Concentration-effect relationships of morphine and morphine-6 beta-glucuronide in the rat

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5′-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G’s behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

Evaluation of amentoflavone metabolites on PARP-1 inhibition and the potentiation on anti-proliferative effects of carboplatin in A549 cells

The present study aims to determine the major metabolites of amentoflavone (AMF) and further evaluate their inhibitory effects on PARP-1. First, different fractions (Frs. 1-9), which were collected according to retention time of AMF metabolites based on UHPLC-QTOF-MS/MS qualitative analysis, were evaluated on their inhibitory effects against PARP-1. Then, two mono-sulfate metabolites in the fractions with potent PARP-1 inhibitory effect were targetedly semi-synthesized. Moreover, three mono-sulfate conjugates (compound 8, 9 and 10), including one disulfate conjugate (compound 10), were isolated and their structures were fully elucidated by UHPLC-QTOF-MS/MS and NMR. Finally, the binding mode of compound 8 (amentoflavone-4‴-O-sulfate) toward PARP-1 and its potentiation on carboplatin (CBP) in A549 cells were investigated. This study was the first report on bioactivity evaluation of AMF metabolites in rat bile on PARP-1 and the potentiation of compound 8 on carboplatin (CBP) in A549 cells in vitro. This paper also provided scientific basis for the AMF metabolites on PARP-1 inhibition and chemosensitization.

Pharmacodynamics and pharmacokinetics of the novel synthetic opioid, U-47700, in male rats

Novel synthetic opioids are appearing in recreational drug markets worldwide as adulterants in heroin or ingredients in counterfeit analgesic medications. Trans-3,4-dichloro-N-[2-(dimethylamino)cyclohexyl]-N-methyl-benzamide (U-47700) is an example of a non-fentanyl synthetic opioid linked to overdose deaths. Here, we examined the pharmacodynamics and pharmacokinetics of U-47700 in rats. Male Sprague-Dawley rats were fitted with intravenous (i.v.) catheters and subcutaneous (s.c.) temperature transponders under ketamine/xylazine anesthesia. One week later, rats received s.c. injections of U-47700 HCl (0.3, 1.0 or 3.0 mg/kg) or saline, and blood samples (0.3 mL) were withdrawn via i.v. catheters at 15, 30, 60, 120, 240, 480 min post-injection. Pharmacodynamic effects were assessed at each blood withdrawal, and plasma was assayed for U-47700 and its metabolites by liquid chromatography tandem mass spectrometry. U-47700 induced dose-related increases in hot plate latency (ED₅₀ = 0.5 mg/kg) and catalepsy (ED₅₀ = 1.7 mg/kg), while the 3.0 mg/kg dose also caused hypothermia. Plasma levels of U-47700 rose linearly as dose increased, with maximal concentration (C_max) achieved by 15-38 min. C_max values for N-desmethyl-U-47700 and N,N-didesmethyl-U-47700 were delayed but reached levels in the same range as the parent compound. Pharmacodynamic effects were correlated with plasma U-47700 and its N-desmethyl metabolite. Using radioligand binding assays, U-47700 displayed high affinity for μ-opioid receptors (Ki = 11.1 nM) whereas metabolites were more than 18-fold weaker. Our data reveal that U-47700 induces typical μ-opioid effects which are related to plasma concentrations of the parent compound. Given its high potency, U-47700 poses substantial risk to humans who are inadvertently exposed to the drug.

Effect of bile salts on the transport of morphine-6-glucuronide in rat brain endothelial cells

Bile salts are known to enhance the permeability of biological barriers but little is known about their effects on drug permeability across the blood-brain barrier (BBB). In this paper, the rat brain endothelial 4 (RBE4) cell monolayer incubated with astrocyte-conditioned medium was used as an in vitro model of the BBB to investigate the effects of cholate (C), 12-monoketocholate (MKC), deoxycholate (DC), and taurocholate (TC) on the transport of the hydrophilic drug, morphine-6-glucuronide (M6G). C, MKC, and TC at a concentration of 5 mM each and DC at 1 mM increased the permeability of M6G through the paracellular pathway based on a similar permeability pattern to that of sucrose. RBE4 cell uptake of M6G was unaffected by 5 mM C and TC, whereas 1 mM DC dramatically increased it due to an effect shown to be cytotoxicity as measured by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay. Surprisingly, 1 mM MKC significantly increased M6G uptake without any cytotoxicity. In summary, all bile salts increased paracellular permeation of M6G but MKC also enhanced transcellular transport with little cytotoxicity. MKC appears to have the potential to modulate biophysical properties of the cell membrane or membrane-bound transporters and may therefore enhance drug delivery to the brain.

Proinflammatory cytokines oppose opioid-induced acute and chronic analgesia

Spinal proinflammatory cytokines are powerful pain-enhancing signals that contribute to pain following peripheral nerve injury (neuropathic pain). Recently, one proinflammatory cytokine, interleukin-1, was also implicated in the loss of analgesia upon repeated morphine exposure (tolerance). In contrast to prior literature, we demonstrate that the action of several spinal proinflammatory cytokines oppose systemic and intrathecal opioid analgesia, causing reduced pain suppression. In vitro morphine exposure of lumbar dorsal spinal cord caused significant increases in proinflammatory cytokine and chemokine release. Opposition of analgesia by proinflammatory cytokines is rapid, occurring < or =5 min after intrathecal (perispinal) opioid administration. We document that opposition of analgesia by proinflammatory cytokines cannot be accounted for by an alteration in spinal morphine concentrations. The acute anti-analgesic effects of proinflammatory cytokines occur in a p38 mitogen-activated protein kinase and nitric oxide dependent fashion. Chronic intrathecal morphine or methadone significantly increased spinal glial activation (toll-like receptor 4 mRNA and protein) and the expression of multiple chemokines and cytokines, combined with development of analgesic tolerance and pain enhancement (hyperalgesia, allodynia). Statistical analysis demonstrated that a cluster of cytokines and chemokines was linked with pain-related behavioral changes. Moreover, blockade of spinal proinflammatory cytokines during a stringent morphine regimen previously associated with altered neuronal function also attenuated enhanced pain, supportive that proinflammatory cytokines are importantly involved in tolerance induced by such regimens. These data implicate multiple opioid-induced spinal proinflammatory cytokines in opposing both acute and chronic opioid analgesia, and provide a novel mechanism for the opposition of acute opioid analgesia.

Morphine reduces spinal c-fos expression dose-dependently during experimental laparotomy in pigs: A combined pharmacokinetic and surgical study

The pharmacokinetics of intravenous morphine 2.5mg/kg (n=4) and 10mg/kg (n=4) in plasma and cerebrospinal fluid (CSF) of pigs was studied. Plasma half-life was 1.0+/-0.1h and the main metabolite was morphine-3-glucuronide, whereas morphine-6-glucuronide was negligible. CSF morphine concentration peaked after 20-30min (2.5mg/kg) and 60-120min (10mg/kg), and elimination half-life was 3.5+/-0.3h. Subsequently, the effect of morphine on surgery-induced spinal nociception in pigs subjected to unilateral laparotomy was evaluated by stereological quantification of the total number of Fos-like-immunoreactive (Fos-LI) spinal neurons of the dorsal horn. Surgery (n=4) induced 91,680+/-14,974 Fos-LI neurons ipsilaterally and morphine reduced this number to 45,771+/-8755 following the 2.5mg/kg dose (p<0.01; n=6) and 14,981+/-2327 following the 10mg/kg dose (p<0.001; n=6). These results indicate that morphine dose-dependently reduces the number of surgery-induced Fos-LI neurons in the spinal cord. As even a high dose of morphine does not reduce spinal c-fos expression to basal level, it may be appropriate to use other analgesics simultaneously with morphine during surgery.

Population pharmacokinetic modelling of non-linear brain distribution of morphine: influence of active saturable influx and P-glycoprotein mediated efflux

BACKGROUND AND PURPOSE

Biophase equilibration must be considered to gain insight into the mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) correlations of opioids. The objective was to characterise in a quantitative manner the non-linear distribution kinetics of morphine in brain.

EXPERIMENTAL APPROACH

Male rats received a 10-min infusion of 4 mg kg(-1) of morphine, combined with a continuous infusion of the P-glycoprotein (Pgp) inhibitor GF120918 or vehicle, or 40 mg kg(-1) morphine alone. Unbound extracellular fluid (ECF) concentrations obtained by intracerebral microdialysis and total blood concentrations were analysed using a population modelling approach.

KEY RESULTS

Blood pharmacokinetics of morphine was best described with a three-compartment model and was not influenced by GF120918. Non-linear distribution kinetics in brain ECF was observed with increasing dose. A one compartment distribution model was developed, with separate expressions for passive diffusion, active saturable influx and active efflux by Pgp. The passive diffusion rate constant was 0.0014 min(-1). The active efflux rate constant decreased from 0.0195 min(-1) to 0.0113 min(-1) in the presence of GF120918. The active influx was insensitive to GF120918 and had a maximum transport (N(max)/V(ecf)) of 0.66 ng min(-1) ml(-1) and was saturated at low concentrations of morphine (C(50)=9.9 ng ml(-1)).

CONCLUSIONS AND IMPLICATIONS

Brain distribution of morphine is determined by three factors: limited passive diffusion; active efflux, reduced by 42% by Pgp inhibition; low capacity active uptake. This implies blood concentration-dependency and sensitivity to drug-drug interactions. These factors should be taken into account in further investigations on PK-PD correlations of morphine.

Dose-related thermal antinociceptive effects of intravenous hydromorphone in cats

OBJECTIVE

To describe the dose-related thermal antinociceptive effects of intravenous (i.v.) hydromorphone in cats.

STUDY DESIGN

Randomized, blinded, crossover design.

ANIMALS

Seven adult cats (3.5-7.4 kg), two spayed females, and five neutered males.

METHODS

Hydromorphone (0.025, 0.05, or 0.1 mg kg(-1)) was administered i.v.. Skin temperature and thermal threshold were measured before and at selected time points to 720 minutes post-administration. Statistical analysis of mean thermal threshold and skin temperatures over time for each dose and between doses was by way of a split-plot model and post hoc Bonferroni t-tests. p < 0.05 was considered significant.

RESULTS

A significant difference from baseline for mean thermal threshold was identified for the 0.05 mg kg(-1) dose (5-80 minutes, peak thermal threshold 46.9 +/- 6.2 degrees C) and 0.1 mg kg(-1) dose (5-200 minutes, peak thermal threshold 54.9 +/-0.2 degrees C). The thermal threshold was significantly greater after the 0.1 mg kg(-1) dose from 5 to 200 minutes compared to the 0.025 mg kg(-1) and 0.5 mg kg(-1) doses. The thermal threshold was significantly greater from 35 to 80 minutes for the 0.05 mg kg(-1) dose when compared with the 0.025 mg kg(-1) dose. Skin temperature was significantly increased from 35 to 140 minutes following the 0.1 mg kg(-1) dose.

CONCLUSIONS

A dose-related antinociceptive effect was demonstrated for i.v. hydromorphone in cats.

CLINICAL RELEVANCE

Hydromorphone at doses less than 0.1 mg kg(-1) has a modest antinociceptive effect and a short duration of action. At a dose of 0.1 mg kg(-1) i.v., onset of analgesia is rapid with a clinically useful duration of effect, but is associated with a rise in skin temperature.

Nociceptive threshold and analgesic response to morphine in aged and young adult rats as determined by thermal radiation and intracerebral electrical stimulation

The present experiment compared the nociceptive threshold and analgesic response to morphine in young (4-5 months) and aged (24 months) rats using peripheral thermal stimulation and intracerebral electrical stimulation. Responses to thermal stimuli were assessed using both the classical tail-flick procedure in which latency of response is the dependent variable and a new method in which threshold in calories of heat is the dependent variable. In the intracerebral nociceptive threshold procedure, electrical stimuli were delivered via an electrode implanted in the mesencephalic reticular formation (MRF), a pain pathway, and the animals were trained to terminate the stimulation by turning a cylindrical manipulandum embedded in one wall of the experimental chamber. For the classical tail-flick method, the aged rats required a greater intensity of stimulation to produce a basal response latency that was between 2.5 and 3.5 s. Using the new psychophysical method for determining the tail-flick threshold, the aged rats' basal thresholds were significantly higher than that of the young rats. However, the basal thresholds obtained by direct stimulation of the MRF failed to show a significant age effect, suggesting that the registration of pain is not different between young and aged rats. These age-related differences in baseline tail-flick response may be due to changes in the spinal reflex associated with aging. Although, there was no difference in the analgesic effects of morphine between young and aged rats using the latency of the tail-flick response, evidence for decreased analgesic response was seen using the tail-flick threshold measure and the intracerebral stimulation threshold method.

Improved Brain Uptake and Pharmacological Activity Profile of Morphine-6-Glucuronide Using a Peptide Vector-Mediated Strategy

Morphine-6-glucuronide (M6G), an active metabolite of morphine, has been shown to have significantly attenuated brain penetration relative to that of morphine. Recently, we have demonstrated that conjugation of various drugs to peptide vectors significantly enhances their brain uptake. In this study, we have conjugated morphine-6-glucuronide to a peptide vector SynB3 to enhance its brain uptake and its analgesic potency after systemic administration. We show by in situ brain perfusion that vectorization of M6G (Syn1001) markedly enhances the brain uptake of M6G. This enhancement results in a significant improvement in the pharmacological activity of M6G in several models of nociception. Syn1001 was about 4 times more potent than free M6G (ED(50) of 1.87 versus 8.74 micromol/kg). Syn1001 showed also a prolonged duration of action compared with free M6G (300 and 120 min, respectively). Furthermore, the conjugation of M6G results in a lowered respiratory depression, as measured in a rat model. Taken together, these data strongly support the utility of peptide-mediated strategies for improving the efficacy of drugs such as M6G for the treatment of pain.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Cerebral and lung kinetics of morphine in conscious sheep after short intravenous infusions

BACKGROUND

The analgesic effects of morphine are delayed relative to its concentration in blood. The rate of equilibration of morphine between blood and brain may contribute to this delay, but the kinetics of this process have not been modelled. This was determined in conscious instrumented sheep. The lung kinetics of morphine were also determined given their importance in defining systemic kinetics after i.v. bolus administration.

METHODS

Sheep were given short i.v. infusions of morphine (30 mg over 4 min). Cerebral kinetics were inferred from arterio-sagittal sinus concentration gradients and cerebral blood flow, and lung kinetics from the pulmonary artery-aortic gradient and cardiac output. These data were fitted to flow- and membrane-limited models of the kinetics in each organ.

RESULTS

Morphine had minimal cardiovascular effects, did not alter cerebral blood flow and caused insignificant respiratory depression. Lung kinetics were best described by a single distribution volume (2036 ml) with a first-order loss (1370 ml min(-1)), which was attributed to deep distribution. The cerebral kinetics of morphine were characterized by a significant permeability barrier. Permeability across the barrier (7.44 ml min(-1)) was estimated with good precision, and was approximately one-fifth of the nominal cerebral blood flow. The distribution volume of morphine in the brain was estimated with less precision, but was described by a brain:blood partition coefficient of approximately 1.4. The time required for 50% equilibration between brain and blood concentrations was approximately 10.3 min.

CONCLUSION

The cerebral equilibration of morphine was relatively slow, and was characterized by significant membrane limitation.

Contribution of morphine-6-glucuronide to antinociception following intravenous administration of morphine to healthy volunteers

This study was performed to develop an integrated pharmacokinetic-pharmacodynamic model for estimating the contribution of morphine-6-glucuronide (M6G) to morphine-associated antinociception in humans. Healthy volunteers (n = 8) received 10 mg of morphine sulfate as a 5-minute i.v. infusion. A Contact Thermode heat probe was placed on the volar forearm to elicitpain. Thermal threshold, defined as the temperature at which pain was first perceived, was measured at fixed time intervals over 8 hours. Serum concentrations of morphine and M6G were determined by LC/MS. Concentration- and effect-time data were analyzed by stepwise nonlinear least-squares regression. The pharmacodynamic parameter estimates were recovered with a linear effect-compartment model and were used to assess the contribution of M6G to morphine-associated analgesia. The estimates (mean +/- SEM) for morphine total clearance and steady-state volume of distribution were 1.0 +/- 0.07 L/h/kg and 1.6 +/- 0.1 L/kg, respectively. The AUC ratio of M6G to morphine was 0.73 +/- 0.06. The contribution of M6G to analgesia ranged from < 0.1% to 66% and was inversely related to the overall effect elicited by the morphine dose (r2 = 0.776). Differences in gender were observed where the contribution (mean +/- SEM) of M6G to analgesia was 32% +/- 19% in males (n = 3) and 13% +/- 8% in females (n = 5). These results suggest that as the overall effect of morphine increases, the fractional contribution of M6G declines and the contribution of M6G to analgesia may differ between males and females. Alterations in the M6G/morphine system may have clinically significant pharmacodynamic consequences.

Morphine-6-glucuronide: an analgesic of the future?

Morphine-6-beta-glucuronide (M6G) is an opioid agonist that plays a role in the clinical effects of morphine. Although M6G probably crosses the blood-brain barrier with difficulty, during long term morphine administration it may reach sufficiently high CNS concentrations to exert clinically relevant opioid effects. As a consequence of its almost exclusive renal elimination, M6G may accumulate in the body of patients with impaired renal function and cause severe opioid adverse effects with insidious onset and long persistence. Its profile of receptor affinities, however, gives reason to speculate that M6G may exhibit analgesic effects while causing fewer adverse effects than morphine. This is supported by reports of the good tolerability of intrathecal and intravenous injections of M6G in humans with intact renal function. M6G may thus be contemplated as an analgesic for short term postoperative analgesia, especially for intrathecal analgesic therapy. In addition, its possibly higher potency than morphine makes M6G a candidate opioid for local or peripheral analgesic therapy. However, current knowledge is too incomplete to finally judge the clinical usefulness of M6G. The next topics for clinical research on M6G should include: (i) a comparison of the potencies of M6G and morphine to cause wanted and unwanted clinical effects; (ii) development of a predictive population pharmacokinetic-pharmacodynamic model of M6G with calculation of the transfer half-life between plasma and effect site; and (iii) identification of cofactors influencing the action of M6G that can serve as predictors for the clinical outcome of morphine/M6G therapy in an individual including the pharmacogenetics of M6G.

Blood-brain barrier transport and brain distribution of morphine-6-glucuronide in relation to the antinociceptive effect in rats--pharmacokinetic/pharmacodynamic modelling

1. The objective of this study was to investigate the contribution of the blood-brain barrier (BBB) transport to the delay in antinociceptive effect of morphine-6-glucuronide (M6G), and to study the equilibration of M6G in vivo across the BBB with microdialysis measuring unbound concentrations. 2. On two consecutive days, rats received an exponential infusion of M6G for 4 h aiming at a target concentration of 3000 ng ml(-1) (6.5 microM) in blood. Concentrations of unbound M6G were determined in brain extracellular fluid (ECF) and venous blood using microdialysis and in arterial blood by regular sampling. MD probes were calibrated in vivo using retrodialysis by drug prior to drug administration. 3. The half-life of M6G was 23+/-5 min in arterial blood, 26+/-10 min in venous blood and 58+/-17 min in brain ECF (P<0.05; brain vs blood). The BBB equilibration, expressed as the unbound steady-state concentration ratio, was 0.22+/-0.09, indicating active efflux in the BBB transport of M6G. A two-compartment model best described the brain distribution of M6G. The unbound volume of distribution was 0.20+/-0.02 ml g brain(-1). The concentration-antinociceptive effect relationships exhibited a clear hysteresis, resulting in an effect delay half-life of 103 min in relation to blood concentrations and a remaining effect delay half-life of 53 min in relation to brain ECF concentrations. 4. Half the effect delay of M6G can be explained by transport across the BBB, suggesting that the remaining effect delay of 53 min is a result of drug distribution within the brain tissue or rate-limiting mechanisms at the receptor level.

Precipitated withdrawal following codeine administration is dependent on CYP genotype

The role of metabolic polymorphism in the development of physical dependence to codeine was assessed in cytochrome P450 2D2 (CYP2D2) deficient Dark Agouti and CYP2D2 intact Sprague-Dawley rats by assessment of the severity of naloxone precipitated withdrawal after codeine and morphine administration. Plasma morphine concentrations after codeine were significantly higher (P<0.01) in Sprague-Dawley than in Dark Agouti rats with metabolic ratios of 0.71 +/- 0.27 and 0.07 +/- 0.04, respectively. Withdrawal after codeine resulted in significantly greater hypothermia (3.5-4 degrees C, P<0.0001) in Sprague-Dawley animals compared to the other groups. Body weight loss was similar for all groups ranging from 6.2 +/- 0.4 to 8.2 +/- 0.6 g. When strain and treatment data were combined, a relationship between body temperature and plasma morphine concentration could be described by the inverse Hill equation (r(2)=0.76, EC(50)=556 +/- 121 ng/ml, n=2.9 +/- 1.5). These data indicate that dependence and withdrawal after codeine administration are dependent on its bioconversion to morphine.

Methadone maintenance patients are cross-tolerant to the antinociceptive effects of morphine

We have previously shown that methadone maintenance patients are hyperalgesic. Very little is known about the antinociceptive effects of additional opioids in these patients. This study (1) compared the intensity and duration of antinociceptive responses, at two pseudo-steady-state plasma morphine concentrations (C(SS1) and C(SS2)), between four patients on stable, once daily, doses of methadone and four matched control subjects; and (2) determined, in methadone patients, whether the antinociceptive effects of morphine are affected by changes in plasma R(-)-methadone concentration that occur during an inter-dosing interval. Two types of nociceptive stimuli were used: (1) a cold pressor test (CP), (2) electrical stimulation (ES). Morphine was administered intravenously to achieve the two consecutive plasma concentrations. Blood samples were collected, concurrently with nociceptive responses, to determine plasma morphine concentrations. Methadone patients achieved mean C(SS1) and C(SS2) of 16 and 55 ng/ml respectively; those of controls were 11 and 33 ng/ml. Methadone patients were hyperalgesic to pain induced by CP but not ES. Despite significantly greater plasma morphine concentrations, methadone patients experienced minimal antinociception in comparison with controls. Furthermore in methadone patients, the antinociception ceased when the infusion ended. In comparison, the duration of effect in control subjects was 3 h. The fluctuations that occurred in plasma R(-)-methadone concentration during an inter-dosing interval had little effect on patients' responses to morphine. Our findings suggest that methadone patients are cross-tolerant to the antinociceptive effects of morphine, and conventional doses of morphine are likely to be ineffective in managing episodes of acute pain amongst this patient group. Further research is needed to determine whether other drugs are more effective than morphine in managing acute pain in this patient population.

Effect of uranyl nitrate-induced renal failure on morphine disposition and antinociceptive response in rats

1. The aims of the present study were to administer morphine (14.0 mumol/kg, s.c.) to male Hooded Wistar rats and to determine the effect of uranyl nitrate-induced renal failure on: (i) the antinociceptive effect of morphine; (ii) the pharmacokinetics of morphine and morphine-3-glucuronide (M3G); and (iii) the relationship between antinociceptive effect and the pharmacokinetics of morphine in plasma and brain. 2. Renal failure was induced by a single s.c. injection of uranyl nitrate and kinetic/dynamic studies were performed 10 days after its administration, when creatinine clearance was 17% of the control group. Antinociceptive effect was measured by the tail-flick method at various times up to 2 h post-drug administration. Concentrations of morphine and M3G in plasma and brain and concentrations of creatinine in urine and serum were determined by specific HPLC methods. 3. After morphine administration, the area under the antinociceptive effect-time curve was decreased by 44% in renal failure rats. There were no differences between control and renal failure rats in: (i) plasma morphine concentration-time curves; (ii) brain morphine concentration-time curves; and (iii) plasma M3G concentration-time curves. Morphine-6-glucuronide was not detected in any plasma or brain sample from rats administered morphine and no M3G was detected in brain. 4. For both control and renal failure rats, the relationships between antinociceptive effect and plasma morphine concentration were characterized by counterclockwise hysteresis loops, probably reflecting a delay for the relatively polar morphine to cross the blood-brain barrier. The relationship between antinociceptive effect and brain morphine concentration in control rats revealed no evidence of acute tolerance and was described by a sigmoidal function. In contrast, the relationship in renal failure rats was characterized by clockwise hysteresis, which is consistent with acute tolerance development.