A compounds of formula I: wherein: a is 0 or an integer of from 1 to 3; each R 1 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 1a, -C OR 1b, -SR 1c, -S R 1d, -S 2 R 1c and -NR 1f R 1g; each of R 1a, R 1b, R 1c, R 1d, R 1c, R 1f and R 1g is independently hydrogen, alkyl or phenyl- alkyl; b is 0 or an integer of from 1 to 3; each R 2 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 2a, -C OR 2b, -SR 2c, -S R 2d, -S 2 R 2e and -NR 2f R 2g. each of R 2a, R 2b, R 2c, R 2d, R 2c, R 3f and R 2g is independently hydrogen, alkyl or phenyl- alkyl; W is attached to the 3- or 4-position with respect to the nitrogen atom in the piperidine ring and represents O or NW a; W a is hydrogen or alkyl; c is 0 or an integer of from 1 to 4; each R 3 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 3a, -C OR 3b, -SR 3c, -S R 3d, -S 2 R 3e and -NR 3f R 3g; or two R 3 groups are joined to form alkylene, alkenylene or oxiran-2,3-diyl; each of R 3a, R 3b, R 3c, R 3d, R 3c, R 3f and R 3g is independently hydrogen or alkyl; R 4 is a divalente group of the formula: -(R 4a) d -(A 1) e -(R 4b) f -Q-(R 4c) g -(A 2) h -(R 4d) i - wherein d, e, f, g, h and i are each independently selected from 0 and 1; R 4a, R 4b, R 4c and R 4d are each independently selected from alkylene, alkenylene and alkynylene, wherein each alkylene, alkenylene or alkynylene group is unsubstituted or substituted with from 1 to 5 substituents independently selected from alkyl, fluoro, hydroxy, phenyl and phenyl- alkyl; A 1 and A 2 are each independently selected from cycloalkylene, arylene, -O- arylene, arylene-O-, heteroarylene, -O- heteroarylene, heteroarylene-O- and heterocyelene, wherein each cycloalkylene is unsubstituted or substituted with from 1 to 4 substitutents selected independently from alkyl, and each arylene, heteroarylene or heterocyclene group is unsubstituted or substituted with from 1 to 4 substituents independently selected from halo, alkyl, alkoxy, -S- alkyl, -S - alkyl, -S 2 - alkyl, -C O alkyl, carboxyl, cyano, hydroxy, nitro, trifluoromethyl and trifluoromethoxy; Q is selected from a bond, -O-, -C O-, -OC -, -S-, -S -, -S 2 -, -N(Q a)C -, -C N(Q b)-, -N(Q c)S 2 -, -S 2 N(Q d)-, -N(Q c)C N(Q f)-, -N(Q g)S 2 N(Q h)-, -OC N(Q i)-, -N(Q j)C O- and -N(Q k); Q a, Q b, Q c, Q d, Q e, Q f, Q g, Q h, Q i, Q j and Q k are each independently selected from hydrogen, alkyl, A 3 and alkylene-A 4, wherein the alkyl group is unsubstituted or substituted with from 1 to 3 substituents independently selected from fluoro, hydroxy and alkoxy; or together with the nitrogen atom and the group R 4b or R 4c to which they are attached, form a 4-6 membered azacycloalkylene group; A 3 and A 4 are each independently selected from cycloalkyl, aryl, heteroaryl and heterocyclyl, wherein each cycloalkyl is unsubstituted or substituted with from 1 to 4 substituents selected independently from alkyl and each aryl, heteroaryl or hetorooyclyl group is unsubstituted or substituted with from 1 to 4 substitutents independently selected from halo, alkyl and alkoxy; provided that the number of contiguous atoms in the shortest chain between the two nitrogen atoms to which R 4 is attached is in the range of from 4 to 16; R 5 represents hydrogen or alkyl; R 6 is -NR 6a CR 6b or -CR 6c R 6d OR 6c and R 7 is hydrogen; or R 6 and R 7 together form -NR 7a C -CR 7b =CR 7c -, -CR 7d =CR 7c -C -NR 7f -, -NR 7g C -CR 7h R 7i -CR 7j R 7k - or - CR 7l R 7m -CR 7n R 7o - C -NR 7p -; each of R 6a, R 6b, R 6c, R 6d and R 6c is independently hydrogen or alkyl; and each of R 7a, R 7b, R 7c, R 7d, R 7c, R 7f, R 7g, R 7h, R 7i, R 7j, R 7k, R 7l, R 7m, R 7n, R 7o and R 7P is independently hydrogen or alkyl; or a pharmaceutically acceptable salt or solvate or stereoisomer thereof. a is 0 or an integer of from 1 to 3; each R 1 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 1a, -C OR 1b, -SR 1c, -S R 1d, -S 2 R 1c and -NR 1f R 1g; each of R 1a, R 1b, R 1c, R 1d, R 1c, R 1f and R 1g is independently hydrogen, alkyl or phenyl- alkyl; b is 0 or an integer of from 1 to 3; each R 2 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 2a, -C OR 2b, -SR 2c, -S R 2d, -S 2 R 2e and -NR 2f R 2g. each of R 2a, R 2b, R 2c, R 2d, R 2c, R 3f and R 2g is independently hydrogen, alkyl or phenyl- alkyl; W is attached to the 3- or 4-position with respect to the nitrogen atom in the piperidine ring and represents O or NW a; W a is hydrogen or alkyl; c is 0 or an integer of from 1 to 4; each R 3 is independently selected from alkyl, alkenyl, alkynyl, cycloalkyl, cyano, halo, -OR 3a, -C OR 3b, -SR 3c, -S R 3d, -S 2 R 3e and -NR 3f R 3g; or two R 3 groups are joined to form alkylene, alkenylene or oxiran-2,3-diyl; each of R 3a, R 3b, R 3c, R 3d, R 3c, R 3f and R 3g is independently hydrogen or alkyl; R 4 is a divalente group of the formula: -(R 4a) d -(A 1) e -(R 4b) f -Q-(R 4c) g -(A 2) h -(R 4d) i -