Modified creatine compounds

A compound of Formula I

• or a pharmaceutically acceptable salt thereof wherein
• Z is —C(═O)NR5—;
• wherein R5 is hydrogen, alkyl, aryl, or heterocyclic;
• Y is a phosphonium group;
• each R1 is independently hydrogen, alkyl, or a phosphate group;
• R2 is alkyl;
• R3 is alkyl, cycloalkyl, alkylcycloalkyl, heterocycloalkyl, or alkylheterocycloalkyl;
• R4 is hydrogen, or alkyl;
• at each occurrence, an alkyl is optionally substituted with 1-3 substituents independently selected from halo, haloalkyl, hydroxyl, amino, thio, ether, ester, oxo, aldehyde, cycloalkyl, nitrile, urea, amide, carbamate and aryl;
• at each occurrence, an aryl is optionally substituted with 1-5 substituents independently selected from halogen, azide, alkyl, haloalkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamide, ketone, aldehyde, ester, heterocyclyl, and CN;
• W is hydrogen or alkyl;
• and with the provisos that Z and Y are not substituted on the same R2 carbon;
• and that Z and the —NR4— moiety are not substituted on the same R3 carbon.