An analysis of 1-(2-chloroethyl)-1-nitrosourea activity at the cellular level

The effect of five different 1-(2-chloroethyl)-1-nitrosoureas on the growth of cultured P388 cells has been analyzed in terms of physical, chemical, and kinetic parameters that are related to the mechanism of action of this class of cancer chemotherapeutic agent. This study correlates structure with activity at the cellular level by using a dose function that is related to the amount of active species, the (2-chloroethyl)diazonium ion, that is formed during the period of exposure of cells to drug rather than to the initial drug dose. 1-(2-Chloroethyl)-1-nitrosourea analogues that rapidly enter the P388 cells are shown to have the same activity relative to the amount of active species formed. When analyzed in this way, activity is not influenced by the structure of the N-3 substituent, lipophilicity, or carbamoylating activity. The agents 1-(2-chloroethyl)-1-nitrosourea (CNU), 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea (PCNU), 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) all produce a 50% cell growth inhibition at 6 to 7 microM active species formed per cell volume. Chlorozotocin required a twofold higher effective dose to produce the same toxic effect. This decreased activity is attributed to the slow uptake of the water-soluble chlorozotocin into P388 and L1210 cells relative to the rate of chlorozotocin conversion to active species in medium. The yields to 2-chloroethanol from CNU, BCNU, and chlorozotocin were shown to be the same, indicating that these agents generate the same yield of alkylating intermediate at 37 degrees C and pH 7.4.