The search for novel compounds that produce analgesia in the absence of tolerance, dependence, addiction, and unpleasant or dangerous side effects is a major goal of pain research. This is an important topic to the Journal of Pharmacology and Experimental Therapeutics (JPET), as indicated by their publication of recent review articles (Coussens et al., 2019; Ashraf et al., 2024) and experiments (Giancotti et al., 2024; Huang et al., 2024; Sato et al., 2024) on analgesic drug development. Despite great advances in understanding pain mechanisms, preclinical research has largely failed to produce clinically useful analgesics (Mogil, 2009). On the one hand, this lack of success is surprising because hundreds of drugs have been reported to produce antinociception in animals (de Cássia da Silveira E Sá et al., 2017; Chen et al., 2021; Laev and Salakhutdinov, 2021; Obeng et al., 2021; Gach-Janczak et al., 2024). On the other hand, meeting the criteria for an ideal analgesic is extremely challenging. These criteria include analgesic efficacy and selectivity, lack of tolerance, dependence, and addiction with repeated administration, minimal side effects, and high oral bioavailability.

Although opioids are the most effective analgesics for most types of pain, the dangers associated with opioids limit their use, as highlighted by high levels of dependence and overdose deaths. Given that opioid efficacy varies greatly depending on the opioid (Kelly et al., 2023) and specific behavioral effect assessed (Azevedo Neto et al., 2020), manipulation of the molecular structure of standard opioids is a promising approach to produce opioids with relatively high analgesic efficacy and low side effect efficacy. The manuscript by Santos and colleagues (2024) assesses the antinociceptive and side effects of C9-substituted phenylmorphans developed at the National Institute on Drug Abuse (Hiebel et al., 2007; Chambers et al., 2022). Seven compounds with high mu-opioid receptor (MOR) specificity and varying signaling efficacies were tested.

There is much to like about the experiment by Santos and colleagues (2024). The primary focus is comparison of mu-opioid receptor agonists with different signaling efficacies. These compounds were screened in male and female mice for both antinociceptive efficacy and common side effects (i.e., constipation and respiratory depression). Alterations in MOR efficacy and signaling are potential pharmacological methods to maximize antinociception and minimize side effects. Many studies have examined this relationship (Azevedo Neto et al., 2020; Kelly et al., 2023). Although the Santos et al. (2024) manuscript is relevant to this issue, the use of a pain-depressed test instead of pain-evoked tests is what distinguishes this study from other drug development studies.

Although one would expect compounds with the highest MOR signaling efficacy to be the best analgesics, the present study shows that two relatively low-efficacy compounds are the most promising. This finding is a direct result of using pain-depressed as opposed to more commonly used pain-evoked tests. Pain-depressed tests assess the ability of a drug to restore a behavior that is depressed by pain—in this case, restoration of movement after a painful intraperitoneal injection of lactic acid. In contrast, pain-evoked tests such as the hot-plate, tail-flick, and von Frey tests measure withdrawal from a noxious stimulus. The value of a pain-depressed test is evident from the results. C9-substituted phenylmorphans with high MOR efficacy produce antinociception when assessed with pain-evoked tests (Chambers et al., 2022; Lutz et al., 2023), but such compounds failed to restore behavior in the present study because they inhibited movement. In other words, high-efficacy compounds produce antinociception but do not restore behavior because of side effects. A decrease in locomotion could be caused by a direct motor effect, sedation, or a range of disruptive psychoactive effects. The lowest-efficacy compounds did not inhibit movement but were also incapable of producing antinociception. Of the seven phenylmorphans tested, only two relatively low-efficacy compounds restored behavior in the absence of inhibitory motor effects. These two low-efficacy compounds also produced less inhibition of gastrointestinal transit and respiratory depression than fentanyl, a high-efficacy opioid. These effects were relatively consistent in male and female mice, although analysis of sex differences was not a primary objective of this study.

The old joke about the person searching for their keys under the streetlamp because that is where the light is best is consistent with how preclinical drug discovery has been approached. Pain-evoked tests like the hot-plate and von Frey tests are commonly used because that is where the light has been shining for over 100 years. The manuscript by Santos et al. (2024) shows the value of looking for your keys in the dark corners of the street. That is, the pain-depressed tests hiding in the shadows provide a unique and much more clinically relevant method to screen potential analgesics.

The pain-depressed test used by Santos and colleagues (2024) measures vertical and horizontal movement as mice shuttle between two chambers. Although this group recently introduced and validated this test (Negus et al., 2023), there is nothing special about this particular task. Many rodent behaviors (e.g., wheel running, eating and drinking, grooming, nest building, and intracranial self-stimulation) are disrupted by pain and have been used to assess nociception (Tappe-Theodor et al., 2019). The advantage of the shuttle task used here over more commonly used locomotor tasks such as wheel running is not clear. In fact, there are several advantages of assessing locomotion with wheel running (Kandasamy and Morgan, 2021). Most notable are that data collection is continuous and occurs in the rat’s home cage. The practical application of this is that the time course and magnitude of antinociception are evident by analyzing recovery of wheel running after drug administration (Kandasamy et al., 2017, 2018; Morgan et al., 2023). In contrast, Santos and colleagues (2024) injected their drugs at four different times, requiring four groups of mice to determine the approximate time course for the analgesic effects.

To be clear, pain-evoked tests are useful if the goal is to understand nociception. Such tests have been used to assess nociception since Charles Sherrington examined withdrawal reflexes in cats well over 100 years ago (Sherrington, 1906) and remain the dominant method today (González-Cano et al., 2020). However, pain-evoked tests are flawed when used to screen potential analgesics (Le Bars et al., 2001), especially when compared with pain-depressed tests (Tappe-Theodor et al., 2019). The defining difference between these two types of tests when used to screen potential analgesics is shown in Fig. 1. A wide range of drugs (e.g., sedatives, paralytics) will inhibit pain-evoked responses. Such a depression of behavior is contrary to the restoration of behavior produced by an analgesic when using a pain-depressed test.

Fig. 1.

Potential effects of different classes of drugs using pain-evoked and pain-depressed tests. (A) Pain-evoked tests cannot distinguish between analgesics, sedatives, and paralytics because they all inhibit behavior. (B) Pain-depressed tests model the human condition by causing pain (e.g., intraperitoneal administration of lactic acid) to reduce behavior (e.g., locomotion). An effective analgesic will restore behavior, whereas drugs with other effects will either have no effect on behavior or cause a reduction.

Pain-depressed tests will not solve the animal to human translation problem. The two 9C-substituted phenylmorphans identified by Santos and colleagues (2024) are clearly the most promising of the seven compounds tested. Nonetheless, these compounds will require vigorous testing in humans and are likely to be derailed by any of a number of problems (e.g., tolerance, dependence, addiction, unforeseen side effects, bioavailability). The value of pain-depressed tests is that they will save time and money by allowing scientists to focus on the most promising compounds. As shown by Santos and colleagues (2024), these are not necessarily the compounds with the highest MOR signaling efficacy. The literature is littered with drugs with excellent antinociceptive efficacy on pain-evoked tests, none of which have been translated to clinical use. And yet, the National Institutes of Health, Big Pharma, and small drug-screening companies continue to rely on pain-evoked tests. A much more efficient and clinically relevant process would follow the lead of Santos and colleagues (2024) and include pain-depressed tests to screen drugs.

Authorship Contributions

Wrote or contributed to the writing of the manuscript: Morgan.

FootnotesReceived April 11, 2024.Accepted April 26, 2024.

This work received no external funding.

The author has no actual or perceived conflicts of interest with the contents of this article.

dx.doi.org/10.1124/jpet.124.002274.

AbbreviationsMORmu-opioid receptorCopyright © 2024 by The American Society for Pharmacology and Experimental Therapeutics