Purpose: In preclinical studies with rodent models of inflammatory diseases, [11C]CS1P1 has been identified as a promising imaging agent targeting sphingosine-1-phosphate receptor 1 (S1P1) in the central nervous system and other tissues. In preparation for USA Food and Drug Administration (FDA) approval of [11C]CS1P1 for human use, an acute biodistribution study in mice and an acute tolerability and toxicity evaluation in rats were conducted.
Procedures: Acute organ biodistribution and excretion data was obtained using male and female Swiss Webster mice intravenously (IV) injected with 4.8-10 MBq of [11C]CS1P1. The organ residence times for each harvested organ were calculated using the animal biodistribution data, and were entered in the program OLINDA/EXM for C-11 to obtain human radiation dosimetry estimates. Acute tolerability and toxicity studies were conducted in male and female Sprague Dawley rats. Rats were administered an IV bolus of either the vehicle control or 0.3 mg/kg CS1P1. Blood samples were collected and a gross post-mortem examination was conducted at day 2 or day 15 post-injection.
Results: The extrapolated human radiation dose estimates revealed that the highest organ dose was received by the liver with 24.05 μGy/MBq in males and 32.70 μGy/MBq in females. The effective dose (ED) estimates of [11C]CS1P1 were calculated at 3.5 μSv/MBq in males and 5.9 μSv/MBq in females. The acute tolerability and toxicity study identified 0.3 mg/kg as a no observable adverse effect level (NOAEL) dose, which is a ~ 300-fold dose multiple of the human equivalent dose of the mass to be injected for positron emission tomography (PET) imaging studies in humans as a no-observable-effect limit.
Conclusions: The toxicity study in rats suggested that injection dose of radiotracer [11C]CS1P1 with mass amount < 10 μg is safe for performing a human PET study. The dosimetry data supported an injection of 0.74 GBq (20 mCi) dose for human studies would be acceptable.
Keywords: Dosimetry; Microdose; Rodent; Sphingosine-1-phosphate receptor 1; [11C]CS1P1.