Analogues and analogue inhibitors of ppp(A2′p)_nA: Their stability biological activity

Higher Oligomers of ppp(A2′p)_n together with (A2′p)_nA, (A2′p)₂A3′OCH₃, (A2′p)₂A2′,3′CH₂, (A2′p)₂dA, (dA2′p)₂dA, their 5′‐monophosphates and 5′‐S‐methylphorothioates have been investigated for relative stability and biological activity in mouse and human cells and mouse, human, and rabbit cell‐free systems. The oligomers from trimer to heptamer inhibited protein and DNA synthesis when introduced into intace mouse cells and activated the ppp(A2′p)_n A‐dependent RNase at below nanomolar concentrations in mouse cell extracts. The 5′‐diphosphates pp(A2′p)₂A and corresponding analogues were active both in cell‐free systems and on introduction into intact cells. The exception to this was the all 3′‐deoxyadenosine analogue pp(dA2′p)₂ dA which failed to activate the ppp(A2′p)_nA‐dependent nuclease in the mouse L and human (Daudi and HeLa) cell extracts tested. Of the active analogues the 3′‐OCH₃ appeared to be the most stable in the cells and systems employed. On the other hand the non‐phosphorylated ‘core’ (A2′p)₂A and its 3′‐sunstituted analogues were inactive in mouse L and Ehrlich acites tumours cell‐free systems and had to effect on intact (non‐permeabilised) 3T3 cells. The 5′‐monophoshate, p(A2′p)₂A, can act as an inhibitor of ppp(A2′p)_nA in mouse cell‐free systems [Torrence, Imai and Johnston (1981) Proc. Natl Acad. Sci. USA, 78, 5993‐59997]. Here in intact mouse L cells or extracts from interferon‐treated human (Daudi) cells, however, it mimicked the action of ppp(A2′p)₂A, possibly through conversion to the 5′‐diphosphate or 5′‐S‐methylphosphorothioate derivatives of the 3′‐substituted analogues are both more stable to exonucleolytic cleavage and unlikely to be converted to the 5′‐diphosphated or 5′‐triphosphates. They are analogue inhibitors of ppp(A2′p)_nA in mouse L cell extracts. How widely they will be effective in a variety of cell‐free systems and intact cells remains to be established. The 5′‐diphosphate pp(A2′p)₂A and corresponding analogues were not equally active, nor was the CH₃Sp(A2′p)₂A2′,3′CH₂ equally effective as an analogue inhibitor, in different cell‐free systems. This emphasises the apparent differences in the properdties of the ppp(A2′p)_nA‐dependent RNases from different sources. Accordingly, in looking for a generally effective analogue inhibitor of ppp(A2′p)₂A its activity in a variety of extracts should be tested and in any search for further analogues for potential clincial use human cells and extracts should be employed.