Variability of morphine disposition during long-term subcutaneous infusion in terminally ill cancer patients

Pharmacological data science perspective on fatal incidents of morphine treatment

Morphine prescribed for analgesia has caused drug-related deaths at an estimated incidence of 0.3% to 4%. Morphine has pharmacological properties that make it particularly difficult to assess the causality of morphine administration with a patient's death, such as its slow transfer between plasma and central nervous sites of action and the existence of the active metabolite morphine-6-glucuronide with opioid agonistic effects, Furthermore, there is no well-defined toxic dose or plasma/blood concentration for morphine. Dosing is often adjusted for adequate pain relief. Here, we summarize reported deaths associated with morphine therapy, including associated morphine exposure and modulating patient factors such as pharmacogenetics, concomitant medications, or comorbidities. In addition, we systematically analyzed published numerical information on the stability of concentrations of morphine and its relevant metabolites in biological samples collected postmortem. A medicolegal case is presented in which the causality of morphine administration with death was in dispute and pharmacokinetic modeling was applied to infer the administered dose. The results of this analytical review suggest that (i) inference from postmortem blood concentrations to the morphine dose administered has low validity and (ii) causality between a patient's death and the morphine dose administered remains a highly context-dependent and collaborative assessment among experts from different medical specialties.

A Prospective Population Pharmacokinetic Study on Morphine Metabolism in Cancer Patients

Background

Oral and subcutaneous morphine is widely used for the treatment of cancer-related pain; however, solid pharmacokinetic data on this practice are lacking. Furthermore, it is largely unknown which factors contribute to the variability in clearances of morphine and its metabolites and whether morphine clearance is related to treatment outcome.

Methods

Blood samples from 49 cancer patients treated with oral and/or subcutaneous morphine were prospectively collected and were used to develop a population pharmacokinetic model for morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). The influence of age, gender, renal function and several polymorphisms possibly related to the pharmacokinetics of the three compounds was investigated. In addition, the relation between treatment failure and morphine and metabolite clearances was explored.

Results

A one-compartment model including an extensive first-pass effect adequately described the data of morphine and its metabolites. Estimated mean area under the plasma concentration–time curve (AUC) ratios following oral versus subcutaneous administration were: M3G/morphine 29.7:1 vs. 11.1:1; M6G/morphine 5.26:1 vs. 1.95:1; and M3G/M6G 5.65:1 vs. 5.70:1. Renal function was significantly correlated with clearance of the metabolites, which increased 0.602 L/h per every 10 mL/min/1.73 m² increase of estimated glomerular filtration rate (eGFR), reaching a plateau for eGFR >90 mL/min/1.73 m². The clearance of morphine or its metabolites was not found to be correlated with treatment failure.

Conclusion

The influence of age-, gender- and pharmacokinetic-related polymorphisms was not identified on the pharmacokinetics of morphine. Clearance of morphine or its metabolites was not found to explain treatment outcome; however, large variations in plasma concentrations of morphine, M3G and M6G support further studies on the relation between plasma concentrations and treatment outcome.

Dutch Trial Register ID: NTR4369.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-016-0471-7) contains supplementary material, which is available to authorized users.

Induction of morphine-6-glucuronide synthesis by heroin self-administration in the rat

RATIONALE

Heroin is rapidly metabolized to morphine that in turn is transformed into morphine-3-glucuronide (M3G), an inactive metabolite at mu-opioid receptor (MOR), and morphine-6-glucuronide (M6G), a potent MOR agonist. We have found that rats that had received repeated intraperitoneal injections of heroin exhibit measurable levels of M6G (which is usually undetectable in this species).

OBJECTIVE

The goal of the present study was to investigate whether M6G synthesis can be induced by intravenous (i.v.) heroin self-administration (SA).

MATERIALS AND METHODS

Rats were trained to self-administer either heroin (50 μg/kg per infusion) or saline for 20 consecutive 6-h sessions and then challenged with an intraperitoneal challenge of 10 mg/kg of heroin. Plasma levels of heroin, morphine, 6-mono-acetyl morphine, M3G, and M6G were quantified 2 h after the challenge. In vitro morphine glucuronidation was studied in microsomal preparations obtained from the liver of the same rats.

RESULTS

Heroin SA induced the synthesis of M6G, as indicated by detectable plasma levels of M6G (89.7 ± 37.0 ng/ml vs. 7.35 ± 7.35 ng/ml after saline SA). Most important, the in vitro V (max) for M6G synthesis was correlated with plasma levels of M6G (r (2) = 0.78). Microsomal preparations from saline SA rats produced negligible amounts of M6G.

CONCLUSION

Both in vivo and in vitro data indicate that i.v. heroin SA induces the synthesis of M6G. These data are discussed in the light of previous studies conducted in heroin addicts indicating that in humans heroin enhances the synthesis of the active metabolite of heroin and morphine.

Pharmacological consequences of long-term morphine treatment in patients with cancer and chronic non-malignant pain

BACKGROUND

In patients with pain of malignant origin morphine may be administered in high and often increasing doses during extended periods of time. In patients with chronic pain of non-malignant origin morphine may be an important remedy, and in these cases the goal is to keep the morphine dose stable. The pharmacokinetic as well as the pharmacodynamic consequences of long-term morphine treatment with special reference to the two most important metabolites of morphine morphine-6-glucuronide (M-6-G) and morphine-3-glucuronide (M-3-G) remain to be settled.

METHODS

Assessments for pain, sedation and other morphine induced side effects were made several times for 19 cancer patients treated with changing doses of oral sustained release (SR) morphine and twice for 17 non-cancer patients treated with stable doses of SR morphine. Blood samples were obtained simultaneously and analysed for contents of morphine, M-3-G and M-6-G by high-performance liquid chromatography (HPLC).

RESULTS

Significant correlations were found between the daily dose of SR morphine and plasma morphine (r = 0.469, p < 0.01), plasma M-6-G (r = 0.677, p < 0.01), and plasma M-3-G ((r = 0.827, p < 0.01), in the cancer patient group, but only between the daily dose of SR morphine and plasma M-3-G (0.662, p < 0.01) and plasma M-6-G (0.571, p < 0.01) in the non-cancer patient group. Normalised M-3-G/M and M-6-G/M ratios for the cancer patient group were independent of duration of treatment and daily dose of SR morphine. Likewise in the non-cancer patient group duration of treatment did not influence the metabolite ratios. Correlations between pain score and plasma morphine, M-6-G and M-6-G/M were weak in the cancer patient as well as in the non-cancer patient group making it impossible to draw any conclusion regarding the potential contributory analgesic effect of M-6-G. Dryness of the mouth was the most frequent adverse effect reported in the non-cancer as well as the cancer patient group. In the latter group patients complaining of dryness of the mouth had significantly higher plasma morphine and M-6-G concentrations than patients who did not suffer from this side effect. This difference persisted (or was close to significance) when excluding patients receiving antidepressants.

CONCLUSION

In the cancer patient group neither dose nor treatment period seems to influence morphine glucuronidation. Likewise in the non-cancer patient group receiving stable doses of morphine duration of treatment does not seem to influence morphine glucuronidation. Dryness of the mouth was positively correlated to high plasma concentrations of morphine and M-6-G.

Explaining the escalation of drug use in substance dependence: models and appropriate animal laboratory tests

Escalation of drug use, a hallmark of drug dependence, has traditionally been interpreted as reflecting the development of tolerance to the drug's effects. However, on the basis of animal behavioral data, several groups have recently proposed alternative explanations, i.e. that such an escalation of drug use might not be based on (1) tolerance, but rather be indicative of (2) sensitization to the drug's reinforcing effect, (3) reward allostasis, (4) an increase in the incentive salience of drug-associated stimuli, (5) an increase in the reinforcing strength of the drug reinforcer relative to alternative reinforcers, or (6) habit formation. From the pharmacological perspective, models 1-3 allow predictions about the change in the shape of drug dose-effect curves that are based on mathematically defined models governing receptor-ligand interaction and signal transduction. These predictions are tested in the present review, which also describes the other currently championed models for drug use escalation and other components of apparent 'reinforcement' (in its original meaning, like 'tolerance' or 'sensitization', a purely descriptive term). It evaluates the animal experimental approaches employed to support or prove the existence of each of the models and reinforcement components, and recapitulates the clinical evidence, which strongly suggests that escalation of drug use is predominantly based on an increase in the frequency of intoxication events rather than an increase in the dose taken at each intoxication event. Two apparent discrepancies in animal experiments are that (a) sensitization to overall reinforcement has been found more often for psychostimulants than for opioids, and that (b) tolerance to the reinforcing and other effects has been observed more often for opioids than for cocaine. These discrepancies are resolved by the finding that cocaine levels seem to be more tightly regulated at submaximum reinforcing levels than opioid levels are. Consequently, animals self-administering opioids are more likely to expose themselves to higher above-threshold doses than animals self-administering psychostimulants, rendering the development of tolerance to opioids more likely than tolerance to psychostimulants. The review concludes by making suggestions on how to improve the current behavioral experimental approaches.

Subcutaneous administration of drugs in the elderly: survey of practice and systematic literature review

OBJECTIVES AND METHOD

Survey of subcutaneous drug use and hypodermoclysis with a standardized questionnaire to 27 nursing teams and 52 physicians in a geriatric hospital department (404 beds). Evaluation of license status (CH, F, D and UK) and systematic literature review of 34 drugs used in the geriatric setting.

RESULTS

Subcutaneous route is used daily with drugs and fluids mostly for patients in palliative care (83%) or who are dehydrated (54%) when oral or IV administration is impossible (73%, 68% respectively). Morphine (98%), haloperidol (90%), furosemide (69%) and hydromorphone (56%) by bolus (36%) or slow injection over 5 min (82%) are the main drugs used and NaCI 0.9% (95%), and glucose 5%/NaCI 0.9% (31 %) are commonly used for rehydration. Among the 34 drugs reviewed, only 13 (38%) are licensed for subcutaneous use in CH, UK, F or D, and only, morphine (14 articles of 68) and rehydration (six articles) are evaluated in high level studies. Haloperidol and furosemide are used off-label and there are no well-designed studies supporting their subcutaneous use.

CONCLUSION

There is a lack of information on drugs widely used by subcutaneous route in the elderly. In that context, physicians carry responsibility for the prescription.

Day-to-day variations during clinical drug monitoring of morphine, morphine-3-glucuronide and morphine-6-glucuronide serum concentrations in cancer patients. A prospective observational study

Background

The feasibility of drug monitoring of serum concentrations of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) during chronic morphine therapy is not established. One important factor relevant to drug monitoring is to what extent morphine, M6G and M3G serum concentrations fluctuate during stable morphine treatment.

Methods

We included twenty-nine patients admitted to a palliative care unit receiving oral morphine (n = 19) or continuous subcutaneous (sc) morphine infusions (n = 10). Serum concentrations of morphine, M6G and M3G were obtained at the same time on four consecutive days. If readmitted, the patients were followed for another trial period. Day-to-day variations in serum concentrations and ratios were determined by estimating the percent coefficient of variation (CV = (mean/SD) ×100).

Results

The patients' median morphine doses were 90 (range; 20–1460) mg/24 h and 135 (range; 30–440) mg/24 h during oral and sc administration, respectively. Intraindividual fluctuations of serum concentrations estimated by median coefficients of day-to-day variation were in the oral group for morphine 46%, for M6G 25% and for M3G 18%. The median coefficients of variation were lower in patients receiving continuous sc morphine infusions (morphine 10%, M6G 13%, M3G 9%).

Conclusion

These findings indicate that serum concentrations of morphine and morphine metabolites fluctuate. The fluctuations found in our study are not explained by changes in morphine doses, administration of other drugs or by time for collection of blood samples. As expected the day-to-day variation was lower in patients receiving continuous sc morphine infusions compared with patients receiving oral morphine.

Clinical pharmacokinetics of morphine

Morphine, the most widely used mu-opioid analgesic for acute and chronic pain, is the standard against which new analgesics are measured. A thorough understanding of the pharmacokinetics of morphine is required in order to safely and effectively use this analgesic in a wide variety of patients with different levels of organ function. A MEDLINE search was conducted to identify literature published between 1966 and January 2002 relevant to the pharmacokinetics of morphine. These publications were reviewed and the literature summarized regarding unique and clinically important elements of morphine disposition relative to its parenteral administration (including intravenous, intramuscular, subcutaneous, epidural and intrathecal administration), absorption profile (immediate release, controlled release, and sublingual/buccal, and rectal administration), distribution, and its metabolism/excretion. Special populations, including infants, elderly, and those with renal/liver failure, have a unique morphine pharmacokinetic profile that must be taken into account in order to maximize analgesic efficacy and reduce the risk of adverse events.

The oral-to-intravenous equianalgesic ratio of morphine based on plasma concentrations of morphine and metabolites in advanced cancer patients receiving chronic morphine treatment

To provide additional pharmacokinetic evidence for the oral-to-parenteral relative potency ratio of 1:2 to 1:3 for chronic morphine use in a palliative care setting, we determined the plasma concentrations of morphine and its major metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), in hospitalized advanced cancer patients maintained on long-term oral or intravenous morphine. There were significant linear correlations between daily doses of morphine and plasma concentrations (molar base) of morphine, M3G and M6G for both routes of administration. The oral-to-intravenous relative ratios of the regression coefficients were 2.9 for morphine and 1.8 for morphine + M6G. The morphine kinetic variables were not significantly influenced by any hepato-renal biochemical markers. These results support the commonly used oral-to-intravenous relative potency ratio of 1:2 to 1:3 in patients with cancer pain receiving chronic morphine treatment.

High levels of morphine-6-glucuronide in street heroin addicts

RATIONALE

In the body, heroin is rapidly transformed to 6-acetylmorphine (6-AM) and then to morphine, that in turn is mainly metabolized to morphine-3-glucuronide (M3G) and, at lesser extent, to morphine-6-glucuronide (M6G). Unlike M3G, M6G is a potent opioid agonist. Intravenous heroin abusers (IHU) are exposed to a wide array of drugs and contaminants that might affect glucuronidation.

OBJECTIVES

We assessed plasma and urine concentrations of M3G and M6G in four groups of subjects: the first two included long-term IHU either exposed to street heroin ( n=8) or receiving a single IV injection of morphine ( n=4), while the other two groups included non-IHU patients receiving acute IV ( n=8) or chronic oral ( n=6) administrations of morphine.

METHODS

After solid phase extraction plasma and urine concentrations of morphine metabolites were determined by HPLC analyses.

RESULTS

M3G accounted for the greater part of morphine glucuronides detected in body fluids of non-IHU patients treated with morphine. This pattern of metabolism remained stable across 15 days of oral administration of incremental doses of morphine. In contrast, the two groups of IHU (street heroin taking or morphine-treated subjects) showed a reduction of blood and urine M3G concentrations in favor of M6G. Consequently, M6G/M3G ratio was significantly higher in the two IHU groups in comparison with the non-IHU groups.

CONCLUSIONS

Chronic exposure to street heroin causes a relative increase in concentrations of the active metabolite, M6G. Since the pattern of M6G action seems closer to heroin than to morphine, the increased synthesis of M6G observed in IHU may prolong the narrow window of heroin effects.

Influences on serum concentrations of morphine, M6G and M3G during routine clinical drug monitoring: a prospective survey in 300 adult cancer patients

BACKGROUND

In order to make treatment decisions physicians should have knowledge about the relations between patient characteristics and drug disposition. Dose, route of administration, gender, age and renal function are reported to influence the serum concentrations of morphine, morphine-6-glucurnide (M6G) and morphine-3-glucuronide (M3G) during chronic treatment of cancer pain. These factors, however, are not evaluated in studies with a sample size sufficient to explore predictive factors.

METHODS

Three hundred consecutive morphine users admitted because of a malignant disease were recruited. The relations of serum concentrations of morphine, M6G and M3G to patient characteristics (gender, age, weight, renal function, liver function, dose, route of administration) were explored, and regression analysis performed to investigate whether these characteristics predicted serum concentrations obtained during routine clinical drug monitoring.

RESULTS

Morphine dose was associated with serum concentrations of morphine (r = 0.69), M6G (r = 0.76) and M3G (r = 0.76). Oral morphine resulted in higher dose-adjusted M6G and M3G serum concentrations compared with s.c. morphine. Creatinine serum concentrations correlated with serum concentrations of M6G and M3G. Dose and route of administration predicted morphine serum concentrations, while dose and renal function predicted M6G and M3G serum concentrations. Age was an additional factor predicting M3G concentrations. Dose was the only factor that explained a clinically significant part of the observed variability.

CONCLUSION

Patient characteristics predict only minor parts of the variability of morphine, M3G and M6G serum concentrations observed during routine clinical drug-monitoring in cancer patients.

Relationships among morphine metabolism, pain and side effects during long-term treatment: an update

The two metabolites of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), have been studied intensively in animals and humans during the past 30 years in order to elucidate their precise action and possible contribution to the desired effects and side effects seen after morphine administration. M3G and M6G are formed by morphine glucuronidation, mainly in the liver, and are excreted by the kidneys. The metabolites are found in the cerebrospinal fluid after single as well as multiple doses of morphine. M6G binds to opioid receptors, and animal studies have demonstrated that M6G may be a more potent analgesic than morphine. Results from human studies regarding the analgesic effect of M6G are not unanimous. The potency ratio between systemic M6G and morphine in humans has not been settled, but is probably lower than previously assumed. Hitherto, only a few studies have found evidence for a contributory effect of M6G to the overall effects observed after morphine administration. Several studies have demonstrated that administration of M6G is accompanied by fewer and a milder degree of opioid-like side effects than observed after morphine administration, but most of the studies have used lower doses of M6G than of morphine. M3G displays very low affinity for opioid receptors and has no analgesic activity. Animal studies have shown that M3G may antagonize the analgesic effect of morphine and M6G, but no human studies have demonstrated this. M3G has also been connected to certain neurotoxic symptoms, such as hyperalgesia, allodynia and myoclonus, which have been observed after administration of M3G or high doses of morphine in animals. The symptoms have been reported sporadically in humans treated primarily with high doses of morphine, but the role of M3G in eliciting the symptoms is not fully elucidated.

Increased plasma morphine metabolites in terminally ill cancer patients with delirium: an intra-individual comparison

Delirium often causes severe distress for terminally ill cancer patients, and treatment of underlying pathologies is important to achieve symptom alleviation. Although accumulation of morphine metabolites may play an important role in development of delirium, empirical findings are conflicting due to a large inter-individual variation in morphine-related concentrations. To explore intra-individual changes of morphine metabolite concentrations before and after occurrence of terminal delirium, a prospective observational study was performed on terminally ill cancer patients. Among 131 consecutive hospice inpatients, 16 samples from 8 patients who received two blood samplings before and after development of delirium were analyzed. Delirium developed a median of 5 days before death, and clinical causes were attributed to multi-organ failure. Plasma concentrations of morphine-6-glucuronide (M-6-G) and morphine-3-glucuronide (M-3-G) significantly increased after development of delirium within the same patient. Mean normalized concentrations of M-6-G and M-3-G elevated from 1.24 +/- 1.06 to 2.94 +/- 3.52 ng/mL/mg (P = 0.016), and from 7.46 +/- 4.75 to 15.4 +/- 13.2 ng/mL/mg (P = 0.016), respectively. Normalized morphine concentration increased with a marginal statistical significance from 0.54 +/- 0.27 to 0.83 +/- 0.22 ng/mL/mg (P = 0.055). In conclusion, plasma concentrations of M-6-G and M-3-G were significantly higher after development of terminal delirium than before. It is suggested that accumulations of morphine metabolites can contribute to development of delirium in cancer patients whose death is impending.

The pharmacokinetics of morphine and morphine glucuronide metabolites after subcutaneous bolus injection and subcutaneous infusion of morphine

AIMS

To investigate the pharmacokinetics of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) in healthy volunteers after the administration of morphine by subcutaneous bolus injection (s.c.b.) and subcutaneous infusion (s.c. i.) over 4 h, and to compare the results with the intravenous bolus (i.v.) administration of morphine.

METHODS

Six healthy volunteers each received 5 mg morphine sulphate by i.v., s.c.b. and short s.c.i. over 4 h, on three separate occasions, in random order, each separated by at least 1 week. Plasma samples were assayed for morphine, M6G and M3G.

RESULTS

After i.v. morphine, the concentrations of morphine, M6G and M3G and their pharmacokinetic parameters were similar to those we have observed previously, in other healthy volunteers (when standardized to nmol l- 1, for a 10 mg dose to a 70 kg subject). After s.c.b. morphine, similar results were obtained except that the median tmax values for morphine and M3G were significantly longer than after i.v. morphine (P< 0.001 and P< 0.05, respectively), with a trend to a longer tmax for M6G (P = 0. 09). The appearance half-lives after s.c.b. morphine for M6G and M3G were also significantly longer than after i.v. morphine (P = 0.03 and P< 0.05, respectively). Comparison of log-transformed AUC values indicated that i.v. and s.c.b. administration of morphine were bioequivalent with respect to morphine, M6G and M3G. In comparison with i.v. morphine, morphine by s.c.i. was associated with significantly longer median tmax values for morphine (P< 0.001), M6G (P< 0.001) and M3G (P< 0.05), and the mean standardized Cmax values significantly lower than after both i.v. and s.c.b. morphine (morphine P< 0.001, M6G P< 0.001 and M3G P< 0.01 for each comparison). Comparison of log-transformed AUC values after i.v. and s.c.i. morphine indicated that the two routes were not bioequivalent for morphine (log-transformed AUC ratio 0.78, 90% CI 0.66-0.93), M6G (0.72, 90% CI 0.63-0.82), or M3G (0.65, 90% CI 0.54-0.78). A small stability study indicated no evidence of adsorptive losses from morphine infused over 4 h using the infusion devices from the study.

CONCLUSIONS

Although bioequivalence was demonstrated between the s. c.b. and i.v. routes of morphine administration, the bioavailabilities of morphine, M6G and M3G after s.c.i. were significantly lower than after i.v. administration. However, despite this, the study demonstrates that the subcutaneous route is an effective method for the parenteral administration of morphine.