Functional Characterization of the Cat and Dog Wild‐Type and Mutant <scp>MDR1</scp> Carrier Proteins and Frequency of the <i>MDR1</i> Gene Mutation in 800 Cats From Germany

The ATP-binding cassette transporter MDR1 P-glycoprotein (syn. ABCB1) is an efflux carrier at the cell membrane that regulates drug absorption, distribution, and elimination. At the blood-brain barrier, MDR1 restricts brain entry of potentially neurotoxic drugs, such as ivermectin. In dogs and cats, MDR1 (syn. ABCB1) gene deletion mutations exist that have been associated with increased neurological toxicity after ivermectin treatment. The present study found an allelic frequency of 0.625% for the MDR1 mutation in 800 cats from Germany. In addition, the canine and feline mutant and wild-type MDR1 proteins were expressed in HEK293 and MDCKII cells, and transport experiments were performed with the fluorescent MDR1 probe substrate rhodamine 123. In both cell lines, significant MDR1-mediated rhodamine 123 efflux was identified for the wild-type MDR1 proteins, but not for the mutant MDR1 proteins, confirming a complete loss-of-function phenotype due to MDR1 gene mutation. Competitive in vitro studies showed inhibition of both wild-type MDR1 carriers with the reference MDR1 inhibitors verapamil (IC₅₀ = 5-9 μM), PSC833 (IC₅₀ = 1-2 μM), and tariquidar (IC₅₀ = 0.1-0.2 μM), as well as with the antiparasitic drugs ivermectin (IC₅₀ = 3-4 μM), eprinomectin (IC₅₀ = 3-4 μM), moxidectin (IC₅₀ = 8-21 μM), selamectin (IC₅₀ = 10-22 μM), lotilaner (IC₅₀ = 11-23 μM), and sarolaner (IC₅₀ = 30-57 μM), clearly demonstrating multi-drug interactions with the MDR1 carriers from both species.