Modulators of ATP-binding cassette transporters

A kit for use in measuring the activity of CFTR in a biological sample in vitro or in vivo, comprising:

• (i) a compound of formula (I):
• or a pharmaceutically acceptable salt thereof,
• wherein:
• Each R1 is an optionally substituted C1-6 aliphatic, an optionally substituted aryl, an optionally substituted heteroaryl, an optionally substituted C3-10 cycloaliphatic, an optionally substituted 3 to 10 membered heterocycloaliphatic, carboxy, amido, amino, halo, or hydroxy, provided that at least one R1 is an optionally substituted cycloaliphatic, an optionally substituted heterocycloaliphatic, an optionally substituted aryl, or an optionally substituted heteroaryl attached to the 5- or 6- position of the pyridyl ring;
• Each R2 is hydrogen, an optionally substituted C1-6 aliphatic, an optionally substituted C3-6 cycloaliphatic, an optionally substituted phenyl, or an optionally substituted heteroaryl;
• Each R3 and R′3 together with the carbon atom to which they are attached form an optionally substituted C3-7 cycloaliphatic or an optionally substituted heterocycloaliphatic;
• Each R4 is an optionally substituted aryl or an optionally substituted heteroaryl;
• and
• Each n is 1,2,3 or 4;
• and
• (ii) instructions for:
• a) contacting the compound with the biological sample;
• and
• b) measuring activity of said CFTR.