br Conclusion In this research

Conclusion In this research we have successfully formulated a PEGylated liposomal formulation encapsulating IRL-1620. The liposomal nanocarriers loaded with IRL-1620 were characterized for their particle size, PDI, zeta potential, and loading efficiency. We established that the treatment with IRL-1620 encapsulated PEGylated liposomes produced a cytoprotective effect better than free IRL-1620 at 1nM concentration, preventing the apoptosis in differentiated PC-12 Losmapimod that have been deprived of NGF. The mechanism of this cytoprotective efficacy was found to be due to increased expression of anti-apoptotic BCL-2 protein and decreased expression of pro-apoptotic BAX protein. IRL-1620 was fond to be neuroprotective leading to neurite outgrowth and found to induce neurogenesis in PC-12 cells without altering the level of NGF in cell culture.
Acknowledgments The authors would like to acknowledge the funding provided by the Alzheimer’s Drug Discovery Foundation (www.alzdiscovery.com) and Midwestern University. Authors would also like to acknowledge the efforts of Dr Elizabeth Langan (CCP, Midwestern University) for carefully reviewing the manuscript.
Introduction Opioids such as morphine and oxycodone are among the most potent analgesics used commonly for the management of moderate to severe pain. Due to their high analgesic efficacy, they are considered to be the drug of choice for numerous clinical situations such as managing acute pain following surgery or physical injury and chronic pain due to cancer or arthritis. However, a major limitation of opioid use is the development of rapid tolerance to its analgesic effect, resulting in inadequate pain relief if a higher dose of the drug is not used. There are multiple hypotheses to explain opioid tolerance. One explanation is opioid receptor down-regulation which reduces the number of receptors available for opioid actions [1]. Another explanation is opioid receptor desensitization, where sustained exposure to opioids produces decoupling of the opioid receptors, which leads to signaling desensitization [1, 2]. In general, tolerance mechanisms are extremely complex and not very well understood. Many drugs that produce tolerance and dependence have been shown to modulate neurogenesis, including methamphetamine [3], cocaine [4] and opioids [5, 6]. Accumulating evidence suggests that opioid drugs have a negative impact on neurogenesis [5, 6, 7, 8, 9]. It was found that morphine decreases the expression of nestin positive cells [7]. Nestin is a neural stem cell marker, which suggests that morphine inhibits self-renewal of neural stem cells [7]. These effects appear to be mediated by opioid receptors since they were reversed with the addition of naloxone, a nonselective opioid receptor antagonist [7]. Neurogenesis is known to persist throughout adult life in the brains of mammals [10, 11, 12]. Chronic administration of morphine markedly decreases neurogenesis in the hippocampus of adult rats [5]. It has been observed that a decrease in opioid agonist analgesic potency is a reflection of decreased neurogenic differentiation 1 (NeuroD1) activity [13]. Further, chronic administration of morphine produced a decrease in NeuroD1 activity and an increase in (effective dose) ED50, while chronic administration of fentanyl did not decrease NeuroD1 activity or increase ED50. It is known that the ability of fentanyl to induce tolerance is lower than that of morphine [14] further implicating involvement of neurogenesis in the development of opioid tolerance. Endothelin (ET) has been shown to increase the release of both neurotrophic and angiogenic factors, such as VEGF and NGF, via stimulation of ETB receptors on astrocytes [15, 16]. Since it has been demonstrated that ETB receptors are involved in angiogenesis and neurogenesis in an opposing manner to opioid receptors, it is therefore possible that ETB receptors may be playing a role in opioid tolerance. It has been shown that stimulation of central ETB receptors plays an important role in providing neuroprotection and leads to neurovascular remodeling [12, 17, 18]. Subsequent studies have demonstrated that ETB receptor induced angiogenesis and neurogenesis occurs, at least in part, via altering expression of VEGF, NGF and PI3K [19, 20]. In previous studies, we have shown that ETA receptor antagonists potentiate morphine analgesia in mice and rats [21, 22] and reverse opioid tolerance via a G-protein mediated mechanism [22, 23, 24]. In an acute study it was found that ETB receptors are not involved in morphine analgesia [25], however, the role of ETB receptors in opioid tolerance has never been investigated. Since, both opioid and ETB receptors are involved in neurogenesis, which is being implicated in the development of opioid tolerance, it is convincing to investigate involvement of ETB receptors in opioid tolerance.