1-β-D-arabinofuranosyl-cytosine (ara-C) is one of the most effective agents in the chemo-therapy of acute non-lymphocytic leukemia (ANLL). Herein, the effects of uracil(U), deoxyuridine (UdR) and uridine (UR) on the anti-tumor activity of ara-C and on ara-C accumulation in the cells were studied. Growth-inhibition activities of ara-C alone and in combination with U, UdR and UR were determined by the trypan blue method. Cell-killing activities against MOLT-4, HL60, human leukemic progenitors (L-CFU) and human colony forming units (G-CSF) were determined by a colonogenic assay. The growth-inhibition activity of ara-C against MOLT-4 and HL60 was not enhanced by U or UdR. On the other hand, 10(-3)mol UR enhanced the growth-inhibition activity of ara-C against both MOLT-4 and HL60. The 50% inhibition dose (ID50) of ara-C was 6.0×10(-7)mol for MOLT-4 and 4.0×10(-7)mol for MOLT-4 and HL60. On the other hand, in the culture medium containing 10(-3)mol UR ID50 was 3.0×10(-8)mol for MOLT-4 and HL60. Cell-killing acticvity of ara-C was enhanced by 10(-3)mol UR. The 50% lethal dose (LD50) of ara-C for MOLT-4 and HL60 was decreased from 9.0×10(-7)mol to 5.0×10(-8)mol and from 5.0×10(-7)mol to 5.0×10(-8)mol after a 72-hour exposure to 10(-3)mol of UR, respectively. Cell-killing activity of ara-C against L-CFU was enhanced by 10(-3)mol UR in 4 of the 9 ANLL patients. On the other hand, the cell-killing activity of ara-C against G-CSF was enhanced in 2 of the 9 healthy individuals. 10(-8)mol ara-C, UR enhanced the cell-killing activity against L-CFU in 2 of the 9 ANLL patients, but not for G-CSF. Accumlation of (3)H-sra-C in MOLT-4 cells at 12, 24 and 48 hours was significantly increased in culture medium containing 10(-8)mol of (3)H-ara-C and 10(-3)mol of UR. Accumulation of 3H-ara-C in HL60 cells at 24 and 48 hours was also significantly increased. It is noteworthy that the cell-killing activity of ara-C against not only human lymphoid and myeloid leukemic cell lines but also L-CFU was enhanced by the combination with a nucleoside (UR), but not with anti-lrukemic agents. These findings provide some information on the enhancement of the anti-tumor activity and mechanims of resistance of ara-C.
enhancement of antitumor effect
in vitro chemotherapy