The milbemycins are a group of

macrolide Macrolides are a class of mostly natural products with a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. Macrolides belong to the polyketide class of natural products. ...

s chemically related to the

avermectin The avermectins are a group of 16-membered Macrolide, macrocyclic lactone derivatives with potent anthelmintic and Insecticide, insecticidal properties. These naturally occurring compounds are generated as fermentation products by ''Streptomyces a ...

s and were first isolated in 1972 from ''

Streptomyces hygroscopicus ''Streptomyces hygroscopicus'' is a bacterial species in the genus '' Streptomyces''. It was first described by Hans Laurits Jensen in 1931. Biochemistry Cultures of different strains of ''S. hygroscopicus'' can be used to produce several che ...

''. They are used in veterinary medicine as

antiparasitic Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, ectoparasites, parasitic fungi, and protozoa, among others. Antiparasitics target the parasitic ...

agents against worms, ticks and fleas.The 2nd International Electronic Conference on Synthetic Organic Chemistry: Insecticides
They also show insecticidal and acaricidal activity. __TOC__

Mechanism of action

Milbemycins have a similar mechanism of action, but a longer half-life than the avermectins. They open

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

-sensitive

chloride channel Chloride channels are a superfamily of poorly understood ion channels specific for chloride. These channels may conduct many different ions, but are named for chloride because its concentration ''in vivo'' is much higher than other anions. Several ...

s in

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s and

myocyte A muscle cell, also known as a myocyte, is a mature contractile Cell (biology), cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal muscle, skeletal, smooth muscle, smooth, and Cardiac muscle, cardiac ...

s of

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s, leading to hyperpolarisation of these cells and blocking of signal transfer. They are in

IRAC IRAC ( ) is an acronym that generally stands for: Issue, Rule, Application, and Conclusion. It functions as a methodology for legal analysis. The IRAC format is mostly used in hypothetical questions in law school and bar exams. Sections of an IR ...

class 6. Milbemycin is effective against some avermectin-resistant insects.

Biosynthesis

Like avermectins, milbemycins are products of

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

by ''

Streptomyces ''Streptomyces'', from στρεπτός (''streptós''), meaning "twisted", and μύκης (''múkés''), meaning "fungus", is the largest genus of Actinomycetota, and the type genus of the family Streptomycetaceae. Over 700 species of ''St ...

'' species. A grand total of five species, namely '' S. hygroscopicus'', '' S. griseochromogenes'', '' S. cyaneogriseus'', '' S. nanchanggensis'' and '' S. bingchenggensis'', are known to produce this family of compounds. The biosynthetic cluster from ''S. bingchenggensis'' and ''S. nanchanggensis'' have been sequenced and analyzed, partly by analogy to the homologous biosynthetic cluster for avermectin. New regulatory elements that control how much milbemycins are being identified in hopes to improve the industrial fermentation yield of milbemycins, which (as of 2021) is much lower than that of avermectin. An alternative pathway to higher yields was demonstrated in 2017, when select genes from ''S. avermitilis'' were swapped out so the avermectin producer makes milbemycin instead.

Examples

References

Anthelmintics Insecticides {{antiinfective-drug-stub