Patent Publication Number: US-5290772-A

Title: Immunosuppressant agent

Description:
This is a continuation of Ser. No. 07/23,481, filed on Oct. 10, 1989, now abandoned, which is a continuation of Ser. No. 07/213,025, filed Jun. 29, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a new immunosuppressant agent, &#34;demethimmunomycin&#34; (L-683,742: also described as 31-desmethoxy-31-hydroxy-L-683,590), a novel fermentation process for its production, utilizing the microorganism Actinoplanacete M (MA 6559), ATCC No. 53771. The process involves culturing the microorganism in the presence of L-683,590 under conditions which demethylates the C 31  methoxy group of L-683,590. Also disclosed is a method of their use in a human host for treatment of autoimmune diseases, infectious diseases and/or prevention of organ transplant rejections. 
     2. Brief Description of Disclosures in the Art 
     In 1983, the US FDA licensed cyclosporin, and extremely effective anti-rejection drug that revolutionized the field of organ transplant surgery. The drug acts by inhibiting the body&#39;s immune system from mobilizing its vast arsenal of natural protecting agents to reject the transplant&#39;s foreign protein. 
     As effective as the drug is in fighting transplantation rejection, it suffers drawbacks in causing kidney failure, liver damage and ulcers which in many cases can be very severe. 
     EPO Publication No. 0184162 to Fujisawa, describes a new macrolide immunosuppressant FK-506 which is reputed to be 100 times more effective than cyclosporin. The macrolide is produced by fermentation of a particular strain of Strentomyces tsukubaensis. Also described is the closely related macrolide immunosuppressant FK-520, produced by S. hygroscopicus subsp. yakushimaensis. 
     U.S. Pat. No. 3,244,592 to T. Arai describes the culturing of Strerptomyces hygroscopicus var, ascomyceticus to produce the antifungal &#34;ascomycin&#34;. 
     There is, however, no description in the literature of the production of any immunosuppressive agents, which substantially lack the side effects of cyclosporin. 
     Newer, safer drugs exhibiting less side effects are constantly being searched for in the field. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is an  1  H nuclear magnetic resonance (NMR) spectrum taken at 400 MHz of &#34;demethimmunomycin&#34; in CDCl 3 . 
     FIG. 2 is an  1  H NMR spectrum taken at 400 MHz of L-683,590 in CDCl 3 . 
     FIG. 3 is the assigned molecular structure of &#34;demethimmunomycin&#34;. 
    
    
     SUMMARY OF THE INVENTION 
     It has been found that a new immunosuppressant, &#34;demethimmunomycin&#34;, can be obtained by the fermentation of the microorganism Actinoplanacete sp., (MA 6559), ATCC No. 53771, with the macrolide immunosuppressant L-683,590, under submerged aerobic conditions in an aqueous carbohydrate medium, containing a nitrogen nutrient, said conditions being conducted at a pH of about 7 which are sufficient to selectively demethylate L-683,590 at the C-31 position. 
     The resultant &#34;demethimmunomycin&#34; exhibits immunosuppressive activity, i.e., positive inhibition of T-cell activation, as demonstrated by the calcium ionophore (ionomycin) plus phorbol myristate acetate (PMA) induced T-cell stimulation assay, also referred to herein as the &#34;T-cell proliferation assay&#34;. 
     The principle of this assay is to measure the proliferation of mouse T lymphocytes stimulated with the combination of ionomycin plus PMA. A positive sample in this assay will inhibit T-cell proliferation, as indicated by reduced tritiated thymidine uptake. 
     In accordance with this invention, there is provided a process for producing an immunosuppressant, identified as &#34;demethimmunomycin&#34;, comprising the step of culturing a strain of Actinoplanacete sp., together with L-683,590 under submerged aerobic fermentation conditions in an aqueous carbohydrate medium, containing a nitrogen nutrient, for a sufficient time to produce product &#34;demethimmunomycin&#34;. 
     Further provided is a new immunosuppressant, &#34;demethimmunomycin&#34;, produced by the above process which exhibits positive inhibition of T-cell activation by the T-cell proliferation assay and exhibits a proton nuclear magnetic resonance spectrum as identified in FIG. 1. 
     Also provided is a pharmaceutical composition containing a therapeutically effective amount of &#34;demethimmunomycin&#34; in combination with a pharmaceutically acceptable, substantially non-toxic carrier or excipient. 
     In addition, there is provided a method of use for treating human host to prevent transplantation rejection, or for treating autoimmune disease or infectious disease comprising administering to said host a therapeutically effective amount of &#34;demethimmunomycin&#34;. 
     DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
     The present invention involves the fermentation of Actinoplanacete sp., MA 6559, together with L-683,590 to produce &#34;demethimmunomycin&#34;. The microorganism is currently on restricted deposit with the American Type Culture Collection, 12301 Parklawn Drive in Rockville, Md. as ATCC No. 53771, and in the Merck Culture Collection in Rahway, N.J. as MA 6559. The physical characteristics and taxonomy, including morphological, cultural, biological and physiological characteristics are briefly described hereinbelow. 
     On the basis of the taxonomic analysis performed thus far, the culture has tentatively been assigned in the order Actinomycetales and in the family Actinoplanacea. Further taxonomic characteristics are being examined to place this organism conclusively within a genus and species. 
     This culture grows well on routine media including trypticase soy agar (28° and 37° C.), yeast malt extract agar, glycerol asparagine agar, inorganic salt starch agar, oatmeal agar, Czapek Dox, Czapek solution agar and peptone agar, and Bennett&#39;s agar, all at 28° C. 
     Morphology--This culture grows as a branched filamentous mycelium with a diameter of 0.2-0.4 microns. Colonies are opaque, raised, and erose. Colony texture is rubbery on yeast malt extract agar but tends to be butyrous on other media where significant fragmentation of the mycelium is observed. The colony surface tends to be powdery in appearance. No diffusable pigments were observed. 
     Sporancia--are predominantly spherical and range in size from 4-25 microns in diameter. Sporangia are generally visible by 21 days and tend to coalesce on glycerol asparagine agar. Spores are rod shaped with blunt ends (0.76×1.9 microns), non-motile and occur in long, unbranched chains of up to 150 microns in length. 
     CULTURAL CHARACTERISTICS OF MA 6559 
     Yeast Extract-Malt Extract Agar (ISP Medium 2) 
     Vegetative mycelium is hyaline to yellow, aerial mycelium develops in 24-72 h and is buff to rose-pink and powdery in appearance. The reverse side is tan to reddish brown. 
     Oatmeal Agar (ISP Medium 3) 
     Vegetative mycelium is hyaline to yellow, the reverse side is hyaline to tan. Aerial growth is white to light rose-beige and powdery in appearance. 
     Inorganic Salts-Starch Agar (ISP Medium 4) 
     Light growth, scant aerial mycelium. Vegetative growth is hyaline and highly fragmented. Clearing of starch occurs at periphery of colonies noted by 7 d. 
     Glycerol Asparagine Agar (ISP Medium 5) 
     Vegetative growth is hyaline to yellow, the reverse side is hyaline to cinnamon brown. Aerial mycelium is powdery and white to rose-pink. 
     Peptide-Iron-Yeast Extract Agar (ISP Medium 6) 
     Vegetative growth is tan. No aerial growth observed, no melanoid pigments produced. 
     Tyrosine (ISP Medium 7) 
     Vegetative growth is tan becoming deep purple as culture ages. Aerial mycelium is velvety to grayed rose-beige. 
     Czapek-Dox Agar 
     Vegetative growth is tan with a pink tone as the culture ages. Aerial mycelia are short and matted with a moist appearance. 
     The present invention process can be practiced with any &#34;demethimmunomycin-producing&#34; strain of Actinoplanacete sy., and particularly preferred is the ATCC No. 53771 strain. 
     In general, &#34;demethimmunomycin&#34; can be produced by culturing (fermentation) the demethimmunomycin substance-producing strain in an aqueous nutrient medium containing sources of assimilable carbon and nitrogen, preferably under submerged aerobic conditions (e.g. shaking culture, submerged culture, etc.). The aqueous medium is preferably maintained at a pH of about 7 at the initiation and termination (harvest) of the fermentation process. A higher pH leads to substantial and/or total loss of product. The desired pH may be maintained by the use of a buffer such as morpholinoethanesulfonic acid (MES), morpholinopropanesulfonic acid (MOPS), and the like, or by choice of nutrient materials which inherently possess buffering properties, such as production media described hereinbelow. 
     The preferred sources of carbon in the nutrient medium are carbohydrates such as glucose, xylose, galactose, glycerin, starch, dextrin, and the like. Other sources which may be included are maltose, rhamnose, raffinose, arabinose, mannose, salicin, sodium succinate, and the like. 
     The preferred sources of nitrogen are yeast extract, meat extract, peptone, gluten meal, cottonseed meal, soybean meal and other vegetable meals (partially or totally defatted), casein hydrolysates, soybean hydrolysates and yeast hydrolysates, corn steep liquor, dried yeast, wheat germ, feather meal, peanut powder, distiller&#39;s solubles, etc., as well as inorganic and organic nitrogen compounds such as ammonium salts (e.g. ammonium nitrate, ammonium sulfate, ammonium phosphate, etc.), urea, amino acids, and the like. 
     The carbon and nitrogen sources, though advantageously employed in combination, need not be used in their pure form, because less pure materials which contain traces of growth factors and considerable quantities of mineral nutrients, are also suitable for use. When desired, there may be added to the medium mineral salts such as sodium or calcium carbonate, sodium or potassium phosphate, sodium or potassium chloride, sodium or potassium iodide, magnesium salts, copper salts, cobalt salts, and the like. If necessary, especially when the culture medium foams seriously, a defoaming agent, such as liquid paraffin, fatty oil, plant oil, mineral oil or silicone may be added. 
     The L-683,590 starting material can be obtained by the fermentation of S. hygroscopicus var, ascomyceticus, ATCC No. 14891, as described in U.S. Pat. No. 3,244,592, and by the fermentation of S. hygroscoricus subs. yakushimaensis No. 7278, (to produce FR-900520, or &#34;FK-520&#34;, which is identical to L-683,590) as described in EPO Publication No. 0184162 to Fujisawa, said above references hereby incorporated by reference for this particular purpose. 
     As to the conditions for the production of demethimmunomycin in massive amounts, submerged aerobic cultural conditions are preferred therefor. For the production in small amounts, a shaking or surface culture in a flask or bottle is employed. Furthermore, when the growth is carried out in large tanks, it is preferable to use the vegetative form of the organism for inoculation in the production tanks in order to avoid growth lag in the process of production of demethimmunomycin. Accordingly, it is desirable first to produce a vegetative inoculum of the organism by inoculating a relatively small quantity of culture medium with spores or mycelia of the organism produced in a &#34;slant&#34; and culturing said inoculated medium, also called the &#34;seed medium&#34;, and then to transfer the cultured vegetative inoculum aseptically to large tanks. The fermentation medium, in which the inoculum is produced, is substantially the same as or different from the medium utilized for the production of demethimmunomycin and is generally autoclaved to sterilize the medium prior to inoculation. The pH of the medium is generally adjusted to about 7.0 prior to the autoclaving step by suitable addition of an acid or base, preferably in the form of a buffering solution. 
     Agitation and aeration of the culture mixture may be accomplished in a variety of ways. Agitation may be provided by a propeller or similar mechanical agitation equipment, by revolving or shaking the fermentor, by various pumping equipment or by the passage of sterile air through the medium. Aeration may be effected by passing sterile air through the fermentation mixture. 
     The fermentation is usually conducted at a temperature between about 20° C. and 40° C., preferably 25°-35° C., for a period of about 10 hours to 20 hours, which may be varied according to fermentation conditions and scales. Preferably, the production cultures are incubated for about 17 hours at 27° C. on a rotary shaker operating at 220 rpm, wherein the pH of the fermentation medium is maintained at 7.0 to harvest. 
     Preferred culturing/production media for carrying out the fermentation include the following media: 
     
         ______________________________________                                    
                  g/l                                                     
______________________________________                                    
Seed Medium A                                                             
Dextrose            1.0                                                   
Dextrin             10.0                                                  
Beef Extract        3.0                                                   
Ardamine pH         5.0                                                   
NZ Amine Type E     5.0                                                   
MgSO.sub.4.7H.sub.2 O                                                     
                    0.05                                                  
K.sub.2 HPO.sub.4   0.37                                                  
Adjust pH to 7.1                                                          
Add CaCO.sub.3 0.5 g/l                                                    
Transformation Medium B                                                   
Glucose             10                                                    
Hycase SF           2                                                     
Beef Extract        1                                                     
Corn Steep Liquor   3                                                     
Adjust pH to 7.0                                                          
Transformation Medium C                                                   
Mannitol            5                                                     
Glycerol            5                                                     
Hycase SF           2                                                     
Beef extract        1                                                     
Corn Steep Liquor   3                                                     
Adjust pH to 7.0                                                          
______________________________________                                    
 
    
     The produced demethimmunomycin can be recovered from the culture medium by conventional means which are commonly used for the recovery of other known biologically active substances. The demethimmunomycin substance produced is found in the cultured mycelium and filtrate, and accordingly can be isolated and purified from the mycelium and the filtrate, which are obtained by filtering or centrifuging the cultured broth, by a conventional method such as concentration under reduced pressure, lyophilization, extraction with a conventional solvent, such as methanol and the like, pH adjustment, treatment with a conventional resin (e.g. anion or cation exchange resin, non-ionic adsorption resin, etc.), treatment with a conventional adsorbent (e.g. activated charcoal, silicic acid, silica gel, cellulose, alumina, etc.), crystallization, recrystallization, and the like. A preferred method is solvent extraction, particularly using methanol. 
     The product demethimmunomycin from the fermentation exhibits positive immunosuppressive activity by the &#34;T-cell proliferation assay&#34; and possesses utility on this basis and exhibits the following physical characteristics: 
     1. White amorphous powder 
     2. Solubility in methanol 
     3. Molecular weight of 777, as determined by FAB mass spectroscopy (observed M +Li=784), is consistent with the assigned structure in FIG. 3, corresponding to the loss of 14 mass units, the CH 2  group, from L-683,590. 
     The key NMR characteristic of the assigned demethimmunomycin structure (FIG. 3), is the loss of a methoxy signal in FIG. 1 from FIG. 2, coupled with the absence of the H-31 resonance at its characteristic chemical shift near 3.1 ppm. Since a CH with an attached OH is generally 0.1-0.3 ppm downfield relative to a CH-OCH 3  it is reasonable to conclude that the 31 methyl is lost and that H-31 is displaced into the 3.3-3.5 ppm multiplet containing H-13, H-21, H-33 and the remaining methoxyls. Further support for assigning the missing methoxyl to C-31 rather than to C-13 or C-15 is the fact that the chemical shifts of H-14 and H-15 in FIG. 1 are identical to those in the parent molecule in FIG. 2. It is also reasonable to infer that the C-31 demethylation is the sole structural change since no new resonances are seen and all recognizable proton signals from H-2 to H-28 are virtually superimposed with those in L-683,590. 
     The demethimmunomycin obtained according to the fermentation processes as explained above can be isolated and purified in a conventional manner, for example, extraction, precipitation, fractional crystallization, recrystallization, chromatography, and the like. 
     Suitable salts of the material may include pharmaceutically acceptable salts such as basic salts, for example, alkali metal salt (e.g. sodium salt, potassium salt, etc.), alkaline earth metal salt (e.g. calcium salt, magnesium salt, etc.), ammonium salt, amine salt (e.g. triethylamine salt, N-benzyl-N-methylamine salt, etc.) and other conventional organic salts. 
     It is to be noted that in the aforementioned fermentation reactions and the post-treatment of the fermentation mixture therein, the conformer and/or stereo isomer(s) of demethimmunomycin due to asymmetric carbon atom(s) or double bond(s) of the demethimmunomycin may occasionally be transformed into the other conformer and/or stereoisomer(s), and such cases are also included within the scope of the present invention. 
     The demethimmunomycin of the present invention possesses pharmacological activity such as immunosuppressive activity, antimicrobial activity, and the like, and therefore are useful for the treatment and prevention of the transplantation rejection of organs or tissues such as heart, kidney, liver, medulla ossium, skin, etc., graft-versus-host diseases by medulla ossium transplantation, autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, Hashimoto&#39;s thyroiditis, multiple sclerosis, myasthenia gravis, type I diabetes, uveitis, and the like. 
     The pharmaceutical composition of this invention can be used in the form of a pharmaceutical preparation, for example, in solid, semisolid or liquid form, which contains the demethimmunomycin, of the present invention, as an active ingredient, in admixture with an organic or inorganic carrier or excipient suitable for external, enteral or parenteral applications. The active ingredient may be compounded, for example, with the usual non-toxic, pharmaceutically acceptable carriers for tablets, pellets, capsules, suppositories, solutions, emulsions, suspensions, and any other form suitable for use. The carriers which can be used are water, glucose, lactose, gum acacia, gelatin, mannitol, starch paste, magnesium trisilicate, talc, corn starch, keratin, colloidal silica, potato starch, urea and other carriers suitable for use in manufacturing preparations, in solid, semisolid, or liquid form, and in addition auxiliary, stabilizing, thickening and coloring agents and perfumes may be used. The active object compound is included in the pharmaceutical composition in an amount sufficient to produce the desired effect upon the process or condition of diseases. 
     For applying this composition to a human, it is preferable to apply if by parenteral or enteral administration. While the dosage of therapeutically effective amount of the demethimmunomycin, varies from, and also depends upon the age and condition of each individual patient to be treated, a daily dose (calculated on the basis of a 70 kg man) of about 0.01-1000 mg, preferably 0.1-500 mg and more preferably 0.5-100 mg, of the active ingredient is generally given for treating diseases, and an average single dose of about 0.5 mg, 1 mg, 5 mg, 10 mg, 50 mg, 100 mg, 250 mg and 500 mg is generally administered. 
     The following examples are given for the purpose of illustrating the present invention and should not be construed as being limitations on the scope or spirit of the instant invention. 
     EXAMPLE 1 
     Microorganism and Culture Conditions 
     The lyophilized culture (MA 6559) ATCC No. 53771 was used to inoculate a 250 ml baffled shake flask containing 50 ml of an autoclaved (sterilized) seed medium consisting of (in units of grams/liter) dextrin 10.0%, dextrose 1.0%, beef extract 3.0%, ardamine PH (Yeast Products, Inc.) 5.0%, N-Z Amine type E 5.0%, MgSO 4 .7H 2  O 0.05%, KH 2  PO 4  0.37%, and CACO 3  0.5%. The pH of the seed medium was adjusted to 7.1 before autoclaving. The seed was incubated in the seed medium at 27° C. for 48 hours on a rotary shaker operating at 220 rpm. Alternatively, when frozen vegetative mycelia or a slant source is used, the culture is incubated in the seed medium at 27° C. for 24 hours at 220 rpm. A 2.5 ml aliquot of the resulting seed medium was used to inoculate a 250 ml non-baffled shake flask containing 50 ml of the following previously autoclaved (sterilized) transformation medium B. L-683,590 was added as a solution in dimethylsulfoxide to achieve a final concentration of 0.1 mg/ml concentration. The shake flask contents were subsequently incubated for 18 hours at 27° C. on a rotary shaker operating at 220 rpm: 
     1. Transformation medium B consisted of (in grams/liter) glucose 10.0; Hycase SF 2.0; beef extract 1.0; corn steep liquor 3.0; where the pH was adjusted to 7.0 before autoclaving. 
     Isolation-and Purification Procedure for the Broth 
     The whole broth (100 ml) of transformation media B was extracted three times with methylene chloride (3×100 ml). Methylene chloride extracts were combined, dried over sodium sulfate, and concentrated under vacuum to an oily residue. The residue was dissolved in acetonitrile and subjected to high performance liquid chromatography (HPLC) purification. 
     HPLC was carried out on Whatman Partisil 10 ODS-3, 9.4 mm×25 cm column and monitored at 205 nm and 225 nm at 60° C. The column was developed with linear gradient from 0.1% aqueous H 3  PO 4  --CH 3  CN, 45:55 to 0.1% aqueous H 3  PO 4  --CH 3  CN, 20:80 in 30 minutes. The compound was collected during repeated injections of the above described extract. The fractions at retention time 24 minutes were pooled, adjusted to pH 6.5 and evaporated to remove acetonitrile. The compound was further purified using a C 18  Sep-Pak (Waters Associates) and acetonitrile-water elution solvent to yield 1.4 mg. of product, designated as L-683,742, &#34;demethimmunomycin&#34;. Similar results were obtained by the use of transformation medium C. 
     Characterization 
     Demethimmunomycin was characterized via NMR spectrometry yielding the NMR spectrum of FIG. 1. The assigned molecular structure is shown in FIG. 3. 
     EXAMPLE 2 
     T-Cell Proliferation Assay 
     1. Sample Preparation 
     Purified demethimmunomycin, as prepared by HPLC above, was dissolved in absolute ethanol at 1 mg/ml. 
     2. Assay 
     Spleens from C57Bl/6 mice were taken under sterile conditions and gently dissocii3ted in ice-cold RPMI 1640 culture medium (GIBCO, Grand Island, N.Y.) supplemented with 10% heat-inactivated fetal calf serum (GIBCO). Cells were pelleted by centrifugation at 1500 rpm for 8 minutes. Contaminating red cells were removed by treating the pellet with ammonium chloride lysing buffer (GIBCO) for 2 minutes at 4° C. Cold medium was added and cells were again centrifuged at 1500 rpm for 8 minutes. T lymphocytes were then isolated by separation of the cell suspension on nylon wool columns as follows: Nylon wool columns were prepared by packing approximately 4 grams of washed and dried nylon wool into 20 ml plastic syringes. The columns were sterilized by autoclaving at 250° F. for 30 minutes. Nylon wool columns were wetted with warm (37° C.) culture medium and rinsed with the same medium. Washed spleen cells resuspended in warm medium were slowly applied to the nylon wool. The columns were then incubated in an upright position at 37° C. for 1 hour. Non-adherent T lymphocytes were eluted from the columns with warm culture medium and the cell suspensions were spun as above. 
     Purified T lymphocytes were resuspended at 2.5×10 5  cells/ml in complete culture medium composed of RPMI 1640 medium with 10% heat-inactivated fetal calf serum, 100 MM glutamine, 1 MM sodium pyruvate, 2×10 -5  M 2-mercaptoethanol and 50 pg/ml gentamycin. Ionomycin was added at 250 ng/ml and PMA at 10 ng/ml. The cell suspension was immediately distributed into 96 well flat-bottom microculture plates (Costar) at 200 μl/well. The control, being the medium without test drug, and various below-indicated dilutions of the sample (above-described purified demethimmunomycin) to be tested were then added in triplicate wells at 20 μI/well. L-679,934 was used as a standard. The culture plates were then incubated at 37° C. in a humidified atmosphere of 5% CO 2  -95% air for 44 hours. The proliferation of T lymphocytes was assessed by measurement of tritiated thymidine incorporation. After 44 hours of culturing, the cells were pulse-labelled with 2 μCi/well of tritiated thymidine (NEN, Cambridge, Mass.). After another 4 hours of incubation, cultures were harvested on glass fiber filters using a multiple sample harvester. Radioactivity of filter discs corresponding to individual wells was measured by standard liquid scintillation counting methods (Betacounter). Mean counts per minute of replicate wells were calculated and the results expressed as percent inhibition of tritiated thymidine uptake (proliferation) as follows: ##EQU1## 
     The results of % inhibition at various concentrations of demethimmunomycin are presented in the following Table: 
     
                       TABLE                                                       
______________________________________                                    
Inhibition of T-Cell Proliferation                                        
by Demethimmunomycin                                                      
Demethimmunomycin (ng/ml                                                  
                    % Inhibition                                          
______________________________________                                    
10                  98.6                                                  
6.6                 97.9                                                  
4.4                 91.9                                                  
2.9                 86.3                                                  
1.9                 72.1                                                  
1.3                 38.2                                                  
0.9                 9.0                                                   
0.6                 0                                                     
______________________________________                                    
 Notes:                                                                   
 1. Mouse T cell cultures were pulsed with .sup.3 Hthymidine for 4 hours  
 prior to harvesting at 48 hours.                                         
 2. Standard L679,934 (10 ng/ml) gave 99% inhibition.                     
 3. IC.sub.50 = 1.4 ng/ml = 1.81 nM, for demethimmunomycin, (L638, 742),  
 and generally in the range of 1.6 to 1.9 × 10.sup.-9 molar.        
 4. Inhibition of Tcell proliferation by demethimmunomycin was reversed by
 the addition of 50 μ/ml of IL2 (recombinant IL2) at the initiation of 
 culture.