Challenges of dermal administration in the juvenile Minipig

Introduction

The Göttingen Minipig has now become widely accepted as a non-rodent species for safety testing, including juvenile safety testing due to its availability and comparability to man. The Minipig has many advantages such as a relatively large litter size, rapid growth rate, rapid achievement of sexual maturity and piglets can be used for laboratory procedures from a very early age, even from postnatal day (PND1) onwards. Due to the relatively recent use of the Minipigs for juvenile studies, only a small number has been conducted and experience with this model is limited. In the work presented here, we set up the dermal administration technique for neonatal Minipigs from as early as PND1.

Method

Three litters of newborn Göttingen Minipigs were allocated to a dermal feasibility study with the aim of starting administration from PND1.

Piglets from the first litter (n=8) were not treated with any compound and were used uniquely for establishing technical procedures and materials. Piglets from the other two litters (n=12 in total) were treated dermally with a gel placebo (volume of 2 mL/kg/day) applied on both flanks, defined with anatomic landmarks (between the scapulae and hipbones). The treated area was occluded for 4 to 6 hours per day.

To evaluate the feasibility of dermal administration, the piglets were observed daily for clinical observations and local tolerance. They were socialized to evaluate the interactions between the mother and piglets and between piglets. The application area and the total body surface were measured between PND1 to 8 weeks of age in order to confirm regulatory compliance1 (application on at least 10% of the body surface).

Several dressings were tested to cover the application area, depending on the weight of piglets. For piglets at birth (body weight <0.5 kg), we used a porous gauze dressing kept in place with a tubular net bandage and surgical tape (Figure 1a). Then, the porous gauze dressing was covered by a jacket of different sizes (normally used for implanted animals): jacket for juvenile animals for piglets >0.5 kg (Figure 1b) and then jacket for adult Minipigs from 4 weeks of age (2-2.5 kg) (Figure 1c).

Results

The dressing materials and placebo were well tolerated locally and there was no effect on body weight and tibia length when compared with background data.

We demonstrated that:

• Anatomical landmarks can be used to determine the application area in the juvenile Minipig at least up to 12 weeks of age.
• At least 10% of the total body surface was treated dermally throughout the study despite the rapid piglet growth.
• Our results confirmed that the theoretical total body surface area calculated using the Spector formula2 was similar or slightly superior to the actual total body surface up to 12 weeks of age (4.5 kg).
• For a more accurate estimation of total body surface area, DeRoth formula3 and Bollen formula4 can be used for piglets depending on the body weight (inferior or superior to 2 kg, respectively) (Figure 2).

Conclusion

In conclusion, dermal administration to litters of suckling piglets is feasible from PND1. The dressing materials and placebo were well tolerated locally and had no impact on physical development. We demonstrated that anatomical landmarks (area between the scapulae and hipbones) can be used to determine the application area in the neonatal and juvenile Minipig, at least up to 12 weeks of age (4.5 kg) and that at least 10 % of the total body surface area was treated dermally throughout the study, despite rapid piglet growth. The brief periods during which the newborn piglets were removed from the mother for technical procedures had no impact on their health status so adequate transfer of colostrum was assumed. Our results also confirmed that the theoretical total body surface area, calculated using DeRoth formula for piglets < 2 kg and then Bollen formula for piglets > 2 kg, was similar to the actual total body surface up to 12 weeks of age.

REFERENCES

1. Regulatory guideline requirements (OECD 410 Repeated Dose Dermal Toxicity Study; EPA-Health Effects Test Guidelines-OPPTS 870,3200)
2. Spector formula: BSA (cm²) = 10 x BW(g)2/3 (Handbook of Biological Data. Philadelphia: W.B. Saunders; 1956)
3. DeRoth formula for pig<2kg:BSA (cm²) = 337,2 + 0,552 x BW(g) (DeRoth and Bisaillon. Determination of body surface area in neonatal swine. Lab. Anim. Sci. 29(2):249-250.1979) (Swindle. Surgery, anesthesia and experimental techniques in swine. 1st Ed. Iowa State University Press. 1998, p230)
4. Bollen formula: BSA (cm²) = 700 x BW(kg)0,75 (Bollen and al. The Laboratory Swine, Second Edition. CRC Press. 2010, p87) (Willard-Mack and al. Dermatotoxicology: Safety Evaluation of Topical Products in Minipigs: Study Designs and Practical Considerations. Toxicol. Pathol., 2016)