Differences between and combinations of opioids re-visited

In vitro profiling of opioid ligands using the cAMP formation inhibition assay and the β-arrestin2 recruitment assay: No two ligands have the same profile

Previously, we showed that no two of seven opioids administered by the intracerebroventricular route had the same potency rank order for evoking antinociception, constipation and respiratory depression in rats. To gain insight at the cellular level, this study was designed to systematically investigate the activity profiles of six commonly used opioid ligands using the forskolin-stimulated cAMP assay and a β-arrestin2 recruitment assay in cultured HEK-293 cells transfected with MOP(μ), DOP(δ) or KOP(κ) receptors(-r). Morphine was a potent agonist at the MOP-r in the cAMP assay whereas it was a weak agonist at the KOP-r and DOP-r. Oxycodone had moderate efficacy and low potency at the MOP-r. Buprenorphine was a potent MOP-r and DOP-r agonist; its efficacy rank order was DOP > MOP > KOP. Fentanyl was a potent agonist at the MOP-r; its efficacy rank order was MOP > DOP > KOP. For DPDPE, its agonist efficacy was confined to the DOP-r, whereas for U69593, its efficacy rank order was KOP>> MOP. For the β-arrestin2 assay, fentanyl had full efficacy at the MOP-r whereas morphine and oxycodone were weak with insignificant efficacy at DOP and KOP receptors. Buprenorphine did not recruit β-arrestin2 at all three opioid-receptors. DPDPE and U69593 had full efficacy for β-arrestin2 recruitment to the DOP-r and KOP-r respectively. Despite the low efficacy and potency of morphine, oxycodone and buprenorphine in recruiting β-arrestin2 to the MOP-r herein, these opioids all evoked respiratory depression and constipation in rats. Together, our findings discount a key role for β-arrestin2 recruitment at the MOP-r in evoking opioid-related side-effects.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Intravenous Oxycodone versus Intravenous Morphine in Cancer Pain: A Randomized, Open-Label, Parallel-Group, Active-Control Study

Objective

To compare efficacy and safety of intravenous continuous infusion of oxycodone with morphine in patients with cancer pain.

Methods

A 5-day, randomized, open-label, exploratory study at 6 sites in the Republic of Korea. Sixty-six adults aged ≥19 years with moderate-to-severe cancer pain (Numeric Rating Scale [NRS] ≥ 4) were enrolled. The study group received intravenous (IV) oxycodone, and the comparator group received IV morphine which were titrated depending on pain intensity. The efficacy endpoint is change in average NRS score from baseline to Day 5. Other assessments included worst, current, and average pain intensity; patient satisfaction; medication dose; and adverse events.

Results

Both groups achieved >50% reduction in average pain intensity: from “moderate” at baseline (oxycodone versus morphine: 6.0 ± 1.8 versus 5.9 ± 1.4) to “mild” at Day 5 (2.5 ± 1.8 versus 2.8 ± 1.6). While this reduction was similar between groups (3.5 ± 2.2 versus 3.1 ± 1.8, P value = 0.562), oxycodone achieved faster pain relief (average pain: 3.0 ± 1.6 versus 3.9 ± 1.6, P value = 0.020) on Day 2 and significant NRS reductions for worst pain on Day 2 (P value = 0.045) and current pain on Day 2 (P value = 0.035) and Day 5 (P value = 0.020) compared to morphine. Patient satisfaction, adverse events, and adverse drug reactions were similar for both groups.

Conclusions

For Asian patients with cancer pain, IV oxycodone is faster acting and showed similar analgesic efficacy and safety profiles as IV morphine. This trial is registered with Clinicaltrials.gov NCT02660229.

A new therapeutic option for postoperative pain management with oxycodone HCI injection

Fentanyl is the most commonly used opioid analgesic in intravenous patient-controlled analgesia (IV PCA) in Korea. IV oxycodone was approved for postoperative IV PCA by the Ministry of Food and Drug Safety of Korea in 2013. The approved dosage regimen for postoperative pain relief with IV oxycodone is IV bolus loading of 2 mg followed by PCA composed of demand boluses of 1 mg and no background infusion with an oxycodone concentration of 1 mg/ml. However, a simulation study indicated that the minimum effective analgesic concentration (MEAC, as indicated by relief of pain by administering rescue analgesics) of oxycodone was reached most quickly with a higher loading dose of 0.1 mg/kg and IV PCA with background infusion. Oxycodone is a therapeutic option as an analgesic for postoperative pain management. It is necessary to reduce the analgesic dose of oxycodone in elderly patients because metabolic clearance decreases with age.

Neuropathic Pain due to Small Fiber Neuropathy in Aging: Current Management and Future Prospects

Over the last 10 years, the diagnosis small fiber neuropathy (SFN) has gained recognition worldwide. Patients often suffer from severe neuropathic pain that may be difficult to treat. A substantial subset of patients with SFN is aged 65 years or older, and these patients often exhibit comorbidities and usage of multiple drugs, making neuropathic pain treatment more challenging. In this review, we highlight relevant pathophysiological aspects and discuss currently used therapeutic strategies for neuropathic pain. Possible pitfalls in neuropathic pain treatment in the elderly will be underlined.

Slow drug delivery decreased total body clearance and altered bioavailability of immediate- and controlled-release oxycodone formulations

Oxycodone is a commonly used analgesic with a large body of pharmacokinetic data from various immediate-release or controlled-release formulations, under different administration routes, and in diverse populations. Longer terminal half-lives from extravascular administration as compared to IV administration have been attributed to flip-flop pharmacokinetics with the rate constant of absorption slower than elimination. However, PK parameters from the extravascular studies showed faster absorption than elimination. Sustained release formulations guided by the flip-flop concept produced mixed outcomes in formulation development and clinical studies. This research aims to develop a mechanistic knowledge of oxycodone ADME, and provide a consistent interpretation of diverging results and insight to guide further extended release development and optimize the clinical use of oxycodone. PK data of oxycodone in human studies were collected from literature and digitized. The PK data were analyzed using a new PK model with Weibull function to describe time-varying drug releases/ oral absorption, and elimination dependent upon drug input to the portal vein. The new and traditional PK models were coded in NONMEM. Sensitivity analyses were conducted to address the relationship between rates of drug release/absorption and PK profiles plus terminal half-lives. Traditional PK model could not be applied consistently to describe drug absorption and elimination of oxycodone. Errors were forced on absorption, elimination, or both parameters when IV and PO profiles were fitted separately. The new mechanistic PK model with Weibull function on absorption and slower total body clearance caused by slower absorption adequately describes the complex interplay between oxycodone absorption and elimination in vivo. Terminal phase of oxycodone PK profile was shown to reflect slower total body drug clearance due to slower drug release/absorption from oral formulations. Mechanistic PK models with Weibull absorption functions, and release rate-dependent saturable total body clearance well described the diverging oxycodone absorption and elimination kinetics in the literature. It showed no actual drug absorption during the terminal phase, but slower drug clearance caused by slower release/absorption producing the appearance of flip-flop and offered new insight for the development of modified release formulations and clinical use of oxycodone.

Synthesis and biological evaluation of some novel 1-substituted fentanyl analogs in Swiss albino mice

Fentanyl [N-(1-phenethyl-4-piperidinyl)propionanilide] is a potent opioid analgesic agent, but a has narrow therapeutic index. We reported earlier on the synthesis and bioefficacy of fentanyl and its 1-substituted analogs (1–4) in mice. Here we report the synthesis and biological evaluation of four additional analogs, viz. N-isopropyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (5), N-t-butyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (6), isopropyl 2-[4-(N-phenylpropionamido)piperidin-1-yl]propionate (7) and t-butyl 2-[4-(N-phenylpropionamido)piperidin-1-yl]propionate (8). The median lethal dose (LD₅₀) determined by intravenous, intraperitoneal and oral routes suggests these analogs to be comparatively less toxic than fentanyl. On the basis of observational assessment on spontaneous activities of the central, peripheral, and autonomic nervous systems, all the analogs were found to be similar to fentanyl. Naloxone hydrochloride abolished the neurotoxic effects of these analogs, thereby ascertaining their opioid receptor-mediated effects. All the analogs displayed significant analgesic effects, measured by formalin-induced hind paw licking and tail immersion tests at their respective median effective dose (ED₅₀). They also exhibited 8–12 fold increase in therapeutic index over fentanyl. However, 5 and 6 alone produced lower ED₅₀ (20.5 and 21.0 µg/kg, respectively) and higher potency ratio (1.37 and 1.33, respectively) compared to fentanyl. They could thus be considered for further studies on pain management.

Efficacy and safety of dual opioid therapy

There is some evidence for a partial opioid switching or an 'add on' approach to opioid dosing strategies. Preclinical and clinical findings suggest different activation profiles for the stimulation of the mu subtypes, raising the questions about what might occur with combinations of these substances. In the postoperative setting, it seems that the analgesic effect of the combination at equivalent doses is similar to that produced by the individual components, not adding particular advantages. However, adverse effects seem to be reduced with the combination of morphine/oxycodone, when given in doses equianalgesic to individual opioids. The reduction of opioid-induced postoperative adverse effects may have important clinical implications, given that adverse effects may prolong length of stay and hospitalization costs. Thus, in the acute postoperative setting, a reduction of adverse effects may be expected. In chronic pain, information is still in the infancy, but opioid combination therapy may have greater advantages in improving the opioid response. The possibility to clinically translate opioid combinations into practice, as demonstrated in some animal models, depends on a broad number of factors implicated in the pain process. More research is needed to better elucidate these issues in the near future.

In vivo profiling of seven common opioids for antinociception, constipation and respiratory depression: no two opioids have the same profile

BACKGROUND AND PURPOSE

For patients experiencing inadequate analgesia and intolerable opioid-related side effects on one strong opioid analgesic, pain relief with acceptable tolerability is often achieved by rotation to a second strong opioid. These observations suggest subtle pharmacodynamic differences between opioids in vivo. This study in rats was designed to assess differences between opioids in their in vivo profiles.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats were given single i.c.v. bolus doses of morphine, morphine-6-glucuronide (M6G), fentanyl, oxycodone, buprenorphine, DPDPE ([D-penicillamine(2,5) ]-enkephalin) or U69,593. Antinociception, constipation and respiratory depression were assessed using the warm water tail-flick test, the castor oil-induced diarrhoea test and whole body plethysmography respectively.

KEY RESULTS

These opioid agonists produced dose-dependent antinociception, constipation and respiratory depression. For antinociception, morphine, fentanyl and oxycodone were full agonists, buprenorphine and M6G were partial agonists, whereas DPDPE and U69,593 had low potency. For constipation, M6G, fentanyl and buprenorphine were full agonists, oxycodone was a partial agonist, morphine produced a bell-shaped dose-response curve, whereas DPDPE and U69,593 were inactive. For respiratory depression, morphine, M6G, fentanyl and buprenorphine were full agonists, oxycodone was a partial agonist, whereas DPDPE and U69,593 were inactive. The respiratory depressant effects of fentanyl and oxycodone were of short duration, whereas morphine, M6G and buprenorphine evoked prolonged respiratory depression.

CONCLUSION AND IMPLICATIONS

For the seven opioids we assessed, no two had the same profile for evoking antinociception, constipation and respiratory depression, suggesting that these effects are differentially regulated. Our findings may explain the clinical success of 'opioid rotation'.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Modulation of peripheral μ-opioid analgesia by σ1 receptors

We evaluated the effects of σ1-receptor inhibition on μ-opioid-induced mechanical antinociception and constipation. σ1-Knockout mice exhibited marked mechanical antinociception in response to several μ-opioid analgesics (fentanyl, oxycodone, morphine, buprenorphine, and tramadol) at systemic (subcutaneous) doses that were inactive in wild-type mice and even unmasked the antinociceptive effects of the peripheral μ-opioid agonist loperamide. Likewise, systemic (subcutaneous) or local (intraplantar) treatment of wild-type mice with the selective σ1 antagonists BD-1063 [1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine dihydrochloride] or S1RA [4-[2-[[5-methyl-1-(2-naphthalenyl)1H-pyrazol-3-yl]oxy]ethyl] morpholine hydrochloride] potentiated μ-opioid antinociception; these effects were fully reversed by the σ1 agonist PRE-084 [2-(4-morpholinethyl)1-phenylcyclohexanecarboxylate) hydrochloride], showing the selectivity of the pharmacological approach. The μ-opioid antinociception potentiated by σ1 inhibition (by σ1-receptor knockout or σ1-pharmacological antagonism) was more sensitive to the peripherally restricted opioid antagonist naloxone methiodide than opioid antinociception under normal conditions, indicating a key role for peripheral opioid receptors in the enhanced antinociception. Direct interaction between the opioid drugs and σ1 receptor cannot account for our results, since the former lacked affinity for σ1 receptors (labeled with [(3)H](+)-pentazocine). A peripheral role for σ1 receptors was also supported by their higher density (Western blot results) in peripheral nervous tissue (dorsal root ganglia) than in several central areas involved in opioid antinociception (dorsal spinal cord, basolateral amygdala, periaqueductal gray, and rostroventral medulla). In contrast to its effects on nociception, σ1-receptor inhibition did not alter fentanyl- or loperamide-induced constipation, a peripherally mediated nonanalgesic opioid effect. Therefore, σ1-receptor inhibition may be used as a systemic or local adjuvant to enhance peripheral μ-opioid analgesia without affecting opioid-induced constipation.

A phase 3, randomized, double-blind comparison of analgesic efficacy and tolerability of Q8003 vs oxycodone or morphine for moderate-to-severe postoperative pain following bunionectomy surgery

Objective

Compare the efficacy and tolerability of the dual-opioid, Q8003® (morphine/oxycodone combination) 12 mg/8 mg to morphine 12 mg or oxycodone 8 mg in subjects following bunionectomy surgery.

Design

This was a randomized, double-blind study.

Setting

Hospitalized patients.

Patients

Healthy men or women aged ≥18 years with moderate or severe pain (score ≥2 on a 4-point Likert scale) and ≥4 on the 11-point numerical pain rating scale following surgery.

Interventions

Study medication was initiated after surgery and was given for 48 hours.

Outcomes

The primary efficacy variable was mean sum of the pain intensity difference (SPID) scores from the postsurgical baseline.

Results

Five hundred twenty-two subjects were randomized; 31 (5.9%) discontinued, including 19 (3.6%) for adverse events. The mean total morphine equivalent dose (MED) was 182.7 mg from Q8003 12 mg/8 mg, 92.4 mg for morphine 12 mg, and 92.1 mg for oxycodone 8 mg. SPID from baseline over 24 hours and SPID from baseline over 48 hours were significantly (P < 0.02) higher for Q8003 12 mg/8 mg vs morphine 12 mg or oxycodone 8 mg. Significantly (P < 0.015) fewer subjects in the Q8003 group required ibuprofen rescue medication, used lower doses of rescue medication, and had a longer median time to first use of rescue medication. Oxygen desaturation <90% occurred in 5.3% with Q8003, 2.8% with morphine 12 mg, and 2.3% with oxycodone 8 mg, and the cumulative median dose at first desaturation was twofold greater with Q8003.

Conclusion

Q8003 provided superior efficacy to its individual components at twice the MED with only a modest increase in the incidence of adverse events.

Acute Pain Management in Opioid-Tolerant Patients: A Growing Challenge

In Australia and New Zealand, in parallel with other developed countries, the number of patients prescribed opioids on a long-term basis has grown rapidly over the last decade. The burden of chronic pain is more widely recognised and there has been an increase in the use of opioids for both cancer and non-cancer indications. While the prevalence of illicit opioid use has remained relatively stable, the diversion and abuse of prescription opioids has escalated, as has the number of individuals receiving methadone or buprenorphine pharmacotherapy for opioid addiction. As a result, the proportion of opioid-tolerant patients requiring acute pain management has increased, often presenting clinicians with greater challenges than those faced when treating the opioid-naïve.

Treatment aims include effective relief of acute pain, prevention of drug withdrawal, assistance with any related social, psychiatric and behavioural issues, and ensuring continuity of long-term care. Pharmacological approaches incorporate the continuation of usual medications (or equivalent), short-term use of sometimes much higher than average doses of additional opioid, and prescription of non-opioid and adjuvant drugs, aiming to improve pain relief and attenuate opioid tolerance and/or opioid-induced hyperalgesia. Discharge planning should commence at an early stage and may involve the use of a ‘Reverse Pain Ladder’ aiming to limit duration of additional opioid use. Legislative requirements may restrict which drugs can be prescribed at the time of hospital discharge. At all stages, there should be appropriate and regular consultation and liaison with the patient, other treating teams and specialist services.

A systematic review of combination step III opioid therapy in cancer pain: an EPCRC opioid guideline project

BACKGROUND

The use of combinations of opioids is a common clinical practice; however, this is not advocated by the World Health Organization (WHO) analgesic ladder. As opioid combination therapy becomes used increasingly, a review of the evidence on this practice was conducted.

AIMS

To carry out a systematic review of the use of strong opioids in combination in cancer pain.

METHODS

The following databases were searched electronically: Embase (1980-2010 week 2), Medline (1950-2010 week 1) and the Cochrane Database of Systematic Reviews (fourth quarter 2009). Only strong opioids as defined by the WHO ladder and full opioid agonists were examined. Only studies conducted in human, adult patients with chronic cancer pain were eligible. Studies must have contained data on efficacy and/or side effects in the key point. Appraisal was conducted using predetermined criteria set by the EAPC guideline development group. All potential papers were reviewed independently by both authors.

RESULTS

In total 596 articles were retrieved resulting in only two eligible studies, which were rated as grade C and grade D evidence. These examined morphine in combination with oxycodone or fentanyl/methadone.

CONCLUSION

Only a weak recommendation can be used to support combination opioid therapy. This recommendation is also based on the caveat that the desirable effects of combination opioid therapy is outweighed by any disadvantages that this would confer. Prospective randomized trials are needed to clarify the benefits and safety of combination opioid therapy.

Morphine versus oxycodone in pancreatic cancer pain: a randomized controlled study

OBJECTIVE

According to experimental findings, oxycodone (OX) could have some advantages over morphine (MO) in clinical models of visceral pain. It was hypothesized that OX could have some advantages over MO in terms of efficacy and dose escalation in pancreatic cancer pain.

METHODS

Sixty patients with pancreatic cancer with a pain intensity rating of 4/10 who required opioids were included in the study. Patients were randomized to receive 30 mg/d of sustained release oral MO or sustained release oral OX (20 mg/d). Opioid doses were increased according to the clinical needs. Daily doses of opioids, pain and symptom intensity were recorded at admission (T0) and at weekly intervals for the subsequent 4 weeks (T1, T2, T3, and T4), with an extension at 8 weeks (T8). Opioid escalation index (OEI) as percentage (OEI %) and in mg (OEI mg) was calculated.

RESULTS

Nineteen and 20 patients in groups OX and MO, respectively, were followed for the entire period of study (T4). No differences between groups were found in age (P=0.400), Karnofsky (P=0.667), or escalation indexes at T4 and T8 (OEImg, P=0.945 and OEI %, P=0.295). No statistical differences in pain and symptoms intensity between the groups were observed.

CONCLUSION

OX and MO provided similar analgesia and adverse effects with similar escalating doses in patients with pancreatic cancer pain, resembling observations reported in the general cancer pain population. The experimental hypothesis that OX would be superior to MO in the clinical model of pancreatic cancer pain was not confirmed.

Oxycodone combinations for pain relief

No single analgesic drug provides the perfect therapeutic/adverse effect profile for every pain condition. In addition to convenience and possibly improved compliance, a combination of analgesic drugs offers the potential, requiring verification, of providing greater pain relief and/or reduced adverse effects than the constituent drugs when used individually. We review here analgesic combinations containing oxycodone. We found surprisingly little preclinical information about the analgesic or adverse effect profiles of the combinations (with acetaminophen, paracetamol, nonsteroidal anti-inflammatory drugs, morphine, gabapentin or pregabalin). Clinical experience and studies suggest that the combinations are safe and effective and may offer certain advantages. As with all combinations, the profile of adverse effects must also be determined in order to provide the clinician with the overall benefit/risk assessment.