A compound of Formula I wherein: R is C 1 -C 6 alkyl; R 1 is selected from the group consisting of n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; R 2 and R 3 are independently selected from the group consisting of hydrogen, optionally substituted alkyl, aralkyl, optionally substituted aryl, and monovalent pharmaceutically acceptable cation, or taken together R 2 and R 3 represent a divalent pharmaceutically acceptable cation; R 4 is selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, and COR 5; R 5 is selected from the group consisting of optionally substituted alkyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; X - is a pharmaceutically acceptable anion, or X - and R 2 are absent and the compound of Formula I is a zwitterion; Z is selected from the group consisting of O and NR 10; and R 10 is selected from the group consisting of hydrogen, optionally substituted alkyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; or a pharmaceutically acceptable hydrate, crystalline form or amorphous form thereof, or a stereoisomer or mixture of stereoisomers thereof. R is C 1 -C 6 alkyl; R 1 is selected from the group consisting of n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; R 2 and R 3 are independently selected from the group consisting of hydrogen, optionally substituted alkyl, aralkyl, optionally substituted aryl, and monovalent pharmaceutically acceptable cation, or taken together R 2 and R 3 represent a divalent pharmaceutically acceptable cation; R 4 is selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, and COR 5; R 5 is selected from the group consisting of optionally substituted alkyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; X - is a pharmaceutically acceptable anion, or X - and R 2 are absent and the compound of Formula I is a zwitterion; Z is selected from the group consisting of O and NR 10; and R 10 is selected from the group consisting of hydrogen, optionally substituted alkyl, aralkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heteroaryl, and optionally substituted heterocyclo; or a pharmaceutically acceptable hydrate, crystalline form or amorphous form thereof, or a stereoisomer or mixture of stereoisomers thereof.