Additivity versus repair inhibition in fractionated treatments combining drugs and X rays: a theoretical analysis

Cobalt Radiation with or without Low-Dose Cisplatin for Treatment of Canine Naso-Sinus Carcinomas

The objective of this study was to determine if low-dose cisplatin could be added safely to radiation therapy for the treatment of naso-sinus carcinomas in dogs. Thirty-one dogs were evaluated; 18 of these dogs received cobalt radiation in combination with low-dose cisplatin while 13 dogs received radiation alone. No difference was observed for acute or late radiation effects. Cisplatin was administered at a dosage of 7.5 mg/m2 20 min prior to every other radiation treatment. An initial dose of 10 mg/m2 was intended but toxicity (primarily azotemia) was unacceptable. Cisplatin was administered as prescribed in 12 of 18 dogs. Cisplatin was discontinued in 2 dogs because of azotemia. In the other 4 dogs cisplatin was not administered as prescribed because the dogs were withdrawn from treatment due to disease progression or radiation effects. There was no long-term renal disease in patients who developed azotemia. The overall median survival was 433 days with 4 (12.9%) dogs still alive at the completion of the study.

Cisplatin-modification of DNA repair and ionizing radiation lethality in yeast, Saccharomyces cerevisiae

Cis-diamminedichloroplatinum II (cisplatin) is a DNA inter- and intrastrand crosslinking agent which can sensitize prokaryotic and eukaryotic cells to killing by ionizing radiation. The mechanism of radiosensitization is unknown but may involve cisplatin inhibition of repair of DNA damage caused by radiation. Repair proficient wild type and repair deficient (rad52, recombinational repair or rad3, excision repair) strains of the yeast Saccharomyces cerevisiae were used to determine whether defects in DNA repair mechanisms would modify the radiosensitizing effect of cisplatin. We report that cisplatin exposure could sensitize yeast cells with a competent recombinational repair mechanism (wild type or rad3), but could not sensitize cells defective in recombinational repair (rad52), indicating that the radiosensitizing effect of cisplatin was due to inhibition of DNA repair processes involving error free RAD52-dependent recombinational repair. The presence or absence of oxygen during irradiation did not alter this radiosensitization. Consistent with this result, cisplatin did not sensitize cells to mutation that results from lesion processing by an error prone DNA repair system. However, under certain circumstances, cisplatin exposure did not cause radiosensitization to killing by radiation in repair competent wild type cells. Within 2 h after a sublethal cisplatin treatment, wild type yeast cells became both thermally tolerant and radiation resistant. Cisplatin pretreatment also suppressed mutations caused by exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a response previously shown in wild type yeast cells following radiation pretreatment. Like radiation, the cisplatin-induced stress response did not confer radiation resistance or suppress MNNG mutations in a recombinational repair deficient mutant (rad52), although thermal tolerance was still induced. These results support the idea that cisplatin adducts in DNA interfere with RAD52-dependent recombinational repair and thereby sensitize cells to killing by radiation. However, the lesions can subsequently induce a general stress response, part of which is induction of RAD52-dependent error free recombinational repair. This stress response confers radiation resistance, thermal tolerance, and mutation resistance in yeast.

The influence of platinum drugs on the radiation response of rat kidneys

The influence of a single bolus injection of platinum drugs on the radiation sensitivity of the kidneys was investigated in WAG/Rij rats. Drugs employed were cis-diammine-dichloroplatinum(II) (cisplatin, CDDP), cis-diammine-1,1-cyclobutanedicarboxylate platinum(II) (carboplatin, CBDCA) and cis-dichloro,trans- dihydroxy-bis-isopropylamine platinum(IV) (iproplatin, CHIP). Both kidneys were irradiated with a range of single X-ray doses while drugs were administered at 1 day or 1 week before irradiation. Maximum tolerated drug doses (defined as the LD1, the dose resulting in a mortality of 1%) were given. Damage inflicted upon the kidneys was monitored by determination of several parameters indicative of kidney function. Isoeffective radiation doses were calculated from these data for each treatment group at 4-8-week intervals up to 80 weeks following treatment. At each assay time, dose modifying factors (DMF) were calculated for each drug/radiation combination. The mean DMFs were highest for CDDP: approximately 1.6. Those for CBDCA and CHIP were lower: approximately 1.1 and 1.2, respectively. The CHIP DMFs were significantly different from unity. When the radiation was given in 4 or 8 daily fractions (4 fractions/week) the DMFs for CDDP were identical to those obtained with single doses. For CBDCA and CHIP, however, the DMFs after fractionated treatments were not significantly different from unity. Analysis in terms of the linear-quadratic (LQ) model indicated that not one of the three drugs had an effect on the alpha/beta ratio, and hence on the fractionation sensitivity of the rat kidney. Consequently, if these data are extrapolated to the clinical setting, the administration of these drugs at the maximum tolerated dose preceding a fractionated radiation treatment should not be expected to result in extra, unexpected, radiation toxicity of the kidney.

A therapeutic benefit from combining normobaric carbogen or oxygen with nicotinamide in fractionated X-ray treatments

The ability of normobaric oxygen and carbogen (95% O2 + 5% CO2) combined with nicotinamide to enhance the radiosensitivity of two rodent adenocarcinomas and of mouse skin and kidneys, using a 10 fraction radiation schedule, was compared with the effect of radiation in air with and without the drug. Tumour response was assayed using local control and regrowth delay, and compared with acute skin reactions, decreased renal 51Cr-EDTA clearance and reduction in haematocrit. Nicotinamide increased the radiation sensitivity of CaNT tumours under all three different oxygen concentrations tested (21, 95 and 100% oxygen). The effect was statistically significant for oxygen and carbogen but not for air; the combination of nicotinamide with carbogen gave the greatest increase in tumour radiosensitivity. Relative to treatments in air without the drug, the enhancement ratios (ER) at the TCD50 level were 1.17, 1.65 and 1.83 for CaNT tumours irradiated in air, oxygen or carbogen and injected with nicotinamide 1 h before each fraction. The ER in CaRH tumours irradiated in carbogen plus the drug was 1.83, which was greater, but statistically not significantly different, to that seen with carbogen alone (ER = 1.68). In skin, relative to air without the drug, the increase in radiosensitivity by nicotinamide was greater in oxygen and carbogen than in air (1.29, 1.36 and 1.08, respectively). The ERs for both assays of renal damage were similar and lower than those in skin: less than or equal to 1.07, less than or equal to 1.13 and less than or equal to 1.16 for irradiations done in air, oxygen and carbogen plus nicotinamide, relative to air alone. A comparison of these results in the tumours and normal tissues showed that a significant therapeutic benefit was obtained with normobaric oxygen and carbogen combined with nicotinamide. This benefit is greater than observed with other radiosensitizers tested so far. Toxic side effects of the treatment are unlikely in a clinical situation, since prolonged administration of nicotinamide is well tolerated in man. The combination of normobaric carbogen with nicotinamide could be an effective method of enhancing tumour radiosensitivity in clinical radiotherapy where hypoxia limits the outcome of treatment.

The influence of cisplatin on the response of mouse kidneys to multifraction irradiation

The aim of these studies was to measure the extent of renal damage after fractionated irradiation in combination with cis-diamminedichloroplatinum(II) (c-DDP) and to determine whether there was any drug-induced inhibition of repair of sublethal X-ray damage. Fractionated irradiations were given either in a short overall treatment time of 2 days or in a total time of one month, in which irradiations were given in the first and fourth weeks only. A single dose of c-DDP was given before the first X-ray dose for treatments in a total time of 2 days and before the first X-ray dose of each week for the split-course treatments. Renal function was measured at monthly intervals (from 18 to 39 weeks after the start of treatment) by clearance of 51Cr-EDTA. The renal damage was always worse after combined treatments (X + c-DDP) than after X-rays alone. A comparison of radiation dose-response curves with and without c-DDP yielded Dose Enhancement Factors (DEFs) of 1.2 to 1.4 for all fractionation schedules, with no trend for an increase in DEF with increasing number of fractions. A Linear-Quadratic (LQ) analysis of the data demonstrated that there was no change in the alpha/beta value for renal radiation damage for the combined treatments compared with X-rays alone (alpha/beta = 2-3 Gy). These results suggest that the increased renal damage which occurred after combined X + c-DDP was due to independent, additive toxicities and not to radiosensitization or inhibition of X-ray repair by the drug.

Effect of simultaneous administration of bleomycin on the acute skin reactions of mice after single and fractionated doses of radiation

The effect of bleomycin on the acute mouse foot skin reactions, occurring after irradiation, was investigated. Bleomycin was delivered simultaneously with the irradiation treatment, either by intraperitoneal injection or by subcutaneous continuous infusion. Experiments were carried out to investigate (a) the modification by bleomycin of the response to single doses of radiation (b) the effect of total drug dose on this modification (c) the influence of the drug on repair of sublethal radiation damage and (d) the interference by the drug with compensatory repopulation after irradiation. In none of the experiments could any influence of the drug on these radiation induced reactions and recovery processes be demonstrated. These results are in direct contrast to what we have observed previously for similar types of experiments in another epithelial system, the mouse lip mucosa.