Nitric oxide and free stable nitroxyl radicals in the mechanism of biological action of the spin-labeled compounds.

A comparison of more important physical, chemical and biological properties of the nitric oxide (NO) and free stable nitroxyl radicals (nitroxides) on the base of their structural similarity is made in the article. The active moiety in the nitroxide molecule represents a sterically hindered nitric oxide. The mechanisms of biological action of the nitroxides and especially of their derivatives with antitumor agents from the groups of nitrogen mustards, nitrosoureas, aziridines and triazenes (spin-labeled compounds) is explained through the biological activities of sterically hindered NO. Similarly to NO, nitroxides also can react with superoxide anion radical (O(2)(-)), they possess superoxide dismutase (SOD) mimetic action. While the interaction of NO with O(2)(-)yields very toxic peroxynitrite (ONOO(-)), its formation is strongly limited in the presence of a nitroxide. It is known that the nitrosourea antitumor drugs, like lomustine (CCNU) and carmustine (BCNU), showed high general toxicity, one of the reasons for that probability is the formation of NO, and subsequently of ONOO(-), during their metabolism. The biological investigations of the nitroxides showed their considerably lower general toxicity that could be explained with the SOD-mimetic action of the nitroxide present in their molecule.

Activation of the anticancer drug CCNU into a free radical intermediate via UV irradiation--an ESR study.

We studied the formation of a free radical induced by UV irradiation of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) in benzene. It was determined a stable nitroxide radical by ESR spectroscopy. We confirmed that sterically hindered cyclic amine 2,2,6,6-tetramethyl-4-piperidone (TMPone) was transformed into a corresponding stable free nitroxyl radical during UV irradiation. When CCNU was present, the rate of free radical formation from TMPone increased considerably.

Nucleophilic targets in carcinogenesis, mutagenesis and chemotherapy of cancer.

A hypothesis is suggested, which emphasizes the role in carcinogenesis of the attack on low molecular nucleophilic substances (LMN) by electrophilic agents - chemical carcinogens, phisical factors, and antitumor alkylating agents. The significance of the degree of nucleophilicity (electronic charge, order of bonds, index of valence) as a locus minoris resistentiae of the LMN in the electrophilic attack on the latter is emphasized as well as the probable role of the hydrogenated pteridines in influencing carcinogenesis by means of ascorbate, tocopherol, SH-containing compounds etc. In support of this hypothesis the preference of electrophilic agents (derivatives of nitrogen mustard and nitrosoureas) for the places with highest degree of nucleophilicity as targets, in experiments in vitro with nucleic bases and pteridines is emphasized.

EPR study on the interaction of spin-labelled hydrazine mustard derivatives with DNA.

The interaction of three spin-labelled compounds, derivatives of bis-(2-chloroethyl)-hydrazine (HMSL), N-methyl,N-chloroethyl-hydrazine (MCEHSL) and bis-(2-bromoethyl)-hydrazine (BEHSL) with DNA was studied by the method of electron paramagnetic resonance (EPR). It was found that HMSL (containing two chloroethyl groups) and MCEHSL (containing one chloroethyl group) gave spin-labelled dsDNA with identical strongly immobilized EPR spectra. The conclusion was drawn that only one of the alkylating groups of HMSL reacted with DNA. In contrast, the EPR spectrum of DNA spin-labelled with BEHSL was non-immobilized due to the strong destabilizing effect of this compound on the double helix. The extent of alkylation of DNA with the three hydrazine mustard derivatives was one and the same. It was found, however, that chloroethyl-containing derivatives (HMSL and MCEHSL) had an expressed base specificity and alkylated preferably the guanilic residues, and their bromo-analogue (BEHSL) did not show any base specificity and alkylated the bases of DNA at random.