WO1992004024A2

WO1992004024A2 - Toxic agent protective compounds, compositions and method

Info

Publication number: WO1992004024A2
Application number: PCT/GB1991/001462
Authority: WO
Inventors: David Glyndwr Upshall; Ian William Lawston
Original assignee: The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1990-08-31
Filing date: 1991-08-30
Publication date: 1992-03-19
Also published as: CA2089158A1; GB9018994D0; GB2262446B; AU652183B2; WO1992004024A3; JPH06500113A; KR930701995A; EP0546063A1; GB2262446A; AU8525591A; GB9300657D0

Abstract

Novel and known compounds are identified which possess protective properties against the toxic effects of electrophilic agents upon human and/or animal bodies, and the use of these compounds for the production of compositions for the protection of such bodies against those electrophiles is particularly provided. The compounds and compositions are effective against topically or systemically administered agents which are themselves electrophiles or which are metabolised to produce electrophilic products in the body. A particularly advantageous aspect of the present invention lies in the application of these compounds and compositions as protectants against inhaled electrophiles and their precursors. Some radioprotectant activity against intracellularly produced electrophiles will be provided due to the ability of the compounds to raise intracellular cysteine levels.

Description

TOXIC AGENT PROTECTIVE COMPOUNDS. COMPOSITIONS AND METHOD.

The present invention relates to novel and known compounds which possess protective properties against the toxic effects of electrophilic agents upon human and/or animal bodies, and to the use of these compounds for the production of compositions for the protection of such bodies against those electrophiles. The compounds and compositions are effective against topically or systemically administered agents which are themselves electrophiles or which are metabolised to produce electrophilic products in the body. A particularly advantageous aspect of the present invention lies in the application of these compounds and compositions as protectants against inhaled electrophiles and their metabolic precursors.

Typical examples of toxic electrophiles include acrolein, formaldehyde, some organofluorine compounds, phosgene, sulphur mustards, bromobenzene, oxygen (in cases of high concentration) and thioureas, but many others will occur to a man skilled in the art. An example of where encounter with electrophilic compounds is common is in fire-fighting environments, particularly of fires involving certain types of wood, plastics and furnishings. Such encounters typically lead to a form of pulmonary oedema which is very difficult to treat with present therapeutics.

Many toxicant chemicals are poisonous because of their intrinsic electrophilic character or because they produce toxic electrophilic compounds within the cells of the body when metabolised. These electrophiles react with the intracellular components such as proteins and nucleic acids to produce cell death and various degrees of damage. In some cases the damage is so severe that fatality may result or a carcinogenic response may be evoked.

It is known that certain nucleophilic compounds will reduce the effects of these compounds and the role of thiols in this process is reasonably well understood. The principle endogenous thiol is glutathione which is present in high concentrations in the liver but at much lower concentrations within other organs, in particular in the lung. In the case of paracetamol poisoning the standard medical treatment is to infuse N-acetyl cysteine in order to reduce the intracellular levels of the toxicant electrophilic metabolites. Problems can arise however because of the toxic properties of the N-acetyl cysteine itself and the limited elevation of intracellular thiol concentration that it evokes. The potential toxicity of such antidotes makes it essential to determine the levels of toxicant present and thus makes their use where toxicant concentrations are unknown unsuitable for all but those cases where serious physical damage would be expected to take place in any event. Obviously routine administration of such compounds for industrial and economic reasons where poisoning of the body has not already occurred would be foolhardy. It would be unreasonable for fire-fighting or rescue and recovery personnel to have to take potentially toxic compounds every time they had reason to operate in an electrophilic environment and this would be especially unacceptable where the concentration and nature of toxicant are unknown and thus not readily matched by the protectant.

The compounds and compositions identified in this invention are of particular interest in so far as they have the ability to raise protective thiol levels in the organs of a human or animal body. The present inventors have found that, unexpectedly, the compounds of the invention selectively raise protective reactive thiol levels in the tissues, particularly the lungs, to a considerably greater extent than previously identified compounds; with increases of 5 to 40 fold being routinely obtained in the most affected tissues.

A particular application of the invention uses the compounds and compositions by way of their preemptive administration to industrial, rescue, recovery or military operatives prior to engaging in operations in an environment where such toxic electrophiles, electrophile producing agents are present, eg: in the atmosphere or on work surfaces, or an environment which is capable of forming the agents within the operatives body (eg. radioactive) .

Typical examples of these toxic agents which are encountered in industrial or fire-fighting environments include acrolein, hexa -fluoropropene, perfluoroisobutene, phosgene, formaldehyde and various pyrolysis products of plastics, woods and furnishing materials. Where such agents are accompanied by compounds such as carbon monoxide or hydrogen cyanide there will of course still be the need to use further protection against these. The ability of the compounds of the invention to raise protective thiol levels provides a supplement or, in suitably low electrophile levels or where weakly toxic and/or low concentrations of electrophiles are encountered, a replacement for other protective devices such as respirators and oxygen masks. Furthermore, the administration of the compounds is capable of raising such levels over a period of some hours without toxic side effects.

The use of the invention has application in the field of radio -protectants in so far as the presently identified compounds have higher thiol elevating properties than previously used agents for such purpose. As such radioprotectant function is connected with the elimination of electrophiles produced by exposure to ionizing levels of radiation it will be recognized that the particular property of increasing intracellular thiol is potentially of great significance in the protection of DNA and proteins which are inaccessible by extra -cellular agents but which may be damaged by agents formed in situ.

The use of the invention still further has medical applications in the therapeutic treatment of patients suffering from poisoning by electrophilic agents or metabolic precursors of these such as the toxicity produced by overdoses of paracetamol whereby toxic electrophilic metabolites are formed. Further examples of such treatable toxicity include cyclophosphamide, bleomycin and oxygen poisoning. Administration must be before electrophile formation is complete. It is envisaged that inclusion of compounds of the present invention within dosages of drugs which are metabolic precursors to electrophiles could be used to preempt overdose toxicity situations.

The present inventors have identified a method, compounds and compositions capable of producing increased levels of intracellular thiols such that incoming or intracellularly generated electrophile is mopped up by reaction therewith thus preventing the initiation of the toxic effect. The approach of the invention has a sparing effect on the endogenous levels of glutathione, eg, such as that caused by metabolic feedback, and thus permits other cellular processes to proceed as normal. The efficacy of the present invention is particularly increased with regard to use against electrophilic agents or their metabolic precursors in gaseous, vapour, droplet, mist or other airborne form due to it's ability to increase protectant levels to particularly high concentrations in the lungs and blood of operatives so that those intending to carry out operations in such toxicant atmospheres are protected at the point of most susceptibility to attack. Thus an internal 'chemical filter' is provided acting as a back-up to any respirator or protective clothing or may even be used instead of these. Such back-up would be potentially life-saving in cases where the respirator or clothing was defective, had become damaged, was badly fitted or became displaced.

The present invention thus provides a composition for use as a pharmaceutical for the protection of a human or animal body from the toxic effects of an electrophilic agent or an agent that is capable of being metabolised to produce an electrophilic agent in said body, said composition comprising a compound of the Formula I:

Formula I

NRR³

Wherein R¹ comprises a group -SH, -S-S-CH--CH-C0-Z or -X-R^A;

!

NR²R³

R² and R³ are independently selected from H, -COR , or an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more halogen atoms or -CF, groups;

R* and R⁵ are independently selected from an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more halogen or -CF groups; Z is an alkoxy, alkenoxy, aryloxy or arylalkoxy group optionally substituted by one or more halogen or -CF groups, but is not tertiary butyloxy; and X is -S-S- or -S0-; Preferably R¹ is -SH or -S-S-CH -CH-CO-Z

wherein R² and R³ are independently selected from H or -CO-R^ and are the same in each half of the molecule.

It will be appreciated that in all cases the terms alkyl or alkoxy above include straight or branched chain aliphatic groups or cycloalkyl and cycloalkoxy groups such as cyclopropyl, cyclopropyloxy, cyclopentyl, cyclopentyloxy, cyclohexyl, cyclohexyloxy, cycloheptyl or similar such groups. Preferred groups are of 1 to 20 carbons.

Examples of compounds for use in the method and compositions of the invention are those where R¹ is -SH, R² and R³ are H and Z is selected from the group methoxy, ethoxy, n-propyloxy, isopropyloxy, n-butyloxy, sec-butyloxy, n-pentyloxy, iso-pentyloxy, cyclopentyloxy, n-hexyloxy, cyclohexyloxy, n-heptyloxy, cycloheptyloxy and benzyloxy. Further examples include those compounds wherein R² and R³ are independently selected from H or an acetyl, n-propionyl, iso-propionyl, n-butyryl, iso-butyryl or sec-butyryl group.

The lipophilicity of the compounds, and thus their affinity for high lipid tissues may be further increased by incorporation of lipophilic groups such as long chain alkyl groups eg, lauryl (C₁₂H₂_-) groups, as alkyl substituents at R², R³ and E¹* and/or as long chain alkoxy groups, eg lauryloxy (C₁₂H₂_0-) at Z. Such long chain groups typically c from 8 to 20 carbon atoms; 1 to 7 carbon chains may be used otherwise.

Both L- and D- or mixed chirality LD- forms of the compounds may be used for the compositions and methods of the invention. Specific examples of compounds for use in the compositions and method of the invention include alkyl esters of the amino acids cysteine and cystine. It should be noted that cysteine and cystine themselves have toxic properties when distributed in the blood stream and cause liver damage whereas the present compounds do not appear to have such effects.

The present invention further provides a method of preparation of compositions for the protection or therapy of a human or animal body from the toxic effects of an electrophilic agent or an agent capable of being, or that has been metabolised to form an electrophilic compound in said body comprising the inclusion of a compound of said formula I above. These compositions may be adapted for parental, oral, topical or aerosol administration.

Particularly advantageous compositions comprise liposomes containing the compounds of Formula I and are used for aerosol administration. Such liposome compositions are effectively prepared, for example, by the methods referred to by Meisner et al (J. Microencapsulation, 1989 Vol 6. No. 3. P379-387), Mayer et al (Chemistry and Physics of Lipids, 40, 1986 p333-3^ ) or Debs et al (Am. Rev. Respir. Dis. 1987. 135, P731-737). Use of such aerosol delivered liposomal compositions provides a means of locating the protective compounds of Formula I within a carrier of liposome form in the lungs of a subject where they remain over a longer period than those administered via alternative administration routes. Typical liposomal compositions comprise one or more compounds of formula I with a phospholipid, eg. dipalmitoyl-phosphatidylcholine, with optional inclusion of cholesterol for reduction of leakage of compound into the plasma.

Topically administrable compositions will be preferred where skin contact is likely or where precursor contamination has occurred and will advantageously incorporate oils, eg: olive or almond oil, isopropyl palmitate or oleoyl alcohol, preferably in conjunction with lipophilically substituted examples of Formula I. Such compositions will have further application in transdermal delivery of compounds of formula I, allowing continuous systemic dosing via the skin.

The present inventors have determined that, whereas previously used skin protectants which are restricted to the extracellular fluid, eg. thiosulphates, are useful for systemic protection, the compounds identified for the use of the present invention are actively taken up into skin cells. Advantageously, it is demonstrable that present compounds are taken up into the particularly important basal epidermal keratinocytes and cause increase in the levels of intracellular cysteine and in many cases, in due course, glutathione. Thus the protective agent is able to exert its effect within the cells before the electrophile can react with intracellular molecules such as DNA and protein.

It should be particularly noted that the varied metabolism of different compounds of formula I offers the prospect of compositions comprising more than one such compound with the effect of a blend of properties with regard to effect on thiol and glutathione levels and the duratio of protection. Examples of such varied effects will be given in the Figures and Tables below (see Table IV and Figure IV) .

The present invention still further provides a method for the preparation of a composition for the treatment of human or animal bodies for electrophilic agent poisoning comprising use of a compound of the Formula I.

The present invention also provides novel compounds falling within the general Formula I and novel compositions containing those compounds.

Such novel compounds include all those compounds of formula I where Z is cycloheptyloxy, cyclohexyloxy, cyclopentyloxy, cyclobutyloxy, cyclopropyloxy and all those compounds of formula I where R_χ is -S-S-CH--CH-C0-Z.

NR²R³

with the exception of dimethyl and ditertiary-butyl cystines.

Several of the compounds herein identified by the present inventors are already in use for a variety of medical or other applications. The cysteine methyl ester is already available under the trade mark VISCLAIR as a ucolytic agent and as such is available in a form readily administered to the respiratory tract. US 3322625 discloses both the methyl and ethyl esters of cysteine as useful for relief of bronchial congestion. JP 63280023 discloses use of the ethyl ester as an orally administered antirheumatic drug while GB 1114313 discloses its use as a brochodilator. FR 1463761 discloses use of the methyl ester as an antibacterial. Further uses include skin whitening application as disclosed by JP 5912830 while JP 6263517 discloses use of compounds where X is SO or -S-S- and R^ is alkyl or alkenyl as 'liver health drugs' . None of these documents relate to the ability of the present compounds to provide intracellular protection of tissues, particularly lung tissue, from electrophilic attack.

The compounds and compositions of the present invention will now be illustrated by way of example only with regard to the following Examples and Figures, whereby other embodiments of said invention will occur to the man skilled in the art.

FIGURES.

Figure 1: Shows the timecourse of plasma total thiol (non protein thiol (NPSH) ) levels on intraperitoneal adminstration of 3 mmol/kg of five compounds of Formula I and the comparative compound cysteine in rats. Compounds represented are: isobutyl cysteine (light +) , n-butyl cysteine (upward pointed open triangle) , n-propyl cysteine (downward pointed open triangle), tert-butyl cysteine (heavy +), iso-propyl cysteine (filled circle) and cysteine (filled triangle) . All compounds were administered as hydrochlorides in 0.3% saline. The plots are umol NPSH/ml in plasma versus time in minutes.

Figures 2A-2B and 3A-3B: Show the increase in total thiol levels in the liver, thymus, heart, lung, testis, spleen, kidney, adrenals and blood on administration of 3_*0 mmol/kg of isopropyl cysteine ester hydrochloride compared to controls administered with just 0.9% saline. Cs, Cm and T indicate sing^le treated control, four dose control and four dose test ester respectively. Injected doses were given at 0 minutes or 0, 30, 60 and 90 minutes and the animals were sacrificed at 120 minutes.

Measurement of thiol levels: Tissue levels of thiols were measured within 1 hour of exposure using the Ellman method. (Archives Biochem. Biophys. __\ (1959) P70-77) to determine total thiol levels. Where necessary glutathione and cysteine levels were determined using the monobromobimane method of derivatization of thiols (Kosower et al (I983), Biochem. Biophys. Toxicological and aspects. Ed. A. Larsson et al, Raven Press, New York p243~250) ; Fahey et al (1983) ibid, p251-26θ. Figures 4A-4D: Mean increase in NPSH, GSH and Cysteine levels in the lungs of rats injected ip with either A: cysteine (3 mmol kg^-1), B: N-acetyl cysteine (5 mmol kg^"1), C: cysteine methyl ester (3 mmol kg^"1) and D: cystine dimethyl ester (1.5 mmol kg^"1). Open circles indicate non-protein thiol (NPSH) , filled circles indicate glutathione and triangles indicate cysteine levels. Each point is mean for three rats killed at that time point.

Figure : Shows the mean % increase in NPSH levels in plasma of rats injected ip with either cysteine (3 mmol kg^"1) (filled circles), N acetyl cysteine (5 mmol kg^"1) (+)^', cysteine methyl ester (3 mmol kg^"1) (open triangles) or cystine dimethyl ester (1.5 mmol kg^"1) (open circles). Each point represents a three rats mean as in Fig 4.

Figures 6A-6C: Show mean levels of cysteine and glutathione in the lungs (A) and livers (B) of four rats injected ip with 3 mmol kg^"1 of cysteine isopropyl ester or physiological saline. Saline controls are given in open circles, cysteine in open triangles, cysteine isopropyl ester in closed circles and glutathione in closed triangles. Figure 70 gives NPSH levels for plasma with saline controls (open circles) and cysteine isopropyl ester (filled circles) for those same rats.

Figures 7 to 10: Show the effect of exposing immortalised cultured human basal epidermal keratinocyte cells to 0.5mM cysteine and a number of its esters upon intracellular cysteine (+) and glutathione (open circles). The test cysteines and esters are: Figure 7-' cysteine; Figure 8: cysteine methyl ester; Figure ' cysteine isopropyl ester; ( note extra trace for cysteine isopropyl ester given in filled triangles); Figure 10: cysteine n-butyl ester. All axes are intracellular thiol cone (pmoles/ug DNA) versus exposure time in minutes.

Synthesis of compounds of Formula I: The compounds of the present invention are conveniently synthesised from L-, D- or race ic LD- cysteine, cystine or their analogues, the particular analogues depending on the desired product.

For synthesis of cysteine compounds where Z is an alkoxy, alkenoxy, aryloxy or arylalkoxy group a suitable analogue of cysteine was dissolved in the alcohol corresponding to the desired group Z and the solution was bubbled with dry hydrogen chloride gas and refluxed to produce esterification. Thus where Z is to be ethoxy, the cysteine analogue is dissolved in, and thus reacted, with ethanol.

Synthesis examples.

Preparation of cysteine ester hydrochlorides: Anhydrous hydrogen chloride (25 g) was passed into propan-2-ol (228 ml) . L-cysteine (15 g) was added and the mixture was heated under reflux for 4 hours, the solution was concentrated to half its original volume and the crystalline product was filtered off and recrystallised from propan-2-ol to yield cysteine isopropyl ester hydrochloride (19-5 S^: 81% yield) ; melting point 155~156°C) . Identity and purity were confirmed by HPLC and IR spectroscopy.

The following esters were also synthesised by analogous technique to that above, with the exception of the tertiary butyl ester which required anhydrous hydrogen chloride to be passed through the refluxing mixture for 5 days.

L-cysteine n-butyl ester hydrochloride (76 yield, mp 84°C) . L-cysteine iso-butyl ester hydrochloride (83% yield, mp 87°C) . L-cysteine n-pentyl ester hydrochloride (55% yield, mp 87°C) . L-cysteine cyclohexyl ester hydrochloride (75% yield, mp 130°C) . L-cysteine tert-butyl ester hydrochloride (78% yield, mp 213-2l4°C) .

EXAMPLE 1. Protection against Perfluoroisobutene (PFIB) .

The efficacy of cysteine and cystine esters of Formula I in protecting rats against the effects of the electrophilic agent perfluoroisobutene (PFIB) , a toxic pyrolysis product of polytetrafluoroethylene, was investigated. PFIB is a colourless, odourless gas with a boiling point of 6°C which is formed either by pyrolysis or by hot-forming in an industrial environment. In man it produces 'polymer fume fever' , a condition characterised by nausea, fever, sore throat, cough and pulmonary oedema. In rats the toxicity is greater than that of phosgene having a Ct-₀ (Concentration at which 50% of test animals die) of 1250mg min^"1 m^"3 and damage to the alveolar type I and II cells and the pulmonary endothelium is evident. Exposure to PFIB reduces total lung thiol (NPSH), glutathione (GSH) and cysteine (CYSH) by 38% to 55% while treatment with buthionine sulfoxamine (BSO) and/or DEM actually increases the toxicity. (NB: BSO is used to deplete glutathione and thus sensitise rats to the effects of the PFIB) .

Protocol: Toxicity testing was carried out by administering doses of various protectant compounds according to Formula I intraperitoneally in 0.9% saline 30 minutes before exposure. Exposure to PFIB was. carried out over 10 minutes followed by observation for 7 days. All protectants of the present invention were administered as hydrochloride salts. Results: results are provided in Table I below. It should be noted that the lack of efficacy of tertiary butyl cysteine is the basis for its ommission from the claims of this document.

EXAMPLE 2.

Protection against phosgene gas.

Phosgene, a well known industrial and military electrophilic gas producing toxic states in both man and animals was used to demonstrate tne efficacy of compounds of the present invention.

Protocol: Test doses of protectant were administered to rats intraperitoneally in 0.9% saline 30 minutes before exposure to phosgene for a period of 10 minutes. Observation was continued for 7 days. All protectants of the present invention were administered as hydrochloride salts. Results: results are provided in Table 2 below. EXAMPLE .

Effect of dose and time of administration of protectant upon mortality of rats exposed to PFIB,

The protocol of Example 1 was used with varying dose of hydrochlorides of the protectant compounds of Formula I to determine the response relationship.

Results: results are summarised in Table III below. Time values prefixed (-) designate pretreatment with protectant while (+) designates post treatment.

TABLE II .

Protection of rats against death from pulmonary oedema induced bv exposure to phosgene by pretreatment with cvsteine and its esters.

TABLE III.

Effect of varying dosage and time of administration of protectant on mortality of rats exposed to PFIB.

Metabolism studies:

Materials and methods: Rat lung and liver homogenates (5 mg/ml) and plasma (50%) were used in Krebs Ringer Phosphate buffer (ICRP) at pH7.4. Incubates contained 1 mM of selected thiol and hydrolysis rates were determined by HPLC of the monobromobimane derivatives of the parent ester and product. Assays were conducted at 0, 1, 2, 5. 10, 15, 30 and 60 minutes, results are expressed per mg weight or per ml of plasma. Estimates of initial rates of hydrolysis are presented in Table IV.

TABLE IV Rates of hydrolysis of L-cysteine esters (umol/mg tissue/minute or umd/ml/min for plasma) .

It can be seen that the cyclohexyl ester is hydrolysed fastest in the lung yet is hydrolysed slowest in plasma. The ineffectiveness of the tertiary butyl ester is reflected in its non-metabolism in the tissues studied, suggesting release of cysteine, at least, is required for the protective effect. Each ester had a quantitatively different effect on cysteine levels in the tissue with time; the isopropryl and cyclohexyl esters led to the most durable increase in lung cysteine.

EXAMPLE 4.

Effect of administration of cysteine ester protectant on thiol levels in cultured human keratinocvte cells.

Cultured human basal epidermal keratinocyte cells, made immortal by infection with SV 40 virus (Taylor-Papadimitriou et al; Cell Differn. H, 169-80) , were exposed to mM of either cysteine or various of its esters. Results are shown in Figures 7 to 10. All esters increased intracellular cysteine levels but the effect on glutathione was highly variable with regard to duration and extent. It should be noted that long chain esters could be detected in the cell free extracts of the cultures but methyl and ethyl esters could not. These experiments demonstrate the value of varying content of two or more such esters in skin protectant preparations in order to blend the effects to a give a desired result. It should be noted that cysteine gives only a brief increase in glutathione before causing the level of that compound to decrease below normal. In contrast cysteine isopropryl ester leads to increased glutathione after 3 hours and cysteine n-butyl ester gave a very stable increase in glutathione throughout the exposure period.

EXAMPLE ,

Human basal keratinocytes supported on 3T3 mouse embryo fibroblasts were exposed to sulphur mustard agent 'H' (an alkylating electrophilic chemical warfare agent) added to their culture solution at 2 uM. The effect of pretreatment with cysteine and cysteine/cystine esters was studied on growth of the cells and toxicity of the challenge with 'H' . Cysteine applied at 2 mM or more increased cell growth and gave some protection against the effects of 'H' . All the cysteine esters tested (cysteine methyl and ethyl ester, cystine dimethyl ester) were mitogenic and further provided complete protection at 0.5 to 1.0 mM. Neither glutathione nor its oxidised dimer form increased cell growth or gave significant protection against 'H' .

Claims

CLAIMS .

1. A composition comprising a compound according to the Formula I

Formula I

wherein R¹ comprises a group -SH, or -X-R^h

R² and R³ are independently selected from H, -C0-R⁵ or an alkyl, alkenyl, aryl, arylalkyl group optionally substituted by one or more halogen atoms or -CF, groups; R^ή and R⁵ are independently selected an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more halogen atoms or -CF_ groups, and Z is an alkoxy, alkenoxy, aryloxy, arylalkoxy group optionally substituted by one or mmoorree hhaallooggeenn aattoommss or -CF groups but is not tertiary-butyloxy ; and X is -S-S- or -SO-

or a pharmaceutically acceptable salt thereof for use as a pharmaceutical for the protection of a human or animal body against the toxic effects of an electrophilic agent or a metabolic precursor of such agent.

2. A composition as claimed in claim 1 comprising a compound of formula I wherein Z is a C_j - C₂₀ alkoxy group.

3. A composition as claimed in claim 1 comprising a compound of formula I wherein Z is a C_j - C. alkoxy group.

4. A composition as claimed in claim 1 comprising a compound of formula I wherein R¹ is a -SH or -S-S-CH.-CH-CO-Z group wherein R² and

NR²R³ R³ are the same in each half of the molecule.

5. A composition as claimed in claim 1 wherein the compound of formula I is a cysteine alkyl ester or a cystine dialkyl ester.

6. A composition as claimed in claim wherein the ester is of L- optically active form.

7- A composition as claimed in claim 1 wherein the compound of formula I is methyl cysteine, ethyl cysteine, iso-propyl cysteine, iso-butyl cysteine, n-butyl cysteine, n-penytyl cysteine, cyclopentyl cysteine or cyclohexyl cysteine.

8. A composition as claimed in claim 1 wherein the compound of formula I is a cystine dimethyl ester.

9. A composition according to any one of the claims 1 to 8 wherein the compound of formula I is provided within a carrier capable of taking the form of liposomes within the lungs of a human or animal body.

10. A composition according to claim 9 wherein the carrier comprises a phospholipid.

11. A composition according to claim 10 further comprising cholesterol.

12. A composition according to any one of the claims 1 to 8 wherein the compound of formula I is provided within a carrier comprising an oil.

13. A composition according to claim 12 wherein the oil comprises olive oil, almond oil, isopropyl palmitate or oleoyl alcohol.

14. A composition according to claim 13 wherein the compound of formula I is one comprising a long chain alkyl group at one or more of R², R³ and R and/or a long chain alkoxy substituent at Z.

15. A composition according to claim 16 wherein the long chain alkyl or alkoxy substituent comprises from 8 to 25 carbon atoms.

16. A composition according to claim 14 wherein the long chain alkyl or alkoxy substituent comprises a lauryl (C₁₂H---) or lauryloxy (C₁₂H₂₅0-) group.

17. A method for protecting a human or animal operative against toxic effects of a toxic agent comprising an electrophile or a metabolic precursor thereof for the purpose of industrial, rescue, recovery or military activity, comprising administering a composition as claimed in any one of claims 1 to 16 to said operative prior to entry into an environment containing said toxic agent, or an environment which is capable of causing formation of such agent in the operative.

18. A method as claimed in claim 17 wherein the toxic agent is in gaseous and/or vapour form and the composition of is administered by aerosol inhalation or by systemic injection.

19. A method as claimed in claim 17 wherein the toxic agent is in gaseous, vapour and/or liquid form and the composition is administered topically to the skin.

20. A method as claimed in claim 17 wherein the composition is applied via a skin patch and comprises a transdermal delivery formulation.

21. A method for treating poisoning by a metabolic precursor of an electrophile comprising administering a composition as claimed in any one of claims 1 to 16 before formation of electrophile is complete.

22. A method as claimed in claim 21 wherein the metabolic precursor comprises paracetamol, cyclophosphamide, bleomycin or oxygen.

23. A prophylactic and/or therapeutic comprising a composition as claimed in any one of claims 1 to 16 and a drug that is a metabolic precursor for an electrophile.

24. A prophylactic and/or therapeutic as claimed in claim 23 wherein the drug comprises paracetamol, cyclophosphamide or bleomycin.

25. Novel compositions comprising a compound of the Formula I

CO-Z

Formula I

HC-CH---RR¹¹

NR²R³

wherein R¹ comprises a group -SH, -S-S-CH₂-CH-C0-Z or -X-R⁴

MRR³

R² and R³ are independently selected from H, -C0-R⁵ or an alkyl, alkenyl, aryl, arylalkyl group optionally substituted by one or more halogen or atoms or -CF, groups; R and R⁵ are independently selected an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more halogen atoms or -CF groups, and Z is an alkoxy, alkenoxy, aryloxy, arylalkoxy group optionally substituted by one or more halogen atoms or -CF, groups, but is not tertiary-butyloxy ; and X is -S-S- or -SO-; wherein

when R² and R³ are both H and R¹ is -SH,

Z is not C₁ to C^ n-alkoxy or iso-alkoxy and

when R² and R³ are both H and R¹ is -S-S-CH -CH-CO-Z

Z is not C₁ alkoxy;

or a pharmaceutically acceptable salt thereof.

26. A composition as claimed in claim 25 further comprising a carrier capable of taking the form of liposomes within the lungs of a human or animal body.

27. A composition as claimed in claim 26 wherein the carrier comprises a phospholipid.

28. A composition as claimed in any one of claims 25 to 27 further comprising cholesterol.

29. Use of a compound of Formula I:

CO-Z

H-C-C^-R¹ Formula I

NR²R³

wherein R¹ comprises a group -SH, -S-S-CH--CH-C0-Z or -X-R^h

NR²R³

R² and R³ are independently selected from H, -C0-R⁵ or an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more haogen atoms and/or -CF, groups; R^h and R⁵ are independently selected from an alkyl, alkenyl, aryl or arylalkyl group which is optionally substituted by one or more halogen atoms and/or -CF- groups; Z is an alkoxy, alkenoxy, aryloxy or arylalkoxy which is optionally substituted by one or more halogen and/or -CF- groups, but is not tertiary butyloxy, and X is -S-S-, -SO-,

or a pharmaceutically acceptable salt thereof for the preparation a composition for the protection of a human or animal body against the toxic effects of exposure to an electrophilic agent or a metabolic precursor to such agent.

30. A compound according to Formula I:

CO-Z

H-C-CH--R¹ Formula I

NR²R³ wherein R¹ comprises a group -SH, -S-S-CH--CH-C0-Z or -X-R⁴

NR²R³

R² and R³ are independently selected from H, -C0-R⁵ or an alkyl, alkenyl, aryl or arylalkyl group optionally substituted by one or more halogen atoms or -CF, groups; R⁴ and R⁵ are independently selected from alkyl, alkenyl, aryl or arylalkyl groups optionally substituted by one or more halogen and/or -CF groups; Z is an alkoxy, alkenoxy, aryloxy, arylalkoxy group which is optionally substituted by one or more halogen and/or -CF groups but is not tertiary butyloxy; and X is -S-S-, -SO-;

provided that when R² and R³ are H and R¹ is -SH Z is not a ^ to C₄ n-alkoxy or isoalkoxy group;

and that when R² and R³ are H and R¹ is -S-S-CH₂-CH-C0-Z

NR²R³ Z is not a C₁ alkoxy group.

31. A compound as claimed in claim 30 wherein R¹ is -SH, R² and R³ are both H, and wherein Z is a cycloalkoxy group, a C_g to C₂₀ n- or iso-alkoxy or n- or iso-alkenoxy group or is an aryl or aralkyl group.

32. A compound as claimed in claim 30 wherein R¹ is -S-S-CH₂-CH-C0-Z

NR²R³

R² and R³ are both H and Z is an alkoxy, alkenoxy, aryloxy or arylalkoxy group.

33> A compound as claimed in claim 32 wherein Z is a cycloalkoxy group.

34. L-cysteine cyclopentyl ester.

35- L-cysteine cyclohexyl ester.