Development of a ¹⁸F‐labeled Diaryl‐Substituted Dihydropyrrolo[3,2,1‐hi]indole as Potential Probe for Functional Imaging of Cyclooxygenase‐2 with PET

Abstract

High COX-2 expression is associated with tumor progression and poor treatment response. Imaging of functional COX-2 expression by PET would provide beneficial information for theranostics. Here we describe precursor synthesis and radiolabeling attempts of (dihydro)pyrrolo [3,2,1-hi]indoles [ 18 F]DHPI and [ 18 F]PI, two novel tricyclic COX-2 inhibitors. Contrary to [ 18 F] PI, [ 18 F]DHPI was accessible by [ 18 F]fluorination under mild basic conditions and McMurry cyclization.Radiopharmacological evaluation was performed in vitro by investigating cellular uptake in human COX-2-positive cells, and in vivo in rats and A2058-melanoma xenograft mice, by focusing on metabolic stability and tumor uptake. To assess COX-2 specificity, blocking experiments with celecoxib were performed. Despite of high COX-2 selectivity and metabolic stability, [ 18 F]DHPI did not show COX-2-dependent cell and tissue uptake, in part explained by high unspecific binding. Since appropriate lipophilicity is a prerequisite for targeting intracellularly localized COX-2, future efforts should focus on use of longer-lived radionuclides and/or targeted delivery systems.

Synthesis and Cyclooxygenase Inhibition of Sulfonamide-Substituted (Dihydro)Pyrrolo[3,2,1-hi]indoles and Their Potential Prodrugs

“…This is in accordance with the behavior of valdecoxib (IC 50 hCOX-2 = 5 nM) and its prodrug parecoxib (IC 50 hCOX-2 = 20 µM) [19]. Taking into account that only the 18 F-labeled 1,2-dihydropyrrolo[3,2,1- hi ]indole but not the respective 18 F-labeled pyrrolo[3,2,1- hi ]indole was accessible via radiosynthesis before [16], compound 3d represents the most promising lead for further PET tracer developments. As a non-COX-active and hydrophilic prodrug this probe could be administered and, hypothetically, be converted into the potent radiolabeled COX-2 inhibitor in the liver to finally distribute comparable to the parent drug afterward.…”

“…Recently, a potent and selective COX-2 inhibitor having a 1,2-dihydropyrrolo[3,2,1- hi ]indole core and a methylsulfonyl group was labeled with fluorine-18 by us and evaluated in vitro and in vivo. This tracer failed to visualize COX-2 in vivo because of its fast hepatobiliary excretion as well as high non-specific binding, too [16,17]. In this study we aimed for the investigation of a set of 1,2-dihydropyrrolo[3,2,1- hi ]indoles and pyrrolo[3,2,1- hi ]indoles with a sulfonamide-group to overcome the previous drawbacks and find novel leads for the development of radiolabeled COX-2 inhibitors.…”

“…Based on our ongoing interest to develop novel probes in this field, we found a set of methylsulfonyl-substituted tricyclic 1,2-dihydropyrrolo[3,2,1- hi ]indoles and pyrrolo[3,2,1- hi ]indoles that show highly potent and selective COX-2 inhibition and radiolabeled one derivative with fluorine-18 [16,17]. Unfortunately, in vitro cell uptake studies as well as in vivo studies using the COX-2-positive human melanoma cell line A2058 revealed that this radiotracer was unable to target COX-2 due to high unspecific binding in cells and off-target tissues caused by its high lipophilicity as well as due to fast hepatobiliary excretion.…”

Deuteration versus ethylation – strategies to improve the metabolic fate of an 18F-labeled celecoxib derivative

“…One reason might be, as shown by Uddin et al, 21 that the [ 18 F]uoromethyl-substituted radiotracer is prone to 18 F-deuorination in vivo and, presumably, binds to carbonic anhydrase in the erythrocytes as it is known for celecoxib and other sulfonamide-substituted Coxibs. 28,32,33 Based on our interest in the development of radiolabeled COX-2 inhibitors [34][35][36][37] and to minimize non-specic binding, we aimed to synthesize the respective methylsulfonyl-substituted celecoxib derivatives and thus we herein describe two attempts to increase their metabolic stability. In general, possibilities to increase the metabolic stability of a radiotracer containing 18 F bonded to an sp 3 -hybridized carbon atom include (i) the deuteration vicinal to the uorine substituent [38][39][40][41] or at other parts of the molecule, [41][42][43] (ii) the replacement of an alkyl chain by a cycloalkyl ring, 44 (iii) the use of CF 2 18 F groups, 20 and (iv) the application of uorophenyl-instead of uoroalkylsubstituted analogs wherever possible.…”

“…S5 †). The log D value (pH 7.4) was determined to be 2.72 for [ 18 F]5a and [D 2 , 18 F]5a, and 2.65 for [ 18 F]5bby an HPLC-based method 35,48.…”