VU-0238429 is a drug which acts as a selective positive allosteric modulator for the muscarinic acetylcholine receptor M<sub>5</sub>. It was the first selective ligand developed for the M<sub>5</sub> subtype, and is structurally derived from older M<sub>1</sub>-selective positive allosteric modulators such as VU-0119498. Replacing the O-methyl- by a phenyl group further improves the receptor subtype selectivity.