Patent Publication Number: US-2023159448-A1

Title: Method for Preparation of N-Acetyl Cysteine Amide and Derivatives Thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 62/561,129, filed Sep. 20, 2017, entitled “Method for Preparation of N-Acetyl Cysteine Amide and Derivatives” and is a Continuation-in-Part of U.S. Ser. No. 16/137,262, filed Sep. 20, 2018, entitled “Method for Preparation of N-Acetyl Cysteine Amide and Derivatives”, now U.S. Pat. No. 10,590,073, and is a Continuation of U.S. Ser. No. 16/818,416, filed Mar. 13, 2020, now U.S. Pat. No. 11,091,433, and is a Continuation-in-Part of U.S. Ser. No. 17/366,913, filed Jul. 2, 2021, now U.S. Pat. No. 11,548,851, the entire contents of each of which are incorporated herein by reference. 
    
    
     STATEMENT OF FEDERALLY FUNDED RESEARCH 
     None. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to the field of methods for preparing N-Acetyl Cysteine Amide (N-acetylcysteine amide, NACA), (2R,2R′)-3,3′-disulfanediyl bis(2-acetamidopropanamide, diNACA), and derivatives thereof. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with synthesis of N-Acetyl Cysteine Amide and diNACA. 
     One such method is taught in U.S. Patent Publication No. 20170183302, filed by Warner, et al., entitled “Method for Preparation of N-Acetyl Cysteine Amide”. Briefly, these inventors teach a process for the preparation of N-acetyl-L-cysteine amide (NACA) starting with N-acetyl-L-cysteine that involves contacting N-acetyl-L-cysteine with an organic alcohol and an inorganic acid to form an organic solution containing N-acetyl-L-cysteine ester; neutralizing the acid in the organic solution with an aqueous solution containing a base to form a neutralized mixture; separating an organic solution containing N-acetyl-L-cysteine ester from the neutralized mixture; removing the N-acetyl-L-cysteine ester from the organic solution under reduced pressure; and contacting the N-acetyl-L-cysteine ester with ammonia. 
     Another such method for the preparation of N-acetyl cysteine amide (NACA) was previously described by Martin, et al., entitled “Amides of N-Acylcysteines as Mucolytic Agents”, J. Med. Chem. 1967, 10, 1172-1176. 
     However, a need remains for developing an efficient method for the effective, large-scale synthesis of (2R,2R′)-3,3′-disulfanediyl bis(2-acetamidopropanamide, diNACA and N-acetyl cysteine amide that provides the product in high chemical yields and, in particular, high chemical and enantiomeric purity, without the need for chromatography. 
     SUMMARY OF THE INVENTION 
     As embodied and broadly described herein, an aspect of the present disclosure relates to an efficient method or process for the preparation of NACA and diNACA in high chemical yields and high enantiomeric purity. Dimethyl 3,3′-disulfanediyl(2R,2′R)-bis(2-aminopropanoate) dihydrochloride is commonly referred to herein as L-cystine dimethyl ester dihydrochloride. Dimethyl 3,3′-disulfanediyl(2R,2′R)-bis(2-acetamidopropanoate) is commonly referred to herein as di-N-acetylcystine dimethyl ester or Di-NACMe. (2R,2′R)-3,3′-disulfanediylbis(2-acetamidopropanamide) is commonly referred to herein as diNACA. (R)-2-acetamido-3-mercaptopropanamide is commonly referred to herein as NACA or NPI-001 or N-acetylcysteine amide. Specifically, disclosed herein is a process comprising: contacting cystine with an alcohol and a chlorinating reagent to form an organic solution containing L-cystine dimethylester dihydrochloride; combining dried or undried L-cystine dimethylester dihydrochloride with a triethylamine, an acetic anhydride, and an acetonitrile to form a di-N-acetylcystine dimethylester; mixing dried di-N-acetylcystine dimethylester with ammonium hydroxide to form a di-N-acetylcystine amide; and reducing dried di-N-acetylcystine dimethylester into N-acetylcysteine amide with dithiothreitol, triethylamine and an alcohol. In another aspect the synthesis of diNACA or NACA can begin with L-cystine dimethylester dihydrochloride. In one aspect, the alcohol is an organic alcohol selected from an alkyl alcohol, methanol, ethanol, propanol, iso-propanol or butanol. In another aspect, the step of contacting L-cystine with an alcohol and a chlorinating reagent to form an organic solution containing L-cystine dimethylester dihydrochloride is conducted at −10 to 10° C. and then heated to reflux at 65 to 70° C. to completion. In another aspect, the chlorinating agent is thionyl chloride. In another aspect, there is a solvent exchange between the contacting and the combining steps. In another aspect, the step of combining is performed at −10 to 10° C. In another aspect, a precipitate formed in the combining step that is filtered and washed with ethyl acetate before drying under vacuum. In another aspect, the step of combining uses at least 15 volumes acetonitrile at −10 to 10° C. before adding at least 4 equivalents of triethylamine followed by at least 2 equivalents of acetic anhydride. In another aspect, the organic solvent is ethyl acetate. In another aspect, the process further comprises drying the organic solution removed from the neutralized mixture with a drying agent. In another aspect, the process further comprises the step of free-basing the triethylamine from the di-N-acetylcystine dimethylester with saturated sodium bicarbonate after reaction completion. In another aspect, the ammonia is provided in the form of aqueous ammonium hydroxide. In another aspect, the contacting of the N-acetyl-L-cystine ester with ammonia is performed at 0° C. In another aspect, no metals are used in the reduction of di-N-acetylcystine dimethylester into N-acetylcysteine amide. In another aspect, the step of removing the organics under reduced pressure is performed at about 45° C. or less. In another aspect, the step of removing the organics under reduced pressure is performed at about 35° C. or less. In another aspect, the step of removing the organics under reduced pressure is performed at about 30° C. or less. In another aspect, the step of removing the organics under reduced pressure is performed at about 45° C. In another aspect, the organic solution removed from the neutralized mixture is filtered to remove solids. In another aspect, the one or more reducing agents is selected from triphenylphosphine, thioglycolic acid or dithiothreitol and the organic solvent is tetrahydrofuran (THF), dichloromethane (DCM), isopropanol (2-propanol), or ethanol. 
     As embodied and broadly described herein, an aspect of the present disclosure relates to a compound having a formula: 
     
       
         
         
             
             
         
       
     
     As embodied and broadly described herein, an aspect of the present disclosure relates to a compound having a formula: 
     
       
         
         
             
             
         
       
     
     As embodied and broadly described herein, an aspect of the present disclosure relates to a process for making N-acetylcysteine amide or diNACA comprising: 
     
       
         
         
             
             
         
       
     
     As embodied and broadly described herein, an aspect of the present disclosure relates to a process for making diNACA starting with L-cystine dimethylester dihydrochloride rather than L-cystine: 
     
       
         
         
             
             
         
       
     
     In another embodiment, the present invention includes a process for making N-acetylcysteine amide or diNACA comprising: contacting L-cystine with an alcohol and a chlorinating reagent to form an organic solution containing L-cystine dimethylester dihydrochloride; combining dried L-cystine dimethylester dihydrochloride with a triethylamine, an acetic anhydride, and an acetonitrile to form a di-N-acetylcystine dimethylester; and mixing dried di-N-acetylcystine dimethylester with ammonium hydroxide to form a di-N-acetylcystine amide; and reducing dried di-N-acetylcystine amide into N-acetylcysteine amide with dithiothreitol, triethylamine, and an alcohol, wherein the reduction is without the presence of a metal. In one aspect, the one or more reducing agents is selected from tris (2-carboxyethyl)phosphine, thioglycolic acid, dithiothreitol, and the organic solvent is THF or dichloromethane (DCM), isopropanol, or ethanol, and the base is triethylamine and or sodium bicarbonate. 
     As embodied and broadly described herein, an aspect of the present disclosure relates to a process for making (2R,2R′)-3,3′-disulfanediyl bis(2-acetamidopropanamide) (DiNACA) comprising: contacting cystine with an alcohol and a chlorinating reagent to form an organic solution containing L-cystine dimethylester dihydrochloride; combining dried or undried L-cystine dimethylester dihydrochloride with a triethylamine, an acetic anhydride, and an acetonitrile to form a di-N-acetylcystine dimethylester; and mixing dried di-N-acetylcystine dimethylester with ammonium hydroxide to form a di-N-acetylcystine amide. 
     As embodied and broadly described herein, an aspect of the present disclosure relates to a process for making di-N-acetylcystine amide (diNACA) comprising: combining a dried L-cystine dimethylester dihydrochloride with triethylamine, acetic anhydride, and acetonitrile to form a di-N-acetylcystine dimethylester and optionally isolating and drying the di-N-acetylcystine dimethylester; mixing a dried di-N-acetylcystine dimethylester with ammonium hydroxide to form a di-N-acetylcystine amide and optionally isolating and drying the di-N-acetylcystine amide. In one aspect, the diNACA is further purified by the steps of: suspending the diNACA in degassed water and heated to reflux, cooling the diNACA solution to ambient temperature, filtering the diNACA, washing with ethanol, and drying to yield diNACA as a final product. 
     As embodied and broadly described herein, an aspect of the present disclosure relates to a process for making purified di-N-acetylcystine amide (diNACA) comprising: combining a dried or undried L-cystine dimethylester dihydrochloride with triethylamine, acetic anhydride, and acetonitrile to form a di-N-acetylcystine dimethylester and optionally isolating and drying the di-N-acetylcystine dimethylester; mixing a dried or undried di-N-acetylcystine dimethylester with ammonium hydroxide to form a di-N-acetylcystine amide and optionally isolating and drying the di-N-acetylcystine amide; and purifying the diNACA by suspending the diNACA in degassed water and heated to reflux, cooling the diNACA to ambient temperature, filtering the diNACA, washing with ethanol, and drying to yield diNACA as a final product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which: 
         FIG.  1    shows a basic chemical synthesis of the present invention. 
         FIG.  2 A  shows a view of NPI-001 (NACA) molecule ‘A’ without atom labelling. All non-hydrogen atoms are shown with thermal ellipsoids set at the 50% probability level. Color code: Carbon, grey; H, white; 0, red; S, yellow. 
         FIG.  2 B  shows a portion of NACA derived from starting material L-Cystine. 
         FIG.  3    shows a view of a unit cell an axis of NPI-001 containing complete molecules. All atoms are shown with thermal ellipsoids set at the 50% probability level. Color code: Carbon, grey; H, white; 0, red; S, yellow. 
         FIG.  4    shows results of liquid chromatography with mass spectrometric detection. 
         FIG.  5    shows Simulated (120 K) XRPD 20 diffractogram of NPI-001. 
         FIG.  6    shows  1 H-NMR of NACA. 
         FIG.  7    shows  1 H-NMR assignments for NACA ( 1 H and  13 C assignments are based on analysis of the 1D  1 H NMR,  1 H- 13 C HSQC and  1 H- 13 C HMBC spectra.), 
         FIG.  8    shows  13 C-NMR of NACA. 
         FIG.  9    shows  13 C-NMR assignments for NACA. ( 1 H and  13 C assignments are based on analysis of the 1D  1 H NMR,  1 H- 13 C HSQC and  1 H- 13 C HMBC spectra). 
         FIG.  10    shows combined thermogravimetry/differential scanning calorimetry scans of NACA, 
         FIG.  11    shows fourier transform-infrared (FT-IR) spectrum of NACA. 
         FIG.  12    is a representative Method-I Chromatogram showing NACA, Intermediates and Starting Material. 
         FIG.  13    is a representative Chromatogram of Method-II Showing B1 and B2. 
         FIG.  14    is a representative chromatogram showing separation of R-NACA and S-NACA. 
         FIG.  15    shows XRPD of NPI-001 Form 1, NPI-001/urea pattern 1 and NPI-001/urea pattern 1 Crash Cool. 
         FIG.  16    shows XRPD Diffractograms of NPI-001 Form 1, diNACA, Sodium Methoxide, NPI-001/sodium methoxide pattern 1 from methanol and NPI-001/sodium methoxide pattern 1 from ethanol. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a”, “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not limit the invention, except as outlined in the claims. 
     The present invention is directed to novel methods for making N-Acetyl Cysteine Amide (NACA) or diNACA, and novel intermediates thereof. More particularly, the present invention takes advantage of the novel intermediates to significantly, and surprisingly, increase the efficiency of preparing diNACA in both a high chemical yield and a high enantiomeric purity. Below is the basic chemical structure of NACA. 
     
       
         
         
             
             
         
       
     
     The following procedures may be employed for the preparation of the compound of the present invention. The starting materials and reagents used in preparing these compounds are either available from commercial suppliers such as the Aldrich Chemical Company (Milwaukee, Wis.), Bachem (Torrance, Calif.), Sigma (St. Louis, Mo.), or are prepared by methods well known to a person of ordinary skill in the art, following procedures described in such references as Fieser and Fieser&#39;s Reagents for Organic Synthesis, vols. 1-17, John Wiley and Sons, New York, N.Y., 1991; Rodd&#39;s Chemistry of Carbon Compounds, vols. 1-5 and supplements, Elsevier Science Publishers, 1989; Organic Reactions, vols. 1-40, John Wiley and Sons, New York, N.Y., 1991; March J.: Advanced Organic Chemistry, 4th ed., John Wiley and Sons, New York, N.Y.; and Larock: Comprehensive Organic Transformations, VCH Publishers, New York, 1989, relevant portions of which are incorporated herein by reference. 
     DiNACA is manufactured as shown in  FIG.  1   . The starting material may be the naturally occurring L-cystine with the L conformation on both cysteine subunits. In the first step the two acid groups are protected by forming the di-methyl ester of L-cystine as the dihydrochloride salt. Alternatively, the synthesis may begin with commercially acquired L-cystine dimethyl ester dihydrochloride with the L conformation on both cysteine subunits. Beginning with L-cystine dimethyl ester dihydrochloride obviates the need for use of thionyl chloride (typically used to convert L-cystine to L-cystine dimethyl ester dihydrochloride). In the second step the two nitrogens on the L-cystine are reacted with acetic anhydride to give diNACME which is in essence L-cystine with both acids protected as methyl esters and both primary amine groups protected with acetyl groups. Step 3 converts the methyl ester groups to primary amides. Step 4 reductively cleaves the diNACA intermediate to give NACA. 
     The preparation of NACA has one starting material that contributes significantly to the backbone of the final structure: L-cystine. In  FIG.  2 B , the portion of NACA that is in the dotted box is derived from L-cystine. L-Cystine provides the chiral center and the amino acid core. Two common reagents, acetic anhydride and ammonium hydroxide, provide atoms by derivatizing the carboxylic acid and primary amine, respectively. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 List of Starting Materials 
               
            
           
           
               
               
               
               
            
               
                   
                 Starting Material 
                 Quality 
                 Other 
               
               
                   
                   
               
               
                   
                 L-cystine 
                 ≥98.5% pure 
                 BSE/TSE-certification 
               
               
                   
                   
               
            
           
         
       
     
     Control of Starting Materials. The quality of L-cystine may be controlled based on the specification provided by the manufacturer and/or incoming testing performed by the Contract Manufacturing Organization (CMO) as shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 L-Cystine Testing 
               
            
           
           
               
               
               
            
               
                 Attribute 
                 Test Method 
                 Specification/Limit 
               
               
                   
               
               
                 Appearance 
                 Visual inspection 
                 White powder 
               
               
                 ID 
                 NMR (USP&lt;761&gt;; D2O) 
                 Conforms to structure 
               
               
                 Purity 1   
                 Titration 
                 98.5-101.0% 
               
               
                 Optical Rotation 
                 Vendor method 
                 −215 to −225° 
               
               
                   
               
               
                   1 A use test may be substituted for the purity test 
               
            
           
         
       
     
     Reagents and Solvents. A list of reagents and solvents used in each step of the manufacturing process is provided in Table 3 and Table 4, respectively. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 List of Reagents 
               
            
           
           
               
               
               
               
            
               
                   
                 Step 
                 Reagent 
                 Quality 
               
               
                   
                   
               
               
                   
                 1 
                 Thionyl chloride 
                 ≥97% 
               
               
                   
                 2, 4 
                 Triethylamine 
                 ≥99% 
               
               
                   
                 2 
                 Acetic anhydride 
                 ≥99% 
               
               
                   
                 2 
                 Sodium bicarbonate 
                 ≥99% 
               
               
                   
                 2 
                 Sodium sulfate (anhydrous) 
                 — 
               
               
                   
                 3 
                 Ammonium Hydroxide 
                 28-32%  
               
               
                   
                 4 
                 Dithiothreitol 
                 ≥98% 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 List of Solvents 
               
            
           
           
               
               
               
            
               
                 Step 
                 Solvent 
                 Quality 
               
               
                   
               
               
                 1 
                 Methanol 
                 ≥99% 
               
               
                 1, 4 
                 methyl t-Butyl Ether (MTBE) 
                 ≥99% 
               
               
                 2 
                 Acetonitrile 
                 ≥99% 
               
               
                 2 
                 Ethyl acetate 
                 ≥99% 
               
               
                 3, 4 
                 EtOH 
                 89.0-92.0%   
               
               
                   
               
            
           
         
       
     
     In the first step, the dimethyl ester of L-cystine was formed. L-cystine and methanol were cooled to −10° C. before slowly adding thionyl chloride. The skilled artisan will recognize methanol can be substituted with other similar reactive alcohols, and/or other alternative carboxylic acid activating agents (e.g., oxalyl chloride, CDI, etc). The slurry was then heated to reflux to produce a solution. Initially, the reaction was held for 4 hours before proceeding to the workup. Using HPLC it was found to not achieve full conversion. Holding the reaction at reflux for 16 hours proved to be effective for conversion. Upon verifying reaction completion, the solution was solvent-exchanged into ethyl ether. Depending on the method of making, other such solvent may also be used, e.g., methyl tert-butyl ether (MTBE). MTBE has proven to be an effective replacement for ethyl ether, as such the solution can be solvent-exchanged successfully into MTBE, and can also be used for washes. The white solids were filtered and washed with MTBE before drying at 45° C. under vacuum to a yield of 95-99% with a purity of 96-99%. 
     In Step 2, the L-cystine dimethylester dichydrochloride is converted to di-N-acetylcystine dimethylester, DiNACME. L-cystine dimethylester dihydrochloride, as a slurry in acetonitrile, is cooled to 0±5° C. To the cooled solution is first added at least 4 equivalents of triethylamine followed by at least 2 equivalents of acetic anhydride while maintaining an internal temperature of ≤5° C. throughout the additions. After aging for not less than 30 minutes, reaction completion is confirmed by HPLC. To the reaction mixture is added ethyl acetate and an aqueous workup is performed. Upon completion of the aqueous workup, the organic solution is fully exchanged into ethyl acetate by vacuum distillation of the acetonitrile and replacement with ethyl acetate, several times. The product, di-N-acetylcystine dimethylester, is isolated by filtration and dried to a 73-75% yield with a purity of 93-97%. Step 2 can be accomplished in a couple of different ways. While not as efficient, the L-cystine dimethylester dihydrochloride was free-based using 300 weigh percent amberlyst-A21 in 10 volumes acetonitrile. The solution of free-based material in acetonitrile was then acetylated using 2.1 equivalents acetic anhydride and 2.5 equivalents triethylamine at room temperature for 1 hour. The crude material was concentrated and dissolved into ethyl acetate before being washed with saturated sodium bicarbonate and brine solutions. The washed organics were concentrated to dryness for a 73-75% yield with a purity of 93-97%. 
     Alternatively, and more efficiently, the Step 2, triethylamine after the free-basing was replaced with amberlyst-A21 for a total of 600 weight percent. The acetonitrile was increased to 15 volumes to facilitate agitation. After free-basing is complete, the acetic anhydride can be added at room temperature or below. Some impurities may be formed when reaction was performed at room temperature, as such, the reaction can be cooled to 0° C. to successfully reduce impurity formation. After reaction completion, the solution was previously concentrated to dryness. Typically, filterable solids may be preferred in certain isolation steps. As such, the acetonitrile solution was solvent-exchanged into ethyl acetate to yield filterable solids. These solids were filtered and washed with ethyl acetate before drying at 45° C. under vacuum for a 45% yield with a purity of 98%. The solvent-exchanged material can also be chilled to 0° C. for 1 hour to increase yield to 65% while achieving similar purity. 
     Typically, triethylamine is cheaper than amberlyst-A21, as such the reaction can also be accomplished using only triethylamine. This was achieved by cooling the mixture of L-cystine dimethylester dihydrochloride in 15 volumes acetonitrile to 0° C. before adding 4.2 equivalents of triethylamine followed by 2.1 equivalents of acetic anhydride. Previously, using amberlyst-A21 to free-base, the hydrochloride salt formed was bound to amberlyst-A21 and filtered off of the material to yield a clear colorless L-cystine dimethylester solution. Using triethylamine as free base led to the formation of triethylamine chloride salts. These were filtered off before salting out triethylamine using 0.5 N HCl in 13% NaCl solution. At this point, the reaction was solvent-exchanged into ethyl acetate and filtered similar to previous methods. The alternative method resulted in 88-92% yield with 93-95% purity. 
     Using only triethylamine for step 2 led to some loss of product before acid treatment, ineffective removal of triethylamine using acid, and degradation of material. When the triethylamine acetylation was first performed, solids were filtered before treating the solution. However, HPLC of the solids revealed the presence of desired product, Di-NACMe. Upon reaction completion, the slurry was now dissolved in 15 volumes ethyl acetate before treatment to maximize Di-NACMe in solution. Initially, 0.5 N HCl in 13% brine solution was used to treat the reaction solution.  1 H-NMR results of acid-treated product revealed significant amounts of triethylamine were still present in step 2 product. In addition to the ineffective removal of triethylamine, the presence of acid in the material before drying degraded the material into a brown taffy when heat was applied. Since acid not only failed to remove triethylamine but also degraded material, acid was avoided. Saturated sodium bicarbonate replaced HCl to free-base triethylamine, acetic acid and HCl after reaction completion. After treating reaction solution with base to neutralize HCl, the belief was that triethylamine could be azeotroped with acetonitrile (ACN) while acetic acid stayed behind in ethyl acetate after the solvent-exchange. 
     The amidation of Di-NACMe into diNACA was performed in ammonium hydroxide. Initially, the solid was charged with at least 3 equivalents of 28-30% aqueous ammonium hydroxide. The solids dissolved in solution as diNACA precipitated. The slurry was agitated for 2 hours after solid formation before filtering and washing with minimal water. The solids were dried at 45° C. under vacuum. This method resulted in 60-70% yield with 70% purity. A solvent-exchange into ethanol after reaction completion further increased yield without sacrificing purity. Higher purity of the diNACA can be used to achieve a higher yield in the final reduction step. Additionally, diNACA may be recrystallized from water to further enhance the purity. 
     The reduction of diNACA to NACA can be accomplished using tris (2-carboxyethyl)phosphine (TCEP), thioglycolic acid, and/or dithiothreitol (DTT). One reduction method uses 1.1 eq TCEP in 15 volumes 10:1 THF:water, heated to reflux. Reduction by thioglycolic acid was performed using 2.5 eq thioglycolic acid. The reaction was attempted using THF and ethanol as solvents. THF reaction yielded ˜42% with a purity of 4%. Ethanol reaction had a yield of 2% with purity of 30%. With the goal of using ethanol, amberlyst-A21 and triethylamine were evaluated as bases to aid reduction. Amberlyst-A21 was determined to be a better base with yields of 45% and purity of 85%. Amberlyst-A21 also reduced reaction time from 2 days in triethylamine-mediated reactions to 3 hours in amberlyst-A21-mediated reactions. Alternatively, DTT can also be used. Various possible solvents can be used, e.g., dichloromethane (DCM), methanol, isopropanol, ethanol and/or other alcoholic solvents. Ethanol was chosen for its greater solubility of diNACA. The use of alternative reducing agents such as, but not limited to triphenylphosphine, TCEP, thioglycolic acid, etc., is contemplated herein as well as solvents such as, but not limited to, THF, EtOH, iPrOH, MeOH, DCM, water, etc. and bases, whether catalytic or stoichiometric, such as, but not limited to, Et3N, NaOH, amberlyst-A21, etc. Initially, to effect the reduction of the disulfide bond, 1.5 equivalents triethylamine were added to diNACA dissolved in ethanol followed by 1.5 equivalents of DTT. The solution was heated to reflux and held for 2 hours before solvent-exchanging into methyl tert-butyl ether to precipitate filterable solids. Further optimization of the reaction led to the reduction of triethylamine to a catalytic 0.1 equivalents, DTT to 1.25 equivalents and the reaction temperature to 62±3° C. with the effect of improving the impurity profile of the isolated NACA. This method yielded 85% of NACA with a purity of ≥98.0%. The purity of NACA may be further enhanced via recrystallization from ethanol. 
     These efforts to increase purity focused on the major impurities as measured by HPLC, namely, diNACA, DTT and cyclic DTT. Oxidation of NACA occurs when exposed to air. Ethanol and methyl t-butyl Ether (MTBE) used in the reduction and solvent-exchange were degassed in an effort to reduce diNACA. MTBE proved to be efficient in removing DTT and cyclic DTT, so an MTBE trituration was performed to improve purity. 
     diNACA purifications were performed with the goal of taking purer material through reduction for purer NACA. 
     Step 1: Formation of L-Cystine Dimethylester Dihydrochloride 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Reagents/Materials 
                 MW 
                 Density 
                 Eqs. 
                 Moles 
                 Weight/Volume 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 L-Cystine, ≥98.5% 
                 240.29 
                 — 
                 1.0 
                 208 
                 50  
                 kg 
               
               
                 Thionyl Chloride, ≥97% 
                 118.97 
                 1.64 
                 2.41 
                 504 
                 60  
                 kg 
               
               
                 Methanol (MeOH), ≥99% 
                 32.04 
                 0.79 
                 12.5  
                 — 
                 625  
                 L 
               
               
                   
                   
                   
                 vol 
                   
                   
                   
               
               
                 Methyl-tert Butyl 
                 88.15 
                 0.74 
                 8  
                 — 
                 400  
                 L 
               
               
                 Ether (MTBE), ≥99% 
                   
                   
                 vol 
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Set-up: A 2000 L glass-lined reactor; 
     50 kg L-Cystine; and 
     625 L Methanol was charged to the reactor and agitated while cooling to −10° C. 
     60 kg Thionyl Chloride was slowly added at ≤−5° C. After addition completion, the reaction material was heated to reflux and held for 16 hours. After the reaction was verified as complete by HPLC (≤0.5% starting material), the reaction was cooled to room temperature. The reaction mixture was concentrated to 6 volumes before solvent-exchanging into 3×8 volumes MTBE. The resulting slurry was agitated at room temperature for 1 hour before being filtered and washed with MTBE. The solids were dried at 45° C. under vacuum. 
     Yield: 68.15 kg (95.9%), Purity: 95.8% 
     Step 2: Formation of di-N-acetyl-1-cystine dimethylester (Di-NACMe) 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Reagents/Materials 
                 MW 
                 Density 
                 Eqs. 
                 Moles 
                 Weight/Volume 
               
               
                   
               
               
                 L-Cystine Dimethylester  
                 341.26 
                 — 
                 1.0 
                 76.2 
                 26 
               
               
                 Dihydrochloride, ≥95% 
                   
                   
                   
                   
                   
               
               
                 Acetonitrile, ≥99% 
                 41.05 
                 0.79 
                 23 
                 — 
                 472 
               
               
                   
                   
                   
                 vol 
                   
                   
               
               
                 Triethylamine (TEA), ≥99% 
                 101.19 
                 0.73 
                 4.2 
                 316.2 
                 32 
               
               
                 Acetic Anhydride, ≥99% 
                 102.09 
                   
                 2.1 
                 156.7 
                 16 
               
               
                 Ethyl Acetate, ≥99% 
                 88.1 
                   
                 41 
                 — 
                 958 
               
               
                   
                   
                   
                 vol 
                   
                   
               
               
                   
               
            
           
         
       
     
     Set-up: A 800 L glass lined reactor 
     26 kg L-Cystine Dimethylester Dihydrochloride and 
     390 L Acetonitrile (ACN) is charged to the reactor and agitated while cooling to 0° C. 
     32 kg Triethylamine is added to the reactor at ≤5° C. 
     16 kg Acetic Anhydride is slowly added to the reactor at ≤5° C. Upon addition completion the reaction is held for 30 minutes at 5±5° C. After reaction was verified as complete by HPLC (≤0.5% starting material), 10 volumes ethyl acetate was charged to the reactor and agitated to ambient. The resulting reaction mixture was washed with 2×2 volumes saturated bicarbonate. The aqueous layer was back extracted with 5 volumes ethyl acetate. All organics were combined and dried over sodium sulfate and polish filtered. The filtrate was concentrated to 5 volumes before azeodrying with 2×4 volumes acetonitrile followed by a solvent-exchange into 4×6 volumes ethyl acetate. The resulting slurry was agitated at 0° C. for 1 hour before being filtered and washed with 2 volumes ethyl acetate. The solids were dried at 25° C. under vacuum. 
     Average Yield: 29.56 kg (100%), Average Purity: 92.2% 
     Step 3: Formation of DiNACA 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Reagents/Materials 
                 MW 
                 Density 
                 Eqs. 
                 Moles 
                 Weight/Volume 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Di-NACMe 
                 352.42 
                 — 
                 1.0 
                 247 
                 87.05  
                 kg 
               
               
                 28-30% Ammonium Hydroxide 
                 35.05 
                 0.9 
                 8.44 
                 2088 
                 244  
                 kg 
               
               
                 Ethanol, absolute 200 proof 
                 46.07 
                 0.79 
                 17 vol  
                 — 
                 1483  
                 L 
               
               
                   
               
            
           
         
       
     
     Set-up: A 800 L glass lined reactor 
     244 kg Degassed 28-30% NH 4 OH (aq) was cooled to 0° C. before 
     87.05 kg Di-NACMe was charged to the reactor and agitated for 4 hours. After reaction was verified as complete by HPLC (≤0.5% starting material), the reaction mixture was solvent-exchanged into 3×5 volumes degassed ethanol. The resulting slurry was agitated at 0° C. for 30 minutes before being filtered and washed with cold degassed ethanol. The solids were dried at 45° C. under vacuum. 
     Average Yield: 52 kg (67.1%), Average Purity: 72.0% 
     Step 3A: Purification of DiNACA 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Weight∧ 
               
               
                 Reagents/Materials 
                 MW 
                 Density 
                 Eqs. 
                 Moles 
                 Volume 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Di-NACA 
                 322.40 
                 — 
                 1.0 
                 161 
                 52  
                 kg 
               
               
                 Process Water, Filtered 
                 18.02 
                 1.0 
                 8 vol 
                 — 
                 416  
                 L 
               
               
                   
               
            
           
         
       
     
     Set-up: A 800 L glass lined reactor 
     52 kg DiNACA and 
     416 L Degassed Water were charged to the flask and agitated while heating to reflux. Upon dissolution, the reaction solution was allowed to cool overnight. The solids were filtered and washed with 2 volumes cold degassed water. The solids were dried at 45° C. under vacuum. This material may be used to continue the synthesis of NACA. 
     Recovery: 36 g (69%), Purity: 86.4% 
     Alternate Purification of diNACA as a Final Product: 
     DiNACA is suspended in degassed water and heated to reflux. After cooling to ambient temperature, the product is filtered, washed with ethanol and dried to yield diNACA as a final product. 
     Step 4: Formation of NACA 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Reagents/Materials 
                 MW 
                 Density 
                 Eqs. 
                 Moles 
                 Weight/Volume 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Di-NACA 
                 322.40 
                 — 
                 1.0 
                 112 
                 36  
                 kg 
               
               
                 Ethanol, absolute, 200 proof 
                 46.07 
                 0.79 
                 20 vo1 
                 — 
                 720  
                 L 
               
               
                 Triethylamine (TEA), ≥99% 
                 101.19 
                 0.73 
                 0.1 
                 8.8 
                 1.1  
                 kg 
               
               
                 Dithiothreitol, ≥98% (aka 1,4- 
                 154.25 
                 — 
                  1.25 
                 143 
                 22  
                 kg 
               
               
                 Dithiothreitol and DL-Dithiothreitol) 
               
               
                   
               
            
           
         
       
     
     Set-up: A 800 L glass lined reactor 
     720 L Degassed Ethanol, 
     1.1 kg Triethylamine, 
     22 kg Dithiothreitol and 
     36 kg of DiNACA were charged to the reactor before heating the reaction to 62±3° C. The reaction is held at temp for 2 hours. After reaction was verified as complete by HPLC (≤0.5% starting material on overloaded column), cool reaction to ambient. The reaction solution was polish filtered and concentrated to 10 volumes before solvent exchanging into 4×10 volumes degassed MTBE. The resulting slurry was agitated overnight before being filtered and washed with 2 volumes degassed MTBE. The solids were dried @45° C. under vacuum. diNACA was recrystallized with ethanol. 
     Yield: 27.8 kg (77.2%), Purity: 98.5% 
       FIG.  1    shows a basic chemical synthesis of the present invention.  FIG.  2 A  shows a view of NPI-001 molecule ‘A’ without atom labeling. All non-hydrogen atoms are shown with thermal ellipsoids set at the 50% probability level. Color code: Carbon, grey; H, white; O, red; S, yellow.  FIG.  2 B  shows a portion of NACA derived from starting material L-Cystine.  FIG.  3    shows a view of unit cell an axis of NPI-001 containing complete molecules. All atoms are shown with thermal ellipsoids set at the 50% probability level. Color code: Carbon, grey; H, white; O, red; S, yellow.  FIG.  4    shows results of liquid chromatography with mass spectrometric detection.  FIG.  5    shows Simulated (120 K) XRPD 2θ diffractogram of NACA.  FIG.  6    shows  1 H-NMR of NACA.  FIG.  7    shows  1 H-NMR assignments for NACA ( 1 H and  13 C assignments are based on analysis of the 1D  1 H NMR,  1 H- 13 C HSQC and  1 H- 13 C HMBC spectra.).  FIG.  8    shows  13 C-NMR of NACA.  FIG.  9    shows  13 C-NMR assignments for NACA. ( 1 H and  13 C assignments are based on analysis of the 1D  1 H NMR,  1 H- 13 C HSQC and  1 H- 13 C HMBC spectra).  FIG.  10    shows combined thermogravimetricy/differential scanning calorimetry scans of NACA.  FIG.  11    shows fourier transform-infrared spectrum (FT-IR) of NACA. 
     Analytical procedures for NACA. The analytical methods for the testing of NACA drug substance are listed in Table 5. Most of the methods are USP compendial tests. The additional NACA drug substance methods (which are not compendial) include two HPLC methods for assay and impurities, and one chiral HPLC method for chiral purity. Each of the non-compendial methods is described in more detail in sections that follow. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 List of Analytical Procedures for NACA Drug Substance 
               
            
           
           
               
               
               
            
               
                 Test 
                 Test Method 
                 Description 
               
               
                   
               
               
                 Appearance 
                 Visual 
                 A sample of the solid material is 
               
               
                   
                   
                 examined visually for form and 
               
               
                   
                   
                 color. 
               
               
                 ID-1 
                 IR 
                 Method follows USP&lt;197A&gt; 
               
               
                 ID-2 
                   1 H-NMR 
                 Method follows USP&lt;761&gt; 
               
               
                 Potency/ 
                 Calculated 
                 Purity = (100 − % HPLC 
               
               
                 Assigned 
                   
                 impurities − % H 2 O − % ROI − 
               
               
                 Purity 
                   
                 % Total Residual Solvents) 
               
               
                 Organic 
                 HPLC-Method I 
                 Reverse phase gradient HPLC 
               
               
                   
                   
                 method. 
               
               
                 Impurities/ 
                 HPLC Method II 
                 Reverse phase gradient HPLC 
               
               
                   
                   
                 method. 
               
               
                 Related 
               
               
                 Substances 
               
               
                 Chirality 
                 Optical Rotation 
                 Method follows 
               
               
                   
                   
                 USP&lt;781&gt; (c 1.00, MeOH) 
               
               
                 Chiral Purity 
                 Chiral HPLC 
                 Chiral HPLC method 
               
               
                 Residue on 
                 USP&lt;281&gt; 
                 Method follows USP&lt;281&gt; 
               
               
                 Ignition 
               
               
                 DSC 
                 USP&lt;891&gt; 
                 Method follows USP&lt;891&gt; 
               
               
                 X-ray Powder 
                 USP&lt;941&gt; 
                 Method follows USP&lt;941&gt; 
               
               
                 Diffraction 
               
               
                 Heavy Metals 
                 USP&lt;233&gt; 
                 Method follows USP&lt;233&gt; 
               
               
                 Residual Solvents 
                 GC (USP&lt;467&gt;) 
                 Method follows USP&lt;467&gt; 
               
               
                 Water Content 
                 Karl Fischer 
                 Method follows USP&lt;921&gt; 
               
               
                   
                   
                 version 1c 
               
               
                 Microbial Limits 
                 Microbial 
                 Method follows USP&lt;61&gt; 
               
               
                   
                 enumeration 
               
               
                   
                 Test for 
                 Method follows USP&lt;62&gt; 
               
               
                   
                 specified 
               
               
                   
                 organisms 
               
               
                   
               
            
           
         
       
     
     HPLC Method for Purity and Related Substances. Analysis of the NACA drug substance for purity and related substances makes use of a reverse phase HPLC with an ultraviolet (UV) detector. The method is summarized in Table 6. This method is also used as the in-process method for each step to follow completion of reaction. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Summary of NACA HPLC Method I 
               
               
                 (Purity and Related Substances) 
               
            
           
           
               
               
            
               
                 Method 
                   
               
               
                 Element 
                 Description 
               
               
                   
               
            
           
           
               
               
            
               
                 Column 
                 Phenomenex Synergi Hydro-RP, 4.6 × 250 mm, 4 μm 
               
               
                 Detection 
                 214 nm 
               
               
                 Column 
                 40° C. 
               
               
                 Temperature 
               
               
                 Injection 
                 25 μL 
               
               
                 Volume 
               
               
                 Flow Rate 
                 1.0 mL/minute 
               
               
                 Mobile Phase A 
                 0.02% H 3 PO 4  in H 2 O 
               
               
                 Mobile Phase B 
                 Acetonitrile (ACN) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Time 
                   
                   
               
               
                 Gradient 
                 (min) 
                 % A 
                 % B 
               
               
                   
               
               
                   
                  0.0 
                 100 
                 0 
               
               
                   
                 15.0 
                 90 
                 10 
               
               
                   
                 25.0 
                 0 
                 100 
               
               
                   
                 30.0 
                 0 
                 100 
               
               
                   
                  30.1a 
                 100 
                 0 
               
               
                   
                   35.0 a 
                 100 
                 0 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 (a) Equilibration time-no integration 
               
               
                 System 
                 1. Specificity: No significant interference in the 
               
               
                 Suitability 
                 blank chromatogram at retention times of interest. 
               
               
                   
                 2. The S/N of the NACA peak in the 0.03% 
               
               
                   
                 sensitivity solution must be ≥10 
               
               
                   
                 3. The % RSD of the RT and peak area of NACA in 
               
               
                   
                 the 5 injections of the first sample must be ≤2.0%. 
               
               
                   
                 4. The recovery of one standard prep (average of 
               
               
                   
                 all injections) versus a second standard prep 
               
               
                   
                 (average of all injection) must be 100.0 ± 2.0%. 
               
               
                   
               
            
           
         
       
     
       FIG.  12    shows a Method I Chromatogram Showing NACA and all Intermediates and Starting Material. 
     HPLC Method for Impurities B1 and B2. Method-I did not always detect impurities B1 and B2. A second method, Method-II, was developed to reliably quantitate these two impurities. The method is summarized in Table 7. A representative chromatogram showing the retention times of B1 and B2 is presented in  FIG.  13   . 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Summary of NACA HPLC Method II (Impurities B1 and B2) 
               
            
           
           
               
               
            
               
                 Method Element 
                 Description 
               
               
                   
               
            
           
           
               
               
            
               
                 Column 
                 Agilent Zorbax SB-Aq, 4.6 × 250 mm, 5 μm 
               
               
                 Detection 
                 214 nm 
               
               
                 Column Temperature 
                 40° C. 
               
               
                 Injection Volume 
                 25 μL 
               
               
                 Flow Rate 
                 1.0 mL/minute 
               
               
                 Mobile Phase A 
                 0.02% H 3 PO 4  in H 2 O 
               
               
                 Mobile Phase B 
                 Acetonitrile (ACN) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Time 
                   
                   
               
               
                 Gradient 
                 (min) 
                 % A 
                 % B 
               
               
                   
               
               
                   
                 0.0 
                 100 
                 0 
               
               
                   
                 5.0 
                 100 
                 0 
               
               
                   
                 20.0 
                 0 
                 100 
               
               
                   
                 25.0 
                 0 
                 100 
               
               
                   
                 25.01 
                 100 
                 0 
               
               
                   
                 35.0 
                 100 
                 0 
               
               
                   
               
            
           
           
               
               
            
               
                 System 
                 1. Specificity: No significant interferences in the 
               
               
                 Suitability 
                 blank chromatogram at retention times of interest. 
               
               
                   
                 2. The S/N of the NACA peak in the 0.03% 
               
               
                   
                 sensitivity solution must be ≥10 
               
               
                   
                 3. The % RSD of the RT and peak area of NACA 
               
               
                   
                 in the 5 injections of the first sample must 
               
               
                   
                 be ≤2.0%. 
               
               
                   
               
            
           
         
       
     
       FIG.  13    is a representative Chromatogram of Method-II Showing B1 and B2 
     Chiral HPLC Method for Measuring Chiral Purity of NACA. A chiral method was developed to assess the chiral purity of NACA. The method is summarized in Table 8. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Summary of Chiral HPLC Method for NACA 
               
            
           
           
               
               
            
               
                 Method Element 
                 Description 
               
               
                   
               
               
                 Column 
                 Diacel Chiralpak IC-3, 4.6 × 150 mm 
               
               
                 Detection 
                 217 nm 
               
               
                 Column Temperature 
                 35° C. 
               
               
                 Injection Volume 
                 20 μL 
               
               
                 Flow Rate 
                 0.8 mL/minute 
               
               
                 Mobile Phase (isocratic) 
                 0.05% H 3 PO 4  in 1:1 n-hexane:IPA 
               
               
                 System 
                 1. Specificity: No significant interferences 
               
               
                 Suitability 
                 in the blank chromatogram at retention times 
               
               
                   
                 of interest. 
               
               
                   
                 2. The resolution between the enantiomers is 
               
               
                   
                 sufficient to allow for accurate integration of 
               
               
                   
                 both peaks. 
               
               
                   
                 3. The S/N of the NACA peak in the 0.03% 
               
               
                   
                 sensitivity solution must be ≥10 
               
               
                   
                 4. The % RSD of the RT and peak area of 
               
               
                   
                 NACA in the 6 injections of the first sample 
               
               
                   
                 must be ≤2.0%. 
               
               
                   
               
            
           
         
       
     
       FIG.  14    is a chromatogram Showing Separation of R-NACA and S-NACA. 
     Preparation of D-NACA. D-Cystine was obtained from a commercial vendor. D-Cystine was dissolved in water and pH was adjusted to pH 9-10 with NaOH. Acetic anhydride was added dropwise at 0° C. and pH maintained at 9-10. Solution was stirred for 4 hours after which it was acidified to pH ˜2 with HCl. The solvent was evaporated under reduced pressure. 20 mL MeOH was added to the flask to dissolve contents. Solution was filtered. Filtrate was concentrated and evaporated. Thin layer chromatography (MeOH:DCM:HOAc, 1:8:1) indicated loss of starting material and appearance of a single new peak for D-acetylcystine. D-Acetylcystine was charged to a round-bottomed flask with 50 mL MeOH to which was added 0.35 mL concentrated H 2 SO 4  via syringe, dropwise, at ambient temperature, with agitation. Solution turned turbid. IPC by TLC (MeOH:DCM, 1:9] showed no starting material. Solvent was evaporated. Fraction was neutralized with NaHCO 3  (saturated), extracted with DCM (50 ml twice), washed with water, dried over Na 2 SO 4 , filtered, purged with nitrogen and concentrated under reduced pressure to yield a net weight of 7.8 g white solid, formed in the refrigerator. N-acetylcysteine methyl ester was charged into a 3-necked, round-bottomed flask with nitrogen bubbler, and magnetic stirrer. Ammonium hydroxide at ambient temperature was added and purged with nitrogen through reaction mixture and agitated at ambient temperature. Solvent was evaporated under vacuum and a white solid formed. Ethanol was added and heated to form clear solution and left to stand overnight. White solid crystallized, was filtered, washed with ethanol and purified by column chromatography. 
     Oral Solution of NACA. 
     A study was conducted to determine the solubilization of NACA in ORA-SWEET® (Ora-Sweet is a commercially available syrup vehicle containing water, sucrose, glycerin, sorbitol, flavoring, buffering agents (citric acid and/or sodium phosphate), methyl paraben and potassium sorbate, pH 4.2 manufactured by Paddock Laboratories, Inc., Minneapolis, Minn.). HPLC equipment and glass containers and stirrers were utilized. 
     NACA at 80 mg/ml did not readily dissolve in ORA-SWEET, rather it required stirring for 20-30 minutes to achieve a solution. However, NACA was readily dissolved in water with shaking for 30 seconds, followed by dilution with an equal volume of ORA-SWEET with shaking for 20-30 seconds. Therefore, NACA was dissolved in 50 mL water followed by 50 mL Ora-Sweet, shaken to dissolve in an opaque plastic bottle with closure and provided to subject for self-administration (oral ingestion). 
     NACA oral solution was also be prepared as 100 ml oral solution in ORA-SWEET. Doses can be achieved ranged from 250 mg to 4000 mg/day/patient. The NACA dissolution in water followed by dilution with ORA-SWEET was found optimal for compounding. ORA-SWEET is a pale pink solution with a cherry syrup flavor. NACA has a mild sulfur odor and a bitter taste (like burned sesame seeds). When dissolved in ORA-SWEET, the odor and taste were masked. 
     The following instructions for preparation of NACA Oral Solution were developed: Weigh NACA [either 250, 750, 1500, 3000 or 4000 mg (±1 mg), as appropriate for the particular dose group] and place into a 125 mL (approximately) capacity opaque high density polyethylene, labeled (see Table 9) bottle with opaque polypropylene screw cap. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Container/closure for NACA Oral Solution Used for Phase 1 Study 
               
            
           
           
               
               
            
               
                 Component 
                 Description 
               
               
                   
               
               
                 Bottle 
                 125 mL opaque white high density polyethylene bottle 
               
               
                 Cap 
                 Polypropylene opaque white cap 
               
               
                 Label 
                 Pharmacy approved label 
               
               
                   
               
            
           
         
       
     
     Measure 50 mL of Purified Water and pour into each bottle containing NACA and shake vigorously by hand (at least 30 seconds) to dissolve. 
     Measure 50 mL Ora-Sweet and pour into each bottle containing NACA and shake vigorously by hand (at least 30 seconds) to dissolve. 
     A subject drinks entire solution, followed by two 20-ml rinses of the container with water, which are also drank. 
     Tablet or Capsule of NACA for clinical phase 1 or clinical phase II trials: 
     The qualitative composition for NACA Tablets is presented in Table 10. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Qualitative Composition of NACA Tablets 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Quality Standard 
               
               
                   
                   
               
               
                   
                 NACA 
                 Nacuity 
               
               
                   
                 Lactose 
                 NF 
               
               
                   
                 Microcrystalline Cellulose 
                 NF 
               
               
                   
                 Croscarmellose Sodium 
                 NF 
               
               
                   
                 Stearic acid 
                 NF 
               
               
                   
                   
               
            
           
         
       
     
     NACA Tablets, 250 mg, are formulated as an immediate release drug product. A roller-compacted blend containing 250 mg NACA is compressed into round, biconvex tablets. 
     The formulation use to manufacture NACA Tablets is a roller compacted blend of common excipients (Table 11). The blend is compressed into round, biconvex shaped tablets. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Quantitative Composition of NACA Tablets 
               
            
           
           
               
               
               
               
               
            
               
                   
                 mg/tablet 
                   
                   
                 Quality 
               
               
                 Component 
                 weight 
                 % 
                 Function 
                 Standard 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 NACA 
                 250.0 
                 50 
                 Drug 
                 Nacuity 
               
               
                   
                   
                   
                 substance 
               
               
                 Lactose 
                 42.875 
                 8.575 
                 Filler 
                 NF 
               
               
                 Microcrystalline 
                 177.125 
                 35.425 
                 Filler 
                 NF 
               
               
                 Cellulose 
               
               
                 Croscarmellose 
                 25 
                 5.0 
                 Disintegrant 
                 NF 
               
               
                 Sodium 
               
               
                 Stearic acid 
                 5 
                 1.0 
                 Lubricant 
                 NF 
               
               
                 TABLET WT 
                 500 
                 100 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     Type of Container and Closure for Dosage Form 
     NACA Tablets, 250 mg, are packaged in high density polyethylene (HDPE) bottles with foil induction seal and a white polyproplyene screw-top closure. Excipients used for the formulation meet compendial standards. Lactose functions as a filler. Microcrystalline cellulose functions as a filler. Croscarmellose Sodium functions as a disintegrant. Stearic Acid functions as a lubricant. Excipients were screened by assessing the stability of NACA in mixtures with each excipient. Mixtures of NACA with either microcrystalline cellulose, lactose monohydrate, croscarmellose sodium and crospovidone/Kollidon CL and hydroxypropyl cellulose exhibited less degradation of NACA compared to other excipients (Table 12). Hydroxypropyl cellulose exhibited greater levels of impurities than the other lead excipients (data not shown). Based on these results as well as the collective experience of the Formulators, microcrystalline cellulose, lactose monohydrate, croscarmellose sodium and steric acid were chosen as excipients for NACA Tablets. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Stability results for NACA/excipient mixtures 
               
               
                 after 4 weeks at 40° C./75% RH 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Ratio of 
                 % 
               
               
                 Sample ID 
                 Excipient 
                 API:Excipient 
                 Assay 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 SPt: 17004Q4; P 
                 NA 
                 1:0 
                 95.3 
               
               
                 SPL-1700405-P 
                 Microcrystalline Cellulose 
                 1:1 
                 94.9 
               
               
                   
                 PH 102 
               
               
                 SPL-1700406-P 
                 Lactose Monohydrate 
                 1:1 
                 93.2 
               
               
                   
                 (Fastflo) 
               
               
                 SPL-1700407-P 
                 Croscarmellose Sodium 
                 4:1 
                 93.6 
               
               
                 SPL-1700408-P 
                 Crospovidone/Kollidon CL 
                 4:1 
                 94.4 
               
               
                 SPl-1700409-P 
                 Sodium Lauryl Sulfate 
                 4:1 
                 26.2 
               
               
                 SPL-1700410-P 
                 Colloidal Silicon Dioxide 
                 4:1 
                 92.7 
               
               
                 SPL-1700411-P 
                 Magnesium Stearate 
                 9:1 
                 77.5 
               
               
                 SPL-1700412-P 
                 Sodium Stearyl Fumarate 
                 9:1 
                 86.7 
               
               
                   
               
            
           
         
       
     
     Tables 13 and 14 were prepared. The use of roller compaction of formulations of NACA with various excipients yielded powders with acceptable flow properties. Roller compaction followed by compression yielded tablets with acceptable properties based on friability and hardness. The clinical formulation was selected based on acceptable 2-week stability data. 
     Dry blending of formulations of NACA with various excipients yielded poorly flowing powders. Dry blending followed by compression yielded tablets that were unsatisfactory based on friability and hardness. 
     The use of roller compaction of formulations of NACA with various excipients yielded powders with acceptable flow properties. Roller compaction followed by compression yielded tablets with acceptable properties based on friability and hardness. 
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Finished Prototype NACA Tablets by Roller Compaction 
               
            
           
           
               
               
               
            
               
                   
                   
                 Packaging 
               
               
                 Batch Formulation 
                 Product Batch # 
                 Configurations* 
               
               
                   
               
            
           
           
               
               
               
            
               
                 NAC1G50%0401 
                 NAC1T250mg0401A 
                 1 
               
               
                 Low Roller Compaction 
                 NAC1T250mg0401B 
                 2 
               
               
                 Force (3 kN) 
               
               
                 NAC1G50%0402 
                 NAC1T250mg0402A 
                 1 
               
               
                 High Roller Compaction 
                 NAC1T250mg0402B 
                 2 
               
               
                 Force (6 kN) 
               
               
                 NAC1G50%0501 
                 NAC1T250mg0501A 
                 1 
               
               
                 Low Roller Compaction 
                 NAC1T250mg0501B 
                 2 
               
               
                 Force (3 kN) 
               
               
                 NAC1G50%0502 
                 NAC1T250mg0502A 
                 1 
               
               
                 High Roller Compaction 
                 NAC1T250mg0502B 
                 2 
               
               
                 Force (6 kN) 
               
               
                   
               
               
                 *Packaging Configuration 1 (With Desiccant): 
               
               
                 Bottle: 60 CC 33/400 W-HDPE ROUND BTL 
               
               
                 Cap: 33 mm SECURX with SG-75M liner 
               
               
                 Desiccant: Desiccant Canister Silica Gel 2GM 
               
               
                 Fill: 20 tablets per bottle 
               
               
                 *Packaging Configuration 2 (Without Desiccant): 
               
               
                 Bottle: 60 CC 33/400 W-HDPE ROUND BTL 
               
               
                 Cap: 33 mm SECURX with SG-75M liner 
               
               
                 Fill: 20 tablets per bottle 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 14 
               
             
            
               
                   
               
               
                 NACA Tablet Prototype Batch Formulations by Roller Compaction 
               
            
           
           
               
               
            
               
                   
                 Component 
               
            
           
           
               
               
               
               
               
            
               
                   
                 NAC1G50% 
                 NAC1G50% 
                 NAC1G50% 
                 NAC1G50% 
               
               
                   
                 0401 mg/ 
                 0402 mg/ 
                 0501 mg/ 
                 0502 mg/ 
               
               
                   
                 tablet weight 
                 tablet weight 
                 tablet weight 
                 tablet weight 
               
               
                   
                 Low Force 
                 High Force 
                 Low Force 
                 High Force 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 g 
                 % 
                 g 
                 % 
                 g 
                 % 
                 g 
                 % 
               
               
                   
                   
               
            
           
           
               
            
               
                 INTRAGRANULAR 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 NACA 
                 750 
                 50 
                 750 
                 50 
                 750 
                 50 
                 0 
                 0 
               
               
                 NACA Direct 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 71.42 
               
               
                 Blend 70% 
               
               
                 (NAC1B70% 01) 
               
               
                 Lactose 
                 330 
                 22 
                 330 
                 22 
                 128.6 
                 8.57 
                 857.0 
                 0 
               
               
                 Microcrystalline 
                 330 
                 22 
                 330 
                 22 
                 128.6 
                 8.57 
                 0 
                 0 
               
               
                 Cellulose 
               
               
                 Croscarmellose 
                 75 
                 5 
                 75 
                 5 
                 53.6 
                 3.57 
                 0 
                 0 
               
               
                 Sodium 
               
               
                 Stearic Acid 
                 7.5 
                 0.5 
                 7.5 
                 0.5 
                 10.6 
                 0.71 
                 0 
                 0 
               
            
           
           
               
            
               
                 EXTRAGRANULAR 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Microcrystalline 
                 0 
                 0 
                 0 
                 0 
                 402.9 
                 26.86 
                 322.3 
                 26.86 
               
               
                 Cellulose 
               
               
                 Croscarmellose 
                 0 
                 0 
                 0 
                 0 
                 21.4 
                 1.43 
                 17.2 
                 1.43 
               
               
                 Sodium 
               
               
                 Stearic Acid 
                 7.5 
                 0.5 
                 7.5 
                 0.5 
                 4.3 
                 0.29 
                 3.5 
                 0.29 
               
               
                 Total 
                 1500 
                 100 
                 1500 
                 100 
                 1500 
                 100 
                 1200 
                 100 
               
               
                   
               
               
                 *NAC1B70% 01 = 70% NACA + 12% Lactose + 12MCC + 5% Croscarmellose Sodium + 1% Stearic Acid 
               
            
           
         
       
     
     NACA Tablet Dry Blend Formulation Development 
     NACA Tablet formulations (Table 15) were dry blended and directly compressed. The flow of formulation from the hopper to the press was not uniform. Also, the resulting tablets suffered poor friability, hardness and capping. As a result dry blending was abandoned. 
     
       
         
           
               
             
               
                 TABLE 15 
               
             
            
               
                   
               
               
                 Composition of Prototype NACA Tablet Dry Blend Formulations 
               
            
           
           
               
               
            
               
                   
                 Prototype Batch Composition 
               
            
           
           
               
               
               
            
               
                   
                 Batch NAC1B50%0101 
                 Batch NAC1B50%201 
               
            
           
           
               
               
               
               
               
            
               
                 Component 
                 g 
                 % 
                 g 
                 % 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 NACA 
                 600 
                 50 
                 600 
                 50 
               
               
                 Lactose 
                 528 
                 44 
                 — 
                 — 
               
               
                 Microcrystalline 
                 — 
                 — 
                 528 
                 44 
               
               
                 Cellulose 
               
               
                 Croscarmellose 
                 60 
                 5 
                 60 
                 5 
               
               
                 Sodium 
               
               
                 Stearic Acid 
                 12 
                 1 
                 12 
                 1 
               
               
                 Total 
                 1200 
                 100 
                 1200 
                 100 
               
               
                   
               
            
           
         
       
     
     The formulations described above can also be used as a dry blend for filling into capsules. 
     The relation between micronization conditions and an initial increase of the degradation product diNACA was further investigated. Comparing stainless steel and zirconium oxide grinding jars gives a clear indication that steel samples undergo a time dependent increase of the degradation product diNACA. In contrast, using zirconium oxide jars and balls did not lead to an initial increase of the degradation product diNACA. Therefore, the zirconium oxide milling process was further investigated and successfully optimized regarding particle size distribution, milling parameters and impurity profile for the 1% NACA formulation. Zirconium oxide milling process was investigated and successfully optimized regarding particle size distribution, milling parameters and impurity profile for the 1% NACA formulation. The particle size distribution by laser diffraction analysis was ×10=1.3 μm, ×50=4.7 μm and ×90=12.3 μm. Batches were prepared and found to be stable for up to 4 weeks at ambient temperature. 
     Single Crystal X-Ray Diffraction. 
     The absolute structure of NACA has been determined by single crystal X-ray diffraction from suitable crystals grown from cooling of a saturated 2-propanol NACA solution to ambient conditions. Single crystal analysis of crystals clearly shows that the material is NACA with the expected bond connectivity. The absolute stereochemistry has been proved in the crystal with excellent confidence, as confirmed by the Flack parameter being −0.02(3). The density of the material is high, reducing the risk that a more stable polymorph is even possible and the hydrogen bonding network observed satisfies the expected functionality observed in NACA. The predicted XRPD from the SC-XRD data is consistent with the Form 1 material, indicating that the crystal was representative of the bulk material. Data was collected and found to be twinned, therefore, was refined accordingly using HKLF5 and BASF commands, locating a two component twin with BASF scales 0.7271(10): 0.2729(10) in the Monoclinic space group P21 where two complete formula units of NACA were found in the asymmetric unit only. No disorder was noted in the final structure with final a R1 [I&gt;2σ(I)] of 3.30% obtained with Flack parameter of −0.02 with e.s.d 0.03 determined using 1549 quotients that is suitable to accurately determine the IUPAC name of NACA as 2R)-2-(acetylamino)-3-sulfanylpropanamide (=N-acetyl-L-cysteine amide=(R)-2-acetylamino)-3-mercapto-propamide). 
     NACA, Form 1 overall structure quality factor: 1 
     Where: 
     1. Strong data set, no disorder, R1 ˜4%. Publishable quality. 
     2. Good data set, contains some minor disorder, R1 ˜6%. Publishable quality. 
     3. Average data set and/or easily modelled disorder or twinning. Publishable with care. 
     4. Weak data and/or major disorder or twinning that is not easily modelled. Publishable in some cases. 
     5. Very weak data and/or unexplained features of data or model. Not of publishable quality. 
     Polymorphism. 
     A detailed polymorph screen of NACA (NACA) (NACA) has been performed using a variety of solvents and experimental conditions. During the primary screen, the most common solid form observed was pattern 1 (Table 16). 
     
       
         
           
               
             
               
                 TABLE 16 
               
               
                   
               
               
                 Crystallographic parameters and refinement indicators of NACA, Form 1. 
               
               
                 NACA, Form 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Empirical formula 
                 C 5 H 10 N 2 O 2 S 
               
               
                 Formula weight 
                 162.21 
               
               
                 Temperature/K 
                 120(1) 
               
               
                 Crystal system 
                 Monoclinic 
               
               
                 Space group 
                 P2 1   
               
               
                 a/Å 
                 7.2832(2) 
               
               
                 b/Å 
                 7.5542(2) 
               
               
                 c/Å 
                 13.9686(4) 
               
               
                 α/° 
                 90 
               
               
                 β/° 
                 98.6983(15) 
               
               
                 γ/° 
                 90 
               
               
                 Volume/Å 3   
                 759.70(4) 
               
               
                 Z, Z′ 
                 4, 2 
               
               
                 ρ calc  g/cm 3   
                 1.418 
               
               
                 μ/mm −1   
                 0.369 
               
               
                 F(000) 
                 344.0 
               
               
                 Crystal size/mm 3   
                 0.384 × 0.207 × 0.131 
               
               
                 Radiation 
                 MoKα (λ = 0.71073) 
               
               
                 2Θ range for data collection/° 
                 2.95 to 56.582 
               
               
                 Index ranges 
                 −9 ≤ h ≤ 9, −10 ≤ k ≤ 10, 
               
               
                   
                 −18 ≤ l ≤ 18 
               
               
                 Reflections collected 
                 5810 
               
               
                 Independent reflections 
                 5810 [R int  = 0.0580, R sigma  = 0.0288] 
               
               
                 Data/restraints/parameters 
                 5810/1/186 
               
               
                 Goodness of Fit 
                 1.057 
               
               
                 Final R indexes [I &gt; 2σ (I)] 
                 R 1  = 0.0330, wR 2  = 0.0869 
               
               
                 Final R indexes [all data] 
                 R 1  = 0.0350, wR 2  = 0.0900 
               
               
                 Δρmax, Δρmin/e Å −3   
                 0.47/−0.53 
               
               
                 Flack Parameter 
                  −0.02(3) 
               
               
                   
               
               
                 R 1  = (Σ |F o | − |F c |)/Σ |F o |); wR 2  = {Σ [ W (F o   2  − F c   2 ) 2 ]/Σ [w(F c   2 ) 2 ]} 1/2 ; S = {Σ [w(F o   2  − F c   2  ) 2 ]/(n − p)} 1/2.   
               
            
           
         
       
     
     Several experiments yielded diffractogram patterns that were different or, more commonly, had extra peaks observed. The extra peaks would indicate the presence of another form, albeit not in a pure phase. 
     Attempts to reproduce these forms failed using both crash cooling, evaporation and anti-solvent addition. Analysis of these attempts by NMR showed that the material was still predominately the NACA material and that it had not oxidized to diNACA. The lack of reproducibility of these potential forms is good evidence for their lack of stability. This study has clearly demonstrated that the NACA material exists as Form 1 and that other forms are difficult, if not impossible, to produce. 
     Approximately 80 mg of NACA was weighed into each of 24 vials. The solvents listed below were added to the appropriate vials. The quantities added were calculated (based on solubility studies) to dissolve approx. 60% of the material. These mixtures were temperature cycled between ambient and 40° C., in 4 hr cycles, for 72 hours. Solids isolated from the slurries are tested by XRPD. The resulting saturated solutions were separated into three separated vials for crash cooling, anti-solvent addition and evaporation experiments. 
     
       
         
           
               
             
               
                 TABLE 17 
               
             
            
               
                   
               
               
                 List of Solvents Used in the Primary Polymorph Screen 
               
            
           
           
               
               
            
               
                   
                 Solvent 
               
               
                   
                   
               
            
           
           
               
               
            
               
                 1 
                 Acetone 
               
               
                 2 
                 Acetone/water (80:20) 
               
               
                 3 
                 Acetone/Heptane (75:25) 
               
               
                 4 
                 Acetonitrile 
               
               
                 5 
                 Acetonitrile/water (80:20) 
               
               
                 6 
                 1-Butanol 
               
               
                 7 
                 1,2-Dimethoxyethane 
               
               
                 8 
                 1,4-Dioxane 
               
               
                 9 
                 Dioxane/water (80:20) 
               
               
                 10 
                 Ethanol 
               
               
                 11 
                 Ethanol/water (80:20) 
               
               
                 12 
                 Ethanol/heptane (75:25) 
               
               
                 13 
                 Ethyl Formate 
               
               
                 14 
                 Isopropyl acetate 
               
               
                 15 
                 Ethyl acetate 
               
               
                 16 
                 Methanol 
               
               
                 17 
                 Methanol/water (80:20) 
               
               
                 18 
                 Methyl Ethyl ketone 
               
               
                 19 
                 Nitromethane 
               
               
                 20 
                 1-Propanol 
               
               
                 21 
                 2-Propanol 
               
               
                 22 
                 2-Propanol/water (80:20) 
               
               
                 23 
                 Tetrahydrofuran 
               
               
                 24 
                 Water 
               
               
                   
               
            
           
         
       
     
     Liquid Chromatography with Mass Spectrometric Detection 
     Column Temperature: 30° C. 
     Mobile Phase A: 0.1% v/v Formic in Water 
     Mobile Phase B: 0.1% v/v Formic in Acetonitrile 
     Diluent: Mobile Phase A: 50:50 Water: Acetonitrile 
     Flow Rate: 1.0 mL/min 
     Runtime: 25 minutes 
     Injection Volume: 10 
     Detection: 190-400 nm 
     Gradient: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Time (minutes) 
                 Solvent B [%] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 0 
                 0 
               
               
                   
                 12 
                 10 
               
               
                   
                 15 
                 100 
               
               
                   
                 15.1 
                 100 
               
               
                   
                 25 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     Instrument: LCQ Advantage Ion Trap MS 
     Sample concentration: 1 mg/ml, +ve ion mode by infusion 
     Source voltage (kV): 4.50 
     Source current (μA): 80.00 
     Sheath gas flow rate: 20.00 
     Aux/Sweep gas flow rate: 0.00 
     Capillary voltage (V): 8.00 
     Capillary temp (° C.): 200 
     Tube lens (V, Sp): 40.00 
     NACA/Urea Co-crystal. A primary co-crystal screen was conducted where 28 potential co-crystal formers (CCFs) were screened (Table 18) in 6 solvent systems under 2 process relevant crystallization conditions, namely, thermal maturation and evaporation. A NACA/urea co-crystal was identified ( FIG.  15   ). The NACA/urea pattern 1 material was successfully scaled up from four solvents as a part of the secondary co-crystal screen, then fully characterized where it was found to be crystalline by XPRD and PLM with the expected XRPD pattern, thermally stable with high purity. NMR analysis confirmed an approximate stoichiometric content of urea contained within the material. The material appeared to be stable under ambient conditions and elevated temperature (80° C.) but unstable when stored at high humidity for prolonged periods, showing degradation to the diNACA. No signs of dissociation were identified in organic solvents and solvent/water mixtures with low water activity but was found to dissociate in deionized water and solvent/water mixtures with a high water activity. An additional DSC experiment with post-XRPD analysis confirmed that the exothermic event observed during the DSC cooling cycle is a recrystallization to NACA. 
     
       
         
           
               
             
               
                 TABLE 18 
               
             
            
               
                   
               
               
                 Primary Co-Crystal Screen Co-Former List 
               
            
           
           
               
               
               
            
               
                   
                 Co-Former 
                 GRAS* 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 1 
                 2-Picolinamide 
                 Yes 
               
               
                 2 
                 5-Methylfurfural 
                 Yes 
               
               
                 3 
                 5-Methylfurfurylamine 
                 Yes 
               
               
                 4 
                 Adenine 
                 Yes 
               
               
                 5 
                 Citric Acid 
                 Yes 
               
               
                 6 
                 Glycine 
                 Yes 
               
               
                 7 
                 Hippuric Acid 
                 Yes 
               
               
                 8 
                 L-Aspartic Acid 
                 Yes 
               
               
                 9 
                 L-Proline 
                 Yes 
               
               
                 10 
                 L-Tyrosine 
                 Yes 
               
               
                 11 
                 Malonic Acid 
                 Yes 
               
               
                 12 
                 Melamine 
                 Yes 
               
               
                 13 
                 Oxalic Acid 
                 Yes 
               
               
                 14 
                 Theophylline 
                 Yes 
               
               
                 15 
                 Tromethamine 
                 Yes 
               
               
                 16 
                 Urea 
                 Yes 
               
               
                 17 
                 Xanthine 
                 Yes 
               
               
                 18 
                 3,4-Dihydroxybenzoic Acid 
                 Yes 
               
               
                 19 
                 Camphoric Acid 
                 Yes 
               
               
                 20 
                 Cytosine 
                 Yes 
               
               
                 21 
                 Formamide 
                 Yes 
               
               
                 22 
                 L-Cysteine 
                 Yes 
               
               
                 23 
                 L-Methionine 
                 Yes 
               
               
                 24 
                 L-Serine 
                 Yes 
               
               
                 25 
                 Threonine 
                 Yes 
               
               
                 26 
                 DiNACA 
                 Yes 
               
               
                 27 
                 N-Acetyl-L-cysteine 
                 Yes 
               
               
                 28 
                 Succinic acid 
                 Yes 
               
               
                   
               
               
                 *GRAS: generally recognized as safe 
               
            
           
         
       
     
     NACA Sodium Salt. A salt screen was conducted using 6 solvent systems under 2 process-relevant crystallization conditions, namely thermal maturation and evaporation. From this study, one sodium salt was identified. The salt was formed using sodium methoxide from either acetonitrile, ethanol, methanol, or tetrahydrofuran as shown in  FIG.  16   . 
     It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method, kit, reagent, or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention. 
     It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims. 
     All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or.” Throughout this application, the term “about” is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects. 
     As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. In embodiments of any of the compositions and methods provided herein, “comprising” may be replaced with “consisting essentially of” or “consisting of”. As used herein, the phrase “consisting essentially of” requires the specified integer(s) or steps as well as those that do not materially affect the character or function of the claimed invention. As used herein, the term “consisting” is used to indicate the presence of the recited integer (e.g., a feature, an element, a characteristic, a property, a method/process step or a limitation) or group of integers (e.g., feature(s), element(s), characteristic(s), property(ies), method/process steps or limitation(s)) only. 
     The term “or combinations thereof” as used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof” is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context. 
     As used herein, words of approximation such as, without limitation, “about”, “substantial” or “substantially” refers to a condition that when so modified is understood to not necessarily be absolute or perfect but would be considered close enough to those of ordinary skill in the art to warrant designating the condition as being present. The extent to which the description may vary will depend on how great a change can be instituted and still have one of ordinary skill in the art recognize the modified feature as still having the required characteristics and capabilities of the unmodified feature. In general, but subject to the preceding discussion, a numerical value herein that is modified by a word of approximation such as “about” may vary from the stated value by at least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%. 
     All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.