The objective of any Master Agreement Orders issued under this Master Agreement will be to determine the probable mechanism(s) of action of potential antitumor agents which ar of interest to the Developmental Therapeutics Program (DTP). The majority of compounds to be studied will have been identified by the DTP in vitro human tumor cell line screen. Those compounds demonstrating specific differential cytotoxic and/or growth inhibitory effects will be considered for further evaluation. Elucidation of the biochemical mechanism of action of such agents will be used to help set priorities for development of compounds to clinical trial. Compounds may also be selected for mechanistic evaluation on the basis of their antimetastatic, photosensitizing or radiosensitizing activities. Thus, the specific biochemical studies performed may vary greatly from compound to compound. Mechanistic evaluations will be conducted wherever possible using cell lines from the in vitro screen. Studies may be performed with cultured cells, cellular extracts, purified cellular components, and/or intact animals and isolated tissues. Specific biochemical effects to be evaluated may include induction of DNA damage, perturbation of RNA processing, changes in nucleotide pool sizes, DNA binding, inhibition of specific cellular enzymes (e.g., DNA polymerases, primases, and methylases; enzymes of de novo purine and pyrimidine synthesis; salvage enzymes; topoisomerases I and II; repair enzymes), inhibition of macromolecular synthesis, effects on tubulin and mitotic spindle, alterations in free radical formation and interactions with cell membrane components such as protein kinase C, G proteins and other receptors involved in signal transduction, growth regulation, differentiation, and oncogene expression.