Radiolabeling, biodistribution evaluation in normal rats and radiation dose estimates of a low molecular weight apoptosis imaging agent, [¹⁸F]-NST ML-10

Abstract

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Objectives: PET imaging to detect the presense of apoptotic cells would be highly benificial for defining the affected areas in stroke patients, as well as for monitoring the effects of neuroprotective treatments. However, such imaging in the CNS requires a yet unavailable low molecular weight tracer that would selectively associate with affective areas and clear quickly from normal tissues. We report herein the radiolabeling and evaluation in healthy rats of a novel PET tracer for imaging of apoptosis, defined as [¹⁸F]-NST ML-10 (a-methyl-fluoropentylmalonate, M.W. 206). The tracer utilizes the ApoSense Technology, which comprises small-molecular weight compounds, capable of detection of apoptosis. These structures, when tritiated or fluorophore-labeled, manifest selective uptake in regions of cell death in animal models of stroke. The study objective was to determine the feasiblity of radiolabeling with fluorine-18, and to determine it’s whole body biodistribution and domsimety of [¹⁸F]-NST ML-10 in normal rats.

Methods: [¹⁸F]-NST ML-10 was labeled by reacting the corresponding BOC-protected alkyl mesylate precursor with anhydrous [¹⁸F]flouide/K222/acetonitrile at 90C for 20 min. After silica gel cartridge removal of unreacted [¹⁸F]fluoride, BOC groups were removed by treatment with 90% trifluoroacetic acid at ambient temperature for 5 min. Purification was by reversed phase HPLC. Whole body distribution (5–240 min) in adult male rats (10-20 µCi in 100 µl saline per rat) was performed. Tissue radiation dose was determined from these biodistribution data.

Results: [¹⁸F]-NST ML-10 was synthesized in good yield (25-40% EOB), high specific activity (> 1.0 Ci/µmol, EOS) and high radiochemical purity (> 99%) with no detectable degradation of the tracer for up to 2 hrs when maintained at ambient temperature. Peak uptake in all tissues occurred early (5-15 min) and was highest in the kidney (2.7 ± 0.9 %ID/g, 5 min), with rapid clearance over time. Excretion appeared mainly through kidneys. Bone uptake was low (0.17 ± 0.05 %ID/g, 5 min) and decreased over time, indicating metabolic stability. Radiation dosimetry estimates of [¹⁸F]-NST ML10 revealed that the brain and kidneys receive about 0.03 mGy/MBq, while other organs receive between 0.01-0.02 mGy/MBq.

Conclusions: These data support that [¹⁸F]-NST ML-10 is a very promising candidate PET probe for imaging apoptosis, manifesting a novel, reproducible radiolabeling method, and favorable biodistribution and dosimetry profile in healthy individuals. These findings support further development of [¹⁸F]-NST ML-10 for PET imaging of apoptosis.

Research Support (if any): NeuroSurvival Technologies LTD