Process validation and preclinical development of a new PET cerebral blood flow tracer [11C]MMP for initial clinical trials

Background

2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) is commonly used for diagnosis of dementia because brain glucose metabolism reflects neuronal activity. However, as [¹⁸F]FDG is an analogue of glucose, accumulation of tracer in the brain is affected by plasma glucose levels. In contrast, cerebral blood flow (CBF) tracers are theoretically unaffected by plasma glucose levels and are therefore expected to be useful alternatives for the diagnosis of dementia in patients with diabetes. The techniques currently used for CBF imaging using single photon emission computed tomography (SPECT) and [¹⁵O]H₂O positron emission tomography (PET), but these are limited by their insufficient resolution and sensitivity for regional brain imaging, especially in patients with brain atrophy. N-isopropyl-4-[¹¹C]methylamphetamine ([¹¹C]MMP) is a possible CBF tracer with high resolution and sensitivity that exhibits comparable performance to that of [¹⁵O]H₂O in conscious monkey brains. We performed process validation of the radiosynthesis and preclinical development of [¹¹C]MMP prior to clinical translation.

Results

The decay-corrected yields of [¹¹C]MMP at the end of synthesis were 41.4 ± 6.5%, with 99.7 ± 0.3% radiochemical purity, and 192.3 ± 22.5 MBq/nmol molar activity. All process validation batches complied with the product specifications. The acute toxicity of MMP was evaluated at a dose of 3.55 mg/kg body weight, which is 10,000 times the potential maximum clinical dose of [¹¹C]MMP. The acute toxicity of [¹¹C]MMP injection at 150 or 200 times, to administer a postulated dose of 740 MBq of [¹¹C]MMP, was also evaluated after the decay-out of ¹¹C. No acute toxicity of MMP and [¹¹C]MMP injection was found. No mutagenic activity was observed for MMP. The effective dose calculated according to the Medical Internal Radiation Dose (MIRD) method was 5.4 µSv/MBq, and the maximum absorbed dose to the bladder wall was 57.6 µGy/MBq. MMP, a derivative of phenylalkylamine, showed binding to the sigma receptor, but had approximately 1/100 of the affinity of existing sigma receptor imaging agents. The affinity for other brain neuroreceptors was low.

Conclusions

[¹¹C]MMP shows acceptable pharmacological safety at the dose required for adequate PET imaging. The potential risk associated with [¹¹C]MMP PET imaging is well within the acceptable dose limit.