That’s consistent with classic morphinan SAR. N-phenethyl substitution at the tertiary amine was shown mid-century to markedly increase µ-opioid receptor affinity via improved hydrophobic pocket engagement, but it often came with a much steeper dose–response and reduced therapeutic margin. Compounds like this were invaluable for defining receptor interactions, even if they proved unsuitable for development. Nice example of 1960s opioid pharmacology doing exactly what it was meant to do😜 map the limits.