The development of molecular tools for the expression of prodrug converting enzymes in Clostridium sporogenes
Despite intensive research, cancer remains one of the major causes of worldwide morbidity. It is widely believed, however, that if currently available anti-cancer drugs could be delivered specifically to tumours then the disease would have been mastered. The delivery of prodrug converting enzymes by clostridial spores specifically to the anoxic centres of tumours is one potential delivery mechanism. This is due to the extreme selectivity of spores to germinate solely in the hypoxic regions of tumours. Once germinated, the expression of a prodrug converting enzyme converts a systemical1y administered prodrug to a highly toxic drug only in the tumour. Previous studies using Clostridium acetobutylicum and Clostridium beijerinckii as the delivery vehicle highlighted that prodrug converting enzyme expression is only found in tumours. However, no significant anti-tumour affect was observed. Two possible reasons were evolved. Firstly, expression of the prodrug converting enzyme may be low, and/or, secondly, the tumours may not be colonised sufficiently to promote an antitumour effect. Preliminary studies identified that Clostridium sporogenes NCIMB 10696 may represent a more suitable host. Higher spore titres could be prepared and, once administered, higher cell counts are found in the colonised tumours. Prodrug converting enzymes with improved kinetics over pre-existing enzymes have also been identified. Once effective gene transfer systems and expression systems had been developed, suitably high levels of several different prodrug converting enzymes, in particular nitroreductases, were obtained. Initial in vivo studies on one of the early recombinant strains identified a definite anti-tumour effect. Since those initial studies, further improvements to expression have been made. It is hoped that a more significant anti-tumour affect would result from using these improved strains. It is the ultimate aim of CDEPT to have the prodrug converting enzymes integrated into the host genome so as to negate the use of antibiotics. Towards this, studies on the use of both classical and novel integrative technologies have been investigated.