Coelenterazine analogues

A method for in vivo bioluminescent imaging, the method comprising:

• contacting a live cell with a compound of Formula I or Formula II:
• wherein R5 and R5a are hydrogen;
• R6 and R6a are aryl optionally substituted with 1, 2, 3, 4, or 5 substituents independently selected from the group consisting of halogen, hydroxy, cyano, nitro, amino, alkylamino, dialkylamino, acyl amino, aminoalkyl, alkyl, fluoroalkyl, alkoxyfluoroalkyl, fluoroalkoxy, alkenyl, alkynyl, haloalkyl, haloalkoxy, heteroalkyl, alkoxy, sulfonylamino, sulfonyl, alkylsulfonyl, amide, carbamate, silyl, substituted silyl, acyl, or aminosulfonyl;
• R8 and R8a are optionally substituted aryl, heteroaryl, alkyl substituted with aryl, or alkyl substituted with heteroaryl;
• Rx is halogen, hydroxy, cyano, nitro, amino, alkylamino, dialkylamino, alkyl, alkenyl, alkynyl, alkoxy, or heteroalkyl;
• and
• p is 0, 1, 2, 3, 4, or 5;
• provided that the following compounds are excluded from formula (I):
• 8-benzyl-2-(furan-2-ylmethyl)-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one;
• and
• 8-benzyl-2-(furan-2-ylmethyl)-6-(4-hydroxyphenyl)imidazo[1,2-a]pyrazin-3(7H)-one;
• and
• Rx is not 4-hydroxy when p is 1, R5a is not hydrogen, or R8a is not benzyl, or any combination thereof;
• and
• detecting luminescence;
• wherein the live cell expresses a coelenterazine-utilizing luciferase.