A peptide-based positron emission tomography probe for in vivo detection of caspase activity in apoptotic cells

Purpose: Apoptosis, or programmed cell death, can be leveraged as a surrogate measure of response to therapeutic interventions in medicine. Cysteine aspartic acid-specific proteases, or caspases, are essential determinants of apoptosis signaling cascades and represent promising targets for molecular imaging. Here, we report development and in vivo validation of [18F]4-fluorobenzylcarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone ([18F]FB-VAD-FMK), a novel peptide-based molecular probe suitable for quantification of caspase activity in vivo using positron emission tomography (PET). Experimental Design: Supported by molecular modeling studies and subsequent in vitro assays suggesting probe feasibility, the labeled pan-caspase inhibitory peptide, [18F]FB-VAD-FMK, was produced in high radiochemical yield and purity using a simple two-step, radiofluorination. The biodistribution of [18F]FB-VAD-FMK in normal tissue and its efficacy to predict response to molecularly targeted therapy in tumors was evaluated using microPET imaging of mouse models of human colorectal cancer. Results: Accumulation of [18F]FB-VAD-FMK was found to agree with elevated caspase-3 activity in response to Aurora B kinase inhibition as well as a multidrug regimen that combined an inhibitor of mutant BRAF and a dual PI3K/mTOR inhibitor in V600EBRAF colon cancer. In the latter setting, [18F]FB-VAD-FMK PET was also elevated in the tumors of cohorts that exhibited reduction in size. Conclusions: These studies illuminate [18F]FB-VAD-FMK as a promising PET imaging probe to detect apoptosis in tumors and as a novel, potentially translatable biomarker for predicting response to personalized medicine.