Carbamates metabolic resistance

Organophosphates (OPs), introduced in 1944, and carbamates, introduced in 1956, remain widely used and effective insecticides although not free from resistance problems. Metabolic resistance to OPs was reported 14 years after their introduction, compared to only 7 years for DDT and 5 for the carbamates. The complex metabolic fate of the OPs, including attack by cytochrome P-450 leading either to activation or detoxification, as well as by glutathione transferases and esterases, may play a role in this delay. Carbamates are not bioactivated they are detoxified by cytochrome P-450. 

In the presence of a continued selection pressure, metabolic resistance may facilitate the evolution of other defenses such as target site resistance, reported for the OPs and carbamates 6 and 10 years after metabolic resistance. Target site resistance to OPs and carbamates resides in modified forms of acetyl-cholinesterases (AChEs) with reduced affinity for the insecticides. AChE-based target site resistance does not necessarily confer cross resistance to all other OPs and carbamates and may be unstable in the absence of a selection pressure. 

Metabolic resistance mechanisms are still the most widely encountered causes for OP and carbamate resistance. Yet, after the first few cases of target site insensitivity were reported in the early 1970 s in mites and ticks, many such cases have been found also in insects. Resistance can also be enhanced by a decreased rate of penetration through the integument. This resistance mechanism by itself is of minor importance, but provides an increased opportunity for detoxification. A 50-fold increase in resistance to carbaryl was seen in a house fly strain, in which a gene for reduced penetration had been combined (by selective breeding) with a gene for increased detoxification (5). 

Target site resistance to OPs was first reported in 1964, 20 years after their introduction, in a spider mite (53) and subsequently in several insect species including the green rice leaf hopper (54), mosquitoes (5 ), house flies (56-58, and an armyworm (59). Target site resistance to the carbamates was first observed in 1971 (60). It is, today, a common form of resistance to these insecticides and always combined with metabolic resistance. 

One of the major pest Insects in rice fields is the green rice leafhopper (Nephotettix cincticeps). Recently acquired resistance to carbamate insecticides has greatly complicated control of this insect. Resistance can develop by a number of different mechanisms such as cuticle thickening to Impede transport, enhanced metabolic degradation or molecular changes within the target enzyme (AChE) (1 ). In the case of carbamate insecticides, resistance occurs mainly at the target enzyme which expresses reduced sensitivity to the inhibitors (2). 

Resistance mechanisms associated with changes in toxicokinetics are predominately cases of enhanced metabolic detoxication. With readily biodegradable insecticides such as pyrethroids and carbamates, enhanced detoxication by P450-based monooxygenase is a common resistance mechanism (see Table 4.3). 

Second-generation juvenoids incorporate more substantial structural departures from neotenin and are more resistant to metabolic and environmental degradation. Epiphenonane, 2-ethyl-3-[3-ethyl-5-(4-ethylphenoxy)-pent-3-en-yl] 2-methyloxirane (131), has a rat oral LD50 of 4000 mg/kg. It and similar juvenoids are used in China and Japan to prolong the last larval instar of the silkworm so that silk production is increased 10—15%. Fenoxycarb, ethyl [2-(4-phenoxyphenoxy)ethyl] carbamate (132) (mp 53°C, vp 0.0078 mPa at 20°C), is soluble in water to 6 mg/L. The rat oral LD50 is >16,800 mg/kg. Fenoxycarb has a wide spectrum of activity, interfering with the developmental processes of fleas, cockroaches, and ants. 

Mixtures of PBO with pyrethroid or carbamate insecticides are often more effective against insect strains resistant to these compounds where oxidative metabolism is responsible for the decreased effect (Davies et at., 1958 Wilkinson, 1968 Glynne-Jones, 1983 MacDonald eta ., 1983 h unaki etai. 1986). There is no evidence to indicate that PBO increases the low toxicity of pyrethrins and pyrethroids l<> mammals. The acceptable daily intake (ADI) of PBO for humans has been established at 0.2 mg per kg body weight (JMPR, 1995),... 

As in vitro methods for studying cytochrome P-450 in insects became available (11-131, it soon became clear that insects with high cytochrome P-450 activities were resistant to carbamates and most other insecticides. This phenomenon is termed metabolic cross resistance and derives from the characteristic of cytochrome P-450 of accepting a very wide range of molecular structures as substrates the cytochrome binds the substrate very loosely by a lipophilic interaction and rapidly oxidizes it by an oxygen free radical-mediated reaction, a very powerful combination. Moreover, the cytochrome occurs in several or many different isoenzymic forms with broadly overlapping substrate preferences. A normally infrequent form may be selectively induced by allelochemicals in the crop plants (14), and if the induced form has survival value in the presence of an insecticide, it could be selected to dominate in the exposed population (15). 

Selection of laboratory strains of insects with insecticides has repeatedly resulted in populations with considerably higher cytochrome P-450 activities than the original ones (16. 17). The role of cytochrome P-450 in the detoxification and metabolism of the carbamates has been studied extensively (18.). High cytochrome P-450 activities are associated with virtually all cases of carbamate resistance. 

Reviews on detoxification. All the enzyme systems involved in carbamate and OP metabolism and detoxification have been described in detail by several recent reviewers (7. 43-45. For an in-depth discussion of the genetics of these resistance mechanisms, see Oppenoorth (28). 

The major difference between OPs and carbamates is in their metabolic fate. This strongly indicates that metabolism and detoxification are the most important factors in resistance evolution. The current emphasis on target site interactions and their associated resistance mechanisms reflects their perceived practical importance, the importance of their study for the development of new insecticidal molecules, and the fundability of such research. But detailed studies of the biochemical characteristics and physiological behavior of the enzymes involved in insecticide metabolism and detoxification are still very important. 

Several prodrugs of Hourouracil were obtained by acylation or carbamoylation of N-1 and/or N-3 atoms of the pyrimidine ring of 1. In particniar, an oral drug Carmofur (2) which is 1-hexylcarbamoyl derivative of 1 was launched in Japan in 1981 and later - in other countries [35], The carbamate moiety in 2 decomposes gradually in neutral water or in basic conditions, but it is strongly resistant to acidic hydrolysis and hence can survive acid in the stomach. The 1-hexylcarbamoyl moiety also facilitates the rapid uptake of 2 through the cell membrane [36]. The metabolic activation of Carmofur involves oxidation and scission of the side-chain with slow release of 1 [37]. Two main routes of the side chain transformation are (o-oxidation and ((o-l)-oxidation metabolites 40-43 were detected after adnunis-tration of Carmofur (Fig. 3) [38]. Non-enzymatic hydrolytic decomposition of 2 and its metabolites also contributes to release of 1. 

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