A comparison of the cellular uptake and binding of radiolabeled 4′-demethyl epipodophyllotoxin thenylidene-β-d-glucoside (VM-26) and 4′-demethyl epipodophyllotoxin ethylidene-β-d-glucoside (VP-16) has been made with the use of mouse leukemia L1210 cells. The initial velocity of influx measured at 2 min for VM-26 and VP-16 was 229 and 24 nmol/min/g dry weight, respectively. The steady-state intracellular accumulation of VM-26 and VP-16 measured after incubation for 15 min was 786 and 86 nmol/g dry weight, respectively. The initial velocity of uptake was linear for either drug over the range of extracellular concentrations studied (6 to 60 µm). The incremental increase in initial velocity of cellular uptake or oil/water partition coefficient for a 10° rise in temperature for uptake of either VM-26 or VP-16 was much smaller than that of drugs known to use carrier-mediated pathways. Furthermore, neither the initial velocity of uptake nor the steady-state accumulation was affected by NaN3, NaF, p-chloromercuribenzoic acid, or vinblastine. The efflux of VM-26 and VP-16 was monoexponential (half-life, 3 min) until a final plateau phase was reached. The accumulation of VM-26 and VP-16 at the plateau phase was 123 and 5 nmol/g dry weight, respectively. The residual plateau level was linearly dependent upon the initial extracellular drug concentration and could be increased by vinblastine but was not affected by incubation in high concentrations of podophyllotoxin aglycones, colchicine, or unradiolabeled VM-26 or VP-16. The intracellular accumulation of VM-26 was associated with various cell organelles. However, only the nuclear fraction demonstrated high-affinity binding. The intracellular accumulation of VM-26 by high-affinity binding paralleled the intracellular accumulation of low-affinity-binding drug.
↵1 This investigation was supported in part by USPHS Grant CA-13038.
↵2 To whom requests for reprints should be addressed, at Division of Oncologic Pharmacology, Department of Oncology, Florida Comprehensive Cancer Center, University of Miami School of Medicine, Miami, Fla. 33152.
- Received January 16, 1978.
- Accepted May 16, 1978.
- ©1978 American Association for Cancer Research.