Charles E. Inturrisi

The nature of opioid responsiveness and its implications for neuropathic pain: new hypotheses derived from studies of opioid infusions

&NA; In recent years, the observation that the response of patients to opioid drugs may be influenced by properties inherent in the pain or pain syndrome, such as its pathophysiology, has evolved into the belief that certain types of pain e.g., neuropathic pains, may be unresponsive to these drugs. This concept has important implications for both clinical practice and basic understanding of opioid mechanisms. We critically evaluate opioid responsiveness, particularly as it relates to neuropathic pain, and propose a clinically relevant definition and a paradigm for its investigation. The paradigm is illustrated by analgesic responses to opioid infusion in 28 patients with neuropathic pains and by a detailed presentation of the pharmacokinetic and pharmacodynamic relationships in one of these patients, whose central pain responded promptly to an infusion of hydromorphone. From this analysis, we hypothesize that (1) opioid responsiveness in man can be defined by the degree of analgesia achieved during dose escalation to either intolerable side effects or the occurrence of ‘complete’ or ‘adequate’ analgesia; (2) opioid responsiveness is a continuum, rather than a quantal phenomenon; (3) opioid responsiveness is determined by a diverse group of patient characteristics and pain‐related factors, as well as drug‐selective effects; and (4) a neuropathic mechanism may reduce opioid responsiveness, but does not result in an inherent resistance to these drugs. Given the complexity of factors contributing to opioid responsiveness and the observation that outcome cannot be reliably predicted, opioids should not be withheld on the assumption that pain mechanism, or any other factor, precludes a favorable response. Both the clinical use of opioids and paradigms to investigate opioid responsiveness should include dose escalation to maximally tolerated levels and repeated monitoring of analgesia and other effects.

Clinical pharmacology of opioids for pain.

The pharmacological effects of the opioid analgesics are derived from their complex interactions with three opioid receptor types (μ, δ, and κ; morphine is an agonist at the μ opioid receptor). These receptors are found in the periphery, at presynaptic and postsynaptic sites in the spinal cord dorsal horn, and in the brain stem, thalamus, and cortex, in what constitutes the ascending pain transmission system, as well as structures that comprise a descending inhibitory system that modulates pain at the level of the spinal cord. The cellular effects of opioids include a decrease in presynaptic transmitter release, hyperpolarization of postsynaptic elements, and disinhibition. The endogenous opioid peptides are part of an endogenous pain modulatory system. A number of opioids are available for clinical use, including morphine, hydromorphone, levorphanol, oxymorphone, methadone, meperidine, oxycodone, and fentanyl, and their advantages and disadvantages for the management of pain are discussed. An understanding of the pharmacokinetic properties, as well as issues related to opioid rotation, tolerance, dependence, and addiction are essential aspects of the clinical pharmacology of opioids for pain.

Morphine-6-glucuronide, a potent mu agonist

The 3- and the 6-glucuronides of morphine have been examined in binding studies and in vivo. The 3-glucuronide had poor affinity in all binding studies whereas the 6-glucuronide potently labeled mu, but not delta or kappa receptors with affinities similar to morphine. Microinjections of the 3-glucuronide directly into the periaqueductal gray were without effect. The 6-glucuronide, on the other hand, was up to 20-fold more potent than morphine following microinjections in the same region. High doses of the 6-glucuronide produced profound seizure activity. All 6-glucuronide actions were sensitive to the opiate antagonist naloxone.

Accumulation of Normeperidine, an Active Metabolite of Meperidine, in Patients with Renal Failure or Cancer

Concentrations of meperidine and its active metabolite, normeperidine, were measured in plasma of patients receiving the drug for analgesia. Meperidine levels in cancer patients were 0.10 to 0.55 microng/ml 1 h after a dose and were 0.05 to 0.14 in patients in the oliguric period after renal transplantation. Normeperidine levels were 0.05 to 0.28 microng/ml in the cancer patients and 0.13 to 0.36 in the renal failure patients. The ratio of normeperidine to meperidine levels was always higher in the renal failure patients than in the cancer patients. Additionally, two patients receiving multiple doses of meperidine had high normeperidine levels and very high normeperidine/meperidine ratios when they showed signs of central nervous system excitation. These data indicate that normeperidine can contribute to the excitatory effects seen after multiple doses of meperidine and suggest that patients with renal failure are particularly susceptible to this problem.

The NMDA Receptor antagonists, LY274614 and MK-801, and the nitric oxide synthase inhibitor, NG-nitro-l-arginine, attenuate analgesic tolerance to the mu-opioid morphine but not to kappa opioids

&NA; Once daily s.c. administration of 5 mg/kg morphine, a mu‐opioid agonist, or U50488H (U50), a kappa1‐opioid agonist, for 5 days in male CD‐1 mice results in a 2–3‐fold shift to the right of the respective analgesic (tail flick) dose‐response curves, indicating the development of tolerance. Concurrent s.c. administration of the competitive NMDA receptor antagonist, LY274614 (LY), at 24 mg/kg/24 h infusion (osmotic pump) or 6 mg/kg i.p. once daily attenuates the development of morphine tolerance, when the response to saline plus morphine is compared on day 5 with LY plus morphine. Using this paradigm, once daily administration of either the non‐competitive NMDA antagonist, MK‐801, at 0.3 mg/kg i.p. or the nitric oxide synthase inhibitor, NG‐nitro‐L‐arginine (NorArg), at 1 mg/kg i.p. twice daily attenuated the development of morphine tolerance. None of these drugs modify the tail‐flick response or alter the ED50 for morphine. In contrast, co‐administration of LY, MK‐801 or NorArg, as above, failed to attenuate the development of tolerance to U50 or to the kappa3‐opioid agonist, naloxone benzoylhydrazone (NalBzoH). These results suggest that mu‐opioid tolerance but not kappa1‐ or kappa3‐opioid tolerance involves the mediation of NMDA receptors and the nitric oxide system.

The d- and l-isomers of methadone bind to the non-competitive site on the N-methyl-D-aspartate (NMDA) receptor in rat forebrain and spinal cord.

Racemic (dl)-methadone has antagonist activity at the N-methyl-D-aspartate (NMDA) receptor. We evaluated dl-methadone, the opioid active (l-) and the opioid inactive (d-) isomers in competition binding assays. dl-Methadone and its d- and l- isomers exhibited low micromolar affinities for the [3H]MK-801-labeled non-competitive site of the NMDA receptor in both rat forebrain and spinal cord synaptic membranes, with Ki values and displacement curves similar to those of dextromethorphan, an established NMDA receptor antagonist. They lacked affinity at the [3H]CGS-19755-labeled competitive site of the NMDA receptor. Therefore, both methadone and its the d- and l- isomers differ from morphine, hydromorphone, and naltrexone in that they have non-competitive antagonist activity at the NMDA receptor. A non-opioid NMDA receptor antagonist, such as d-methadone, may improve the efficacy of morphine by attenuating the development of tolerance.

Treatment of opioid-induced constipation with oral naloxone: A pilot study

Opioids cause constipation by binding to specific opioid receptors in the enteric and central nervous systems. First‐pass glucuronidation limits systemic bioavailability of oral naloxone. This study was designed to determine if oral naloxone could reverse opioid‐induced constipation without precipitating abstinence or recrudescence of pain in opioid‐dependent individuals. Concentrations of unmetabolized and total naloxone, including naloxone glucuronide, were measured by radioimmunoassay. A dose‐related increase in symptoms of laxation resulted in all three opioid‐dependent patients studied that paralleled the increase in active and total naloxone plasma levels. Withdrawal symptoms occurred with plasma naloxone area under the plasma concentration—time curves above 550 ng · min/ml and with dosing intervals less than 3 hours. Peak plasma levels did not predict withdrawal. Oral naloxone ameliorates opioid‐induced constipation in opioid‐dependent persons. Titration of dose to a maximum of 12 mg at least 6 hours apart may be needed to avoid adverse reactions.

The Pharmacokinetics of Heroin in Patients with Chronic Pain

We measured blood concentrations of heroin and its active metabolites, 6-acetylmorphine and morphine, serially in 11 patients with chronic pain (9 of whom had cancer) after intravenous injection, intravenous infusion, intramuscular injection, and an oral dose of heroin hydrochloride. Parenteral heroin provided measureable blood levels of heroin, 6-acetylmorphine, and morphine. Blood levels of heroin and 6-acetylmorphine reached their maximal concentrations within minutes and were cleared rapidly. The mean half-life of heroin (+/- S.D.) after intravenous injection or infusion was only 3.0 +/- 1.3 minutes, and the mean clearance of heroin from the blood at apparent steady state was 30.8 +/- 2.1 ml per kilogram of body weight per minute. Morphine levels rose more gradually, and morphine was cleared much more slowly. Oral administration of heroin resulted in measurable blood levels of morphine but not of heroin or 6-acetylmorphine. The amount of circulating morphine provided by an oral dose of heroin was only 79 per cent of that available from an equal amount of morphine. We conclude that heroin is a pro-drug that serves to determine the distribution of its active metabolites. Parenteral heroin is rapidly converted to 6-acetylmorphine, which contributes to rapid pain relief. Oral heroin is converted to morphine and appears to be an inefficient means of providing morphine to the systemic circulation.

Evidence from opiate binding studies that heroin acts through its metabolites.

The relative affinity to opiate receptors of heroin, 6-acetylmorphine and morphine was estimated by determining their ability to displace specifically bound 3H-naltrexone from rat brain opiate binding sites. In vitro hydrolysis of heroin to 6-acetylmorphine was monitored in the binding assay filtrate by use of a quantitative HPLC procedure. The rate of heroin hydrolysis was significantly slower at 0 degrees C than at 37 degrees C. The displacement of 1 nM 3H-naltrexone by unlabeled ligand at concentrations ranging from 7 to 500 nM was measured at 0 degrees C for 120 minutes, yielding IC50 values of heroin = 483 nM, 6-acetylmorphine = 73 nM and morphine = 53 nM. When the binding data for heroin were recalculated to include the displacement that could be attributed to the 6-acetylmorphine derived from heroin degradation during the incubation, all of the apparent heroin binding was accounted for by the 6-acetylmorphine. These results are consistent with previous reports of the low binding affinity of morphine congeners (e.g., codeine) that lack a free phenolic 3-hydroxyl group and support the view that heroin is a prodrug which serves to determine the distribution of its intrinsically active metabolites, 6-acetylmorphine and morphine.

Transdermal Fentanyl for Cancer Pain Repeated Dose Pharmacokinetics

BackgroundThe transdermal therapeutic system (fentanyl), or TTS(fentanyl), continuously delivers fentanyl for up to 72 h. The transdermal therapeutic system (fentanyl)-lOO delivers approximately 100 μg/h. The repeated dose pharmacokinetics of this drug using the recommended dosing interval have not been evaluated previously and were determined in the present study. MethodsBlood samples were obtained from ten opioid-tolerant cancer patients who received five applications of TTS(fentanyl) at 72-h intervals. A sample of venous blood was taken before each dose; multiple samples were taken during and after the fifth application. A gas chromatographic/mass spectrometry method was used to assay fentanyl (limit of detection 0.2 ng/ml). ResultsFor the fifth dose, the mean (SD) maximum concentration was 2.6 (1.3) ng/ml and the mean (SD) area under the serum fentanyl concentration-time curve (0–72 h) was 116.9 (59.9). Following removal of the system, the mean (SD) apparent half-life was 21.9 (8.9) h. There were no differences among the serum fentanyl concentrations measured before the second through fifth doses. Fentanyl absorption was 47% complete at 24 h, 88% complete at 48 h, and 94% complete at 72 h. The mean (SD) dose delivered during the 72-h period was 4.3 (1.1) mg. A first-dose trough concentration predicted from fifth-dose kinetics and the actual first-dose trough concentration were very similar. Adverse effects ascribed to the transdermal system were minimal. ConclusionsThese results suggest that steady-state serum concentrations are approached by the second dose of TTS(fentanyl) and that the kinetics are stable with repeated dosing. The apparent half-life following system removal is relatively long, indicating ongoing absorption from a subcutaneous depot.