Thanks @conudel. Agree to some extent. It looks like the most potent among all N-nitroso piperazines is 1-methyl-4-nitrosopiperazine. There is difference between 1,2,6-trimethyl-4-nitrosopiperazine and 1-methyl-4-nitrosopiperazine in terms of carcinogenicity.
Structurally 1-methyl-4-nitrosopiperazine looks more closer to N-nitroso Ciprofloxacin.
with absence of beta methyl substituents.