Respiratory, analgesic and endocrine responses to an enkephalin analogue in normal man

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

Differential roles of opioid receptors in respiration, respiratory disease, and opiate-induced respiratory depression

In summary, these findings indicate the importance of designing future experiments that delineate between opioid and nonopioid forms of respiratory disease and dysfunction, and the need to identify means of diagnosing them in order to achieve successful recovery. Apparently there is great diversity between animal species in terms of contributions of endogenous opioids to tonic control of ventilation, and future work should strive to identify which species is most appropriate as a model of human ventilatory control and disease. Certain opioid receptor types appear to be linked to independent respiratory functions. For instance, mu receptors in the brain stem produce strong inhibitory actions on respiratory parameters, including RR, VT, VE, and CO2 sensitivity. These effects have been observed in vivo and by electrophysiologic recordings in vitro. Delta receptors may also exert some inhibitory effect on respiration, especially in the NTS. In the CNS, the ventral surfaces of the medulla and pons, especially the NTS and NA, seem to be important sites for opioid-induced inhibition of respiration, whereas the spinal cord probably is not involved in opioid-mediated ventilatory depression. Kappa receptors appear to be devoid of respiratory depressant activity, whereas sigma receptors may stimulate some ventilatory parameters. Morphine and similar pure mu agonists, such as fentanyl and oxymorphine, probably produce their analgesic and respiratory depressant effects through stimulation of mu receptors. Mixed agonists/antagonists that have mu antagonist (or partial agonist) activity plus kappa agonist and/or sigma agonist activity show a ceiling effect for respiratory depression. Future tests need to determine which opioid receptor may be responsible for the ceiling effect. In addition, the effects of mu, delta, kappa, and sigma selective agonists on hypoxic drive should also be determined, as a drug that stimulates hypoxic sensitivity in the face of hypercapnic depression may produce less overall respiratory depression due to counteractive effects. In the future, clinically optimal opiates should have more specificity of action than those available now. This may be achieved by creating drugs selective for single receptors or by creating drugs with desirable combinations of receptor selectivities. The combinations of mixed agonists/antagonists with pure mu agonists currently in use today are promising, as they provide analgesia with reduced respiratory depression. In the early days of opiate research and development, combination drug regimens were thoroughly tested to determine the "ideal ratios" that would retain analgesic properties but not the other undesirable effects such as respiratory depression (196).(ABSTRACT TRUNCATED AT 400 WORDS)

Analgesic, central, cardiovascular and endocrine effects of the enkephalin analogue Tyr-D.Arg-Gly-Phe(4NO2)-Pro-NH2 (443C81) in healthy volunteers

443C81 is a synthetic enkephalin thought to act on peripheral opiate receptors. The analgesic, central, cardiovascular and endocrine effects of two i.v. doses of 443C81 were investigated in 12 healthy male volunteers. Its effects were compared with those of placebo and the classical opiate dipipanone given orally using a double dummy design. 443C81 produced dose-related analgesia; dipipanone 10 mg had a greater effect than the high dose 443C81. In contrast to dipipanone, 443C81 did not cause significant miosis or reduce minute volume on rebreathing CO2 and there was no evidence of sedation. Dry mouth was reported frequently and associated with reduced salivation after all active treatments. Both 443C81 and dipipanone increased circulating prolactin and growth hormone and reduced cortisol levels. This novel enkephalin appears to possess analgesic activity and some other properties of opiates but is devoid of clinically relevant narcotic effects.

The acute effects of the enkephalin analogue BW 942C in humans

BW 942C hydrochloride is an enkephalin analogue that has exhibited a wide separation between antidiarrheal dosages and dosages inducing adverse effects in animals. This has likewise been the case in humans when administered orally. In this study, the safety and tolerance of single 0.5-mg doses of intravenous BW 942C compared with placebo were assessed in humans. Four healthy male volunteers received BW 942C, and two received placebo. The effects of BW 942C on serum growth hormone (GH), luteinizing hormone (LH), prolactin (PR), and follicle-stimulating hormone (FSH) were assessed in three of these volunteers. No significant changes were apparent in vital signs, in clinical chemistry, hematologic and urine studies following BW 942C administration. BW 942C did not appear to alter mood as assessed by two psychologic mood scales. Prolactin levels tended to increase in volunteers receiving BW 942C two hours postinfusion. Luteinizing hormone concentrations decreased slightly at two and six hours. No trends in FSH or GH could be identified. Pulmonary function testing did not reveal any significant changes in oximetry, spirometry, or plethysmography in any of the subjects. A marked decrease in CO2 responsiveness in two subjects may indicate that BW 942C has mild ventilatory depressant effects. Untoward effects experienced in volunteers receiving BW 942C included heaviness in the limbs, nasal stuffiness, mouth dryness, facial flushing, skin rash, and prickling sensations. These effects bear a striking similarity to those experienced after parenteral administration of other enkephalin analogues. Intravenous administration of BW 942C up to 0.5 mg appears safe from a laboratory, physiologic, and clinical perspective with unusual untoward effects that may preclude rational use of the drug by the parenteral route.

Analgesic, respiratory and endocrine responses in normal man to THIP, a GABA-agonist

In a controlled study, the effects of THIP (a synthetic gamma-aminobutyric-acid-agonist) on respiratory function (ventilatory response to CO2), first detection of stimulation (electrical stimulation of a tooth), pain threshold, magnitude of maximal tolerated pain stimulation, and plasma cortisol, prolactin and glucose were investigated in six normal men. Intramuscular injection of THIP in dosages of both 10 mg and 20 mg increased the magnitude of stimulus before first detection, and the pain threshold as well as the maximal tolerance of pain stimulation. THIP did not lead to changes in respiratory function, or in plasma cortisol, prolactin or glucose, suggesting an analgesic action independent of opiate receptors.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.