascular safety profile, underlining its suitability for application as a nanodrug carrier in vivo. These current findings indicate the potential of 1-PG as a useful photosensitizer candidate for clinical PDT.