Pentalysine beta-carbonylphthalocyanine zinc: an effective tumor-targeting photosensitizer for photodynamic therapy

Abstract

Unsymmetrical phthalocyanine derivatives have been widely studied as photosensitizers for photodynamic therapy (PDT), targeting various tumor types. However, the preparation of unsymmetrical phthalocyanines is always a challenge due to the presence of many possible structural isomers. Herein we report a new unsymmetrical zinc phthalocyanine, pentalysine beta-carbonylphthalocyanine zinc (ZnPc-(Lys)(5)), that was prepared in large quantity and high purity. This is a water-soluble cationic photosensitizer and maintains a high quantum yield of singlet oxygen generation similar to that of unsubstituted zinc phthalocyanine (ZnPc). Compared with anionic ZnPc counterparts, ZnPc-(Lys)(5) shows a higher level cellular uptake and 20-fold higher phototoxicity toward tumor cells. Pharmacokinetics and PDT studies of ZnPc-(Lys)(5) in S180 tumor-bearing mice showed a high ratio of tumor versus skin retention and significant tumor inhibition. This new molecular framework will allow synthetic diversity in the number of lysine residues incorporated and will facilitate future QSAR studies.

Figure 1

The C₁₈ reversed-phase HPLC chromatogram of ZnPc-(Lys)₅ (structure shown) and the UV/Vis absorption spectrum in DMSO (inset).

Figure 2

Cellular uptake of the ZnPc photosensitizers (● ZnPc-(Lys)₅, ○ ZnPc-S₂P₂, ▼ ZnPc-S₄) in a) BGC-823 cells and b) HELF cells after 2 h incubation with each photosensitizer at various concentrations. Values represent the mean of three separate experiments; bars represent standard error of the mean (SEM). c) The internalization of ZnPc photosensitizers by K562 cells after 2 h incubation.

Figure 3

Phototoxicity (light dosage: 1.5 J cm⁻² at λ 670 nm, left) and dark toxicity (right) after incubation with ZnPc photosensitizers (● ZnPc-(Lys)₅, ○ ZnPc-S₂P₂, ▼ ZnPc-S₄): a) K562 cells, 2 h incubation; b) K562 cells, 24 h incubation; c) HELF cells, 2 h incubation. Data represent the mean ± SEM of three independent experiments, each performed in triplicate.

Figure 4

Plasma concentration of ZnPc-(Lys)₅ (2 mg kg⁻¹ i.v.) as a function of time in S180 tumor-bearing mice.

Figure 5

Tissue concentrations (mean ± SEM) of ZnPc-(Lys)₅ as a function of time after i.v. injection (2 mg kg⁻¹) in S180 tumor-bearing mice (n =5).

Scheme 1

Synthesis of pentalysine β-carbonylphthalocyanine zinc (7, ZnPc-(Lys)₅) from the previously described monosubstituted β-carboxyphthalocyanine zinc (5, ZnPc-COOH).[20] Complex 5 was purified on a self-packed silica gel column. Reagents and conditions: a) (NH₂)₂CO, Zn(OAc)₂, (NH₄)₂MoO₄, 170 °C, 4 h; b) KOH (1 M), 100 °C, 24 h; c) DCC, HOBt, 25 °C, 24 h; d) TFA (95 %), 25 °C, 3 h. The final yield of 7 is 33.8% based on 5 (ZnPc-COOH).