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A Fridland, MC Connelly and R Ashmun
Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee 38101.
The metabolism and cytostatic effects of 3'-azido-3'-deoxythymidine (AZT), one of the most effective agents being used in the treatment of acquired immunodeficiency syndrome, were investigated in the CCRF-CEM line of human T lymphoid cells. The concentration of drug required to inhibit cell growth by 50% (CD50) was significantly lower when the cells were exposed to AZT for 24 hr (CD50 = 50 microM), as compared with 48 or 96 hr (CD50 = 225 and greater than 300 microM, respectively). AZT at 25 microM blocked the progression of cells in S phase for about 12 hr, but this effect was reversed by 24 hr, despite the continued presence of drug in the medium. At this drug concentration, the level of dTTP decreased to about 75% of the control level by 4 hr but rebounded to 30% above normal by 8 hr of drug exposure. dGTP and dATP pool sizes were unchanged, whereas the dCTP pool increased 5-fold. The time course of these biochemical changes indicated that the onset of S phase arrest was not directly related to the decrease in deoxynucleoside triphosphate pools. CCRF-CEM cells incubated with 25 microM AZT accumulated about 0.9 mM 5'-monophosphate (AZTMP) after 4 hr whereas levels of the 5'-di- and 5'-triphosphates (AZTDP and AZTTP) plateaued at about 2 and 5 microM, respectively. After this period, there was a rapid decrease in AZTMP accumulation, to one third its initial level by 24 hr, whereas AZTDP and AZTTP pools decreased to only about 70%. The loss in AZT nucleotide formation with time of drug exposure was associated with a concomitant accumulation of AZTMP in the medium. Cellular excretion of AZTMP was not associated with any detectable cell lysis or leakage of other cellular metabolites. The ability of CCRF-CEM cells to excrete AZTMP may be an important factor limiting the biochemical and biological effects of the drug.
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