Preclinical evaluation of [18 F]MA3: a CB2 receptor agonist radiotracer for PET

Abstract

Background and purpose: Non-invasive in vivo imaging of cannabinoid CB₂ receptors using PET is pursued to study neuroinflammation. The purpose of this study is to evaluate the in vivo binding specificity of [¹⁸ F]MA3, a CB₂ receptor agonist, in a rat model with local overexpression of human (h) CB₂ receptors.

Methods: [¹⁸ F]MA3 was produced with good radiochemical yield and radiochemical purity. The radiotracer was evaluated in rats with local overexpression of hCB₂ receptors and in a healthy non-human primate using PET.

Key results: Ex vivo autoradiography demonstrated CB₂ -specific binding of [¹⁸ F]MA3 in rat hCB₂ receptor vector injected striatum. In a PET study, increased tracer binding in the hCB₂ receptor vector-injected striatum compared to the contralateral control vector-injected striatum was observed. Binding in hCB₂ receptor vector-injected striatum was blocked with a structurally non-related CB₂ receptor inverse agonist, and a displacement study confirmed the reversibility of tracer binding. This study identified the utility of mutated inactive vector model for evaluation of CB₂ receptor agonist PET tracers. [¹⁸ F]MA3 PET scans in the non-human primate showed good uptake and fast washout from brain, but no CB₂ receptor-specific binding was observed.

Conclusion and implications: Evaluation of [¹⁸ F]MA3 in a rat model with local overexpression of hCB₂ receptors showed CB₂ receptor-specific and reversible tracer binding. [¹⁸ F]MA3 showed good brain uptake and subsequent washout in a healthy non-human primate, but no specific binding was observed. Further clinical evaluation of [¹⁸ F]MA3 in patients with neuroinflammation is warranted.

Linked articles: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Figure 1

Chemical structures of [¹¹C]NE40, [¹¹C]MA2 and [¹⁸F]MA3, PET radiotracers for in vivo CB₂ receptor imaging.

Figure 2

PET study with [¹¹C]NE40 in rat model with local overexpression of hCB₂ receptors. Left, summed images of 0–90 min after injection; right, corresponding TACs of the right and left striatum. Data are expressed as SUV. Error bars correspond to SD.

Figure 3

PET data of [¹⁸F]MA3 in an hCB₂ receptor rat model: the right striatum was stereotactically injected with AAV‐hCB₂ receptor; and the left striatum was injected with control vector AAV‐eGFP. (A) Baseline study (n = 3): summed image of 0–120 min post‐injection of [¹⁸F]MA3 with corresponding TACs of the right and left striatum. (B) Pretreatment study with NE40 (n = 3): summed image of 0–120 min post‐injection of [¹⁸F]MA3 with corresponding TACs of the right and left striatum. NE40 (in vehicle) was injected i.p. at a dose of 1 mg kg⁻¹ 30 min before tracer injection. In the baseline scan, only vehicle was administered i.p. 30 min before tracer injection. Data are expressed as SUV. Error bars correspond to SD.

Figure 4

PET data of [¹⁸F]MA3 displacement study in a hCB₂ receptor rat model. The right striatum was stereotactically injected with AAV‐hCB₂ receptor; and the left striatum was injected with control vector AAV‐eGFP. NE40 (1 mg kg⁻¹) was injected i.v. (arrow) 20 min after tracer injection. Images (from transverse sections) corresponding to the chase experiment: summed image (0–20 min after tracer injection) before chase injection (A) and summed image (20–120 min after tracer injection) after chase injection (B), with corresponding TACs of the right and left striatum. Data are expressed as SUV. Error bars correspond to SD.

Figure 5

PET data of study with rimonabant blockade of CB₁ receptors. (A) Baseline study (n = 3) of 0–120 min post‐injection of [¹⁸F]MA3 TACs of whole brains and pretreatment study with rimonabant (n = 3) of 0–120 min after injection of [¹⁸F]MA3 TACs of whole brains. Rimonabant (1 mg kg⁻¹ in vehicle) was injected i.v. 15 min before tracer injection. In the baseline scan, only vehicle was administered i.v. 15 min before tracer injection. Data are expressed as SUV. Error bars correspond to SD.

Figure 6

Transverse section of an ex vivo autoradiography study performed in a rat model with local overexpression of hCB₂ receptors in the right striatum (n = 1). [¹⁸F]MA3 was injected i.v. via the tail vein, and the rat was killed at 30 min after injection. The autoradiogram shows specific binding of [¹⁸F]MA3 in the right striatum. Binding is expressed as digital light units (DLU) mm⁻² of the right and left striatum. Max, maximum; min, minimum.

Figure 7

PET data of [¹⁸F]MA3 in Rhesus monkey. (A) Transverse, sagittal and frontal PET sections of monkey brain. Summed images of 10–30, 30–60, 60–90 and 90–120 min after tracer injection for the baseline study (left images) and for the pretreatment study with NE40 (1 mg kg⁻¹; i.v.) 10 min before tracer injection (right images). (B) Corresponding TACs of total monkey brain at baseline (control) and after pretreatment (block).

Figure 8

Monkey plasma radiometabolite analysis and quantification of intact tracer in control and block conditions at 10, 30 and 60 min after tracer injection.

Figure 9

(A) Baseline TACs of [¹¹C]NE40 and [¹⁸F]MA3 in the left and right striatum of rats overexpressing hCB₂ receptors. (B) Right‐to‐left striatum SUV ratios as a function of time after injection of [¹¹C]NE40 and [¹⁸F]MA3.