A Compound of Formula or: or a pharmaceutically acceptable salt thereof, wherein: (a) each of Z 1, Z 2 and Z 3 is N; (b) Y is CH 2, O, or S; (c) Xa, Xb, Xc and Xd are independently selected from CH, CH 2, O, N, NH, S, carbonyl, fluoromethylene, and difluoromethylene selected so as to satisfy valence, wherein each bond to an X group is either a single bond or a double bond; (d) X 2 is halogen, aryl, alkynyl, or amino; (e) X 4 is hydrogen or halogen; and (f) R is straight-chain- or branched- substituted or unsubstituted alkyl, straight-chain- or branched- substituted or unsubstituted alkenyl, straight-chain- or branched- substituted or unsubstituted alkynyl, or substituted or unsubstituted cycloalkyl wherein the R group is interrupted by one or more -S N(R A)-, -NR A S -, -SO 2 N(R A)-, -NR A SO 2 -, -C N(R A)-, or -NR A C - groups, and/or terminated by an -S NR A R B, -NR A S R B, -SO2NR A R B, -NR A SO 2 R B, -C NR A R B, or -NR A C R B group, wherein each R A and R B is independently selected from hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, cycloalkyl, heteroalkyl, heterocycloalkyl, aryl, heteroaryl, alkylaryl, arylalkyl, alkylheteroaryl, heteroarylalkyl, and alkylheteroarylalkyl, wherein in a "substituted" group, each substituent is independently selected from the group comprising halo, -OH, -SH, -CN, -NO 2, -NH2, trihalomethyl, pentahaloethyl, C 1 -C 10 alkyl, C 0 -C 10 alkyloxyC 0 -C 10 alkyl, C 0 -C 10 alkylthioC 0 -C 10 alkyl, C 0 -C 10 alkylaminoC 0 -C 10 alkyl, N-aryl-N-C 0 -C 10 alkylaminoC 0 -C 10 alkyl, C 1 -C 10 alkylcarbonylC 0 -C 10 alkyl, C 1 -C 10 alkylcarboxyC 0 -C 10 alkyl, arylC 1 -C 10 alkylcarboxyC 0 -C 10 alkyl, C 1 -C 10 alkylcarbonylaminoC 0 -C 10 alkyl, -Co-C 10 alkylC OR x, and -C 0 -C 10 alkylC NRyR z wherein R X, R Y and R Z are independently selected from hydrogen, alkyl, and aryl or R Y and R Z are taken together with the nitrogen to which they are attached to form a saturated cyclic or unsaturated cyclic system having 3, 4, 5, 6, 7, or 8 carbon atoms with at least one substituent as defined above, wherein a "C 0 alkyl," as in C 0 -C 10 alkyl, is a covalent bond. (a) each of Z 1, Z 2 and Z 3 is N; (b) Y is CH 2, O, or S; (c) Xa, Xb, Xc and Xd are independently selected from CH, CH 2, O, N, NH, S, carbonyl, fluoromethylene, and difluoromethylene selected so as to satisfy valence, wherein each bond to an X group is either a single bond or a double bond; (d) X 2 is halogen, aryl, alkynyl, or amino; (e) X 4 is hydrogen or halogen; and (f) R is straight-chain- or branched- substituted or unsubstituted alkyl, straight-chain- or branched- substituted or unsubstituted alkenyl, straight-chain- or branched- substituted or unsubstituted alkynyl, or substituted or unsubstituted cycloalkyl wherein the R group is interrupted by one or more -S N(R A)-, -NR A S -, -SO 2 N(R A)-, -NR A SO 2 -, -C N(R A)-, or -NR A C - groups, and/or terminated by an -S NR A R B, -NR A S R B, -SO2NR A R B, -NR A SO 2 R B, -C NR A R B, or -NR A C R B group, wherein each R A and R B is independently selected from hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, cycloalkyl, heteroalkyl, heterocycloalkyl, aryl, heteroaryl, alkylaryl, arylalkyl, alkylheteroaryl, heteroarylalkyl, and alkylheteroarylalkyl, wherein in a "substituted" group, each substituent is independently selected from the group comprising halo, -OH, -SH, -CN, -NO 2, -NH2, trihalomethyl, pentahaloethyl, C 1 -C 10 alkyl, C 0 -C 10 alkyloxyC 0 -C 10 alkyl, C 0 -C 10 alkylthioC 0 -C 10 alkyl, C 0 -C 10 alkylaminoC 0 -C 10 alkyl, N-aryl-N-C 0 -C 10 alkylaminoC 0 -C 10 alkyl, C 1 -C 10 alkylcarbonylC 0 -C 10 alkyl, C 1 -C 10 alkylcarboxyC 0 -C 10 alkyl, arylC 1 -C 10 alkylcarboxyC 0 -C 10 alkyl, C 1 -C 10 alkylcarbonylaminoC 0 -C 10 alkyl, -Co-C 10 alkylC OR x, and -C 0 -C 10 alkylC NRyR z wherein R X, R Y and R Z are independently selected from hydrogen, alkyl, and aryl or R Y and R Z are taken together with the nitrogen to which they are attached to form a saturated cyclic or unsaturated cyclic system having 3, 4, 5, 6, 7, or 8 carbon atoms with at least one substituent as defined above, wherein a "C 0 alkyl," as in C 0 -C 10 alkyl, is a covalent bond.