A.W.E. Wright

Hydromorphone-3-glucuronide: A more potent neuro-excitant than its structural analogue, morphine-3-glucuronide

In humans, hydromorphone (HMOR) is metabolised principally by conjugation with glucuronic acid to form hydromorphone-3-glucuronide (H3G), a close structural analogue of morphine-3-glucuronide (M3G), the major metabolite of morphine. In a previous study we described the biochemical synthesis of H3G together with a preliminary evaluation of its pharmacology which revealed that it is a neuroexcitant in rats in a manner analogous to M3G. Thus the aims of the current study were to quantify the neuro-excitatory behaviours evoked by intracerebroventricular (icv) H3G in the rat and to define its potency relative to M3G. Groups of adult male Sprague-Dawley rats received icv injections (1 microL) of H3G (1 - 3 microg), M3G (2 - 7 microg) or vehicle via a stainless steel guide cannula that had been implanted stereotaxically seven days prior to drug administration. Behavioural excitation was monitored by scoring fifteen different behaviours (myoclonic jerks, chewing, wet-dog-shakes, rearing, tonic-clonic-convulsions, explosive motor behaviour, grooming, exploring, general activity, eating, staring, ataxia, righting reflex, body posture, touch evoked agitation) immediately prior to icv injection and at the following post-dosing times: 5, 15, 25, 35, 50, 65 and 80 min. H3G produced dose-dependent behavioural excitation in a manner analogous to that reported previously for M3G by our laboratory and reproduced herein. H3G was found to be approximately 2.5-fold more potent than M3G, such that the mean (+/- S.D.) ED50 values were 2.3 (+/- 0.1) microg and 6.1 (+/- 0.6) microg respectively. Thus, our data clearly imply that if H3G crosses the BBB with equivalent efficiency to M3G, then the myoclonus, allodynia and seizures observed in some patients dosed chronically with large systemic doses of HMOR, are almost certainly due to the accumulation of sufficient H3G in the central nervous system, to evoke behavioural excitation.

Quantitation of Morphine, Morphine-3-Glucuronide, and Morphine-6-Glucuronide in Plasma and Cerebrospinal-Fluid Using Solid-Phase Extraction and High-Performance Liquid-Chromatography with Electrochemical Detection

An original, sensitive, and specific high-performance liquid chromatographic (HPLC) assay was developed for the quantitation of morphine and its two major metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), in human plasma and cerebrospinal fluid (CSF) and in rat plasma, using hydromorphone as the internal standard. Solid-phase extraction was used to separate morphine and its glucuronide metabolites from plasma constituents. Extraction efficiencies of morphine, M3G, and M6G from human plasma samples (0.5 ml) were 84, 87, and 88%, respectively. Extraction efficiencies of morphine, M3G, and M6G did not differ significantly (p > 0.05) between human plasma and CSF or rat plasma. Morphine, M3G, M6G, and hydromorphone were separated on a 10 μ C8 Resolve radially compressed cartridge using a mobile phase comprising methanol:acetonitrile:phosphate buffer, (0.0125M pH 7.5; 10:10:80), in which 11 mg/L of cetyltrimethylammo-nium bromide (cetrimide) was dissolved. Quantitation was achieved using a single electrochemical detector at ambient temperature (23°C). Standard curves were linear over the ranges 0.020–2.190, 0.027–2.709, and 0.027–0.542 μ for morphine, M3G, and M6G, respectively. Lower limits of detection for morphine, M3G, and M6G in human plasma and CSF samples (0.5 ml) were 0.020, 0.027, and 0.027 μM, respectively. Corresponding lower limits of detection in rat plasma (0.1 ml) were 0.102, 0.135, and 0.135 μM, respectively. Intraassay precision for low and high concentrations of morphine, M3G, and M6G were <23 and <8% respectively. Similarly, interassay accuracy for low and medium concentrations of morphine, M3G, and M6G were <17% and were <9% for high concentrations.

Determination of the Serum-Protein Binding of Oxycodone and Morphine Using Ultrafiltration

Protein binding of oxycodone and morphine in human serum was determined in vitro using ultrafiltration. Binding studies were also performed using both purified human serum albumin and human α1-acid glycoprotein (AAG). Albumin was found to be the major binding protein for both oxycodone and morphine. The serum protein binding of both oxycodone and morphine was independent of drug concentration in the therapeutic range (5–100 ng/ml), but was dependent on protein concentration. In addition, bound fractions of oxycodone and morphine increased with increasing concentrations of both albumin and A AG. At physiological pH and temperature, the mean (± SD) serum protein binding of oxycodone was 45.1% (± 0.4%) and that of morphine was 35.3% (± 0.2%) A decrease in temperature from 37 to 23°C significantly increased the serum protein binding of oxycodone and morphine by 8–9% (p < 0.0001) and 7–10% (p < 0.0001), respectively, indicating the importance of maintaining the temperature at 37° C during protein binding experiments. A reduction in pH from 7.75–8.85 to 7.4 significantly reduced serum protein binding of both oxycodone and morphine by 4–5% (p < 0.0001) and 4–7% (p < 0.0001), respectively. Serum samples, to which known concentrations of oxycodone had been added and which were stored at −20° C, showing a gradual but significant decline (p < 0.0001) in serum protein binding of oxycodone from –45 to 39% during the 4-week storage period. Although serum protein binding of oxycodone and morphine is dependent on protein concentration and pH – factors that may vary in different disease states–any changes in the binding of oxycodone or morphine are unlikely to alter the pharmacological effects of these drugs because of their normally low extent of binding (45 and 35%, respectively).

Cerebrospinal fluid and plasma concentrations of morphine, morphine-3-glucuronide, and morphine-6-glucuronide in patients before and after initiation of intracerebroventricular morphine for cancer pain management

UNLABELLED Twenty-three patients treated with intracerebroventricular (ICV) morphine in this study not only obtained excellent pain relief without rapid increases in dose, but also experienced a reduction in morphine-related side effects. By 24 h after initiation of ICV morphine, the mean trough cerebrospinal fluid (CSF) morphine concentration (approximately 20 microM) was 50-fold higher than the baseline concentration (approximately 0.4 microM), and the CSF concentration of morphine-6-glucuronide (M6G) was undetectable (<0.01 microM). The mean CSF concentration of morphine-3-glucuronide (M3G) decreased 90%, from a baseline concentration of 1 microM to 0.1 microM by Day 7 postventriculostomy. Thereafter, the mean trough CSF M3G concentration remained relatively constant while ICV morphine was continued, although the concomitant M3G plasma concentrations were undetectable (<0.01 microM). The large increase in the CSF morphine concentration in patients receiving ICV morphine strongly suggests that increased CSF morphine levels are unlikely to be the primary cause of analgesic tolerance or undesirable excitatory side effects (hyperalgesia, myoclonus, seizures) experienced by some patients receiving chronic large-dose systemic morphine. IMPLICATIONS After initiation of intracerebroventricular morphine, cancer patients experienced excellent pain relief. Although the mean morphine concentration in cerebrospinal fluid increased 50-fold relative to preventriculostomy levels, rapid dose increases did not occur, which suggests that increased cerebrospinal fluid morphine levels are unlikely to be the main cause of analgesic tolerance.

Monitoring salivary lamotrigine concentrations

Lamotrigine concentrations were measured simultaneously (as far as was feasible) in stimulated and unstimulated saliva samples, and in plasma, from seven adult volunteers over a 32 h period following a single 50 mg dose of the drug, and in 20 children and adolescents during the course of routine antiepileptic therapy. In individuals there was a close correlation between the measurements at least 2 h after ingestion of the drug. Concentrations in stimulated and unstimulated saliva were similar; the stimulation produced little change in the saliva secretion rate. The saliva-to-plasma concentration ratio increased linearly by 0.78% for each 1 mg/L plasma lamotrigine concentration, with a mean value of 48.8% at a plasma lamotrigine concentration of 10 mg/L. With appropriate precautions as to the timing of saliva collections, and a single plasma lamotrigine concentration measurement to calibrate the salivary values in the individual, salivary lamotrigine concentration measurement appears to be a practicable approach to therapeutic drug monitoring. This has significant implications for the elucidation of the pharmacokinetics of lamotrigine in the paediatric population.

Solid-phase extraction method with high-performance liquid chromatography and electrochemical detection for the quantitative analysis of oxycodone in human plasma

A sensitive and reproducible solid-phase extraction (SPE) method for the quantification of oxycodone in human plasma was developed. Varian Certify SPE cartridges containing both C8 and benzoic acid functional groups were the most suitable for the extraction of oxycodone and codeine (internal standard), with consistently high (> or =80%) and reproducible recoveries. The elution mobile phase consisted of 1.2 ml of butyl chloride-isopropanol (80:20, v/v) containing 2% ammonia. The quantification limit for oxycodone was 5.3 pmol on-column. Within-day and inter-day coefficients of variation were 1.2% and 6.8% respectively for 284 nM oxycodone and 9.5% and 6.2% respectively for 28.4 nM oxycodone using 0.5-ml plasma aliquots.