Effects of bis(6-mercaptopurine-9-beta-D-ribofuranoside)-5',5"-phosphate and its butyryl derivative on mouse leukaemia L1210 and a 6-mercaptopurine-resistant subline in culture

Bis(6-mercaptppurine-9-β-d-ribofuranoside)-5′-5″′-monophosphate (bis(MPR)P) and its butyryl derivative, bis(O²,$\text{O}{}^3\text{-}\text{dibutyryl}\text{-6-}\text{mercaptopurine}\text{-9-β-}\text{d}\text{-}\text{ribofuranoside}$)-5′,5″′-monophosphate (bis(dibutyrylMPR)P) were synthesized from 6-mercaptopurine-9-β-d-ribofuranoside (MPR). Bis(MPR)P (ec₅₀ = 0.014 μM) and MPR (ec₅₀ = 0.022 μM) were essentially equivalent in their growth inhibitory activities against L1210/0 cell cultures, whilst bis(dibutyrylMPR)P (ec₅₀ = 1.1 μM) was considerably less effective. L1210/MPR cells grew normally in the presence of 1 mM MPR but were inhibited by bis(MPR)P (ec₅₀ = 580 μM) and (bis(dibutyrylMPR)P (ec₅₀ = 42 μM). Bis(dibutyrylMPR)P was less readily broken down to MPR by enzymes in the serum component of the culture medium than was bis(MPR)P, and leukaemia cells did not contribute to the extracellular degradation of the acylated derivative. The delayed cytotoxic effects of bis(MPR)P and bis(dibutyrylMPR)P on L1210/0 cells were those of the MPR breakdown product. Exposure to bis(MPR)P resulted in delayed cytotoxicity in L1210/MPR cultures whilst bis(dibutyrylMPR)P produced only acute growth inhibition and no delayed effect on the MPR-resistant subline. MPR was incorporated into DNA of L1210/0 cells as 6-thioguanine deoxyribonucleotide whilst bis(MPR)P was not incorporated into L1210/MPR cell DNA. These results suggested that the ultimate mechanisms of action of bis(MPR)P and bis(dibutyrylMPR)P in L1210/ MPR cells may have been different from that of MPR in sensitive L1210/0 cells and therefore might not represent true circumvention of resistance to MPR.