Imiquimod-induced pruritus in female wild-type and knockin Wistar rats: underscoring behavioral scratching in a rat model for antipruritic treatments

Abstract

Objectives: Animal models of skin disease are used to evaluate therapeutics to alleviate disease. One common clinical dermatological complaint is pruritus (itch), but there is a lack of standardization in the characterization of pre-clinical models and scratching behavior, a key itch endpoint, is often neglected. One such model is the widely used imiquimod (IMQ) mouse model of psoriasis. However, it lacks characterized behavioral attributes like scratching, nor has widely expanded to other species like rats. Given these important attributes, this study was designed to broaden the characterization beyond the expected IMQ-induced psoriasis-like skin inflammatory skin changes and to validate the role of a potential therapeutic agent for pruritus in our genetic rat model. The study included female Wistar rats and genetically modified knockin (humanized proteinase-activated receptor 2 (F2RL1) female rats, with the widely used C57BL/6 J mice as a methodology control for typical IMQ dosing.

Results: We demonstrate that the IMQ model can be reproduced in rats, including their genetically modified derivatives, and how scratching can be used as a key behavioral endpoint. We systemically delivered an anti-PAR2 antibody (P24E1102) which reversed scratching bouts-validating this behavioral methodology and have shown its feasibility and value in identifying effective antipruritic drugs.

Keywords: Imiquimod; Pruritus; Psoriasis; Scratching bout; Wistar rat.

Fig. 1

Study design. A Female Wistar rats received from nine daily topical administrations of imiquimod (IMQ) or vehicle (Vaseline^®). Test antibodies or vehicle formulations were administered on Day 1, 3, and Day 6. Development of psoriasis-like dermatitis B appeared starting Day 4 continuing through Day 10. On Day 10, severity of the back inflammation was monitored on a modified clinical Psoriasis Area and Severity Index (PASI) scoring system. Scratching behavior was captured by videotape and serum and back skin samples were collected for cytokine measurements and immunohistochemistry to visualize IMQ induced morphological changes. Number of scratching bouts in IMQ-treated rats was significantly (P < 0.0001) higher than animals exposed to vehicle in C female Wistar rats and D F2RL1 knockin female Wistar rat animals from various studies under control conditions (n = 15 total). Data is expressed as the mean (SEM). Asterisk (*) denotes P values of IMQ compared to vehicle, *P < 0.0001 (Student’s t-test, followed by Mann–Whitney post-test)

Fig. 2

Effect of IMQ administration on total scratching bouts and severity of psoriasis-like activity in F2RL1 rats. A Rats with total scratching bouts treated with IMQ were significantly higher than animals exposed to vehicle (n = 5/group). Rats with scratching bouts after exposure to IMQ show decreased scratching upon administration of P24E1102 at 30 mg/kg IV at Day 10 as compared with vehicle and isotype control-dosed rats (P < 0.05) (n = 5/group). B IMQ induces scratching activity, but not biting or licking. Scratching is the main activity observed. C Rats with dermatology severity symptoms of erythema, scaling and thickening of lesions after exposure to IMQ show decreased severity upon administration with P24E1102. Data is expressed as the mean (SEM). Asterisk (*) denotes P values of IMQ compared to vehicle, *P < 0.05 (Student’s t-test, followed by Mann–Whitney post-test

Fig. 3

Confirming specific targeting effects of P24E1102 A Rats administered with the negative control isotype antibody may show decreased severity scores after exposure to IMQ and may induce activation of non-antigen specific Fc receptors interactions. Therefore, we made F(ab)’2 antibody fragments A and retested them in the IMQ rat model. B Target specific attenuation of IMQ-induced scratching activity with P24E1102 F(ab)’2 30 mg/kg IV similar to the whole IgG molecule, while the isotype F(ab)’2 showed no targeting effect (n = 5/group). C IMQ induces scratching activity, but not biting or licking. Data is expressed as the mean (SEM). Statistical significance was determined using Student’s t test followed by Mann–Whitney post-test