This review focuses on recent developments in the physicochemical profiling of morphine and other opioids. The acid‐base properties and lipophilicity of these compounds is discussed at the microscopic, species‐specific level. Examples are provided where this type of information can reveal the mechanism of pharmacokinetic processes at the submolecular level. The role of lipophilicity in quantitative structure–activity relationship (QSAR) studies of opioids is reviewed. The physicochemical properties and pharmacology of the main metabolites of morphine are also discussed. Recent studies indicate that the active metabolite morphine‐6‐glucuronide (M6G) can contribute to the analgesic activity of systemically administered morphine. The unexpectedly high lipophilicity of M6G partly accounts for its analgesic activity. When administered parenterally, another suspected minor metabolite, morphine‐6‐sulfate (M6S) has superior antinociceptive effects to those of morphine. However, because sulfate esters of morphine derivatives cannot cross the blood‐brain barrier these esters may be good candidates to develop peripheral analgesic drugs. Who is in charge ? In solutions, naloxone exists in four differently charged forms. Its overall lipophilicity profile indicated by the broad black line is the sum of the contributions of its four forms. Although the non‐charged form of naloxone is less lipophilic than naltrexone, its relative concentration at physiological pH is higher. This explains the observation that naloxone has a more rapid onset for antagonist activity and a shorter duration of action.
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