Preferential radiosensitization of G1 checkpoint-deficient cells by methylxanthines.

PURPOSE: To develop a checkpoint-based strategy for preferential radiosensitization of human tumors with deficient and/or mutant p53. METHODS AND MATERIALS: A549 human lung adenocarcinoma cell lines differing in their expression of the p53 tumor suppressor gene were produced by transduction with the E6 oncogene from human papilloma virus type 16. The cells expressing E6 (E6+) lack a G1 arrest in response to ionizing radiation, are deficient in p53 and p21 expression, and exhibit a fivefold greater clonogenic survival following 10 Gy radiation. RESULTS: Postirradiation incubation with millimolar concentrations of the methylxanthine pentoxifylline (PTX) results in preferential radiosensitization of the E6+ cells compared to the LXSN+ vector transduced controls. There is a threefold sensitization of the LXSN+ cells and a 15-fold sensitization of the E6+ cells, which results in equal clonogenic survival of the two lines. Flow cytometry reveals PTX abrogation of the radiation induced G2 arrest for both cell lines. PTX also prolongs G1 transit for both cell lines. Preliminary results are presented using a novel methylxanthine, lisofylline (LSF), which has similar cell cycle effects on G1 and G2 and achieves differential radiosensitization at micromolar concentrations that are sustainable in humans. CONCLUSION: This checkpoint-based strategy is a promising approach for achieving preferential radiosensitization of p53- tumors relative to p53+ normal tissues.

Abrogation of the G2 checkpoint results in differential radiosensitization of G1 checkpoint-deficient and G1 checkpoint-competent cells.

We have examined the effect of abrogation of the G2 checkpoint on the radiosensitivity of G1 checkpoint-proficient and G1 checkpoint-deficient cells. A549 human lung adenocarcinoma cells were transduced with the E6 oncogene of the human papillomavirus type 16 to eliminate their radiation-induced G1 arrest. These E6+ cells exhibited a dose-dependent increase in radiation resistance compared to control A549 cells transduced with the vector alone. Treatment (96 h) with 2 mM caffeine resulted in an abrogation of the cellular G2 checkpoint in both E6+ and control cells and a differential radiosensitizing effect on the two cell lines such that the E6+ clones and the vector controls became equally radiosensitive. These data show that human tumors which are radioresistant due to the loss of the p53-mediated G1 checkpoint can be made radiosensitive by abrogation of the G2 checkpoint. The implications of these results for cancer therapy are discussed.

Inhibition of radiation cataractogenesis by WR-77913.

The radioprotective drug S-3-amino-2-hydroxypropylphosphorothioic acid (WR-77913) has been tested as an inhibitor of radiation cataractogenesis. Animals treated with 15 Gy whole-head 137Cs gamma radiation developed mature cataracts 10-12 weeks after irradiation. Intraperitoneal pretreatment with 815 mg/kg WR-77913 30 min before irradiation delayed the development of cataracts; mature cataracts required 42 weeks for development. Doses as low as 350 mg/kg, substantially below the toxic range, resulted in graded but incomplete protection and a significant delay in the development of cataracts. Drug treatment combined with radiation doses of 12.5 or 10 Gy showed less pronounced protection. The optimum time of drug delivery was found to be between 30 min and 2 h before irradiation; protective action diminished if longer times were used or if the drug was given after irradiation. These results are discussed in relation to those obtained with other chemical radioprotective agents and in terms of possible mechanisms of the action of the drug.

Prostate specific antigen in the management of patients with localized adenocarcinoma of the prostate treated with primary radiation therapy.

The records of 143 patients treated at 5 institutions with external beam megavoltage irradiation for localized prostatic cancer were reviewed to evaluate post-treatment changes in prostate specific antigen (PSA) in the context of subsequent events. Complete responders were defined as patients clinically well with normal PSA, clinical failures were patients with documented local tumor recurrence or distant metastases and chemical failures were patients clinically well but with a PSA level above the upper limits of normal. Correlations with pre-treatment PSA values were also made for the 50 of 143 patients for whom pre-treatment PSA data were available. Median patient followup was 27 months (range 18 to 91 months). The data were analyzed with parametric and nonparametric univariate and multivariate statistical procedures. Pre-treatment PSA levels increased with increasing tumor stage (p = 0.004) but not with increasing summed Gleason pattern scores (p = 0.15). The probability of remaining a complete responder decreased with increasing stage (p = 0.008) but not with increasing Gleason score (p = 0.14). Increasing pre-treatment PSA correlated with clinical failure (p = 0.01) and chemical failure (p = 0.006). Of the patients with a pre-treatment PSA level of less than 4 times the upper limits of normal 83% remained as complete responders compared to 30% of those with a higher pre-treatment PSA (p = 0.0002). The return of PSA levels to the normal range within 6 months after treatment was strongly correlated with a favorable outcome when analyzed by multivariate logistic regression. The status at last followup of patients who had a normal PSA level at 6 months versus those with an elevated PSA level 6 months after treatment is 94% versus 8% for complete responders (p = 0.0001), 0% versus 60% for clinical failures (p = 0.002) and 6% versus 32% for chemical failures (p = 0.14). Similar results occurred when analyzing outcomes in relationship to PSA normalization within 12 months after treatment (p = 0.001 for clinical failures, p = 0.02 for chemical failures and p = 0.001 for complete responders). We conclude that the pre-treatment level of PSA is an independent prognostic factor for prostate cancer patients treated with primary radiation therapy, and that the failure of PSA to return to the normal range within 1 year after completion of treatment identifies a group of patients at high risk for tumor recurrence.

Transient and chronic neurological complications of fast neutron radiation for adenocarcinoma of the prostate.

The records of 132 patients participating in clinical trials using fast neutron (n = 94), mixed neutron and photon (n = 16), or conventional photon (n = 22) irradiation for primary management of prostatic cancer were retrospectively reviewed to assess treatment-related neurological complications. With a median follow-up of 14 months (range 1 to 101 months), 31/132 patients (26 neutron, 3 mixed beam, 2 photon) have experienced either sciatica beginning during or shortly after treatment, or diminished bladder or bowel continence that developed at a median time of 6.5 months following treatment. Sciatica responded to oral steroids and was usually self-limited, whereas sphincter dysfunction appears to be permanent. Pre-treatment risk factors for complications included a history of hypertension, diabetes, cigarette smoking or peripheral vascular disease, with 81% of affected patients having one or more risk factors compared with 55% of unaffected patients (p = 0.01). Seven patients have moderate (5) or severe (2) residual problems, all in the cohorts receiving neutrons (6/7) or mixed beam therapy (1/7).