While the primary focus of pharmacogenetic studies has been on the mu class of opioid receptors, research has shown that naltrexone also binds to delta and kappa opioid receptors (Takemori et al., 1988, Takemori and Portoghese, 1992), although with lower affinity than for mu. Specifically, preferential binding for mu over kappa receptors has been shown to be in the range of three to ten times greater (Ananthan et al., 1999, Ko et al., 1998). For mu receptors versus delta, the affinity for mu may be as much as 63 times greater (Emmerson et al., 1994).
