Novel thermosensitive 5-fluorouracil-cyclotriphosphazene conjugates: Synthesis, thermosensitivity, degradability, and in vitro antitumor activity

A novel thermosensitive macromolecular prodrug of 5-fluorouracil (5-FU) was synthesized using cyclotriphosphazene, and its thermosensitivity, degradability, and in vitro antitumor activity were studied. A series of a-substituted glycine derivatives of 5-FU containing carboxylic groups were prepared, and cyclotriphosphazenes with amino groups were synthesized via the stepwise substitution of hexachlorocyclotriphosphazene (NPCl2)(3) With methoxy-poly(ethylene glycol) (MPEG) or alkoxy ethylene oxide and lysine ethyl ester (LysOEt). The coupling reaction of the two derivatives, and their subsequent deprotection, yielded a thermosenstive 5-FU-cyclotriphosphazene conjugate, which exhibited a unique octopus-shaped molecular structure, in which the three hydrophilic PEG groups (or alkoxy ethylene oxides) were oriented in one direction, opposing the other three hydrophobic groups containing 5-FU, with respect to the trimer ring plane. This conjugate exhibited a reversible and thermosensitive phase transition in an aqueous medium, from soluble to insoluble states. The lower critical solution temperature (LCST) of the conjugate was controlled by substitution with different hydrophilic/hydrophobic side groups, and a few of the conjugates displayed LCSTs which were just below body temperature. This, of course, implies possible applications for local drug delivery by direct intratumoral injection. The conjugate exhibited gradual degradation at 37 degrees C in both neutral and acidic buffer solutions, and high temperature significantly facilitated its hydrolytic degradation. All of the conjugates displayed dose-dependent cytotoxicity against the leukemia L1210 cell line and exhibited more pronounced cytotoxic effects than did 5-FU.