Process for the preparation and purification of thiol-containing maytansinoids

Abstract

The present invention provides a process for the preparation and purification of thiol-containing maytansinoids comprising the steps of: (1) reductive hydrolysis of a maytansinoid C-3 ester with a reducing agent selected from the group consisting lithium trimethoxyaluminum hydride (LiAl (OMe) 3 H), lithium triethoxyaluminum hydride (LiAl(OEt) 3 H), lithium tripropoxyaluminum hydride (LiAl (OPr) 3 H), sodium trimethoxyaluminum hydride (NaAl (OMe) 3 H), sodium triethoxyaluminum hydride (NaAl(OEt) 3 H) and sodium tripropoxyaluminum hydride (NaAl(OPr) 3 H) to yield a maytansinol; (2) purifying the maytansinol to remove side products when present; (3) esterifying the purified maytansinol with a carboxylic acid to yield a mixture of an L- and a D-aminoacyl ester of maytansinol; (4) separating the L-aminoacyl ester of maytansinol from the reaction mixture in (3); (5) reducing the L-aminoacyl ester of maytansinol to yield a thiol-containing maytansinoid; and (5) purifying the thiol-containing maytansinoid.

1. A process for preparing a thiol-containing maytansinoid wherein the thiol group is part of the ester moiety at C-3, comprising the steps of:
 (1) conducting reductive hydrolysis of a maytansinoid C-3 ester with a reducing agent selected from the group consisting of lithium trimethoxyaluminum hydride (LiAl(OMe) 3 H), lithium triethoxyaluminum hydride (LiAl(OEt) 3 H) and lithium tripropoxyaluminum hydride (LiAl(OPr) 3 H), to yield a maytansinol;  
 (2) purifying the maytansinol to remove side products when present;  
 (3) esterifying the purified maytansinol with a carboxylic acid to yield a reaction mixture of an L- and a D-aminoacyl ester of maytansinol;  
 (4) separating the L-aminoacyl ester of maytansinol from the reaction mixture in (3);  
 (5) reducing the L-aminoacyl ester of maytansinol to yield a thiol-containing maytansinoid; and  
 (6) purifying the thiol-containing maytansinoid.  
 
     
     
       2. The process of  claim 1 , wherein the reducing agent in (1) is lithium trimethoxyaluminum hydride. 
     
     
       3. The process of  claim 1 , wherein the reducing agent in (1) is used in a concentration of from about 5 to 100 equivalents per mole of the maytansinoid C-3 ester. 
     
     
       4. The process of  claim 1 , wherein the reducing agent in (1) is used in a concentration of from about 7.5 to 30 equivalents per mole of the maytansinoid C-3 ester. 
     
     
       5. The process of  claim 1 , wherein the reducing agent in (1) is used in a concentration of from about 10 to 20 equivalents per mole of the maytansinoid C-3 ester. 
     
     
       6. The process of  claim 1 , wherein the reductive hydrolysis in (1) is conducted at a temperature of from about −80° C. to 0° C. 
     
     
       7. The process of  claim 1 , wherein the reductive hydrolysis in (1) is conducted at a temperature of from about −45° C. to −27.5° C. 
     
     
       8. The process of  claim 1 , wherein the reductive hydrolysis in (1) is conducted at a temperature of from about −35° C. to −30° C. 
     
     
       9. The process of  claim 1 , wherein the reducing agent in (1) is added over a period of from about 5 to 40 minutes. 
     
     
       10. The process of  claim 1 , wherein the reducing agent in (1) is added over a period of from about 7 to 20 minutes. 
     
     
       11. The process of  claim 1 , wherein the reducing agent in (1) is added over a period of from about 8 to 12 minutes. 
     
     
       12. The process of  claim 1 , wherein the maytansinol is purified in (2) by chromatography. 
     
     
       13. The process of  claim 12 , wherein the chromatography is silica gel column chromatography, preparative thin-layer chromatography on silica gel or cyano-bonded silica HPLC column chromatography. 
     
     
       14. The process of  claim 12 , wherein the chromatography is silica gel column chromatography. 
     
     
       15. The process of  claim 12 , wherein the purification is performed at ambient temperature. 
     
     
       16. The process of  claim 12 , wherein the maytansinol is purified to a purity of about 95%. 
     
     
       17. The process of  claim 1 , wherein the carboxylic acid in (3) is selected from the group consisting of N-methyl-N-methyldithioacetyl-L-alanine, N-methyl-N-(3-methyldithio-propanoyl)-L-alanine, N-methyl-N-(3-methyldithio-butanoyl)-L-alanine, N-methyl-N-(4-methyldithio-butanoyl)-L-alanine, N-methyl-N-(5-methyldithio-pentanoyl)-L-alanine, N-methyl-N-(3-phenyldithio-propanoyl)-L-alanine, N-methyl-N-(3-(4-nitrophenyldithio)propanoyl)L-alanine, N-acetyl-N-methylmethyldithiocysteine and N-acetyl-N-methyl-methyldithiohomocysteine. 
     
     
       18. The process of  claim 1 , wherein the carboxylic acid in (3) is N-methyl-N-(3-methyldithio-propanoyl)-L-alanine. 
     
     
       19. The process of  claim 1 , wherein the esterification in (3) is conducted at ambient temperature. 
     
     
       20. The process of  claim 1 , wherein the esterification in (3) further comprises the use of dicyclohexylcarbodiimide and zinc chloride. 
     
     
       21. The process of  claim 1 , wherein the separating in (4) is carried out by passing the reaction mixture over a cyano-bonded silica HPLC column. 
     
     
       22. The process of  claim 1 , wherein the separating in (4) is carried out at about 25° C. 
     
     
       23. The process of  claim 1 , wherein the reduction in (5) uses dithiothreitol as the reducing agent. 
     
     
       24. The process of  claim 1 , wherein the reduction in (5) is carried out in a mixture of ethyl acetate-methanol-aqueous buffer which is capable of keeping buffer salts, dithiothreitol, unreduced maytansinoids and reduced maytansinoids in solution. 
     
     
       25. The process of  claim 24 , wherein the mixture of ethyl acetate-methanol-aqueous buffer is 1:1.5:1, v/v/v, ethyl acetate:methanol:aqueous buffer. 
     
     
       26. The process of  claim 24 , wherein the concentration of the thiol-containing maytansinoid is such that the thiol-containing maytansinoid remains soluble in ethyl acetate-methanol-aqueous buffer. 
     
     
       27. The process of  claim 26 , wherein the concentration of the thiol-containing maytansinoid is about 4 g/L. 
     
     
       28. The process of  claim 1 , wherein the reduction in (5) is carried out in an oxygen-free atmosphere. 
     
     
       29. The process of  claim 1 , wherein the reduction in (5) is carried out at about 25° C. 
     
     
       30. The process of  claim 1 , wherein the purifying of the thiol-containing maytansinoid in (6) is by chromatography. 
     
     
       31. The process of  claim 30 , wherein the purifying of the thiol-containing maytansinoid in (6) is by a cyano-bonded HPLC column chromatography. 
     
     
       32. The process of  claim 31 , wherein the chromatography is by a cyano-bonded HPLC column equilibrated and run in an organic solvent. 
     
     
       33. The process of  claim 32 , wherein the organic solvent is a mixture of hexanes:2-propanol:ethyl acetate. 
     
     
       34. The process of  claim 33 , wherein the organic solvent is a 78.0:5.5:16.5, v/v/v, mixture of hexanes:2-propanol:ethyl acetate. 
     
     
       35. A process for isolating maytansinol from a mixture containing unreduced and over- reduced maytansinoids by separating the maytansinol by normal - phase high performance liquid chromatography  ( HPLC )  on a chemically modified silica stationary phase.   
     
     
       36. The process of  claim 35 , wherein the chemically modified silica is cyano- bonded silica.   
     
     
       37. The process of  claim 35 , further comprising converting the maytansinol prepared by the process into a cell- binding agent maytansinoid complex.   
     
     
       38. A process for isolating maytansinol from a mixture containing unreduced and over- reduced maytansinoids by separating the maytansinol by large - scale preparative normal - phase high performance liquid chromatography  ( HPLC )  on a chemically modified silica stationary phase.   
     
     
       39. The process of  claim 38  wherein the chemically modified silica is cyano- bonded silica.   
     
     
       40. The process of  claim 38  further comprising converting the maytansinol prepared by the process into a cell- binding agent maytansinoid complex.