There is provided herein compounds of formula I, wherein R1 is H or linear or branched C1-C6 lower alkyl； R2 is H, linear or branched C1-C6 lower alkyl, or a proteinogenic amino acid side chain； R3 is H, linear or branched C1-C6 lower alkyl, or a proteinogenic amino acid side chain； R4 is ——OH or —O—R5 wherein R5 is linear or branched C1-C6 lower alkyl； and each R6 is independently H or —CH3； or a pharmaceutically acceptable prodrug, salt, or ester thereof. Compositions containing said compounds, and uses thereof, are also provided.
