The mean graph patterns of many alkylating agents are similar to the eye. However, subtle differences in these patterns permit COMPARE to separate alkylating agents into various subtypes. Selected examples taken from COMPARE listings by using the standard agent database are provided, analyzed at the GI50 level.
Chloroethylating Agents
When methylcyclohexyl-(chloroethyl)-nitrosourea, a nitrosourea chloroethylating agent with carbamoylating potential, was used as the seed in the Table, the two closest matches were its analogues (all with carbamoylating potential), bis(chloroethyl)nitrosourea (BCNU) and cyclohexyl-(chloroethyl)-nitrosourea (CCNU). After CCNU came cyclodisone, a cyclic bissulfonate of ethylene glycol that presumably can ethylate DNA. Another chloroethylating nitrosourea, 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea (PCNU), appeared next. The fifth match was clomesone, the 2-chloroethyl ester of (methylsulfonyl)methanesulfonic acid. This is a chloroethylating agent that induces DNA interstrand cross-links (55, 56, 57), but it is not a nitrosourea and cannot carbamoylate. The sixth match was the nitrosourea chloroethylating agent chlorozotocin. The seventh ranked drug, the platinum complex CBDCA, is not a chloroethylating agent, although it does react with DNA (58). Mitozolamide is a chloroethylating agent but not a nitrosourea. The ninth match, triazinate, is an triazine antifolate with the sulfonyl fluoride group characteristic of Baker and Ashton's (59) active site directed irreversible inhibitors of dihydrofolate reductase.
Bioreductive Agents
Porfiromycin and mitomycin C are closely related in structure. Their activation as alkylating agents requires biological reduction (60), and their cytotoxicity is enhanced by hypoxia (61, 62). These similarities indicated that COMPARE should match them, and mitomycin C was the closest match to porfiromycin in the table. The third was AZQ, an aziridinyl benzoquinone known to be reduced in cancer cells (64, 65, 66, 67, 68, 69, 70).
Aziridines
At least 25 alkylating agents are among the 171 compounds in the database. Only five of these agents contain the aziridine moiety: porfiromycin, mitomycin C, AZQ, thio-tepa, and triethylenemelamine. Three of these agents were discussed previously (porfiromycin table). The remaining two, thio-tepa and triethylenemelamine, are trisaziridines. When thio-tepa was used as a seed in the table, triethylenemelamine appeared as a close match (PCC = 0.923). Only the bisepoxide dianhydrogalactitol and piperazinedione, an agent that potentially reacts through an aziridine intermediate, were closer to the seed. The next seven matches were various types of alkylating agents, with 0.848 the lowest PCC. Notably absent among the top-ranked matches to thio-tepa were the bioreductive aziridines ( porfiromycin table) and the chloroethylating agents (methyl-CCNU table).