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The stability of the P388 mouse leukemia model over 15 years was examined. Eleven natural products and one synthetic compound (5-FU) tested repetitively under the same conditions of dose and schedule were associated with no significant change in their activity. The P388 model, used in conjunction with other in vivo and in vitro tumor models, continues to be of value in the anticancer drug screening process.

A major problem related to in vitro testing of new investigational anticancer compounds is the lack of accurate pharmacokinetic data for the drugs in man. Based on the concept of a relationship between certain toxicologic endpoints in animal systems and maximally tolerated doses in man, we hypothesized that a comparable agreement might exist between these experimental animal toxicology data and clinically achievable peak plasma concentrations (PPCs). A retrospective analysis of the known data pairs of 28 commonly used cytotoxic compounds supports the existence of a reasonably good correlation between ip LD50 values (Pearson test) in nontumored mice and PPCs in man (r2 = 0.501; P less than 0.0001). Our data suggest that by use of the resultant statistical regression model a rational starting point can be selected for in vitro screening of entirely new agents for which human PPCs are not available. Additionally, application of this approach may also prove useful in relation to selecting in vitro doses for chemosensitivity assays intended for predictive correlation with clinical response in cancer patients.

Data are presented from two cell culture systems which support the notion that a multi-drug-resistant phenotype occurs in human tumor cell populations. Human small cell lung cancer cell lines derived from patients in relapse following intensive combination chemotherapy demonstrate broad cross-resistance to nine standard drugs in vitro. However, analysis of [14C]glucosamine-labeled glycoproteins in the small cell lung cancer cell lines failed to identify any consistent association between a specific glycoprotein marker and the drug-resistant phenotype. Evaluation of drug sensitivity of human tumor cells in primary culture (colony-forming assay) has indicated that multidrug-resistant cells may be present in tumor cell populations even in the absence of prior drug therapy. Several features of the multidrug-resistant phenotype, as observed in these human tumor cell populations, differ from those observed in Chinese hamster cell systems. In particular, the variability in patterns of resistance to various agents and in expression of glycoprotein markers suggests that a substantial amount of genetic heterogeneity underlies this phenotype in human tumors.