CSF and plasma pharmacokinetics of pethidine and norpethidine in man after epidural and intrathecal administration of pethidine

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Abstract
The disposition of pethidine and its main metabolite, norpethidine, in cerebrospinal fluid (CSF) and plasma was studied in 11 thoracic surgery patients after lumbar epidural (100 mg;n=6) or lumbar intrathecal (25 mg;n=5) administration of pethidine. Pethidine appeared more slowly in plasma after intrathecal than after epidural administration (tmax 2.3 h and 14 min, respectively), but systemic bioavailability was similar. The CSF concentrations of pethidine were higher than those in plasma after both routes of administration. The maximal CSF/plasma concentration ratio was 6000 to 45000 after intrathecal administration but was only 26 to 97 after the epidural route. Pethidine was rapidly distributed in CSF; nine to ten h after the intrathecal and epidural injections the CSF/plasma concentration ratios were 12 to 89 and 2 to 33, respectively. The calculated bioavailability in CSF of epidural pethidine was 10.3%. The terminal elimination half-life of pethidine was 6.0 h (CSF) and 5.4 h (plasma) after intrathecal administration and 8.6 h (CSF) and 8.8 h (plasma) after epidural injection. The volume of distribution of unchanged pethidine in the subarachnoid space was 13 ml·kg−1 and clearance from the CSF was 15 µl·kg−1·min−1. In all patients receiving intrathecal pethidine and in some patients after epidural pethidine, CSF norpethidine concentrations were higher than those in plasma; the maximum CSF norpethidine was 102 to 1211 ng·ml−1 and 14 to 210 ng·ml−1 and the maximum CSF/plasma norpethidine concentration ratios were 21 to 652 and 0.6 to 14 times after intrathecal and epidural administration, respectively. Norpethidine was rapidly distributed and its level in CSF was about the same or lower than in plasma during the terminal elimination phase. The maximum CSF norpethidine level was 1.2±1.0% of that of pethidine after intrathecal injection. Thus, epidural pethidine enters the CSF more rapidly and to a greater extent than has been previously shown for epidural morphine, but pethidine is more rapidly redistributed from CSF. The terminal elimination half-life of pethidine was found to be long in relation to the reported duration of analgesia after a single spinal dose of pethidine, which suggests a potential risk of accumulation within the CSF on multiple spinal injections of pethidine. Pethidine is partly metabolised within the subarachnoid space by N-dealkylating enzymes in the CNS. After intrathecal injection of more than 25 mg pethidine, the concentration of the principle metabolite, norpethidine, in CSF may be higher than that associated with CNS toxicity in man.