Patent ID: 8008492

Claim:
A compound of formula (I), or a pharmaceutically acceptable salt thereof, including any stereoisomer or mixtures of stereoisomers, wherein: R 1 and R 3 are radicals independently selected from the group consisting of H and (C 1 -C 4 )-alkyl, wherein either R 1 or R 3 is methyl; or alternatively, R 1 and R 3 are both methyl; R 2 is phenyl, cyclohexyl,2-napthtyl, or 1-pyrenyl; X 1 is —CH 2 or CH 2 —CH 2 ; and X 2 is a biradical selected from the group consisting of —NH—, —NH—(CH 2 ) 1-3 —COO—, —NH—(CH 2 ) 1-3 —S—, and —NH—CO—(CH 2 ) 1-3 —S—.