¹⁸F-ICMT-11, a Caspase-3–Specific PET Tracer for Apoptosis: Biodistribution and Radiation Dosimetry

Radiopharmaceutical Preparation

¹⁸F-ICMT-11 was synthesized from the precursor as previously described (15). The radiochemical purity of ¹⁸F-ICMT-11 was 100%, with a mean (±SD) specific activity of 1,951.5 ± 4,084 GBq/μmol (range, 110–12,032 GBq/μmol; the high specific activity in 1 subject was due to a lower cold concentration of ¹⁸F-ICMT-11, below the limit of quantification) and a pH of 5.41 ± 0.16 (range, 5.16–5.71).

Subjects

Eight healthy volunteers (4 men, 4 women), with a mean age of 63.1 ± 2.58 y (range, 59–68 y) and an average weight of 74 ± 15.4 kg (range, 52.1–98.7 kg), were enrolled. Inclusion criteria included age above 50 y, the ability to provide informed written consent, and a normal medical history (including physical examination, electrocardiogram, hematology, and biochemistry). Exclusion criteria included pregnancy and lactation. No specific fasting or food protocol was implemented for subjects. Ethical approval for the study was granted by the West-London Research Ethics Committee. All volunteers gave written informed consent to participate in the study, according to the Declaration of Helsinki guidelines. The administration of radioactivity for the PET/CT scans was approved by the Administration of Radioactive Substances Advisory Committee, United Kingdom.

Safety

Safety data collected up to 72 h after injection included adverse events (AEs), graded according to common toxicity criteria (version 4.03; http://www.eortc.be/services/doc/ctc/CTCAE_4.03_2010-0614_QuickReference_5x7.pdf); vital signs (blood pressure, respiratory rate, heart rate, and body temperature); physical examination; cardiovascular, lung, abdomen, and neurologic examinations; electrocardiogram; and laboratory parameters (serum biochemistry, hematology, coagulation, and urinalysis). Blood samples were collected through an in-dwelling cannula, and to avoid occlusion, heparinized saline was used for line flushing.

Image Acquisition and In Vivo Activity Measurement

Images were acquired on a Siemens Biograph 6 TruePoint PET/CT scanner (with TrueV; extended field of view) with 21.6-cm axial and 60.5-cm transaxial fields of view. Before administration of ¹⁸F-ICMT-11, an attenuation CT scan of each subject was obtained from the vertex to mid thigh (CT settings: 130 kV; 15 effective mAs; pitch, 1.5; slice thickness, 5 mm; rotation time, 0.6 s; and effective dose [ED], 2.5 mSv). The CT scan was followed by 4 emission scans consisting of contiguous static images acquired at 6–7 bed positions, with the inferior border set to mid thigh and the superior border set at the vertex. The subjects were permitted to rise and move after this and were encouraged to void to enhance tracer clearance. After this break, a second attenuation CT scan and 2 further emission scans were obtained. In total, 6 emission scans were taken at time points up to 4 h after injection, with nominal start times of 0, 8, 22, 56, 121, and 170 min after injection. To compensate for physical decay of ¹⁸F, the acquisition for each bed position was 1, 2, 5, 5, 7, and 7 min per individual bed position for these 6 time points after injection. The mean injected ¹⁸F-ICMT-11 activity and ICMT-11 cold dose in the subjects were 159 ± 3 MBq (range, 154–161 MBq) and 2.18 ± 1.39 μg (range, 0.1–4.44 μg), respectively.

Emission data were reconstructed using the ordered-subsets expectation maximization algorithm (3 iterations and 21 subsets). Volumes of interest (VOIs) were delineated manually on the attenuation CT scan, using a circle of a fixed diameter, depending on the size of the organ. To avoid tissue inhomogeneity, particularly near organ boundaries, the VOIs were defined within the boundaries of the normal-tissue organs, as visualized on the CT images and mapped to the corresponding emission scans using shape-based interpolation to extract the ¹⁸F activities with the ANALYZE analysis software (version 11; Biomedical Imaging Resource, Mayo Clinic). For the urinary bladder where the volume changes over time, the organ was outlined manually on the emission scans. All the volumes were outlined by a single investigator to avoid any interobserver variation. ¹⁸F activity concentrations in the images were decay-corrected to the time of injection. VOIs included brain, thyroid, thymus, breast, lungs, heart wall, aortic lumen, liver, gallbladder, spleen, stomach wall, pancreas, adrenals, kidneys, small intestine wall and contents, large intestine wall and contents, urinary bladder contents, uterus, testes/ovaries, gluteus maximus muscle, and red marrow (iliac crest).

Measurement of Blood Activity

Venous blood samples were taken at nominal times of 5, 10, 15, 30, 60, 90, 150, and 240 min after injection. Single aliquots, each of whole blood and plasma, were obtained from each sample, and ¹⁸F activity concentration was measured in a well counter previously cross-calibrated to the PET scanner. Urine was collected as voided up to 240 min after injection, and the volume and time of each micturition were recorded. Dual aliquots of urine were sampled from each void, and the mean ¹⁸F activity was measured; the resulting ¹⁸F activity concentration was multiplied by the volume of voided urine to provide the ¹⁸F activity excreted.

Biodistribution and Dosimetry

For each subject and for each source region, the non–decay-corrected ¹⁸F activity concentration over the 6 time points was generated. The integrated activity concentrations (area under the curve [AUC]) were calculated for all organ VOIs by applying a trapezoidal integration to the non–decay-corrected time–activity curves over the duration of the scan and the projection of decay in activity that was extrapolated beyond the last time point for imaging. The total number of disintegrations in each organ normalized to injected activity (subsequently referred to as the residence time [RT]: kBq·h·kBq⁻¹) was calculated as follows (Eq. 1):Eq. 1where τ is the RT, AUC is the area under the curve of the non–decay-corrected time–activity curve, and V_organ is the tabulated organ volume as used in the OLINDA/EXM version 1.1 (16). As the volume of the urinary bladder and the gallbladder changed over time, the image-derived organ volume at each scan time point was used to estimate the RT, rather than the tabulated organ volume. Lung activity was corrected for tissue density using a density value of 0.33 g/mL (17). The tissue density in the other organs was assumed to be 1.0 g/mL. With the exception of the urinary bladder, extrapolation of the integration past the last image data point (4 h) assumed only physical decay of the ¹⁸F label without biologic clearance—that is, an additional contribution to the AUC of time–activity curve_l/λ where time–activity curve_l is the last time–activity curve value measured and λ the decay constant for ¹⁸F. For the bladder, the measured voided activities were also included to form an extended 8-time-point time–activity curve (Supplemental Fig. 1; supplemental materials are available at http://jnm.snmjournals.org). The function:Eq. 2was used to model the bladder time–activity curve where A, B, C are the estimated parameters, t the time since administration of the tracer, and A_i the urine radioactivity voided at time t_i (18,19). Residual bladder activity was not included in the model because it was not known if complete voiding had occurred for these subjects, although any residual activities must have been small, compared with the voided activities. The sum of the squared differences between the function and the extended time–activity curve was minimized for each subject using the solver function in Excel 2010 (Microsoft, Inc.). Across all 8 subjects, the AUCs derived from the fitted function matched the measured AUCs calculated above with a fractional mean difference of −3% and SD of 6%. The decay constant C is usually assumed to be the same as λ on physical grounds, but in this case a larger value was observed. This larger value may be attributed to activity becoming trapped in other tissues of the body, most notably the gallbladder. As a result, for t greater than 5 h, Equation 2 predicts a faster decline in bladder activity than is physically possible. This is, however, beyond the observation time in the present study for which the bladder activity accumulation rate was always positive and was predicted to be very small after 4 h. The fitted functions were extrapolated to estimate the bladder RT for each subject for the following voiding scenarios: complete bladder voids every hour, every 2 h, and every 4 h after tracer administration. Finally, the RT of the remainder term (assumed to be homogeneously distributed in the body) was obtained by subtracting the sum of all defined organ RTs from the inverse of the decay constant for ¹⁸F.

The internal radiation dosimetry was evaluated through the RTs for the organs in each subject, provided as input to the OLINDA/EXM version 1.1 (16). The organ absorbed doses and ED for each individual subject were subsequently averaged. Because ICRP 103 (20) tissue-weighting factors (W_T) are not implemented in OLINDA/EXM 1.1, the male and female EDs are based on W_T from ICRP 60 (21).

An additional correction to the dose estimates arises because OLINDA/EXM 1.1 assumes that the stomach activity is all in the contents rather than the organ wall. The former was considered unlikely in this case. This only affects the β- (positron) contribution to the dose and is corrected by noting that the energy deposited in the wall from the contents is assumed within OLINDA/EXM to be half that calculated for the contents.