Central Nervous System Biodistribution and Pharmacokinetics of Radiolabeled Tofersen in Rodents, Nonhuman Primates, and Humans

Antisense oligonucleotides (ASOs) are an important therapeutic modality across several therapeutic areas, offering currently available and potential future treatment options for patients. ASO pharmacokinetics, biodistribution, and regional brain uptake are not fully characterized, particularly in humans. Here, we report preclinical studies and the first-in-human imaging trial measuring the biodistribution of [^99mTc]Tc-MAG3-tofersen. The tracer was designed to be a proxy for tofersen (Qalsody; Biogen), an ASO approved for the treatment of amyotrophic lateral sclerosis in adults who have a variant in the <i>SOD1</i> gene (<i>SOD1</i>-ALS). <b>Methods:</b> Tofersen was conjugated to a MAG3 moiety, which chelates ^99mTc to yield [^99mTc]Tc-MAG3-tofersen. [^99mTc]Tc-MAG3-tofersen and unlabeled tofersen were intrathecally injected in rats, nonhuman primates (NHPs), and healthy human volunteers (<i>n</i> = 3) via lumbar puncture, followed by SPECT/CT imaging. Tofersen was coadministered at a therapeutic dose. The tracer [^99mTc]Tc-MAG3-tofersen was prepared with greater than 99% purity. <b>Results:</b> Findings in rats demonstrated that [^99mTc]Tc-MAG3-tofersen was a proxy measure of unlabeled tofersen, and dosimetry was calculated from NHP imaging data. In a clinical study, unlabeled tofersen coadministered with a microdose of [^99mTc]Tc-MAG3-tofersen (≤129.5 MBq [3.5 mCi]) was well-tolerated. Human dosimetry estimates were within safe radiation dose levels. Imaging showed consistent distribution of radiolabeled ASO throughout the spinal cord and brain across species, with clearance patterns diverging in humans. Although rats and NHPs demonstrated declining brain concentrations over the study duration, human brain uptake increased during the first 4 h after injection. Additionally, tracer clearance from the spine in rodents and NHPs plateaued after 6 h but continued to decrease in humans. Radiolabeled ASO clearance from the lumbar spine was observed across all species, with peripheral clearance mediated primarily through the liver and kidneys. Broad uptake of the ASO in the brain and spinal cord is consistent with the clinical effects of tofersen observed in individuals with the <i>SOD1</i>-ALS variation. <b>Conclusion:</b> In preclinical and human SPECT/CT studies, [^99mTc]Tc-MAG3-tofersen mirrored unlabeled drug distribution, showing broad spinal cord and brain uptake, with some differences in kinetics among species.