DNA topoisomerase I and II are enzymes that bind to supercoiled DNA, forming a ‘cleavable complex’ and, through strand breakage, passage and re-ligation, allow a wide variety of essential DNA metabolic reactions, including replication and repair, to take place. These enzymes are functionally related, work together and appear to be essential to maintain cellular viability throughout the cell cycle. Since it was shown in the 1980s that the cleavable complex could be stabilized by known cytotoxic drugs such as dox-orubicin, etoposide and camptothecin, resulting in interference with the strand breakage-re-ligation catalytic cycle and subsequent cell death, much research has taken place into the development of agents that exploit this novel nuclear target.
Established inhibitors of topoisomerase II include the anthracycline antibiotics (e.g. doxorubicin) and the epipodophyllotoxins (etoposide), which were not developed on the basis of rational drug design against this specific cellular target, and are in fact not ‘pure’ topoisomerase II inhibitors. Specific topoisomerase I inhibitors with anti-tumour activity include the heterocyclic alkaloid camptothecin and its analogues.
Isolated in 1966 from the tree Camptotheca acuminata, early studies with camptothecin demonstrated unpredictable and severe toxicity with limited efficacy. However, the development of water-soluble synthetic/semi-synthetic analogues of camptothecin, such as irinotecan (CPT-11) and topotecan, and the discovery that topoisomerase I levels were higher in some tumours compared to normal tissues have led to renewed interest in the topoisomerases as important targets for anti-cancer agents.
Topotecan exerts its cytotoxic effects by stabilizing the covalent DNA-topoisomerase I cleavable complex, thus blocking DNA repair. When DNA replicates in the presence of this complex, double-strand breaks occur and the resulting DNA fragmentation causes cell death. The major side effect of topotecan is myelosuppression, which can be severe, is schedule dependent and may be associated with infectious complications. Thrombocytopenia and anaemia are also common, and non-haematological toxici-ties are otherwise mild – emesis, diarrhoea, oropharangeal ulceration and alopecia. Significant activity has been observed in refractory ovarian cancer, small-cell lung cancer, breast cancer, lymphoma, and head and neck cancer. It has only modest activity against gastrointestinal tumours, unlike CPT-11 (irinotecan). Combination studies of topotecan with other cytotoxic agents, including platinum and paclitaxel, demonstrate dose-limiting myel-osuppression, generally requiring dose reductions of both agents.
CPT-11 was developed in Japan and is used for the treatment of 5-FU-relapsed colorectal cancer. In addition to colorectal cancer, activity has been observed in ovarian cancer, lung cancer (small cell and non-small cell), gastric cancer, oesophageal cancer and cervical cancer. It is converted by hepatic carboxylesterases to a major metabolite, SN-38, which is up to 2000-fold more effective at topoisomerase I inhibition than the parent compound. Like topotecan, the formation of a cleavable complex effects damage to the DNA through double-strand breaks. After intravenous infusion, CPT-11 concentrations decline in a multi-exponential manner, with a mean terminal half-life of 6 hours, whereas the equivalent SN-38 half-life is 10 hours. SN-38 is much more highly protein bound than CPT-11. The main toxicities are myelosuppression, which can be severe, and diarrhoea. CPT-11 can induce both early and late diarrhoea, which appears to be mediated by different mechanisms. Early diarrhoea (within 24 hours) is cholinergic in nature and can be avoided by atropine administration. Late diarrhoea can be prolonged and severe and requires prompt instigation of lopera-mide as it can quickly lead to dehydration and electrolyte imbalance.
ETOPOSIDE
Podophyllin and podophyllotoxin are derived from the mandrake root, and act as mitotic poisons in a similar fashion to vinca alkaloids. Early clinical trials were stopped because of significant toxicities, and researchers looked at analogue development as a way of proceeding. VP-16 (etoposide) is an epipodophyllotoxin analogue that is not only less toxic, but also has a different mechanism of action from the parent compound. It interferes with the re-ligation of topoisomerase II, causing stabilization of DNA-topoisomerase II complexes that are cleavable and result in DNA strand breaks. Another possible contributory mechanism of cytotoxicity is the generation of free radicals.
Oral VP-16 bioavailability is approximately 50 per cent (range 17-100 per cent), and approximately 50 per cent of the VP-16 dose is eliminated as unchanged drug or glu-curonide within 24 hours of administration. Protein binding is approximately 90 per cent. VP-16 clearance appears to be related to renal clearance, and therefore increased toxicity may be expected in patients with poor renal function. Bone-marrow and gastrointestinal toxicity dominate the side effect profile, but other common side effects include alopecia, nausea, vomiting and diarrhoea. VP-16 is active in small-cell lung cancer, germ-cell tumours, ovarian cancer, choriocarcinoma and haematological cancers. Dosing schedules may involve intravenous administration, but many studies have demonstrated that chronic oral dosing is feasible and safe.
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