PET imaging of antisense oligonucleotide distribution in rat and nonhuman primate brains using click chemistry

Determination of a drug's biodistribution is critical to ensure that it reaches the target tissue of interest. This is particularly challenging in the brain, where invasive sampling methods may not be possible. Here, we present a pretargeted positron emission tomography (PET) imaging methodology that uses bioorthogonal click chemistry to determine the distribution of an antisense oligonucleotide (ASO) in the brains of rats and nonhuman primates after intrathecal dosing of ASO. A PET tracer, [¹⁸F]BIO-687, bearing a click-reactive <i>trans</i>-cyclooctene was developed and tested in conjunction with a test Malat1 ASO conjugated with a methyltetrazine group. PET imaging in rats demonstrated that the tracer had good kinetic properties for PET imaging in the rodent central nervous system and could react to form a covalent linkage with high specificity to the methyltetrazine-conjugated ASO in vivo. Furthermore, the amount of PET tracer reacted by cycloaddition with the methyltetrazine was determined to be dependent on the concentration of ASO-methyltetrazine in rat brain tissue, as determined by comparing the PET imaging signal with the liquid chromatography-mass spectrometry signal in the tissue homogenates. The approach was evaluated in cynomolgus macaques using both the Malat1 test ASO and a candidate therapeutic ASO, BIIB080, targeting the microtubule-associated protein tau (<i>MAPT</i>) gene. PET imaging showed favorable tracer kinetics and specific binding to both ASOs in nonhuman primate (NHP) brain in vivo. These results suggest that the PET imaging tracer [¹⁸F]BIO-687 could show the distribution of intrathecally delivered ASOs in the rat and NHP brains.