Studies Towards a pH-Sensitive Anticancer Prodrug Model

Tumour-activated prodrug (TAP) is designed to aim at increasing the prodrug selectivity to kill cancer cells. One strategy to is to design a TAP containing an amine cytotoxin, present as an amide function, which could be released more rapidly in the low pH environment of tumour tissues when amide undergoes hydrolysis. The prodrug model (1) was the subject of the current study. At lower pH its un-ionised carboxylic acid group provides neighbouring catalysis of hydrolysis of the adjacent amide. It was synthesised via ring-opening of the imide (2) which itself was directly synthesised from endo-bicyclo[2.2.2]octa-5-ene-2,3-dicarboxylic anhydride and p-methoxyaniline. The pH-rate profile of (1) was established over the pH range of 3-10, covering rapid hydrolysis of un-ionised acid-amide at lower pH but slower imide formation above pH 8 from the ionised acid-amide. From the kinetic data were calculated the dissociation constant for (1) (pKa: 5.1 at 30 C) and limiting lower pH rate constant for hydrolysis of (1) in its fully neutral form (klim: 0.44 min-1 at 30 C). The data in the pH range of 8-10 provided klow (0.067 min-1) representing formation of (2) from fully ionised (1). The following equilibrium reaction was also investigated at high pH, at which (1) was in its fully ionised amide carboxylate form, by kinetic studies on (2) in hydroxide solutions. Imide + OH- Amide carboxylate The second order rate constant for the forward reaction, kf, was 74 L mol-1 min-1 which with klow for the reverse reaction gave K as 1100 L mol-1