Publisher Summary The existence of any substantial interaction between different agents that are used clinically or to which man is exposed environmentally is potentially of great importance. One agent may affect another's absorption, metabolism, or excretion. It may alter tissue sensitivity to another agent, and may react with it physically or chemically. A variety of different criteria have been devised for deciding whether the agents in a combination interact pharmacologically. The purpose of this chapter is to examine these criteria critically for the progress in this field. To summarize, the effect–summation criterion may be used when the effects of all the agents in a combination are directly proportional to dose. The key to any criterion for examining interactions between different agents lies in the definition of zero interaction. The interactions that can be analyzed by constructing isoboles or calculating interaction indices are not restricted to cases in which all agents in a combination produce the effect under consideration. The therapeutic significance of interactions between agents requires careful consideration.

Criteria for Analyzing Interactions between Biologically Active Agents

Mutants of Saccharomyces cerevisiae with enhanced sensitivity to the DNA cross-linking agent nitrogen mustard (HN2) have been isolated and partially characterized with respect to their phenotypic and genetic properties. The screening technique, based on HN2-sensitivity as sole criterion, yields approximately 1 sensitive isolate in 200 clones when applied to an intensively mutagenized population of a resistant parent strain. Mutants characterized so far are all due to recessive nuclear genes and represent at least seven complementation groups. They exhibit different degrees as well as different patterns of sensitivity towards monofunctional and bifunctional alkylating agents, and ultraviolet light.

Isolation of Yeast Mutants Sensitive to the Bifunctional Alkylating Agent Nitrogen Mustard

— The induction of mutations (reversion to tryptophan independence) by various UV (254, 313, 334 and 365 nm) and visible (405 and 434 nm) wavelengths was measured in exponential phase populations of Escherichia coli B/r thy trp and B/r thy trp uvrA by assay of irradiated populations on semi-enriched media. No mutations were induced in the repair proficient strain at wavelengths longer than 313 nm. Mutations were induced in the excisionless strain at wavelengths as long as 405 nm but less than expected from the known amount of DNA damage induced. Irradiation at the longer wavelengths (434, 405, 365 and 334 nm) suppressed the appearance of 254- or 313-nm-induced mutations in the repair competent strain but not in the excision deficient strain. The relative dose-requirement for mutation suppression was related to the relative efficiency of these wavelengths in inducing growth delay. These results suggest that the growth delay induced by near-UV and visible wavelengths allows more time for the ‘error-free” excision repair process to act on the potentially mutagenic lesions induced by 254- and 313-nm radiations, thereby reducing the mutation frequency observed in the repair-proficient strain. The level of near-UV mutation induced in the excision deficient strain is lower than expected from the DNA damage known to be induced. It is possible that near-UV radiation induces a class of lethal lesions that are not susceptible to error-prone repair.

Mutation induction by and mutational interaction between monochromatic wavelength radiations in the near-ultraviolet and visible ranges.

Repair of ultraviolet damage in human cells also exposed to agents that cause strand breaks, crosslinks, monoadducts and alkylations

Unscheduled DNA synthesis and excision of pyrimidine dimers in human cells exposed to ultraviolet light were inhibited by exposure to methyl methanesulfonate (MMS, 1–2 mM), and repair of MMS damage was not inhibited bu UV light. Beacause the pathway for excision of pyramidine dimers and alkylation damage have previously been shown to be different, this observation implies a direct effect of alkylation on repair enzymes. We estimate that if inhibition is due to protein alkylation, the UV repair system must present an extremely large target to alkylation and may involve a complex of protein subunitsin the order of 1 million daltons such that 1 or more alkylations occur per complex at the concentrations used. These results also indicate that the method of exposing cells to 2 DNA-damaging agents to determine whether they are repaired by common or different pathways can be quite unreliable because of other effects on the repair systems themselves.

Inhibition of excision-repair of ultraviolet damage in human cells by exposure to methyl methanesulfonate.

— The lethal interaction between monochromatic radiation at various wavelengths and methyl methane sulphonate was tested in strains of Escherichia coli proficient and deficient in DNA repair. In the repair proficient wild-type strain K12 AB1157, the efficiency of sensitization to MMS as a function of dose (at 334 nm, 365 nm and 405 nm) was found to be directly correlated with the dose necessary to remove the shoulder from the survival curve at the wavelength employed. The 365 nm: MMS interaction was also observed in other repair proficient E. coli strains (W3110 and B/r) but was absent in a recA and a polA strain. Pre-treatment of AB1157 with MMS leads to a much larger interaction than pre-irradiation with 365 nm. It is concluded that dose-dependent damage to DNA repair by the near-UV radiation is involved in the interaction and possibly that MMS causes irreversible damage 10 repair enzymes.

Lethal interaction between ultraviolet-violet radiations and methyl methane sulphonate in repair proficient and repair deficient strains of Escherichia coli.

— The modification of the lethal interaction between near-UV (365 nm) radiation and a second DNA-damaging agent by incubation between treatments in either a minimal salts medium or complete growth medium has been studied in the wild-type bacterial strain Escherichia coli K12 AB 1157. The results indicate that the lethal interaction may be separated into at least two distinct processes whose evaluation may help in classifying DNA-damaging agents in terms of the repairability of the DNA lesions induced. An observation of changes when methyl methane sulphonate is given prior to the irradiation treatment indicates that this chemical irreversibly damages repair enzymes.

Ivanosca S. Correia

Papers

Lethal interaction between ultraviolet-violet radiations and methyl methane sulphonate in repair proficient and repair deficient strains of Escherichia coli.

Modification of lethal interactions between near-ultraviolet (365 nm) radiation and DNA-damaging agents.