Rat Paw-Lick/Muscle Irritation Model for Evaluating Parenteral Formulations for Pain-on-Injection and Muscle Damage

Rat Paw-Lick/Muscle Irritation Model for Evaluating ParenteralFormulations for Pain-on-Injection and Muscle Damage. CHELLMAN,G. J., FAUROT, G. F., LOLLINI, L. O., AND MCCULLOUGH, T. E.(1990). Fundam. Appl. Toxicol. 15, 697–709. A two-phaseassay was developed in the rat to evaluate parenteral formulationsintended for intramuscular administration for the inductionof both acute pain-on-injection and delayed pain/discomfortat the injection site (secondary to muscle damage). Phase 1of the assay assessed pain-on-injection using a modified versionof the previously published rat paw-lick assay. Adult male CDrats (10/group) were given subplantar (footpad) injections of0.1 ml and then observed for 15 min for paw-lick responses.To increase assay sensitivity, responses more subtle than pawlicks (ie., paw lifts) were scored, and injection-site clinicalsigns were recorded 6, 24, and 48 hr after injection. Phase2 of the assay assessed myotoxic potential, using the same ratsafter a 1-week recovery period. The rats were injected intramuscularlyin the anterior thigh with 0.2 ml, bled from the orbital sinusat 2, 6, and 24 hr for analysis of serum creatine kinase (CK),and then necropsied at 24 hr to prepare tissue sections of theinjection site for microscopic examination. A series of cephalosporin-typeantibiotics produced pain-on-injection and muscle damage consistentwith reported clinical experience (cefazolin < cephalothin< cefoxitin). Several nonantibiotic parenteral formulations(diazepam, digoxin, phenytoin, and lidocaine) tested in thepaw-lick/muscle irritation model also produced responses thatcorrelated with the clinic, i.e., virtually no acute pain butmoderate to marked muscle damage. The results indicate thatthe two-phase rat paw-lick/muscle irritation model is effectivein evaluating parenteral formulations for clinical acceptability,and that both phases of the assay are necessary to optimizepredictability of the assay for human clinical experience.