Screening and evaluation of anticancer agents

The screening and evaluation procedures for the development of anticancer agents indicated that the entire process is a rather difficult task. This is particularly true in choosing screening models and criteria for activity. If the criteria were set too low, then some clinically false-positive results may be faced; and if the criteria were set too high, some agents could be missed which might be effective against certain types of human cancer. Presently, active compounds are selected by prescreening and screening against transplanted mouse tumors and human tumor xenografts as well as by the in vitro systems. Xenografts of human tumor in athymic nude animals represent metabolic characteristics of human malignant disease which appear to be of value in the preclinical screening. Human tumor cloning assays have gained increased attention as a promising in vitro test system for the screening as well as for the prediction of patient responses. Application of chemosensitivity tests in the prediction of the responses of individuals to chemotherapy, especially in the identification of drug resistant tumors are, in general, quite reliable. Human tumor xenografts, human tumor cloning assays and chemosensitivity tests may be regarded as the major impetus in screening during the past decade. After promising agents are selected from the screening procedures, before the filing of an investigational new drug application, the preclinical toxicology and pharmacology should be completed. Information on the nature of toxicity, dose-response effects, and dose schedule are necessary for predicting the effects of the drug in man. The new drugs then go through three phases of clinical trials to assure safety, effectiveness, and reliability of the drugs. During the past fifteen years eight-three antineoplastic drugs were evaluated clinically under the NCI sponsorship and twenty-four are active in at least one disease. Among these active drugs, eleven possess novel clinical structure, the remaining thirteen are analogues of known active compounds already in clinical trials. After an investigational anticancer drug is approved by the U.S. Food and Drug Administration it becomes a commercial product on the market and the benefits can be shared by the general public. The time required for evaluation and development from the first discovery of activity to final FDA approval for fifty-two therapeutic drugs are tabled. The average interval is 8.8 years, but in the 1950s the average was only 2.8 years, in the 1960s, 6.5 years; in the 1970s, 13.9 years; and in the 1980s, 16.0 years. This reflects the increasing stricter requirements for an antineoplastic drug to be officially recognize