The immunomodulator drug Gilenya (FTY720), marketed as the first oral sphingosine-l-phosphate receptor (S1P-R) modulator for treatment of Multiple Sclerosis (MS) also inhibits lysosomal acid sphingomyelinase (ASMase). Treatment of cultured cells for 24 h with FTY720 (up to 10 mu M) inhibited ASMase by >80% and this could be reversed by pre-treatment with the cathepsin protease inhibitor leupeptin (5 mu M). In contrast, neutral sphingomyelinase activity was unaffected and sphingosine-l-phosphate treatment had no effect on ASMase. RT-PCR revealed no inhibition of ASMase mRNA and there was no direct (in vitro) inhibition of ASMase by either FTY720 or FTY720-phosphate. This suggests that its mechanism of inhibition is similar to that of tricyclic anti-depressants such as desipramine, which are also amphiphilic cationic drugs. Both Desipramine and FTY720 treatment reduced ASMase without significant inhibition of other lysosomal hydrolases but most hydrolases showed increased secretion (up to a 50% increase) providing more evidence of lysosomal disruption by these drugs. (C) 2010 Elsevier Inc. All rights reserved.