Brain cyclosporin A levels are determined by ontogenic regulation of mdr1a expression

Cyclosporin A (CyA) toxicity is a common occurrence in pediatric organ transplant patients. We hypothesized that reduced mdr1a expression in newborn and developing mice would affect CyA accumulation within organs and/or toxicity. For functional studies, CyA was administered (5 mg kg^-1 i.p.) to 1-, 12-, and 19-day, and adult male and female mdr1a^+/+ and mdr1a ^-/- mice. Peak blood CyA was lower in 1-, 12-, and 19-day-old (1000 ng ml^-1) versus adult (1500 ng ml^-1) mice but was similar in mdr1a^+/+ and mdr1a^-/- mice. Kidney mdr1a expression (measured by quantitative polymerase chain reaction) increased 2.5-fold in 19-day-old male and female mice and increased another 4-fold in adult females compared with adult males. Liver mdr1a expression increased 6-fold by day 12 compared with neonatal mice. Thereafter, maintenance of hepatic mdr1a expression in females and a reduction to neonatal levels in males was observed. Kidney/blood (8- to 9-fold) and liver/blood (12- to 15-fold) CyA levels were highest on days 12 and 19 and were not dependent on maturational changes in mdr1a mRNA levels. Adults had higher brain expression of mdr1a mRNA (3-fold), a corresponding 5-fold increase in immunodetectable P-glycoprotein, and 80% lower brain accumulation of CyA compared with 1-day-old mice. Conversely, in mdr1a-null mice, brain/blood CyA was similar in newborn and adult mice. A similar pattern was observed for the brain accumulation of the mdr1a substrate ³H-digoxin. We conclude that the risk for central nervous system drug toxicity could be higher in neonates or young children as a consequence of underdeveloped P-glycoprotein.