Sphingosine kinase inhibitors

A compound of the formula I

• or a pharmaceutically acceptable salt thereof, wherein:
• R1 is phenyl, 4-chlorophenyl or 4-fluorophenyl;
• R2 is 4-pyridyl, optionally substituted with up to 4 groups that are independently (C1-C6) alkyl, halogen, haloalkyl, —OC(O)(C1-C6 alkyl), —C(O)O(C1-C6 alkyl), —CONR′R″, —OC(O)NR′R″, —NR′C(O)R″, —CF3, —OCF3, —OH, C1-C6 alkoxy, hydroxyalkyl, —CN, —CO2H, —SH, —S-alkyl, —SOR′R″, —SO2R′, —NO2, or NR′R″, wherein R′ and R″ are independently H or (C1-C6) alkyl, and wherein each alkyl portion of a substituent is optionally further substituted with 1, 2, or 3 groups independently selected from halogen, CN, OH, and NH2;
• R4 is H or alkyl;
• and
• n is 1 or 2.