The mouse-formalin pain model stands as a chemically-evoked acute pain model within our research arsenal at NuChem Sciences. Well-known for its capability for rapid screening, it’s particularly well-suited for most analgesic targets. The model delves into both central and peripheral mechanisms of pain, with its Phase 2 often regarded as a ‘surrogate’ for neuropathic pain models, thereby expanding its utility.

The induction of this model involves the careful injection of a diluted formalin solution into the mouse’s paw. Post-injection, observable behaviors such as biting, licking, and guarding provide crucial indicators of pain reactions. The model’s response is classified into two distinct phases, 0-5 minutes representing Phase 1, and the duration from 15-60 minutes designated as Phase 2 while 5-15 minutes represents a quiescent interphase.

NuChem’s standard study design for this model generally comprises 4-6 groups, with each group containing 10 subjects to maintain data integrity and robustness. A range of positive controls, including Duloxetine, Gabapentin, Mexiletine, and Diclofenac, have been tested to ascertain the model’s reliability and scope. To ensure consistent results and uphold the highest standards of scientific investigation, the model is meticulously validated for the C57BL6 mouse species.

In the comprehensive evaluation of the mouse-formalin pain model, a salient observation is the pronounced pain response exhibited by male mice compared with the females, a factor that underpins our choice to utilize males for the model. Their heightened sensitivity offers a more robust platform to discern and quantify pain reactions, making the analysis more nuanced and informative.

The four clinically used prototype analgesic drugs were then tested in male mice in this model. For rapid screening purpose, 0-5 minutes and 20-40 minutes are often taken as representation of Phase 1 and Phase 2, respectively, of this model. In the Phase 1 of formalin pain model, except for Mexiletine producing a very minor analgesic effect, none of the other analgesic drugs could produce any analgesia. In the Phase 2 of formalin pain model, all four analgesic drugs produced significant and strong analgesic effects indicating higher back-transitional face value of the Phase 2 of formalin pain model. The order of analgesia of the four analgesic drugs was as follows: duloxetine ˃ mexiletine ˃ gabapentin ˃ diclofenac, which is also consistent with their clinical efficacy often seen in chronic pain. In this regard, it’s crucial to accentuate the predictive prowess of Phase 2 within the model. This phase, replicating certain facets of neuropathic pain, proves especially potent in forecasting the potential of analgesic targets. The pronounced responsiveness in Phase 2 serves as an invaluable predictor, often aligning closely with the efficacy and mode of action of analgesic compounds. In essence, the male mouse as the chosen specimen, coupled with the distinct utility of Phase 2, augments the mouse-formalin pain model’s credibility and value in our quest to decipher and modulate pain pathways.