Ovarian cancer, from the laboratory to the clinic: challenges for the future

Over the past 20 years ovarian cancer has provided a vivid illustration of the successes, failures and challenges for the medical oncologist. During that time the results of treatment have substantially improved; in the West of Scotland for example, for women aged under 55, 3-year survival rates have increased from 36% to 50%. One reason for this was probably the introduction of effective agents such as cisplatin in the mid-1970s and then carboplatin in the mid-1980s. The recent introduction of taxoids promises further improvement in the future. It is important to remember, however, that the best results will be obtained by an optimal organization for the delivery of treatment; national audit studies have shown that factors such as management in integrated clinics can have a major impact on outcome. Nevertheless, the majority of patients still die from the disease; when relapse occurs, clinical drug resistance eventually proves fatal despite further treatment. What are the fundamental mechanisms by which this resistance develops, and what means are available to attempt its circumvention? Factors involved could be described as pharmacological or cellular. Pharmacological resistance might best be addressed by increasing the doses of the drugs used, particularly, cis- or carboplatin. Three years ago we published the results of a randomized trial of 2 doses of cisplatin in 191 patients. At that stage a highly significant median survival advantage for the higher dose (100 mg/m2) of cisplatin was seen. However, a recent updated analysis with a median follow-up of 4 1/2 years shows a reduction in the survival benefit, with 4-year overall survival rates for high- and low-dose cisplatin of 32.4% and 26.6%, respectively. This suggests that a population of drug resistant ovarian cancer cells will eventually emerge despite the use of initial higher doses of cisplatin. A more dose-intensive approach is being pursued with carboplatin, and it seems clear that dose-increments over standard therapy of at least 4-fold will be necessary, to justify further randomized trials. Meanwhile, the alternative approach to delivering high drug concentrations, i.e. intraperitoneal (i.p.) chemotherapy, clearly merits further study, particularly in the light of a recently reported study in patients with minimal disease, which showed a significant survival benefit for i.p. cisplatin treatment. Cellular factors will probably prove to be crucial; studies using various cell lines suggest that multiple mechanisms are likely to be involved and these will need to be examined in relevant clinical material. After DNA damage induced by a range of cytotoxic agents has taken place in ovarian cancer cells, the key to sensitivity/resistance may well be the ability of these cells to engage the process of apoptosis. Several genes are involved in control of this process; these include the p53 gene, mutations of which have been linked to cisplatin resistance in our laboratory studies, as well as in clinical trials with carboplatin. We have also demonstrated an association in ovarian cancer cell lines between cisplatin resistance and microsatellite instability (indicative of defective mismatch repair) and the clinical relevance of this link is also being pursued. A thorough understanding of underlying mechanisms may lead to the rational development of therapeutic means for circumventing cisplatin-resistance in ovarian cancer; the emergence of new classes of drug such as taxoids as topoisomerase I inhibitors offers further promise of improvement in outcome in the next few years.