Predicting hematological toxicity (myelosuppression) of cytotoxic drug therapy from in vitro tests.

Several clinical oncology units are studying the roles of in vitro hematotoxicology in phase I evaluations. At the same time, the European Center for the Validation of Alternative Methods (ECVAM) is supporting a validation study of the CFU-GM assay. It is important that these activities be coordinated so that high performance, optimized technical protocols are used for prospective and retrospective clinical evaluations. The EROTC, the NCI and ECVAM could provide support for these coordinated efforts. There is an opportunity for medical oncologists involved in early clinical trials to participate in the evaluation of in vitro tests and their clinical application . Fundamental to acceptance of these assays by oncologists and regulatory scientists, they must predict clinical outcome for myelosuppressive agents and then improve phase I design and performance. These achievements would justify more aggressive dose escalation schemes using guidance from in vitro studies without compromising patient safety. Success in predicting neutropenia might also stimulate the research required to understand how to predict other hematologic toxicities, such as a thrombocytopenia. The complexity of a validation study in hematotoxicology is that it seeks to predict the level of exposure that causes neutropenia, in contrast to other validation studies that have sought to classify a xenobiotic as toxic or not. It may be that the clinical relevance of a new assay is not just a yes-no answer. This important distinction came from the realization that the xenobiotic tolerance in other organ systems of the body must be the same or greater than marrow in order for myelosuppression to be a clinical consequence of exposure. Pharmacological principles of system exposure and toxicity that are integrated into the prediction model provide the links to clinical oncology. It is also important to anticipate future applications of in vitro hematotoxicology. If the maximum tolerated level of drug exposure for human hematopoietic cells can be predicted, then in vitro hematotoxicology could play an important role in new drug discovery. One concept involves screening for compounds that show efficacy at the IC level that predicts maximum tolerated exposure levels in the human. 'Therapeutic index based' drug discovery has been applied to the tallimustine family with some success.