The clinical value of a non-opioid analgesic is defined relative to the established agents it is intended to replace, so its preclinical evaluation is comparative from the outset. The relevant comparisons are analgesic efficacy relative to the current standard, duration of effect, effects on behavior and alertness, and any reduction in subsequent opioid requirement. The FDA's draft guidance on non-opioid analgesics for acute pain addresses labeling claims of this kind directly, and the September 2025 draft guidance for chronic pain extends the framework to longer treatment. Evidence for each of these comparisons originates in the preclinical package, and the pig post-operative pain models were characterized against these clinical questions.
The interpretive value of a comparator arm depends on the clinical performance of the comparator being known. In the pig flank incision model, an analysis of analgesic effect over time ranked liposomal bupivacaine above bupivacaine hydrochloride and above ropivacaine, the same order observed in human postoperative data. In the leg incision model, perineural injection of all three produces a complete nerve block that reverses with time, with the liposomal formulation producing the longest block. A candidate tested alongside these agents is therefore measured against reference compounds whose preclinical and clinical performance are both established. A candidate that outperforms liposomal bupivacaine in the pig provides a directly interpretable basis for clinical planning.
An extended analgesia claim requires longitudinal measurement, and the models were designed for repeated assessment over days. Von Frey testing is repeated over hours and days after surgery. In this test, an animal without pain tolerates the maximum filament force without responding, and an animal with pain withdraws at a fraction of that force. The bupivacaine and meloxicam extended release formulation later approved for postsurgical pain demonstrated analgesia through 72 hours in the flank model, well beyond the 12 to 24 hours over which local anesthetics are usually effective. An oleogel bupivacaine formulation was followed to roughly five days of analgesia by local instillation and by nerve block. The distress behavior score and the approaching test are run on the same schedule, so evoked and non-evoked pain are assessed together across the full duration of the claim.
A mechanism established in the pig can be tested directly in the clinic. Surgical inflammation lowers tissue pH, which leaves only a small fraction of bupivacaine in the unionized form that crosses the nerve membrane. Meloxicam restores tissue pH and increases that fraction more than tenfold, and the pig flank incision study showed the combination outperforming either component alone. A human bunionectomy trial reproduced the pattern, and the product was approved with reduced opioid consumption among its findings. The preclinical study therefore demonstrated both the analgesic effect of the formulation and its mechanism, and the mechanism was subsequently confirmed in patients.
Analgesia must be distinguished from sedation, so the behavioral effects of a non-opioid candidate are assessed alongside its analgesic effect. The open field assay records distance traveled, speed and time in the central zone, and confirms that an animal that no longer responds to von Frey stimulation is moving normally rather than impaired. In the pig peripheral neuritis trauma model, the assay also captures a disease-specific gait pattern. Neuropathic pigs walk the perimeter with the operated leg toward the wall and avoid the center. Gabapentin normalizes this pattern, morphine produces dose-dependent aggressiveness in the same animals, and buprenorphine produces no measurable analgesic or behavioral effect. A candidate developed as an opioid alternative, a partial agonist or a biased ligand is compared against all three, so that its efficacy and its behavioral profile are obtained from a single study.
A formulation that remains in a surgical wound for days is present throughout the early healing period, so the package must characterize the local tissue effects. Pharmacokinetic sampling is performed alongside the behavioral testing. Incision healing is assessed in life and by histology, and for the oleogel bupivacaine formulation the pharmacokinetic and histology data supported safety and acceptable healing at doses several times the bupivacaine hydrochloride control. Sex is a relevant variable in this assessment. In Göttingen minipigs given bupivacaine by wound infiltration, females reached higher plasma concentrations than males and reached them later, with a thicker subcutaneous fat layer at the injection site. This finding has implications for the cardiovascular safety margin of any local anesthetic formulation.
A model has predictive validity when compounds that were effective in patients are effective in the model and compounds that failed in patients fail in the model. The NK1 antagonist aprepitant produced no analgesic effect in the pig peripheral neuritis trauma model, consistent with the compound's clinical record in neuropathic pain. Together, the liposomal bupivacaine ranking, the meloxicam mechanism and the aprepitant result provide a model in which both positive and negative results have been compared against clinical outcomes.
Some non-opioid programs are intended to reduce opioid exposure rather than replace it. A preclinical design intended to demonstrate opioid sparing requires a co-administered opioid and a measure of how the candidate modifies the effect of the opioid. In a mouse study of morphine tolerance, repeated subcutaneous morphine was given with or without a live biotherapeutic candidate. Tolerance was followed on the tail flick test and on sensory evoked potentials recorded in the same animals. The candidate attenuated the development of tolerance on both measures, an electrophysiological result concordant with the behavioral one, supporting its use as an adjunct that limits the morphine exposure required over time.
MD Biosciences runs the pig flank and leg incision models, the peripheral neuritis trauma model and the rodent incisional and nerve injury models. Von Frey, distress behavior scoring, approaching test, open field, electrophysiology, PK, incision healing and histology are collected in the same animals. The comparators described here, liposomal bupivacaine, bupivacaine hydrochloride, ropivacaine, morphine, gabapentin and buprenorphine, are characterized in these models. Candidates of any technology type, from small molecules and biologics to cell based therapies, AAVs, live biotherapeutics and devices, are tested against them. Study design conversations are welcome at neuro@mdbiosciences.com.
References:
Development of Non-Opioid Analgesics for Acute Pain (2022) and for Chronic Pain (September 2025). FDA draft guidance for industry.
Castel D et al. Journal of Pain Research, 2017 and 2018.
Ottoboni T et al. Regional Anesthesia and Pain Medicine, 2019.
Wojtalewicz S et al. Drug Delivery and Translational Research, 2024.