S-(+)-Methoprene: Discovery and Health Effects

General Description

The discovery of S-(+)-Methoprene began with research on juvenile hormones (JHs) and the synthesis of terpenoid analogs, leading to the foundation of Zoecon Corporation in 1968. Researchers aimed to create stable JH mimics to control insect pests effectively. This effort culminated in the synthesis of racemic methoprene in 1971, with significant effectiveness against Aedes aegypti larvae, resulting in its registration as Altosid Liquid Larvicide in 1975. S-(+)-Methoprene is noted for its low toxicity to humans and animals, showing safety in veterinary applications and minimal risk to vertebrate development, thus establishing its role as a reliable insect growth regulator.

Figure 1. S-(+)-Methoprene

Discovery

Early Exploration of Juvenoid Analogues

The journey toward the discovery of S-(+)-Methoprene begins with the research on juvenile hormones (JHs), particularly the synthesis of terpenoid analogs before the actual structure of JH I was unveiled. Initial studies by chemists indicated that compounds like farnesol and farnesal exhibited weak juvenile hormone activity against larvae such as Tenebrio molitor, a finding that sparked further investigation. Despite the lack of potency in early analogs, such as farnesyl methyl ether, these efforts provided critical insights into the potential for creating more effective juvenoids. Researchers like Henrick and others contributed to this foundation by establishing connections between structural similarity and biological activity, including the work on natural products like (+)-juvabione, which showcased effective properties in certain insect species. ¹

The Role of Zoecon Corporation and the Discovery 

The pivotal moment in the discovery of S-(+)-Methoprene was catalyzed by the publication of the structure of JH I, which led to the establishment of Zoecon Corporation in 1968. Founded by Syntex Corporation, Zoecon aimed to develop synthetic mimics of natural JH to selectively control insect pests while minimizing environmental harm. The scientists' understanding that natural JHs were unstable and insufficient for commercial use directed their research toward creating more stable alternatives. By modifying JH I's structure, they aimed to enhance both environmental persistence and metabolic resistance in insects, ultimately leading to the discovery of S-(+)-Methoprene. This compound showcased remarkable bioactivity, particularly against the yellow fever mosquito Aedes aegypti, marking a significant advancement in insect pest control. ¹

Commercial Success and Regulatory Milestones

The first synthetic sample of racemic methoprene was completed in March 1971, and subsequent bioassay results underscored its significance, demonstrating an impressive LC50 against Aedes aegypti larvae. This marked S-(+)-Methoprene as Zoecon Corporation's initial proprietary commercial product and the first juvenoid registered for insect control. Following limited trials for floodwater mosquito management in August 1973, S-(+)-Methoprene gained full commercial registration as Altosid Liquid Larvicide from the U.S. Environmental Protection Agency in March 1975. Initially classified as a conventional pesticide, it was later re-evaluated and designated as a biochemical pesticide, reflecting its distinct mode of action. The successful registration of S-(+)-Methoprene paved the way for further developments in juvenile hormone analogs, with kinoprene and hydroprene following suit in subsequent years, showcasing the potential of juvenoids in pest management. ¹

Health Effects

Safety to Humans and Animals

S-(+)-Methoprene is recognized for its very low toxicity to humans and other vertebrates, making it a safe option for pest control. Extensive studies have shown that S-(+)-Methoprene can be applied directly to pets, livestock, and even zoo animals to effectively manage fleas, mites, and other parasites. Notably, there have been no reported adverse effects in humans following accidental exposure to S-(+)-Methoprene. This safety profile extends to veterinary applications, where S-(+)-Methoprene has been used for decades, including on pregnant and nursing mammals, without any documented developmental effects on animals or their offspring. 2

Developmental Studies and Toxicity Levels

While S-(+)-Methoprene and its breakdown products are sometimes utilized in research aimed at disrupting embryonic development in vertebrates, the concentrations required to induce such effects are significantly higher—often over 100 times the amounts used in mosquito control. This indicates that S-(+)-Methoprene is unlikely to pose a risk to vertebrate development at the levels typically employed in pest management. Overall, the extensive safety data surrounding S-(+)-Methoprene supports its use as a reliable and low-risk insect growth regulator in both agricultural and veterinary contexts. ²

Reference

1. Henrick CA. Methoprene. J Am Mosq Control Assoc. 2007; 23(2 Suppl): 225-239.

2. Lawler SP. Environmental safety review of methoprene and bacterially-derived pesticides commonly used for sustained mosquito control. Ecotoxicol Environ Saf. 2017; 139: 335-343.

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