Dosimetry of 11C-carfentanil, a micro-opioid receptor imaging agent

11C-carfentanil is a radiopharmaceutical that selectively binds the mu-opiate receptor of the central nervous system. However, its dosimetry throughout the body and other organs has never been reported in the literature. The purpose of this study was to measure the radiation dosimetry of 11C-carfentanil in healthy human volunteers. The study was conducted within a regulatory framework that required its pharmacological safety to be assessed simultaneously.

The sample included two male and three female participants ranging in age from 28 to 49 years. Three to four scans were obtained over approximately 2 h starting immediately after the intravenous administration of 0.03 microg/kg of [C]carfentanil injected as a slow bolus (mean activity injected was 280+/-68 MBq). The fraction of the administered dose in 10 regions of interest was quantified from the attenuation-corrected counts obtained on the axial images. Monoexponential functions were fit to each time-activity curve using a nonlinear, least-squares regression algorithm. These curves were numerically integrated to yield the number of disintegrations per unit activity administered in source organs. Sex-specific radiation doses were then estimated with the medical internal radiation dose technique.

A few participants reported mild pharmacological effects of the radiotracer, primarily mild drowsiness, which is an expected side effect. The dose-limiting organ was the bladder wall, which received a mean of 3.65E-02 mGy/MBq. The mean effective dose equivalent and effective dose for 11C-carfentanil were 5.38E-03 and 4.59E-03 mSv/MBq, respectively.

The observed dosimetry values for 11C-carfentanil indicate that it is safe for imaging micro-opiate receptors in the central nervous system and periphery.