A prodrug conjugate of Formula I or Ia: wherein a is 2, or 3; each R 1 and R 2 is independently selected from hydrogen; X 1 is selected from O or S; X 2 is selected from direct bond, O or NR 20 wherein R 20 is selected from hydrogen, halogen, aliphatic or aryl; C1 is selected from optionally substituted C 1 -C 30 alkyl, optionally substituted C 2 -C 30 alkenyl and optionally substituted C 2 -C 30 alkynyl; n is an integer between 4 and 26; each R 10 and R 11 is independently selected from hydrogen, halogen, -OR 20, -SR 20,-NR 20 R 21, -C R 20, -C OR 20, -C NR 20 R 21, -N(R 20)C R 21, -CF 3, -CN, -NO 2,-N 3, acyl, alkoxy, optionally substituted alkylamino, optionally substituted dialkylamino, optionally substituted alkylthio, optionally substituted alkylsulfonyl, optionally substituted aliphatic, optionally substituted aryl or optionally substituted heterocyclyl; wherein each R 20 and R 21 is independently selected from hydrogen, halogen, aliphatic, substituted aliphatic, aryl or substituted aryl; alternatively two R 10 and R 11 together with the atoms to which they are attached may form an optionally substituted 3, 4, 5, 6 or 7 membered ring; the term "substituted" refers to the replacement of one or more hydrogen radicals in a given structure with the radical of a substituent selected from halo, alkyl, alkenyl, alkynyl, aryl, heterocyclyl, thiol, alkylthio, arylthio, alkylthioalkyl, arylthioalkyl, alkysulfonyl, alkylsulfonylalkyl, arylsulfonyalkyl, alkoxy, aryloxy, aralkoxy, aminocarbonyl, alkylaminocarbonyl, arylaminocarbonyl, alkoxycarbonyl, aryloxycarbonyl, haloalkyl, amino, trifluoromethyl, cyano, nitro, alkylamino, arylamino, alkylaminoalkyl, arylaminoalkyl, aminoalkylamino, hydroxy, alkoxyalkyl, carboxyalkyl, alkoxycarbonylalkyl, aminocarbonylalkyl, acyl, aralkoxycarbonyl, carboxylic acid, sulfonic acid, sulfonyl, phosphonic acid, aryl, heteroaryl, heterocyclic, and aliphatic; and each API, API-1 and API-2 is independently a biologically active moiety selected from Tables 1, 5, 12 and 14: Table 1 1 7 2 8 3 9 4 10 5 11 6 12 Table 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 wherein a is 2, or 3; each R 1 and R 2 is independently selected from hydrogen; X 1 is selected from O or S; X 2 is selected from direct bond, O or NR 20 wherein R 20 is selected from hydrogen, halogen, aliphatic or aryl; C1 is selected from optionally substituted C 1 -C 30 alkyl, optionally substituted C 2 -C 30 alkenyl and optionally substituted C 2 -C 30 alkynyl; n is an integer between 4 and 26; C1 is selected from optionally substituted C 1 -C 30 alkyl, optionally substituted C 2 -C 30 alkenyl and optionally substituted C 2 -C 30 alkynyl; n is an integer between 4 and 26; each R 10 and R 11 is independently selected from hydrogen, halogen, -OR 20, -SR 20,-NR 20 R 21, -C R 20, -C OR 20, -C NR 20 R 21, -N(R 20)C R 21, -CF 3, -CN, -NO 2,-N 3, acyl, alkoxy, optionally substituted alkylamino, optionally substituted dialkylamino, optionally substituted alkylthio, optionally substituted alkylsulfonyl, optionally substituted aliphatic, optionally substituted aryl or optionally substituted heterocyclyl; wherein each R 20 and R 21 is independently selected from hydrogen, halogen, aliphatic, substituted aliphatic, aryl or substituted aryl; wherein each R 20 and R 21 is independently selected from hydrogen, halogen, aliphatic, substituted aliphatic, aryl or substituted aryl; alternatively two R 10 and R 11 together with the atoms to which they are attached may form an optionally substituted 3, 4, 5, 6 or 7 membered ring; the term "substituted" refers to the replacement of one or more hydrogen radicals in a given structure with the radical of a substituent selected from halo, alkyl, alkenyl, alkynyl, aryl, heterocyclyl, thiol, alkylthio, arylthio, alkylthioalkyl, arylthioalkyl, alkysulfonyl, alkylsulfonylalkyl, arylsulfonyalkyl, alkoxy, aryloxy, aralkoxy, aminocarbonyl, alkylaminocarbonyl, arylaminocarbonyl, alkoxycarbonyl, aryloxycarbonyl, haloalkyl, amino, trifluoromethyl, cyano, nitro, alkylamino, arylamino, alkylaminoalkyl, arylaminoalkyl, aminoalkylamino, hydroxy, alkoxyalkyl, carboxyalkyl, alkoxycarbonylalkyl, aminocarbonylalkyl, acyl, aralkoxycarbonyl, carboxylic acid, sulfonic acid, sulfonyl, phosphonic acid, aryl, heteroaryl, heterocyclic, and aliphatic; and each API, API-1 and API-2 is independently a biologically active moiety selected from Tables 1, 5, 12 and 14: Table 1 1 7 2 8 3 9 4 10 5 11 6 12 Table 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 each API, API-1 and API-2 is independently a biologically active moiety selected from Tables 1, 5, 12 and 14: Table 1 1 7 2 8 3 9 4 10 5 11 6 12 Table 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21