327~ B457 T~\ present invention relates to 2',3'-dideoxy-nucleosides9 pharmaceutically acceptable derivatives thereof, and their use in therapy, particularly for the treatment or prophylaxis of certain viral infections. This Application is a division of Canadian Patent Application, Serial ~umber 509,129, filed May 14, 1986. In the comparatively new field of antiviral chemotherapy, few drugs exist which effectively combat the virus ~_ se, owing to the difficulty of at~acking the virus while leaving uninfected host cells unimpaired. It has recently been establishedthat certain stages in the virus life-cycle, which vary from species to species,are specified by the virus itself. These sta~es may prove susceptible ~o attack where they differ sufficiently from any corresponding host-cell function. However, owing to great similarity between viral and host functions, effective treatments haYe proven very difficult to identify. One group o~ viruses which has recently assumed a particular importanc~ are the retrovirllses. Retr~viruses form a sub-group of RNA viruses which, in order to r0plicate, must first 'reverse transcribe' the RNA of their genome into DNA ('transcription' conventionally describes the synthesis of RNA from DNA). Once in th~ form of DNA, the viral genome is incorporated into the host cell genome, allùwing it to take full advantage of the host cell's transcription/translation machinery for the purposes of replication. Once incorporated, the viral DNA is virtually indistinguishable from the host's DNA and, in this state, the virus may persist for as long as the cell lives. As it is virtually invulnerable to attack in this form, any treatment must be directed at another stage of the virus li~e cycle and will, of necessity, have to be continued until all virus-infected cells have died. HTLV-I and HTLV-II are both retroviruses and are known ~o be causative agents of leukaemia in man. HTL~/-I infections are especially widespread and are responsible for many deaths world-wide each year. A species of retrovirus has also been reproducibl~ isolated Erom patients with AIDS. The virus was originally known as human T-cell lymphotropic virus III (HTLV III), AIDS associated retrovirus (ARV), lymphadenopathy associated with (LAV), acquired immune deficiency virus (AIDV). m e inter- nationally agreed name for the virus is now Human Im~unodeficiency Virus (~IV). This virus (referred to herein as HIV) has been shown preferentiall~ to infect and destroy 1327~ -2- B457 .. , T-cells bearing the oKT4 surfaca marker and is now generally accepted as the aetiologic agent of AIDS. The patient progressively loses this set of T-cells, upsetting the overall balance of the immune system, reducing his ability to combat other infections, and predisposing him to opportunistic infections which frequently prove fatal. Thus, the usual cause of death in AIDS victims is by opportunistic infection, such as pneumonia or virally induced cancers, and not as a direct result of }t~V infection. Recently, HIV has alao been recovered from other tissue types, including B- cells expressing the T4 marker, macrophages and non-blood associated tissue in the central nervous system. This infection of the central nervous system has been discovered in patients expressing classical AIDS symptoms and is associatedwith progressive demyelination, leading to wasting and such symptoms as encephalopathy, progressive dysarthria, ataxia and disorientation. Further conditions associated with HrV infection arP the asymptomatic carrier state, progressive generalised Iymphadenopathy (PGL) and AlDS-related complex (ARC). Reports have described the testing of compounds against various retroviruses, for example, Murine Leukaemia Virus (MuLV) a mouse retrovirus. M.A. Waqar et al. [0. Cell. Phys., 121 (1984) 40~-408) found that the 2',3'- dideoxyribonucleosides of adenine, cytosine, thymine and guanine inhibited infection5 of cell-lines by MuLV, but no clear indication of therapeutic potential w~s given. . 1327~ - 2a We have now discovered that 2',3'-dideoxynucleosides, as referred to below, are useful for the treatment or prophylaxis of viral infections, particularly retroviral infections and especially AIDS. The present invention is p æ ticularly concerned with 2',3'-dideoxy- guanosine, 2',3'-dideoxycytosine, 2',3'-dideoxyadenosine and 2',3'-di- deoxyinosine and their use in the treatment or prophylaxis of Human Immunodeficiency Virus Infections or hepatitis-B virus infections, as well as to their use in producing antiviral formulations, and anti- viral formulations containing them. There is further disclosed herein 2',3'-dideoxynucleosides having the follow m g formula. ' -3- B457 B 1327~05 (I) wherein B represents a 6-methoxypurine, 6~methylthiopurine, 2-aminopurine or 2,6-diamunopurine base linked to the sugar residue at the 9-position of the base, or a pharmaceutically acceptable salt, ester or salt of such esters thereof. ... i ln vitro testing has shown that the compounds according to the invention have particularly good activity against the Following viruses: human T-cell lymphotropic viruses (HTLV), especially HTLV-19 HTLV-II and HIV (HTLV-III); feline leukaemia virus, equine infectious anaemia virus and other lentiviruses, as well as other human viruses such as hepatitis B virus and Epstein-Barr virus (EBV). Tl~e--invention accordingly provides the compounds according to-the invention for use in the treatment or prophylaxis of any of the above infections. Particularly good activity has been observed against those viruses which are retroviruses and also those DNA viruses which, like retroviruses, are incorporated into the host genome duriny their life-cycle, i.e. retrovirus-like DNA viruses. Thus, there is further provided the compounds according to the ~invention for use in the treatment or prophylaxis of retroviral, or retrovirus-like infections. It will be appreciated that the componds according to the invention may also be usecl in the manufacture of a medicament for the treatment or prophylaxis of any of the above-mentioned medical indications. . Preferred esters of the compoundsof formula (I) include carboxylic acid esters in which the non-carbonyl moiety of the ester grouping is selected from straight orbranched chain alkyl, alkoxyalkyl (e.g. methoxymethyl), aralkyl (e.g. benzyl), aryloxyalkyl (e.g. phenoxymethyl), aryl (e.g. phenyl optionally substituted by HDL/OLM/30th April 1986 1 3 2 7 ~ ~ . ? -4- B457 halogen, Cl 4 alkyl or Cl 4 alkoxy); sulphonate esters such as alkyl- or sralkylsulphonyl (e.g. methanesulphonyl); and mono-, di- or tri-phosphate esters. Any referenee to any of the above compounds also includes a reference to a pharmaceutically acceptable salt thereof. With regard to the above-described esters, unless otherwise specified, any alkylmoiety present advantageously contains 1 to 18 carbon atoms, particularly 1 to 4carbon atoms. Any aryl moiety present in such esters advantageously comprises a phenyl group. Examples of pharmaceutically acceptable salts of the compounds of formula (I) include base salts, e.g., derived fr~ an appropriate base, such as alkali metal (e.g., sodium), alkaline earth metal (e.g. magnesium) salts9 ammonium and NX4 (wherein X is Cl 4 alkyl). Physiologically acceptable salt~ of sn hydrogen atom or an amino group include salts of organic carboxylic acid~ such as acetic5 lactic, tartaric, malic, isethionie, lactobionic and succlnic aeids; organie sulfonic acids such as methanesulfonic, ethanesulfonic, benzenesulfonic and p-tolunesulfonic acids and inorganie acids such aa hydrochloric, sulfuric, phosphoric and sulfamic acids. Physiologieally aeceptable salts of a compound of an hydroxy group include the anion of said eompound in combination with a suitable eation such as Na+, NH4+, and NX4~ (wherein X is a Cl 4 alkyl group). Examples of ~Gmpounds according to the invention include:- 2'?3'-dideoxy-cytosine, 2',3'-dideoxy-adenosine, 2',3'-dideoxy-guanosine, ~327~ - 4a - 2',3'-dideoxy-inosine. Novel compounds of formula (I) disclosed herein include: 6-methylthiopurine-9thD-2',3'-dideoxyribofuranoside, 6-methoxypurine-9-PLD-2',3'-dideoxyribofuranoside, 2,6-diaminopurine-9-~-D-2',3'-dideoxyribofuranoside and 2-aminopurine-9-~-D-2',3'-dideoxyribofuranoside. mere is thus further provided the novel compounds of formula (I) wherein B represents a 2,6-diaminopurine, 6-methylthiopurine, 6-methoxypurine or 2-aminopurine base and their pharmaceutically acceptable derivatives, particularly for use in therapy. ., - 1327~ B457 . Specific examples of pharmaceutically acceptable esters of the compounds of formula (I) that may be used in accordance with the present invention includ~ the followin~ 5' esters: rl~nophosphate; disodium ~onophospha-te; diphosphate; triphospha~e; acetate; 3-methyl-butyrate; octanoate; palmitate; 3-chloro benzoate; benzoate; 4-~ethyl benzoate; hydrogen succinate; pivalate; and mesylate. Specific example of a pharmaceutically acceptable ~alt is the monosodium salt. The compounds according to the invention, also referred to herein as the actlve ingredient, may be administered for therapy by any suitable route including oral, rectal, nasal, topical (including buccal and sublingual), vaginal and parenteral~including subcutaneous, intramuscular, intravenous and intradermal). It will beappreciated that the preferred route will vary with the condition and age of therecipient, the nature of the infection and the chosen active inyredient. In general a suitable dose will be in the range of 3.0 to 120 mg per kilogram body weight of the recipient per day, preferably in the range of 6 to 90 mg per kilogram body weight per day and most preferably in the range 15 to 60 mg per kilogram body weight per day. The desired dose is preferably presented as two, three, four, five, six or or more sub-doses administered at appropriate intervals throughout the day. These sub-doses may be administered in unit dosage forms, for example, containing 10 to 1500 mg, preferably 20 to 1000 mg, and most preferably 50 So 7~0 mg of active ingredient per unit dosage form. Ideally, the active ingredient should be administered to achieve peak plasma concentrations of the active compound of from about 1 to about 75 ~JM, preferably about 2 to 50 t M, most preferably about 3 to about 30 1JM. This may be achieved, for example, by the intravenous injection of a 0.1 to 5% solution of the active ingredient, optionally in saline, or orally administered as a bolus containing about 1 to about 100 mg/kg of the active ingredient. Desirable blood levels may be maintained by a continuous infusion to provide about 0.01 to about 5.0 mg/kg/hour or by intermittent infusions containing about0.4 to about 15 mg/kg of the active ingredient. While it is possible for the active ingredient to be administered alone it is preferable to present it as a pharmaceutical formulation. The formulations of the present inYention cornprise at least one active ingredient, as above defined, together with one or more acceptable carriers thereof and optionally other therapeutic agents. Each carrier must be "acceptable" in the sense of being compatible with the other ingredients of the formulation and not injurious to the HDL/OLM/30th April 19~6 -6- ~327~ B457 patient. Formulations include those suitable for oral, rectal, nasal, topical (including buccal and sublingual), vaginal or parenteral (including subcutaneous, intramuscular, intravenous and intradermal) administration. The for~ulations may conveniently be presented in unit dosags form and may be prepared by any methods well known in the art of pharmacy. Such methods include the step of bringing into association the active ingredient with the carrier which constitutes one or more accessory ingredients. In general, the formulations are prepared by uniformly and intimately bringing into association the active ingredient with liquid carriers or finely divided solid carriers or both, and then if necessary shaping the product. Formulations of the present invention suitable for oral administration may be presented as discrete units such as capsules, cachets or tablets each containing a predetermined amount of the active ingredient; as a powder or granules; as a solution or a suspension in an aqueous or non-aqueous liquid; or as an oil-in-water Iiquid emulsion or a water-in-oil liquid emulsion. The active ingredient may also be presented as a bolus, electuary or paste. A tablet may be made by compression or moulding, optionally with one or more access~ry ingredients. Compressed tablets may be prepared by compressing in a suitable machine the active ingredient in a free-flowing form such as a powder or granules, optionally mixed with a binder (e.g. povidone, gelatin, hydroxypropylmethyl cellulose), lubricant, ir:ert diluent, preservative, disintegrant ~e.g. sodium starch glycollate, cross-linked povidone, cross-linkedsodium carboxymethyl c011ulose) surface-active or dispersing agent. Moulded tablets may be made by moulding in a suitable machine a mixture of the powdsred compound moistened with an inert liquid diluent. The tablets may optionally be coated or scored and may be ~ormulated so as to provide slow or controlled release of the active ingredient therein using, for example, hydroxypropylmethyl cellulose in varying proportions to provide the desired release profiie. Tablets may optionally be provided with an enteric coating, to provide release in parts of tha gut other than the stomach. This is particularlyadvantageous for the conpounds of formula ~I), as such compounds are susceptible to acid hydrolysis. Formulations suitable for topical adrninistration in the mouth include lozenges comprising the active ingredient in a flavoured basis, usually sucrose and acacia or tragacanth; pastilles comprising the active ingredient in an inert basis such as gelatin and glycerin, or sucrose and acacia; and mouthwashes comprising the active ingredient in a suitable liquid carrier. HDL/OLM/30th April l9a6 _7_ ~327~ B457 Formulations for rectal administration may be presented as a suppository with a suitsble base comprising for example cocoa butter or a salicylate. Formulations suitable for vaginal administration may be presented as pessaries, tampons, creams, gels, pastes, foams or spray formulations containing in addition to the active ingredient such carriers as are known in the art to be appropriate. Formulations suitable for parenteral administration include aqueous and non- aqueous isotonic sterile injection solutions which may contain anti-oxidants, buffers, bacteriostats and solutes which render the formulation isotonic with the blood of the intended recipient; and aqueous and non-aqueous sterile suspensionswhich may include suspending agents and thickening agents. The formulations may be presented in unit-dose or multi-dose sealed containers, for example, ampoules and vials, and may be stored in a freeze-dried (Iyophilized) condition requiring only the addition of the sterile liquid carrier, for example water forinjections, immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules and tablets of the kind previously described. Preferred unit dosage formulations are those containing a daily dose or unit, daily sub-dose, as herein above recited, or an appropriate fraction thereof, of an active ingredient. The compounds according to the invention may also be presented for use in the form of veterinary formulations, which may be prepared, for example, by methods that are conventional in the art. Examples of such veterinary formulations include those adapted for:- (a) oral administration, external application, for example drenches (e~g.aqueous or non-aqueous solutions or suspensions); tablets or boluses; powders, granules or pellets for admixture with feed stuffs; pastes for application to the tongue; (b) parenteral administration for example by sub-cutaneous, intramuscular or intravenous injection e.g. as a sterile solution or suspension; or (when appropriate) by intramammary injection where a suspension or solution is introduced into the udder via the teat; HDL/OLM/30th April 1986 -8- 132700~ B4$7 (c) topical application, e.g. as a cream, ointment or spray applied to the skin; or (d) intravaginally, e.g. as a pessary, cream or foam. The administered ingredients may also be used in therapy in conjunction with other medicaments such as 9-[[2-hydroxy-1-(hydroxy- methyl)ethoxy]methyl]guanine, 9-t2-hydroxyethoxymethyl)guanine (acyclovir), 2- amino-9-(2-hydroxyethoxymethyl)purine, interferon, e.g., a interferon, interleukin Il, and phosphonoformate, or in conjunction with other immune modulating therapy including bone marrow or 1ymphocyte transplants or medications such as levamisol or thymosin which would increase Iymphocyte numbers and/or function as is appropriate. It should be understood that in addition to the ingredients particularly mentioned above the formulations of this invention may include other agents conventional in the art having regard to the type of formulation in question, for example, those suitable for oral administration may include such further agents as sweeteners~ thickeners and flavouring agents. 2',3'-dideoxyadenosine, 2',3'~dideoxyinosine~ 2'-3'-dideoxyguanosine and 2',3'-dideoxycytidine are available frorn P.L. Biochemicals and may be prepared in conventional manner for example as described in Horwitz et al., ~. Org. Chem. 32(3), 817-18 (1967) illustrating the preparation of 2',3'-dideoxycytidine, and Prisbe et al., Synth. Commun. 1985, lS (5), 401-9 illustrating the preparation of 2'~3'-dideoxyadenosine and 2',3'-dideoxyguanosine. Other compounds according to the invention may be prepared in conventional manner, e.g. as described in the Examples. The Presant ~DP further includes a process for the preparation of a compound of formula (I) and pharmaceutically acceptable sal~s, esters or sa1ts of such esters, which co~prises either: (A) reacting a compound of formula: B R O j (ll~ HDL/OLM/3ûth April 1986 ~ -9- ~327~ E3457 (wherein B is as hereinbefore defined and R represents a precursor group for thehydroxy group, or for a pharmaceutically acceptable derivative group thereof) with an agent or under conditions serving to convert the said precursor group into the corresponding desired group; or (B) reacting a purine base of for~ula B - H (111) (wherein B is as hereinbefore defined). or a functional equivalent thereof, with a compound serving to introduce the desir2d ribofuranosyl ring at the 9- position respectively of the purine base of formula (III); and thersafter, or simultaneously therewith, effecting one or more of the ~ollowing optional conversions:- (i) when a compound of formula (I) is formed, converting it into apharmaceutically acceptable salt, ester or salt of such ester thereof, (Il) when a pharmaceutically acceptable salt, ester or salt of such ester of a compound of formula (I) is formed, converting the said salt, ester or salt of such ester into a compound of fonmula ~I), or a different salt, : ester or ~alt of such ester thereof. In the above-described process according to the invention, it will be appreciated that the precursor compounds of formula (Il) as well as the above-mentioned agents and conditions, will be selected from those that are known in the art of nucleoside synthetic chemistry. Examples of such conversion procedures are described hereinafter for guidance and it will be understood that they can be modified in conventional manner depending on the desired compound of formula (I). In particular, where a conversion is described which would otherwise resultin the undesired reaction of labile groups then such groups rnay be protected inoonventional manner, with subsequent removal of the protecting groups after cnmpletion of the conversion. With regard to process (A~, R may represent a protected hydroxy group e.g. an ester grouping of the type referred to above in relation to formula (I) HDL/OLM/30th April 1986 -lo- ~327~ B457 ,, particularly scetoxy, or an ether group such as a trialkylsilyloxy group, e.g. t- butyldirnethylsilyloxy or an aralkoxy group e.g. triphenylmethoxy. Such groups may be converted for example by hydrolysis to the desired hydroxy group or, by transesterification, to an alternative ester group. With regard to process (B), this may be effected for example by treating an appropriate purine base of fonmula (III) or a salt or ~ortected derivative thereof, with 2',3'-dideoxythymidine for example in the presence of the appropriate pentosyl transferring enzyme. The coNpoun~s of formula (I) may be prepared, for example, using purine nucleoside phosphorylase and thymidine phosphorylase. A compound of formula (I) may be converted into a pharmaceutically acceptable phosphate or other ester by reaction with respectively a phosphorylating agent, e.g. POCl3 or an appropriate esterifying agent, e.g. an acid halide or anhydride~ I The compound of formula (1), including esters thereof, may be converted into pharmaceutically acceptable salts thereof in conventional manner, e.g. by treatment with an appropriate base. An ester or salt of a compound of formula (I) may be converted into the parent compound, e.g. by hydrolysis. The following Examples are intended for illustration only and are not intended to limit the scop~ of the invention in any way. The term 'active ingredient' as used in the Examples means a compound of formula (I) or a pharmaceutically acceptable derivative thereof. ' Example 1 Tablet Formulations . The following formulations A and B were prepared by wet granulation of the ingredients with a solution of povidone, followed by addition of magnesium stearate and compression. HDL/OLM/30th April 1986 .. ... _ .. . .... . .. . .. . ...... .... . . . . . . . . . B457 1327~ mq/tabletmq~tablet Formulatlon A (a) Active ingredient 250 250 (b) Lactose B.P. 210 26 (c) Povidone B.P. 15 9 (d) Sodium Starch Glycollate 20 12 (e) Magnesium Steara~e 5 3 500 300 Formulation B mq/tabletmq/tablet (a) Active ingredient 250 250 (b) Lactose 150 (c) Avicel PH 101 6D 26 (d) Povidone B.P. 15 9 (e) Sodium Starch Glycollate 20 12 (f) Magnesium Stearate 5 3 500 300 Formulati_n C. mq/tablet Active ingredient 100 Lactose 2G0 Starch 50 Povidone 5 Magnesium stearate 4 359 The following formulations, D and E, were prepared by direct compression of the admixed ingredients. The lactose used in formulation E was of the direct compression type (Dairy Crest - "Zeparox"). Formulation D mq/ ablet Active Ingredient 250 Pregalatinised Starch NF15 150 400 * Trade Mark HDLtOLM/30th April 1~86 ~327al~ -12- B457 ,. , F ormulation E mq/ t~bl~ Active Ingredient 250 Lactose 150 Avicel * 100 500 Formulation F (Controlled Release Formulation) The formulation was prepared by wet qranulation of the ingredients (below) with a solution of povidone followed by the addition of magnesium stearate and compression. mq/tablet (a) Active Ingredient 500 ~b) Hydroxypropylmethylcellulose 112 (Methocel K4M Pramium)* (c) Lactose B.P. 53 (d) Povidone B.P.C. 28 (e) Magnesiurn Stearate 7 700 Drug release touk place over a period of about 6-8 hours and was complets after 12 hours. Example 2: Capsule Formulations Formulation A A capsule formulation was prepared by admixing the ingredients of Formulation D in Example 1 abova and filliny into a two-part hard gelatin capsule. Formulation B (infra) was prepared in a similar manner. ~ormulation B mq/capsule (a) Active ingrediant 250 (b) Lactose B.P. 143 (c) Sodium Starch Glycollate 25 (d) Magnesium Stearate 2 42 * Trade Mark HDL/OLM/30th April 1986 -13- 1 327~ B457 Formulation C mq/capsule (a) Active ingredient 250 (b) Macrogol 4D00 BP 350 600 Capsules were prepared by melting the macro~o1*4000 gp,dispersing the active ingredient in the melt and filling the melt into a two-part hard gelatin capsule. Formu!ation D mg/capsule Active ingredient 250 Lecithin 100 Arachis Oil 1oo 450 Capsules were prepared by dispersing the active ingredient in the lecithin and arachis oil and filling the dispersion into soft, elastic gelatin capsules. Formulation E (Controlled Release Capsule) The following controlled release capsule formulation was prepared by extruding ingredients a, b and c using an extruder, followed by spheronisation of the sxtrudate and drying. The dried pellets were then coatsd with release~ controlling membrane (d) and filled into a two-piece, hard gelatin capsule. mq/capsule (a) Active Ingredient 250 (b) Microcrystalline Cellulose 125 (c~ Lactose BP 125 (d) Ethyl Cellulose 13 513 * Trade Mark HDL/OLM/30th April 19û6 -14- 1327~ B457 Example 3: Injectable Formulation Formulation A. Actlve ingredient 0.2009 Hydrochloric acid solution1 0.1M q.s. to pH 4.0 to 7.0 Sodiurn hydroxide solution, 0.1M q.s. to pH 4.0 to 7.0 Sterile water q.s. to 10ml The active ingredient was dissolved in most of the water (35 -40 C) and the pH adjusted to between 4.0 and 7.û with the hydrochloric acid or the sodium hydroxide as appropriate. The- batch was then made up to volume with the water and filtered through a sterile micropore filter into a sterile 10ml amber glass vial (type 1) and sealed with sterile closures and overseals. Form_lation 8. Active ingredient 0.125 9 Sterile, pyrogsn-free, pH 7 phosphate buffer, q.s. to 25 ml Example 4- Intramuscular inLection Active Ingredient 0.20 9 Benzyl Alcohol 0.10 9 Glycofurol 75 * 1.45 9 Water for Injection q.s. to 3.00 ml The active ingredient was dissolved in the glycofurol. The benzyl alcohol was then added and dissolved, and water added to 3 ml. The mixture was then filtered through a sterile micropore filter and sealed in sterile 3 ml amber glass vials (type 1). Example 5: Syrup Active Ingredient 0.2500 9 Sorbitol Solution 1.5000 9 Glycerol 2.0000 9 Sodium Benzoate 0.0050 9 Flavour, Peach 17.42.3169 û.0125 ml Purified Water q.s. to 5.0000 ml * Trade Mark HDL/OLM/30th April 1986 -15- 13270~ B457 I he active ingredient was dissolved in a mixture of the glycerol and most of the purified water. An aqueous solution of the sodium ben~oate was then added to the solution, followed by addition of the sorbitol solution and finally the flavour. The volume was made up with purified water and mixed well. Example 6: Suppository m~/suppository Active Ingredient (63~m)* 250 Hard Fat, BP (Witepsol H15 - Dynamit NoBel) 1770 2020 *The actlve ingredient was used as a powder wherein at least 90% of theparticles were of 63~1m diameter or less. One-fifth of the Witepsol H15 was melted in a steam-jacketed pan at 45 ~C maximum. The active ingredient was sifted through a 2001Jm sieve and added to the molten base with mixing, using a silverson fitted with a cutting head, until a smooth dispersion was achieved. Maintaining the mixture at 45 C, the remaining Witepsol H15 was added to the suspension and stired to ensure a homogenous mix. The entire suspension was passed through a 250t)m stainless steel screen and, with continuous stirring, was allowed to cool to 40 C. At a temperature of 38 C to 40 C, 2.D2g of the mixture was filled into suitable, 2 ml plastic moulds. The suppositories were allowed to cool to room temperature. Example 7: Pessaries m~/pessary Active ingredient (631~m) 250 Anhydrate Dextrose 380 Potato Starch 363 Magnesium Stearate 7 1000 The above ingredients were mixed directly and pessaries prepared by direct compression of the resulting mixture. ** qrade Mark HDL/OLM/30th April 1986 -16- ~327~ B457 ~xample 8: 2,6-Diaminopurine-9- ~-D-2',3'-dideoxyribofuranoside 2,6-Diaminopurine (9.26 mmoles, 1.39 9) and 2',3'-dideoxythymidine (4.42 mmoles, 19) were suspended in 50 ml deionized water containing 0.4 ml lM K2HPO4. The pH of the suspension was adjusted to 7.6 with the addition of 0.1 M of KH2PO4. The enzyme catalysts purified frorn Escherichia coli, purine nucleoside phosphorylase (1340 I.U.) and thymidine phosphorylase (4450 I.U.) (Krenitsky et al., Biochemistry, 20 3615, 1981 and US Patent 4, 381, 344) were added and the suspension stirred at 35C. After 18 hours, an additional 2225 I.U. of thymidine phosphorylase was added. Two days later, the reaction was filtered and the filtrate stored at -20C. Upon thawing, the suspension was adjusted to apH of 10.6 with concentrated ammonium hydroxide and chromatographed on a column of Dowex~-formate resin (2.5 x 9 cm) with water as the elutant. Fractions containing product were combined and the solvent removed under vacuum. The residue was recrystallised from hot water yielding 2,6- diaminopurine-9-B-D-2',3'-dideoxyribofuranoside that analysed as a half hydrate (m.p. 192). Anal. Calcd- for CloH14N62 0.5H2O: C, 46.33; H, 5.83; N, 32.41. Found; C, 46.27; H, 5.83; N, 32.39 Example 9: 2-Aminopurine-9- 8-D-2',3'-dideoxyribofuranoside 2',3'-Dideoxythymidine (4.42 mmoles, 19), and 2-aminopurine (~.73 moles, 1.189) were combined in 50 ml deionised water containing 0.4 ml lM K2HPO4. The suspension had a pH of 7.8. Purine nucleoside phosphorylase (2680 I.U.) and thymidine phosphorylase (4500 I.U.) were added and the reaction stirred at 35C.On day two, additional thymidine phorsphorylase (2225 I.U.) was added and one day later the solids were filtered off and the filtrate stored at -20C. Upon thawing, solids wera removed and combined with the original reaction cake. The filtrate was adjusted to pH 10.5 with concentrated ammonium hydroxide and chromatographed on a Dowex*-l-formate column (2.5 x 11 cm). The product was eluted from the resin with water. After recrystallising from boiling water, a small quantity of 2-amino-9-~-D-2',3'-dideoxyribofuranoside was isolated. The remaining solids from the reaction were heated in 25 ml of water to a boil and filtered. This solution was combined with all the liquors from above, the volumereduced, and the solid recrystallised from water. The crystals were combined with the those obtained above to yield a final crop of 2-aminopurine-~ -2',3'- dideoxyribofuranoside half hydrate, m.p. 162C. Anal. Calcd. for * Trade Mark HDL/OLM/30th April 198 -17- 1327~5 B457 CloH13N502. 0.5H20~ , 49.17; H, 5.78; N, 28.67. Found; C, 48.97; H, 5.80; N, 28.67. Example 10: 6-Methoxypurine-9-~-D-2',3'-dideoxyribofuranoside 6-Methoxypurine (8.2 mmoles, 1.23 9) and 2',3'-dideoxythymidine (4.5 mmoles, l.û g) were suspended in 50 ml of a 30 mM potassium phosphate solution with a pH of 7.1. Purine nucleoside phosphorylase tl984 I.U.) and thymidine phosphorylase (7B92 I.U.) were added and the reaction incubated at 37 ~C. After 6 days the reaction mixture was filtered and the filtrate stored frozen at -20 C. Upon thawing, solids were removed by filtration and combined with the original reaction cake. The filtrate was adjusted to pH 10.6 with concentrated ammonium hydroxide and chromatographed on a Dowex*l-formate column (2.5 x 10 cm). The product was eluted from the resin with water. After rernoving the water by vacuum, the product was dissolved in 30% n-propanol (v/v) and chromatographed on a column containing P-2 resin (BioRad) (5 x 90 cm). The product was eluted from the column with 30% n-propanol/water (v/v) yielding the title compound after solvent removal (m.p. = 157 - 165 C). Anal. Calc. for CllH14N4O3: 52-79; H, 5-64; N~ 22.39. Found: C, 52.57; H, 5.70; N, 22.32 ~: 6-Methylthiopurine-9- ~-D-2's3'-dideoxyribofuranoside 6-Methylthiopurine (7.2 mrnoles, 1.2 9) 1.2 9) and 2',3'-dideoxythymidine (4.5 mmoles, 1 9) were added to 50 ml of deionized water containing 0.4 ml lM K2HPO4. The suspension had a pH of 7.7. Thymidine phosphorylase (4450 I.U.) and purine nucleoside phosphorylase (1340 I.U.) were added and the reaction stlrred at 37 C~ After 4 days the reaction mixture was filtered and the filtrate stored at -20 'CO Upon thawing, the solution was filtered and the filtrate chrornatographed on a column containing P-2 resin (5 x 90 cm)~ The column was eluted with 30% n-propanol/water (v/v). Product-containing fractions were combined and to this was added the original reaction cake. After removal of the solvent the residue was dissolved in 30% metilanol/water (v/v). The pH was adjusted to 10.3 with concentrated ammonium hydroxide and chromatographed on a Dowex-~-lFormate column (2.5 x 8 cm). The product was eluted with 30% methanol/water (v/v), dried under vacuum and redissolved in 30% n-propanol. The sample was chrornatographed on a colurnn containing P-2 resin (5 x 90 cm) * Trade .~rk I~DL/OLM/30th April 1986 ': ,~ -18- ~13 2 7 ~ ~ ~ B457 and eluted with 309~ n-propanol/water. Product containing fractions were combined and solvent removed to yield the title compound (m.p. = 105 ~C~. for C11~11425 ~ 49-61; H, 5.30; N, 21.04;5,12.04. Found: C, 49.63; H, 5.35; N, 20.99; 5,12.04. Example 12: Antiviral Activity . a) Feline Leukaemia Virus Susceptible feline embryo lung fibroblasts (FLF-3) were seeded onto multiwell slides (105 cells/ml, 0.05 ml/well) with Dulbecco-modified Eagle's essential medium (DME) and incubated at 37C overnight. Each of the 32 wells was then infected with 40-60 focus forming units (ffu) of feline leukaemia virus (FeLV) for one hour after which the medium was replaced with fresh DME with varying concentrations of 2,6-diaminopurine- 9-B-D-2',3'-dideoxyribofuranoside per 4 wells. Concentrations were 0, loO~ 10, 50,100, 200 and 400 ~JM. After 3 days incubation at 37C, the cultures were assayed by the indirect fluorescent antigen test for the production of FeLV. Complete absence of FeLV was seen at concentrations of 100 ~M and above, demonstrating total efficacy at these levels of drug. At 50 vM, 60% inhibition was seen, at 10 t~M, 45% inhibition, and 31% inhibition was noted at 0.1 ~M 2,6-diaminopurine-9-~-D-2',3'-dideoxyribofuranoside. b) HIV The ability of 2,6-diaminopurine-9-~-D-2',3'-dideoxyribofuranoside to block infection of cells by HIV was determined as follows. Cloned T4 positive tetanus specific T helper lymphocytes were infected with a pool of HIV isolates [at challenge doses of up to 5000 virions/cell] and cell survival after infection was rnonitored. After 10 days in culture no viral cytopathic effects were seen in infected T cells treated with 10 and 2 I~M 2,6- diaminopurine-9-~-D-2',3'-dideoxyribofuranoside, while untreated, infected cells were ~-fold decreased. This protective effect was seen on both days 10 and 13 of the experirnent. HDL/OLM/30th April 1986 1327~0~ B457 c) F_end Leukaemia virus The compounds have been tested for retroviral activity against friend leukaemia virus with the following results. Compound ED50 (~' M) 2,6-diaminopurine-9- ~-D-2',3'-dideoxyribofuranoside 22.35 2',7'-dideoxycytidine 15.32 2',3'-dideoxyadenosine 19~25 2',3'-dideoxyguanosine 25.89 Example 13 Cytotoxicity The following compounds were tested for cytotoxic effect at 10-4 M against human D-98 and mouse L cells. Figures shown are % growth relative to the control. Compound % Growth D-98 L 2',3'-dideoxycytidine 81 67 2',3'-dideoxyadenosine 94 59 2,6-diamino-9-(2',3'-dideoxy- ~-D-ribofuranosyl)-9H-purine Y7 41 2-amino-9-(2',3'-dideoxy- 8-D-ribofuranosyl)-9H-purine 97 107 9-(2',3'-dideoxy- ~-D-ribofuranosyl)-6-methylthio-9H-purine 98 91 2',3'-dideoxyguanosine 87 93 9-~2',3'-dideoxy-8-D-ribofuranosyl)-6 methoxy-9H-purine 92 90 HDL/OLM/30th April 1986