Enhanced potency of intravenous, but not intrathecal, morphine and morphine-6-glucuronide after burn trauma

Morphine-6-glucuronide (M6G) for postoperative pain relief

Morphine-6-glucuronide (M6G) is morphine's active metabolite acting at the mu-opioid receptor. Recent experimental human studies and 5 of 6 randomized clinical trials indicate that M6G causes adequate and long lasting pain relief comparable to morphine. There are various observations that M6G is associated with a reduction in the severity of side effects normally associated with opioid use, such as reduced postoperative nausea and vomiting (PONV) and reduced respiratory depression. The present drug profile provides a review of the pharmacological properties of M6G, the clinical evidence relating to its efficacy and safety, and discusses its future role in the treatment of postoperative pain.

Role of morphine's metabolites in analgesia: concepts and controversies

The metabolites of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G), have been extensively studied for their contribution to clinical effects following administration of morphine. Those contributions to both the desired effect (ie, analgesia) and the undesired effects (eg, nausea, respiratory depression) are the subject of clinical controversy. Much attention and effort have been directed at investigating the properties of M6G because of interest in this substance as a possible substitute for morphine. It exhibits increased potency and the possibility of a better side effect profile compared with morphine, although the reported relative benefits vary widely. M3G is not analgesic, but its role in producing side effects, including the development of clinical tolerance, has been proposed. This review is focused on M6G and the factors that contribute to its clinical utility. The formation and distribution of M6G are presented, as are the analgesic effect and the onset of this effect. The impact of genetics, age, and gender on M6G and its effects is also reviewed.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Controlled partial skin thickness burns: an animal model for studies of burnwound progression

A practicable, reliable and reproducible model for infliction of partial skin thickness burn lesions in rabbits is presented. The model is dedicated to experimental studies investigating the influence of drugs on burn wounds. A round aluminium stamp with a contact area of 4 cm2, weight 85 g, was heated up to 80 degrees C and applied for 14 s without additional pressure on the depilated dorsal skin of rabbits. This procedure produced the desired partial skin thickness burn injury. The depth of the burn lesions was investigated by HE-stained paraffin sections. The border of the necrotic zone was found in the central third of the dermis in 80% of cases, and in the central two quarters in 100%. These results are achieved when the rabbit's hair at the site of infliction is in the anagen phase of the hair growth cycle. For obtaining reproducible results we recommend using rabbits of the same strain and weight, anagen hair growth phase, the described procedure of infliction, an identical stamp and the specified temperature and infliction time.

Analgesia for trauma and burns

This article has described the physiologic impact of trauma- and burn-related pain as well as the effect of a clinician's choice of analgesic method, using the specific example of regional analgesia for pain caused by chest trauma. It has been observed that trauma exerts a holistic influence upon the organism, marshalling reflexes, multi-system physiologic stress responses, and psychologic responses--some adaptive and others maladaptive. There is reason to consider that timely analgesia can intervene in this dynamic process and interdict the establishment of a debilitated state. A key finding of these studies is that a report of pain relief may not be the best outcome measure since the choice of analgesic method(s) has a significant impact on the secondary effects of pain. Although extrapolated from studies of perioperative pain, findings do suggest that there may be a critical period of time during which the secondary effects of a painful stimulus may be attenuated or reversed. How long this period of reversibility exists has not been determined, so planning for the level and goals of analgesia intervention should occur early on. Analgesia should be viewed not only as a humanitarian gesture, but also a therapeutic maneuver with the goal being the early restoration of function and the mitigation of a chronic debilitated state. There is scattered evidence that regional analgesic techniques using local anesthetics have some advantages over other analgesic modalities, particularly in the trauma patient with pulmonary compromise; however, as with other medical interventions, one should develop a strategic plan of application which includes consideration of potential complications and side effects, in addition to the potential therapeutic effects. The traumatized body, as well as the attending physician, must deal with inflammation, the neurohumoral reaction, musculoskeletal reflex responses, and numerous other reactions designed to stabilize an acutely destabilized systemic entity. Multimodal analgesia, with the balanced use of systemic and regional medications, has given the best short- and long-term results in studies of postthoracotomy pain. The use of a similar combined plan for posttraumatic analgesia seems logical; however, many questions remain as yet unanswered. In particular, what are the optimal combinations of techniques/medications to employ to maximize analgesia and minimize secondary effects of trauma? Can an aggressive multimodal approach intervene effectively in the development of chronic pain states, and if so, for how long? What are the long-term benefits to be derived from making a significant impact on the stress response? Last, but not least, can analgesic interventions be shown to be cost-effective according to current societal pressures to reduce the cost of health care? These and other questions are not easy to answer. Trauma strikes, in a variable fashion, patients of all ages, with all forms of comorbidity, and is treated by a technology that continues to evolve. Previous research related to the effects of analgesic treatments has been hampered by the limitations that arise when isolated groups embark on vast projects with limited numbers of patients available. It is time for investigators at multiple centers to embark on coordinated efforts to address long-term questions related to trauma and the therapeutic efficacy of analgesia.