**3.2 Influence of structural (stereochemical) features of 3**′**-fluoroanalogs 2-5A on human NK cell lytic activity**

The 2-5A system is not probably responsible for all interferon (INF) biological effects, however, varied evidence suggest that 2-5A, as well as 5′-dephosphorylated analogs are involved in various biochemical processes in animal cells including INF antiviral effect [35]. The INF ability to regulate lytic activity of parent killer cells (NK cells) [36, 37] is considered one of the key factors of INF antitumor effect [38]. It is noteworthy that NK cells are responsible for lysis of virtually any tumor and virally infected cells irrespective of antibodies or complement with no prior immunization needed [39, 40].

(2′-5′)Oligoadenylates, similarly to IFN, increase the NK cell lytic activity at an optimum concentration of 50 μM thus mimicking IFN action [41]. The study of effects produced by the parent trimer, A3, and 3′-deoxyadenosine analog (3′dA3) on NK cells showed that the increased NK cell activity is typical only for oligomers with a (2′-5′) phosphodiester bond. The (3′-5′)oligoadenylic acid trimer, (3′-5′)A3 produced no effect on NK cell lytic activity even at a concentration of 300 μM. Adenosine and 3′-deoxyadenosine at a concentration of 150 μM also did not change NK cell activity, which rules out the effect of trimers as depot forms of nucleosides. It was assumed that the stereochemistry of (3′-5′)- and (2′-5′) phosphodiester bonds caused differences in the effects on NK cell lytic activity of these oligomers [41].

Based on these data, similar activity of A3 and (3′dA)3 in respect to NK cells seems unexpected. Indeed, both trimers are widely different in their spatial structure [31, 42]—the parent trimer being a conformationally flexible molecule, and for (3′dA)3, only one spatial structure is predominantly occupied. Apparently, the (3′dA)3 molecule is not more rigid thermodynamically compared to the parent trimer, A3, and is capable of taking a spatial arrangement similar to that of the parent trimer when interacting with NK cells.

The effect of conformationally different molecules of *xylo* and *ribo* 3′-fluorodeoxyanalogs of (2′-5′)oligoadenylic acid on human parent killer (NK cells) was studied [43, 44]. Treatment of human effector NK cells with fluorodeoxyanalogs 2-5A has been generally shown to result in a significantly augmented cytotoxic activity toward target cells. Moreover, the degree of augmentation in NK cell activity varied significantly and depended on the conformation of the fluorodeoxyanalog (**Table 1**). Stereochemistry of (2′-5′)(A<sup>F</sup> )A2 and (2′-5′)A(AF )A trimers is determined by the predominant population of the furanose ring N-conformation and *syn*-orientation of the heterocyclic base around the glycosidic bond of AF fragment. Relevant *ribo*isomers, (2′-5′)(AF)A2 and (2′-5′)A(AF)A are similar to the parent trimer, (2′-5′)A3, having *anti-*conformation of all heterobases yet rigid S-conformation of AF furanose ring. All 2-5A *ribo*-fluorodeoxyanalogs were much more active on NK cells compared to *xylo*-analogs or the parent mediator, which is obviously due to a closer conformational resemblance of *ribo-* fluorodeoxyanalogs with the parent oligomer compared to isomeric *xylo-*analogs.
