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I am not a chemist and I didn't find any data on this at all but from a look over the two dimensional diagrams of the two molecules trazodone and risperidone I see a bit of resembelence, especially in the number and features of the rings. Besides the similarity in names (done suffix) they are both used by psychiatrists for various mood off-label uses.

Are trazodone and risperidone chemically related?

Update

In a well recieved comment it was written that they are related in that they both have two aromatic groups with a basic centre between them and they are both brain-penetrant.

  • Is this in Red the basic center and is it "basic" in the sense that it's not acidic?

  • What makes the two molecules brain-penetrant?

Trazodone:

enter image description here

Risperidone:

enter image description here

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    $\begingroup$ Chemically they are related in that they both have two aromatic groups with a basic centre between them and they are both brain-penetrant. They are different in their pharmacology - Trazadone's most important interactions are at 5HT2a and alpha 1, Risperidone is also highly active at 5HT2a but has significant activity at 5HT7 and Dopamine receptors. $\endgroup$
    – Waylander
    Commented Feb 5 at 13:13
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    $\begingroup$ The carbon chains you've highlighted isn't what makes these molecules basic. It's those few nitrogen atoms. The two chains are also not really that similar. For most (= almost all) intents and purposes I would regard them as chemically different. They are probably similarly-ish in size, but that's basically it. $\endgroup$ Commented Feb 6 at 18:48

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Generally and historically drugs ending with -odone are in the opiods family (see Oxycodone, Hydrocodone, ecc), but it's not a hard rule. In this case Trazodone is an antidepressant and Risperidone is an antipsycothic, but they are not opiods (so they do not derive/mimic natural opiods found in opium).

Being both neuroactive drugs they need to/tend to have some pretty common chemical properties for being capable of passing the brain-blood barrier. They typically need to be water-soluble as they need to reach the brain via bloodstream (so they need to have some polar groups), but not too much ( so as to allow the drug to pass the brain-blood barrier. (see Lipinski's rule of five)

You can see that both molecules do not have strong polar groups (no alcohols -OH, no amines -NH2, ecc) and both have some basic centers (the central nitrogen atoms of both molecules). Generally, many drugs that act on the brain display this feature (basic nitrogen atoms); see alkaloids for more info.

The carbon chains you highlighted in red are simply some carbon linkers that bind the two "sections" of each molecule together AND gives the molecule some degree of flexibility, needed to better interact with the active sites present in the proteins in the brain.

Other than these pretty general characteristic of neuroactive drugs they are not particularly related.

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