interaction of platinum drugs with clinically relevant x-ray doses in mammalian cells: a comparison...

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Inr. J. Radiorron Oncology Bid Phys., Vol. 20. pp. 221-225 036@3016/91 $3.00 + .oO Printed in the U.S.A. All ri&s reserved. copyright 0 I99 I Pelgamon Prm plc l Session I INTERACIION OF PLATINUM DRUGS WITH CLINICALLY RELEVANT X-RAY DOSES IN MAMMALIAN CELLS: A COMPARISON OF CISPLATIN, CARBOPLATIN, IPROPLATIN, AND TETRAPLATIN KIRSTEN SKOV, PH.D. AND SUSAN MACFHAIL, M.Sc. Medical Biophysics Unit, B.C. Cancer Research Centre, 601 West 10th Ave., Vancouver, B.C., V5Z IL3 Canada Whereas the interaction between radiation and platinum complexes has never been pronounced in radiobiological experiments (to 30 Gy in mammalian cells), there have been reports of interest in this combination in the clinic, where fractionated doses of approximately 2 Gy are used. Our studies on the marked interaction in hypoxia at the 80% survival level (l-2.5 Gy) with cisplntin have been extended to second generation platinum drugs of clinical interest. The studies in the lower radiation dose region have been facilitated by the use of the cell analyzer DMIPS to identify individual cells and follow them microscopically to assess for clonogenic ability. Chinese hamster V79 ceils were used, which were exposed to drug for 1 hr prior to irradiation in hypoxia (or air). None of the drugs give an enhancement ratio (ER) greater than 1.3 in the high radiation dose region, whereas all can produce Eb (ER calculated at iso-survival of 80%) of 2 or higher at low doses in hypoxic cells. The enhancement of radiation kill in oxic V79 cells (ER’s to 1.1 at l-24 S) disappears at low doses (E%oa, = 1.0) except for tetraplatin, where a moderate E& (to 1.64) was measured. Comparison of the hypoxic interaction on a concentration basis suggests that cisplatin is the best drug at low x-ray doses and low concentrations, but the interaction reaches a plateau at ER,- 2.0. Tetraphttin continues to give better interaction with increasing concentration (up to EL = 3.7 at 25 PM). Interaction of radiation with the less toxic drugs, iproplatin and carboplatitin, used at around 100 PM can be improved by longer exposure times prior to irradiation. Comparison on the basis of toxicity, for which the plating efficiency was used, suggests that cisplatin gives a better interaction than the three newer drugs for a given level of toxicity in hypoxic V79 cells. Hypoxia, Low doses, Irradiation, Interaction, V79 cells, Sensitization, Iproplati, Cisplatin, Carboplatin, Tetraplatin. INTRODUCTION Radiosensitization by cisplatin (cispt) and analogues is generally reported to be low to moderate in hypoxic cells, when assessed in radiobiology laboratories using mam- malian cells: enhancement ratios (ER’s) only to 1.3 are obtained at 1% survival, or up to 30 Gy; some effect on the response of oxic cells is often reported (3, 4, 10). Re- suits emerging from clinical trials where much lower doses are used have recently been reviewed (3, 4) and are not conclusive. In the laboratory, recent studies on cisplatin (9) and its tram (19) isomer show that both platinum complexes are better sensitizers in the low dose region in hypoxic cells; for example, enhancement ratios up to 2.2 troimidazoles (13, 18, 20), and even oxygen itself, (12, 15) which are less effective in the clinically relevant ra- diation dose range. The oxic interaction of cisplatin or its trans isomer with X rays actually disappears at low doses (ERIs - 1.1; ERsos = 1 .O) (9). The mechanisms of these interactions are not yet understood, but preliminary ex- periments did not support the possibility that it was a cell cycle effect; inhibition of repair of damage induced by X rays in hypoxia is a possibility, as full sensitization was achieved if the drug were added after hypoxic irradiation, in the low dose region only. These studies in which the ER’s are the ratios for isoeffects (at 2% or 80% surviving fraction) have now been extended to include other plat- inum complexes of clinical interest: at 4-5 PM cisplatin were obtained at 80% survival (l-4 Gy). This is unlike the electron affinic sensitizers, the ni- Carboplatin, the second generation Pt” complex (JM- 8; CBDCA; paraplatin; cisdiammine-( 1, l-cyclobutane- Presented at the Third International Conference on the In- teraction of Radiation Therapy and Systemic Therapy, Asilomar Conference Center, Monterey, CA 9- I2 March 1990. Reprint requests to: K. A. Skov. Acknowledgements-We are grateful to Dr. M. Korbelik for his input at the beginning of these studies and to his colleagues (14) in Cancer Imaging for making the Cell Analyzer available to us. 221 The platinum drugs were gifts from M. Abrams, Johnson- Matthey Inc., West Chester, PA (JM-8 and JM-9), P. Simpson, Bull Laboratories, Mulgrave Australia (cisplatin) and N. Lomax, Drug Synthesis & Chemistry Branch, Division of Cancer Treat- ment, National Cancer Institute, Bethesda, MD, (tetrapIatin). These studies are supported by NC1 of Canada (#1707). Accepted for publication 24 August 1990.

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Page 1: Interaction of platinum drugs with clinically relevant X-ray doses in mammalian cells: A comparison of cisplatin, carboplatin, iproplatin, and tetraplatin

Inr. J. Radiorron Oncology Bid Phys., Vol. 20. pp. 221-225 036@3016/91 $3.00 + .oO

Printed in the U.S.A. All ri&s reserved. copyright 0 I99 I Pelgamon Prm plc

l Session I

INTERACIION OF PLATINUM DRUGS WITH CLINICALLY RELEVANT X-RAY DOSES IN MAMMALIAN CELLS: A COMPARISON OF CISPLATIN,

CARBOPLATIN, IPROPLATIN, AND TETRAPLATIN

KIRSTEN SKOV, PH.D. AND SUSAN MACFHAIL, M.Sc.

Medical Biophysics Unit, B.C. Cancer Research Centre, 601 West 10th Ave., Vancouver, B.C., V5Z IL3 Canada

Whereas the interaction between radiation and platinum complexes has never been pronounced in radiobiological experiments (to 30 Gy in mammalian cells), there have been reports of interest in this combination in the clinic, where fractionated doses of approximately 2 Gy are used. Our studies on the marked interaction in hypoxia at the 80% survival level (l-2.5 Gy) with cisplntin have been extended to second generation platinum drugs of clinical interest. The studies in the lower radiation dose region have been facilitated by the use of the cell analyzer DMIPS to identify individual cells and follow them microscopically to assess for clonogenic ability. Chinese hamster V79 ceils were used, which were exposed to drug for 1 hr prior to irradiation in hypoxia (or air). None of the drugs give an enhancement ratio (ER) greater than 1.3 in the high radiation dose region, whereas all can produce Eb (ER calculated at iso-survival of 80%) of 2 or higher at low doses in hypoxic cells. The enhancement of radiation kill in oxic V79 cells (ER’s to 1.1 at l-24 S) disappears at low doses (E%oa, = 1.0) except for tetraplatin, where a moderate E& (to 1.64) was measured. Comparison of the hypoxic interaction on a concentration basis suggests that cisplatin is the best drug at low x-ray doses and low concentrations, but the interaction reaches a plateau at ER,- 2.0. Tetraphttin continues to give better interaction with increasing concentration (up to EL = 3.7 at 25 PM). Interaction of radiation with the less toxic drugs, iproplatin and carboplatitin, used at around 100 PM can be improved by longer exposure times prior to irradiation. Comparison on the basis of toxicity, for which the plating efficiency was used, suggests that cisplatin gives a better interaction than the three newer drugs for a given level of toxicity in hypoxic V79 cells.

Hypoxia, Low doses, Irradiation, Interaction, V79 cells, Sensitization, Iproplati, Cisplatin, Carboplatin, Tetraplatin.

INTRODUCTION

Radiosensitization by cisplatin (cispt) and analogues is generally reported to be low to moderate in hypoxic cells, when assessed in radiobiology laboratories using mam- malian cells: enhancement ratios (ER’s) only to 1.3 are obtained at 1% survival, or up to 30 Gy; some effect on the response of oxic cells is often reported (3, 4, 10). Re- suits emerging from clinical trials where much lower doses are used have recently been reviewed (3, 4) and are not conclusive. In the laboratory, recent studies on cisplatin (9) and its tram (19) isomer show that both platinum complexes are better sensitizers in the low dose region in hypoxic cells; for example, enhancement ratios up to 2.2

troimidazoles (13, 18, 20), and even oxygen itself, (12, 15) which are less effective in the clinically relevant ra- diation dose range. The oxic interaction of cisplatin or its trans isomer with X rays actually disappears at low doses (ERIs - 1.1; ERsos = 1 .O) (9). The mechanisms of these interactions are not yet understood, but preliminary ex- periments did not support the possibility that it was a cell cycle effect; inhibition of repair of damage induced by X rays in hypoxia is a possibility, as full sensitization was achieved if the drug were added after hypoxic irradiation, in the low dose region only. These studies in which the ER’s are the ratios for isoeffects (at 2% or 80% surviving fraction) have now been extended to include other plat- inum complexes of clinical interest:

at 4-5 PM cisplatin were obtained at 80% survival (l-4 Gy). This is unlike the electron affinic sensitizers, the ni-

Carboplatin, the second generation Pt” complex (JM- 8; CBDCA; paraplatin; cisdiammine-( 1, l-cyclobutane-

Presented at the Third International Conference on the In- teraction of Radiation Therapy and Systemic Therapy, Asilomar Conference Center, Monterey, CA 9- I2 March 1990.

Reprint requests to: K. A. Skov. Acknowledgements-We are grateful to Dr. M. Korbelik for his input at the beginning of these studies and to his colleagues (14) in Cancer Imaging for making the Cell Analyzer available to us.

221

The platinum drugs were gifts from M. Abrams, Johnson- Matthey Inc., West Chester, PA (JM-8 and JM-9), P. Simpson, Bull Laboratories, Mulgrave Australia (cisplatin) and N. Lomax, Drug Synthesis & Chemistry Branch, Division of Cancer Treat- ment, National Cancer Institute, Bethesda, MD, (tetrapIatin).

These studies are supported by NC1 of Canada (#1707). Accepted for publication 24 August 1990.

Page 2: Interaction of platinum drugs with clinically relevant X-ray doses in mammalian cells: A comparison of cisplatin, carboplatin, iproplatin, and tetraplatin

222 I. J. Radiation Oncology 0 Biology 0 Physics February 199 1, Volume 20, Number 2

dicarboxylato)platinum(II)) which differs from cisplatin in that the chloride leaving groups are replaced with car- boxyls joined through a butane ring; this change affects the kinetics of the reaction with DNA but not necessarily the final products (8). The interaction of this complex with radiation has been reported (6, 11) and clinical trials are in progress (3).

Iproplatin, a platinum (IV) complex (known also as CHIP; JM-9; cisdichloro-trans-dihydroxo-cis-bis(isopro- pylamine)platinum (IV)) which has also reached clinical trials, has also been examined in vitro as a radiosensitizer (10, 11).

Tetraplatin, a dach complex of PttV, (TPN; tetrachloro 1,2-diaminocyclohexane platinum (IV)), can exist as two isomers due to the optical activity at the 1,2 carbon centers of the cyclohexane rings (1). The activity of this complex is receiving considerable interest ( 17), and has recently been compared with the above drugs in several human carcinoma lines (7).

METHODS AND MATERIALS

The platinum drugs were donated; fresh solutions were prepared just prior (45 min) to each experiment:’ The methods used to compare radiation response in the low and high dose regions were as previously described (9) and references therein. Briefly, V79 cells were grown as monolayers in Eagle’s MEM with 10% fetal bovine serum*, trypsinized and suspended (1.5 X lo5 cells/ml) in medium for 1 hr at 37°C. Irradiation vessels with freshly prepared solutions of the drugs at prescribed concentra- tions were placed in a 37°C water bath and gassed (1 l/ min) with humidified oxygen-free nitrogen+ for at least 45 min before the addition of a concentrated cell suspen- sion. After incubation with or without drug and with or without nitrogen flow at 37°C (for 60 min unless otherwise noted), the vessels were individually irradiated at 4°C with graded x-ray doses with continuous stirring and nitrogen flow (except for the oxic response). After each dose in- crement, samples were taken, diluted, and the unbound drug was removed by centrifugation and resuspension of the cells in fresh, cold medium, which was followed by plating of the cells for colony growth (i.e., identical sus- pensions and treatment for both dose ranges).

points in the two dose ranges were the same, within ex- perimental error. The error using the standard method is larger (+ 10%) because of uncertainties regarding the actual number of cells plated (pipetting error, Coulter counts that include debris, doublets, etc. resulting in control plating efficiencies (PE) of 65-85%) whereas the identi- fication of each individual cell by microscopic methods, and revisiting to assess survival (? 1%) give high plating efficiencies for control cells (90%). The survival levels were corrected for toxicity of the drugs (as shown in Table 1). Toxicity is expressed as PE rather than converting to sur- viving fraction for cells which were not irradiated in order for comparison purposes. Enhancement ratios were de- termined at 80% (ERsos) and 2% (ERw) survival levels for comparison in the two dose ranges where ER = (x- ray dose in control to produce given S)/(dose with drug to give same S). Data points are averages of 3-10 exper- iments (except for some data in Table 1, shown without errors, which are from one to two experiments and some of the cisplatin points which agree within experimental error with previous data from this laboratory (9)). Data obtained in the two overlapping dose ranges were fitted using an LY/~ linear quadratic equation.

RESULTS

As has been shown previously ( 12, 15,20), the oxygen effect is lower at low doses (OERsos = 2.6 cf OER*% = 2.9 in this study). Figure 1 shows the effect of drug concen- tration on the enhancement ratios for all four drugs at high (solid symbols) and low (open symbols) x-ray doses.

Cisplatin: Results from a previous study (9) showed that at concentrations of l-2.5 PM which produce very little toxicity and no effect at high doses (ERlm, = 1.0) cisplatin gives good low dose enhancement ratios (to 2.2), but the ERsos’s do not further increase over 5 PM. Very low concentrations (0.1 PM) still give measurable effect ( 1.05). Prolonged incubation prior to irradiation does not markedly affect the ERsW for cisplatin (unlike carboplatin, below), but does increase toxicity (Table 1).

Radiation responses at high doses (to 25 Gy) were as- sessed using conventional clonogenic methods. The sur- vival of cells irradiated to low doses (up to 4 Gy for ni- trogen controls) was determined using a cell analyzer ( 14)$ to follow 300-450 individual cells, per sample, from the time of plating to the time of colony scoring 4-6 days later. In both dose ranges, a survivor was defined by a colony of 50 or more cells on day 6. As described previ- ously (9), the survival levels thus obtained at overlapping

Carboplatin: On a molar basis, this second generation Pt” complex is less toxic than cisplatin, and thus is used at a higher level in both the laboratory and the clinic. At 100 PM, carboplatin gives an ERsosb of 2.0 after 1 hr in- cubation (comparable to 2 PM cisplatin). Longer incu- bation with carboplatin before irradiation markedly im- proves the interaction at low doses to ER - 2.7 (Fig. 2), giving data point (*) shown in Figure 1.

Tetraplatin shows the most marked effect at low doses, with an ERsoYO - 3.7 at 25 PM (Fig. 3). This concentration gives a plating efficiency of 0.15-0.30 after 1 hr at 37°C. For tetraplatin, like cisplatin, the high dose ER2% reaches a plateau at about 1.25; however, the tetraplatin ERsOSb

* GIBCO, Burlington, Ontario. t Linde, Union Carbide, Toronto, Ontario.

* DMIPS Cell Analyzer, Cancer Imaging, B.C. Cancer Re- search Centre, Vancouver.

Page 3: Interaction of platinum drugs with clinically relevant X-ray doses in mammalian cells: A comparison of cisplatin, carboplatin, iproplatin, and tetraplatin

Platinum drugs: interaction with low x-ray doses in hypoxic cells 0 K. SKOV AND S. MACPHAIL 223

Table 1. Effects of longer incubation on ER’s, and aerobic responses

Enhancement ratio

Drug

Carboplatin

Iproplatin

Tetraplatin

Cisplatin

Concentration

100 PM 1OOpM 75 PM 75 PM

100 PM 100 PM

10pM 10 PM

25 PM 25 PM

2pM 2pM 2rM

Toxicity (P.E.)

0.58 0.30 0.41 0.61 0.70 0.30 0.46 0.49

0.22 0.28

0.82 0.74 0.6 I

Conditions Low dose High dose (drug treatment, and irradiation) (at 80% S) (at 2% S)

I hr, hypoxia 1.99 * .2 1.20 + .1 90 min, hypoxia 2.76 -+ .2 1.27 + .1 1 hr, hypoxia 1.52 - 90 min, hypoxia 1.61 - 1 hr, hypoxia 1.73 f .2 1.17 f .l 90 min, hypoxia 2.1 - 1 hr, hypoxia 2.06 f .05 1.19 f .1 1 hr, air vs. O2 1.13 1.14 (1 hr vs. N2) (2.94) (3.36) 1 hr, hypoxia 3.75 f .I 1.13 f .1 1 hr, air vs. O2 1.64 k .05 1.1 f .1 ( 1 hr, air vs. N,) (4.28 + .2) (2.83 f .l) 1 hr, hypoxia 1.8 - 90 min, hypoxia 2.0 - 120 min, hypoxia 2.0 -

continues to increase with concentration (toxicity) (Figs. I,3 and 4). Tetraplatin is the only complex thus far which interacts with X rays in air in the low dose region (Table I), giving an enhancement of 1.6 (in air, versus air).

Iproplatin is not as effective in its interaction with ra- diation as the above complexes (Figs. 1, 4), and is com- parable to trans-DDP in its effectiveness ( 19). Longer in- cubation before irradiation improved the ERgos but to a lesser degree than carboplatin (Table 1).

Toxicity The ERsosb increases with decreasing plating efficiency

for the second generation drugs, as shown in Figure 4. With cisplatin (open diamonds), however, toxicity does not appear to be a prerequisite for low dose radiosensi- tization as found previously (9). In the high radiation dose range, no sensitization was observed at these non-toxic concentrations of cisplatin (9); the solid symbols show that none of the newer drugs produce a significant ERzw .

0.30 1.0 2.0 3.0 4.0

DOSE <Gray>

Fig. 1. The enhancement ratios in hypoxic V79 cells, determined Fig. 2. Interaction between carboplatin (100 PM) and radiation at low doses (80% survival: 0, q , A, 0) and at high doses (2% (O-4 Gy Cl, with insert at higher doses) after 90 min incubation S; solid symbols), as a function of concentration of cisplatin (0, prior to irradiation in hypoxia. Oxic (V, v) and hypoxic (A, A) +); carboplatin (Cl, n ); iproplatin (A, A) and tetraplatin (0, l ), responses determined in the same experiment are shown. Error all determined after 1 hr incubation at 37°C prior to irradiation bars are standard deviations from 3 (high dose) to 8 (low dose) except for carboplatin after 90 min (*). experiments.

DISCUSSION

Each of the platinum drugs is considerably more effec- tive at low radiation doses than at high doses in hypoxic

7 0.90 0 0.60

2 0.70 IL

0.60

z H e.se

z > 5 0.48

v,

Carboplatin

Page 4: Interaction of platinum drugs with clinically relevant X-ray doses in mammalian cells: A comparison of cisplatin, carboplatin, iproplatin, and tetraplatin

224 I. J. Radiation Oncology 0 Biology 0 Physics

Tetraplatin

0.30 1 0 1.0 2.0 3.0 4.0

DOSE CGray>

Fig. 3. Tetraplatin effect on survival after X-irradiation in hyp- oxia: cells exposed to 5, 10 and 25 PM drug for 1 hr were irra- diated to doses shown, and assessed for survival using the cell analyzer (open circles). The insert shows the high dose results (solid circles) with little or no concentration dependence in this range. The hypoxic (A, A) and oxic (V, ‘I) data from the same experiments are shown. Error bars indicate standard deviations from 3-6 experiments.

cells (Figs. 1 and 4). Of the four drugs studied here, only tetraplatin shows some sensitization in the low dose region in oxic cells (Table 1). The relative efficiency of the drugs to achieve the low dose interaction depends on the basis of comparison (concentration or toxicity (this paper)), relative effectiveness or absolute sensitivity.”

Cisplatin approaches oxygen in its effectiveness at low doses, but appears to reach a plateau; the ER’s of carbo- platin and tetraplatin continue to increase with concen- tration, and appear to reach higher values than oxygen itself. Iproplatin does not appear to be as effective as the other drugs.

When compared on the basis of the best overall ER, tetraplatin, though relatively toxic, far surpasses the other drugs (ERson, to 3.7). At concentrations that produce only moderate toxicity (PE 0.6), tetraplatin gives a relatively low ERBm ( 1.3) and thus cisplatin is better in this respect (2.2). These ER’s are both considerably higher than would

February 1991, Volume 20, Number 2

CL--- 1 c P. E. (Toxicity)

1

Fig. 4. Enhancement ratios at 80% S (open symbols) and at 2% S (solid symbols) in hypoxic cells as a function of the plating efficiency, used as a measure of toxicity, produced by cisplatin (0, +); carboplatin (0, n ); iproplatin (A, A) and tetraplatin (0, 0). The dotted line is from data in ‘.

be achieved by such low (micromolar) concentrations of electron affinic drugs such as misonidazole and etanida- zole (2). Furthermore, the clinically achievable levels (millimolar) of these nitroimidazole drugs are also ex- pected to give ER,% of only 1.5 to 2.2 (2), assessed in the high dose range. Cisplatin, even at levels which may be 10X lower than achieved in the clinic (5) (e.g., 1 PM cis- platin), still produces a measurable interaction. Note, however, that clinical trials with radiation, to date, favor high dose levels of cisplatin (3).

The differences between carboplatin and cisplatin upon prolonged incubation are interesting in view of the fact that their mechanisms of toxicity are expected to be sim- ilar, with differences mainly in binding kinetics (8). This suggests that the lesions responsible for toxicity (e.g., in- trastrand cross-link (16)) may not be the same as those required for the interactions described here. This was pre- viously pointed out since trans-DDP, which is unable to form intrastrand crosslinks, also interacts with X rays in the low dose region (19).

The parameter “relative effectiveness” (Rel. Eff. = (ERsORD - l)/(ER,% - 1)) was introduced (18) as a means of comparing sensitization at low versus high radiation doses. Electron affinic compounds gave values around 0.6, whereas the platinum complexes give values > 1 (9). It was suggested that another means of comparing effec- tiveness at low versus high doses is required, because there was no enhancement at high doses by 1 and 2.5 PM cis- platin. To avoid the errors introduced by normalizing with

fi Koch, C.; Skov, K.; Comparisons of cellular radiation re- sponse using absolute rather than relative sensitivity parameters; Presented at the 38th Annual Meeting of the Radiation Research

Society, New Orleans, April, 1990. Submitted to Radiation Re- search.

Page 5: Interaction of platinum drugs with clinically relevant X-ray doses in mammalian cells: A comparison of cisplatin, carboplatin, iproplatin, and tetraplatin

the hypoxic control data (used to calculate ER), it may be preferable in some instances to compare directly on the basis of absolute sensitivity - the inverse of the dose required to give a certain response. On this basis (i.e., I/ dose for S = 80% vs. l/dose for S = 2%), tetraplatin shows the greatest differential, of the Pt drugs examined to date, between the interaction at high and low doses.

Clinical studies with cisplatin as a modifier of radiation response have not been conclusive, with respect to addi- tivity or synergism. One explanation which we would like to suggest as a result of the studies to date in the low dose region may be the variable degree of hypoxia in human tumors, since the interaction with cisplatin and the newer drugs is much more pronounced in hypoxic (than oxic) cells at the doses used in fractionated schedules (this study and (9, 19)). This could be important in the response of normal tissue to these combinations, which must be con- sidered to assess therapeutic gain. Thus, these platinum drugs require further studies in combinations at clinically relevant doses of both drug and X rays. The effectiveness of these individual drugs as well as future Pt complexes can be compared on the basis of drug concentration (in the in vitro situation), and toxicity (again, in vitro as a

first approximation) or other parameters to be described

in a subsequent paper. In vitro assessment to determine the “best” Pt drug for interaction with radiation doses of approximately 2 Gy will depend on the basis of compar- ison, and the most meaningful comparison will come from clinical trials in which effects on normal tissues as well as tumor response are monitored.

CONCLUSIONS As with cisplatin (9) and trans-DDP (19) three second

generation drugs show the marked interaction at low doses in cells irradiated under hypoxic conditions. Three of the four drugs examined here, plus trans-DDP, show no in- teraction with low doses in aerobic cells. Thus, platinum complexes require further consideration for their effects in clinically relevant combinations. Cisplatin gives good enhancement ratios at levels that are relatively low in tox- icity, and higher levels do not improve the interaction. Tetraplatin, on the other hand, produces enhancement ratios that are much higher, but at more toxic levels. Ipro- platin and carboplatin are more effective at longer prein- cubation times. Further studies are needed to understand mechanisms, and to determine a preferred basis of com- parison of drugs for their interaction with low radiation doses.

Platinum drugs: interaction with low x-ray doses in hypoxic cells 0 K. SKOV AND S. MACPHAIL 225

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