Jeffrey W. Allen

Chronically Infused Intrathecal Morphine in Dogs

Background Despite the extensive use of intrathecal morphine infusion for pain, no systematic safety studies exist on its effects in high concentrations. The authors assessed the effects of morphine and clonidine given 28 days intrathecally in dogs. Methods Beagles with lumbar intrathecal catheters received solutions delivered by a vest-mounted infusion pump. Six groups (n = 3 each) received infusions (40 &mgr;l/h) of saline or 1.5, 3, 6, 9, or 12 mg/day of morphine for 28 days. Additional groups received morphine at 40 &mgr;l/h (1.5 mg/day) plus clonidine (0.25–1.0 mg/day) or clonidine alone at 100 &mgr;g/h (4.8 mg/day). Results In animals receiving 9 or 12 mg/day morphine, allodynia was observed shortly after initiation of infusion. A concentration-dependent increase in hind limb dysfunction evolved over the infusion interval. Necropsy revealed minimal reactions in saline animals. At the higher morphine concentrations (all dogs receiving 12 mg/day), there was a local inflammatory mass at the catheter tip that produced significant local tissue compression. All animals with motor dysfunction displayed masses, although all animals with masses did not show motor dysfunction. The mass, arising from the dura-arachnoid layer, consisted of multifocal accumulations of neutrophils, monocytes, macrophages, and plasma cells. Inflammatory cells and endothelial cells displayed significant IL1&bgr;, TNF&agr;, iNOS, and eNOS immunoreactivity. No evidence of bacterial or fungal involvement was detected. There were no other changes in spinal morphologic characteristics. In four other groups of dogs, clonidine alone had no effect and in combination with morphine reduced the morphine reaction. Conclusions The authors found that high intrathecal morphine concentrations lead to aseptic intrathecal inflammatory masses. The lack of effect of clonidine and the possible suppressive effects of clonidine on the local reaction suggest the utility of such coadministration.

The Use of Intrathecal Midazolam in Humans: A Case Study of Process

Early preclinical work demonstrated the potential role of spinal benzodiazepine pharmacology in regulating spinal nociceptive transmission. We review this preclinical activity and the evolving implementation of intrathecal midazolam in humans for pain management. Important elements in this development for use in humans are issues pertinent to safety and the preclinical reports that have increased our understanding of intrathecal midazolam toxicity. We seek to emphasize the time course of these studies and how they merged to provide enabling data that drove the clinical implementation. In the case of midazolam, we point to the potential issues that arose when preclinical safety data were unreasonably ignored and how consideration of preclinical safety data can serve to facilitate drug development by demonstrating reasonable safety profiles that document the minimal degree of potential risk to the patient. Issues that are of continuing relevance to the use of intrathecal midazolam, including issues of formulation and kinetics, are considered.

Opiate Pharmacology of Intrathecal Granulomas

Background:Chronic intrathecal morphine infusion produces intradural granulomas. The authors examined a variety of opioids infused intrathecal for analgesic activity and toxicity. Methods:Two sets of experiments were undertaken in dogs with chronic intrathecal catheters: (1) Six-hour intrathecal infusions were used to determine the full analgesic dose and the maximum tolerated dose. (2) To establish toxicity, the maximum tolerated dose was given for up to 28 days by continuous intrathecal infusion. Drugs examined were morphine sulfate, hydromorphone, d/l-methadone, l-methadone, d-methadone, fentanyl, [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), naloxone, or saline. Results: Analgesia and tolerability:Six-hour intrathecal infusion of agonists resulted in a time-dependent increase in thermal escape latency. At higher concentrations, dose-limiting motor dysfunction and sedation occurred, and hypersensitivity occurred. The concentrations, in mg/ml, for full analgesic dose/maximum tolerated dose were as follows: morphine, 0.9/12.0; hydromorphone, 1.0/3.0; d/l-methadone, 2.8/3; l-methadone, 1.0/> 1.0; fentanyl, 0.3/2.0; DAMGO, 0.1/> 2.0; d-methadone, > 1/> 1; naloxone, > 10/> 10. Spinal pathology: Chronic intrathecal infusion of the maximum tolerated dose revealed 100% intradural granuloma formation after morphine, hydromorphone, l-methadone, and naloxone. DAMGO induced a mass in only a single animal (one of three). d/l- and d-methadone produced intradural granulomas but were also associated with parenchymal necrosis. Saline and fentanyl animals displayed no granulomas. Conclusions:Intrathecal opiate–induced granulomas are not strictly dependent on opioid receptor activation. Therefore, opiates at equianalgesic doses present different risks for granuloma formation. Importantly, d/l- and d-methadone also resulted in parenchymal necrosis, an affect associated with the N-methyl-d-aspartate antagonist action of the d-isomer.

Time Course and Role of Morphine Dose and Concentration in Intrathecal Granuloma Formation in Dogs A Combined Magnetic Resonance Imaging and Histopathology Investigation

Background:Intrathecal morphine infusion leads to intrathecal granulomas. In dogs, the authors examined time course of granuloma formation and the role of concentration in granuloma development. Methods:Dogs were prepared with lumbar intrathecal catheters and vest-mounted pumps. To define the time course of granuloma formation, serial magnetic resonance imaging was performed in animals receiving 10 or 31 days of morphine infusion (12.5 mg/ml at 40 &mgr;l/h). At these times, morphine was removed from the infusate, and further magnetic resonance images were acquired over 14–35 additional days. To assess dose versus concentration, dogs received 28-day infusions of vehicle, 12 mg morphine/day as 12.5 mg/ml at 40 &mgr;l/h, or 1.5 mg/ml at 334 &mgr;l/h (12 mg/day) for 28 days. Additional dogs received 3 mg/day as 12.5 mg/ml at 10 &mgr;l/h. Results:Serial magnetic resonance images in dogs receiving morphine (12.5 mg/ml at 40 &mgr;l/h) revealed pericatheter-enhancing tissues as early as 3 days with a prominent signal by 10 days. Removal of morphine reduced the mass volume within 7 days. At a fixed infusion rate, the incidence of granuloma formation with the continuous intrathecal infusion of morphine ranged from 0 in vehicle-treated dogs to 100% in dogs treated with 12.5 mg/ml at 40 &mgr;l/h (12 mg/day). Infusion of 12 mg/day at 1.5 mg/ml (334 &mgr;l/h) resulted in granuloma in one of four animals. The authors found that infusion of morphine in different concentrations at a fixed rate resulted in a dose-dependent increase in concentration, with the granuloma-producing, dose-yielding lumbar cerebrospinal fluid morphine concentrations around 40 &mgr;g/ml. Conclusions:Serial magnetic resonance imaging showed a rapid formation and regression of the masses initiated by intrathecal morphine infusion. These masses are dependent on local concentration.

An assessment of the antinociceptive efficacy of intrathecal and epidural contulakin-G in rats and dogs.

Contulakin-G is a novel conopeptide with an incompletely defined mechanism of action. To assess nociceptive activity we delivered Contulakin-G as a bolus intrathecally (0.03, 0.1, 0.3, 3 nmol) or epidurally (10, 30, 89 nmol) in rats. Intrathecal Contulakin G significantly decreased Phase II and, to a lesser degree, Phase I paw flinching produced by intradermal formalin. Intrathecal and epidural doses of ED50s were 0.07 nmol and 45 nmol, respectively, giving an epidural/intrathecal ED50 ratio = 647). In dogs, intrathecal Contulakin-G (50-500 nmoL) produced a dose-dependent increase in the thermally evoked skin twitch latency by 30 min after administration, as did morphine (150 and 450 nmol). Epidural morphine (750 and 7500 nmol), but not epidural 1000 nmol Contulakin-G, also significantly decreased skin twitch in dogs. No changes in motor function were seen in any rats or dogs receiving these doses of Contulakin-G. In dogs, no physiologically significant dose-dependent changes in motor function, heart rate, arterial blood pressure, or body temperature were found. Contulakin-G is a potent antinociceptive drug when delivered intrathecally with no observable negative side effects in rats or dogs and may provide an alternative to opioid spinal analgesics.

Preclinical Insights into the Implementation of Intrathecal Midazolam: A Cautionary Tale

Editor’s note: Please refer to the editorial by Cousins and Miller (pp. 1507–8) and the articles by Tucker et al. (pp. 1512–20 and 1521–7), Johansen et al. (pp. 1528–35), and Yaksh and Allen (pp. 1536–45) in this issue. I n the present issue of the journal, there are four papers focusing on intrathecal (IT) midazolam: a preclinical assessment of safety in sheep and pigs (1), a population evaluation of postoperative side effects in humans receiving perioperative IT midazolam in combination with local anesthetics and fentanyl (2), a human study examining IT midazolam and fentanyl in labor (3), and a review related to the issues pertinent to the development of the use of spinal midazolam (4). These papers reflect a continuing process that began almost 20 years ago. As reviewed (4), insights in the late 1970s regarding the spinal actions of -aminobutyric acid and the pharmacology of the benzodiazepine receptors led to the initial preclinical work suggesting the activity of spinal midazolam in regulating spasticity and pain processing. These insights, given the zeitgeist of the late 1970s, formed by the appreciation of the clinical benefits arising from the spinal delivery of drugs such as opiates and baclofen provided a foundation for considering the spinal delivery of midazolam as a therapeutic approach to manage pain and spasticity. Tempering the enthusiasm for IT therapy, spinal drugs have long been known for having the potential for producing local injury leading to functional deficits. Accordingly, even early work emphasized that the initial delivery of any drug into the human spinal canal must be preceded by some assessments of safety and the assertion of no toxicity. What justifies this emphasis on preclinical work? First, the preclinical models have been reliable in predicting the analgesic efficacy, physiological effects, and pharmacology of IT delivered drugs ranging from opiates through blockers of N-type calcium channels in humans (5,6). Second, they provide a readout of any potential changes in function that are deleterious. Third, they allow a direct examination of the drug-exposed spinal tissue. Ethical issues aside, the first two aims could be achieved by careful titration of doses in humans. Obtaining normal patient spinal tissue for histopathology poses obvious limitations. Why is the third component so important? The principal concern of a preclinical spinal safety assessment is that the drug therapy induces no deleterious changes in spinal morphology. Whereas persistent changes in behavior and function may indicate underlying events, a lack of change in behavior does not exclude underlying tissue pathology. Normal function is at best only a surrogate marker for the absence of underlying tissue effect. Thus, whereas the lack of deleterious or irreversible effects of a treatment upon autonomic and behavioral function is a required aspect of safety assessment, it is not sufficient. Evolving deficits may not be revealed by functional indices for an extended period of time, whereas histological examination demonstrates a continuing event. How then does the evolution of IT midazolam compare with that of other spinal drugs. IT morphine, frequently used in humans since 1978, initially underwent extensive behavioral investigations in species ranging from the rat through the primate. In all instances, the bolus delivery resulted in predictable, reversible behavioral effects. Although anecdotal comments on tissue pathology were made, it was only later that IT pathology was systematically examined after single dosing in primates (7) and repeated dosing in rats, cats (8), and dogs (9). In each case, the studies provided a convergent assertion that bolus delivery was without evident toxicity. In contrast, the first spinal delivery of many drugs such as clonidine, d-ala2d-leu5-enkephalin, and neostigmine were preceded not only by a host of preclinical studies on behavioral and pharmacological characteristics, but also by systematic studies assessing the histopathological effects of the drug (6). Again, in each instance, although the Accepted for publication January 28, 2004. Address correspondence and reprint requests to Tony L. Yaksh, PhD, Department of Anesthesiology, University of California-San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0818. Address e-mail to tyaksh@ucsd.edu.

The pharmacokinetics of the conopeptide Contulakin-G (CGX-1160) after intrathecal administration: An analysis of data from studies in beagles

BACKGROUND:The synthetic peptide agent Contulakin-G (CGX-1160), isolated from the toxin of the snail Conus Geographus, produces significant analgesia in animals. Its peptide structure requires intrathecal administration for effectiveness, therefore we determined the intrathecal pharmacokinetics of CGX-1160 after bolus dose and multiple day infusions to beagles. METHODS:For the bolus dose study, eight animals received a dose ranging from 16.7 to 1000 nmol under isoflurane anesthesia. Cerebral spinal fluid sampling for drug assay occurred up to 24 h. For the multiple day infusion study, three animals received infusions of 10, 40, and 160 &mgr;g/h respectively for 24 h at each rate. Cerebral spinal fluid sampling occurred during the infusion rate and the washout period after the 72 h of cumulative drug delivery. Data from the two study designs were modeled separately using NONMEM. RESULTS:The results showed a biexponential disposition profile for both experiments with a rapid rate constant that was an order of magnitude greater than the slow rate constant. The bolus results showed a nonlinear dependence of the slow rate constant on administered dose due to the large bolus range used in the study. CONCLUSION:These data, coupled with clinical pharmacology results, provide a basis for determining appropriate dosing strategies to achieve therapeutic intrathecal concentrations of Contulakin-G.

Tissue Injury Models of Persistent Nociception in Rats

The purpose of this chapter is to provide guidance to the novice investigator as to two models of ongoing nociception in rats. The models described herein are the formalin test, in which an irritant is injected subcutaneously into a dorsal paw and the numbers of flinches produced over 60 min are counted, and a mild burn model that produces a transitory primary and secondary thermal and mechanical hyperalgesia lasting approx 90 min. These models allow assessment of spinal sensitization, which may be an important factor when considering plasticity associated with human pain states. Detailed protocols using both manual and automated counting for the formalin test are included, as are methods concerning data analysis.