In vitro radiosensitization by pentoxifylline does not depend on p53 status

PURPOSE

The mode by which the xanthine derivative, pentoxifylline, induces a radiosensitizing effect in cell cultures is a key and controversial radiobiological issue and requires further elucidation.

MATERIALS AND METHODS

Six human glioblastoma cell lines were tested for the effect of pentoxifylline treatment at maximum G2/M block on the basis of cell survival, mitotic activity, and micronucleus formation after exposure to gamma radiation. Cell survival was measured by the colony-forming assay. Micronucleus formation (an indicator of DNA damage) and the proportion of binucleated cells (a representation of mitotic activity) were determined using the cytokinesis-block assay.

RESULTS

Remarkably, exposure to a single dose of 4 Gy produced strong G2/M blocks in both p53 mutant and wild-type cells. Addition of pentoxifylline at the peak of radiation-induced G2/M blocks resulted in a p53-independent reduction in cell survival in all cell lines. This radiosensitization was strongly correlated with the magnitude of the radiation-induced G2/M block. The changes observed in mitotic activity and micronucleus yield were also p53-independent.

CONCLUSIONS

These results are at variance with the view that pentoxifylline preferentially sensitizes p53 mutant cells, and that sensitization occurs only when cells are irradiated in the presence of the drug. The data suggest that the effectiveness of pentoxifylline as radiosensitizer depends on the proportion of cells that are arrested in the G2/M phase transition following exposure to ionizing radiation. These findings can assist in the identification of cancers that may benefit from therapies using G2/M checkpoint abrogators.

Changes in lognormal shape parameter guide design of patient-specific radiochemotherapy cocktails

Uptake of radiopharmaceuticals and chemotherapeutic drugs is nonuniform at the microscopic level. Their distributions are typically lognormal, suggesting that failure in chemotherapy and targeted radionuclide therapy may be attributable, in part, to the characteristics of this biologically ubiquitous distribution. The lognormal problem can be overcome by using cocktails of 2 or more agents, tailored such that at least 1 agent is strongly incorporated by every cell in the target population. Therefore, critical assessment of the tissue uptake of each cocktail component is warranted.

METHODS

Cellular incorporation of the α-particle-emitting radiochemical ((210)Po-citrate) and 2 anticancer drugs (daunomycin and doxorubicin) was determined using flow cytometry. The role of their lognormal distribution in clonogenic cell survival was evaluated.

RESULTS

The shape parameter of the lognormal distribution was found to be correlated to both intracellular agent concentration and cell survival. Although no difference emerged between the shape parameters for citrate within the first 2 logs of cell kill, those for daunomycin and doxorubicin changed significantly.

CONCLUSION

Changes in the value of the lognormal shape parameter and slope of the cellular drug uptake curves can be used to rapidly screen radiopharmaceuticals and other cytotoxic agents to formulate more effective cocktails for cancer therapy.