Rubitecan

Abstract

The only approved camptothecins for use in patients to date (topotecan and irinotecan) are delivered intravenously. Thus, an oral camptothecin analogue that would provide the convenience of oral delivery with the flexibility for a variety of prolonged treatment schedules would be advantageous. Rubitecan is an orally available camptothecin analogue that also has potential for delivery transdermally or by inhalation. Like all of the camptothecins, its antitumour activity is mediated through the inhibition of DNA topoisomerase I, which is involved in relaxing supercoiled DNA, which is important for the process of DNA replication and RNA transcription. Rubitecan exists in equilibrium as 9-nitro-camptothecin (9-NC) and 9-amino-camptothecin (9-AC), a metabolite that is thought to be active although it failed in clinical trials. Both 9-NC and 9-AC contain a lactone ring that is required for optimal activity with the carboxylic acid (open ring) forms being significantly less active or inactive. A more acidic environment favours the lactone ring structure, whereas neutral or basic conditions favour the conversion to the carboxylic acid form. In addition to issues of lactone ring stability at physiological pH (true for all of the camptothecin analogues), there is pharmacokinetic variability that has had to be dealt with during the development of rubitecan. Preclinically, rubitecan has shown activity against a broad spectrum of tumour types in in vitro and in vivo human tumour xenograft models. Frustratingly, the level of activity of an agent in preclinical models has not always translated into similar activity against human tumours in clinical trials. To date, with the exception of pancreatic and possibly ovarian cancer, rubitecan has had disappointing activity against a number of other solid tumours in relatively small Phase I/II trials; however, it has shown sufficient activity against pancreatic cancer, a malignancy that remains difficult to treat, to continue to be evaluated in clinical trials for this indication. Results of clinical trials in the next few years should determine whether rubitecan can find a role in cancer therapy.