Diet gel-based oral drug delivery system for controlled dosing of small molecules for microglia depletion and inducible Cre recombination in mice

Small molecules such as PLX5622 for microglia depletion and tamoxifen for inducible Cre recombination are commonly used in mouse research. Traditional application methods such as drug-infused chow, oral gavage or injections have limitations, including uncontrolled dosing (chow) or risk of injury and/or stress (gavage or injections). Here, to address these issues, we have developed an alternative oral drug delivery system using a gel-based rodent maintenance diet that allows for controlled consumption and adjustment of dosage and is suitable for water-insoluble small molecules. We tested DietGel 93M (93M) infused with PLX5622 (0.8 mg/g and 2.0 mg/g) in the Cx3cr1^gfp/+ retinal microglia reporter mouse and tamoxifen-infused 93M (0.3125 mg/g) in the Rlbp1-CreERT2;Rosa^ai14 mouse with an inducible tdTomato reporter in retinal Müller glia. Mice were single caged and received daily batches of PLX5622-infused 93M over 14 days or tamoxifen-infused 93M for 1 or 3 days followed by a 14-day observation period. Longitudinal scanning laser ophthalmoscopy in vivo and fixed-tissue imaging were used to track GFP and tdTomato expression. Following evaluation of a suitable 93M consumption rate to sustain body weight, the PLX5622-93M diet at both concentrations tested showed a 94% microglia depletion rate at 3 days and >99% after 1 and 2 weeks. The tamoxifen-93M diet confirmed suitability for inducible Cre recombination, with significant treatment time-dependent efficacy and a positive correlation between total tamoxifen dose and tdTomato expression. This study demonstrates that a diet gel-based drug delivery system offers a minimally invasive alternative to current drug application methods for PLX5622 and tamoxifen. This approach could be useful for other drugs or tissues beyond the retina.