Improving Preclinical Development of Novel Interventions to Treat Pain: Insanity Is Doing the Same Thing Over and Over and Expecting Different Results

Making sense of feelings

Internal feeling states such as pain, hunger, and thirst are widely assumed to be drivers of behaviours essential for homeostasis and animal survival. Call this the 'causal assumption'. It is becoming increasingly apparent that the causal assumption is incompatible with the standard view of motor action in neuroscience. While there is a well-known explanatory gap between neural activity and feelings, there is also a disjuncture in the reverse direction-what role, if any, do feelings play in animals if not to cause behaviour? To deny that feelings cause behaviours might thus seem to presage epiphenomenalism-the idea that subjective experiences, including feelings, are inert, emergent and, on some views, non-physical properties of brain processes. Since epiphenomenalism is antagonistic to fundamental commitments of evolutionary biology, the view developed here challenges the standard view about the function of feelings without denying that feelings have a function. Instead, we introduce the 'sense making sense' hypothesis-the idea that the function of subjective experience is not to cause behaviour, but to explain, in a restricted but still useful sense of 'explanation'. A plausible framework is derived that integrates commonly accepted neural computations to blend motor control, feelings, and explanatory processes to make sense of the way feelings are integrated into our sense of how and why we do and what we do.

The endocannabinoid system as a therapeutic target in neuropathic pain: a review

INTRODUCTION

This review highlights the critical role of the endocannabinoid system (ECS) in regulating neuropathic pain and explores the therapeutic potential of cannabinoids. Understanding the mechanisms of the ECS, including its receptors, endogenous ligands, and enzymatic routes, can lead to innovative treatments for chronic pain, offering more effective therapies for neuropathic conditions. This review bridges the gap between preclinical studies and clinical applications by emphasizing ECS modulation for better pain management outcomes.

AREAS COVERED

A review mapped the existing literature on neuropathic pain and the effects of modulating the ECS using natural and synthetic cannabinoids. This analysis examined ECS components and their alterations in neuropathic pain, highlighting the peripheral, spinal, and supraspinal mechanisms. This review aimed to provide a thorough understanding of the therapeutic potential of cannabinoids in the management of neuropathic pain.

EXPERT OPINION

Advances in cannabinoid research have shown significant potential for the management of chronic neuropathic pain. The study emphasizes the need for high-quality clinical trials and collaborative efforts among researchers, clinicians, and regulatory bodies to ensure safe and effective integration of cannabinoids into pain management protocols. Understanding the mechanisms and optimizing cannabinoid formulations and delivery methods are crucial for enhancing therapeutic outcomes.

Strategies for measuring non-evoked pain in preclinical models of neuropathic pain: Systematic review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

Eleven neurology-related proteins measured in serum are positively correlated to the severity of diabetic neuropathy

About 20% of patients with diabetes suffer from chronic pain with neuropathic characteristics. We investigated the multivariate associations between 92 neurology-related proteins measured in serum from 190 patients with painful and painless diabetic neuropathy. Participants were recruited from the Pain in Neuropathy Study, an observational cross-sectional multicentre study in which participants underwent deep phenotyping. In the exploration cohort, two groups were defined by hierarchical cluster analyses of protein data. The proportion of painless vs painful neuropathy did not differ between the two groups, but one group had a significantly higher grade of neuropathy as measured by the Toronto Clinical Scoring System (TCSS). This finding was replicated in the replication cohort. Analyzing both groups together, we found that a group of 11 inter-correlated proteins (TNFRSF12A, SCARB2, N2DL-2, SKR3, EFNA4, LAYN, CLM-1, CD38, UNC5C, GFR-alpha-1, and JAM-B) were positively associated with TCSS values. Notably, EFNA4 and UNC5C are known to be part of axon guidance pathways. To conclude, although cluster analysis of 92 neurology-related proteins did not distinguish painful from painless diabetic neuropathy, we identified 11 proteins which positively correlated to neuropathy severity and warrant further investigation as potential biomarkers.

Lost in Translation: Failure of Preclinical Studies to Accurately Predict the Effect of Regional Analgesia on Cancer Recurrence

The major goal of translational research is to evaluate the efficacy and effectiveness of treatments and interventions that have emerged from exhaustive preclinical evidence. In 2007, a major clinical trial was started to investigate the impact of paravertebral analgesia on breast cancer recurrence. The trial was based on preclinical evidence demonstrating that spinal anesthesia suppressed metastatic dissemination by inhibiting surgical stress, boosting the immunological response, avoiding volatile anesthetics, and reducing opioid use. However, that trial and three more recent randomized trials with a total of 4,770 patients demonstrate that regional analgesia does not improve survival outcomes after breast, lung, and abdominal cancers. An obvious question is why there was an almost complete disconnect between the copious preclinical investigations suggesting benefit and robust clinical trials showing no benefit? The answer is complex but may result from preclinical research being mechanistically driven and based on reductionist models. Both basic scientists and clinical investigators underestimated the limitations of various preclinical models, leading to the apparently incorrect hypothesis that regional anesthesia reduces cancer recurrence. This article reviews factors that contributed to the discordance between the laboratory science, suggesting that regional analgesia might reduce cancer recurrence and clinical trials showing that it does not-and what can be learned from the disconnect.

Practical approaches to improving translatability and reproducibility in preclinical pain research

Pain research continues to face the challenge of poor translatability of pre-clinical studies. In this short primer, we are summarizing the possible causes, with an emphasis on practical and constructive solutions. In particular, we stress the importance of increased heterogeneity in animal studies; formal or informal pre-registration to combat publication bias; and increased statistical training in order to help pre-clinical scientists appreciate the usefulness of available experimental design and reporting guidelines.

Angiotensin II type 2 receptor signalling as a pain target: Bench, bedside and back-translation

Translating promising preclinical pain relief data for novel molecules from drug discovery to positive clinical trial outcomes is challenging. The angiotensin II type 2 (AT2) receptor is a clinically-validated target based upon positive proof-of-concept clinical trial data in patients with post-herpetic neuralgia. This trial was conducted because AT2 receptor antagonists evoked pain relief in rodent models of neuropathic pain. EMA401 was selected as the drug candidate based upon its suitable preclinical toxicity and safety profile and good pharmacokinetics. Herein, we provide an overview of the discovery, preclinical and clinical development of EMA401, for the alleviation of peripheral neuropathic pain.

Discovering chronic pain treatments: better animal models might help us get there

Only three classes of pain medications have made it into clinical use in the past 60 years despite intensive efforts and the need for nonaddictive pain treatments. One reason for the failure involves the use of animal models that lack mechanistic similarity to human pain conditions, with endpoint measurements that may not reflect the human pain experience. In this issue of the JCI, Ding, Fischer, and co-authors developed the foramen lacerum impingement of trigeminal nerve root (FLIT) model of human trigeminal neuralgia that has improved face, construct, and predictive validities over those of current models. They used the FLIT model to investigate the role that abnormal, hypersynchronous cortical activity contributed to a neuropathic pain state. Unrestrained, synchronous glutamatergic activity in the primary somatosensory cortex upper lip and jaw (S1ULp-S1J) region of the somatosensory cortex drove pain phenotypes. The model establishes a powerful tool to continue investigating the interaction between the peripheral and central nervous systems that leads to chronic pain.