The nonpeptidyl caspase binding radioligand (S)-1-(4-(2-[18F]Fluoroethoxy)-benzyl)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin ([18F]CbR) as potential positron emission tomography-compatible apoptosis imaging agent.
OBJECTIVE
Radiolabeled Annexin V-derivatives are well characterized phosphatidylserine-targeting biomarkers and considered as state-of-the-art tracers for non-invasive molecular imaging of apoptosis. In contrast to Annexin V-derived imaging agents being surrogate markers of apoptosis, activated cysteinyl aspartate-specific proteases (caspases) represent the common final path of apoptosis being a suitable in vivo target for the exclusive imaging of apoptotic tissues in vivo.


METHODS
We suggest 5-pyrrolidinylsulfonyl isatins as a potential nonpeptidyl class of caspase inhibitors for the design of caspase binding radioligands (CbRs), that could be used for in vivo visualization of activated effector caspases. The caspase inhibitor (S)-(+)-5-[1-(2-Methoxy-methylpyrrolidinyl)sulfonyl]isatin 1 (K(i, caspase)-3 (1)=60 nM) was chosen as lead structure for the development of nonpeptidyl CbRs. Its structural expansion at the N-1-position the yields moderate lipophilic p-(2-fluoroethoxy)benzyl variant 2 (log D=2.2), without loss of caspase binding potency (IC(50, caspase)-3 (2)=36.4 nM).


RESULTS
Subsequent automated radiosynthesis of the corresponding (18)F-labeled target CbR [(18)F]2 was performed by direct (18)F-labeling of tosylate precursor 4.


CONCLUSIONS
As shown by biodistribution studies and small animal positron emission tomography a nonpeptidyl 5-pyrrolidinylsulfonyl isatin-type caspase inhibitor (S)-1-(4-(2-[(18)F]Fluoroetho-xy)benzyl)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin [(18)F]2 with rapid blood clearance characteristics could potentially detect apoptosis in vivo.
Faust
A
A
Department of Nuclear Medicine, University Hospital Muenster, Muenster, Germany.
Wagner
S
S
Law
M P
MP
Hermann
S
S
Schnöckel
U
U
Keul
P
P
Schober
O
O
Schäfers
M
M
Levkau
B
B
Kopka
K
K
D000818:Animals; D017209:Apoptosis; D020169:Caspases / Q000378:metabolism*; D007510:Isatin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000493:pharmacokinetics; D007553:Isotope Labeling / Q000379:methods; D051379:Mice; D008819:Mice, Nude; D049268:Positron-Emission Tomography / Q000379:methods*; D011759:Pyrrolidines / Q000737:chemistry* / Q000493:pharmacokinetics*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis
C516218:1-(4-(2-fluoroethoxy)benzyl)-5-(1-(2-methoxymethylpyrrolidinyl)sulfonyl)isatin; D011759:Pyrrolidines; D019275:Radiopharmaceuticals; D007510:Isatin; D020169:Caspases