Hypoxic cell sensitization to radiation damage by a new radiosensitizer: cis-dichloro-bis(1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole-N3)platinum(II) (flap)

Assessment of DNA binding of platinum-radiosensitizer complexes by inhibition of restriction enzymes

A simple and rapid method has been used to compare the binding of platinum complexes to DNA, in a relatively qualitative manner. A compound bound at or near the restriction site inhibits enzymatic cleavage of DNA; inhibition of BamHI and EcoRI activity by complexes was assessed in this study using linearized pSV2-gpt plasmid. Our particular interest was in DNA binding by complexes of platinum (Pt) with known organic radiosensitizers (RS), to determine whether the Pt was able to target the RS to the DNA. Although the Pt-RS complexes investigated themselves have moderate radiosensitizing ability (like the inorganic complexes, cis- or trans-diamminedichloroplatinum(II), c- or t-DDP) none of the Pt-RS inhibit to the same extent as c- or t-DDP. However, there appears to be some correlation between enhanced radiosensitization by Pt-RS over Pt(RS)2, with the degree of Pt binding (as assessed by our assay). Our results using isolated DNA suggest that not all complexes bind well (e.g. Pt with two RS ligands), but that in certain cases (e.g. Pt with only one RS), it is possible to target the drug to the DNA. An ammine or amine ligand may be required in order to target a radiosensitizer to DNA using platinum.

The interaction between radiation and complexes of cis-Pt(II) and Rh(II): studies at the molecular and cellular level

A range of Rh(II) carboxylates and cis-Pt(II) complexes have been examined for their ability to increase the radiation sensitivity of aerobic and hypoxic V79 cells in vitro. The transition metal complexes sensitize in both air and nitrogen, with the greater effect generally occurring in nitrogen. The cis-Pt(II) complexes only show small levels of sensitization with dose modification factors (DMFs) of no more than 1.2. In contrast, the Rh(II) complexes can give DMFs of 2.0. Radiation chemical experiments show the transition metal complexes to have substantially lower redox potentials than metronidazole and, in addition, neither type of complex undergoes electron transfer reaction or adduct formation on interaction with radicals derived from DNA bases. Thus, the inorganic complexes do not operate by mechanisms similar to those occurring with electron affinic or stable free radical sensitizers. The increase in radiation sensitivity for cells treated with the Rh(II) carboxylates, but not the cis-Pt(II) complexes, is attributed to the ability of the Rh compounds to deplete intracellular thiols. Further, the efficiency of sensitization by the Rh(II) complexes and their ability to interact with cellular thiols depends upon the nature of the carboxylate ligand and follows the order butyrate greater than propionate greater than acetate greater than methoxyacetate. The differences between the carboxylates may be due to differences in drug uptake. A combination of the Rh(II) complexes with misonidazole given to hypoxic cells irradiated in vitro gives an additive response. However, it was not possible to demonstrate a similar effect in tumours in mice given the combination of Rh(II) methoxyacetate and the misonidazole analogue RSU 1070.

Radiation and platinum drug interaction

Platinum drugs have chemical as well as biochemical and biological effects on cells, all of which may interact with radiation effects. They inhibit recovery from sublethal and potentially lethal radiation damage. They produce a pattern of chromosome aberrations analogous to that from alkylating agents. Cellular sensitivity to platinum is increased when glutathione levels are reduced, just as is radiosensitivity. There is a pattern of drug sensitivity throughout the phases of the cell cycle which is different from that for radiosensitivity. The ideal platinum drug-radiation interaction would achieve radiosensitization of hypoxic tumour cells with the use of a dose of drug which is completely non-toxic to normal tissues. Electron-affinic agents are employed with this aim, but the commoner platinum drugs are only weakly electron-affinic. They do have a quasi-alkylating action however, and this DNA targeting may account for the radiosensitizing effect which occurs with both pre- and post-radiation treatments. Because toxic drug dosage is usually required for this, the evidence of the biological responses to the drug and to the radiation, as well as to the combination, requires critical analysis before any claim of true enhancement, rather than simple additivity, can be accepted. The amount of enhancement will vary with both the platinum drug dose and the time interval between drug administration and radiation. Clinical schedules may produce an increase in tumour response and/or morbidity, depending upon such dose and time relationships.

Carboplatin as a potentiator of radiation therapy

A rationale for coordinating the administration of carboplatin with radiation to achieve enhancement of cancer therapy is developed. This approach is based upon a review of the reports of effects in a variety of systems, effects attributed to interactions between cisplatin or other platinum analogs and radiation. Two major effects include radiosensitization (RS) of hypoxic cells with platinum present during irradiation and potentiation of cell kill with platinum complexes administered after irradiation. Both these effects are expected to result in an improved therapeutic ratio. The latter effect may include inhibition of recovery from radiation-induced potentially lethal damage (PLD) and sublethal damage (SLD). Evidence for RS by carboplatin with an enhancement ratio (ER) of 1.8 is presented in Chinese hamster lung cells (V79) irradiated in culture under hypoxic conditions. Potentiation of radiation therapy in mice bearing a transplanted mouse mammary tumor (MTG-B) is reported as a supra-additive tumor growth delay when 60 mg/kg carboplatin is administered either 30 minutes before or immediately after 20 Gy of X-irradiation. Improved efficacy resulting from ongoing clinical trials coordinating cisplatin with radiation should support the role for carboplatin as a potentiator of radiation therapy since this second generation complex of platinum also interacts with radiation and larger concentrations of platinum should be attainable in tumors using the new drug.

Radiosensitization of EMT6 cells by four platinum complexes

The greatest research effort in producing radiation sensitizers has been directed toward organic compounds. However, platinum complexes also have radiosensitizing capabilities, perhaps because they bind to DNA. The compound described here are dichloro complexes of bivalent platinum with one or two potentially radiosensitizing ligands. The radiosensitization of oxygenated and hypoxic exponentially growing EMT6 cells in vitro was measured. The dose modifying factors obtained with 200 microM and 400 microM trans-bis(2-nitroimidazole)dichloroplatinum II (NIPt) in hypoxic cells were 1.5 and 2.1, respectively. For trans-bis(2-amino-5-nitrothiazole)dichloroplatinum II (Plant) under the same conditions, the dose modifying factor was 1.5 at 200 microM and 1.8 at 400 microM. Neither compound sensitized oxygenated cells when tested similar protocols. Unlike the trans complexes, (1,2-diamino-4-nitrobenzene)dichloroplatinum II (Plato) was cytotoxic toward the hypoxic cells in the absence of X rays. The time course of cytotoxicity for 100 microM Plato in exponentially growing cells showed rapid killing of hypoxic cells, and much less toxicity toward oxygenated cells. In radiosensitization studies, dose modifying factors of 1.6 and 2.0 were found with 200 microM and 400 microM Plato in hypoxic cells. The compound did not sensitize aerobic cells. The well-known platinum complex cis-dipyridinedichloroplatinum II (PyPt) represents a cis-platinum heterocyclic aromatic complex that does not have a nitro-functionality. The dose modifying factor obtained with 400 microM PyPt in hypoxic cells was 1.7. On a molar basis, the nitro-functional platinum complexes appear to be more effective as hypoxic cell radiosensitizers than the corresponding free ligands.

Radiosensitization of mammalian cells by transition metal complexes containing nitroimidazole ligands

Various transition metal complexes containing nitroimidazoles as ligands have been shown to act as radiosensitizers of hypoxic cells in vitro. Sensitization by cis-Pt(II)Cl2 (nitroimidazole)2 complexes is no greater than that which can be demonstrated by the free nitroimidazole ligand alone. These results differ from those previously described for the compound FLAP where an ER of 2.4 was obtained at a concentration of 50 microM. We report that, even using the same treatment technique, the maximum ER that can be achieved is 1.2. Further, the sensitizing efficiency of FLAP cannot be improved when cells are kept in contact with the compound for 12 hours in air prior to deoxygenation and irradiation. In contrast, Rh(II) complexes show much greater sensitization than can be obtained with the free ligand and these compounds are in turn more efficient sensitizers than the comparable Pt(II) complexes.

Sister chromatid exchanges induced by two radiosensitizing platinum compounds (cis-dichloro-bis isopropylamine trans dihydroxy platinum IV (CHIP) and cis platinum metronidazole2Cl2(FLAP)) in CHO cells in vitro

Sister chromatid exchange (SCE) induction by two radiosensitizing platinum compounds (cis-dichloro-bis isopropylamine trans dihydroxy platinum IV (CHIP) and cis-platinum metronidazole2 Cl2 (FLAP] was studied in CHO cells in vitro. Both drugs induced SCE in a dose dependent manner. CHIP was a much more potent inducer of SCE than FLAP and produced almost 4 times as many SCE as FLAP at equimolar concentrations and twice as many at equitoxic dosage. Induction of SCE by a component of the FLAP molecule--metronidazole--was also examined. It did not cause any increase of SCE frequency over the control level when applied at 10 times the highest concentration of FLAP which was used.