Date Published: May 29, 2019
Publisher: Public Library of Science
Author(s): Shin-Hong Shiao, Shih-Che Weng, Liqiang Luan, Maria da Graça H. Vicente, Xiong-Jie Jiang, Dennis K. P. Ng, Bala Krishna Kolli, Kwang Poo Chang, Michael R. Hamblin.
Mosquitoes are significant vectors, responsible for transmitting serious infectious diseases, including the recent epidemics of global significance caused by, for example, Zika, Dengue and Chikungunya viruses. The chemical insecticides in use for mosquito control are toxic and ineffective due to the development of resistance to them. The new approach to reduce mosquito population by releasing genetically modified males to cause female infertility is still under environmental safety evaluation. Photodynamic insecticides (PDI) have long been known as a safe and effective alternative by using dyes as the photosensitizers (PS) for activation with light to generate insecticidal singlet oxygen and reactive oxygen species. This approach warrants re-examination with advances in the chemical synthesis of novel PS, e.g. phthalocyanines (PC). Nine PC were compared with five porphyrin derivatives and two classic PS of halogenated fluoresceins, i.e. cyanosine and rose bengal experimentally for photodynamic treatment (PDT) of the larvae of laboratory-reared Aedes mosquitoes and their cell lines. Groups of 2nd instar larvae were first exposed overnight to graded concentrations of each PS in the dark followed by their exposure to dim light for up to 7 hours. Larvae of both experimental and control groups were examined hourly for viability based on their motility. Monolayers of mosquito cells were similarly PS-sensitized and exposed briefly to light at the PS-specific excitation wavelengths. Cell viability was assessed by MTT reduction assays. Of the 16 PS examined for photodynamic inactivation of the mosquito larvae, effective are three novel PC, i.e. amino-Si-PC1 and -PC2, anilinium Zn-PC3.4, pyridyloxy Si-PC14 and two porphyrin derivatives, i.e. TPPS2 and TMAP. Their EC50 values were determined, all falling in the nanomolar range lower than those of rose bengal and cyanosine. All PS effective in vivo were also found to dose-dependently inactivate mosquito cells photodynamically in vitro, providing cellular basis for their larvicidal activities. The present findings of novel PC with effective photodynamic larvicidal activities provide fresh impetus to the development of PDI with their established advantages in safety and efficacy. Toward that end, the insect cell lines are of value for rapid screening of new PC. The optimal excitability of PC with insect-invisible red light is inferred to have the potential to broaden the range of targetable insect pests.
Photodynamic therapy or treatment (PDT) is referred to the use of dyes as photosensitizers (PS) for light excitation to produce biocidal oxidative radicals in the presence of oxygen. PDT has long been used clinically for treating patients with solid tumors, certain skin diseases, infection and other medical conditions . The application of PDT to control insect pests has been studied since the early 1900’s . From 1987 to1995, the American Chemical Society published three symposium volumes devoted to light-activated pesticides [3–5]. Since then, follow-up investigations have been sporadic, as summarized in a handful of reviews [6–8]. Halogenated fluoresceins and some natural dyes were the classic PS used in the early work for experimental and field trials of PDT against various insects, including mosquito larvae and Mediterranean fruit flies. Industrial application (PhotoDye International, Inc) progressed to the stage of using aerial spray of dye mixtures (xanthenes) (Red Dye #28 and Yellow Dye #8) or SureDye  in attempt to control the latter pest of agricultural importance.
The major contribution of the present study is the identification of several novel PS (photosensitizers) as effective light-activated mosquito larvicides by screening a total of 16 different dyes from three chemical groups, i.e. phthalocyanines (PC), porphyrins and halogenated fluoresceins (Figs 1–3). The relative efficacy of these PS is validated by their side-by-side comparisons under identical laboratory conditions. Similar methodology used previously [18–21] was adopted here to simulate natural conditions, i.e. illumination of PS-preloaded larvae with white light of dim intensity for increasing time periods. The PDT (photodynamic therapy)-effective PS were identified by the increasing larvicidal activities with increasing PS pre-loading concentrations and increasing periods of illumination. The decrease in their effectiveness with increasing larva instars is an expected observation, consistent with the previous report .