Polyacetal-diethylstilboestrol: A polymeric drug designed for pH-triggered activation

Those polymer anticancer-drug conjugates currently undergoing clinical evaluation have a tripartite structure; a water-soluble polymer, an anticancer agent and a pendant linker. To simplify the construct it would be attractive to develop anticancer polymer therapeutics that contain the bioactive agent as an integral part of the polymer backbone. The aim of this study was to utilise the reaction between a divinyl ethers and diols, to synthesise polyacetals incorporating a drug with bis -hydroxyl functionality into the polymer backbone. Degradation of the polymer backbone in the acidic environment of the lysosome or the extracellular fluid of some tumours would then trigger drug release eliminating the need for a biodegradable linker. A tert -polymerisation approach was used to incorporate non-steroidal oestrogen diethylstilboestrol (DES) into the mainchain of water-soluble polyacetals synthesised using as co-monomer PEG of M-w 2900 or 3400 g/mol. When PEG 2900 was used the resultant polymer had a M-w of 18,900 g/mol, a M-w/M-n of 1.9 and a DES loading 4.3 wt.%. With PEG 3400 the polymer M-w was 43,000 g/mol, M-w/M-n =1.8 and it had a DES loading 4.7 wt.%. H-1-NMR confirmed the presence of two distinct sets of acetal peaks, which correspond to the two possible mainchain acetals; from PEG at 1.25-1.3(d) and 4.7-4.8(q) ppm and from DES at 1.55-1.6(d) and 5.4-5.5(q) ppm. These were consistent with the acetal signals observed for the non-water-soluble co-polymer DES: tri(ethylene glycol) divinyl ether (TEGDVE) (1:2, M-w =6859 g/mol, M-w / M-n =1.3). When evaluated in vitro , the DES-polyacetal displayed greater cytotoxicity than DES against human and murine tumour cell lines ( IC50 =48 and 420 mug/ml against MCF-7 human breast cancer cells and IC50 =97 and 560 mug/ml against B16F10 murine melanoma cells, respectively). These polymers showed no significant haemolysis at concentrations up to 20 mg/ml confirming suitability for further in vivo evaluation. An enhanced rate of hydrolytic degradation of the polymer backbone was seen at pH 5.5, (65% trans -DES released in 96 h), compared to pH 7.4 (4% trans -DES released in 96 h). These bioresponsive DES-polyacetals tert -polymers are the first water-soluble anticancer polymeric drugs designed for acidic pH-triggered release of a drug incorporated into the polymer mainchain. Their in vitro characteristics suggest further in vivo evaluation is warranted.