Intranasal diamorphine as an alternative to intramuscular morphine: pharmacokinetic and pharmacodynamic aspects

Diamorphine is a semisynthetic derivative of morphine that is currently licensed for use in the treatment of moderate to severe acute pain, administered by the intramuscular, intravenous or subcutaneous routes. It is highly water-soluble and has a number of properties that render it suitable for administration via the nasal route. Administration via the intranasal route is well described for other drugs, but has only recently been evaluated in a clinical setting for diamorphine. A well-tolerated and rapidly effective analgesic agent has proven elusive in the paediatric setting. The pharmacokinetic profile of intranasal diamorphine in adults has been systematically studied. It is rapidly and dose-dependently absorbed as a dry powder, with peak plasma concentrations occurring within 5 minutes, and has a similar pharmacokinetic profile to that of intramuscular diamorphine. It is rapidly converted to 6-acetylmorphine (peak concentrations within 5-10 minutes) and thence to morphine (peak concentrations within 1 hour). The pharmacodynamic properties of intranasal diamorphine have also been studied in comparison with intramuscular diamorphine. Intranasal and intramuscular administration of diamorphine resulted in similar physiological responses (including pupil diameter, respiration rate and temperature). Changes in behavioural measures (including euphoria, sedation and dysphoria) were also similar. Intranasal administration of diamorphine, therefore, produces the expected drug effects on the same timescale and of the same magnitude as intramuscular injection. Intranasal diamorphine has been clinically evaluated in a randomised controlled trial versus intramuscular morphine in the setting of acute orthopaedic pain in children with fractures. Intranasal diamorphine provided the same overall degree of pain relief as intramuscular morphine, but with a quicker onset of action. It was found to be well tolerated with an acceptable safety profile. It has also been studied in the setting of patient-controlled analgesia for postoperative pain in adults, with encouraging results. The pharmacokinetic and pharmacodynamic properties of intranasal diamorphine, and particularly the ability to administer it without a needle (and therefore reduce the incidence of transmissible infection), have made this a popular route for abuse amongst opioid addicts. In this setting, however, the intranasal route is not free from adverse events, including deaths. The primary clinical need in the paediatric population is for a well tolerated, effective and expedient analgesic agent that is safe to use; intranasal diamorphine has pharmacokinetic properties that would make it suitable for such a clinical indication and, in clinical evaluations to date, appears to be promising.