Rat paw-lick/muscle irritation model for evaluating parenteral formulations for pain-on-injection and muscle damage

A two-phase assay was developed in the rat to evaluate parenteral formulations intended for intramuscular administration for the induction of both acute pain-on-injection and delayed pain/discomfort at the injection site (secondary to muscle damage). Phase 1 of the assay assessed pain-on-injection using a modified version of the previously published rat paw-lick assay. Adult male CD rats (10/group) were given subplantar (footpad) injections of 0.1 ml and then observed for 15 min for paw-lick responses. To increase assay sensitivity, responses more subtle than paw licks (ie., paw lifts) were scored, and injection-site clinical signs were recorded 6, 24, and 48 hr after injection. Phase 2 of the assay assessed myotoxic potential, using the same rats after a 1-week recovery period. The rats were injected intramuscularly in the anterior thigh with 0.2 ml, bled from the orbital sinus at 2, 6, and 24 hr for analysis of serum creatine kinase (CK), and then necropsied at 24 hr to prepare tissue sections of the injection site for microscopic examination. A series of cephalosporin-type antibiotics produced pain-on-injection and muscle damage consistent with reported clinical experience (cefazolin < cephalothin < cefoxitin). Several nonantibiotic parenteral formulations (diazepam, digoxin, phenytoin, and lidocaine) tested in the paw-lick/muscle irritation model also produced responses that correlated with the clinic, i.e., virtually no acute pain but moderate to marked muscle damage. The results indicate that the two-phase rat paw-lick/muscle irritation model is effective in evaluating parenteral formulations for clinical acceptability, and that both phases of the assay are necessary to optimize predictability of the assay for human clinical experience.