EFFECTS OF 8-CHLORO-CYCLIC ADENOSINE-MONOPHOSPHATE ON THE GROWTH AND SENSITIVITY TO DOXORUBICIN OF MULTIDRUG-RESISTANT TUMOR-CELL LINES

We examined the in vitro effects of 8-chloro-adenosine 3':5'-monophosphate (8-Cl-cAMP), a reportedly stable, potent and site-selective analogue of cAMP, on the proliferation and sensitivity to doxorubicin (DXR) of two mouse cell lines, the B16 melanoma and Friend leukaemia, both as wild-type (B16, FLC) and DXR-resistant (B16/DXR, FLC/DXR) variants. The latter strains had characteristics of 'typical' multidrug resistance (MDR), including the over-expression of P-glycoprotein. Encouragingly, 8-Cl-cAMP affected almost equally the growth of the chemosensitive and chemoresistant variants of both cell lines. Its activity proved to be much more elevated on cells cultivated with fresh rather than heat-inactivated calf serum. In fact, the IC50 values for B-16 and B16/DXR were about 4.7 mu M in fresh serum and 215 mu M in heat-inactivated serum; the IC50 values for FLC and FLC/DXR were about 12 mu M in fresh serum and 70 mu M in heat-inactivated serum. Furthermore, experiments with B16 showed that cotreatments with isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor, or adenosine deaminase (ADA) greatly reduce the activity of 8-Cl-cAMP bringing it to comparable levels in fresh and heat-inactivated serum. These results indicate that the antiproliferative effects of 8-Cl-cAMP may be due principally to metabolites formed by the enzymic activities of the serum, most probably including 8-chloro-adenosine (8-Cl-adenosine), as suggested by other authors. Moreover, the dose-response curves and the ICS, values of the latter compound for the various cell lines were compatible with those observed for 8-Cl-cAMP in fresh serum. Finally, there was no evidence that 8-Cl-cAMP, either in the presence of fresh or heat-inactivated serum, or 8-Cl-adenosine may increase the sensitivity to DXR of the MDR variants of B16 melanoma and Friend leukaemia.