Function of the anthracycline amino group in cellular transport and cytotoxicity

Using a number of derivatives of doxorubicin (Adriamycin) and daunomycin, we have examined how substitution of the anthracycline amine affected net cellular accumulation and cytotoxic potency in HL-60 leukemia cells. Octanol/buffer partitioning demonstrated that each of the derivatives had an amino group titratable between pH 5 and 8, with the exception of derivatives containing a cyanomorpholino-substituted amine, which had a significantly lower pKa value. The steady state cellular drug levels for the Adriamycin and daunomycin series decreased in the following order: N,N-dimethyl-greater than morpholino-greater than parent greater than cyanomorpholino-. Thus, the net cellular accumulation of an anthracycline was found to be influenced by the basicity of the amino group; drugs with a non-basic amino group exhibited reduced uptake. Soft agar clonogenic assays showed the following order of cytotoxicity for both series: cyanomorpholino-much greater than parent greater than morpholino-approximately equal to N,N-dimethyl-. The data demonstrate an inverse correlation between uptake and potency; thus, differences in net cellular accumulation do not account for the order of anthracycline potency.