Intracellular activation of cytotoxic agents: kinetic models for methylnitrosoureas and N-methyl-N'-nitro-N-nitrosoguanidine in cell culture.

The cytotoxic activity of N-methyl-N-nitrosourea (MNU), streptozotocin, and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was determined in cell culture by using a P388 cell growth rate inhibition assay. These agents appear to have very different activities when inhibition is related to the agent concentration in the culture medium: ED50(C0) = 40 microM for MNNG to 875 microM for streptozotocin. The mechanism of action of these three agents involves conversion to the active methanediazonium ion and subsequent methylation of cellular macromolecules. As a consequence, the rates of conversion of the parent agent to the methylating species in the medium and within the cell are important parameters that also need to be considered to reach a more detailed understanding of the mechanism of action. In order to do this, a kinetic model has been developed to calculate the concentration of drug that is converted to active methylating species within the cell during the assay incubation period. The use of cell culture kinetic models was extended from simple compounds activated through solvolytic reactions (nitrosoureas) to an agent that undergoes selective intracellular activation (MNNG). By use of measured values for initial drug concentration, incubation time, and cell volume, as well as extracellular and intracellular chemical activation rate constants, the intracellular concentration, [P4], which represents the cumulative intracellular reaction products formed during the incubation period, was calculated and related to cytotoxicity. All three agents showed an ED50[P4] between 140 and 180 microM, and for MNNG, this ED50 was independent of extracellular sulfhydryl concentration.(ABSTRACT TRUNCATED AT 250 WORDS)