USE OF IN-VITRO ASSAYS TO PREDICT THE EFFICACY OF CHEMOPREVENTIVE AGENTS IN WHOLE ANIMALS

Five in vitro assays have been applied to screen the efficacy of poten tial chemopreventive agents. These assays measure a) inhibition of mor phological transformation in rat tracheal epithelial (RTE) cells, b) i nhibition of anchorage independence in human lung tumor (A427) cells, c) inhibition of hyperplastic alveolar nodule formation in mouse mamma ry organ cultures (MMOC), d) inhibition of anchorage independence in m ouse JB6 epidermal cells, and e) the inhibition of calcium tolerance i n human foreskin epithelial cells. The efficacy of many of these same agents in whole animal studies of lung, colon, mammary gland, skin, an d urinary bladder carcinogenesis has also been measured. The aim herei n is to estimate the positive and negative predictive values of these in vitro assays against whole animal chemopreventive efficacy data usi ng the same chemicals. For three of these assays-using RTE, A427 cells and mouse mammary organ culture (MMOC)-enough data are available to a llow the estimate to be made. Such extrapolations of in vitro data to the in vivo situation are difficult at best. There are many dissimilar ities between the two assay systems. The in vitro assays use respirato ry and mammary epithelial cells, while the in vivo assays use respirat ory, mammary, colon, bladder and skin cells. The in vitro assays use t he carcinogens benzo(a)pyrene (B(a)P) and 7,12-dimethylbenz(a)anthrace ne (DMBA), while the in vivo assays use B(a)P, DMBA, N-methyl-N-nitros ourea (MNU), N,N'-diethylnitrosamine (DEN), azoxymethane (AOM), and N- butyl-N-(4-hydroxybutyl)nitrosoamine (OH-BBN). There are vast differen ces in pharmacodynamics and pharmacokinetics in vitro and in vivo, yet it is possible to rapidly screen chemicals in vitro for efficacy at o ne-tenth the cost and complete tests in weeks instead of months. A pos itive in vitro assay was defined as a 20% inhibition (compared with co ntrol) for the RTE and A427 assays and a 60% inhibition for the MMOC a ssay at nontoxic concentrations. For in vivo assays, the criterion for a positive result was a statistically significant inhibition of incid ence, multiplicity or a significant increase in latency (mean time to first tumor). For an agent to be considered negative in animals, it re quired negative results in at least two different organ systems and no positive results. Using the battery of three in vitro tests, the posi tive predictive value for having one, two, or three positive in vitro assays and at least one positive whole animal test was 76%, 80%, and 8 3% respectively. The negative predictive values for one, two or all th ree in vitro assays was 25%, 27%, and 50%. From these data it is obser ved that in vitro assays give valuable positive predictive values and less valuable negative predictive values. The mechanisms of chemopreve ntion are not well understood. Seven categories of agents were examine d for their cancer preventing activity both in vitro and in vivo: anti inflammatories, antioxidants, arachadonic acid metabolism inhibitors, GSH inducers, GST inducers, ODC inhibitors, and PKC inhibitors. Three or even five in vitro assays cannot be all-inclusive of the many mecha nisms of cancer prevention. However, three assays help to predict whol e animal efficacy with reasonable positive predictive values. Much wor k and development remains to be done to rapidly identify new chemoprev entive drugs.