Special Report: Efficient agents for cancer treatment

fig1The Organic Chemistry Group of Coimbra Chemistry Centre discuss highly efficient theranostics agents for cancer.

A project focused on the development of novel photosensitisers to be used as near-infrared (NIR) agents for targeted photodynamic therapy and optical imaging of cancer tumours is underway at the Organic Chemistry Group, Coimbra Chemistry Centre, University of Coimbra, Portugal. The project, PDT-NanoBullet, funded by FCT, was awarded to Teresa MVD Pinho e Melo in 2015.

The research team has previously reported the synthesis of a new type of stable 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins and bacteriochlorins via an [8π+2π] cycloaddition of diazafulvenium methides with porphyrins and chlorins.1 Absorption spectra of these chlorins and bacteriochlorins revealed intense absorption bands within the therapeutic spectral window, at 650nm and 730nm, respectively. These favourable photophysical characteristics, combined with high thermal and photochemical stability, led us to evaluate the phototoxicity of some of the tetrahydropyrazolo[1,5-a]pyridine-fused chlorins in cancer cell lines.

Photodynamic therapy (PDT) has been successfully used in the treatment of skin cancers. However, treatment of melanoma with this method can be compromised due to the natural resistance mechanism of some melanoma cancer cells. In particular, high melanin levels in such pigmented tumours can lead to optical interference via competition with the photosensitiser for light absorption. This, together with the antioxidant effect of melanin, can affect the efficiency of PDT. Melanin is the dominant absorber in the 500-600nm spectral window, therefore photosensitisers that absorb at longer wavelengths are interesting targets.

In this context, preliminary studies on the photodynamic activity of 4,5,6,7-tetrahydropyrazolo[1,5- a]pyridine-fused chlorins against melanoma cells proved this class of compounds to be very active as photodynamic agents against melanotic (A375) and amelanotic (C32) cancer cells.2 Interestingly, dihydroxymethyl-chlorin 1 was particularly active against human melanocytic melanoma cells (IC50 = 31nm) (Fig. 1).

fig2Following from this, metabolic activity studies of A375 and C32 cells after photodynamic treatment with the chlorins, both in the presence and absence of the singlet oxygen quencher sodium azide and in the presence of the superoxide scavenger D-mannitol, were carried out. The addition of these inhibitors resulted in a decrease in the growth inhibition rate. Thus, both singlet oxygen and superoxide must be involved in the observed photodynamic activity. Experiments with A375 and C32 cells in the absence of light demonstrated that the cytotoxicity is light-dependent. PDT studies with different irradiances demonstrated that the cytotoxicity is also light-dose dependent. The generation of the active oxygen species only in the presence of light is one of the major advantages of PDT. Furthermore, if these cytotoxic species are generated near the target, high selectivity towards cancer cells can be achieved, minimising the side effects usually observed with common systemic drugs.

The research team is working on the development of targeted photodynamic therapy using nanobody-photosensitiser conjugates. Sabrina Oliveira, research fellow, Utrecht University and co-principal investigator of the project, has recently introduced a novel form of targeted PDT by conjugating nanobodies to photosensitisers.3 Nanobodies are the smallest naturally-derived antigen binding fragment, obtained from a different sort of antibodies that are composed only of heavy chains.4 Nanobodies are known to accumulate rapidly and specifically in tumours in xenograft models.5 Furthermore, such immunoconjugates can bind tightly with strong specificity in an efficient manner to cells over-expressing the target. For example the epidermal growth factor receptor (EGFR) is over-expressed in cancers relevant to this study, and this offers potential for clinical diagnosis and biomedical research for the investigation of biochemical processes at the cellular and subcellular levels. NIR emitters are particularly important as their light output is in a region where organisms are highly transparent. The incorporation of high atomic number metals, such as platinum, can enhance the triplet state properties of tetrapyrrolic macrocycles, and in many cases leads to long wavelength, room temperature phosphorescence. In this context, NIR luminescent compounds based on platinum(II) derivatives of 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins have been prepared and preliminary studies indicate that they are very promising theranostic cancer agents.6

fig3From a photophysical point of view, these platinum complexes possess the ideal properties of a NIR-emitting complex for time-resolved microscopy. Moreover, the phosphorescence intensity of the platinum complexes is strongly quenched in the presence of oxygen. In contrast, the fluorescence is relatively unaffected, thus providing the possibility for their use as ratiometric oxygen sensors in chemical and biological media. The phosphorescence lifetime is also reduced in the presence of oxygen, making these compounds suitable for applications in fluorescence lifetime imaging microscopy. Hence, these novel and stable NIR luminescent compounds based on platinum(II) derivatives of 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins are likely to be useful for theranostics not only to melanoma but also to oesophageal, bladder and colorectal cancers.

The PDT-NanoBullet project is being carried out by an interdisciplinary research team, which includes Filomena Botelho’s research group at IBILI, Faculty of Medicine, University of Coimbra and Oliveira’s research group, at the Department of Biology, Faculty of Science, Utrecht University and has the contribution of Mathias Senge (SFI Tetrapyrrole Laboratory, School of Chemistry, Trinity College Dublin, the University of Dublin, Ireland) as the project consultant.

The main objective for the next two years is to potentiate the results already achieved in order to develop efficient theranostic agents for cancer treatment.

PTDC/QEQ-MED/0262/2014 (Project’s Acronym: PDT-NanoBullet)

References

CCC logoU C logo
Teresa MVD Pinho e Melo
Associate Professor with Habilitation
Group Leader – Organic Chemistry Group of ‘Coimbra Chemistry Centre’ (I&D, FCT)
Department of Chemistry, University of Coimbra
+351 239854475

https://apps.uc.pt/mypage/faculty/tmelo/en