Synthesis and opioid receptor binding properties of a highly potent 4-hydroxy analogue of naltrexone.

Very high affinity for opioid receptors (e.g., K(i)=0.052nM for mu) has been observed in the rationally designed naltrexone analogue 5. SAR and physical data supports the hypothesis that the 4-OH group of 5 stabilizes the 3-carboxamido group in the putative bioactive conformation.