SUMMARY Background and purpose Ziconotide is a peptide that blocks N-type calcium channels and is anti-hyperalgesic after intrathecal delivery. We here characterize the spinal kinetics of intrathecal bolus and infused ziconotide in dog. Experimental approach Male beagle dogs (N = 5) were prepared with chronic intrathecal (IT) lumbar injection and cerebrospinal fluid (LCSF) sampling catheters connected to vest-mounted pumps. Each dog received: i) IT bolus ziconotide (10 µg + 1 µCi 3H-inulin), ii) IT infusion for 48 hr of ziconotide (1 µg/100 µL/hr), iii) IT infusion for 48 hr of ziconotide (5 µg/100 µL/hr), and iv) intravenous injection of ziconotide (0.1 mg/kg). After IT bolus, LCSF ziconotide and inulin showed an initial peak and biphasic (distribtution/elimination) clearance (ziconotide T1/2 α / ß = 0.14 and 1.77 hr, and inulin T1/2 α / ß = 0.16 and 3.88 hr, respectively). The LCSF: plasma ziconotide concentration ratio was 20,000: 1 at 30 min, and 30: 1 at 8 hr. IT infusion of 1 and then 5 µg/hr resulted in LCSF concentrations that peaked by 8 hr and remained stable at 343 and 1380 ng/mL, respectively, to the end of the 48-hr infusions. Terminal elimination T1/2 after termination of continuous infusion was 2.47 hr. Ziconotide LCSF: cisternal CSF: plasma concentration ratios after infusion of 1 µg/hr and 5 µg/hr were 1: 0.017: 0.001 and 1: 0.015: 0.003, respectively. IT infusion of ziconotide at 1 µg/hr inhibited thermal skin twitch by 24 hr, and produced modest trembling, ataxia, and decreased arousal. Effects continued through the 48-hr infusion period, increased in magnitude during the subsequent 5 µg/hr infusion periods, and disappeared after drug clearance. Conclusions and Implications After intrathecal bolus or infusion, ziconotide displays linear kinetics that are consistent with a hydrophilic molecule of approximately 2500 Da that is cleared slightly more rapidly than inulin from the LCSF. Behavioral effects were dose dependent and reversible.
Oxycodone is used as a potent analgesic medication. Oxycodone is extensively metabolized. To fully describe its metabolism, the oxygenation of oxycodone to oxycodone N-oxide was investigated in hepatic preparations. The hypothesis tested was that oxycodone N-oxygenation was enzymatic and the amount of N-oxide detected was a consequence of both oxygenation and retro-reduction. Methods for testing the hypothesis included both in vitro and in vivo studies. Results indicated that oxycodone was N-oxygenated by the flavin-containing monooxygenase. Oxycodone N-oxide is chemically quite stable but in the presence of hepatic preparations and NADPH was retro-reduced to its parent compound oxycodone. Subsequently, oxycodone was metabolized to other metabolites including noroxycodone, noroxymorphone, and oxymorphone via cytochrome P-450. Retro-reduction of oxycodone N-oxide to oxycodone was facilitated by quinone reductase, aldehyde oxidase, and hemoglobin but not to a great extent by cytochrome P-450 or the flavin-containing monooxygenase. To confirm the in vitro observations, oxycodone was administered to rats and humans. In good agreement with in vitro results, substantial oxycodone N-oxide was observed in urine after oxycodone administration to rats and humans. Administration of oxycodone N-oxide to rats showed substantial amount of recovered oxycodone N-oxide. In vivo, noroxycodone was formed as a major rat urinary metabolite from oxycodone N-oxide presumably after retro-reduction to oxycodone and oxidative N-demethylation. To a lesser extent, oxycodone, noroxymorphone, and oxymorphone were observed as urinary metabolites. SIGNIFICANCE STATEMENTThis manuscript describes the N-oxygenation of oxycodone in vitro as well as in small animals and humans. A new metabolite was quantified as oxycodone N-oxide. Oxycodone N-oxide undergoes extensive retro-reduction to oxycodone. This re-establishes the metabolic profile of oxycodone and introduces new concepts about a metabolic futile cycle related to oxycodone metabolism.
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