Opioids and Their Receptors: Present and Emerging Concepts in Opioid Drug Discovery

Dualism, allosteric modulation, and biased signaling of opioid receptors: Future therapeutic potential

Opioids are still the drugs of choice for the treatment of acute post-surgical pain and chronic cancer pain. Overprescribing of these drugs has given rise to an "opioid crisis" in some countries. In this context, attention has been drawn to the therapeutic potential of various ligands that act as allosteric modulators of orthosteric binding sites and modulate the drug's activity, affinity, potency, and even efficacy.

Synthesis and Modification of Morphine and Codeine, Leading to Diverse Libraries with Improved Pain Relief Properties

Morphine and codeine, two of the most common opioids, are widely used in the clinic for different types of pain. Morphine is one of the most potent agonists for the μ-opioid receptor, leading to the strongest analgesic effect. However, due to their association with serious side effects such as respiratory depression, constriction, euphoria, and addiction, it is necessary for derivatives of morphine and codeine to be developed to overcome such drawbacks. The development of analgesics based on the opiate structure that can be safe, orally active, and non-addictive is one of the important fields in medicinal chemistry. Over the years, morphine and codeine have undergone many structural changes. The biological investigation of semi-synthetic derivatives of both morphine and codeine, especially morphine, shows that studies on these structures are still significant for the development of potent opioid antagonists and agonists. In this review, we summarize several decade-long attempts to synthesize new analogues of morphine and codeine. Our summary placed a focus on synthetic derivatives derived from ring A (positions 1, 2, and 3), ring C (position 6), and N-17 moiety.

Melatonin as preemptive analgesic for intraoperative pain

Several anti-inflammatory and analgesic drugs have been used to reduce pain and discomfort during periodontal surgeries. This study evaluates the efficacy of using melatonin and ketorolac for pain prevention during open-flap debridement surgery. This prospective randomized controlled trial was performed in patients who presented with chronic periodontitis after non-surgical periodontal therapy, requiring flap surgery. Group 1: Flap surgery following non-surgical periodontal therapy after one month with no oral administration of analgesic. Group 2: Flap surgery following non-surgical periodontal therapy after one month with oral administration of Ketorolac 400mg one hour prior to the surgery. Group 3: Flap surgery following non-surgical periodontal therapy after one month with oral administration of 2 mg Melatonin one hour prior to the surgery. VAS and FLACC score along with blood pressure, SPO2 and heart rate. Statistical analysis was done using SPSS software. The intragroup comparisons (control-test drug) demonstrated that melatonin and ketorolac showed positive preemptive effect which compared to the control with mean differences significantly different from zero. However, when melatonin and ketorolac were compared there was no significant difference in postoperative pain among patients. The adoption of a preemptive medication protocol using either melatonin or ketorolac may be considered effective for pain and discomfort prevention during and post open-flap debridement surgeries. Melatonin showed similar effect to gold standard ketorolac in terms of its preemptive analgesic effect additional to having anti-inflammatory effect. Further studies are required to standardize the protocol for using melatonin as preemptive analgesic for dental surgical procedures.

Recent Chemical and Pharmacological Developments on 14-Oxygenated-N-methylmorphinan-6-ones

Adequate pain management, particularly chronic pain, remains a major challenge associated with modern-day medicine. Current pharmacotherapy offers unsatisfactory long-term solutions due to serious side effects related to the chronic administration of analgesic drugs. Morphine and structurally related derivatives (e.g., oxycodone, oxymorphone, buprenorphine) are highly effective opioid analgesics, mediating their effects via the activation of opioid receptors, with the mu-opioid receptor subtype as the primary molecular target. However, they also cause addiction and overdose deaths, which has led to a global opioid crisis in the last decades. Therefore, research efforts are needed to overcome the limitations of present pain therapies with the aim to improve treatment efficacy and to reduce complications. This review presents recent chemical and pharmacological advances on 14-oxygenated-N-methylmorphinan-6-ones, in the search of safer pain therapeutics. We focus on drug design strategies and structure-activity relationships on specific modifications in positions 5, 6, 14 and 17 on the morphinan skeleton, with the goal of aiding the discovery of opioid analgesics with more favorable pharmacological properties, potent analgesia and fewer undesirable effects. Targeted molecular modifications on the morphinan scaffold can afford novel opioids as bi- or multifunctional ligands targeting multiple opioid receptors, as attractive alternatives to mu-opioid receptor selective analgesics.