A novel fast green method for the preparation of the squaraine dye 3-oxo-4[(1,3,3-trimethyl-3H-indol-1-ium-2-yl)methylene]-2-[(1,3,3-trimethylindolin-2-ylidene)methyl]cyclobut-1-enolate, inner salt

D-(+)-Biotinylated squaraine dyes: A journey from synthetic conception, photophysical and -chemical characterization, to the exploration of their photoantitumoral action mechanisms

Biotin is primarily taken up by cells through sodium-dependent multivitamin transporter, which is highly expressed in aggressive cancer cell lines, often at levels surpassing those of the folate receptor. This makes biotin an attractive ligand for tumor-targeted drug delivery. Building on this rationale, this study presents a series of six D-(+)-biotin-conjugated squaraine dyes derived from benzothiazole, indolenine, and benz[e]indole, with N-ethyl and N-hexyl chains. These compounds were thoroughly characterized in terms of their photophysical and photochemical properties, revealing strong absorption in the so-called "phototherapeutic window", notable fluorescence, especially the benzothiazole derivatives, aqueous stability, particularly the indolenine-based dyes, and moderate to high photostability. Computational studies further indicated a strong binding affinity to human serum albumin and avidin proteins. All dyes exhibited photodynamic activity, with indolenine derivatives showing remarkable tumor selectivity and benz[e]indole analogs evidencing superior photocytotoxicity. The most promising compounds preferentially accumulated in mitochondria, and both singlet oxygen and other reactive oxygen species were found to play a role in their photobiological effects. Additionally, they were non-genotoxic in the absence of irradiation, and apoptosis was the primary mechanism of cell death upon light activation. This was evidenced by preserved cytoplasmic membrane integrity, nuclear fragmentation, and caspase-3/7 activation, reinforcing the safety and potential of these compounds as phototherapeutic agents. Although cellular uptake via the sodium-dependent multivitamin transporter was not established, and diffusion is expected to be the predominant mechanism, the high predicted avidin-binding affinity of these dyes opens exciting new avenues for photodynamic therapy-combined strategies.

'Lights, squaraines, action!' - the role of squaraine dyes in photodynamic therapy

Since they were first synthesized in 1965 by Treibs and Jacob, squaraine dyes have revolutionized the polymethine dyes' 'universe' and their potential applications due to their indisputable physical, chemical and biological properties. After 30 years and up to the present, various research teams have dedicated themselves to studying the squaraines' photodynamic therapy application using in vitro and in vivo models. The various structural modifications made to these compounds, as well as the influence they have shown to have in their phototherapeutic activity, are the main focus of the present review. Finally, the most evident limitations of this class of dyes, as well as future perspectives in the sense of hypothetically successfully overcoming them, are suggested by the authors.