Cyclopyrimorate: The First HST-Inhibiting Herbicide and What It Means for Weed Control

HPPD inhibitors came onto the market in the 1990s, and for roughly three decades after that, no new herbicide target followed them. Cyclopyrimorate (CAS 499231-24-2) broke the streak. In 2018, researchers confirmed the molecular target of this selective rice herbicide: homogentisate solanesyltransferase, or HST. It was the first commercial herbicide ever built around that enzyme, and it gave weed control a fresh front under HRAC Group 33.

What is cyclopyrimorate?

Cyclopyrimorate is a selective bleaching herbicide developed by Mitsui Chemicals Agro for weed control in transplanted rice, including paddies infested with ALS-resistant weeds such as sedges. Treated plants turn white, the symptom you would expect from older bleaching chemistry. The biochemistry underneath is different. Older bleaching herbicides block either phytoene desaturase (PDS) or 4-hydroxyphenylpyruvate dioxygenase (4-HPPD). Cyclopyrimorate cuts the plastoquinone (PQ) biosynthesis pathway one step further down, at HST. A plant without PQ cannot build carotenoids or shield its chlorophyll from light damage. Photosynthesis collapses, and the weed bleaches out and dies.

Why a new mode of action matters

Resistance has been spreading since the 1970s, when ALS-inhibiting herbicides became standard tools in wheat, rice, and soybean production. Survey data as of June 2026 show weeds resistant to 21 of the 31 known herbicide sites of action, involving 168 herbicides across 275 species in 76 countries. The 2026 HRAC mode-of-action classification removed dozens of obsolete active ingredients and added only five new ones. On the commercial side, truly novel mechanisms have been scarce since HPPD inhibitors appeared. A new validated target is what the industry has been missing most, and HST is the first to clear the market bar in decades.

Following the symptom back to the pathway

The first stop was Arabidopsis thaliana. Seedlings treated with cyclopyrimorate turned white and stopped growing, closely matching the symptoms caused by the PDS inhibitor norflurazon and the 4-HPPD inhibitor mesotrione.

Biochemical profiling then exposed the difference. In treated Arabidopsis, mustard (Brassica juncea), and the paddy weed Schoenoplectus juncoides, two things happened at once:

1. Homogentisate (HGA), an upstream intermediate in plastoquinone biosynthesis, piled up to high levels.
2. Plastoquinone itself collapsed, becoming undetectable at higher treatment concentrations.

A 4-HPPD inhibitor produces the reverse pattern. It blocks HGA formation, so HGA drops to zero. Since HGA accumulated here, cyclopyrimorate had to be acting downstream, at the step that converts HGA into PQ.

DMC: the active metabolite behind the prodrug design

Analyzing plant metabolism turned up something important: cyclopyrimorate is rapidly converted inside plants into des-morpholinocarbonyl cyclopyrimorate, or DMC. Four days after treatment of Arabidopsis leaves, tissue treated at 7.8 ppm held 1.50 pmol/mg fresh weight of DMC alongside 7.81 pmol/mg of the parent compound. At 31.3 ppm the figures were 4.01 versus 80.1. At 125 ppm, 13.6 versus 421. The parent accumulates to higher levels, but DMC rose in proportion to treatment dose, and the data pointed to DMC as the active entity. Cyclopyrimorate works as a prodrug. DMC is the molecule that actually shuts the target down.

Proving HST is the direct target

The team cloned and expressed Arabidopsis HST, then built an in vitro assay with farnesyl diphosphate (FPP) as the substrate, measuring formation of the reaction product MFBQH (2-methyl-6-farnesyl-1,4-benzoquinol). The numbers settled the question:

• DMC inhibited HST with an IC50 of 3.93 μM. That is potent.
• The parent compound was weak, with an IC50 above 500 μM, which fits its prodrug behavior in plants.
• Haloxydine, a known HST inhibitor that was never commercialized, came in at 9.19 μM. DMC is more than twice as active in vitro.
• Mesotrione, norflurazon, and clomazone did not touch HST at all, even at 1 mM.

The pattern held across compounds. HST inhibition measured in the test tube tracked whole-plant bleaching in vivo. The enzyme in the assay is the same enzyme getting hit in the field.

What structure-activity relationships revealed

Derivatives of DMC showed what a good HST inhibitor needs. A hydrophobic group at the 5-position of the pyridazine ring raises potency. The ethyl analog (compound 1) reached an IC50 of 2.39 μM, stronger than DMC itself at 3.93 μM, and its whole-plant chlorophyll EC50 improved from 708 to 352 μM. Methylation at the 3- or 4-position helped too (compound 5, IC50 1.65 μM, EC50 63.6 μM). Swapping the pyridazine core for a pyridine ring, on the other hand, wrecked the molecule: IC50 fell to 463 μM (compound 9). How rigid the heteroaromatic core is matters as much as what hangs off it.

Why cyclopyrimorate synergizes with HPPD inhibitors

The study also explained a field observation: cyclopyrimorate performs strongly when mixed with 4-HPPD inhibitors such as pyrazolynate. The two chemistries attack the same biosynthetic pathway at consecutive steps. HPPD inhibitors cut off HGA production at the top. HST inhibitors block the conversion of HGA into plastoquinone below. Applied together, they shut down PQ synthesis from both directions, and the plant loses its photoprotective system faster than either chemistry alone could manage. The double blockade raises weeding efficiency and, at the same time, lowers the chance of resistance evolving to either component.

A new foundation for resistance management

This work closed a decades-old mystery about how cyclopyrimorate kills weeds, and it did something bigger: it established HST as a commercially validated herbicide target. Researchers had known since 2010 that haloxydine inhibits HST, but no HST inhibitor had ever reached the market until cyclopyrimorate. HST now sits beside ALS, ACCase, HPPD, and PPO as a proven target, classified as HRAC Group 33.

For rice growers in Asia battling ALS-resistant weeds, that means a new line of defense. For the agrichemical industry, the lesson is just as practical. Re-examining older compounds for new targets may be one of the most realistic ways to bring fresh modes of action to market now that inventing them from scratch has become so hard.

Frequently asked questions

What is the mode of action of cyclopyrimorate? Cyclopyrimorate inhibits homogentisate solanesyltransferase (HST), an enzyme in the plastoquinone biosynthesis pathway. Blocking HST stops plastoquinone synthesis, which destroys carotenoid formation and chlorophyll, so weeds bleach and die. It is classified under HRAC Group 33.

Is cyclopyrimorate a PDS or HPPD inhibitor? No. Treated plants show the same whitening symptoms, but cyclopyrimorate acts downstream of both targets. HGA accumulates in treated plants instead of disappearing, which sets it apart from PDS and 4-HPPD inhibitors.

What is HST (homogentisate solanesyltransferase)? HST is the enzyme that attaches a solanesyl side chain to homogentisate, the final committed step in plastoquinone biosynthesis in plants. Cyclopyrimorate is the first commercial herbicide to target it.

What is DMC? DMC (des-morpholinocarbonyl cyclopyrimorate) is the metabolite into which plants rapidly convert cyclopyrimorate. DMC inhibits HST with an IC50 of 3.93 μM, while the parent compound is weak, making cyclopyrimorate a prodrug and DMC the active inhibitor.

Why does cyclopyrimorate work against ALS-resistant weeds? HST is a completely different target from acetolactate synthase. Weeds carrying ALS resistance mutations stay fully susceptible to cyclopyrimorate, and cross-resistance between the two modes of action is not expected.

Why is cyclopyrimorate synergistic with HPPD inhibitors? Both chemistries attack the plastoquinone pathway at consecutive steps: HPPD inhibitors stop HGA formation upstream, and HST inhibitors block HGA-to-PQ conversion downstream. Together they impose a double blockade that kills weeds faster and lowers resistance risk.