How exactly is a drugs mechanism of action on the brain determined scientifically by pharmaceutical companies or the FDA?


As in an antidepressant being called an SSRI or THC being a partial canabinoid receptor antagonist when other compounds are full receptor antagonist. With the SSRI, which to my understanding, refers to the chemical selectively blocks serotonin receptors on the receiving side of the synapse therefore causing an excess of serotonin in the gap so to say in lay terms. How is this selectivity determined as serotonin is one chemical? More specifically I don’t understand the science that says this is what is actually happening? ELI5

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A lot of it has been theoretical, and there are still a lot of studies to determine exactly what happens. One common thing is to do [experiments on mice to see what is actually happening with your genes and your neurotransmitters]( Otherwise, a lot of medicines have effects that we don’t necessarily understand – researchers determine that a drug does a certain thing, then they run experiments to determine if these effects are consistent, and if there are negative side effects. While they may have an idea of why things work the way they do, and sometimes even develop drugs specifically because they have an idea of what effects it should have on our cells/genes/etc., it’s not uncommon for the actual mechanism of how a drug works to be worked out long after it has started being used to treat something.

TLDR; This is an extremely complicated process involving multiple fields with large uncertainties that take years to adress.

Without going into details I’ll try to generalize.

System observation. Administer a drug observe the effect and use pre existing knowledge to explain the effect. Often combined with mutations that inactivate potential targets the drug may act on.

Ex vivo (not in the organism). mix the drug with something and see what it does. ie isolate a protein causing a disease mix it with the drug and see if the drug binds to the protein.

in vivo (inside the organism). administer the drug often to animals then examine things like where in the organism the drug ends up.

In silico (computer modeling). create advanced computer models og the organism or parts of it and see what the drug does.

The problem is that all these have blind spots and weaknesses that make it impossible to correct for all the interactions hapening inside the body. So collecting evidence of a mexhanism takes alot of time, money and brains