Targeting tumor hypoxia: a third generation 2-nitroimidazole-indocyanine dye-conjugate with improved fluorescent yield

Abstract

Tumor hypoxia is associated with the rapid proliferation and growth of malignant tumors, and the ability to detect tumor hypoxia is important for predicting tumor response to anti-cancer treatments. We have developed a class of dye-conjugates that are related to indocyanine green (ICG, 1) to target tumor hypoxia, based on in vivo infrared fluorescence imaging using nitroimidazole moieties linked to indocyanine fluorescent dyes. We previously reported that linking 2-nitroimidazole to an indocyanine dicarboxylic acid dye derivative (2) using an ethanolamine linker (ethanolamine-2-nitroimidazole-ICG, 3), led to a dye-conjugate that gave promising results for targeting cancer hypoxia in vivo. Structural modification of the dye conjugate replaced the ethanolamine unit with a piperazineacetyl unit and led a second generation dye conjugate, piperzine-2-nitroimidazole-ICG (4). This second generation dye-conjugate showed improved targeting of tumor hypoxia when compared with 3. Based on the hypothesis that molecules with more planar and rigid structures have a higher fluorescence yield, as they could release less absorbed energy through molecular vibration or collision, we have developed a new 2-nitroimidazole ICG conjugate, 12, with two carbon atoms less in the polyene linker. Dye-conjugate 12 was prepared from our new dye (8), and coupled to 2-nitroimidazole using a piperazine linker to produce this third-generation dye-conjugate. Spectral measurements showed that the absorption/emission wavelengths of 657/670 were shifted ∼100 nm from the second-generation hypoxia dye of 755/780 nm. Its fluorescence quantum yield was measured to be 0.467, which is about 5 times higher than that of 4 (0.083). In vivo experiments were conducted with balb/c mice and 12 showed more than twice the average in vivo fluorescence intensity in the tumor beyond two hours post retro-orbital injection as compared with 4. These initial results suggest that 12 may significantly improve in vivo tumor hypoxia targeting.

The Structure and Activity of Double-Nitroimidazoles. A Mini-Review

“…As a result, the measured maximum fluorescence concentration of the tumors injected with compound 16 was two-fold higher than that after injection with ethanolamine-2-nitroimidazole-ICG within 3 h. Piperazine-2-nitroimidazole-ICG (16) is a novel second-generation tumor hypoxia-targeted conjugate. The results of biological evaluation suggest that the molecules which are more rigid and planar are characterized by higher fluorescence yield, and they might release less absorbed energy [16]. The same research group has reported a new substance after further chemical modifications with an improved level of fluorescence quantum yield.…”

“…Experiments in vivo were also promising. A new compound (17) showed more than two times the fluorescence intensity in the tumor as compared with a second-generation conjugate (16).…”

“…Another interesting approach to the synthesis and evaluation of bis-nitroimidazoles has been presented by Xu et al [15], in which the authors described the method for obtaining a more expanded structure of novel nitroimidazole indocyanine dye conjugate (2-nitroimidazole-ICG) (16) (Figure 8) using a piperazine linker for tumor-targeted hypoxia fluorescence tomography [15]. The activity of the new compound (16) was compared with that of the earlier known analogue of ethanolamine-2-nitroimidazole-ICG.…”

Treasure hunt for peptides with undefined chemical modifications: Proteomics identification of differential albumin adducts of 2‐nitroimidazole‐indocyanine green in hypoxic tumor

“…Selectively monitoring an exogenous bioreducible and traceable marker is one of the mainstream methods for hypoxia assessment, where 2‐nitroimidazole derivatives are typically used as the hypoxia tracer for indirect measurements . In contrast to 2‐nitroimidazole derivatives as established radiotracers for positron emission tomography (PET) to image tumor hypoxia, indocyanine green (ICG) conjugates of 2‐nitroimidazole are a novel class of near‐infrared (NIR) fluorescent tumor hypoxia tracers demonstrating superb in vivo detectability and biocompatibility . Built on a fluorophore analogous to the commercial ICG as the scaffold structure, this class of compounds has evolved for three generations as ICG conjugates differing in linker and imidazole structures (Scheme S1).…”

Antisense Cancer Therapy: Do Antisense Oligonucleotides Hold Promise as a Cure for Cancer?

“…Several groups have been working on fluorescence imaging techniques in biomedical sciences to design effective and cheap strategies to image cancer. Innus Mohammad and co-workers developed a strategy to image hypoxic regions in tumor cells utilizing Indocyanine Green bis-acid [3]. Several other groups are working in this area to improve the effectiveness of non-invasive detection of cancer at early stage [4][5][6][7].…”