Biodistribution, pharmacokinetics and metabolism of interleukin-1 receptor antagonist (IL-1RA) using [18F]-IL1RA and PET imaging in rats

BACKGROUND AND PURPOSE Positron emission tomography (PET) has the potential to improve our understanding of the preclinical pharmacokinetics and metabolism of therapeutic agents, and is easily translated to clinical studies in humans. However, studies involving proteins radiolabelled with clinically relevant PET isotopes are currently limited. Here we illustrate the potential of PET imaging in a preclinical study of the biodistribution and metabolism of 18F-labelled IL-1 receptor antagonist ([18F]IL-1RA) using a novel [18F]-radiolabelling technique. EXPERIMENTAL APPROACH IL-1RA was radiolabelled by reductive amination on lysine moieties with [18F]fluoroacetaldehyde. Sprague-Dawley rats were injected intravenously with [18F]IL-1RA and imaged with a PET camera for 2 h. For the study of IL-1RA metabolites by ex vivo gamma-counting of samples, rats were killed 20 min, 1 h or 2 h after injection of [18F]IL-1RA. KEY RESULTS [18F]IL-1RA distribution into the major organs of interest was as follows: kidneys >> liver > lungs >> brain. In lungs and liver, [18F]IL-1RA uptake peaked within 1 min post-injection then decreased rapidly to reach a plateau from 10 min post-injection. In the brain, the uptake exhibited slower pharmacokinetics with a smaller post-injection peak and a plateau from 6 min onward. IL-1RA was rapidly metabolized and these metabolites represented similar to 40% of total activity in plasma and similar to 80% in urine, 20 min after injection. CONCLUSIONS AND IMPLICATIONS Preclinical PET imaging is a feasible method of assessing the biodistribution of new biological compounds of therapeutic interest rapidly. The biodistribution of [18F]IL-1RA reported here is in agreement with an earlier study suggesting low uptake in the normal brain, with rapid metabolism and excretion via the kidneys.