Understanding the Screening Process of New Molecules

Abstract

Globally there is a huge market for herbicides. Syngenta and its competitors spend large amounts of money in trying to develop new herbicides which are highly effective (kills the plants that the farmer wants to get rid of), selective (does not harm the crops that the farmer is trying to protect), safe (not harmful to humans or the environment) and cheap to produce.
Every year Syngenta chemists develop around one thousand new compounds that could be used as herbicides. Accurately evaluating the efficacy, selectivity, safety and ease of production of each one of these compounds would be extremely expensive. Initially synthesizing compounds even in tiny quantities requires a lot of work. Different compounds will then require different levels of dosage, may target certain species of plants better than others, may work more effectively in conjugation with different solvents etc. A huge amount of time and money could be spent optimizing the application of a particular compound, which then turns out to be very poor compared to existing herbicide products on the market.
In order to attempt to select only the best performing compounds in a cost effective way, Syngenta use a screening cascade. This process begins by testing all of the candidate compounds in laboratory experiments, referred to as assays, to see if they can (e.g.) target a particular enzyme, or penetrate a leaf. These lab experiments only require a tiny quantity of each compound to be synthesized. Compounds which perform poorly in these first experiments are discarded from the trial. We call this the first screen.
Compounds which pass this first round are then passed to a second round in which a small amount of the compound is applied to several small pots containing a few different species of plant. Compounds which perform poorly in this second experiment are discarded from the trial process. We call this the second screen.
In subsequent screens larger quantities of the compounds are used in the experiments, which makes them a lot more expensive. By using more compounds the candidate herbicides can be tested on a larger range or species, at different dosages, in conjunction with various different conditions, and with less sampling error.
As the screening process progresses, the screens become steadily more rigorous and consequently more expensive. At the same time the number of compounds remaining in the trial goes down. Finally a small number of compounds are taken to the final level, called the field trial. In this trial the compounds are applied outdoors in the way they would be used if they were eventually developed into a commercial product.