A compound of formula: or a pharmaceutically acceptable salt thereof, wherein, Het is selected from the group consisting of P is selected from the group consisting of R 1 is selected from hydrogen, halogen, cyano, hydroxyl, substituted or unsubstituted alkyl, alkoxy, haloalkyl, haloalkoxy, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, heterocyclic ring and heterocyclylalkyl; at each occurrence, R 2, which may be the same or different, is independently selected from halogen, cyano, hydroxyl, nitro, -NR e R f, substituted or unsubstituted alkyl, alkoxy, haloalkyl, haloalkoxy, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, heterocyclic ring and heterocyclylalkyl; R 3 is selected from -NR 6 R 7, substituted or unsubstituted alkyl, aryl, heterocyclic ring, heterocyclylalkyl and heteroaryl; R 4 is selected from hydrogen, substituted or unsubstituted alkyl, hydroxyalkyl, arylalkyl heterocyclylalkyl, heteroarylalkyl, -(CR g R h) p -C -C 1 -C 4 alkyl, -(CR g R h) p -C -NR e R f, -(CR g R h) p -NR e R f, -(CR g R h) p -NR e -C -NR e R f and -(CR g R h) p -NR e -C(=NH)-NR e R f; R 6 and R 7, which may be the same or different, are independently selected from hydrogen, substituted or unsubstituted alkyl and hydroxyalkyl; or R 6 and R 7 together with the nitrogen atom to which they are attached, form a cyclic ring, which may be monocyclic, bicyclic or tricyclic rings, which is substituted or unsubstituted and wherein the cyclic ring optionally contains one or more heteroatoms selected from O, N or S; R 8 is selected from hydrogen, C 1 -C 4 alkyl, arylalkyl and pharmaceutically acceptable cation (M + or M 2+); R e and R f, which may be the same or different, are independently selected from hydrogen, substituted or unsubstituted alkyl and hydroxyalkyl; or R e and R f together with the nitrogen atom to which they are attached, form a cyclic ring, which may be monocyclic, bicyclic or tricyclic rings, which is substituted or unsubstituted and wherein the cyclic ring optionally contains one or more heteroatoms selected from O, N or S; R g and R h, which may be the same or different, are each independently hydrogen or C 1 -C 4 alkyl; 'm' is an integer ranging from 0 to 5, both inclusive; and 'p' is an integer ranging from 1 to 3, both inclusive. P is selected from the group consisting of R 1 is selected from hydrogen, halogen, cyano, hydroxyl, substituted or unsubstituted alkyl, alkoxy, haloalkyl, haloalkoxy, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, heterocyclic ring and heterocyclylalkyl; at each occurrence, R 2, which may be the same or different, is independently selected from halogen, cyano, hydroxyl, nitro, -NR e R f, substituted or unsubstituted alkyl, alkoxy, haloalkyl, haloalkoxy, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heteroaryl, heteroarylalkyl, heterocyclic ring and heterocyclylalkyl; R 3 is selected from -NR 6 R 7, substituted or unsubstituted alkyl, aryl, heterocyclic ring, heterocyclylalkyl and heteroaryl; R 4 is selected from hydrogen, substituted or unsubstituted alkyl, hydroxyalkyl, arylalkyl heterocyclylalkyl, heteroarylalkyl, -(CR g R h) p -C -C 1 -C 4 alkyl, -(CR g R h) p -C -NR e R f, -(CR g R h) p -NR e R f, -(CR g R h) p -NR e -C -NR e R f and -(CR g R h) p -NR e -C(=NH)-NR e R f; R 6 and R 7, which may be the same or different, are independently selected from hydrogen, substituted or unsubstituted alkyl and hydroxyalkyl; or R 6 and R 7 together with the nitrogen atom to which they are attached, form a cyclic ring, which may be monocyclic, bicyclic or tricyclic rings, which is substituted or unsubstituted and wherein the cyclic ring optionally contains one or more heteroatoms selected from O, N or S; R 8 is selected from hydrogen, C 1 -C 4 alkyl, arylalkyl and pharmaceutically acceptable cation (M + or M 2+); R e and R f, which may be the same or different, are independently selected from hydrogen, substituted or unsubstituted alkyl and hydroxyalkyl; or R e and R f together with the nitrogen atom to which they are attached, form a cyclic ring, which may be monocyclic, bicyclic or tricyclic rings, which is substituted or unsubstituted and wherein the cyclic ring optionally contains one or more heteroatoms selected from O, N or S; R g and R h, which may be the same or different, are each independently hydrogen or C 1 -C 4 alkyl; 'm' is an integer ranging from 0 to 5, both inclusive; and 'p' is an integer ranging from 1 to 3, both inclusive.