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Monoamine oxidase (MAO) is an important drug target as the MAO isoforms play key roles in neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, as well as in neuropsychiatric diseases such as depression. Methylene blue is an inhibitor of MAO-A, while azure B, the major metabolite of methylene blue, and various other structural analogues retain the ability to inhibit MAO-A. Based on this, the present study evaluated 22 dyes, many of which are structurally related to methylene blue, as potential inhibitors of human MAO-A and MAO-B. The results highlighted three dye compounds as good potency competitive and reversible MAO inhibitors, and which exhibit higher MAO inhibition than methylene blue: acridine orange, oxazine 170 and Darrow red. Acridine orange was found to be a MAO-A specific inhibitor (IC 50  = 0.017 μM), whereas oxazine 170 is a MAO-B specific inhibitor (IC 50  = 0.0065 μM). Darrow red was found to be a non-specific MAO inhibitor (MAO-A, IC 50  = 0.059 μM; MAO-B, IC 50  = 0.065 μM). These compounds may be advanced for further testing and preclinical development, or be used as possible lead compounds for the future design of MAO inhibitors., (© 2019 John Wiley & Sons A/S.)

A method of marking adult Culex quinquefasciatus by feeding the larvae commercial hog chow dyed with methylene blue, Giemsa, and crystal violet was evaluated under laboratory conditions. Of 243 mosquitoes fed the dyed food, 230 had visible marks (94.6%). The dyed food increased the egg-adult development time from 11.4 to 12.1 d. After 9 d, 82.5% of adult mosquitoes dyed as larvae could be identified, and remained detectable for up to 15 d, their maximum laboratory life.

The photodynamic antibacterial properties of a closely related series of phenothiazinium dyes were tested against several pathogenic strains of Staphylococcus aureus, four of which were methicillin-resistant. Illumination of the photosensitisers at a fluence rate of 1.75 mW cm-2 generally resulted in the enhancement of antibacterial activity in liquid culture and in greater efficacy than the methicillin analogue flucloxacillin. For methylene blue, dimethyl methylene blue and new methylene blue illumination led to increases in bactericidal activity < or = 16-fold, typically 4-fold. In addition dimethyl methylene blue and new methylene blue were active against epidemic strains of methicillin-resistant Staphylococcus aureus at concentrations lower than that of vancomycin (> or = 0.5 microM).

Twenty-seven compounds were screened for their ability to sensitize Streptococcus sanguis to killing by light from a 7.3-mW Helium/Neon (HeNe) laser. Bacteria were mixed with various concentrations of the test compounds, spread over the surfaces of agar plates, and then exposed to light from the HeNe laser for various time periods. The plates were then incubated and examined for zones of inhibition. Those compounds found to be effective photosensitizers were then tested against Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, and Fusobacterium nucleatum. Toluidine blue O, azure B chloride, and methylene blue at concentrations of 0.005% (wt/vol) were effective photosensitizers of all four species, enabling killing of bacteria following exposure to laser light for only 30 s.

Experiments on rabbits showed that intravenous injection of dyestuffs of the thionine series (toluidine blue, azur A, 1:9 dimethylene blue) was accompanied by hypofibrinogenemia, a decrease in the concentration of prothrombin complex factors, thrombocytopenia and the lowering of the blood platelets adhesion against the background of delayed blood thrombogenesis. The blood heparin tolerance and the amount of free heparin were sharply lowered. The authors consider that the hypocoagulative effect of cationic dyestuffs on the blood was caused by the thrombocytopenia and by the lowering of the platelet aggregation activity.