Substituted hydroxamic acids and uses thereof

Abstract

This invention provides compounds of formula (I): wherein R 1a , R 1b , R a , R 2a , R 2b , R 1 , and X have values as described in the specification, useful as inhibitors of HDAC6. The invention also provides pharmaceutical compositions comprising the compounds of the invention and methods of using the compositions in the treatment of proliferative, inflammatory, infectious, neurological or cardiovascular diseases or disorders.

What is claimed is: 
     
         1 . A compound of formula (I): 
       
         
           
           
               
               
           
         
         or a pharmaceutically acceptable salt thereof; 
         wherein: 
         each occurrence of R 1  is independently hydrogen, chloro, fluoro, —O—C 1-4  alkyl, cyano, hydroxy, C 1-4  alkyl, or C 1-4  fluoroalkyl; 
         one of R 2a  and R 2b  is R 1 , and the other is —C(O)—NH—OH; 
         X is C(R 1c )(R 1d ) or N—R b ; 
         R 1d  is taken together with R 1b  to form a double bond; 
         R 1c  is hydrogen, fluoro, bromo, —CH 2 —OR 4aa , —CH 2 N(R 4aa ) 2 , or C 1-4  alkyl; 
         R b  is hydrogen, C 1-6  alkyl, benzyl, or phenyl; or R b  is taken together with R 1b  to form a double bond; 
         when X is C(R 1c )(R 1d ), R 1b  is taken together with R 1d  to form a double bond; and when X is N—R b , R 1b  is taken together with R b  to form a double bond, or is taken together with R a  to form a double bond, or is taken together with R 1a  to form C═O; 
         R 1a  is hydrogen, fluoro, C 1-4  alkyl, C 1-4  fluoroalkyl, or G 1 ; or when X is N—R b , R 1a  can be taken together with R 1b  to form C═O; 
         R a  is hydrogen, C 1-4  alkyl, or G; or when X is N—R b , R 1b  can be taken together with R a  to form a double bond; 
         G is hydrogen, —R 3 , —L 1 —R 3 , or —L 2 —V 1 —R 3 ; 
         G 1  is —NH 2 , —R 3 , —L 1 —R 3 , —V 2 —R 3 , —L 1 —V 2 —R 3 , or —V 2 —L 1 —R 3 ; 
         V 1  is —O—, —N(R 4a )—, —S—, —N(R 4a )—C(O)—, —C(O)—N(R 4a )—, —SO 2 —N(R 4a )—, —N(R 4a )—SO 2 —, —C(O)—, —SO 2 —, or —N(R 4a )—C(O)—N(R 4a )—; 
         V 2  is —O—, —N(R 4a )—, —C(O)—N(R 4a )—, —N(R 4a )—C(O)—N(R 4a )—, or —N(R 4a )—C(O)—O—; 
         L 1  is an unsubstituted or substituted C 1-4  alkylene chain; 
         L 2  is an unsubstituted or substituted C 2-4  alkylene chain; 
         each occurrence of R 4aa  is independently hydrogen or C 1-4  alkyl; 
         R 3  is unsubstituted or substituted C 1-6  aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur; and 
         each occurrence of R 4a  is independently hydrogen, unsubstituted or substituted C 1-4 aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur. 
       
     
     
         2 . The compound of  claim 1 , wherein:
 R 2a  is C(O)—NH—OH;   R 2b  is R 1 ;   X is N—R b ;   R b  is taken together with R 1b  to form a double bond; and   R a  is G.   
     
     
         3 . The compound of  claim 1 , wherein: R 2a  is C(O)—NH—OH;
 R 2b  is R 1 ; 
 X is C(R 1c )(R 1d ); 
 R 1d  is taken together with R 1b  to form a double bond; 
 R 1c  is hydrogen; and 
 R a  is G. 
 
     
     
         4 . The compound of  claim 1 , wherein:
 each occurrence of R 1  is independently hydrogen, chloro, fluoro, cyano, hydroxy, methoxy, ethoxy, trifluoromethyl, methyl, or ethyl.   
     
     
         5 . The compound of  claim 1 , wherein: each substitutable carbon chain atom in R 3  is unsubstituted or substituted with 1-2 occurrences of —R 5dd ;
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with ═O, ═C(R 5 ) 2 , or —R 5aa ; 
 each substitutable unsaturated ring carbon atom in R 3  is unsubstituted or is substituted with —R 5a ; 
 each substitutable ring nitrogen atom in R 3  is unsubstituted or substituted with —R 9b ;
 each R 5a  is independently halogen, —NO 2 , —CN, —C(R 5 )═C(R 5)   2 , —C═C—R 5 , —OR 5 , —SR 6 , —S(O)R 6 , —SO 2 R 6 , —SO 2 N(R 4 ) 2 , —N(R 4 ) 2 , —NR 4 C(O)R 5 , —NR 4 C(O)N(R 4 ) 2 , —NR 4 CO 2 R 6 , —OC(O)N(R 4 ) 2 , —C(O)R 6 , —C(O)N(R 4 ) 2 , —N(R 4 )SO 2 R 6 , —N(R 4 )SO 2 N(R 4 ) 2 , unsubstituted or substituted Q.6 aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur; or two adjacent R 5a , taken together with the intervening ring atoms, form an unsubstituted or substituted fused 5-10 membered aromatic ring or an unsubstituted or substituted 4-10 membered non-aromatic ring having 0-3 heteroatoms independently selected from nitrogen, oxygen, and sulfur; 
 each occurrence of R 5aa  is independently chloro, fluoro, hydroxy, unsubstituted or substituted C 1-6  aliphatic, —O(C 1-6  alkyl), —C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, cyano, —N(R 4 ) 2 , —C(O)(C 1-6  alkyl), —CO 2 H, —C(O)NH 2 , —C(O)NH(C 1-6  alkyl), —C(O)N(C 1-6  alkyl) 2 , —NHC(O)C 1-6  alkyl, —NHC(O)OC 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, or —NHS(O) 2 C 1-6  alkyl; 
 each occurrence of R 5dd  is independently fluoro, hydroxy, —O(C 1-6  alkyl), cyano, —N(R 4 ) 2 , —C(O)(C 1-6  alkyl), —CO 2 H, —C(O)NH 2 , —C(O)NH(C 1-6  alkyl), —C(O)N(C 1-6  alkyl) 2 , —NHC(O)C 1-6  alkyl, —NHC(O)OC 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, or —NHS(O) 2 C 1-6  alkyl; 
 each R 4  is independently hydrogen, unsubstituted or substituted C 1-6  aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur; or two R 4  on the same nitrogen atom, taken together with the nitrogen atom, form an unsubstituted or substituted 5- to 6-membered heteroaryl or an unsubstituted or substituted 4- to 8-membered heterocyclyl having, in addition to the nitrogen atom, 0-2 ring heteroatoms selected from nitrogen, oxygen, and sulfur; 
 each R 5  is independently hydrogen, unsubstituted or substituted C 1-6  aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur. 
 each R 6  is independently unsubstituted or substituted C 1-6  aliphatic, unsubstituted or substituted 3-10-membered cycloaliphatic, unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur; 
 each R 9b  is independently —C(O)R 6 , —C(O)N(R 4 ) 2 , —CO 2 R 6 , —SO 2 R 6 , —SO 2 N(R 4 ) 2 , unsubstituted C 3-10  cycloaliphatic, C 3-10  cycloaliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted C 1-6  aliphatic, or C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 ; 
 each R 7  is independently unsubstituted or substituted 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur, unsubstituted or substituted 6-10-membered aryl, or unsubstituted or substituted 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur; and 
 each R 8  is independently chloro, fluoro, —OR 5 , —CN, —N(R 4 ) 2 ) —C(O)(C 1-6  alkyl), —CO 2 H, —CO 2 (C 1-6  alkyl), —C(O)NH 2 , —C(O)NH(C 1-6  alkyl), or —C(O)N(C 1-6  alkyl) 2 . 
 
 
     
     
         6 . The compound of  claim 5 , wherein:
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with —R 5aa ;   the total number of R 5a  and R 5aa  substituents is p;   p is 1-4;
 each R 5a  is independently halogen, cyano, nitro, hydroxy, unsubstituted C 1-6  aliphatic, C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted —O—C 1-6  alkyl, —O—C 1-6  alkyl substituted with 1-2 independent occurrences of R 7  or R 8 , C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, —NHC(O)R 6 , —C(O)NH(R 4 ), —NHC(O)O—C 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, —NHS(O) 2 C 1-6  alkyl, —NHC 1-6 alkyl, —N(C 1-6  alkyl) 2 , 3-10-membered cycloaliphatic substituted with 0-2 occurrences of —R 7a , 4-10-membered heterocyclyl having 1 -4 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a , 6-10-membered aryl substituted with 0-2occurrences of —R 7a , or 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a ; and 
 each occurrence of R 7a  is independently chloro, fluoro, C 1-6  aliphatic, C 1-6  fluoroalkyl, —O—C 1-6  alkyl, —O—C 1-6  fluoroalkyl, cyano, hydroxy, —CO 2 H, —NHC(O)C 1-6  alkyl, —NH 2 , —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , —C(O)NHC 1-6  alkyl, —C(O)N(C 1-6  alkyl) 2 , —NHC(O)NHC 1-6  alkyl, —NHC(O)N(C 1-6  alkyl) 2 , or —NHS(O) 2 C 1-6  alkyl. 
   
     
     
         7 . The compound of  claim 5 , represented by formula (IV-a-i-a): 
       
         
           
           
               
               
           
         
         wherein: 
         R 1a′  is hydrogen, fluoro, or C 1-4  alkyl. 
       
     
     
         8 . The compound of  claim 7 , wherein:
 G is —L 1 —R 3 , or —L 2 —V 1 —R 3 ; and   V 1  is —O— or —N(R 4a )—.   
     
     
         9 . The compound of  claim 8 , wherein:
 each occurrence of R 1  is independently hydrogen, chloro, fluoro, cyano, hydroxy, methoxy, ethoxy, trifluoromethyl, methyl, or ethyl.   
     
     
         10 . The compound of  claim 8 , wherein:
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with —R 5aa ;   the total number of R 5a  and R 5aa  substituents is p;   p is 1-4;
 each R 5a  is independently halogen, cyano, nitro, hydroxy, unsubstituted C 1-6  aliphatic, C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted —O—C 1-6  alkyl, —O—C 1-6  alkyl substituted with 1-2 independent occurrences of R 7  or R 8 , C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, —NHC(O)R 6 , —C(O)NH(R 4 ), —NHC(O)O—C 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, —NHS(O) 2 C 1-6  alkyl, —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , 3-10-membered cycloaliphatic substituted with 0-2 occurrences of —R 7a , 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a , 6-10-membered aryl substituted with 0-2occurrences of —R 7a , or 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a ; and 
 each occurrence of R 7a  is independently chloro, fluoro, C 1-6  aliphatic, C 1-6  fluoroalkyl, —O—C 1-6  alkyl, —O—C 1-6  fluoroalkyl, cyano, hydroxy, —CO 2 H, —NHC(O)C 1-6  alkyl, —NH 2 , —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , —C(O)NHC 1-6  alkyl, —C(O)N(C 1-6  alkyl) 2 , —NHC(O)NHC 1-6  alkyl, —NHC(O)N(C 1-6  alkyl) 2 , or —NHS(O) 2 C 1-6  alkyl. 
   
     
     
         11 . The compound of  claim 10 , wherein:
 R 1a′  is hydrogen; and   R 1  is hydrogen.   
     
     
         12 . The compound of  claim 5 , represented by formula (IV-b-i-a): 
       
         
           
           
               
               
           
         
         wherein: 
         R 1a′  is hydrogen, fluoro, or C 1-4  alkyl. 
       
     
     
         13 . The compound of  claim 12 , wherein:
 G is —L 1 —R 3 , or —L 2 —V 1 —R 3 ; and   V 1  is —O— or —N(R 4a )—.   
     
     
         14 . The compound of  claim 13 , wherein:
 each occurrence of R 1  is independently hydrogen, chloro, fluoro, cyano, hydroxy, methoxy, ethoxy, trifluoromethyl, methyl, or ethyl.   
     
     
         15 . The compound of  claim 13 , wherein:
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with —R 5aa ;   the total number of R 5a  and R 5aa  substituents is p;   p is 1-4;
 each R 5a  is independently halogen, cyano, nitro, hydroxy, unsubstituted C 1-6  aliphatic, C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted —O—C 1-6  alkyl, —O—C 1-6  alkyl substituted with 1-2 independent occurrences of R 7  or R 8 , C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, —NHC(O)R 6 , —C(O)NH(R 4 ), —NHC(O)O—C 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, —NHS(O) 2 C 1-6  alkyl, —NHC 1-6  alkyl, —N(C 1-6 alkyl) 2 , 3-10-membered cycloaliphatic substituted with 0-2 occurrences of —R 7a , 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a , 6-10-membered aryl substituted with 0-2 occurrences of —R 7a , or 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a ; and 
 each occurrence of R 7a  is independently chloro, fluoro, C 1-6  aliphatic, C 1-6  fluoroalkyl, —O—C 1-6  alkyl, —O—C 1-6  fluoroalkyl, cyano, hydroxy, —CO 2 H, —NHC(O)C 1-6  alkyl, —NH 2 , —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , —C(O)NHC 1-6  alkyl, —C(O)N(C 1-6  alkyl) 2 , —NHC(O)NHC 1-6  alkyl, —NHC(O)N(C 1-6  alkyl) 2 , or —NHS(O) 2 C 1-6  alkyl. 
   
     
     
         16 . The compound of  claim 15 , wherein:
 R 1a′  is hydrogen; and   R 1  is hydrogen.   
     
     
         17 . The compound of  claim 5 , represented by formula (IV-b-ii-a): 
       
         
           
           
               
               
           
         
         wherein: 
         R 1a′  is hydrogen, fluoro, or C 1-4  alkyl. 
       
     
     
         18 . The compound of  claim 17 , wherein:
 G is —L 1 —R 3 , or —L 2 —V 1 —R 3 ; and   V 1  is —O— or —N(R 4a )—.   
     
     
         19 . The compound of  claim 18 , wherein:
 each occurrence of R 1  is independently hydrogen, chloro, fluoro, cyano, hydroxy, methoxy, ethoxy, trifluoromethyl, methyl, or ethyl.   
     
     
         20 . The compound of  claim 18 , wherein:
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with —R 5aa .   the total number of R 5a  and R 5aa  substituents is p;   p is 1-4;
 each R 5a  is independently halogen, cyano, nitro, hydroxy, unsubstituted C 1-6  aliphatic, C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted —O—C 1-6  alkyl, —O—C 1-6  alkyl substituted with 1-2 independent occurrences of R 7  or R 8 , C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, —NHC(O)R 6 , —C(O)NH(R 4 ), 
 —NHC(O)O—C 1-6  alkyl, —NHC(O)NHC 1-6  alkyl, —NHS(O) 2 C 1-6  alkyl, —NHC 1-6  alkyl, —N(C 1-6 alkyl) 2 , 3-10-membered cycloaliphatic substituted with 0-2 occurrences of —R 7a , 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a , 6-10-membered aryl substituted with 0-2 occurrences of —R 7a , or 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a ; and 
 each occurrence of R 7a  is independently chloro, fluoro, C 1-6  aliphatic, C 1-6  fluoroalkyl, —O—C 1-6  alkyl, —O—C 1-6  fluoroalkyl, cyano, hydroxy, —CO 2 H, —NHC(O)C 1-6  alkyl, —NH 2 , —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , —C(O)NHC 1-6  alkyl, —C(O)N(C 1-6 alkyl) 2 , —NHC(O)NHC 1-6  alkyl, —NHC(O)N(C 1-6  alkyl) 2 , or —NHS(O) 2 C 1-6  alkyl. 
   
     
     
         21 . The compound of  claim 20 , wherein:
 R 1a  is hydrogen; and   R 1  is hydrogen.   
     
     
         22 . The compound of  claim 5 , represented by formula (IV-b-i-b): 
       
         
           
           
               
               
           
         
         wherein: 
         R a′  is hydrogen, or C 1-4  alkyl. 
       
     
     
         23 . The compound of  claim 22 , wherein:
 G is —R 3 , or —L 1 —R 3 .   
     
     
         24 . The compound of  claim 23 , wherein:
 each occurrence of R 1  is independently hydrogen, chloro, fluoro, cyano, hydroxy, methoxy, ethoxy, trifluoromethyl, methyl, or ethyl.   
     
     
         25 . The compound of  claim 23 , wherein:
 each substitutable saturated ring carbon atom in R 3  is unsubstituted or substituted with —R 5aa ;   the total number of R 5a  and R 5aa  substituents is p;   p is 1-4;
 each R 5a  is independently halogen, cyano, nitro, hydroxy, unsubstituted C 1-6  aliphatic, C 1-6  aliphatic substituted with 1-2 independent occurrences of R 7  or R 8 , unsubstituted —O—C 1-6  alkyl, —O—C 1-6  alkyl substituted with 1-2 independent occurrences of R 7  or R 8 , C 1-6  fluoroalkyl, —O—C 1-6  fluoroalkyl, —NHC(O)R 6 , —C(O)NH(R 4 ), —NHC(O)O—C 1-6  alkyl, —NHC(O)NH 1-6  alkyl, —NHS(O) 2 C 1-6  alkyl, —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , 3-10-membered cycloaliphatic substituted with 0-2 occurrences of —R 7a , 4-10-membered heterocyclyl having 1-4 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of −R 7a , 6-10-membered aryl substituted with 0-2 occurrences of —R 7a , or 5-10-membered heteroaryl having 1-5 heteroatoms independently selected from nitrogen, oxygen, and sulfur substituted with 0-2 occurrences of —R 7a ; and 
 each occurrence of R 7a  is independently chloro, fluoro, C 1-6  aliphatic, C 1-6  fluoroalkyl, —O—C 1-6 alkyl, —O—C 1-6  fluoroalkyl, cyano, hydroxy, —CO 2 H, —NHC(O)C 1-6  alkyl, —NH 2 , —NHC 1-6  alkyl, —N(C 1-6  alkyl) 2 , —C(O)NHC 1-6  alkyl, —C(O)N(C 1-6  alkyl) 2 , —NHC(O)NHC 1-6  alkyl, —NHC(O)N(C 1-6  alkyl) 2 , or —NHS(O) 2 C 1-6  alkyl. 
   
     
     
         26 . The compound of  25 , wherein:
 R a′  is hydrogen; and   R 1  is hydrogen.   
     
     
         27 . A pharmaceutical composition comprising a compound of  claim 1  and a pharmaceutically acceptable carrier. 
     
     
         28 . A method of treating a proliferative disorder in a patient comprising administering to said patient a therapeutically effective amount of a compound of  claim 1 . 
     
     
         29 . The method of  claim 28 , wherein the proliferative disorder is breast cancer, lung cancer, ovarian cancer, multiple myeloma, acute myelogenous leukemia, or acute lymphoblastic leukemia. 
     
     
         30 . A method for inhibiting HDAC6 activity in a patient comprising administering a pharmaceutical composition comprising an amount of a compound of  claim 1  effective to inhibit HDAC6 activity in the patient.