Sedative tolerance accompanies tolerance to the analgesic effects of fentanyl in infant rats

Piperidine propionamide as a scaffold for potent sigma-1 receptor antagonists and mu opioid receptor agonists for treating neuropathic pain

We designed and synthesized a novel series of piperidine propionamide derivatives as potent sigma-1 (σ1) receptor antagonists and mu (μ) opioid receptor agonists, and measured their affinity for σ1 and μ receptors in vitro through binding assays. The basic scaffold of the new compounds contained a 4-substituted piperidine ring and N-aryl propionamide. Compound 44, N-(2-(4-(4-fluorobenzyl) piperidin-1-yl) ethyl)-N-(4-methoxy-phenyl) propionamide, showed the highest affinity for σ1 receptor (Ki σ1 = 1.86 nM) and μ receptor (Ki μ = 2.1 nM). It exhibited potent analgesic activity in the formalin test (ED50 = 15.1 ± 1.67 mg/kg) and had equivalent analgesic effects to S1RA (σ1 antagonist) in a CCI model. Therefore, Compound 44, which has mixed σ1/μ receptor profiles, may be a potential candidate for treating neuropathic pain.

Conversion from prolonged intravenous fentanyl infusion to enteral methadone in critically ill children

AIM

To describe our institutional experience with conversion from intravenous (IV) fentanyl infusion directly to enteral methadone and occurrence of withdrawal in critically ill mechanically ventilated children exposed to prolonged sedation and analgesia.

METHODS

With Institutional Review Board approval, we retrospectively studied consecutively admitted invasively mechanically ventilated children (0-18 years) sedated with IV fentanyl infusion > 5 d and subsequently converted directly to enteral methadone. Data were obtained on subject demographics, illness severity, daily IV fentanyl and enteral methadone dosing, time to complete conversion, withdrawal scores (WAT-1), pain scores, and need for rescue opioids. Patients were classified as rapid conversion group (RCG) if completely converted ≤ 48 h and slow conversion group (SCG) if completely converted in > 48 h. Primary outcome was difference in WAT-1 scores at 7 d. Secondary outcomes included differences in overall pain scores, and differences in daily rescue opioids.

RESULTS

Compared to SCG (n = 21), RCG (n = 21) had lower median WAT-1 scores at 7 d (2.5 vs 5, P = 0.027). Additionally, RCG had lower overall median pain scores (3 vs 6, P = 0.007), and required less median daily rescue opioids (3 vs 12, P = 0.003) than SCG. The starting daily median methadone dose was 2.3 times the daily median fentanyl dose in the RCG, compared to 1.1 times in the SCG (P = 0.049).

CONCLUSION

We observed wide variation in conversion from IV fentanyl infusion directly to enteral methadone and variability in withdrawal in critically ill mechanically ventilated children exposed to prolonged sedation. In those children who converted successfully from IV fentanyl infusion to enteral methadone within a period of 48 h, a methadone:fentanyl dose conversion ratio of approximately 2.5:1 was associated with less withdrawal and reduced need for rescue opioids.

Ventilation and the Response to Hypercapnia after Morphine in Opioid-naive and Opioid-tolerant Rats

BACKGROUND

Opioid-related deaths are a leading cause of accidental death, with most occurring in patients receiving chronic pain therapy. Respiratory arrest is the usual cause of death, but mechanisms increasing that risk with increased length of treatment remain unclear. Repeated administration produces tolerance to opioid analgesia, prompting increased dosing, but depression of ventilation may not gain tolerance to the same degree. This study addresses differences in the degree to which chronic morphine (1) produces tolerance to ventilatory depression versus analgesia and (2) alters the magnitude and time course of ventilatory depression.

METHODS

Juvenile rats received subcutaneous morphine for 3 days (n = 116) or vehicle control (n = 119) and were then tested on day 4 following one of a range of morphine doses for (a) analgesia by paw withdraw from heat or (b) respiratory parameters by plethysmography-respirometry.

RESULTS

Rats receiving chronic morphine showed significant tolerance to morphine sedation and analgesia (five times increased ED50). When sedation was achieved for all animals in a dose group (lowest effective doses: opioid-tolerant, 15 mg/kg; opioid-naive, 3 mg/kg), the opioid-tolerant showed similar magnitudes of depressed ventilation (-41.4 ± 7.0%, mean ± SD) and hypercapnic response (-80.9 ± 15.7%) as found for morphine-naive (-35.5 ± 16.9% and -67.7 ± 15.1%, respectively). Ventilation recovered due to tidal volume without recovery of respiratory rate or hypercapnic sensitivity and more slowly in morphine-tolerant.

CONCLUSIONS

In rats, gaining tolerance to morphine analgesia does not reduce ventilatory depression effects when sedated and may inhibit recovery of ventilation.

A survey for pain and sedation medications in pediatric patients during extracorporeal membrane oxygenation

Routine administration of large amounts of pain and sedative medication is common to critically ill pediatric patients undergoing extracorporeal membrane oxygenation (ECMO) for cardiopulmonary failure. It has been our experience that pediatric patients are the most difficult age group in which to achieve an ideal pain and sedative control due to the narrow margin of safety. The purpose of this study was to determine the general practice guideline used for pain and anxiolytic pharmacotherapy for pediatric patients at ECMO centers. We sent a survey questionnaire to all ECMO centers in the USA that treat pediatric respiratory failure patients. Of the 46 responding centers (including telephone follow-ups), 37 (80%) centers had an active pediatric ECMO programs for patients with severe respiratory failure. Fentanyl was the most commonly used pain medication and continuous infusion, administered directly to the patient, was preferred. Subjective effectiveness of various pharmacological agents was variable without clear consensus; however, midazolam was considered to be the most effective agent used.

Changing mechanisms of opiate tolerance and withdrawal during early development: animal models of the human experience

Human infants may be exposed to opiates through placental transfer from an opiate-using mother or through the direct administration of such drugs to relieve pain (e.g., due to illness or neonatal surgery). Infants of many species show physical dependence and tolerance to opiates. The magnitude of tolerance and the nature of withdrawal differ from those of the adult. Moreover, the mechanisms that contribute to the chronic effects of opiates are not well understood in the infant but include biological processes that are both common to and distinct from those of the adult. We review the animal research literature on the effects of chronic and acute opiate exposure in infants and identify mechanisms of withdrawal and tolerance that are similar to and different from those understood in adults. These mechanisms include opioid pharmacology, underlying neural substrates, and the involvement of other neurotransmitter systems. It appears that brain circuitry and opioid receptor types are similar but that NMDA receptor function is immature in the infant. Intracellular signaling cascades may differ but data are complicated by differences between the effects of chronic versus acute morphine treatment. Given the limited treatment options for the dependent infant patient, further study of the biological functions that are altered by chronic opiate treatment is necessary to guide evidenced-based treatment modalities.

Tolerance and withdrawal from prolonged opioid use in critically ill children

OBJECTIVE

After prolonged opioid exposure, children develop opioid-induced hyperalgesia, tolerance, and withdrawal. Strategies for prevention and management should be based on the mechanisms of opioid tolerance and withdrawal.

PATIENTS AND METHODS

Relevant manuscripts published in the English language were searched in Medline by using search terms "opioid," "opiate," "sedation," "analgesia," "child," "infant-newborn," "tolerance," "dependency," "withdrawal," "analgesic," "receptor," and "individual opioid drugs." Clinical and preclinical studies were reviewed for data synthesis.

RESULTS

Mechanisms of opioid-induced hyperalgesia and tolerance suggest important drug- and patient-related risk factors that lead to tolerance and withdrawal. Opioid tolerance occurs earlier in the younger age groups, develops commonly during critical illness, and results more frequently from prolonged intravenous infusions of short-acting opioids. Treatment options include slowly tapering opioid doses, switching to longer-acting opioids, or specifically treating the symptoms of opioid withdrawal. Novel therapies may also include blocking the mechanisms of opioid tolerance, which would enhance the safety and effectiveness of opioid analgesia.

CONCLUSIONS

Opioid tolerance and withdrawal occur frequently in critically ill children. Novel insights into opioid receptor physiology and cellular biochemical changes will inform scientific approaches for the use of opioid analgesia and the prevention of opioid tolerance and withdrawal.

Ontogeny of respiratory sensitivity and tolerance to the mu-opioid agonist fentanyl in rat

Whereas developmental changes in analgesic sensitivity and tolerance to the mu-opioid agonist fentanyl have been reported, knowledge of respiratory responses to that drug is lacking. Using 7- and 14-day-old (P7, P14) and adult conscious rats, we first established, using whole body plethysmography, the fentanyl dose that decreased minute ventilation by 50% (ED50) at each age. ED50 increased with postnatal age (40, 60 and 120 microg/kg sc, respectively), indicating a high sensitivity to fentanyl in the youngest rats that decreased with maturation. In separate rat groups of the 3 ages, we injected each ED50 dose, once a day, for several consecutive days, until tolerance was established. Tolerance was defined as a reduction in respiratory depression from 50% to 75% of baseline. All age groups reached tolerance in minute ventilation, respiratory frequency, tidal volume and instantaneous flow (equivalent to respiratory drive). The P14 rat pups attained tolerance more rapidly (at 2.6 days) than did either the younger (5.1 days) or the adult rats (4.4 days). These results indicate that respiratory sensitivity and tolerance to fentanyl in rat vary in a distinct manner during maturation.

Mu-opioid self-administration vs passive administration in heroin abusers produces differential EEG activation

Psychoactive drug self-administration (SA) produces different neurobiological effects than passive administration (PA) in non-human animals; however, such consequences have never been examined in human drug abusers. The present study compared electroencephalographic (EEG) activation produced by intravenous PA and SA of the mu-opioid fentanyl in eight heroin-dependent, methadone-stabilized male participants. In phase 1, participants received cumulative PA of fentanyl (up to 1.5 mg/70 kg; session 1), then bolus PA of placebo and fentanyl 1.5 mg/70 kg (session 2). High-dose fentanyl significantly increased the amplitude of slow-frequency (delta- and theta-band) EEG activity. In phase 2, bolus fentanyl 1.5 mg/70 kg was available for SA, requiring the participant to complete 1500 responses, in each of two sessions after saline or naloxone pretreatment. Delta EEG peak amplitude increases were greater following fentanyl SA than fentanyl PA, primarily over the central midline region, and were attenuated by naloxone pretreatment. The EEG increase and its attenuation by naloxone agree with preclinical evidence and suggest that SA-related EEG responses were mediated by opioid receptors.

Vulnerability of the developing brain. Neuronal mechanisms

Despite the improved survival of tiny preterm neonates, their neurodevelopmental outcomes remain a cause for grave concern. The authors propose two primary mechanisms leading to enhanced neuronal cell death in the immature brain: (1) NMDA-mediated excitotoxicity resulting from repetitive or prolonged pain, and (2) enhanced naturally occurring neuronal apoptosis during early development due to multiple metabolic stresses or lack of social stimulation. The pattern and magnitude of abnormalities will depend on genetic variability as well as the timing, intensity, and duration of adverse environmental experiences. Thus, cumulative brain damage during infancy will finally lead to reductions in brain volume, abnormal behavioral and neuroendocrine regulation, and poor cognitive outcomes during childhood and adolescence. The public health and economic importance of preventing or ameliorating the subtle brain damage caused by these mechanisms cannot be overestimated. This certainly justifies concerted efforts by neuroscientists and clinicians to investigate the mechanisms underlying early neuronal injury, to minimize the impact of adverse experiences and environmental factors in neonates, and to develop novel therapeutic strategies for improving the cognitive and behavioral outcomes of ex-preterm neonates.

Dihydroetorphine: a potent analgesic: pharmacology, toxicology, pharmacokinetics, and clinical effects

Dihydroetorphine (DHE) is one of the strongest analgesic opioid alkaloids known; it is 1000 to 12000 times more potent than morphine. Several in vitro and in vivo studies have shown that DHE is a selective mu-opioid receptor (OP(3)) agonist that also binds and activates all human recombinant mu-, delta-, and kappa-opioid receptors (OP(3), OP(1), and OP(2)). The onset of the analgesic effect of DHE in rodents is rapid, 5 to 15 min after parenteral administration; the duration of action is short, the analgesic effect disappears within 120 min after administration. By oral administration much higher doses of DHE are required to produce analgesic effects. These characteristics are accounted for by the pharmacokinetic properties of DHE in the rat, namely, by rapid distribution of DHE from the injection site to the brain and rapid metabolism by glucuronidation in the gut and liver followed by elimination into the bile. Continuous infusion and repeated administration of DHE lead to the development of tolerance to analgesia, physical dependence, and a rewarding effect in normal rats but not in animals with formalin-induced inflammation. Although formalin-induced inflammation is only one type of pain stimulus, these findings suggest that DHE addiction would be observed only in the case of pain-free conditions. Clinical reports in China show that sublingual doses of DHE, 20 to 180 microg, produce a potent analgesic effect with only mild side effects, including dizziness, somnolence, nausea, vomiting, constipation, and shortness of breath. To improve the short-lasting effect following sublingual administration, transdermal delivery of DHE via a patch has been investigated. The patch formulation of DHE produces continuous analgesic effect with minimal physical dependence and rewarding effect in rats suffering from chronic pain. This patch formulation, which is very suitable for DHE, may be viable for the treatment of severe pain and is likely to improve patients' quality of life.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.